Microbial network of the carbonate precipitation process induced by microbial consortia and the potential application to crack healing in concrete.

PubMed

Zhang, Jiaguang; Zhou, Aijuan; Liu, Yuanzhen; Zhao, Bowei; Luan, Yunbo; Wang, Sufang; Yue, Xiuping; Li, Zhu

2017-11-06

Current studies have employed various pure-cultures for improving concrete durability based on microbially induced carbonate precipitation (MICP). However, there have been very few reports concerned with microbial consortia, which could perform more complex tasks and be more robust in their resistance to environmental fluctuations. In this study, we constructed three microbial consortia that are capable of MICP under aerobic (AE), anaerobic (AN) and facultative anaerobic (FA) conditions. The results showed that AE consortia showed more positive effects on inorganic carbon conversion than AN and FA consortia. Pyrosequencing analysis showed that clear distinctions appeared in the community structure between different microbial consortia systems. Further investigation on microbial community networks revealed that the species in the three microbial consortia built thorough energetic and metabolic interaction networks regarding MICP, nitrate-reduction, bacterial endospores and fermentation communities. Crack-healing experiments showed that the selected cracks of the three consortia-based concrete specimens were almost completely healed in 28 days, which was consistent with the studies using pure cultures. Although the economic advantage might not be clear yet, this study highlights the potential implementation of microbial consortia on crack healing in concrete.

Intrinsic fatigue crack propagation in aluminum-lithium alloys - The effect of gaseous environments

NASA Technical Reports Server (NTRS)

Piascik, Robert S.; Gangloff, Richard P.

1989-01-01

Gaseous environmental effects on intrinsic fatigue crack growth are significant for the Al-Li-Cu alloy 2090, peak aged. For both moderate Delta K-low R and low Delta K-high R regimes, crack growth rates decrease according to the environment order: purified water vapor, moist air, helium and oxygen. Gaseous environmental effects are pronounced near threshold and are not closure dominated. Here, embrittlement by low levels of H2O (ppm) supports hydrogen embrittlement and suggests that molecular transport controlled cracking, established for high Delta K-low R, is modified near threshold. Localized crack tip reaction sites or high R crack opening shape may enable the strong, environmental effect at low levels of Delta K. Similar crack growth in He and O2 eliminates the contribution of surface films to fatigue damage in alloy 2090. While 2090 and 7075 exhibit similar environmental trends, the Al-Li-Cu alloy is more resistant to intrinsic corrosion fatigue crack growth.

Study on Corrosion-induced Crack Initiation and Propagation of Sustaining Loaded RCbeams

NASA Astrophysics Data System (ADS)

Zhong, X. P.; Li, Y.; Yuan, C. B.; Yang, Z.; Chen, Y.

2018-05-01

For 13 pieces of reinforced concrete beams with HRB500 steel bars under long-term sustained loads, at time of corrosion-induced initial crack of concrete, and corrosion-induced crack widths of 0.3mm and 1mm, corrosion of steel bars and time-varying behavior of corrosion-induced crack width were studied by the ECWD (Electro-osmosis - constant Current – Wet and Dry cycles) accelerated corrosion method. The results show that when cover thickness was between 30 and 50mm,corrosion rates of steel bars were between 0.8% and 1.7% at time of corrosion-induced crack, and decreased with increasing concrete cover thickness; when corrosion-induced crack width was 0.3mm, the corrosion rate decreased with increasing steel bar diameter, and increased with increasing cover thickness; its corrosion rate varied between 0.98% and 4.54%; when corrosion-induced crack width reached 1mm, corrosion rate of steel bars was between 4% and 4.5%; when corrosion rate of steel bars was within 5%, the maximum and average corrosion-induced crack and corrosion rate of steel bars had a good linear relationship. The calculation model predicting the maximum and average width of corrosion-induced crack is given in this paper.

Fatigue crack closure: a review of the physical phenomena

PubMed Central

Pippan, R.

2017-01-01

Abstract Plasticity‐induced, roughness‐induced and oxide‐induced crack closures are reviewed. Special attention is devoted to the physical origin, the consequences for the experimental determination and the prediction of the effective crack driving force for fatigue crack propagation. Plasticity‐induced crack closure under plane stress and plane strain conditions require, in principle, a different explanation; however, both types are predictable. This is even the case in the transition region from the plane strain to the plane stress state and all types of loading conditions including constant and variable amplitude loading, the short crack case or the transition from small‐scale to large‐scale yielding. In contrast, the prediction of roughness‐induced and oxide‐induced closures is not as straightforward. PMID:28616624

Evolution of Residual-Strain Distribution through an Overload-Induced Retardation Period during Fatigue Crack Growth

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lee, S. Y.; Sun, Yinan; An, Ke

2010-01-01

Neutron diffraction was employed to investigate the crack-growth retardation phenomenon after a single tensile overload by mapping both one-dimensional and two-dimensional residual-strain distributions around the crack tip in a series of compact-tension specimens representing various crack-growth stages through an overload-induced retardation period. The results clearly show a large compressive residual-strain field near the crack tip immediately after the overload. As the fatigue crack propagates through the overload-induced plastic zone, the compressive residual strains are gradually relaxed, and a new compressive residual-strain field is developed around the propagating crack tip, illustrating that the subsequent fatigue-induced plastic zone grows out of themore » large plastic zone caused by the overloading. The relationship between the overload-induced plastic zone and subsequent fatigue-induced plastic zone, and its influence on the residual-strain distributions in the perturbed plastic zone are discussed.« less

Effect of Thermomechanical Treatment on the Environmentally Induced Cracking Behavior of AA7075 Alloy

NASA Astrophysics Data System (ADS)

Ghosh, Rahul; Venugopal, A.; Sankaravelayudham, P.; Panda, Rajiv; Sharma, S. C.; George, Koshy M.; Raja, V. S.

2015-02-01

The influence of thermomechanical treatment on the stress corrosion cracking behavior of AA7075 aluminum alloy forgings was examined in 3.5% NaCl solution by varying the extent of thermomechanical working imparted to each of the conditions. The results show that inadequate working during billet processing resulted in inferior corrosion and mechanical properties. However, more working with intermediate pre-heating stages also led to precipitation of coarse particles resulting in lowering of mechanical properties marginally and a significant reduction in the general/pitting corrosion resistance. The results obtained in the present study indicate that optimum working with controlled pre-heating levels is needed during forging to achieve the desired properties. It is also demonstrated that AA7075 in the over aged condition does not show any environmental cracking susceptibility in spite of the microstructural variations in terms of size and volume fraction of the precipitates. However, the above microstructural variations definitely affected the pitting corrosion and mechanical properties significantly and hence a strict control over the working and pre-heating stages during billet processing is suggested.

Hydrogen-Induced Delayed Cracking in TRIP-Aided Lean-Alloyed Ferritic-Austenitic Stainless Steels.

PubMed

Papula, Suvi; Sarikka, Teemu; Anttila, Severi; Talonen, Juho; Virkkunen, Iikka; Hänninen, Hannu

2017-06-03

Susceptibility of three lean-alloyed ferritic-austenitic stainless steels to hydrogen-induced delayed cracking was examined, concentrating on internal hydrogen contained in the materials after production operations. The aim was to study the role of strain-induced austenite to martensite transformation in the delayed cracking susceptibility. According to the conducted deep drawing tests and constant load tensile testing, the studied materials seem not to be particularly susceptible to delayed cracking. Delayed cracks were only occasionally initiated in two of the materials at high local stress levels. However, if a delayed crack initiated in a highly stressed location, strain-induced martensite transformation decreased the crack arrest tendency of the austenite phase in a duplex microstructure. According to electron microscopy examination and electron backscattering diffraction analysis, the fracture mode was predominantly cleavage, and cracks propagated along the body-centered cubic (BCC) phases ferrite and α'-martensite. The BCC crystal structure enables fast diffusion of hydrogen to the crack tip area. No delayed cracking was observed in the stainless steel that had high austenite stability. Thus, it can be concluded that the presence of α'-martensite increases the hydrogen-induced cracking susceptibility.

Hydrogen-Induced Delayed Cracking in TRIP-Aided Lean-Alloyed Ferritic-Austenitic Stainless Steels

PubMed Central

Papula, Suvi; Sarikka, Teemu; Anttila, Severi; Talonen, Juho; Virkkunen, Iikka; Hänninen, Hannu

2017-01-01

Susceptibility of three lean-alloyed ferritic-austenitic stainless steels to hydrogen-induced delayed cracking was examined, concentrating on internal hydrogen contained in the materials after production operations. The aim was to study the role of strain-induced austenite to martensite transformation in the delayed cracking susceptibility. According to the conducted deep drawing tests and constant load tensile testing, the studied materials seem not to be particularly susceptible to delayed cracking. Delayed cracks were only occasionally initiated in two of the materials at high local stress levels. However, if a delayed crack initiated in a highly stressed location, strain-induced martensite transformation decreased the crack arrest tendency of the austenite phase in a duplex microstructure. According to electron microscopy examination and electron backscattering diffraction analysis, the fracture mode was predominantly cleavage, and cracks propagated along the body-centered cubic (BCC) phases ferrite and α’-martensite. The BCC crystal structure enables fast diffusion of hydrogen to the crack tip area. No delayed cracking was observed in the stainless steel that had high austenite stability. Thus, it can be concluded that the presence of α’-martensite increases the hydrogen-induced cracking susceptibility. PMID:28772975

Effects of UV Aging on the Cracking of Titanium Oxide Layer on Poly(ethylene terephthalate) Substrate: Preprint

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Chao; Gray, Matthew H.; Tirawat, Robert

Thin oxide and metal films deposited on polymer substrates is an emerging technology for advanced reflectors for concentrated solar power applications, due to their unique combination of light weight, flexibility and inexpensive manufacture. Thus far, there is little knowledge on the mechanical integrity or structural persistence of such multi-layer thin film systems under long-term environmental aging. In this paper, the cracking of a brittle titanium dioxide layer deposited onto elasto-plastic poly(ethylene terephthalate) (PET) substrate is studied through a combination of experiment and modeling. In-situ fragmentation tests have been conducted to monitor the onset and evolution of cracks both on pristinemore » and on samples aged with ultraviolet (UV) light. An analytical model is presented to simulate the cracking behavior and to predict the effects of UV aging. Based on preliminary experimental observation, the effect of aging is divided into three aspects and analyzed independently: mechanical property degradation of the polymer substrate; degradation of the interlayer between substrate and oxide coating; and internal stress-induced cracks on the oxide coating.« less

Evaluation of Stress Corrosion Resistance Properties of 15CrMoR(H) in H2S Environment

NASA Astrophysics Data System (ADS)

Zhang, Yiliang; Wang, Jing; Wu, Mingyao; Li, Shurui; Liu, Wenbin

To evaluate the hydrogen resistant properties of the 15CrMoR(H) with new smelting process, according to NACE and National Standards, three tests including NACE standard tensile test, NACE standard bent-beam test and hydrogen induced cracking test are executed in saturated hydrogen sulfide(H2S) environment. Stress-life mathematical model of this material is given by analyzing and fitting the results of tensile test. Test results show that the threshold sth of tensile test is 0.7R eL(252MPa); the threshold nominal stress SC of bent-beam is higher than 4.5 R eL (1620MPa); for HIC test, the crack length rate CLR is 4.40%, the crack thickness rate CTR is 0.87% and the crack sensitive rate CSR is 0.04%. Compare with EFC standard, the safety margin of HIC test is 3.4, 3.4 and 37.5 times respectively. All the experimental results show that the new 15CrMoR(H) material has excellent H2S environmental cracking resistance properties.

Hydrogen-enhanced cracking revealed by in situ micro-cantilever bending test inside environmental scanning electron microscope.

PubMed

Deng, Yun; Hajilou, Tarlan; Barnoush, Afrooz

2017-07-28

To evaluate the hydrogen (H)-induced embrittlement in iron aluminium intermetallics, especially the one with stoichiometric composition of 50 at.% Al, a novel in situ micro-cantilever bending test was applied within an environmental scanning electron microscope (ESEM), which provides both a full process monitoring and a clean, in situ H-charging condition. Two sets of cantilevers were analysed in this work: one set of un-notched cantilevers, and the other set with focused ion beam-milled notch laying on two crystallographic planes: (010) and (110). The cantilevers were tested under two environmental conditions: vacuum (approximately 5 × 10 -4  Pa) and ESEM (450 Pa water vapour). Crack initiation at stress-concentrated locations and propagation to cause catastrophic failure were observed when cantilevers were tested in the presence of H; while no cracking occurred when tested in vacuum. Both the bending strength for un-notched beams and the fracture toughness for notched beams were reduced under H exposure. The hydrogen embrittlement (HE) susceptibility was found to be orientation dependent: the (010) crystallographic plane was more fragile to HE than the (110) plane.This article is part of the themed issue 'The challenges of hydrogen and metals'. © 2017 The Author(s).

Fatigue Crack Propagation from Notched Specimens of 304 SS in elevated Temperature Aqueous Environment

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wire, G. L.; Mills, W. J.

2002-08-01

Fatigue crack propagation (FCP) rates for 304 stainless steel (304SS) were determined in 24 degree C and 288 degree C air and 288 degree C water using double-edged notch (DEN) specimens of 304 stainless steel (304 SS). Test performed at matched loading conditions in air and water at 288 degree C with 20-6- cc h[sub]2/kg h[sub]2O provided a direct comparison of the relative crack growth rates in air and water over a wide range of crack growth rates. The DEN crack extension ranged from short cracks (0.03-0.25 mm) to long cracks up to 4.06 mm, which are consistent with conventionalmore » deep crack tests. Crack growth rates of 304 SS in water were about 12 times the air rate. This 12X environmental enhancement persisted to crack extensions up to 4.06 mm, far outside the range associated with short crack effects. The large environmental degradation for 304 SS crack growth is consistent with the strong reduction of fatigue life in high hydrogen water. Further, very similar environmental effects w ere reported in fatigue crack growth tests in hydrogen water chemistry (HWC). Most literature data in high hydrogen water show only a mild environmental effect for 304 SS, of order 2.5 times air or less, but the tests were predominantly performed at high cyclic stress intensity or equivalently, high air rates. The environmental effect in low oxygen environments at low stress intensity depends strongly on both the stress ratio, R, and the load rise time, T[sub]r, as recently reported for austenitic stainless steel in BWR water. Fractography was performed for both tests in air and water. At 288 degree C in water, the fracture surfaces were crisply faceted with a crystallographic appearance, and showed striations under high magnification. The cleavage-like facets on the fracture surfaces suggest that hydrogen embrittlement is the primary cause of accelerated cracking.« less

Time-dependent corrosion fatique crack propagation in 7000 series aluminum alloys. M.S. Thesis

NASA Technical Reports Server (NTRS)

Mason, Mark E.

1995-01-01

The goal of this research is to characterize environmentally assisted subcritical crack growth for the susceptible short-longitudinal orientation of aluminum alloy 7075-T651, immersed in acidified and inhibited NaCl solution. This work is necessary in order to provide a basis for incorporating environmental effects into fatigue crack propagation life prediction codes such as NASA-FLAGRO (NASGRO). This effort concentrates on determining relevant inputs to a superposition model in order to more accurately model environmental fatigue crack propagation.

Failures in Hybrid Microcircuits During Environmental Testing. History Cases

NASA Technical Reports Server (NTRS)

Teverovsky, Alexander

2008-01-01

This purpose of this viewgraph presentation is to discuss failures in hermetic hybrids observed at the GSFC PA Lab during environmental stress testing. The cases discussed are: Case I. Substrate metallization failures during Thermal cycling (TC). Case II. Flex lid-induced failure. Case Ill. Hermeticity failures during TC. Case IV. Die metallization cracking during TC. and how many test cycles and parts is necessary? Case V. Wire Bond failures after life test. Case VI. Failures caused by Au/In IMC growth.

Assessment Criteria for Environmental Cracking of High-Strength Steels in Seawater.

DTIC Science & Technology

1983-03-18

OF HIGH-STRENGTH STEELS IN SEAWATER T. W. Crooker and J. A. Hauser II Material Science and Technology Division Naval Research Laboratory Washington, DC...AGSTPACT (Continued) environmental cracking. This report provides a summary of state-of-theart technology for assessing environmental cracking of high...This report will address this issue, both in terms of existing technology and needs for further

Earthquake-induced ground failures in Italy from a reviewed database

NASA Astrophysics Data System (ADS)

Martino, S.; Prestininzi, A.; Romeo, R. W.

2013-05-01

A database (Italian acronym CEDIT) of earthquake-induced ground failures in Italy is presented, and the related content is analysed. The catalogue collects data regarding landslides, liquefaction, ground cracks, surface faulting and ground-level changes triggered by earthquakes of Mercalli intensity 8 or greater that occurred in the last millennium in Italy. As of January 2013, the CEDIT database has been available online for public use (URL: http://www.ceri.uniroma1.it/cn/index.do?id=230&page=55) and is presently hosted by the website of the Research Centre for Geological Risks (CERI) of the "Sapienza" University of Rome. Summary statistics of the database content indicate that 14% of the Italian municipalities have experienced at least one earthquake-induced ground failure and that landslides are the most common ground effects (approximately 45%), followed by ground cracks (32%) and liquefaction (18%). The relationships between ground effects and earthquake parameters such as seismic source energy (earthquake magnitude and epicentral intensity), local conditions (site intensity) and source-to-site distances are also analysed. The analysis indicates that liquefaction, surface faulting and ground-level changes are much more dependent on the earthquake source energy (i.e. magnitude) than landslides and ground cracks. In contrast, the latter effects are triggered at lower site intensities and greater epicentral distances than the other environmental effects.

Effects of External Hydrogen on Hydrogen Transportation and Distribution Around the Fatigue Crack Tip in Type 304 Stainless Steel

NASA Astrophysics Data System (ADS)

Chen, Xingyang; Zhou, Chengshuang; Cai, Xiao; Zheng, Jinyang; Zhang, Lin

2017-10-01

The effects of external hydrogen on hydrogen transportation and distribution around the fatigue crack tip in type 304 stainless steel were investigated by using hydrogen microprint technique (HMT) and thermal desorption spectrometry. HMT results show that some silver particles induced by hydrogen release are located near the fatigue crack and more silver particles are concentrated around the crack tip, which indicates that hydrogen accumulates in the vicinity of the crack tip during the crack growth in hydrogen gas environment. Along with the crack propagation, strain-induced α' martensite forms around the crack tip and promotes hydrogen invasion into the matrix, which will cause the crack initiation and propagation at the austenite/ α' martensite interface. In addition, the hydrogen content in the vicinity of the crack tip is higher than that at the crack edge far away from the crack tip, which is related to the stress state and strain-induced α' martensite.

Fatigue crack sizing in rail steel using crack closure-induced acoustic emission waves

NASA Astrophysics Data System (ADS)

Li, Dan; Kuang, Kevin Sze Chiang; Ghee Koh, Chan

2017-06-01

The acoustic emission (AE) technique is a promising approach for detecting and locating fatigue cracks in metallic structures such as rail tracks. However, it is still a challenge to quantify the crack size accurately using this technique. AE waves can be generated by either crack propagation (CP) or crack closure (CC) processes and classification of these two types of AE waves is necessary to obtain more reliable crack sizing results. As the pre-processing step, an index based on wavelet power (WP) of AE signal is initially established in this paper in order to distinguish between the CC-induced AE waves and their CP-induced counterparts. Here, information embedded within the AE signal was used to perform the AE wave classification, which is preferred to the use of real-time load information, typically adopted in other studies. With the proposed approach, it renders the AE technique more amenable to practical implementation. Following the AE wave classification, a novel method to quantify the fatigue crack length was developed by taking advantage of the CC-induced AE waves, the count rate of which was observed to be positively correlated with the crack length. The crack length was subsequently determined using an empirical model derived from the AE data acquired during the fatigue tests of the rail steel specimens. The performance of the proposed method was validated by experimental data and compared with that of the traditional crack sizing method, which is based on CP-induced AE waves. As a significant advantage over other AE crack sizing methods, the proposed novel method is able to estimate the crack length without prior knowledge of the initial crack length, integration of AE data or real-time load amplitude. It is thus applicable to the health monitoring of both new and existing structures.

Understanding the Risk of Chloride Induced Stress Corrosion Cracking of Interim Storage Containers for the Dry Storage of Spent Nuclear Fuel: Evolution of Brine Chemistry on the Container Surface.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Enos, David; Bryan, Charles R.

Although the susceptibility of austenitic stainless steels to chloride-induced stress corrosion cracking is well known, uncertainties exist in terms of the environmental conditions that exist on the surface of the storage containers. While a diversity of salts is present in atmospheric aerosols, many of these are not stable when placed onto a heated surface. Given that the surface temperature of any container storing spent nuclear fuel will be well above ambient, it is likely that salts deposited on its surface may decompose or degas. To characterize this effect, relevant single and multi-salt mixtures are being evaluated as a function ofmore » temperature and relative humidity to establish the rates of degassing, as well as the likely final salt and brine chemistries that will remain on the canister surface.« less

Characterizing the effect of creep on stress corrosion cracking of cold worked Alloy 690 in supercritical water environment

NASA Astrophysics Data System (ADS)

Zhang, Lefu; Chen, Kai; Du, Donghai; Gao, Wenhua; Andresen, Peter L.; Guo, Xianglong

2017-08-01

The effect of creep on stress corrosion cracking (SCC) was studied by measuring crack growth rates (CGRs) of 30% cold worked (CW) Alloy 690 in supercritical water (SCW) and inert gas environments at temperatures ranging from 450 °C to 550 °C. The SCC crack growth rate under SCW environments can be regarded as the cracking induced by the combined effect of corrosion and creep, while the CGR in inert gas environment can be taken as the portion of creep induced cracking. Results showed that the CW Alloy 690 sustained high susceptibility to intergranular (IG) cracking, and creep played a dominant role in the SCC crack growth behavior, contributing more than 80% of the total crack growth rate at each testing temperature. The temperature dependence of creep induced CGRs follows an Arrhenius dependency, with an apparent activation energy (QE) of about 225 kJ/mol.

Indentation studies on Y[sub 2]O[sub 3]-stabilized ZrO[sub 2]; 1: Development of indentation-induced cracks

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kaliszewski, M.S.; Behrens, G.; Heuer, A.H.

1994-05-01

The development of Vickers indent-induced cracks with increasing indent load has been studied in two Y[sub 2]O[sub 3]-stabilized ZrO[sub 2] ceramics. Such cracks form as radial or Palmqvist cracks at low loads, assume kidney'' shapes at intermediate loads, and finally form median (half-penny) cracks at high loads. The plastic zone directly beneath the indent is uncracked; a significant portion of the plasticity induced by indentation occurs by martensitic transformation.

Inspecting cracks in foam insulation

NASA Technical Reports Server (NTRS)

Cambell, L. W.; Jung, G. K.

1979-01-01

Dye solution indicates extent of cracking by penetrating crack and showing original crack depth clearly. Solution comprised of methylene blue in denatured ethyl alcohol penetrates cracks completely and evaporates quickly and is suitable technique for usage in environmental or structural tests.

Effective Crack Control of Concrete by Self-Healing of Cementitious Composites Using Synthetic Fiber

PubMed Central

Choi, Heesup; Inoue, Masumi; Kwon, Sukmin; Choi, Hyeonggil; Lim, Myungkwan

2016-01-01

Although concrete is one of the most widely used construction materials, it is characterized by substantially low tensile strength in comparison to its compression strength, and the occurrence of cracks is unavoidable. In addition, cracks progress due to environmental conditions including damage by freezing, neutralization, and salt, etc. Moreover, detrimental damage can occur in concrete structures due to the permeation of deteriorating elements such as Cl− and CO2. Meanwhile, under an environment in which moisture is being supplied and if the width of the crack is small, a phenomenon of self-healing, in which a portion of the crack is filled in due to the rehydration of the cement particles and precipitation of CaCO3, is been confirmed. In this study, cracks in cementitious composite materials are effectively dispersed using synthetic fibers, and for cracks with a width of more than 0.1 mm, a review of the optimal self-healing conditions is conducted along with the review of a diverse range of self-healing performance factors. As a result, it was confirmed that the effective restoration of watertightness through the production of the majority of self-healing products was achieved by CaCO3 and the use of synthetic fibers with polarity, along with the effect of inducing a multiple number of hairline cracks. In addition, it was confirmed that the self-healing conditions of saturated Ca(OH)2 solution, which supplied CO2 micro-bubbles, displayed the most effective self-healing performance in the surface and internal sections of the cracks. PMID:28773372

Laser-induced cracks in ice due to temperature gradient and thermal stress

NASA Astrophysics Data System (ADS)

Yang, Song; Yang, Ying-Ying; Zhang, Jing-Yuan; Zhang, Zhi-Yan; Zhang, Ling; Lin, Xue-Chun

2018-06-01

This work presents the experimental and theoretical investigations on the mechanism of laser-induce cracks in ice. The laser-induced thermal gradient would generate significant thermal stress and lead to the cracking without thermal melting in the ice. The crack density induced by a pulsed laser in the ice critically depends on the laser scanning speed and the size of the laser spot on the surface, which determines the laser power density on the surface. A maximum of 16 cracks within an area of 17 cm × 10 cm can be generated when the laser scanning speed is at 10 mm/s and the focal point of the laser is right on the surface of the ice with a laser intensity of ∼4.6 × 107 W/cm2. By comparing the infrared images of the ice generated at various experimental conditions, it was found that a larger temperature gradient would result in more laser-induced cracks, while there is no visible melting of the ice by the laser beam. The data confirm that the laser-induced thermal stress is the main cause of the cracks created in the ice.

Influence of fatigue crack wake length and state of stress on crack closure

NASA Technical Reports Server (NTRS)

Telesman, J.; Fisher, D. M.

1986-01-01

The location of crack closure with respect to crack wake and specimen thickness under different loading conditions was determined. The rate of increase of K sub CL in the crack wake was found to be significantly higher for plasticity induced closure in comparison to roughness induced closure. Roughness induced closure was uniform throughout the thickness of the specimen while plasticity induced closure levels were 50 percent higher in the near surface region than in the midthickness. The influence of state of stress on low-high load interaction effects was also examined. Load interaction effects differed depending upon the state of stress and were explained in terms of delta K sub eff.

Influence of fatigue crack wake length and state of stress on crack closure

NASA Technical Reports Server (NTRS)

Telesman, Jack; Fisher, Douglas M.

1988-01-01

The location of crack closure with respect to crack wake and specimen thickness under different loading conditions was determined. The rate of increase of K sub CL in the crack wake was found to be significantly higher for plasticity induced closure in comparison to roughness induced closure. Roughness induced closure was uniform throughout the thickness of the specimen while plasticity induced closure levels were 50 percent higher in the near surface region than in the midthickness. The influence of state of stress on low-high load interaction effects was also examined. Load interaction effects differed depending upon the state of stress and were explained in terms of delta K sub eff.

Separating the Influence of Environment from Stress Relaxation Effects on Dwell Fatigue Crack Growth

NASA Technical Reports Server (NTRS)

Telesman, Jack; Gabb, Tim; Ghosn, Louis J.

2016-01-01

Seven different microstructural variations of LSHR were produced by controlling the cooling rate and the subsequent aging and thermal exposure heat treatments. Through cyclic fatigue crack growth testing performed both in air and vacuum, it was established that four out of the seven LSHR heat treatments evaluated, possessed similar intrinsic environmental resistance to cyclic crack growth. For these four heat treatments, it was further shown that the large differences in dwell crack growth behavior which still persisted, were related to their measured stress relaxation behavior. The apparent differences in their dwell crack growth resistance were attributed to the inability of the standard linear elastic fracture mechanics (LEFM) stress intensity parameter to account for visco-plastic behavior. Crack tip stress relaxation controls the magnitude of the remaining local tensile stresses which are directly related to the measured dwell crack growth rates. It was hypothesized that the environmentally weakened grain boundary crack tip regions fail during the dwells when their strength is exceeded by the remaining local crack tip tensile stresses. It was shown that the classical creep crack growth mechanisms such as grain boundary sliding did not contribute to crack growth, but the local visco-plastic behavior still plays a very significant role by determining the crack tip tensile stress field which controls the dwell crack growth behavior. To account for the influence of the visco-plastic behavior on the crack tip stress field, an empirical modification to the LEFM stress intensity parameter, Kmax, was developed by incorporating into the formulation the remaining stress level concept as measured by simple stress relaxation tests. The newly proposed parameter, Ksrf, did an excellent job in correlating the dwell crack growth rates for the four heat treatments which were shown to have similar intrinsic environmental cyclic fatigue crack growth resistance.

Overload retardation due to plasticity-induced crack closure

NASA Technical Reports Server (NTRS)

Fleck, N. A.; Shercliff, H. R.

1989-01-01

Experiments are reported which show that plasticity-induced crack closure can account for crack growth retardation following an overload. The finite element method is used to provide evidence which supports the experimental observations of crack closure. Finally, a simple model is presented which predicts with limited success the retardation transient following an overload.

Gaseous hydrogen-induced cracking of Ti-5Al-2.5Sn.

NASA Technical Reports Server (NTRS)

Williams, D. P.; Nelson, H. G.

1972-01-01

Study of the kinetics of hydrogen-induced cracking in the Ti-5Al-2.5Sn titanium alloy, which has a structure of acicular alpha platelets in a beta matrix. The crack-growth rate at low stress-intensity levels was found to be exponentially dependent on stress intensity but essentially independent of temperature. The crack-growth rate at intermediate stress-intensity levels was found to be independent of stress intensity but dependent on temperature in such a way that crack-growth rate was controlled by a thermally activated mechanism having an activation energy of 5500 cal/mole and varied as the square root of the hydrogen pressure. The crack-growth rate at stress-intensity levels very near the fracture toughness is presumed to be independent of environment. The results are interpreted to suggest that crack growth at high stress intensities is controlled by normal, bulk failure mechanisms such as void coalescence and the like. At intermediate stress-intensity levels the transport of hydrogen to some interaction site along the alpha-beta boundary is the rate-controlling mechanism. The crack-growth behavior at low stress intensities suggests that the hydrogen interacts at this site to produce a strain-induced hydride which, in turn, induces crack growth by restricting plastic flow at the crack tip.

Earthquake-induced ground failures in Italy from a reviewed database

NASA Astrophysics Data System (ADS)

Martino, S.; Prestininzi, A.; Romeo, R. W.

2014-04-01

A database (Italian acronym CEDIT) of earthquake-induced ground failures in Italy is presented, and the related content is analysed. The catalogue collects data regarding landslides, liquefaction, ground cracks, surface faulting and ground changes triggered by earthquakes of Mercalli epicentral intensity 8 or greater that occurred in the last millennium in Italy. As of January 2013, the CEDIT database has been available online for public use (http://www.ceri.uniroma1.it/cn/gis.jsp ) and is presently hosted by the website of the Research Centre for Geological Risks (CERI) of the Sapienza University of Rome. Summary statistics of the database content indicate that 14% of the Italian municipalities have experienced at least one earthquake-induced ground failure and that landslides are the most common ground effects (approximately 45%), followed by ground cracks (32%) and liquefaction (18%). The relationships between ground effects and earthquake parameters such as seismic source energy (earthquake magnitude and epicentral intensity), local conditions (site intensity) and source-to-site distances are also analysed. The analysis indicates that liquefaction, surface faulting and ground changes are much more dependent on the earthquake source energy (i.e. magnitude) than landslides and ground cracks. In contrast, the latter effects are triggered at lower site intensities and greater epicentral distances than the other environmental effects.

Separating the Influence of Environment from Stress Relaxation Effects on Dwell Fatigue Crack Growth in a Nickel-Base Disk Alloy

NASA Technical Reports Server (NTRS)

Telesman, J.; Gabb, T. P.; Ghosn, L. J.

2016-01-01

Both environmental embrittlement and crack tip visco-plastic stress relaxation play a significant role in determining the dwell fatigue crack growth (DFCG) resistance of nickel-based disk superalloys. In the current study performed on the Low Solvus High Refractory (LSHR) disk alloy, the influence of these two mechanisms were separated so that the effects of each could be quantified and modeled. Seven different microstructural variations of LSHR were produced by controlling the cooling rate and the subsequent aging and thermal exposure heat treatments. Through cyclic fatigue crack growth testing performed both in air and vacuum, it was established that four out of the seven LSHR heat treatments evaluated, possessed similar intrinsic environmental resistance to cyclic crack growth. For these four heat treatments, it was further shown that the large differences in dwell crack growth behavior which still persisted, were related to their measured stress relaxation behavior. The apparent differences in their dwell crack growth resistance were attributed to the inability of the standard linear elastic fracture mechanics (LEFM) stress intensity parameter to account for visco-plastic behavior. Crack tip stress relaxation controls the magnitude of the remaining local tensile stresses which are directly related to the measured dwell crack growth rates. It was hypothesized that the environmentally weakened grain boundary crack tip regions fail during the dwells when their strength is exceeded by the remaining local crack tip tensile stresses. It was shown that the classical creep crack growth mechanisms such as grain boundary sliding did not contribute to crack growth, but the local visco-plastic behavior still plays a very significant role by determining the crack tip tensile stress field which controls the dwell crack growth behavior. To account for the influence of the visco-plastic behavior on the crack tip stress field, an empirical modification to the LEFM stress intensity parameter, Kmax, was developed by incorporating into the formulation the remaining stress level concept as measured by simple stress relaxation tests. The newly proposed parameter, Ksrf, did an excellent job in correlating the dwell crack growth rates for the four heat treatments which were shown to have similar intrinsic environmental cyclic fatigue crack growth resistance.

Analysis of cracks induced by elevated temperature in rock using micro-focus X-ray CT

NASA Astrophysics Data System (ADS)

Cheon, D. S.; Park, E. S.

2016-12-01

Thermal energy storage facilities and deep borehole nuclear waste disposal in the underground are repeatedly applied by heat. The thermal stress induced by heat can generate micro-cracks and extend the existing micro-cracks of rocks. For long-term stabilities of the above facilities, the features of thermal induced cracks should be investigated. In this paper, we investigated occurred the features of thermal cracks using micro-focus X-ray CT before and after thermal experiments. Two different kinds of rock core specimens (limestone, granite) were heated within the furnace with the elevated temperatures of 250 °C, 400 °C and 550 °C. In thermal experiments, we heated rocks with the speed of 1.5 ºC /min to avoid thermal shock. Total 16 cases were subjected to X-ray imaging and post-processing to observe thermally induced fractures. Micro-cracks induced by thermal loading may not be extractable by a thresholding method such that the manual tracking within the ROI (Region of Interest) was implemented by using the VG Studio Software. Identified fractures were grouped by each object whose orientation was fitted by 3D plane. And then, its normal vector was computed and visualized. Nominal fractures (less than 10 voxel size) were excluded. Each fracture was projected on the 3D sphere and its volume was represented by color map. Thermal induced cracks in the limestone observed on CT images were very small. On the other hand, they could be more clearly observed in the granite. In case of limestone, the number of cracks is only 4 after heating up 550 °C and most of them occurred within the mineral. In case of granite, 157 cracks are detected both at the boundaries of minerals and within the mineral. In both rocks, the development of thermal cracks within a certain mineral was superior to them that occurred along the interface between minerals. After heating up to 550 °C the occurred cracks significantly increased. Crack volume was also similar pattern to the number of cracks. However the average volume of cracks in limestone is larger than granite. The normal vector of the cracks is similar to the bedding plane of limestone and texture of granite. These cracks affected the physical(density, elastic wave velocity) and mechanical properties(uniaxial compression strength , elastic modulus.

78 FR 2195 - Airworthiness Directives; Rolls-Royce Deutschland Ltd & Co KG Turbofan Engines

Federal Register 2010, 2011, 2012, 2013, 2014

2013-01-10

... of silver chloride-induced stress corrosion cracking of the HP compressor stages 1 to 6 rotor disc...: Silver chloride-induced stress corrosion cracking was identified during overhaul of a BR700-715 engine... a report of silver chloride-induced stress corrosion cracking of the high-pressure (HP) compressor...

Diffraction-based study of fatigue crack initiation and propagation in aerospace aluminum alloys

NASA Astrophysics Data System (ADS)

Gupta, Vipul K.

The crack initiation sites and microstructure-sensitive growth of small fatigue cracks are experimentally characterized in two precipitation-hardened aluminum alloys, 7075-T651 and 7050-T7451, stressed in ambient temperature moist-air (warm-humid) and -50°C dry N2 (cold-dry) environmental conditions. Backscattered electron imaging (BSE) and energy dispersive spectroscopy (EDS) of the fracture surfaces showed that Fe-Cu rich constituent particle clusters are the most common initiation sites within both alloys stressed in either environment. The crack growth within each alloy, on average, was observed to be slowed in the cold-dry environment than in the warm-humid environment, but only at longer crack lengths. Although no overwhelming effects of grain boundaries and grain orientations on small-crack growth were observed, crack growth data showed local fluctuations within individual grains. These observations are understood as crack propagation through the underlying substructure at the crack surface and frequent interaction with low/high-angle grain and subgrain boundaries, during cyclic loading, and, are further attributed to periodic changes in crack propagation path and multiple occurrences of crack-branching observed in the current study. SEM-based stereology in combination with electron backscattered diffraction (EBSD) established fatigue crack surface crystallography within the region from ˜1 to 50 mum of crack initiating particle clusters. Fatigue crack facets were parallel to a wide variety of crystallographic planes, with pole orientations distributed broadly across the irreducible stereographic triangle between the {001} and {101}-poles within both warm-humid and cold-dry environments. The results indicate environmentally affected fatigue cracking in both cases, given the similarity between the observed morphology and crystallography with that of a variety of aerospace aluminum alloys cracked in the presence of moist-air. There was no evidence of crystallographic {111} slip-plane cracking typical of the Stage I crack growth mode observed in single crystals and high purity polycrystals of face centered cubic metals, and which has presently been assumed for the present materials within fatigue crack initiation models. Rather, the facets tend to have near-Mode I spatial orientation, which is another indicator of the importance of environmentally affected fatigue damage. The results provide a physical basis to develop microstructurally-based next generation multi-stage fatigue (MSF) models that should include a new crack decohesion criteria based upon environmental fatigue cracking mechanisms. EBSD study of small-cracks in alloy 7050-T7451, stressed in warm-humid environment, showed that crack-path orientation changes and crack-branching occurred at both low/high-angle grain and subgrain boundaries. Single surface trace analysis suggests that the crack-path differs substantially from crystallographic slip-planes. EBSD-based observations of small-crack propagation through subgrain structure, either formed by cyclic plastic strain accumulation or pre-existing (typical of unrecrystallized grain structure in the present materials), suggest that subgrain structure plays a crucial role in small fatigue crack propagation. As mentioned earlier, local fluctuations in small-crack growth rates appear to be caused by frequent interaction with subgrain boundaries, and multiple occurrences of crack-branching and crack-path orientation changes at low/high-angle grain and subgrain boundaries. The aforementioned deviation from low-index {001}/{101}-planes and the occurrence of high-index cracking planes observed by EBSD/Stereology, in this study and others, are interpreted as trans-subgranular decohesion or inter-subgranular cracking, due to trapped hydrogen. In summary, the results provide a firmer experimental foundation for, and clearer understanding of, the mechanisms of environmental fatigue cracking of aluminum alloys, especially the role of inter-subgranular cracking, which had previously been advanced based upon fracture surface observations alone.

Use of lightweight concrete for reducing cracks in bridge decks.

DOT National Transportation Integrated Search

2016-04-01

Cracks in bridge decks can be due to many factors related to environmental effects, chemical reactions, and structural : loads. Careful selection of materials and mixture proportions can minimize cracking to some degree. To reduce cracking, : shrinka...

Environmentally induced crack propagation in Inconel alloys 600 and 690 under hydrogen supersaturated steam

NASA Astrophysics Data System (ADS)

Ali, Mehboob Muzzammil

Intergranular stress corrosion cracking (IGSCC) of Inconel alloys 600 and 690 was investigated by exposing them to 300--400°C in deaerated hydrogen supersaturated steam. Crack growth rates were measured in-situ for the above alloys using modified wedge-opening-loaded (M-WOL) linear elastic fracture specimens under constant displacement conditions. The applied stress intensity factors (K) used varied from 29--90 MPam1/2. An activation energy of 120 kJ/mol was found for crack growth rates as a function of temperature. This activation energy is close to the one corresponding to grain boundary self diffusion of nickel. In addition, it was found that the apparent crack growth rates (da/dt) exhibited a linear dependence with KI, given by (da/dt) = A.KIn, where n = 1 in our case. Microstructurally, crack propagation in both the alloys was predominantly along the grain boundaries. It is suggested that high fugacity hydrogen was generated at the tip of the crack, as a result of the reaction of H2O with Cr2O3 on the fracture surface leading to IGSCC. It was found that the rates of crack propagation in both alloys 600 and 690 are very similar. Moreover, under the applied KIs, both alloys 600 and 690 show a similar tendency to crack intergranularly in a direction perpendicular to the applied stress. Crack branching was also exhibited by both the alloys 600 as well as 690. The difference in the crack growth rates of alloys 600 and 690 was found to be only about 2%, which indicates that the crack growth rates in these alloys is independent of the alloy chromium content and that, possibly very similar crack growth mechanisms are active in both the alloys 600 and 690 under similar conditions of KI and temperature. HREM images at the crack tip of alloy 690 exhibit two distinct regions---a crystalline region, and an adjacent amorphous region, which is likely to be either a hydroxide or an amorphous oxide layer. This layer is expected to result from passivation reactions as the crack surface is exposed to the corrosive environment.

Fiber Grating Environmental Sensing System

DOEpatents

Schulz, Whitten L.; Udd, Eric

2003-07-29

Fiber grating environmental measurement systems are comprised of sensors that are configured to respond to changes in moisture or chemical content of the surrounding medium through the action of coatings and plates inducing strain that is measured. These sensors can also be used to monitor the interior of bonds for degradation due to aging, cracking, or chemical attack. Means to multiplex these sensors at high speed and with high sensitivity can be accomplished by using spectral filters placed to correspond to each fiber grating environmental sensor. By forming networks of spectral elements and using wavelength division multiplexing arrays of fiber grating sensors may be processed in a single fiber line allowing distributed high sensitivity, high bandwidth fiber optic grating environmental sensor systems to be realized.

Use of Slow Strain Rate Tensile Testing to Assess the Ability of Several Superalloys to Resist Environmentally-Assisted Intergranular Cracking

NASA Technical Reports Server (NTRS)

Gabb, Timothy P.; Telesman, Jack; Banik, Anthony; McDevitt, Erin

2014-01-01

Intergranular fatigue crack initiation and growth due to environmental degradation, especially at notched features, can often limit the fatigue life of disk superalloys at high temperatures. For clear comparisons, the effects of alloy composition on cracking in air needs to be understood and compared separately from variables associated with notches and cracks such as effective stress concentration, plastic flow, stress relaxation, and stress redistribution. The objective of this study was to attempt using simple tensile tests of specimens with uniform gage sections to compare the effects of varied alloy composition on environment-assisted cracking of several powder metal and cast and wrought superalloys including ME3, LSHR, Udimet 720(TradeMark) ATI 718Plus(Registered TradeMark) alloy, Haynes 282(Trademark), and Inconel 740(TradeMark) Slow and fast strain-rate tensile tests were found to be a useful tool to compare propensities for intergranular surface crack initiation and growth. The effects of composition and heat treatment on tensile fracture strain and associated failure modes were compared. Environment interactions were determined to often limit ductility, by promoting intergranular surface cracking. The response of various superalloys and heat treatments to slow strain rate tensile testing varied substantially, showing that composition and microstructure can significantly influence environmental resistance to cracking.

A film-rupture model of hydrogen-induced, slow crack growth in alpha-beta titanium

NASA Technical Reports Server (NTRS)

Nelson, H. G.

1975-01-01

The appearance of the terrace like fracture morphology of gaseous hydrogen induced crack growth in acicular alpha-beta titanium alloys is discussed as a function of specimen configuration, magnitude of applied stress intensity, test temperature, and hydrogen pressure. Although the overall appearance of the terrace structure remained essentially unchanged, a distinguishable variation is found in the size of the individual terrace steps, and step size is found to be inversely dependent upon the rate of hydrogen induced slow crack growth. Additionally, this inverse relationship is independent of all the variables investigated. These observations are quantitatively discussed in terms of the formation and growth of a thin hydride film along the alpha-beta boundaries and a qualitative model for hydrogen induced slow crack growth is presented, based on the film-rupture model of stress corrosion cracking.

40 CFR 419.20 - Applicability; description of the cracking subcategory.

Code of Federal Regulations, 2010 CFR

2010-07-01

... cracking subcategory. 419.20 Section 419.20 Protection of Environment ENVIRONMENTAL PROTECTION AGENCY (CONTINUED) EFFLUENT GUIDELINES AND STANDARDS PETROLEUM REFINING POINT SOURCE CATEGORY Cracking Subcategory § 419.20 Applicability; description of the cracking subcategory. The provisions of this subpart are...

40 CFR 419.20 - Applicability; description of the cracking subcategory.

Code of Federal Regulations, 2011 CFR

2011-07-01

... cracking subcategory. 419.20 Section 419.20 Protection of Environment ENVIRONMENTAL PROTECTION AGENCY (CONTINUED) EFFLUENT GUIDELINES AND STANDARDS PETROLEUM REFINING POINT SOURCE CATEGORY Cracking Subcategory § 419.20 Applicability; description of the cracking subcategory. The provisions of this subpart are...

An Industrial Perspective on Environmentally Assisted Cracking of Some Commercially Used Carbon Steels and Corrosion-Resistant Alloys

NASA Astrophysics Data System (ADS)

Ashida, Yugo; Daigo, Yuzo; Sugahara, Katsuo

2017-08-01

Commercial metals and alloys like carbon steels, stainless steels, and nickel-based super alloys frequently encounter the problem of environmentally assisted cracking (EAC) and resulting failure in engineering components. This article aims to provide a perspective on three critical industrial applications having EAC issues: (1) corrosion and cracking of carbon steels in automotive applications, (2) EAC of iron- and nickel-based alloys in salt production and processing, and (3) EAC of iron- and nickel-based alloys in supercritical water. The review focuses on current industrial-level understanding with respect to corrosion fatigue, hydrogen-assisted cracking, or stress corrosion cracking, as well as the dominant factors affecting crack initiation and propagation. Furthermore, some ongoing industrial studies and directions of future research are also discussed.

Thermal Cycling and High-Temperature Corrosion Tests of Rare Earth Silicate Environmental Barrier Coatings

NASA Astrophysics Data System (ADS)

Darthout, Émilien; Gitzhofer, François

2017-12-01

Lutetium and yttrium silicates, enriched with an additional secondary zirconia phase, environmental barrier coatings were synthesized by the solution precursor plasma spraying process on silicon carbide substrates. A custom-made oven was designed for thermal cycling and water vapor corrosion testing. The oven can test four specimens simultaneously and allows to evaluate environmental barrier performances under similar corrosion kinetics compared to turbine engines. Coatings structural evolution has been observed by SEM on the polished cross sections, and phase composition has been analyzed by XRD. All coatings have been thermally cycled between 1300 °C and the ambient temperature, without spallation, due to their porosity and the presence of additional secondary phase which increases the thermal cycling resistance. During water vapor exposure at 1200 °C, rare earth disilicates showed a good stability, which is contradictory with the literature, due to impurities—such as Si- and Al-hydroxides—in the water vapor jets. The presence of vertical cracks allowed the water vapor to reach the substrate and then to corrode it. It has been observed that thin vertical cracks induced some spallation after 24 h of corrosion.

Theoretical predicting of permeability evolution in damaged rock under compressive stress

NASA Astrophysics Data System (ADS)

Vu, M. N.; Nguyen, S. T.; To, Q. D.; Dao, N. H.

2017-05-01

This paper outlines an analytical model of crack growth induced permeability changes. A theoretical solution of effective permeability of cracked porous media is derived. The fluid flow obeys Poisseuille's law along the crack and Darcy's law in the porous matrix. This solution exhibits a percolation threshold for any type of crack distribution apart from a parallel crack distribution. The physical behaviour of fluid flow through a cracked porous material is well reproduced by the proposed model. The presence of this effective permeability coupling to analytical expression of crack growth under compression enables the modelling of the permeability variation due to stress-induced cracking in a porous rock. This incorporation allows the prediction of the permeability change of a porous rock embedding an anisotropic crack distribution from any initial crack density, that is, lower, around or upper to percolation threshold. The interaction between cracks is not explicitly taken into account. The model is well applicable both to micro- and macrocracks.

Bioconcrete: next generation of self-healing concrete.

PubMed

Seifan, Mostafa; Samani, Ali Khajeh; Berenjian, Aydin

2016-03-01

Concrete is one of the most widely used construction materials and has a high tendency to form cracks. These cracks lead to significant reduction in concrete service life and high replacement costs. Although it is not possible to prevent crack formation, various types of techniques are in place to heal the cracks. It has been shown that some of the current concrete treatment methods such as the application of chemicals and polymers are a source of health and environmental risks, and more importantly, they are effective only in the short term. Thus, treatment methods that are environmentally friendly and long-lasting are in high demand. A microbial self-healing approach is distinguished by its potential for long-lasting, rapid and active crack repair, while also being environmentally friendly. Furthermore, the microbial self-healing approach prevails the other treatment techniques due to the efficient bonding capacity and compatibility with concrete compositions. This study provides an overview of the microbial approaches to produce calcium carbonate (CaCO3). Prospective challenges in microbial crack treatment are discussed, and recommendations are also given for areas of future research.

Dielectric cracking produced by electromigration in microelectronic interconnects

NASA Astrophysics Data System (ADS)

Chiras, S.; Clarke, D. R.

2000-12-01

The development of stress during electromigration along Al lines, constrained within a dielectric in a coplanar test configuration, is measured. It is shown that, above a certain threshold current density, cracking of the dielectric is induced in the vicinity of the anode. Cracking of the dielectric leads to loss of mechanical constraint on the aluminum conductor which, in turn, leads to increases in electrical resistance with continued current flow. The electromigration-induced stresses are determined from the measured frequency shifts induced in a novel ruby strain sensor embedded immediately beneath the interconnect line on a sapphire substrate. The transparency of the sapphire substrate also facilitated the observation of a hitherto unreported form of dielectric cracking, namely cracking from the interconnect along internal interfaces. The observations of dielectric cracking are in agreement with a recent fracture mechanics model. Analysis of the stress data, together with the results of finite element calculations of the strain energy release rate for crack extension, gives a quantitative estimate of the effective valence Z*(=1.3±0.2) for aluminum.

Effect of crack openings on carbonation-induced corrosion

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ghantous, Rita Maria, E-mail: rita-maria.ghantous@yncrea.fr; LMDC, Université de Toulouse, INSA, UPS, Toulouse; Poyet, Stéphane

Reinforced concrete is widely used in the construction of buildings, historical monuments, infrastructures and nuclear power plants. For a variety of reasons, many concrete structures are subject to unavoidable cracks that accelerate the diffusion of atmospheric carbon dioxide to the steel/concrete interface. Carbonation at the interface induces steel corrosion that could cause the development of new cracks in the structure, a determining factor for its durability. The aim of this article is to study the effect of existing cracks on the development of carbonation-induced corrosion. The results indicate that, after the initiation phase, the corrosion kinetics decreases with time andmore » the free corrosion potential increases independently of the crack opening. In addition, the corroded zone matches the carbonated one. The interpretation of these results allows the authors to conclude that, during the corrosion process, corrosion products seal the crack and act as a barrier to oxygen and water diffusion. Consequently, the influence of crack opening on corrosion development is masked and the corrosion development is limited.« less

Reduction of the 355-nm laser-induced damage initiators by removing the subsurface cracks in fused silica

NASA Astrophysics Data System (ADS)

Yang, Minghong; Qi, Hongji; Zhao, Yuanan; Yi, Kui

2012-01-01

The 355 nm laser-induced damage thresholds (LIDTs) of polished fused silica with and without the residual subsurface cracks were explored. HF based wet etching and magnetorheological finishing was used to remove the subsurface cracks. To isolate the effect of subsurface cracks, chemical leaching was used to eliminate the photoactive impurities in the polishing layer. Results show that the crack number density decreased from~103 to <1cm-2, and the LIDT was improved as high as 2.8-fold with both the subsurface cracks and the polishing layer being removed. Subsurface cracks play a significant role in laser damage at fluencies between 15~31 J/cm2 (355nm, 8ns). HF Etching of the cracks was shown to increase the damage performance as nearly high as that of the samples in which subsurface cracks are well controlled.

The interaction of fatigue cracks with a residual stress field using thermoelastic stress analysis and synchrotron X-ray diffraction experiments

PubMed Central

Amjad, Khurram; Asquith, David; Sebastian, Christopher M.; Wang, Wei-Chung

2017-01-01

This article presents an experimental study on the fatigue behaviour of cracks emanating from cold-expanded holes utilizing thermoelastic stress analysis (TSA) and synchrotron X-ray diffraction (SXRD) techniques with the aim of resolving the long-standing ambiguity in the literature regarding potential relaxation, or modification, of beneficial compressive residual stresses as a result of fatigue crack propagation. The crack growth rates are found to be substantially lower as the crack tip moved through the residual stress zone induced by cold expansion. The TSA results demonstrated that the crack tip plastic zones were reduced in size by the presence of the residual compressive stresses induced by cold expansion. The crack tip plastic zones were found to be insignificant in size in comparison to the residual stress zone resulting from cold expansion, which implied that they were unlikely to have had a notable impact on the surrounding residual stresses induced by cold expansion. The residual stress distributions measured along the direction of crack growth, using SXRD, showed no signs of any significant stress relaxation or redistribution, which validates the conclusions drawn from the TSA data. Fractographic analysis qualitatively confirmed the influence on crack initiation of the residual stresses induced by the cold expansion. It was found that the application of single compressive overload caused a relaxation, or reduction in the residual stresses, which has wider implications for improving the fatigue life. PMID:29291095

International Conference/Workshop on Small Fatigue Cracks (2nd) Held in Santa Barbara, California on 5-10 January 1986.

DTIC Science & Technology

1986-03-31

critical issues thus pertain to the determination of crack tip conditions, as a function of crack length, in terms of the coupled processes of fluid...transport and chemical/electrochemical reactions within the crack, and the determination of the origin of the environmentally-enhanced cracking rates in...Depth in Determining Crack Electrochemistry and Crack Growth" A. Turnbull, National Physical Laboratory, U.K., and R. C. Newmann, UMIST, U.K. 7:30 p.m.-7

Comparing the floquet stability of open and breathing fatigue cracks in an overhung rotordynamic system

NASA Astrophysics Data System (ADS)

Varney, Philip; Green, Itzhak

2017-11-01

Rotor cracks represent an uncommon but serious threat to rotating machines and must be detected early to avoid catastrophic machine failure. An important aspect of analyzing rotor cracks is understanding their influence on the rotor stability. It is well-known that the extent of rotor instability versus shaft speed is exacerbated by deeper cracks. Consequently, crack propagation can eventually result in an unstable response even if the shaft speed remains constant. Most previous investigations of crack-induced rotor instability concern simple Jeffcott rotors. This work advances the state-of-the-art by (a) providing a novel inertial-frame model of an overhung rotor, and (b) assessing the stability of the cracked overhung rotor using Floquet stability analysis. The rotor Floquet stability analysis is performed for both an open crack and a breathing crack, and conclusions are drawn regarding the importance of appropriately selecting the crack model. The rotor stability is analyzed versus crack depth, external viscous damping ratio, and rotor inertia. In general, this work concludes that the onset of instability occurs at lower shaft speeds for thick rotors, lower viscous damping ratios, and deeper cracks. In addition, when comparing commensurate cracks, the breathing crack is shown to induce more regions of instability than the open crack, though the open crack generally predicts an unstable response for shallower cracks than the breathing crack. Keywords: rotordynamics, stability, rotor cracks.

Fatigue crack propagation of nickel-base superalloys at 650 deg C

NASA Technical Reports Server (NTRS)

Gayda, J.; Gabb, T. P.; Miner, R. V.

1988-01-01

The 650 C fatigue crack propagation behavior of two nickel-base superalloys, Rene 95 and Waspaloy, is studied with particular emphasis placed on understanding the roles of creep, environment, and two key grain boundary alloying additions, boron and zirconium. Comparison of air and vacuum data shows the air environment to be detrimental over a wide range of frequencies for both alloys. More in-depth analysis on Rene 95 shows at lower frequencies, such as 0.02 Hz, failure in air occurs by intergranular, environmentally-assisted creep crack growth, while at higher frequencies, up to 5.0 Hz, environmental interaction are still evident but creep effects are minimized. The effect of B and Zr in Waspaloy is found to be important where environmental and/or creep interactions are presented. In those instances, removal of B and Zr dramatically increases crack growth and it is therefore plausible that effective dilution of these elements may explain a previously observed trend in which crack growth rates increase with decreasing grain size.

Fatigue crack propagation of nickel-base superalloys at 650 deg C

NASA Technical Reports Server (NTRS)

Gayda, J.; Gabb, T. P.; Miner, R. V.

1985-01-01

The 650 C fatigue crack propagation behavior of two nickel-base superalloys, Rene 95 and Waspaloy, is studied with particular emphasis placed on understanding the roles of creep, environment, and two key grain boundary alloying additions, boron and zirconium. Comparison of air and vacuum data shows the air environment to be detrimental over a wide range of frequencies for both alloys. More in-depth analysis on Rene 95 shows at lower frequencies, such as 0.02 Hz, failure in air occurs by intergranular, environmentally-assisted creep crack growth, while at higher frequencies, up to 5.0 Hz, environmental interactions are still evident but creep effects are minimized. The effect of B and Zr in Waspaloy is found to be important where environmental and/or creep interactions are presented. In those instances, removal of B and Zr dramatically increases crack growth and it is therefore plausible that effective dilution of these elements may explain a previously observed trend in which crack growth rates increase with decreasing grain size.

Hydrogen-Induced Cold Cracking in High-Frequency Induction Welded Steel Tubes

NASA Astrophysics Data System (ADS)

Banerjee, Kumkum

2016-04-01

Detailed investigation was carried out on 0.4C steel tubes used for the telescopic front fork of two-wheelers to establish the root cause for the occurrence of transverse cracks at the weld heat-affected zone of the tubes. Fractographic and microstructural observations provide evidences of delayed hydrogen-induced cracking. The beneficial microstructure for avoiding the transverse cracks was found to be the bainitic-martensitic, while martensitic structure was noted to be deleterious.

The MISSE 7 Flexural Stress Effects Experiment After 1.5 Years of Wake Space Exposure

NASA Technical Reports Server (NTRS)

Snow, Kate E.; De Groh, Kim K.; Banks, Bruce A.

2017-01-01

Low Earth orbit space environment conditions, including ultraviolet radiation, thermal cycling, and atomic oxygen exposure, can cause degradation of exterior spacecraft materials over time. Radiation and thermal exposure often results in bond- breaking and embrittlement of polymers, reducing mechanical strength and structural integrity. An experiment called the Flexural Stress Effects Experiment (FSEE) was flown with the objective of determining the role of space environmental exposure on the degradation of polymers under flexural stress. The FSEE samples were flown in the wake orientation on the exterior of International Space Station for 1.5 years. Twenty-four samples were flown: 12 bent over a 0.375 in. mandrel and 12 were over a 0.25 in. mandrel. This was designed to simulate flight configurations of insulation blankets on spacecraft. The samples consisted of assorted polyimide and fluorinated polymers with various coatings. Half the samples were designated for bend testing and the other half will be tensile tested. A non-standard bend-test procedure was designed to determine the surface strain at which embrittled polymers crack. All ten samples designated for bend testing have been tested. None of the control samples' polymers cracked, even under surface strains up to 19.7%, although one coating cracked. Of the ten flight samples tested, seven show increased embrittlement through bend-test induced cracking at surface strains from 0.70%to 11.73%. These results show that most of the tested polymers are embrittled due to space exposure, when compared to their control samples. Determination of the extent of space induced embrittlement of polymers is important for designing durable spacecraft.

The kinetic and mechanical aspects of hydrogen-induced failure in metals. Ph.D. Thesis, 1971

NASA Technical Reports Server (NTRS)

Nelson, H. G.

1972-01-01

Premature hydrogen-induced failure observed to occur in many metal systems involves three stages of fracture: (1) crack initiation, (2) stable slow crack growth, and (3) unstable rapid crack growth. The presence of hydrogen at some critical location on the metal surface or within the metal lattice was shown to influence one or both of the first two stages of brittle fracture but has a negligible effect on the unstable rapid crack growth stage. The relative influence of the applied parameters of time, temperature, etc., on the propensity of a metal to exhibit hydrogen induced premature failure was investigated.

Creep and cracking of concrete hinges: insight from centric and eccentric compression experiments.

PubMed

Schlappal, Thomas; Schweigler, Michael; Gmainer, Susanne; Peyerl, Martin; Pichler, Bernhard

2017-01-01

Existing design guidelines for concrete hinges consider bending-induced tensile cracking, but the structural behavior is oversimplified to be time-independent. This is the motivation to study creep and bending-induced tensile cracking of initially monolithic concrete hinges systematically. Material tests on plain concrete specimens and structural tests on marginally reinforced concrete hinges are performed. The experiments characterize material and structural creep under centric compression as well as bending-induced tensile cracking and the interaction between creep and cracking of concrete hinges. As for the latter two aims, three nominally identical concrete hinges are subjected to short-term and to longer-term eccentric compression tests. Obtained material and structural creep functions referring to centric compression are found to be very similar. The structural creep activity under eccentric compression is significantly larger because of the interaction between creep and cracking, i.e. bending-induced cracks progressively open and propagate under sustained eccentric loading. As for concrete hinges in frame-like integral bridge construction, it is concluded (i) that realistic simulation of variable loads requires consideration of the here-studied time-dependent behavior and (ii) that permanent compressive normal forces shall be limited by 45% of the ultimate load carrying capacity, in order to avoid damage of concrete hinges under sustained loading.

Stress corrosion cracking of duplex stainless steels in caustic solutions

NASA Astrophysics Data System (ADS)

Bhattacharya, Ananya

Duplex stainless steels (DSS) with roughly equal amount of austenite and ferrite phases are being used in industries such as petrochemical, nuclear, pulp and paper mills, de-salination plants, marine environments, and others. However, many DSS grades have been reported to undergo corrosion and stress corrosion cracking in some aggressive environments such as chlorides and sulfide-containing caustic solutions. Although stress corrosion cracking of duplex stainless steels in chloride solution has been investigated and well documented in the literature but the SCC mechanisms for DSS in caustic solutions were not known. Microstructural changes during fabrication processes affect the overall SCC susceptibility of these steels in caustic solutions. Other environmental factors, like pH of the solution, temperature, and resulting electrochemical potential also influence the SCC susceptibility of duplex stainless steels. In this study, the role of material and environmental parameters on corrosion and stress corrosion cracking of duplex stainless steels in caustic solutions were investigated. Changes in the DSS microstructure by different annealing and aging treatments were characterized in terms of changes in the ratio of austenite and ferrite phases, phase morphology and intermetallic precipitation using optical micrography, SEM, EDS, XRD, nano-indentation and microhardness methods. These samples were then tested for general and localized corrosion susceptibility and SCC to understand the underlying mechanisms of crack initiation and propagation in DSS in the above-mentioned environments. Results showed that the austenite phase in the DSS is more susceptible to crack initiation and propagation in caustic solutions, which is different from that in the low pH chloride environment where the ferrite phase is the more susceptible phase. This study also showed that microstructural changes in duplex stainless steels due to different heat treatments could affect their SCC susceptibility. Annealed and water quenched specimens were found to be immune to SCC in caustic environment. Aging treatment at 800°C gave rise to sigma and chi precipitates in the DSS. However, these sigma and chi precipitates, known to initiate cracking in DSS in chloride environment did not cause any cracking of DSS in caustic solutions. Aging of DSS at 475°C had resulted in '475°C embrittlement' and caused cracks to initiate in the ferrite phase. This was in contrast to the cracks initiating in the austenite phase in the as-received DSS. Alloy composition and microstructure of DSS as well as solution composition (dissolved ionic species) was also found to affect the electrochemical behavior and passivation of DSS which in turn plays a major role in stress corrosion crack initiation and propagation. Corrosion rates and SCC susceptibility of DSS was found to increase with addition of sulfide to caustic solutions. Corrosion films on DSS, characterized using XRD and X-ray photoelectron spectroscopy, indicated that the metal sulfide compounds were formed along with oxides at the metal surface in the presence of sulfide containing caustic environments. These metal sulfide containing passive films are unstable and hence breaks down under mechanical straining, leading to SCC initiations. The overall results from this study helped in understanding the mechanism of SCC in caustic solutions. Favorable slip systems in the austenite phase of DSS favors slip-induced local film damage thereby initiating a stress corrosion crack. Repeated film repassivation and breaking, followed by crack tip dissolution results in crack propagation in the austenite phase of DSS alloys. Result from this study will have a significant impact in terms of identifying the alloy compositions, fabrication processes, microstructures, and environmental conditions that may be avoided to mitigate corrosion and stress corrosion cracking of DSS in caustic solutions.

Microcracks, micropores, and their petrologic interpretation for 72415 and 15418

NASA Technical Reports Server (NTRS)

Richter, D.; Simmons, G.; Siegfried, R.

1976-01-01

Lunar samples 72415 and 15418 have complex microstructures that indicate a series of fracturing and healing events. Both samples contain relatively few open microcracks but many sealed and healed microcracks. Dunite 72415 contains abundant healed cracks that formed tectonically, symplectic intergrowths spatially and probably genetically related to microcracks, and a cataclastic matrix that has been extensively sintered. Metamorphosed breccia 15418 contains many post-metamorphic healed cracks, large shock induced cracks that have been sealed with glass, and a few younger, thin, open shock induced cracks.

Biological Dimensions of Crack Morphology in Dryland Soils

NASA Astrophysics Data System (ADS)

DeCarlo, K. F.; Spiegel, M.; Caylor, K. K.

2014-12-01

Macropores and cracks have an integral role in soil hydrology, and the physicochemical factors that induce them have been the subject of much laboratory research. How these processes translate to field soils, however, is often obfuscated by the biological elements present that complicate its formation and dynamics. In this study, we investigated the biological influence of herbivores and vegetation on 3D crack morphology in a dryland swelling soil (black cotton/vertisol). Fieldwork was conducted at and near the Kenya Long-Term Exclosure Experiment (KLEE) plots in Mpala, central Kenya, where three different soil regions were identified: highly vegetated areas, animal trails, and termite mounds. Crack networks were physically characterized by pouring liquid resin into the soil and excavating them when dry, after which they were imaged and quantified using medical magnetic resonance imaging (MRI). Cracking intensity of each cast was corrected via soil moisture and bulk density measurements at 5 cm intervals over 30 cm. 3D characterization of the soil system shows that mechanical compaction is a major influence in the formation of extensive and deep cracks in animal trails, with megaherbivores (e.g. elephants) inducing the most extreme cracks. Bioturbation is seen as a major influence in the formation of shallower cracks in termite mounds, as termites loosen and aerate the soil and reduce the soil's cohesive properties. Highly vegetated soils show a large degree of variability: small, disconnected soil patches induced by vegetative cover and a larger root network results in smaller and shallower cracks, but full vegetative cover induces deep and irregular cracks, possibly due to diverted rainfall. Our results highlight the intricate connections between the biology and physics that dictate soil processes in a complex soil system at the field scale.

Laser-driven mechanical fracture in fused silica

NASA Astrophysics Data System (ADS)

Dahmani, Faiz

1999-10-01

Fused silica, widely used as optical-window material in high-fluence requirements on glass and KrF lasers, experiences optical damage. Under fatigue conditions, the damage is initiated by slow crack growth and culminates, if not arrested, with catastrophic crack growth and implosive failure when the stress intensity approaches the critical value. Since laser-induced cracks cannot be eliminated entirely, the behavior of cracked structures under service conditions must be quantified to be predicted. Systematic scientific rules must be devised to characterize laser-induced cracks and their effects, and to predict if and when it may become necessary to replace the damaged components. This thesis makes a contribution toward this end. Measurements of fatigue failure strength of laser-cracked fused silica in air at room temperature for different number of laser pulses and laser fluences are presented. The failure-strength variability is found to be due mainly to the spectrum of crack depths. Agreement with theory suggests the incorporation of a residual term into the failure-strength equation. Experiments on residual stresses induced in fused silica by the presence of a laser-induced crack are carried out using two different techniques. Theoretical modelings show that this residual stress field is of shear nature and mouth-opening. A correlation between the reduction in fracture strength of fused silica and the increase of the residual-stress field is established, providing laser systems designers and operators with guidance on the rate of crack growth as well as on the stress-related ramifications such as laser-driven cracks entail. Specifically, a hoop-stress in the immediate vicinity of a crack growing along the beam propagation direction is identified as strongly coupling to both the laser fluence and the crack growth. This coupling prompted the question of whether or not breaking the hoop stress symmetry by some external perturbation will accelerate or stymie crack growth. Experimental results on stress-inhibited laser-driven crack growth and stress-delayed-laser-damage initiation thresholds in fused silica and borosilicate glass (BK7) are presented. The results obtained show that, for very low compressive stresses (<10 psi), the damage initiation threshold is raised by as much as 78%, while the crack growth is arrested by 70%. Different loading- geometries are tested, giving different crack growth rates and raising the distinction between uniaxial and biaxial states of stresses.

Environmentally Assisted Cracking: Overview of Evidence for an Adsorption-Induced Localised-Slip Process,

DTIC Science & Technology

1986-12-01

Prior to examination of LME fractures, liquid or solid metals were removed from fracture surfaces as follows: Mercury was evaporated from fractures in a...1 mm/s. Under these conditions, the appearance of fracture surfaces was identical to that produced by rapid fracture (-1 mm/s) in liquid mercury ...Furthermore, the appearance of fractures depended somewhat on the orientation of crystals but was the same in hydrogen and mercury environments for each

Stress Corrosion Cracking Behavior of Multipass TIG-Welded AA2219 Aluminum Alloy in 3.5 wt pct NaCl Solution

NASA Astrophysics Data System (ADS)

Venugopal, A.; Sreekumar, K.; Raja, V. S.

2012-09-01

The stress corrosion cracking (SCC) behavior of the AA2219 aluminum alloy in the single-pass (SP) and multipass (MP) welded conditions was examined and compared with that of the base metal (BM) in 3.5 wt pct NaCl solution using a slow-strain-rate technique (SSRT). The reduction in ductility was used as a parameter to evaluate the SCC susceptibility of both the BM and welded joints. The results showed that the ductility ratio ( ɛ NaCl/( ɛ air) was 0.97 and 0.96, respectively, for the BM and MP welded joint, and the same was marginally reduced to 0.9 for the SP welded joint. The fractographic examination of the failed samples revealed a typical ductile cracking morphology for all the base and welded joints, indicating the good environmental cracking resistance of this alloy under all welded conditions. To understand the decrease in the ductility of the SP welded joint, preexposure SSRT followed by microstructural observations were made, which showed that the decrease in ductility ratio of the SP welded joint was caused by the electrochemical pitting that assisted the nucleation of cracks in the form of corrosion induced mechanical cracking rather than true SCC failure of the alloy. The microstructural examination and polarization tests demonstrated a clear grain boundary (GB) sensitization of the PMZ, resulting in severe galvanic corrosion of the SP weld joint, which initiated the necessary conditions for the localized corrosion and cracking along the PMZ. The absence of PMZ and a refined fusion zone (FZ) structure because of the lesser heat input and postweld heating effect improved the galvanic corrosion resistance of the MP welded joint greatly, and thus, failure occurred along the FZ.

Comparison of hydrogen gas embrittlement of austenitic and ferritic stainless steels

NASA Astrophysics Data System (ADS)

Perng, T. P.; Altstetter, C. J.

1987-01-01

Hydrogen-induced slow crack growth (SCG) was compared in austenitic and ferritic stainless steels at 0 to 125 °Cand 11 to 216 kPa of hydrogen gas. No SCG was observed for AISI 310, while AISI 301 was more susceptible to hydrogen embrittlement and had higher cracking velocity than AL 29-4-2 under the same test conditions. The kinetics of crack propagation was modeled in terms of the hydrogen transport in these alloys. This is a function of temperature, microstructure, and stress state in the embrittlement region. The relatively high cracking velocity of AISI 301 was shown to be controlled by the fast transport of hydrogen through the stress-induced α' martensite at the crack tip and low escape rate of hydrogen through the γ phase in the surrounding region. Faster accumulation rates of hydrogen in the embrittlement region were expected for AISI 301, which led to higher cracking velocities. The mechanism of hydrogen-induced SCG was discussed based upon the concept of hydrogen-enhanced plasticity.

Environmentally assisted cracking in light water reactors.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chopra, O. K.; Chung, H. M.; Clark, R. W.

2007-11-06

This report summarizes work performed by Argonne National Laboratory on fatigue and environmentally assisted cracking (EAC) in light water reactors (LWRs) from January to December 2002. Topics that have been investigated include: (a) environmental effects on fatigue crack initiation in carbon and low-alloy steels and austenitic stainless steels (SSs), (b) irradiation-assisted stress corrosion cracking (IASCC) of austenitic SSs in BWRs, (c) evaluation of causes and mechanisms of irradiation-assisted cracking of austenitic SS in PWRs, and (d) cracking in Ni-alloys and welds. A critical review of the ASME Code fatigue design margins and an assessment of the conservation in the currentmore » choice of design margins are presented. The existing fatigue {var_epsilon}-N data have been evaluated to define the effects of key material, loading, and environmental parameters on the fatigue lives of carbon and low-alloy steels and austenitic SSs. Experimental data are presented on the effects of surface roughness on fatigue crack initiation in these materials in air and LWR environments. Crack growth tests were performed in BWR environments on SSs irradiated to 0.9 and 2.0 x 10{sup 21} n x cm{sup -2}. The crack growth rates (CGRs) of the irradiated steels are a factor of {approx}5 higher than the disposition curve proposed in NUREG-0313 for thermally sensitized materials. The CGRs decreased by an order of magnitude in low-dissolved oxygen (DO) environments. Slow-strain-rate tensile (SSRT) tests were conducted in high-purity 289 C water on steels irradiated to {approx}3 dpa. The bulk S content correlated well with the susceptibility to intergranular SCC in 289 C water. The IASCC susceptibility of SSs that contain >0.003 wt. % S increased drastically. bend tests in inert environments at 23 C were conducted on broken pieces of SSRT specimens and on unirradiated specimens of the same materials after hydrogen charging. The results of the tests and a review of other data in the literature indicate that IASCC in 289 C water is dominated by a crack-tip grain-boundary process that involves S. An initial IASCC model has been proposed. A crack growth test was completed on mill annealed Alloy 600 in high-purity water at 289 C and 320 C under various environmental and loading conditions. The results from this test are compared with data obtained earlier on several other heats of Alloy 600.« less

Lead induced stress corrosion cracking of Alloy 690 in high temperature water

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chung, K.K.; Lim, J.K.; Moriya, Shinichi

1995-12-31

Recent investigations of cracked steam generator tubes at nuclear power plants concluded that lead significantly contributed to cracking the Alloy 600 materials. In order to investigate the stress corrosion cracking (SCC) behavior of Alloy 690, slow strain rate tests (SSRT) and anodic polarization measurements were performed. The SSRTs were conducted in a lead-chloride solution (PbCl{sub 2}) and in a chloride but lead free solution (NaCl) at pH of 3 and 4.5 at 288 C. The anodic polarization measurements were carried out at 30 C using the same solutions as in SSRT. The SSRT results showed that Alloy 690 was susceptiblemore » to SCC in both solutions. In the lead chloride solution, cracking had slight dependence on lead concentration and pH. Cracking tend to increase with a higher lead concentration and a lower pH and was mainly intergranular and was to be a few tens to hundreds micrometers in length. In the chloride only solution, cracking was similar to the lead induced SCC. The results of anodic polarization measurement and electron probe micro analysis (EPMA) helped to understand lead induced SCC. Lead was a stronger active corrosive element but had a minor affect on cracking susceptibility of the alloy. While, chloride was quite different from lead effect to SCC. A possible mechanism of lead induced SCC of Alloy 690 was also discussed based on the test results.« less

40 CFR Table 8 to Subpart Uuu of... - Organic HAP Emission Limits for Catalytic Cracking Units

Code of Federal Regulations, 2011 CFR

2011-07-01

... Catalytic Cracking Units 8 Table 8 to Subpart UUU of Part 63 Protection of Environment ENVIRONMENTAL... Refineries: Catalytic Cracking Units, Catalytic Reforming Units, and Sulfur Recovery Units Pt. 63, Subpt. UUU, Table 8 Table 8 to Subpart UUU of Part 63—Organic HAP Emission Limits for Catalytic Cracking Units As...

40 CFR Table 8 to Subpart Uuu of... - Organic HAP Emission Limits for Catalytic Cracking Units

Code of Federal Regulations, 2010 CFR

2010-07-01

... Catalytic Cracking Units 8 Table 8 to Subpart UUU of Part 63 Protection of Environment ENVIRONMENTAL... Refineries: Catalytic Cracking Units, Catalytic Reforming Units, and Sulfur Recovery Units Pt. 63, Subpt. UUU, Table 8 Table 8 to Subpart UUU of Part 63—Organic HAP Emission Limits for Catalytic Cracking Units As...

Environmental fatigue of an Al-Li-Cu alloy. Part 3: Modeling of crack tip hydrogen damage

NASA Technical Reports Server (NTRS)

Piascik, Robert S.; Gangloff, Richard P.

1992-01-01

Environmental fatigue crack propagation rates and microscopic damage modes in Al-Li-Cu alloy 2090 (Parts 1 and 2) are described by a crack tip process zone model based on hydrogen embrittlement. Da/dN sub ENV equates to discontinuous crack advance over a distance, delta a, determined by dislocation transport of dissolved hydrogen at plastic strains above a critical value; and to the number of load cycles, delta N, required to hydrogenate process zone trap sites that fracture according to a local hydrogen concentration-tensile stress criterion. Transgranular (100) cracking occurs for process zones smaller than the subgrain size, and due to lattice decohesion or hydride formation. Intersubgranular cracking dominates when the process zone encompasses one or more subgrains so that dislocation transport provides hydrogen to strong boundary trapping sites. Multi-sloped log da/dN-log delta K behavior is produced by process zone plastic strain-hydrogen-microstructure interactions, and is determined by the DK dependent rates and proportions of each parallel cracking mode. Absolute values of the exponents and the preexponential coefficients are not predictable; however, fractographic measurements theta sub i coupled with fatigue crack propagation data for alloy 2090 established that the process zone model correctly describes fatigue crack propagation kinetics. Crack surface films hinder hydrogen uptake and reduce da/dN and alter the proportions of each fatigue crack propagation mode.

Test methods for environment-assisted cracking

NASA Astrophysics Data System (ADS)

Turnbull, A.

1992-03-01

The test methods for assessing environment assisted cracking of metals in aqueous solution are described. The advantages and disadvantages are examined and the interrelationship between results from different test methods is discussed. The source of differences in susceptibility to cracking occasionally observed from the varied mechanical test methods arises often from the variation between environmental parameters in the different test conditions and the lack of adequate specification, monitoring, and control of environmental variables. Time is also a significant factor when comparing results from short term tests with long exposure tests. In addition to these factors, the intrinsic difference in the important mechanical variables, such as strain rate, associated with the various mechanical tests methods can change the apparent sensitivity of the material to stress corrosion cracking. The increasing economic pressure for more accelerated testing is in conflict with the characteristic time dependence of corrosion processes. Unreliable results may be inevitable in some cases but improved understanding of mechanisms and the development of mechanistically based models of environment assisted cracking which incorporate the key mechanical, material, and environmental variables can provide the framework for a more realistic interpretation of short term data.

Analytical insight into "breathing" crack-induced acoustic nonlinearity with an application to quantitative evaluation of contact cracks.

PubMed

Wang, Kai; Liu, Menglong; Su, Zhongqing; Yuan, Shenfang; Fan, Zheng

2018-08-01

To characterize fatigue cracks, in the undersized stage in particular, preferably in a quantitative and precise manner, a two-dimensional (2D) analytical model is developed for interpreting the modulation mechanism of a "breathing" crack on guided ultrasonic waves (GUWs). In conjunction with a modal decomposition method and a variational principle-based algorithm, the model is capable of analytically depicting the propagating and evanescent waves induced owing to the interaction of probing GUWs with a "breathing" crack, and further extracting linear and nonlinear wave features (e.g., reflection, transmission, mode conversion and contact acoustic nonlinearity (CAN)). With the model, a quantitative correlation between CAN embodied in acquired GUWs and crack parameters (e.g., location and severity) is obtained, whereby a set of damage indices is proposed via which the severity of the crack can be evaluated quantitatively. The evaluation, in principle, does not entail a benchmarking process against baseline signals. As validation, the results obtained from the analytical model are compared with those from finite element simulation, showing good consistency. This has demonstrated accuracy of the developed analytical model in interpreting contact crack-induced CAN, and spotlighted its application to quantitative evaluation of fatigue damage. Copyright © 2018 Elsevier B.V. All rights reserved.

Cyclic stress effect on stress corrosion cracking of duplex stainless steel in chloride and caustic solutions

NASA Astrophysics Data System (ADS)

Yang, Di

Duplex stainless steel (DSS) is a dual-phase material with approximately equal volume amount of austenite and ferrite. It has both great mechanical properties (good ductility and high tensile/fatigue strength) and excellent corrosion resistance due to the mixture of the two phases. Cyclic loadings with high stress level and low frequency are experienced by many structures. However, the existing study on corrosion fatigue (CF) study of various metallic materials has mainly concentrated on relatively high frequency range. No systematic study has been done to understand the ultra-low frequency (˜10-5 Hz) cyclic loading effect on stress corrosion cracking (SCC) of DSSs. In this study, the ultra-low frequency cyclic loading effect on SCC of DSS 2205 was studied in acidified sodium chloride and caustic white liquor (WL) solutions. The research work focused on the environmental effect on SCC of DSS 2205, the cyclic stress effect on strain accumulation behavior of DSS 2205, and the combined environmental and cyclic stress effect on the stress corrosion crack initiation of DSS 2205 in the above environments. Potentiodynamic polarization tests were performed to investigate the electrochemical behavior of DSS 2205 in acidic NaCl solution. Series of slow strain rate tests (SSRTs) at different applied potential values were conducted to reveal the optimum applied potential value for SCC to happen. Room temperature static and cyclic creep tests were performed in air to illustrate the strain accumulation effect of cyclic stresses. Test results showed that cyclic loading could enhance strain accumulation in DSS 2205 compared to static loading. Moreover, the strain accumulation behavior of DSS 2205 was found to be controlled by the two phases of DSS 2205 with different crystal structures. The B.C.C. ferrite phase enhanced strain accumulation due to extensive cross-slips of the dislocations, whereas the F.C.C. austenite phase resisted strain accumulation due to cyclic strain hardening. Cyclic SSRTs were performed under the conditions that SCC occurs in sodium chloride and WL solutions. Test results show that cyclic stress facilitated crack initiations in DSS 2205. Stress corrosion cracks initiated from the intermetallic precipitates in acidic chloride environment, and the cracks initiated from austenite phase in WL environment. Cold-working has been found to retard the crack initiations induced by cyclic stresses.

Characteristics of thermally-induced transverse cracks in graphite epoxy composite laminates

NASA Technical Reports Server (NTRS)

Adams, D. S.; Bowles, D. E.; Herakovich, C. T.

1983-01-01

The characteristics of thermally induced transverse cracks in T300/5208 graphite-epoxy cross-ply and quasi-isotropic laminates were investigated both experimentally and analytically. The formation of transverse cracks and the subsequent crack spacing present during cool down to -250 F (116K) and thermal cycling between 250 and -250 F (116 and 394K) was investigated. The state of stress in the vicinity of a transverse crack and the influence of transverse cracking on the laminate coefficient of thermal expansion (CTE) was predicted using a generalized plane strain finite element analysis and a modified shear lag analysis. A majority of the cross-ply laminates experienced transverse cracking during the initial cool down to -250 F whereas the quasi-isotropic laminates remained uncracked. The in situ transverse strength of the 90 degree layers was more than 1.9 times greater than the transverse strength of the unidirectional 90 degree material for all laminates investigated.

Creep, Fatigue and Environmental Interactions and Their Effect on Crack Growth in Superalloys

NASA Technical Reports Server (NTRS)

Telesman, J.; Gabb, T. P.; Ghosn, L. J.; Smith, T.

2017-01-01

Complex interactions of creep/fatigue/environment control dwell fatigue crack growth (DFCG) in superalloys. Crack tip stress relaxation during dwells significantly changes the crack driving force and influence DFCG. Linear Elastic Fracture Mechanics, Kmax, parameter unsuitable for correlating DFCG behavior due to extensive visco-plastic deformation. Magnitude of remaining crack tip axial stresses controls DFCG resistance due to the brittle-intergranular nature of the crack growth process. Proposed a new empirical parameter, Ksrf, which incorporates visco-plastic evolution of the magnitude of remaining crack tip stresses. Previous work performed at 704C, extend the work to 760C.

Development of a novel non-contact inspection technique to detect micro cracks under the surface of a glass substrate by thermal stress-induced light scattering method

NASA Astrophysics Data System (ADS)

Sakata, Yoshitaro; Terasaki, Nao; Nonaka, Kazuhiro

2017-05-01

Fine polishing techniques, such as a chemical mechanical polishing treatment, are important techniques in glass substrate manufacturing. However, these techniques may cause micro cracks under the surface of glass substrates because they used mechanical friction. A stress-induced light scattering method (SILSM), which was combined with light scattering method and mechanical stress effects, was proposed for inspecting surfaces to detect polishing-induced micro cracks. However, in the conventional SILSM, samples need to be loaded with physical contact, and the loading point is invisible in transparent materials. Here, we introduced a novel non-contact SILSM using a heating device. A glass substrate was heated first, and then the light scattering intensity of micro cracks was detected by a cooled charge-couple device camera during the natural cooling process. Results clearly showed during the decreasing surface temperature of a glass substrate, appropriate thermal stress is generated for detecting micro cracks by using the SILSM and light scattering intensity from micro cracks changes. We confirmed that non-contact thermal SILSM (T-SILSM) can detect micro cracks under the surface of transparent materials.

A demonstration of mitigation of environmentally-assisted cracking by the application of a tensile overload

DOE Office of Scientific and Technical Information (OSTI.GOV)

James, L.A.

1997-02-01

Environmentally-assisted cracking (EAC) of low-alloy steels in high-temperature aqueous environments typical of those employed in light-water reactor (LWR) systems has been a subject of considerable interest since the pioneering work of Kondo et al demonstrated significantly higher fatigue crack propagation (FCP) rates in water than would be expected in an air environment under similar conditions. Here, environmentally-assisted cracking (EAC) of low-alloy steels in elevated temperature aqueous environments is readily observed in many laboratory experiments conducted in autoclaves, yet the observation of EAC in actual components operating in the same environments is quite rare. Mass transport of sulfides from the crackmore » enclave by diffusion and convection occurring in operating components provides one plausible explanation to this apparent paradox. Another contribution to EAC mitigation may also arise from the non-constant stress amplitudes typical for many operating components. This paper provides a demonstration of how a single tensile overload to 40% above a steady-state maximum fatigue stress can retard subsequent crack growth at the steady-state level for a sufficient period of time that diffusion mass transport can reduce the crack-tip sulfide concentration to a level below that necessary to sustain EAC.« less

Environmentally assisted cracking in light water reactors. Semiannual report, July 1998-December 1998.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chopra, O. K.; Chung, H. M.; Gruber, E. E.

This report summarizes work performed by Argonne National Laboratory on fatigue and environmentally assisted cracking (EAC) in light water reactors from July 1998 to December 1998. Topics that have been investigated include (a) environmental effects on fatigue S-N behavior of primary pressure boundary materials, (b) irradiation-assisted stress corrosion cracking of austenitic stainless steels (SSs), and (c) EAC of Alloys 600 and 690. Fatigue tests have been conducted to determine the crack initiation and crack growth characteristics of austenitic SSs in LWR environments. Procedures are presented for incorporating the effects of reactor coolant environments on the fatigue life of pressure vesselmore » and piping steels. Slow-strain-rate tensile tests and posttest fractographic analyses were conducted on several model SS alloys irradiated to {approx}0.3 and 0.9 x 10{sup 21} n {center_dot} cm{sup -2} (E > 1 MeV) in helium at 289 C in the Halden reactor. The results have been used to determine the influence of alloying and impurity elements on the susceptibility of these steels to irradiation-assisted stress corrosion cracking. Fracture toughness J-R curve tests were also conducted on two heats of Type 304 SS that were irradiated to {approx}0.3 x 10{sup 21} n {center_dot} cm{sup -2} in the Halden reactor. Crack-growth-rate tests have been conducted on compact-tension specimens of Alloys 600 and 690 under constant load to evaluate the resistance of these alloys to stress corrosion cracking in LWR environments.« less

Advanced Flaw Manufacturing and Crack Growth Control

NASA Astrophysics Data System (ADS)

Kemppainen, M.; Pitkänen, J.; Virkkunen, I.; Hänninen, H.

2004-02-01

Advanced artificial flaw manufacturing method has become available. The method produces true fatigue cracks, which are representative of most service-induced cracks. These cracks can be used to simulate behaviour of realistic cracks under service conditions. This paper introduces studies of the effects of different thermal loading cycles to crack opening and residual stress state as seen at the surface of the sample and in the ultrasonic signal. In-situ measurements were performed under dynamic thermal fatigue loading of a 20 mm long artificial crack.

Control of serpentinisation rate by reaction-induced cracking

NASA Astrophysics Data System (ADS)

Malvoisin, Benjamin; Brantut, Nicolas; Kaczmarek, Mary-Alix

2017-10-01

Serpentinisation of mantle rocks requires the generation and maintenance of transport pathways for water. The solid volume increase during serpentinisation can lead to stress build-up and trigger cracking, which ease fluid penetration into the rock. The quantitative effect of this reaction-induced cracking mechanism on reactive surface generation is poorly constrained, thus hampering our ability to predict serpentinisation rate in geological environments. Here we use a combined approach with numerical modelling and observations in natural samples to provide estimates of serpentinisation rate at mid-ocean ridges. We develop a micromechanical model to quantify the propagation of serpentinisation-induced cracks in olivine. The maximum crystallisation pressure deduced from thermodynamic calculations reaches several hundreds of megapascals but does not necessary lead to crack propagation if the olivine grain is subjected to high compressive stresses. The micromechanical model is then coupled to a simple geometrical model to predict reactive surface area formation during grain splitting, and thus bulk reaction rate. Our model reproduces quantitatively experimental kinetic data and the typical mesh texture formed during serpentinisation. We also compare the model results with olivine grain size distribution data obtained on natural serpentinised peridotites from the Marum ophiolite and the Papuan ultramafic belt (Papua New Guinea). The natural serpentinised peridotites show an increase of the number of olivine grains for a decrease of the mean grain size by one order of magnitude as reaction progresses from 5 to 40%. These results are in agreement with our model predictions, suggesting that reaction-induced cracking controls the serpentinisation rate. We use our model to estimate that, at mid-ocean ridges, serpentinisation occurs up to 12 km depth and reaction-induced cracking reduces the characteristic time of serpentinisation by one order of magnitude, down to values comprised between 10 and 1000 yr. The increase of effective pressure with depth also prevents cracking, which positions the peak in serpentinisation rate at shallower depths, 4 km above previous predictions.

Block 2 solar cell module environmental test program

NASA Technical Reports Server (NTRS)

Holloway, K. L.

1978-01-01

Environmental tests were performed of on 76 solar cell modules produced by four different manufacturers. The following tests were performed: (1) 28 day temperature and humidity; (2) rain and icing; (3) salt fog; (4) sand and dust; (5) vacuum/steam/pressure; (6) fungus; (7) temperature/altitude; and (8) thermal shock. Environmental testing of the solar cell modules produced cracked cells, cracked encapsulant and encapsulant delaminations on various modules. In addition, there was some minor cell and frame corrosion.

Characterization of Environmentally Assisted Cracking for Design: State of the Art.

DTIC Science & Technology

1982-01-01

Barsom, J.M., Effect of cyclic stress form on corrosion fatigue crack propagation below Kiscc in a high yield strength steel , in Corrosion Fatigue... Effect of Prestressing on the Stress Corrosion Resistance of Two High Strength Steels , Boeing Document D6-25275, Boeing Company, Seattle, Washington...sT’e Residual stress Crack growth High strength steel Seawater Crack initiation Hydrogen embrittlement Stress corrosion Design Linear elastic fracture

Monitoring micro-crack healing in an engineered cementitious composite using the environmental scanning electron microscope

DOE Office of Scientific and Technical Information (OSTI.GOV)

Suryanto, B., E-mail: b.suryanto@hw.ac.uk; Buckman

Environmental Scanning Electron Microscopy (ESEM) is used to study the origin of micro-crack healing in an Engineered Cementitious Composite (ECC). ESEM images were acquired from ECC specimens cut from pre-cracked, dog-bone samples which then subjected to submerged curing followed by exposure to the natural environment. The mineralogical and chemical compositions of the healing products were determined using the EDX facility in the ESEM. It is shown that the precipitation of calcium carbonate is the main contributor to micro-crack healing at the crack mouth. The healing products initially appeared in an angular rhombohedral morphology which then underwent a discernable transformation inmore » size, shape and surface texture, from relatively flat and smooth to irregular and rough, resembling the texture of the original surface areas surrounding the micro-cracks. It is also shown that exposure to the natural environment, involving intermittent wetting/drying cycles, promotes additional crystal growth, which indicates enhanced self-healing capability in this environment. - Highlights: •ESEM with EDX used to characterize the origin of micro-crack healing in an ECC •Evolution of healing precipitates studied at three specific locations over four weeks •Specimens exposed to laboratory environment, followed by the natural environment •Calcium carbonate is the main contributor to crack healing at the crack mouth. •Outdoor exposure involving intermittent rain promotes additional crystal growth.« less

Simulation of Low Velocity Impact Induced Inter- and Intra-Laminar Damage of Composite Beams Based on XFEM

NASA Astrophysics Data System (ADS)

Sun, Wei; Guan, Zhidong; Li, Zengshan

2017-12-01

In this paper, the Inter-Fiber Fracture (IFF) criterion of Puck failure theory based on the eXtended Finite Element Method (XFEM) was implemented in ABAQUS code to predict the intra-laminar crack initiation of unidirectional (UD) composite laminate. The transverse crack path in the matrix can be simulated accurately by the presented method. After the crack initiation, the propagation of the crack is simulated by Cohesive Zoom Model (CZM), in which the displacement discontinuities and stress concentration caused by matrix crack is introduced into the finite element (FE) model. Combined with the usage of the enriched element interface, which can be used to simulate the inter-laminar delamination crack, the Low Velocity Impact (LVI) induced damage of UD composite laminate beam with a typical stacking of composite laminates [05/903]S is studied. A complete crack initiation and propagation process was simulated and the numerical results obtained by the XFEM are consistent with the experimental results.

Fractographic analysis of gaseous hydrogen induced cracking in 18Ni maraging steel

NASA Technical Reports Server (NTRS)

Gangloff, R. P.; Wei, R. P.

1978-01-01

Electron microscope fractographic analysis supplemented an extensive study of the kinetics of gaseous hydrogen assisted cracking in 18Ni maraging steel. Temperature determined the crack path morphology in each steel which, in turn, was directly related to the temperature dependence of the crack growth rate. Crack growth in the low temperature regime proceeded along prior austenite grain boundaries. Increasing the temperature above a critical value produced a continuously increasing proportion of transgranular quasi-cleavage associated with lath martensite boundaries. The amount of transgranular cracking was qualitatively correlated with the degree of temperature-induced deviation from Arrhenius behavior. Fractographic observations are interpreted in terms of hypothesized mechanisms for gaseous hydrogen embrittlement. It is concluded that hydrogen segregation to prior austenite and lath martensite boundaries must be considered as a significant factor in developing mechanisms for gaseous embrittlement of high strength steels.

Rock-Salt Growth-Induced (003) Cracking in a Layered Positive Electrode for Li-Ion Batteries

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Hanlei; Omenya, Fredrick; Yan, Pengfei

For the first time, the (003) cracking is observed and determined to be the major cracking mechanism for the primary particles of Ni-rich layered dioxides as the positive electrode for Li-ion batteries. Using transmission electron microscopy techniques, here we show that the propagation and fracturing of platelet-like rock-salt phase along the (003) plane of the layered oxide are the leading cause for the cracking of primary particles. The fracturing of the rock-salt platelet is induced by the stress discontinuity between the parent layered oxide and the rock-salt phase. The high nickel content is considered to be the key factor formore » the formation of the rock-salt platelet and thus the (003) cracking. The (003)-type cracking can be a major factor for the structural degradation and associated capacity fade of the layered positive electrode.« less

The tensile effect on crack formation in single crystal silicon irradiated by intense pulsed ion beam

NASA Astrophysics Data System (ADS)

Liang, Guoying; Shen, Jie; Zhang, Jie; Zhong, Haowen; Cui, Xiaojun; Yan, Sha; Zhang, Xiaofu; Yu, Xiao; Le, Xiaoyun

2017-10-01

Improving antifatigue performance of silicon substrate is very important for the development of semiconductor industry. The cracking behavior of silicon under intense pulsed ion beam irradiation was studied by numerical simulation in order to understand the mechanism of induced surface peeling observed by experimental means. Using molecular dynamics simulation based on Stillinger Weber potential, tensile effect on crack growth and propagation in single crystal silicon was investigated. Simulation results reveal that stress-strain curves of single crystal silicon at a constant strain rate can be divided into three stages, which are not similar to metal stress-strain curves; different tensile load velocities induce difference of single silicon crack formation speed; the layered stress results in crack formation in single crystal silicon. It is concluded that the crack growth and propagation is more sensitive to strain rate, tensile load velocity, stress distribution in single crystal silicon.

A Contribution to Proof the Component Integrity Taking Into Account the Corrosion-Assisted Crack Growth

DOE Office of Scientific and Technical Information (OSTI.GOV)

Roos, Eberhard; Otremba, Frank; Huttner, Frank

2002-07-01

The proof of the component integrity is fundamental for a safe and reliable operation of Nuclear Power Plants (NPP). The concept of the Material Testing Institute (MPA) for integrity assessment is based on fracture mechanic analysis which results in detailed regulations for nondestructive examination. This approach has to account for the main damage mechanisms as fatigue and corrosion. This paper focuses on the influence of corrosion-assisted crack growth which strongly depends on corrosion and environmental conditions (e.g. coolant purity). Up to stress intensity of approximately 60 MPam for ferritic low-alloy steels in high-purity water (acc. to specification) under constant loadmore » conditions the analysis can be based on a crack extension of max. 70 for each load cycle. Related to a test duration of 1000 hours this is equivalent to a formally calculated crack growth rate (CGR) of = 2 10{sup -8} mm/s. For austenitic stainless steels more complex dependences on material, environmental and mechanical parameters exist. Particularly, for stabilized austenitic steels the crack growth rate data base is relatively weak. Under unfavourable environmental conditions in single cases crack growth rates up to 6 mm/a have been measured. Based on experimental results an arithmetic mean value of 0.95 mm/a and a median value of 0.6 mm/a have been determined. A further improvement of data base is desirable. (authors)« less

Shock-induced Plasticity and Brittle Cracks in Aluminum Nitride

NASA Astrophysics Data System (ADS)

Branicio, Paulo; Kalia, Rajiv

2005-03-01

Two hundred and nine million atom molecular-dynamics simulation of hypervelocity projectile impact in aluminum nitride reveals strong interplay between shock-induced structural phase transformation, plastic deformation and brittle cracks. The shock wave splits into an elastic precursor and a wurtzite-to-rocksalt structural transformation wave. When the elastic wave reflected from the boundary of the sample interacts with the transformation wave front, nanocavities are generated along the penetration path of the projectile and dislocations in adjacent regions. The nanocavities coalesce to form mode I brittle cracks while dislocations generate kink bands that give rise to mode II cracks. These simulations provide a microscopic view of defects associated with simultaneous tensile and shear cracking at the structural phase transformation boundary due to shock impact in high-strength ceramics.

Thermally induced transverse cracking in graphite-epoxy cross-ply laminates

NASA Technical Reports Server (NTRS)

Adams, D. S.; Bowles, D. E.; Herakovich, C. T.

1986-01-01

Thermally induced transverse cracking in T300/5208 graphite-epoxy cross-ply laminates was investigated experimentally and theoretically. The six laminate configurations studied were: 0/90(3)s, 0(2)/90(2)s, 0(3)/90s, 90/0(3)s, 90(2)/0(2)s, and 90(3)/0s. The thermal load required to initiate transverse cracking was determined experimentally and compared to a theoretical prediction. Experimental results for the accumulation of transverse cracks under cyclic thermal loading between - 250 and 250 F for up to 500 thermal cycles are presented. The calculated in situ transverse-lamina strength was determined to be at least 1.9 times the unidirectional-lamina transverse tensile strength. All laminate configurations exhibited an increase in crack density with increasing thermal cycles.

Seeding Cracks Using a Fatigue Tester for Accelerated Gear Tooth Breaking

NASA Technical Reports Server (NTRS)

Nenadic, Nenad G.; Wodenscheck, Joseph A.; Thurston, Michael G.; Lewicki, David G.

2011-01-01

This report describes fatigue-induced seeded cracks in spur gears and compares them to cracks created using a more traditional seeding method, notching. Finite element analysis (FEA) compares the effective compliance of a cracked tooth to the effective compliance of a notched tooth where the crack and the notch are of the same depth. In this analysis, cracks are propagated to the desired depth using FRANC2D and effective compliances are computed in ANSYS. A compliance-based feature for detecting cracks on the fatigue tester is described. The initiated cracks are examined using both nondestructive and destructive methods. The destructive examination reveals variability in the shape of crack surfaces.

Corrosion fatigue crack propagation in metals

NASA Technical Reports Server (NTRS)

Gangloff, Richard P.

1990-01-01

This review assesses fracture mechanics data and mechanistic models for corrosion fatigue crack propagation in structural alloys exposed to ambient temperature gases and electrolytes. Extensive stress intensity-crack growth rate data exist for ferrous, aluminum and nickel based alloys in a variety of environments. Interactive variables (viz., stress intensity range, mean stress, alloy composition and microstructure, loading frequency, temperature, gas pressure and electrode potential) strongly affect crack growth kinetics and complicate fatigue control. Mechanistic models to predict crack growth rates were formulated by coupling crack tip mechanics with occluded crack chemistry, and from both the hydrogen embrittlement and anodic dissolution/film rupture perspectives. Research is required to better define: (1) environmental effects near threshold and on crack closure; (2) damage tolerant life prediction codes and the validity of similitude; (3) the behavior of microcrack; (4) probes and improved models of crack tip damage; and (5) the cracking performance of advanced alloys and composites.

Mesos-scale modeling of irradiation in pressurized water reactor concrete biological shields

DOE Office of Scientific and Technical Information (OSTI.GOV)

Le Pape, Yann; Huang, Hai

Neutron irradiation exposure causes aggregate expansion, namely radiation-induced volumetric expansion (RIVE). The structural significance of RIVE on a portion of a prototypical pressurized water reactor (PWR) concrete biological shield (CBS) is investigated by using a meso- scale nonlinear concrete model with inputs from an irradiation transport code and a coupled moisture transport-heat transfer code. RIVE-induced severe cracking onset appears to be triggered by the ini- tial shrinkage-induced cracking and propagates to a depth of > 10 cm at extended operation of 80 years. Relaxation of the cement paste stresses results in delaying the crack propagation by about 10 years.

Crack injection in silver gold alloys

NASA Astrophysics Data System (ADS)

Chen, Xiying

Stress corrosion cracking (SCC) is a materials degradation phenomena resulting from a combination of stress and a corrosive environment. Among the alphabet soup of proposed mechanism of SCC the most important are film-rupture, film-induced cleavage and hydrogen embrittlement. This work examines various aspects of film-induced cleavage in gold alloys for which the operation of hydrogen embrittlement processes can be strictly ruled out on thermodynamic grounds. This is so because in such alloys SCC occurs under electrochemical conditions within which water is stable to hydrogen gas evolution. The alloy system examined in this work is AgAu since the corrosion processes in this system occur by a dealloying mechanism that results in the formation of nanoporous gold. The physics behind the dealloying process as well as the resulting formation of nanoporous gold is today well understood. Two important aspects of the film-induced cleavage mechanism are examined in this work: dynamic fracture in monolithic nanoporous gold and crack injection. In crack injection there is a finite thickness dealloyed layer formed on a AgAu alloy sample and the question of whether or not a crack that nucleates within this layer can travel for some finite distance into the un-corroded parent phase alloy is addressed. Dynamic fracture tests were performed on single edge-notched monolithic nanoporous gold samples as well as "infinite strip" sample configurations for which the stress intensity remains constant over a significant portion of the crack length. High-speed photography was used to measure the crack velocity. In the dynamic fracture experiments cracks were observed to travel at speeds as large as 270 m/s corresponding to about 68% of the Raleigh wave velocity. Crack injection experiments were performed on single crystal Ag77Au23, polycrystalline Ag72Au28 and pure gold, all of which had thin nanoporous gold layers on the surface of samples. Through-thickness fracture was seen in both the single crystal and polycrystalline samples and there was an indication of ~ 1 mum injected cracks into pure gold. These results have important implications for the operation of the film-induced cleavage mechanism and represent a first step in the development of a fundamental model of SCC.

Investigation of Thermal High Cycle and Low Cycle Fatigue Mechanisms of Thick Thermal Barrier Coatings

NASA Technical Reports Server (NTRS)

Zhu, Dongming; Miller, Robert A.

1998-01-01

Thick thermal barrier coating systems in a diesel engine experience severe thermal low cycle fatigue (LCF) and high cycle fatigue (HCF) during engine operation. In this paper, the mechanisms of fatigue crack initiation and propagation in a ZrO2-8wt.% Y2O3 thermal barrier coating, under simulated engine thermal LCF and HCF conditions, are investigated using a high power CO2 laser. Experiments showed that the combined LCF/HCF tests induced more severe coating surface cracking, microspallation and accelerated crack growth, as compared to the pure LCF test. Lateral crack branching and the ceramic/bond coat interface delaminations were also facilitated by HCF thermal loads, even in the absence of severe interfacial oxidation. Fatigue damages at crack wake surfaces, due to such phenomena as asperity/debris contact induced cracking and splat pull-out bending during cycling, were observed especially for the combined LCF/HCF tests. It is found that the failure associated with LCF is closely related to coating sintering and creep at high temperatures, which induce tensile stresses in the coating after cooling. The failure associated with HCF process, however, is mainly associated with a surface wedging mechanism. The interaction between the LCF, HCF and ceramic coating creep, and the relative importance of LCF and HCF in crack propagation are also discussed based on the experimental evidence.

On the Influence of Nb/Ti Ratio on Environmentally-Assisted Crack Growth in High-Strength Nickel-Based Superalloys

NASA Astrophysics Data System (ADS)

Németh, A. A. N.; Crudden, D. J.; Collins, D. M.; Kuksenko, V.; Liebscher, C. H.; Armstrong, D. E. J.; Wilkinson, A. J.; Reed, R. C.

2018-05-01

The effect of Nb/Ti ratio on environmentally-assisted crack growth of three prototype Ni-based superalloys is studied. For these alloys, the yield strength is unaltered with increasing Nb/Ti ratio due to an increase in grain size. This situation has allowed the rationalization of the factors influencing damage tolerance at 700 °C. Primary intergranular cracks have been investigated using energy-dispersive X-ray spectroscopy in a scanning transmission electron microscope and the analysis of electron back-scatter diffraction patterns. Any possible detrimental effect of Nb on the observed crack tip damage due to Nb-rich oxide formation is not observed. Instead, evidence is presented to indicate that the tertiary γ'-precipitates are dissolving ahead of the crack consistent with the formation of oxides such as alumina and rutile. Our results have implications for alloy design efforts; at any given strength level, both more and less damage-tolerant variants of these alloys can be designed.

Micro-mechanisms of Surface Defects Induced on Aluminum Alloys during Plastic Deformation at Elevated Temperatures

NASA Astrophysics Data System (ADS)

Gali, Olufisayo A.

Near-surface deformed layers developed on aluminum alloys significantly influence the corrosion and tribological behavior as well as reduce the surface quality of the rolled aluminum. The evolution of the near-surface microstructures induced on magnesium containing aluminum alloys during thermomechanical processing has been investigated with the aim generating an understanding of the influence of individual forming parameters on its evolution and examine the microstructure of the roll coating induced on the mating steel roll through material transfer during rolling. The micro-mechanisms related to the various features of near-surface microstructure developed during tribological conditions of the simulated hot rolling process were identified. Thermomechanical processing experiments were performed with the aid of hot rolling (operating temperature: 550 to 460 °C, 4, 10 and 20 rolling pass schedules) and hot forming (operating temperature: 350 to 545 °C, strain rate: 4 x 10-2 s-1) tribo-simulators. The surface, near-surface features and material transfer induced during the elevated temperature plastic deformation were examined and characterized employing optical interferometry, SEM/EDS, FIB and TEM. Near-surface features characterized on the rolled aluminum alloys included; cracks, fractured intermetallic particles, aluminum nano-particles, oxide decorated grain boundaries, rolled-in oxides, shingles and blisters. These features were related to various individual rolling parameters which included, the work roll roughness, which induced the formation of shingles, rolling marks and were responsible for the redistribution of surface oxide and the enhancements of the depth of the near-surface damage. The enhanced stresses and strains experienced during rolling were related to the formation and propagation of cracks, the nanocrystalline structure of the near-surface layers and aluminum nano-particles. The mechanism of the evolution of the near-surface microstructure were determined to include grain boundary sliding which induced the cracks at the surface and subsurface of the alloy, magnesium diffusion to free surfaces, crack propagation from shear stresses and the shear strains inducing the nanocrystalline grain structure, the formation of shingles by the shear deformation of micro-wedges induced by the work roll grooves, and the deformation of this oxide covered micro-wedges inducing the rolled-in oxides. Magnesium diffusion to free surfaces was identified as inducing crack healing due to the formation of MgO within cracks and was responsible for the oxide decorated grain boundaries. An examination of the roll coating revealed a complex layered microstructure that was induced through tribo-chemical and mechanical entrapment mechanisms. The microstructure of the roll coating suggested that the work roll material and the rolled aluminum alloy were essential in determining its composition and structure. Subsequent hot forming processes revealed the rich oxide-layer of the near-surface microstructure was beneficial for reducing the coefficient of friction during tribological contact with the steel die. Damage to the microstructure include cracks induced from grain boundary sliding of near-surface grains and the formation of oxide fibres within cracks of the near-surface deformed layers.

Experimental study on the crack detection with optimized spatial wavelet analysis and windowing

NASA Astrophysics Data System (ADS)

Ghanbari Mardasi, Amir; Wu, Nan; Wu, Christine

2018-05-01

In this paper, a high sensitive crack detection is experimentally realized and presented on a beam under certain deflection by optimizing spatial wavelet analysis. Due to the crack existence in the beam structure, a perturbation/slop singularity is induced in the deflection profile. Spatial wavelet transformation works as a magnifier to amplify the small perturbation signal at the crack location to detect and localize the damage. The profile of a deflected aluminum cantilever beam is obtained for both intact and cracked beams by a high resolution laser profile sensor. Gabor wavelet transformation is applied on the subtraction of intact and cracked data sets. To improve detection sensitivity, scale factor in spatial wavelet transformation and the transformation repeat times are optimized. Furthermore, to detect the possible crack close to the measurement boundaries, wavelet transformation edge effect, which induces large values of wavelet coefficient around the measurement boundaries, is efficiently reduced by introducing different windowing functions. The result shows that a small crack with depth of less than 10% of the beam height can be localized with a clear perturbation. Moreover, the perturbation caused by a crack at 0.85 mm away from one end of the measurement range, which is covered by wavelet transform edge effect, emerges by applying proper window functions.

Numerical simulation of stress amplification induced by crack interaction in human femur bone

NASA Astrophysics Data System (ADS)

Alia, Noor; Daud, Ruslizam; Ramli, Mohammad Fadzli; Azman, Wan Zuki; Faizal, Ahmad; Aisyah, Siti

2015-05-01

This research is about numerical simulation using a computational method which study on stress amplification induced by crack interaction in human femur bone. Cracks in human femur bone usually occur because of large load or stress applied on it. Usually, the fracture takes longer time to heal itself. At present, the crack interaction is still not well understood due to bone complexity. Thus, brittle fracture behavior of bone may be underestimated and inaccurate. This study aims to investigate the geometrical effect of double co-planar edge cracks on stress intensity factor (K) in femur bone. This research focuses to analyze the amplification effect on the fracture behavior of double co-planar edge cracks, where numerical model is developed using computational method. The concept of fracture mechanics and finite element method (FEM) are used to solve the interacting cracks problems using linear elastic fracture mechanics (LEFM) theory. As a result, this study has shown the identification of the crack interaction limit (CIL) and crack unification limit (CUL) exist in the human femur bone model developed. In future research, several improvements will be made such as varying the load, applying thickness on the model and also use different theory or method in calculating the stress intensity factor (K).

An energy-consistent fracture model for ferroelectrics

NASA Astrophysics Data System (ADS)

Miao, Hongchen; Li, Faxin

2017-02-01

The fracture behavior of ferroelectrics has been intensively studied in recent decades, though currently a widely accepted fracture mechanism is still lacking. In this work, enlightened by previous experimental observations that crack propagation in ferroelectrics is always accompanied by domain switching, we propose a micromechanical model in which both crack propagation and domain switching are controlled by energy-based criteria. Both electric energy and mechanical energy can induce domain switching, while only mechanical energy can drive crack propagation. Furthermore, constrained domain switching is considered in this model, leading to the gradient domain switching zone near the crack tip. Analysis results show that stress-induced ferroelastic switching always has a toughening effect as the mechanical energy release rate serves as the driving force for both fracture and domain switching. In comparison, the electric-field-induced switching may have either a toughening or detoughening effect. The proposed model can qualitatively agree with the existing experimental results.

Repair of distortion-induced fatigue damage in bridge no. 135-87 (043SB and 044NB) using newly-developed strengthening schemes : summary.

DOT National Transportation Integrated Search

2016-08-01

A steel girder twin bridge structure located near Park City, Kansas, has experienced : extensive distortion-induced fatigue cracking in its web-gap regions. Due to : the bridges skewed, staggered configuration, the majority of these cracks have : ...

Evaluation of Thermo-Mechanical Stability of COTS Dual-Axis MEMS Accelerometers for Space Applications

NASA Technical Reports Server (NTRS)

Sharma, Ashok K.; Teverovksy, Alexander; Day, John H. (Technical Monitor)

2000-01-01

Microelectromechanical systems in MEMS is one of the fastest growing technologies in microelectronics, and is of great interest for military and aerospace applications. Accelerometers are the earliest and most developed representatives of MEMS. First demonstrated in 1979, micromachined accelerometers were used in automobile industry for air bag crash- sensing applications since 1990. In 1999, N4EMS accelerometers were used in NASA-JPL Mars Microprobe. The most developed accelerometers for airbag crash- sensing are rated for a full range of +/- 50 G. The range of sensitivity for accelerometers required for military or aerospace applications is much larger, varying from 20,000 G (to measure acceleration during gun and ballistic munition launches), and to 10(exp -6) G, when used as guidance sensors (to measure attitude and position of a spacecraft). The presence of moving parts on the surface of chip is specific to MEMS, and particularly, to accelerometers. This characteristic brings new reliability issues to micromachined accelerometers, including cyclic fatigue cracking of polysilicon cantilevers and springs, mechanical stresses that are caused by packaging and contamination in the internal cavity of the package. Studies of fatigue cracks initiation and growth in polysilicon showed that the fatigue damage may influence MEMS device performance, and the presence of water vapor significantly enhances crack initiation and growth. Environmentally induced failures, particularly, failures due to thermal cycling and mechanical shock are considered as one of major reliability concerns in MEMS. These environmental conditions are also critical for space applications of the parts. For example, the Mars pathfinder mission had experienced 80 mechanical shock events during the pyrotechnic separation processes.

Effect of irradiation-induced plastic flow localization on ductile crack resistance behavior of a 9%Cr tempered martensitic steel

NASA Astrophysics Data System (ADS)

Chaouadi, R.

2008-01-01

This paper examines the effect of irradiation-induced plastic flow localization on the crack resistance behavior. Tensile and crack resistance measurements were performed on Eurofer-97 that was irradiated at 300 °C to neutron doses ranging between 0.3 and 2.1 dpa. A severe degradation of crack resistance behavior is experimentally established at quasi-static loading, in contradiction with the Charpy impact data and the dynamic crack resistance measurements. This degradation is attributed to the dislocation channel deformation phenomenon. At quasi-static loading rate, scanning electron microscopy observations of the fracture surfaces revealed a significant change of fracture topography, mainly from equiaxed dimples (mode I) to shear dimples (mode I + II). With increasing loading rate, the high peak stresses that develop inside the process zone activate much more dislocation sources resulting in a higher density of cross cutting dislocation channels and therefore an almost unaffected crack resistance. These explanations provide a rational to all experimental observations.

Degradation and mechanism of the mechanics and durability of reinforced concrete slab in a marine environment

NASA Astrophysics Data System (ADS)

Wu, Sheng-xing; Liu, Guan-guo; Bian, Han-bing; Lv, Wei-bo; Jiang, Jian-hua

2016-04-01

An experimental research was conducted to determine the corrosion and bearing capacity of a reinforced concrete (RC) slab at different ages in a marine environment. Results show that the development of corrosion-induced cracks on a slab in a marine environment can be divided into three stages according to crack morphology at the bottom of the slab. In the first stage, cracks appear. In the second stage, cracks develop from the edges to the middle of the slab. In the third stage, longitudinal and transverse corrosion-induced cracks coexist. The corrosion ratio of reinforcements nonlinearly increases with the age, and the relationship between the corrosion ratio of the reinforcements and the corrosion-induced crack width of the concrete is established. The flexural capacity of the corroded RC slab nonlinearly decreases with the age, and the model for the bearing capacity factor of the corroded RC slab is established. The mid-span deflection of the corroded RC slab that corresponds to the yield of the reinforcements linearly increases with the increase in corrosion ratio. Finally, the mechanisms of corrosion morphology and the degradation of the mechanical properties of an RC slab in a marine environment are discussed on the basis of the basic theories of steel corrosion in concrete and concrete structure design.

40 CFR Table 8 to Subpart Uuu of... - Organic HAP Emission Limits for Catalytic Cracking Units

Code of Federal Regulations, 2014 CFR

2014-07-01

... 40 Protection of Environment 13 2014-07-01 2014-07-01 false Organic HAP Emission Limits for Catalytic Cracking Units 8 Table 8 to Subpart UUU of Part 63 Protection of Environment ENVIRONMENTAL..., Subpt. UUU, Table 8 Table 8 to Subpart UUU of Part 63—Organic HAP Emission Limits for Catalytic Cracking...

40 CFR Table 8 to Subpart Uuu of... - Organic HAP Emission Limits for Catalytic Cracking Units

Code of Federal Regulations, 2013 CFR

2013-07-01

... 40 Protection of Environment 13 2013-07-01 2012-07-01 true Organic HAP Emission Limits for Catalytic Cracking Units 8 Table 8 to Subpart UUU of Part 63 Protection of Environment ENVIRONMENTAL..., Subpt. UUU, Table 8 Table 8 to Subpart UUU of Part 63—Organic HAP Emission Limits for Catalytic Cracking...

40 CFR Table 8 to Subpart Uuu of... - Organic HAP Emission Limits for Catalytic Cracking Units

Code of Federal Regulations, 2012 CFR

2012-07-01

... 40 Protection of Environment 13 2012-07-01 2012-07-01 false Organic HAP Emission Limits for Catalytic Cracking Units 8 Table 8 to Subpart UUU of Part 63 Protection of Environment ENVIRONMENTAL..., Subpt. UUU, Table 8 Table 8 to Subpart UUU of Part 63—Organic HAP Emission Limits for Catalytic Cracking...

Recent progress to understand stress corrosion cracking in sodium borosilicate glasses: linking the chemical composition to structural, physical and fracture properties

NASA Astrophysics Data System (ADS)

Rountree, Cindy L.

2017-08-01

This topical review is dedicated to understanding stress corrosion cracking in oxide glasses and specifically the SiO_2{\\text-B_2O_3{\\text-}Na_2O} (SBN) ternary glass systems. Many review papers already exist on the topic of stress corrosion cracking in complex oxide glasses or overly simplified glasses (pure silica). These papers look at how systematically controlling environmental factors (pH, temperature...) alter stress corrosion cracking, while maintaining the same type of glass sample. Many questions still exist, including: What sets the environmental limit? What sets the velocity versus stress intensity factor in the slow stress corrosion regime (Region I)? Can researchers optimize these two effects to enhance a glass’ resistance to failure? To help answer these questions, this review takes a different approach. It looks at how systemically controlling the glass’ chemical composition alters the structure and physical properties. These changes are then compared and contrasted to the fracture toughness and the stress corrosion cracking properties. By taking this holistic approach, researchers can begin to understand the controlling factors in stress corrosion cracking and how to optimize glasses via the initial chemical composition.

Influence and Modeling of Residual Stresses in Thick Walled Pressure Vessels with Through Holes

DTIC Science & Technology

2012-02-28

9 FIGURE 4 ENVIRONMENTAL CRACKING OBSERVED IN EVACUATOR HOLE .......... 9 FIGURE 5 STRESSES PRESENT IN STRAIGHT EVACUATOR... ASSESMENT OF INITIAL DAMAGE Through investigation was undertaken on vessels similar in size and strength level to pressure vessels 85A and 85B...suggesting that the source of the residual stresses required to initiate and propagate these environmental cracks is not a resultant of the typical

Volume analysis of heat-induced cracks in human molars: A preliminary study

PubMed Central

Sandholzer, Michael A.; Baron, Katharina; Heimel, Patrick; Metscher, Brian D.

2014-01-01

Context: Only a few methods have been published dealing with the visualization of heat-induced cracks inside bones and teeth. Aims: As a novel approach this study used nondestructive X-ray microtomography (micro-CT) for volume analysis of heat-induced cracks to observe the reaction of human molars to various levels of thermal stress. Materials and Methods: Eighteen clinically extracted third molars were rehydrated and burned under controlled temperatures (400, 650, and 800°C) using an electric furnace adjusted with a 25°C increase/min. The subsequent high-resolution scans (voxel-size 17.7 μm) were made with a compact micro-CT scanner (SkyScan 1174). In total, 14 scans were automatically segmented with Definiens XD Developer 1.2 and three-dimensional (3D) models were computed with Visage Imaging Amira 5.2.2. The results of the automated segmentation were analyzed with an analysis of variance (ANOVA) and uncorrected post hoc least significant difference (LSD) tests using Statistical Package for Social Sciences (SPSS) 17. A probability level of P < 0.05 was used as an index of statistical significance. Results: A temperature-dependent increase of heat-induced cracks was observed between the three temperature groups (P < 0.05, ANOVA post hoc LSD). In addition, the distributions and shape of the heat-induced changes could be classified using the computed 3D models. Conclusion: The macroscopic heat-induced changes observed in this preliminary study correspond with previous observations of unrestored human teeth, yet the current observations also take into account the entire microscopic 3D expansions of heat-induced cracks within the dental hard tissues. Using the same experimental conditions proposed in the literature, this study confirms previous results, adds new observations, and offers new perspectives in the investigation of forensic evidence. PMID:25125923

Fracture mechanics and surface chemistry investigations of environment-assisted crack growth

NASA Technical Reports Server (NTRS)

Wei, R. P.; Klier, K.; Simmons, G. W.; Chou, Y. T.

1984-01-01

It is pointed out that environment-assisted subcritical crack growth in high-strength steels and other high-strength alloys (particularly in hydrogen and in hydrogenous environments) is an important technological problem of long standing. This problem is directly related to issues of structural integrity, durability, and reliability. The terms 'hydrogen embrittlement' and 'stress corrosion cracking' have been employed to describe the considered phenomenon. This paper provides a summary of contributions made during the past ten years toward the understanding of environmentally assisted crack growth. The processes involved in crack growth are examined, and details regarding crack growth and chemical reactions are discussed, taking into account crack growth in steels exposed to water/water vapor, the effect of hydrogen, reactions involving hydrogen sulfide, and aspects of fracture surface morphology and composition. Attention is also given to the modeling of crack growth response, crack growth in gas mixtures, and the interaction of solute atoms with the crack-tip stress field.

Modeling the Interactions Between Multiple Crack Closure Mechanisms at Threshold

NASA Technical Reports Server (NTRS)

Newman, John A.; Riddell, William T.; Piascik, Robert S.

2003-01-01

A fatigue crack closure model is developed that includes interactions between the three closure mechanisms most likely to occur at threshold; plasticity, roughness, and oxide. This model, herein referred to as the CROP model (for Closure, Roughness, Oxide, and Plasticity), also includes the effects of out-of plane cracking and multi-axial loading. These features make the CROP closure model uniquely suited for, but not limited to, threshold applications. Rough cracks are idealized here as two-dimensional sawtooths, whose geometry induces mixed-mode crack- tip stresses. Continuum mechanics and crack-tip dislocation concepts are combined to relate crack face displacements to crack-tip loads. Geometric criteria are used to determine closure loads from crack-face displacements. Finite element results, used to verify model predictions, provide critical information about the locations where crack closure occurs.

Generating Fatigue Crack Growth Thresholds with Constant Amplitude Loads

NASA Technical Reports Server (NTRS)

Forth, Scott C.; Newman, James C., J.; Forman, Royce G.

2002-01-01

The fatigue crack growth threshold, defining crack growth as either very slow or nonexistent, has been traditionally determined with standardized load reduction methodologies. Some experimental procedures tend to induce load history effects that result in remote crack closure from plasticity. This history can affect the crack driving force, i.e. during the unloading process the crack will close first at some point along the wake, reducing the effective load at the crack tip. One way to reduce the effects of load history is to propagate a crack under constant amplitude loading. As a crack propagates under constant amplitude loading, the stress intensity factor, K, will increase, as will the crack growth rate, da/dN. A fatigue crack growth threshold test procedure is developed and experimentally validated that does not produce load history effects and can be conducted at a specified stress ratio, R.

Surface Cracking and Interface Reaction Associated Delamination Failure of Thermal and Environmental Barrier Coatings

NASA Technical Reports Server (NTRS)

Zhu, Dong-Ming; Choi, Sung R.; Eldridge, Jeffrey I.; Lee, Kang N.; Miller, Robert A.

2003-01-01

In this paper, surface cracking and interface reactions of a BSAS coating and a multi-layer ZrO2-8wt%Y2O3 and mullite/BSAS/Si thermal and environmental barrier coating system on SiC/SiC ceramic matrix composites were characterized after long-term combined laser thermal gradient and furnace cyclic tests in a water vapor containing environment. The surface cracking was analyzed based on the coating thermal gradient sintering behavior and thermal expansion mismatch stress characteristics under the thermal cyclic conditions. The interface reactions, which were largely enhanced by the coating surface cracking in the water vapor environment, were investigated in detail, and the reaction phases were identified for the coating system after the long-term exposure. The accelerated coating delamination failure was attributed to the increased delamination driving force under the thermal gradient cyclic loading and the reduced interface adhesion due to the detrimental interface reactions.

Surface Cracking and Interface Reaction Associated Delamination Failure of Thermal and Environmental Barrier Coatings

NASA Technical Reports Server (NTRS)

Zhu, Dongming; Choi, Sung R.; Eldridge, Jeffrey I.; Lee, Kang N.; Miller, Robert A.

2003-01-01

In this paper, surface cracking and interface reactions of a BSAS coating and a multi-layer ZTO2-8wt%Y2O3 and mullite/BSAS/Si thermal and environmental barrier coating system on SiC/SiC ceramic matrix composites were characterized after long-term combined laser thermal gradient and furnace cyclic tests in a water vapor containing environment. The surface cracking was analyzed based on the coating thermal gradient sintering behavior and thermal expansion mismatch stress characteristics under the thermal cyclic conditions. The interface reactions, which were largely enhanced by the coating surface cracking in the water vapor environment, were investigated in detail, and the reaction phases were identified for the coating system after the long- term exposure. The accelerated coating delamination failure was attributed to the increased delamination driving force under the thermal gradient cyclic loading and the reduced interface adhesion due to the detrimental interface reactions.

Load environment of rail joint bars - phase I, effects of track parameters on rail joint stresses and crack growth.

DOT National Transportation Integrated Search

2013-04-01

The load environment of joint bars was assessed under a variety of loading and track conditions. Bending stresses, thermal stresses, and residual stresses were measured on commonly used joint bars. Crack growth rates from artificially induced cracks ...

Analysis of Adhesively Bonded Ceramics Using an Asymmetric Wedge Test

DTIC Science & Technology

2008-12-01

4 Figure 2. Average crack ...flexure specimen. The flaw, indicated by the white arrow, is a subsurface semi-elliptical crack induced by surface machining damage...strength-limiting orthogonal surface machining crack in an alumina flexure specimen coated with a single layer of film adhesive. The white arrow

Towards a better understanding of the cracking behavior in soils

USDA-ARS?s Scientific Manuscript database

Understanding and modeling shrinkage-induced cracks helps bridge the gap between flow problem in the laboratory and at the field. Modeling flow at the field scale with Darcian fluxes developed at the laboratory scales is challenged with preferential flows attributed to the cracking behavior of soils...

New methods to quantify the cracking performance of cementitious systems made with internal curing

NASA Astrophysics Data System (ADS)

Schlitter, John L.

The use of high performance concretes that utilize low water-cement ratios have been promoted for use in infrastructure based on their potential to increase durability and service life because they are stronger and less porous. Unfortunately, these benefits are not always realized due to the susceptibility of high performance concrete to undergo early age cracking caused by shrinkage. This problem is widespread and effects federal, state, and local budgets that must maintain or replace deterioration caused by cracking. As a result, methods to reduce or eliminate early age shrinkage cracking have been investigated. Internal curing is one such method in which a prewetted lightweight sand is incorporated into the concrete mixture to provide internal water as the concrete cures. This action can significantly reduce or eliminate shrinkage and in some cases causes a beneficial early age expansion. Standard laboratory tests have been developed to quantify the shrinkage cracking potential of concrete. Unfortunately, many of these tests may not be appropriate for use with internally cured mixtures and only provide limited amounts of information. Most standard tests are not designed to capture the expansive behavior of internally cured mixtures. This thesis describes the design and implementation of two new testing devices that overcome the limitations of current standards. The first device discussed in this thesis is called the dual ring. The dual ring is a testing device that quantifies the early age restrained shrinkage performance of cementitious mixtures. The design of the dual ring is based on the current ASTM C 1581-04 standard test which utilizes one steel ring to restrain a cementitious specimen. The dual ring overcomes two important limitations of the standard test. First, the standard single ring test cannot restrain the expansion that takes place at early ages which is not representative of field conditions. The dual ring incorporates a second restraining ring which is located outside of the sample to provide restraint against expansion. Second, the standard ring test is a passive test that only relies on the autogenous and drying shrinkage of the mixture to induce cracking. The dual ring test can be an active test because it has the ability to vary the temperature of the specimen in order to induce thermal stress and produce cracking. This ability enables the study of the restrained cracking capacity as the mixture ages in order to quantify crack sensitive periods of time. Measurements made with the dual ring quantify the benefits from using larger amounts of internal curing. Mixtures that resupplied internal curing water to match that of chemical shrinkage could sustain three times the magnitude of thermal change before cracking. The second device discussed in this thesis is a large scale slab testing device. This device tests the cracking potential of 15' long by 4" thick by 24" wide slab specimens in an environmentally controlled chamber. The current standard testing devices can be considered small scale and encounter problems when linking their results to the field due to size effects. Therefore, the large scale slab testing device was developed in order to calibrate the results of smaller scale tests to real world field conditions such as a pavement or bridge deck. Measurements made with the large scale testing device showed that the cracking propensity of the internally cured mixtures was reduced and that a significant benefit could be realized.

Prediction of Failure Due to Thermal Aging, Corrosion and Environmental Fracture in Amorphous and Titanium Alloys

DOE Office of Scientific and Technical Information (OSTI.GOV)

Farmer, J C

2003-04-15

DARPA is exploring a number of advanced materials for military applications, including amorphous metals and titanium-based alloys. Equipment made from these materials can undergo degradation due to thermal aging, uniform corrosion, pitting, crevice corrosion, denting, stress corrosion cracking, corrosion fatigue, hydrogen induced cracking and microbial influenced corrosion. Amorphous alloys have exceptional resistance to corrosion, due in part to the absence of grain boundaries, but can undergo crystallization and other phase instabilities during heating and welding. Titanium alloys are extremely corrosion resistant due to the formation of a tenacious passive film of titanium oxide, but is prone to hydrogen absorption inmore » crevices, and hydrogen induced cracking after hydrogen absorption. Accurate predictions of equipment reliability, necessary for strategic planning, requires integrated models that account for all relevant modes of attack, and that can make probabilistic predictions. Once developed, model parameters must be determined experimentally, and the validity of models must be established through careful laboratory and field tests. Such validation testing requires state-of-the-art surface analytical techniques, as well as electrochemical and fracture mechanics tests. The interaction between those processes that perturb the local environment on a surface and those that alter metallurgical condition must be integrated in predictive models. The material and environment come together to drive various modes of corrosive attack (Figure 1). Models must be supported through comprehensive materials testing capabilities. Such capabilities are available at LLNL and include: the Long Term Corrosion Test Facility (LTCTF) where large numbers of standard samples can be exposed to realistic test media at several temperature levels; a reverse DC machine that can be used to monitor the propagation of stress corrosion cracking (SCC) in situ; and banks of potentiostats with temperature controlled cells for potentiostatic and potentiodynamic testing (Figure 2).« less

On Generating Fatigue Crack Growth Thresholds

NASA Technical Reports Server (NTRS)

Forth, Scott C.; Newman, James, Jr.; Forman, Royce G.

2003-01-01

The fatigue crack growth threshold, defining crack growth as either very slow or nonexistent, has been traditionally determined with standardized load reduction methodologies. These experimental procedures can induce load history effects that result in crack closure. This history can affect the crack driving force, i.e. during the unloading process the crack will close first at some point along the wake or blunt at the crack tip, reducing the effective load at the crack tip. One way to reduce the effects of load history is to propagate a crack under constant amplitude loading. As a crack propagates under constant amplitude loading, the stress intensity factor range, Delta K, will increase, as will the crack growth rate. da/dN. A fatigue crack growth threshold test procedure is experimentally validated that does not produce load history effects and can be conducted at a specified stress ratio, R. The authors have chosen to study a ductile aluminum alloy where the plastic deformations generated during testing may be of the magnitude to impact the crack opening.

Effect of Measured Welding Residual Stresses on Crack Growth

NASA Technical Reports Server (NTRS)

Hampton, Roy W.; Nelson, Drew; Doty, Laura W. (Technical Monitor)

1998-01-01

Welding residual stresses in thin plate A516-70 steel and 2219-T87 aluminum butt weldments were measured by the strain-gage hole drilling and X-ray diffraction methods. The residual stress data were used to construct 3D strain fields which were modeled as thermally induced strains. These 3D strain fields were then analyzed with the WARP31) FEM fracture analysis code in order to predict their effect on fatigue and on fracture. For analyses of fatigue crack advance and subsequent verification testing, fatigue crack growth increments were simulated by successive saw-cuts and incremental loading to generate, as a function of crack length, effects on crack growth of the interaction between residual stresses and load induced stresses. The specimen experimental response was characterized and compared to the WARM linear elastic and elastic-plastic fracture mechanics analysis predictions. To perform the fracture analysis, the plate material's crack tearing resistance was determined by tests of thin plate M(T) specimens. Fracture analyses of these specimen were performed using WARP31D to determine the critical Crack Tip Opening Angle [CTOA] of each material. These critical CTOA values were used to predict crack tearing and fracture in the weldments. To verify the fracture predictions, weldment M(T) specimen were tested in monotonic loading to fracture while characterizing the fracture process.

Corrosion Mitigation Strategies - an Introduction

DTIC Science & Technology

2009-02-05

formed • Stress corrosion cracking Leaders in Corrosion Control Technology • Overpressure • Pressure of a gas over a liquid- solubility of gases in...Power surges • Crack protective films, fretting, fatique Design – Chemistry • Used to eliminate candidate materials • pH acidic (H+) basic (OH...Technology • Laboratory tests • Published data Mechanical Properties • Strength • Ductility • Environmental cracking Methods of Corrosion Control–Materials

Elasticity-Driven Backflow of Fluid-Driven Cracks

NASA Astrophysics Data System (ADS)

Lai, Ching-Yao; Dressaire, Emilie; Ramon, Guy; Huppert, Herbert; Stone, Howard A.

2016-11-01

Fluid-driven cracks are generated by the injection of pressurized fluid into an elastic medium. Once the injection pressure is released, the crack closes up due to elasticity and the fluid in the crack drains out of the crack through an outlet, which we refer to as backflow. We experimentally study the effects of crack size, elasticity of the matrix, and fluid viscosity on the backflow dynamics. During backflow, the volume of liquid remaining in the crack as a function of time exhibits a transition from a fast decay at early times to a power law behavior at late times. Our results at late times can be explained by scaling arguments balancing elastic and viscous stresses in the crack. This work may relate to the environmental issue of flowback in hydraulic fracturing. This work is supported by National Science Foundation via Grant CBET-1509347 and partially supported by Andlinger Center for Energy and the Environment at Princeton University.

Causes of Early-Age Thermal Cracking of Concrete Foundation Slabs and their Reinforcement to Control the Cracking

NASA Astrophysics Data System (ADS)

Bilčík, Juraj; Sonnenschein, Róbert; Gažovičová, Natália

2017-09-01

This paper focuses on the causes and consequences of early-age cracking of mass concrete foundation slabs due to restrained volume changes. Considering the importance of water leaking through cracks in terms of the serviceability, durability and environmental impact of watertight concrete structures, emphasis is placed on the effect of temperature loads on foundation slabs. Foundation slabs are usually restrained to some degree externally or internally. To evaluate the effect of external restraints on foundation slabs, friction and interaction models are introduced. The reinforcement of concrete cannot prevent the initiation of cracking, but when cracking has occurred, it may act to reduce the spacing and width of cracks. According to EN 1992-1-1, results of calculating crack widths with local variations included in National Annexes (NAs) vary considerably. A comparison of the required reinforcement areas according to different NAs is presented.

X-ray scattering to probe cracks in rubbers

NASA Astrophysics Data System (ADS)

Creton, Costantino; Demassieux, Quentin; Berghezan, Daniel

Natural rubber is a well-known very tough elastomer and its toughness is generally attributed to its aptitude to crystallize under strain. Yet the mechanism linking the extent of strain induced crystallization to the toughness gamma is still unclear. We mapped by scanning microbeam X-ray diffraction (20 microns resolution), the strain induced crystallization near the crack tip of highly crosslinked and carbon-black filled natural rubbers. Experiments were carried out on static cracks loaded at different values of energy release rates G and for different filler and crosslinker concentrations. We specifically investigated the effect of the crosslinking density, the effect of thermal (oxygen-free) aging and the effect of temperature (between 23 and 100 °C). Several novel findings are reported : a significant amount of crystallization was still present at the crack tip at 100°C, thermal aging (in the absence of oxygen) greatly reduces the amount of crystallization at the crack tip without much effect on the room temperature resistance to fatigue crack propagation of the material, and an increase in crosslinking density reduces the extent of crystallinity at the crack tip for the same applied G. We acknowledge the financial support of Michelin.

Environmental-Induced Damage in Tin (Sn) and Aluminum (Al) Alloys

NASA Astrophysics Data System (ADS)

Vallabhaneni, Venkata Sathya Sai Renuka

Sn and Al alloys are widely used in various industries. Environmental-induced damage resulting in whiskering in Sn and corrosion in Al account for numerous failures globally every year. Therefore, for designing materials that can better withstand these failures, a comprehensive study on the characterization of the damage is necessary. This research implements advanced characterization techniques to study the above-mentioned environmental-induced damage in Sn and Al alloys. Tin based films are known to be susceptible to whisker growth resulting in numerous failures. While the mechanisms and factors affecting whisker growth have been studied extensively, not much has been reported on the mechanical properties of tin whiskers themselves. This study focuses on the tensile behavior of tin whiskers. Tensile tests of whiskers were conducted in situ a dual beam focused ion beam (FIB) with a scanning electron microscope (SEM) using a micro electro-mechanical system (MEMS) tensile testing stage. The deformation mechanisms of whiskers were analyzed using transmission electron microscopy (TEM). Due to the heterogenous nature of the microstructure of Al 7075, it is susceptible to corrosion forming corrosion products and pits. These can be sites for cracks nucleation and propagation resulting in stress corrosion cracking (SCC). Therefore, complete understanding of the corrosion damaged region and its effect on the strength of the alloy is necessary. Several studies have been performed to visualize pits and understand their effect on the mechanical performance of Al alloys using two-dimensional (2D) approaches which are often inadequate. To get a thorough understanding of the pits, it is necessary for three-dimensional (3D) studies. In this study, Al 7075 alloys were corroded in 3.5 wt.% NaCl solution and X-ray tomography was used to obtain the 3D microstructure of pits enabling the quantification of their dimensions accurately. Furthermore, microstructure and mechanical property correlations helped in a better understanding of the effect of corrosion. Apart from the pits, a surface corrosion layer also forms on Al. A subsurface damage layer has also been identified that forms due to the aggressive nature of NaCl. Energy dispersive X-ray spectroscopy (EDX) and nanoindentation helped in identifying this region and understanding the variation in properties.

Fracture behavior of human molars.

PubMed

Keown, Amanda J; Lee, James J-W; Bush, Mark B

2012-12-01

Despite the durability of human teeth, which are able to withstand repeated loading while maintaining form and function, they are still susceptible to fracture. We focus here on longitudinal fracture in molar teeth-channel-like cracks that run along the enamel sidewall of the tooth between the gum line (cemento-enamel junction-CEJ) and the occlusal surface. Such fractures can often be painful and necessitate costly restorative work. The following study describes fracture experiments made on molar teeth of humans in which the molars are placed under axial compressive load using a hard indenting plate in order to induce longitudinal cracks in the enamel. Observed damage modes include fractures originating in the occlusal region ('radial-median cracks') and fractures emanating from the margin of the enamel in the region of the CEJ ('margin cracks'), as well as 'spalling' of enamel (the linking of longitudinal cracks). The loading conditions that govern fracture behavior in enamel are reported and observations made of the evolution of fracture as the load is increased. Relatively low loads were required to induce observable crack initiation-approximately 100 N for radial-median cracks and 200 N for margin cracks-both of which are less than the reported maximum biting force on a single molar tooth of several hundred Newtons. Unstable crack growth was observed to take place soon after and occurred at loads lower than those calculated by the current fracture models. Multiple cracks were observed on a single cusp, their interactions influencing crack growth behavior. The majority of the teeth tested in this study were noted to exhibit margin cracks prior to compression testing, which were apparently formed during the functional lifetime of the tooth. Such teeth were still able to withstand additional loading prior to catastrophic fracture, highlighting the remarkable damage containment capabilities of the natural tooth structure.

Detection and Analysis of Enamel Cracks by Quantitative Light-induced Fluorescence Technology.

PubMed

Jun, Mi-Kyoung; Ku, Hye-Min; Kim, Euiseong; Kim, Hee-Eun; Kwon, Ho-Keun; Kim, Baek-Il

2016-03-01

The ability to accurately detect tooth cracks and quantify their depth would allow the prediction of crack progression and treatment success. The aim of this in vitro study was to determine the capabilities of quantitative light-induced fluorescence (QLF) technology in the detection of enamel cracks. Ninety-six extracted human teeth were selected for examining naturally existing or suspected cracked teeth surfaces using a photocuring unit. QLF performed with a digital camera (QLF-D) images were used to assess the ability to detect enamel cracks based on the maximum fluorescence loss value (ΔFmax, %), which was then analyzed using the QLF-D software. A histologic evaluation was then performed in which the samples were sectioned and observed with the aid of a polarized light microscope. The relationship between ΔFmax and the histology findings was assessed based on the Spearman rank correlation. The sensitivity and specificity were calculated to evaluate the validity of using QLF-D to analyze enamel inner-half cracks and cracks extending to the dentin-enamel junction. There was a strong correlation between the results of histologic evaluations of enamel cracks and the ΔFmax value, with a correlation coefficient of 0.84. The diagnostic accuracy of QLF-D had a sensitivity of 0.87 and a specificity of 0.98 for enamel inner-half cracks and a sensitivity of 0.90 and a specificity of 1.0 for cracks extending to the dentin-enamel junction. These results indicate that QLF technology would be a useful clinical tool for diagnosing enamel cracks, especially given that this is a nondestructive method. Copyright © 2016 American Association of Endodontists. Published by Elsevier Inc. All rights reserved.

Edge-Cracking Behavior of CoCrFeMnNi High-Entropy Alloy During Hot Rolling

NASA Astrophysics Data System (ADS)

Won, Jong Woo; Kang, Minju; Kwon, Heoun-Jun; Lim, Ka Ram; Seo, Seong Moon; Na, Young Sang

2018-05-01

This work investigated edge-cracking behavior of equiatomic CoCrFeMnNi high-entropy alloy during hot rolling at rolling temperatures 500 ≤ T R ≤ 1000 °C. Edge cracks did not form in the material rolled at 500 °C, but widened and deepened into the inside of plate as T R increased from 500 °C. Edge cracks were most severe in the material rolled at 1000 °C. Mn-Cr-O type non-metallic inclusion and oxidation were identified as major factors that caused edge cracking. The inclusions near edge region acted as preferential sites for crack formation. Connection between inclusion cracks and surface cracks induced edge cracking. Rolling at T R ≥ 600 °C generated distinct inclusion cracks whereas they were not serious at T R = 500 °C, so noticeable edge cracks formed at T R ≥ 600 °C. At T R = 1000 °C, significant oxidation occurred at the crack surface. This accelerated edge crack penetration by embrittling the crack tip, so severe edge cracking occurred at T R = 1000 °C.

Environmentally assisted cracking in light water reactors : semiannual report, July 2000 - December 2000.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chopra, O. K.; Chung, H. M.; Gruber, E. E.

This report summarizes work performed by Argonne National Laboratory on fatigue and environmentally assisted cracking (EAC) in light water reactors (LWRs) from July 2000 to December 2000. Topics that have been investigated include (a) environmental effects on fatigue S-N behavior of primary pressure boundary materials, (b) irradiation-assisted stress corrosion cracking (IASCC) of austenitic stainless steels (SSs), and (c) EAC of Alloys 600 and 690. The fatigue strain-vs.-life data are summarized for the effects of various material, loading, and environmental parameters on the fatigue lives of carbon and low-alloy steels and austenitic SSs. Effects of the reactor coolant environment on themore » mechanism of fatigue crack initiation are discussed. Two methods for incorporating the effects of LWR coolant environments into the ASME Code fatigue evaluations are presented. Slow-strain-rate tensile tests and posttest fractographic analyses were conducted on several model SS alloys irradiated to {approx}0.9 x 10{sup 21} n {center_dot} cm{sup -2} (E > 1 MeV) in He at 289 C in the Halden reactor. The results were used to determine the influence of alloying and impurity elements on the susceptibility of these steels to IASCC. A fracture toughness J-R curve test was conducted on a commercial heat of Type 304 SS that was irradiated to {approx}2.0 x 10{sup 21} n {center_dot} cm{sup -2} in the Halden reactor. The results were compared with the data obtained earlier on steels irradiated to 0.3 and 0.9 x 10{sup 21} n {center_dot} cm{sup -2} (E > 1 MeV) (0.45 and 1.35 dpa). Neutron irradiation at 288 C was found to decrease the fracture toughness of austenitic SSs. Tests were conducted on compact-tension specimens of Alloy 600 under cyclic loading to evaluate the enhancement of crack growth rates in LWR environments. Then, the existing fatigue crack growth data on Alloys 600 and 690 were analyzed to establish the effects of temperature, load ratio, frequency, and stress intensity range on crack growth rates in air.« less

Helium-induced weld cracking in irradiated 304 stainless steel

DOE Office of Scientific and Technical Information (OSTI.GOV)

Birchenall, A.K.

1989-01-01

This report consists of slide notes for presentation to The Metallurgical Society of the American Institute of Mining, Metallurgical and Petroleum Engineers (AIME). The meeting in question will be held October 3, 1989 in Indianapolis. This presentation will be the second of three consecutive talks contributed by SRL personnel dealing with helium-induced weld cracking.

Concrete Cracking Prediction Including the Filling Proportion of Strand Corrosion Products.

PubMed

Wang, Lei; Dai, Lizhao; Zhang, Xuhui; Zhang, Jianren

2016-12-23

The filling of strand corrosion products during concrete crack propagation is investigated experimentally in the present paper. The effects of stirrups on the filling of corrosion products and concrete cracking are clarified. A prediction model of crack width is developed incorporating the filling proportion of corrosion products and the twisting shape of the strand. Experimental data on cracking angle, crack width, and corrosion loss obtained from accelerated corrosion tests of concrete beams are presented. The proposed model is verified by experimental data. Results show that the filling extent of corrosion products varies with crack propagation. The rust filling extent increases with the propagating crack until a critical width. Beyond the critical width, the rust-filling extent remains stable. Using stirrups can decrease the critical crack width. Stirrups can restrict crack propagation and reduce the rust filling. The tangent of the cracking angle increases with increasing corrosion loss. The prediction of corrosion-induced crack is sensitive to the rust-filling extent.

Concrete Cracking Prediction Including the Filling Proportion of Strand Corrosion Products

PubMed Central

Wang, Lei; Dai, Lizhao; Zhang, Xuhui; Zhang, Jianren

2016-01-01

The filling of strand corrosion products during concrete crack propagation is investigated experimentally in the present paper. The effects of stirrups on the filling of corrosion products and concrete cracking are clarified. A prediction model of crack width is developed incorporating the filling proportion of corrosion products and the twisting shape of the strand. Experimental data on cracking angle, crack width, and corrosion loss obtained from accelerated corrosion tests of concrete beams are presented. The proposed model is verified by experimental data. Results show that the filling extent of corrosion products varies with crack propagation. The rust filling extent increases with the propagating crack until a critical width. Beyond the critical width, the rust-filling extent remains stable. Using stirrups can decrease the critical crack width. Stirrups can restrict crack propagation and reduce the rust filling. The tangent of the cracking angle increases with increasing corrosion loss. The prediction of corrosion-induced crack is sensitive to the rust-filling extent. PMID:28772367

Effects of Different R ratios on Fatigue Crack Growth in Laser Peened Friction Stir Welds

NASA Technical Reports Server (NTRS)

Hatamleh, Omar; Hackel, Lloyd; Forth, Scott

2007-01-01

The influence of laser peening on the fatigue crack growth behavior of friction stir welded (FSW) Aluminum Alloy (AA) 7075-T7351 sheets was investigated. The surface modification resulting from the peening process on the fatigue crack growth of FSW was assessed for two different R ratios. The investigation indicated a significant decrease in fatigue crack growth rates resulting from using laser shock peening compared with unpeened, welded and unwelded specimens. The slower fatigue crack growth rate was attributed to the compressive residual stresses induced by the peening.

Computational Modeling of Micro-Crack Induced Attenuation in CFRP Composites

NASA Technical Reports Server (NTRS)

Roberts, R. A.; Leckey, C. A. C.

2012-01-01

A computational study is performed to determine the contribution to ultrasound attenuation in carbon fiber reinforced polymer composite laminates of linear elastic scattering by matrix micro-cracking. Multiple scattering approximations are benchmarked against exact computational approaches. Results support linear scattering as the source of observed increased attenuation in the presence of micro-cracking.

Replacement of corrosion protection chromate primers and paints used in cryogenic applications on the Space Shuttle with wire arc sprayed aluminum coatings

NASA Technical Reports Server (NTRS)

Daniel, R. L.; Sanders, H. L.; Zimmerman, F. R.

1995-01-01

With the advent of new environmental laws restricting volatile organic compounds and hexavalent chrome emissions, 'environmentally safe' thermal spray coatings are being developed to replace the traditional corrosion protection chromate primers. A wire arc sprayed aluminum coating is being developed for corrosion protection of low pressure liquid hydrogen carrying ducts on the Space Shuttle Main Engine. Currently, this hardware utilizes a chromate primer to provide protection against corrosion pitting and stress corrosion cracking induced by the cryogenic operating environment. The wire are sprayed aluminum coating has been found to have good potential to provide corrosion protection for flight hardware in cryogenic applications. The coating development, adhesion test, corrosion test and cryogenic flexibility test results will be presented.

Hydrogen assisted cracking and CO2 corrosion behaviors of low-alloy steel with high strength used for armor layer of flexible pipe

NASA Astrophysics Data System (ADS)

Liu, Zhenguang; Gao, Xiuhua; Du, Linxiu; Li, Jianping; Zhou, Xiaowei; Wang, Xiaonan; Wang, Yuxin; Liu, Chuan; Xu, Guoxiang; Misra, R. D. K.

2018-05-01

In this study, hydrogen induced cracking (HIC), sulfide stress corrosion cracking (SSCC) and hydrogen embrittlement (HE) were carried out to study hydrogen assisted cracking behavior (HIC, SSCC and HE) of high strength pipeline steel used for armor layer of flexible pipe in ocean. The CO2 corrosion behavior of designed steel with high strength was studied by using immersion experiment. The experimental results demonstrate that the corrosion resistance of designed steel with tempered martensite to HIC, SSCC and HE is excellent according to specific standards, which contributes to the low concentration of dislocation and vacancies previously formed in cold rolling process. The corrosion mechanism of hydrogen induced cracking of designed steel, which involves in producing process, microstructure and cracking behavior, is proposed. The designed steel with tempered martensite shows excellent corrosion resistance to CO2 corrosion. Cr-rich compound was first formed on the coupon surface exposed to CO2-saturated brine condition and chlorine, one of the corrosion ions in solution, was rich in the inner layer of corrosion products.

Effect of pre-strain on creep of three AISI 316 austenitic stainless steels in relation to reheat cracking of weld-affected zones

NASA Astrophysics Data System (ADS)

Auzoux, Q.; Allais, L.; Caës, C.; Monnet, I.; Gourgues, A. F.; Pineau, A.

2010-05-01

Microstructural modifications induced by welding of 316 stainless steels and their effect on creep properties and relaxation crack propagation were examined. Cumulative strain due to multi-pass welding hardens the materials by increasing the dislocation density. Creep tests were conducted on three plates from different grades of 316 steel at 600 °C, with various carbon and nitrogen contents. These plates were tested both in the annealed condition and after warm rolling, which introduced pre-strain. It was found that the creep strain rate and ductility after warm rolling was reduced compared with the annealed condition. Moreover, all steels exhibited intergranular crack propagation during relaxation tests on Compact Tension specimens in the pre-strained state, but not in the annealed state. These results confirmed that the reheat cracking risk increases with both residual stress triaxiality and pre-strain. On the contrary, high solute content and strain-induced carbide precipitation, which are thought to increase reheat cracking risk of stabilised austenitic stainless steels did not appear as key parameters in reheat cracking of 316 stainless steels.

Transply crack density detection by acousto-ultrasonics

NASA Technical Reports Server (NTRS)

Hemann, John H.; Bowles, Kenneth J.; Kautz, Harold; Cavano, Paul

1987-01-01

The acousto-ultrasonic method was applied to a PMR-15 8-harness, satin Celion 3000 fabric composite to determine the extent of transply cracking. A six-ply 0/90 laminate was also subjected to mechanical loading, which induced transply cracking. The stress wave factor (SWF) is defined as the energy contained in the received signal from a 2.25-MHz center frequency transducer. The correlation of the SWF with transply crack density is shown.

Cracking the Crack Dance: A Case Report on Cocaine-induced Choreoathetosis.

PubMed

Narula, Naureen; Siddiqui, Faraz; Katyal, Nakul; Krishnan, Nithya; Chalhoub, Michel

2017-12-22

Movement disorders represent one of the less common presentations of cocaine toxicity observed in clinical practice. Given the magnitude of crack cocaine use, it is vital to understand the underlying pathogenesis. We present a case of a patient who clinically exhibited cocaine-induced choreoathetosis. The diagnosis was confirmed after ruling out all other organic causes of de novo choreoathetoid movement. This case highlights the association of cocaine with choreoathetoid movements. We propose a preliminary understanding of the underlying pathogenesis, which may help intensivists better recognize this uncommon phenomenon.

Multiple Fatigue Failure Behaviors and Long-Life Prediction Approach of Carburized Cr-Ni Steel with Variable Stress Ratio

PubMed Central

Deng, Hailong; Li, Wei; Zhao, Hongqiao; Sakai, Tatsuo

2017-01-01

Axial loading tests with stress ratios R of −1, 0 and 0.3 were performed to examine the fatigue failure behavior of a carburized Cr-Ni steel in the long-life regime from 104 to 108 cycles. Results show that this steel represents continuously descending S-N characteristics with interior inclusion-induced failure under R = −1, whereas it shows duplex S-N characteristics with surface defect-induced failure and interior inclusion-induced failure under R = 0 and 0.3. The increasing tension eliminates the effect of compressive residual stress and promotes crack initiation from the surface or interior defects in the carburized layer. The FGA (fine granular area) formation greatly depends on the number of loading cycles, but can be inhibited by decreasing the compressive stress. Based on the evaluation of the stress intensity factor at the crack tip, the surface and interior failures in the short life regime can be characterized by the crack growth process, while the interior failure with the FGA in the long life regime can be characterized by the crack initiation process. In view of the good agreement between predicted and experimental results, the proposed approach can be well utilized to predict fatigue lives associated with interior inclusion-FGA-fisheye induced failure, interior inclusion-fisheye induced failure, and surface defect induced failure. PMID:28906454

Mechanical weathering and rock erosion by climate-dependent subcritical cracking

NASA Astrophysics Data System (ADS)

Eppes, Martha-Cary; Keanini, Russell

2017-06-01

This work constructs a fracture mechanics framework for conceptualizing mechanical rock breakdown and consequent regolith production and erosion on the surface of Earth and other terrestrial bodies. Here our analysis of fracture mechanics literature explicitly establishes for the first time that all mechanical weathering in most rock types likely progresses by climate-dependent subcritical cracking under virtually all Earth surface and near-surface environmental conditions. We substantiate and quantify this finding through development of physically based subcritical cracking and rock erosion models founded in well-vetted fracture mechanics and mechanical weathering, theory, and observation. The models show that subcritical cracking can culminate in significant rock fracture and erosion under commonly experienced environmental stress magnitudes that are significantly lower than rock critical strength. Our calculations also indicate that climate strongly influences subcritical cracking—and thus rock weathering rates—irrespective of the source of the stress (e.g., freezing, thermal cycling, and unloading). The climate dependence of subcritical cracking rates is due to the chemophysical processes acting to break bonds at crack tips experiencing these low stresses. We find that for any stress or combination of stresses lower than a rock's critical strength, linear increases in humidity lead to exponential acceleration of subcritical cracking and associated rock erosion. Our modeling also shows that these rates are sensitive to numerous other environment, rock, and mineral properties that are currently not well characterized. We propose that confining pressure from overlying soil or rock may serve to suppress subcritical cracking in near-surface environments. These results are applicable to all weathering processes.

Environmental fatigue of an Al-Li-Cu alloy. Part 2: Microscopic hydrogen cracking processes

NASA Technical Reports Server (NTRS)

Piascik, Robert S.; Gangloff, Richard P.

1992-01-01

Based on a fractographic analysis of fatigue crack propagation (FCP) in Al-Li-Cu alloy 2090 stressed in a variety of inert and embrittling environments, microscopic crack paths are identified and correlated with intrinsic da/dN-delta K kinetics. FCP rates in 2090 are accelerated by hydrogen producing environments (pure water vapor, moist air, and aqueous NaCl), as defined in Part 1. For these cases, subgrain boundary fatigue cracking (SGC) dominates for delta K values where the crack tip process zone, a significant fraction of the cyclic plastic zone, is sufficiently large to envelop 5 micron subgrains in the unrecrystallized microstructure. SGC may be due to strong hydrogen trapping at T1 precipitates concentrated at sub-boundaries. At low delta K, the plastic zone diameter is smaller than the subgrain size and FCP progresses along (100) planes due to either local lattice decohesion or aluminum-lithium hydride cracking. For inert environments (vacuum, helium, and oxygen), or at high delta K where the hydrogen effect on da/dN is small, FCP is along (111) slip planes; this mode does not transition with increasing delta K and plastic zone size. The SGC and (100) crystallographic cracking modes, and the governing influence of the crack tip process zone volume (delta K), support hydrogen embrittlement rather than a surface film rupture and anodic dissolution mechanism for environmental FCP. Multi-sloped log da/dN-log delta K behavior is produced by changes in process zone hydrogen-microstructure interactions, and not by purely micromechanical-microstructure interactions, in contradiction to microstructural distance-based fatigue models.

The creep and intergranular cracking behavior of Ni-Cr-Fe-C alloys in 360{degree}C water

DOE Office of Scientific and Technical Information (OSTI.GOV)

Angeliu, T.M.; Paraventi, D.J.; Was, G.S.

1995-09-01

Mechanical testing of controlled-purity Ni-xCr-9Fe-yC alloys at 360 C revealed an environmental enhancement in IG cracking and time-dependent deformation in high purity and primary water over that exhibited in argon. Dimples on the IG facets indicate a creep void nucleation and growth failure mode. IG cracking was primarily located at the interior of the specimen and not necessarily linked to direct contact with the environment. Controlled potential CERT experiments showed increases in IG cracking as the applied potential decreased, suggesting that hydrogen is detrimental to the mechanical properties. It is proposed that the environment, through the presence of hydrogen, enhancesmore » IG cracking by enhancing the matrix dislocation mobility. This is based on observations that dislocation-controlled creep controls the IG cracking of controlled-purity Ni-xCr-9Fe-yC in argon at 360 C and grain boundary cavitation and sliding results that show the environmental enhancement of the creep rate is primarily due to an increase in matrix plastic deformation. However, controlled potential CLT experiments did not exhibit a change in the creep rate as the applied potential decreased. While this does not clearly support hydrogen assisted creep, the material may already be saturated with hydrogen at these applied potentials and thus no effect was realized. Chromium and carbon decrease the IG cracking in high purity and primary water by increasing the creep resistance. The surface film does not play a significant role in the creep or IG cracking behavior under the conditions investigated.« less

Creep and intergranular cracking behavior of nickel-chromium-iron-carbon alloys in 360 C water

DOE Office of Scientific and Technical Information (OSTI.GOV)

Angeliu, T.M.; Paraventi, D.J.; Was, G.S.

1995-11-01

Mechanical testing of controlled-purity Ni-x% Cr-9% Fe-y% C alloys at 360 C revealed an environmental enhancement in intergranular (IG) cracking and time-dependent deformation in high-purity (HP) and primary water (PW) over that exhibited in argon. Dimples on the IG facets indicated a creep void nucleation and growth failure mode. IG cracking was located primarily in the interior of the specimen and was not necessarily linked to the environment. Controlled-potential constant extension rate tensile (CERT) experiments showed increases in IG cracking as the applied potential decreased, suggesting that hydrogen was detrimental to the mechanical properties. It was proposed that the environment,more » through the presence of hydrogen, enhanced IG cracking by enhancing the matrix dislocation mobility. This conclusion was based on observations that dislocation creep controlled IG cracking of controlled-purity Ni-x% Cr-9% Fe-y% C in argon at 360 C. Grain-boundary cavitation (GBC) and sliding (GBS) results showed environmental enhancement of the creep rate primarily resulted from an increase in matrix plastic deformation. However, controlled-potential constant load tensile (CLT) experiments did not indicate a change in the creep rate as the applied potential decreased. While this result did not support hydrogen-assisted creep, the material already may have been saturated with hydrogen at these applied potentials, and thus, no effect was realized. Chromium and carbon decreased IG cracking in HP and PW by increasing the creep resistance. The surface film did not play a significant role in the creep or IG cracking behavior under the conditions investigated.« less

Monitoring the corrosion process of reinforced concrete using BOTDA and FBG sensors.

PubMed

Mao, Jianghong; Chen, Jiayun; Cui, Lei; Jin, Weiliang; Xu, Chen; He, Yong

2015-04-15

Expansion and cracking induced by the corrosion of reinforcement concrete is the major factor in the failure of concrete durability. Therefore, monitoring of concrete cracking is critical for evaluating the safety of concrete structures. In this paper, we introduce a novel monitoring method combining Brillouin optical time domain analysis (BOTDA) and fiber Bragg grating (FBG), based on mechanical principles of concrete expansion cracking. BOTDA monitors concrete expansion and crack width, while FBG identifies the time and position of cracking. A water-pressure loading simulation test was carried out to determine the relationship between fiber strain, concrete expansion and crack width. An electrical accelerated corrosion test was also conducted to evaluate the ability of this novel sensor to monitor concrete cracking under practical conditions.

Impact Cratering Physics al Large Planetary Scales

NASA Astrophysics Data System (ADS)

Ahrens, Thomas J.

2007-06-01

Present understanding of the physics controlling formation of ˜10^3 km diameter, multi-ringed impact structures on planets were derived from the ideas of Scripps oceanographer, W. Van Dorn, University of London's, W, Murray, and, Caltech's, D. O'Keefe who modeled the vertical oscillations (gravity and elasticity restoring forces) of shock-induced melt and damaged rock within the transient crater immediately after the downward propagating hemispheric shock has processed rock (both lining, and substantially below, the transient cavity crater). The resulting very large surface wave displacements produce the characteristic concentric, multi-ringed basins, as stored energy is radiated away and also dissipated upon inducing further cracking. Initial calculational description, of the above oscillation scenario, has focused upon on properly predicting the resulting density of cracks, and, their orientations. A new numerical version of the Ashby--Sammis crack damage model is coupled to an existing shock hydrodynamics code to predict impact induced damage distributions in a series of 15--70 cm rock targets from high speed impact experiments for a range of impactor type and velocity. These are compared to results of crack damage distributions induced in crustal rocks with small arms impactors and mapped ultrasonically in recent Caltech experiments (Ai and Ahrens, 2006).

Design and performance of crack-free environmentally friendly concrete "crack-free eco-crete".

DOT National Transportation Integrated Search

2014-08-01

High-performance concrete (HPC) is characterized by high content of cement and supplementary cementitious materials (SCMs). : Using high binder content, low water-to-cementitious material ratio (w/cm), and various chemical admixtures in the HPC can r...

Embedded Shape Memory Alloy Particles for the Self-Sensing of Fatigue Crack Growth in an Aluminum Alloy

NASA Astrophysics Data System (ADS)

Leser, William Paul

Future aerospace vehicles will be built using novel materials for mission conditions that are difficult to replicate in a laboratory. Structural health monitoring and condition-based maintenance will be critical to ensure the reliability of such vehicles. A multi-functional aluminum alloy containing embedded shape memory alloy (SMA) particles to detect fatigue crack growth is proposed. The regions of intensified strain near the tip of a growing fatigue crack cause the SMA particles to undergo a solid-to-solid phase transformation from austenite to martensite, releasing a detectable and identifiable acoustic emission (AE) signal that can be used to locate the crack in the affected component. This study investigates the AE response of two SMA systems, Ni-Ti, and Co-Ni-Al. Tensile (Ni-Ti) and compressive (Co-Ni-Al) tests were conducted to study the strain-induced transformation response in both of the alloy systems. It was found that the critical stress for transformation in both SMA systems was easily identified by a burst of AE activity during both transformation and reverse transformation. AE signals from these experiments were collected for use as training data for a Bayesian classifier to be used to identify transformation signals in a Al7050 matrix with embedded SMA particles. The Al/SMA composite was made by vacuum hot pressing SMA powder between aluminum plates. The effect of hot pressing temperature and subsequent heat treatments (solutionizing and peak aging) on the SMA particles was studied. It was found that, at the temperatures required, Co-Ni-Al developed a second phase that restricted the transformation from austenite to martensite, thus rendering it ineffective as a candidate for the embedded particles. Conversely, Ni-Ti did survive the embedding process and it was found that the solutionizing heat treatment applied after hot pressing was the main driver in determining the final transformation temperatures for the Ni-Ti particles. The effect of hot pressing on the transformation temperatures was negated upon solutionizing and peak aging occurred at a sufficiently low temperature to as not affect the properties of the Ni-Ti. Strain-induced transformation was confirmed in the Ni-Ti particles by digital image correlation (DIC) using an environmental scanning electron microscope (ESEM). Specimens were fatigue pre-cracked until a crack was produced and observed to be approaching a particle that could be monitored on the surface, at which point it was put into the ESEM for DIC under tensile loading. Acoustic emission activity was observed during this experiment. In order to distinguish AE signals arising due to phase transformation in the particles from those due to crack extension in the matrix, a Bayesian classifier was constructed based on frequency parameters calculated using the Hilbert-Huang transform (HHT). Using this classifier, AE signals consistent with those arising from phase transformation in bulk Ni-Ti were identified during phase transformation in the particles as observed with DIC. In addition to tensile crack growth in the ESEM, a fatigue crack was grown through a specimen with particles interspersed along the specimen center line. Several low amplitude AE events were observed as the crack grew through the aluminum. As the fatigue crack passed through the line of particles AE events increased dramatically in rate of occurance and amplitude. Amplitudes were 6-10 times higher as the crack passed near the particles. These AE events were also shown to be consistent with Ni-Ti phase transformation. A successful proof-of-concept was demonstrated for an aluminum alloy with embedded particles that emit an identifiable and repeatable AE signal in the presence of a fatigue crack, allowing for quick diagnosis of fatigue crack damage in this material.

Fracture Behavior of Ceramics Under Displacement Controlled Loading

NASA Technical Reports Server (NTRS)

Calomino, Anthony; Brewer, David; Ghosn, Louis

1994-01-01

A Mode I fracture specimen and loading method has been developed which permits the observation of stable crack extension in monolithic and in situ toughened ceramics. The developed technique was used to conduct room temperature tests on commercial grade alumina (Coors' AD-995) and silicon nitride (Norton NC-132). The results of these tests are reported. Crack growth for the alumina remained subcritical throughout testing revealing possible effects of environmental stress corrosion. The crack growth resistance curve for the alumina is presented. The silicon nitride tests displayed a series of stable (slow) crack growth segments interrupted by dynamic (rapid) crack extension. Crack initiation and arrest stress intensity factors, K(sub Ic) and K(sub Ia), for silicon nitride are reported. The evolution of the specimen design through testing is briefly discussed.

Fractography and mechanisms of environmentally enhanced fatigue crack propagation of a reactor pressure vessel steel

DOE Office of Scientific and Technical Information (OSTI.GOV)

Torronen, K.; Kemppainen, M.

1981-10-01

This paper describes the findings and interpretations of the fractographic survey conducted for the International Cyclic Crack Growth Rate (ICCGR) cooperative group round-robin specimens. Specimens of A533B pressure vessel steel were tested at several laboratories in the United States and elsewhere with the same nominal test parameters. A rather wide scatter of the results was found. A fractographic and metallographic survey was carried out in order to clarify the scatter and to evaluate the micromechanism of the crack growth. The fractographic findings are reported in detail and correlated to the crack growth behavior. A hydrogen-assisted crack propagation mechanism based onmore » the fractography is proposed and applied to the observed crack growth behavior.« less

Tufted capuchin monkeys (Sapajus sp) learning how to crack nuts: does variability decline throughout development?

PubMed

Resende, Briseida Dogo; Nagy-Reis, Mariana Baldy; Lacerda, Fernanda Neves; Pagnotta, Murillo; Savalli, Carine

2014-11-01

We investigated the process of nut-cracking acquisition in a semi-free population of tufted capuchin monkeys (Sapajus sp) in São Paulo, Brazil. We analyzed the cracking episodes from monkeys of different ages and found that variability of actions related to cracking declined. Inept movements were more frequent in juveniles, which also showed an improvement on efficient striking. The most effective behavioral sequence for cracking was more frequently used by the most experienced monkeys, which also used non-optimal sequences. Variability in behavior sequences and actions may allow adaptive changes to behavior under changing environmental conditions. Copyright © 2014 Elsevier B.V. All rights reserved.

Near-IR imaging of cracks in teeth

NASA Astrophysics Data System (ADS)

Fried, William A.; Simon, Jacob C.; Lucas, Seth; Chan, Kenneth H.; Darling, Cynthia L.; Staninec, Michal; Fried, Daniel

2014-02-01

Dental enamel is highly transparent at near-IR wavelengths and several studies have shown that these wavelengths are well suited for optical transillumination for the detection and imaging of tooth decay. We hypothesize that these wavelengths are also well suited for imaging cracks in teeth. Extracted teeth with suspected cracks were imaged at several wavelengths in the near-IR from 1300-1700-nm. Extracted teeth were also examined with optical coherence tomography to confirm the existence of suspected cracks. Several teeth of volunteers were also imaged in vivo at 1300-nm to demonstrate clinical potential. In addition we induced cracks in teeth using a carbon dioxide laser and imaged crack formation and propagation in real time using near-IR transillumination. Cracks were clearly visible using near-IR imaging at 1300-nm in both in vitro and in vivo images. Cracks and fractures also interfered with light propagation in the tooth aiding in crack identification and assessment of depth and severity.

Fatigue life and crack growth prediction methodology

NASA Technical Reports Server (NTRS)

Newman, J. C., Jr.; Phillips, E. P.; Everett, R. A., Jr.

1993-01-01

The capabilities of a plasticity-induced crack-closure model and life-prediction code to predict fatigue crack growth and fatigue lives of metallic materials are reviewed. Crack-tip constraint factors, to account for three-dimensional effects, were selected to correlate large-crack growth rate data as a function of the effective-stress-intensity factor range (delta(K(sub eff))) under constant-amplitude loading. Some modifications to the delta(K(sub eff))-rate relations were needed in the near threshold regime to fit small-crack growth rate behavior and endurance limits. The model was then used to calculate small- and large-crack growth rates, and in some cases total fatigue lives, for several aluminum and titanium alloys under constant-amplitude, variable-amplitude, and spectrum loading. Fatigue lives were calculated using the crack growth relations and microstructural features like those that initiated cracks. Results from the tests and analyses agreed well.

Development of chloride-induced corrosion in pre-cracked RC beams under sustained loading: Effect of load-induced cracks, concrete cover, and exposure conditions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yu, Linwen; Université de Sherbrooke, Quebec; François, Raoul, E-mail: raoul.francois@insa-toulouse.fr

2015-01-15

This paper deals with corrosion initiation and propagation in pre-cracked reinforced concrete beams under sustained loading during exposure to a chloride environment. Specimen beams that were cast in 2010 were compared to specimens cast in 1984. The only differences between the two sets of beams were the casting direction in relation to tensile reinforcement and the exposure conditions in the salt-fog chamber. The cracking maps, corrosion maps, chloride profiles, and cross-sectional loss of one group of two beams cast in 2010 were studied and their calculated corrosion rates were compared to that of beams cast in 1984 in order tomore » investigate the factors influencing the natural corrosion process. Experimental results show that, after rapid initiation of corrosion at the crack tip, the corrosion process practically halted and the time elapsing before corrosion resumed depended on the exposure conditions and cover depth.« less

Guided cracking of electrodes by stretching prism-patterned membrane electrode assemblies for high-performance fuel cells.

PubMed

Ahn, Chi-Yeong; Jang, Segeun; Cho, Yong-Hun; Choi, Jiwoo; Kim, Sungjun; Kim, Sang Moon; Sung, Yung-Eun; Choi, Mansoo

2018-01-19

Guided cracks were successfully generated in an electrode using the concentrated surface stress of a prism-patterned Nafion membrane. An electrode with guided cracks was formed by stretching the catalyst-coated Nafion membrane. The morphological features of the stretched membrane electrode assembly (MEA) were investigated with respect to variation in the prism pattern dimension (prism pitches of 20 μm and 50 μm) and applied strain (S ≈ 0.5 and 1.0). The behaviour of water on the surface of the cracked electrode was examined using environmental scanning electron microscopy. Guided cracks in the electrode layer were shown to be efficient water reservoirs and liquid water passages. The MEAs with and without guided cracks were incorporated into fuel cells, and electrochemical measurements were conducted. As expected, all MEAs with guided cracks exhibited better performance than conventional MEAs, mainly because of the improved water transport.

Corrosion Product Film-Induced Stress Facilitates Stress Corrosion Cracking

PubMed Central

Wang, Wenwen; Zhang, Zhiliang; Ren, Xuechong; Guan, Yongjun; Su, Yanjing

2015-01-01

Finite element analyses were conducted to clarify the role of corrosion product films (CPFs) in stress corrosion cracking (SCC). Flat and U-shaped edge-notched specimens were investigated in terms of the CPF-induced stress in the metallic substrate and the stress in the CPF. For a U-shaped edge-notched specimen, the stress field in front of the notch tip is affected by the Young’s modulus of the CPF and the CPF thickness and notch geometry. The CPF-induced tensile stress in the metallic substrate is superimposed on the applied load to increase the crack tip strain and facilitate localized plasticity deformation. In addition, the stress in the CPF surface contributes to the rupture of the CPFs. The results provide physical insights into the role of CPFs in SCC. PMID:26066367

Corrosion Development of Carbon Steel Grids and Shear Connectors in Cracked Composite Beams Exposed to Wet–Dry Cycles in Chloride Environment

PubMed Central

Xue, Wen; Chen, Ju; Jiang, Ao-yu

2018-01-01

The corrosion development of the reinforcement and shear stud connectors in the cracked steel–concrete composite beams under the salt-fog wet–dry cycles is presented in this investigation. Seven identical composite beams with load-induced concrete cracks were exposed to an aggressive chloride environment. The reinforcement and shear connectors were retrieved after specimens underwent a specified number of wet–dry cycles to obtain the corrosion pattern and the cross-section loss at different exposure times and their evolutions. The crack map, the corrosion pattern and the cross-section loss were measured and presented. Based on the experimental results, the influence of crack characteristics, including crack widths, orientations and positions on the corrosion rate and distribution, were accessed. Moreover, the effects of the connecting weldments on the corrosion initiations and patterns were analyzed. It was shown that the corrosion rate would increase with the number of wet–dry cycles. The characteristics of load-induced cracks could have different influences on the steel grids and shear stud connectors. The corrosion tended to initiate from the connecting weldments, due to the potential difference with the parent steel and the aggressive exposure environment, leading to a preferential weldment attack. PMID:29565836

Remote detection of stress corrosion cracking: Surface composition and crack detection

NASA Astrophysics Data System (ADS)

Lissenden, Cliff J.; Jovanovic, Igor; Motta, Arthur T.; Xiao, Xuan; Le Berre, Samuel; Fobar, David; Cho, Hwanjeong; Choi, Sungho

2018-04-01

Chloride induced stress corrosion cracking (SCC) of austenitic stainless steel is a potential issue in long term dry storage of spent nuclear fuel canisters. In order for SCC to occur there must be a corrosive environment, a susceptible material, and a driving force. Because it is likely that the material in the heat affected zone (HAZ) of welded stainless steel structures has been sensitized as a result of chromium depletion at the grain boundaries and a thermal residual stress driving force is likely present if solution annealing is not performed, two issues are critical. Is the environment corrosive, i.e., are chlorides present in solution on the surface? And then, are there cracks that could propagate? Remote detection of chlorides on the surface can be accomplished by laser induced breakdown spectroscopy (LIBS), while cracks can be detected by shear horizontal guided waves generated by electromagnetic acoustic transducers (EMATs). Both are noncontact methods that are amenable to robotic delivery systems and harsh environments. The sensitivity to chlorine on stainless steel of a LIBS system that employs optical fiber for pulse delivery is demonstrated. Likewise, the ability of the EMAT system to detect cracks of a prescribed size and orientation is shown. These results show the potential for remote detection of Cl and cracks in dry storage spent fuel canisters.

Real time monitoring of environmental crack growth in BWRs

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hale, D.; Diehl, C.G.

1988-01-01

A comprehensive field test program was recently completed at several Boiling Water Reactors (BWR) to quantify the effect of coolant impurities on the initiation and growth of stress corrosion cracks. A new technology was utilized which allows for real time monitoring of stress corrosion crack growth rates. The BWR environments were characterized using Ion Chromatography and Electro Chemical Potential (ECP) measurements. The effects of typical water chemistry transients and startups were quantified.

Hygrothermal effects on the mechanical behaviour of graphite fibre-reinforced epoxy laminates beyond initial failure

NASA Technical Reports Server (NTRS)

Ishai, O.; Garg, A.; Nelson, H. G.

1986-01-01

The critical load levels and associated cracking beyond which a multidirectional laminate can be considered as structurally failed has been determined by loading graphite fiber-reinforced epoxy laminates to different strain levels up to ultimate failure. Transverse matrix cracking was monitored by acoustic and optical methods. The residual stiffness and strength parallel and perpendicular to the cracks were determined and related to the environmental/loading history. Within the range of experimental conditions studied, it is concluded that the transverse cracking process does not have a crucial effect on the structural performance of multidirectional composite laminates.

Effective spring stiffness for a periodic array of interacting coplanar penny-shaped cracks at an interface between two dissimilar isotropic materials

PubMed Central

Lekesiz, Huseyin; Katsube, Noriko; Rokhlin, Stanislav I.; Seghi, Robert R.

2013-01-01

An effective spring stiffness approximation is proposed for a hexagonal array of coplanar penny shaped cracks located at the interface between two dissimilar solids. The approximation is based on the factorization of the solution on the material dissimilarity factor, the crack interaction factor and the effective spring stiffness solution for non-interacting cracks in a homogeneous material. Such factorization is exact and was validated for 2D collinear cracks between two dissimilar solids. The crack interaction factor is obtained using a recently developed model for stress intensity factors for an array of coplanar penny shaped cracks in a homogeneous material; also the material dissimilarity function recently obtained for non-interacting penny shaped crack at the interface between two dissimilar materials is employed. The obtained solution is useful for an assessment by ultrasonic measurements of the interface stiffness in bonded structures for monitoring the interfacial microdamage growth due to mechanical loading and environmental factors. PMID:27175036

Effective spring stiffness for a periodic array of interacting coplanar penny-shaped cracks at an interface between two dissimilar isotropic materials.

PubMed

Lekesiz, Huseyin; Katsube, Noriko; Rokhlin, Stanislav I; Seghi, Robert R

2013-08-15

An effective spring stiffness approximation is proposed for a hexagonal array of coplanar penny shaped cracks located at the interface between two dissimilar solids. The approximation is based on the factorization of the solution on the material dissimilarity factor, the crack interaction factor and the effective spring stiffness solution for non-interacting cracks in a homogeneous material. Such factorization is exact and was validated for 2D collinear cracks between two dissimilar solids. The crack interaction factor is obtained using a recently developed model for stress intensity factors for an array of coplanar penny shaped cracks in a homogeneous material; also the material dissimilarity function recently obtained for non-interacting penny shaped crack at the interface between two dissimilar materials is employed. The obtained solution is useful for an assessment by ultrasonic measurements of the interface stiffness in bonded structures for monitoring the interfacial microdamage growth due to mechanical loading and environmental factors.

Stress generated modifications of epitaxial ferroelectric SrTiO3 films on sapphire

NASA Astrophysics Data System (ADS)

Hollmann, E.; Schubert, J.; Kutzner, R.; Wördenweber, R.

2009-06-01

The effect of lattice-mismatch induced stress upon the crystallographic structure, strain, strain relaxation, and the generation of different types of defects in heteroepitaxially grown SrTiO3 films on CeO2 buffered sapphire is examined. Depending on the thickness of the SrTiO3 layer, characteristic changes in the structural perfection of the layers, their crystallographic orientation with respect to the substrate system, and their strain is observed. For thin films misfit dislocations partially compensate the stress in the SrTiO3 layer, whereas cracks develop in thicker SrTiO3 films. The cracks are orientated along two predominant crystallographic orientations of the sapphire. The structural modifications and the formation of misfit defects and cracks are explained in a model based on lattice misfit induced stress, on the one hand, and energy considerations taking into account the stress release due to crack formation and the energy necessary for the formation of new surfaces at the crack, on the other hand. The impact of lattice misfit is discussed in two steps, i.e., intrinsic and thermal induced misfits during heteroepitaxial film growth at a given temperature and the subsequent cooling of the sample, respectively. The comparison of the theoretical predictions and the experimental observations demonstrate that intrinsic mismatch and thermal mismatch have to be considered in order to explain strain dependent effects in complex heteroepitaxial layer systems such as induced ferroelectricity of SrTiO3 on sapphire.

Corrosion and stress corrosion cracking in supercritical water

NASA Astrophysics Data System (ADS)

Was, G. S.; Ampornrat, P.; Gupta, G.; Teysseyre, S.; West, E. A.; Allen, T. R.; Sridharan, K.; Tan, L.; Chen, Y.; Ren, X.; Pister, C.

2007-09-01

Supercritical water (SCW) has attracted increasing attention since SCW boiler power plants were implemented to increase the efficiency of fossil-based power plants. The SCW reactor (SCWR) design has been selected as one of the Generation IV reactor concepts because of its higher thermal efficiency and plant simplification as compared to current light water reactors (LWRs). Reactor operating conditions call for a core coolant temperature between 280 °C and 620 °C at a pressure of 25 MPa and maximum expected neutron damage levels to any replaceable or permanent core component of 15 dpa (thermal reactor design) and 100 dpa (fast reactor design). Irradiation-induced changes in microstructure (swelling, radiation-induced segregation (RIS), hardening, phase stability) and mechanical properties (strength, thermal and irradiation-induced creep, fatigue) are also major concerns. Throughout the core, corrosion, stress corrosion cracking, and the effect of irradiation on these degradation modes are critical issues. This paper reviews the current understanding of the response of candidate materials for SCWR systems, focusing on the corrosion and stress corrosion cracking response, and highlights the design trade-offs associated with certain alloy systems. Ferritic-martensitic steels generally have the best resistance to stress corrosion cracking, but suffer from the worst oxidation. Austenitic stainless steels and Ni-base alloys have better oxidation resistance but are more susceptible to stress corrosion cracking. The promise of grain boundary engineering and surface modification in addressing corrosion and stress corrosion cracking performance is discussed.

Environmental Barrier Coating (EBC) Durability Modeling; An Overview and Preliminary Analysis

NASA Technical Reports Server (NTRS)

Abdul-Aziz, A.; Bhatt, R. T.; Grady, J. E.; Zhu, D.

2012-01-01

A study outlining a fracture mechanics based model that is being developed to investigate crack growth and spallation of environmental barrier coating (EBC) under thermal cycling conditions is presented. A description of the current plan and a model to estimate thermal residual stresses in the coating and preliminary fracture mechanics concepts for studying crack growth in the coating are also discussed. A road map for modeling life and durability of the EBC and the results of FEA model(s) developed for predicting thermal residual stresses and the cracking behavior of the coating are generated and described. Further initial assessment and preliminary results showed that developing a comprehensive EBC life prediction model incorporating EBC cracking, degradation and spalling mechanism under stress and temperature gradients typically seen in turbine components is difficult. This is basically due to mismatch in thermal expansion difference between sub-layers of EBC as well as between EBC and substrate, diffusion of moisture and oxygen though the coating, and densification of the coating during operating conditions as well as due to foreign object damage, the EBC can also crack and spall from the substrate causing oxidation and recession and reducing the design life of the EBC coated substrate.

Self-healing of early age cracks in cement-based materials by mineralization of carbonic anhydrase microorganism

PubMed Central

Qian, Chunxiang; Chen, Huaicheng; Ren, Lifu; Luo, Mian

2015-01-01

This research investigated the self-healing potential of early age cracks in cement-based materials incorporating the bacteria which can produce carbonic anhydrase. Cement-based materials specimens were pre-cracked at the age of 7, 14, 28, 60 days to study the repair ability influenced by cracking time, the width of cracks were between 0.1 and 1.0 mm to study the healing rate influenced by width of cracks. The experimental results indicated that the bacteria showed excellent repairing ability to small cracks formed at early age of 7 days, cracks below 0.4 mm was almost completely closed. The repair effect reduced with the increasing of cracking age. Cracks width influenced self-healing effectiveness significantly. The transportation of CO2and Ca2+ controlled the self-healing process. The computer simulation analyses revealed the self-healing process and mechanism of microbiologically precipitation induced by bacteria and the depth of precipitated CaCO3 could be predicted base on valid Ca2+. PMID:26583014

Contribution of precipitate on migrated grain boundaries to ductility-dip cracking in Alloy 625 weld joints

NASA Astrophysics Data System (ADS)

Lee, Dong Jin; Kim, Youn Soo; Shin, Yong Taek; Jeon, Eon Chan; Lee, Sang Hwa; Lee, Hyo-Jong; Lee, Sung Keun; Lee, Jun Hee; Lee, Hae Woo

2010-10-01

We investigated the crack properties in Alloy 625 weld metals and their characteristics using experimentally designed filler wires fabricated by varying the niobium and manganese contents in the flux with the shield metal arc welding (SMAW) process. The fast diffusivity of niobium on the migrated grain boundary (MGB) under strong restraint tensile stress, which was induced by the hardened matrix in weld metal containing high niobium and manganese, accelerated the growth of niobium carbide (NbC) in multipass deposits. Coalescence of microvoids along with incoherent NbC and further propagation induced ductility-dip cracking (DDC) on MGB.

Monitoring the Corrosion Process of Reinforced Concrete Using BOTDA and FBG Sensors

PubMed Central

Mao, Jianghong; Chen, Jiayun; Cui, Lei; Jin, Weiliang; Xu, Chen; He, Yong

2015-01-01

Expansion and cracking induced by the corrosion of reinforcement concrete is the major factor in the failure of concrete durability. Therefore, monitoring of concrete cracking is critical for evaluating the safety of concrete structures. In this paper, we introduce a novel monitoring method combining Brillouin optical time domain analysis (BOTDA) and fiber Bragg grating (FBG), based on mechanical principles of concrete expansion cracking. BOTDA monitors concrete expansion and crack width, while FBG identifies the time and position of cracking. A water-pressure loading simulation test was carried out to determine the relationship between fiber strain, concrete expansion and crack width. An electrical accelerated corrosion test was also conducted to evaluate the ability of this novel sensor to monitor concrete cracking under practical conditions. PMID:25884790

Prediction of thermal cycling induced matrix cracking

NASA Technical Reports Server (NTRS)

Mcmanus, Hugh L.

1992-01-01

Thermal fatigue has been observed to cause matrix cracking in laminated composite materials. A method is presented to predict transverse matrix cracks in composite laminates subjected to cyclic thermal load. Shear lag stress approximations and a simple energy-based fracture criteria are used to predict crack densities as a function of temperature. Prediction of crack densities as a function of thermal cycling is accomplished by assuming that fatigue degrades the material's inherent resistance to cracking. The method is implemented as a computer program. A simple experiment provides data on progressive cracking of a laminate with decreasing temperature. Existing data on thermal fatigue is also used. Correlations of the analytical predictions to the data are very good. A parametric study using the analytical method is presented which provides insight into material behavior under cyclical thermal loads.

Prediction of thermal cycling induced cracking in polmer matrix composites

NASA Technical Reports Server (NTRS)

Mcmanus, Hugh L.

1994-01-01

The work done in the period August 1993 through February 1994 on the 'Prediction of Thermal Cycling Induced Cracking In Polymer Matrix Composites' program is summarized. Most of the work performed in this period, as well as the previous one, is described in detail in the attached Master's thesis, 'Analysis of Thermally Induced Damage in Composite Space Structures,' by Cecelia Hyun Seon Park. Work on a small thermal cycling and aging chamber was concluded in this period. The chamber was extensively tested and calibrated. Temperatures can be controlled very precisely, and are very uniform in the test chamber. Based on results obtained in the previous period of this program, further experimental progressive cracking studies were carried out. The laminates tested were selected to clarify the differences between the behaviors of thick and thin ply layers, and to explore other variables such as stacking sequence and scaling effects. Most specimens tested were made available from existing stock at Langley Research Center. One laminate type had to be constructed from available prepreg material at Langley Research Center. Specimens from this laminate were cut and prepared at MIT. Thermal conditioning was carried out at Langley Research Center, and at the newly constructed MIT facility. Specimens were examined by edge inspection and by crack configuration studies, in which specimens were sanded down in order to examine the distribution of cracks within the specimens. A method for predicting matrix cracking due to decreasing temperatures and/or thermal cycling in all plies of an arbitrary laminate was implemented as a computer code. The code also predicts changes in properties due to the cracking. Extensive correlations between test results and code predictions were carried out. The computer code was documented and is ready for distribution.

Cracks growth behaviors of commercial pure titanium under nanosecond laser irradiation for formation of nanostructure-covered microstructures (with sub-5-μm)

NASA Astrophysics Data System (ADS)

Pan, A. F.; Wang, W. J.; Mei, X. S.; Zheng, B. X.; Yan, Z. X.

2016-11-01

This study reported on the formation of sub-5-μm microstructures covered on titanium by cracks growth under 10-ns laser radiation at the wavelength of 532 nm and its induced light modification for production of nanostructures. The electric field intensity and laser power density absorbed by commercial pure titanium were computed to investigate the self-trapping introduced by cracks and the effect of surface morphology on laser propagation characteristics. It is found that nanostructures can form at the surface with the curvature radius below 20 μm. Meanwhile, variable laser fluences were applied to explore the evolution of cracks on commercial pure titanium with or without melt as spot overlap number increased. Experimental study was first performed at the peak laser fluence of 1.063 J/cm2 to investigate the microstructures induced only by cracks growth. The results demonstrated that angular microstructures with size between 1.68 μm and 4.74 μm was obtained and no nanostructure covered. Then, at the peak laser fluence of 2.126 J/cm2, there were some nanostructures covered on the melt-induced curved microstructured surface. However, surface molten material submerged in the most of cracks at the spot overlap number of 744, where the old cracks disappeared. The results indicated that there was too much molten material and melting time at the peak laser fluence of 2.126 J/cm2, which was not suitable for obtainment of perfect micro-nano structures. On this basis, peak laser fluence was reduced down to 1.595 J/cm2 and the sharp sub-5 μm microstructures with nanostructures covered was obtained at spot overlap number of 3720.

Damage in gamma titanium aluminides due to small particle impacts

NASA Astrophysics Data System (ADS)

Stief, P. S.; Rubal, M. P.; Gray, G. T., III; Pereiras, J. M.

1998-10-01

Initiation of cracking due to small particle impacts on low ductility intermetallics is investigated experimentally and theoretically. The gamma titanium aluminide alloys of interest which are being considered for elevated temperature structural applications in aircraft engines exhibit tensile ductilities on the order of 1-2%. Cracking due to any source, including small particle impacts, is of concern given the rapid growth of cracks in fatigue. This investigation focuses on a model geometry which reproduces the rear face cracking that is induced by a small particle impinging on an air foil leading edge. Small steel spheres are projected onto thin plates at velocities ranging from 76 to 305 ms ; cracking is thereby induced on the rear surface of the plates. Through finite element analyses of the dynamic impact event and some analytical estimates, we examine the hypothesis that crack initiation due to small particle impacts can be correlated with material ductility and with the severity and spatial extent of the straining during the impact event. In addition, with the use of static indentation tests in which similar strain distributions are present, some insight is gained into the difference in ductility between high and low strain rates. 1998 Elsevier Science Ltd.

Environment enhanced fatigue crack propagation in metals: Inputs to fracture mechanics life prediction models

NASA Technical Reports Server (NTRS)

Gangloff, Richard P.; Kim, Sang-Shik

1993-01-01

This report is a critical review of both environment-enhanced fatigue crack propagation data and the predictive capabilities of crack growth rate models. This information provides the necessary foundation for incorporating environmental effects in NASA FLAGRO and will better enable predictions of aerospace component fatigue lives. The review presents extensive literature data on 'stress corrosion cracking and corrosion fatigue.' The linear elastic fracture mechanics approach, based on stress intensity range (Delta(K)) similitude with microscopic crack propagation threshold and growth rates, provides a basis for these data. Results are presented showing enhanced growth rates for gases (viz., H2 and H2O) and electrolytes (e.g. NaCl and H2O) in aerospace alloys including: C-Mn and heat treated alloy steels, aluminum alloys, nickel-based superalloys, and titanium alloys. Environment causes purely time-dependent accelerated fatigue crack growth above the monotonic load cracking threshold (KIEAC) and promotes cycle-time dependent cracking below (KIEAC). These phenomenon are discussed in terms of hydrogen embrittlement, dissolution, and film rupture crack tip damage mechanisms.

Fracture mechanics by three-dimensional crack-tip synchrotron X-ray microscopy

PubMed Central

Withers, P. J.

2015-01-01

To better understand the relationship between the nucleation and growth of defects and the local stresses and phase changes that cause them, we need both imaging and stress mapping. Here, we explore how this can be achieved by bringing together synchrotron X-ray diffraction and tomographic imaging. Conventionally, these are undertaken on separate synchrotron beamlines; however, instruments capable of both imaging and diffraction are beginning to emerge, such as ID15 at the European Synchrotron Radiation Facility and JEEP at the Diamond Light Source. This review explores the concept of three-dimensional crack-tip X-ray microscopy, bringing them together to probe the crack-tip behaviour under realistic environmental and loading conditions and to extract quantitative fracture mechanics information about the local crack-tip environment. X-ray diffraction provides information about the crack-tip stress field, phase transformations, plastic zone and crack-face tractions and forces. Time-lapse CT, besides providing information about the three-dimensional nature of the crack and its local growth rate, can also provide information as to the activation of extrinsic toughening mechanisms such as crack deflection, crack-tip zone shielding, crack bridging and crack closure. It is shown how crack-tip microscopy allows a quantitative measure of the crack-tip driving force via the stress intensity factor or the crack-tip opening displacement. Finally, further opportunities for synchrotron X-ray microscopy are explored. PMID:25624521

Prediction of thermal cycling induced cracking in polymer matrix composites

NASA Technical Reports Server (NTRS)

Mcmanus, Hugh L.

1993-01-01

This report summarizes the work done in the period February 1993 through July 1993 on the 'Prediction of Thermal Cycling Induced Cracking In Polymer Matrix Composites' program. An oral presentation of this work was given to Langley personnel in September of 1993. This document was prepared for archival purposes. Progress studies have been performed on the effects of spatial variations in material strength. Qualitative agreement was found with observed patterns of crack distribution. These results were presented to NASA Langley personnel in November 1992. The analytical methodology developed by Prof. McManus in the summer of 1992 (under an ASEE fellowship) has been generalized. A method for predicting matrix cracking due to decreasing temperatures and/or thermal cycling in all plies of an arbitrary laminate has been implemented as a computer code. The code also predicts changes in properties due to the cracking. Experimental progressive cracking studies on a variety of laminates were carried out at Langley Research Center. Results were correlated to predictions using the new methods. Results were initially mixed. This motivated an exploration of the configuration of cracks within laminates. A crack configuration study was carried out by cutting and/or sanding specimens in order to examine the distribution of cracks within the specimens. These investigations were supplemented by dye-penetrant enhanced X-ray photographs. The behavior of thin plies was found to be different from the behavior of thicker plies (or ply groups) on which existing theories are based. Significant edge effects were also noted, which caused the traditional metric of microcracking (count of cracks on a polished edge) to be very inaccurate in some cases. With edge and configuration taken into account, rough agreement with predictions was achieved. All results to date were reviewed with NASA Langley personnel in September 1993.

Crack detection and leakage monitoring on reinforced concrete pipe

NASA Astrophysics Data System (ADS)

Feng, Qian; Kong, Qingzhao; Huo, Linsheng; Song, Gangbing

2015-11-01

Reinforced concrete underground pipelines are some of the most widely used types of structures in water transportation systems. Cracks and leakage are the leading causes of pipeline structural failures which directly results in economic losses and environmental hazards. In this paper, the authors propose a piezoceramic based active sensing approach to detect the cracks and the further leakage of concrete pipelines. Due to the piezoelectric properties, piezoceramic material can be utilized as both the actuator and the sensor in the active sensing approach. The piezoceramic patch, which is sandwiched between protective materials called ‘smart aggregates,’ can be safely embedded into concrete structures. Circumferential and axial cracks were investigated. A wavelet packet-based energy analysis was developed to distinguish the type of crack and determine the further leakage based on different stress wave energy attenuation propagated through the cracks.

Small Crack Growth and Fatigue Life Predictions for High-Strength Aluminium Alloys. Part 1; Experimental and Fracture Mechanics Analysis

NASA Technical Reports Server (NTRS)

Wu, X. R.; Newman, J. C.; Zhao, W.; Swain, M. H.; Ding, C. F.; Phillips, E. P.

1998-01-01

The small crack effect was investigated in two high-strength aluminium alloys: 7075-T6 bare and LC9cs clad alloy. Both experimental and analytical investigations were conducted to study crack initiation and growth of small cracks. In the experimental program, fatigue tests, small crack and large crack tests A,ere conducted under constant amplitude and Mini-TWIST spectrum loading conditions. A pronounced small crack effect was observed in both materials, especially for the negative stress ratios. For all loading conditions, most of the fatigue life of the SENT specimens was shown to be crack propagation from initial material defects or from the cladding layer. In the analysis program, three-dimensional finite element and A weight function methods were used to determine stress intensity factors and to develop SIF equations for surface and corner cracks at the notch in the SENT specimens. A plastisity-induced crack-closure model was used to correlate small and large crack data, and to make fatigue life predictions, Predicted crack-growth rates and fatigue lives agreed well with experiments. A total fatigue life prediction method for the aluminum alloys was developed and demonstrated using the crack-closure model.

Fretting Fatigue with Cylindrical-On-Flat Contact: Crack Nucleation, Crack Path and Fatigue Life

PubMed Central

Noraphaiphipaksa, Nitikorn; Manonukul, Anchalee; Kanchanomai, Chaosuan

2017-01-01

Fretting fatigue experiments and finite element analysis were carried out to investigate the influence of cylindrical-on-flat contact on crack nucleation, crack path and fatigue life of medium-carbon steel. The location of crack nucleation was predicted using the maximum shear stress range criterion and the maximum relative slip amplitude criterion. The prediction using the maximum relative slip amplitude criterion gave the better agreement with the experimental result, and should be used for the prediction of the location of crack nucleation. Crack openings under compressive bulk stresses were found in the fretting fatigues with flat-on-flat contact and cylindrical-on-flat contacts, i.e., fretting-contact-induced crack openings. The crack opening stress of specimen with flat-on-flat contact was lower than those of specimens with cylindrical-on-flat contacts, while that of specimen with 60-mm radius contact pad was lower than that of specimen with 15-mm radius contact pad. The fretting fatigue lives were estimated by integrating the fatigue crack growth curve from an initial propagating crack length to a critical crack length. The predictions of fretting fatigue life with consideration of crack opening were in good agreement with the experimental results. PMID:28772522

Exploring How Weathering Related Stresses and Subcritical Crack Growth May Influence the Size of Sediment Produced From Different Rock Types.

NASA Astrophysics Data System (ADS)

Eppes, M. C.; Hallet, B.; Hancock, G. S.; Mackenzie-Helnwein, P.; Keanini, R.

2016-12-01

The formation and diminution of rock debris, sediment and soil at and near Earth's surface is driven in large part by in situ, non-transport related, rock cracking. Given the relatively low magnitude stresses that arise in surface and near-surface settings, this production and diminution of granular material is likely strongly influenced and/or driven by subcritical crack growth (Eppes et al., 2016), cracking that occurs under stress loading conditions much lower than a rock's strength as typically measured in the laboratory under rapid loading. Despite a relatively sound understanding of subcritical crack growth through engineering and geophysical studies, its geomorphic and sedimentologic implications have only been minimally explored. Here, based on existing studies, we formulate several hypotheses to predict how weathering-induced stresses combined with the subcritical crack growth properties of rock may influence sediment size distribution. For example, subcritical crack growth velocity (v) can be described by v = CKIn where KI is the mode I (simple opening mode) stress intensity factor, a function of tensile stress at the crack tip and crack length; C is a rock- and environment-dependent constant; and n is material constant, the subcritical crack growth index. Fracture length and spacing in rock is strongly dependent on n, where higher n values result in fewer, more distally spaced cracks (e.g. Olsen, 1993). Thus, coarser sediment might be expected from rocks with higher n values. Weathering-related stresses such as thermal stresses and mineral hydration, however, can disproportionally stress boundaries between minerals with contrasting thermal or chemical properties and orientation, resulting in granular disintegration. Thus, rocks with properties favorable to inducing these stresses might produce sediment whose size is reflective of its constituent grains. We begin to test these hypotheses through a detailed examination of crack and rock characteristics in outcrops of granite, sandstone, and quartzite found in Shenandoah National Park, Virginia. Preliminary results reveal that many observed cracking characteristics are consistent with our hypotheses linking subcritical crack growth, weathering stresses and the production of different sized sediment from different rock types.

40 CFR 63.1568 - What are my requirements for HAP emissions from sulfur recovery units?

Code of Federal Regulations, 2012 CFR

2012-07-01

... emissions from sulfur recovery units? 63.1568 Section 63.1568 Protection of Environment ENVIRONMENTAL... Petroleum Refineries: Catalytic Cracking Units, Catalytic Reforming Units, and Sulfur Recovery Units Catalytic Cracking Units, Catalytic Reforming Units, Sulfur Recovery Units, and Bypass Lines § 63.1568 What...

Crack tip field and fatigue crack growth in general yielding and low cycle fatigue

NASA Technical Reports Server (NTRS)

Minzhong, Z.; Liu, H. W.

1984-01-01

Fatigue life consists of crack nucleation and crack propagation periods. Fatigue crack nucleation period is shorter relative to the propagation period at higher stresses. Crack nucleation period of low cycle fatigue might even be shortened by material and fabrication defects and by environmental attack. In these cases, fatigue life is largely crack propagation period. The characteristic crack tip field was studied by the finite element method, and the crack tip field is related to the far field parameters: the deformation work density, and the product of applied stress and applied strain. The cyclic carck growth rates in specimens in general yielding as measured by Solomon are analyzed in terms of J-integral. A generalized crack behavior in terms of delta is developed. The relations between J and the far field parameters and the relation for the general cyclic crack growth behavior are used to analyze fatigue lives of specimens under general-yielding cyclic-load. Fatigue life is related to the applied stress and strain ranges, the deformation work density, crack nucleus size, fracture toughness, fatigue crack growth threshold, Young's modulus, and the cyclic yield stress and strain. The fatigue lives of two aluminum alloys correlate well with the deformation work density as depicted by the derived theory. The general relation is reduced to Coffin-Manson low cycle fatigue law in the high strain region.

Salinity effects on the dynamics and patterns of desiccation cracks

NASA Astrophysics Data System (ADS)

Shokri, N.; Zhou, P.

2012-12-01

Cracking arising from desiccation is a ubiquitous phenomenon encountered in various industrial and geo-environmental applications including drying of clayey soil, cement, ceramics, gels, and many more colloidal suspensions. Presence of cracks in muddy sediments modifies the characteristics of the medium such as pore structure, porosity, and permeability which in turn influence various flow and transport processes. Thus it remains a topic of great interest in many disciplines to describe the dynamics of desiccation cracking under various boundary conditions. To this end, we conducted a comprehensive study to investigate effects of NaCl concentrations on cracking dynamics and patterns during desiccation of Bentonite. Mixtures of Bentonite and NaCl solutions were prepared with NaCl concentration varying from 2 to 10 percent in 0.5 percent increment (totally 17 configurations). The slurry was placed in a Petri dish mounted on a digital balance to record the evaporation dynamics. The atmospheric conditions were kept constant using an environmental chamber. An automatic camera was used to record the dynamics of macro-cracks (mm scale) at the surface of desiccating clay each minute. The obtained results illustrate the significant effects of salt concentration on the initiation, propagation, morphology and general dynamics of macro-cracks. We found that higher salt concentrations results in larger macro cracks' lengths attributed to the effects of NaCl on compressing the electric double layer of particles at increasing electrolyte concentrations which reduce considerably the repulsive forces among the particles and causing instability of the slurry and flocculation of the colloidal particles. Rheological measurements by means of a stress controlled rheometer revealed that the yield stress of the slurry decreases as NaCl concentration increases which may indicate aggregation of larger units in the slurry as a result of flocculation causing larger cracks' lengths due to drying. At the end of each round of the experiment, a detailed visualization was conducted using Scanning Electron Microscopy to investigate the patterns and morphology of cracks at micro-scale as influenced by the salt concentration. Our results provide new insights and finding about the effects of salt concentrations on desiccation cracks at different scales ranging from a few mm to few microns.

Experimental investigation on centrifugal compressor blade crack classification using the squared envelope spectrum.

PubMed

Li, Hongkun; Zhang, Xuefeng; Xu, Fujian

2013-09-18

Centrifugal compressors are a key piece of equipment for modern production. Among the components of the centrifugal compressor, the impeller is a pivotal part as it is used to transform kinetic energy into pressure energy. Blade crack condition monitoring and classification has been broadly investigated in the industrial and academic area. In this research, a pressure pulsation (PP) sensor arranged in close vicinity to the crack area and the corresponding casing vibration signals are used to monitor blade crack information. As these signals cannot directly demonstrate the blade crack, the method employed in this research is based on the extraction of weak signal characteristics that are induced by blade cracking. A method for blade crack classification based on the signals monitored by using a squared envelope spectrum (SES) is presented. Experimental investigations on blade crack classification are carried out to verify the effectiveness of this method. The results show that it is an effective tool for blade crack classification in centrifugal compressors.

Experimental Investigation on Centrifugal Compressor Blade Crack Classification Using the Squared Envelope Spectrum

PubMed Central

Li, Hongkun; Zhang, Xuefeng; Xu, Fujian

2013-01-01

Centrifugal compressors are a key piece of equipment for modern production. Among the components of the centrifugal compressor, the impeller is a pivotal part as it is used to transform kinetic energy into pressure energy. Blade crack condition monitoring and classification has been broadly investigated in the industrial and academic area. In this research, a pressure pulsation (PP) sensor arranged in close vicinity to the crack area and the corresponding casing vibration signals are used to monitor blade crack information. As these signals cannot directly demonstrate the blade crack, the method employed in this research is based on the extraction of weak signal characteristics that are induced by blade cracking. A method for blade crack classification based on the signals monitored by using a squared envelope spectrum (SES) is presented. Experimental investigations on blade crack classification are carried out to verify the effectiveness of this method. The results show that it is an effective tool for blade crack classification in centrifugal compressors. PMID:24051521

Prediction of corrosion fatigue crack initiation behavior of A7N01P-T4 aluminum alloy welded joints

NASA Astrophysics Data System (ADS)

An, J.; Chen, J.; Gou, G.; Chen, H.; Wang, W.

2017-07-01

Through investigating the corrosion fatigue crack initiation behavior of A7N01P-T4 aluminum alloy welded joints in 3.5 wt.% NaCl solution, corrosion fatigue crack initiation life is formulated as Ni = 6.97 × 1012[Δσeqv1.739 - 491.739]-2 and the mechanism of corrosion fatigue crack initiation is proposed. SEM and TEM tests revealed that several corrosion fatigue cracks formed asynchronously and the first crack does not necessarily develop into the leading crack. The uneven reticular dislocations produced by fatigue loading are prone to piling up and tangling near the grain boundaries or the second phases and form the “high dislocation-density region” (HDDR), which acts as an anode in microbatteries and dissolved to form small crack. Thus the etching pits, HDDR near the grain boundaries and second phases are confirmed as the main causes inducing the initiation of fatigue crack.

Numerical model of water flow and solute accumulation in vertisols using HYDRUS 2D/3D code

NASA Astrophysics Data System (ADS)

Weiss, Tomáš; Dahan, Ofer; Turkeltub, Tuvia

2015-04-01

Keywords: dessication-crack-induced-salinization, preferential flow, conceptual model, numerical model, vadose zone, vertisols, soil water retention function, HYDRUS 2D/3D Vertisols cover a hydrologically very significant area of semi-arid regions often through which water infiltrates to groundwater aquifers. Understanding of water flow and solute accumulation is thus very relevant to agricultural activity and water resources management. Previous works suggest a conceptual model of dessication-crack-induced-salinization where salinization of sediment in the deep section of the vadose zone (up to 4 m) is induced by subsurface evaporation due to convective air flow in the dessication cracks. It suggests that the salinization is induced by the hydraulic gradient between the dry sediment in the vicinity of cracks (low potential) and the relatively wet sediment further from the main cracks (high potential). This paper presents a modified previously suggested conceptual model and a numerical model. The model uses a simple uniform flow approach but unconventionally prescribes the boundary conditions and the hydraulic parameters of soil. The numerical model is bound to one location close to a dairy farm waste lagoon, but the application of the suggested conceptual model could be possibly extended to all semi-arid regions with vertisols. Simulations were conducted using several modeling approaches with an ultimate goal of fitting the simulation results to the controlling variables measured in the field: temporal variation in water content across thick layer of unsaturated clay sediment (>10 m), sediment salinity and salinity the water draining down the vadose zone to the water table. The development of the model was engineered in several steps; all computed as forward solutions by try-and-error approach. The model suggests very deep instant infiltration of fresh water up to 12 m, which is also supported by the field data. The paper suggests prescribing a special atmospheric boundary to the wall of the crack (so that the solute can accumulate due to evaporation on the crack block wall, and infiltrating fresh water can push the solute further down) - in order to do so, HYDRUS 2D/3D code had to be modified by its developers. Unconventionally, the main fitting parameters were: parameter a and n in the soil water retention curve and saturated hydraulic conductivity. The amount of infiltrated water (within a reasonable range), the infiltration function in the crack and the actual evaporation from the crack were also used as secondary fitting parameters. The model supports the previous findings that significant amount (~90%) of water from rain events must infiltrate through the crack. It was also noted that infiltration from the crack has to be increasing with depth and that the highest infiltration rate should be somewhere between 1-3m. This paper suggests a new way how to model vertisols in semi-arid regions. It also supports the previous findings about vertisols: especially, the utmost importance of soil cracks as preferential pathways for water and contaminants and soil cracks as deep evaporators.

Preventing Cracking of Anodized Coatings

NASA Technical Reports Server (NTRS)

He, Charles C.; Heslin, Thomas M.

1995-01-01

Anodized coatings have been used as optical and thermal surfaces in spacecraft. Particulate contamination from cracked coatings is a concern for many applications. The major cause for the cracking is the difference in the coefficient of thermal expansion between the oxide coatings and the aluminum substrate. The loss of water when the coating is exposed to a vacuum also could induce cracking of the coating. Hot-water sealing was identified as the major cause for the cracking of the coatings because of the large temperature change when the parts were immersed in boiling water and the water was absorbed in the coating. when the hot-water sealing process was eliminated, the cracking resistance of the anodized coatings was greatly improved. Also, it was found that dyed black coatings were more susceptible than clear coatings to cracking during thermo-vacuum cyclings.

Further damage induced by water in micro-indentations in phosphate laser glass

NASA Astrophysics Data System (ADS)

Yu, Jiaxin; Jian, Qingyun; Yuan, Weifeng; Gu, Bin; Ji, Fang; Huang, Wen

2014-02-01

Using a microhardness tester, artificial flaws were made by micro-indentation in N31 Nd-doped phosphate laser glass. Indentation fracture toughness, KIC, was estimated as 0.45-0.53 MPa m1/2 from these indentations. The glasses with indentations were then immersed in ultrapure water to investigate further water-induced damage of these indentations. Stress-enhanced hydrolysis leads to the propagations of radial crack, lateral cracks and microcracks in the subsurface. These crack propagations therefore cause deformation in subsurface to form annular reflections regions around the indentations and further material collapse within imprints. After the residual stresses are exhausted, the leaching plays a more dominated role in glass corrosion in the further immersion. After immersion, the material structure slackens around micro-indentation, which decreases the contact stiffness and results in a lower nano-hardness. For the surface far away from flaws, water immersion presents a weak effect on the near-surface mechanical since the matrix leaching in phosphate glass restricts the formation of hydration layer. During first 20 min immersion, due to higher chemical activity and lower fracture toughness, the radial cracks show a faster propagation in phosphate glass compared with that in K9 silicate glass. For further immersion, crack healing occurs in silicate glass but not in phosphate glass. Analysis shows that the formation of hydration layer on crack walls plays an important role in crack healing in glasses.

A Microstructure-Based Time-Dependent Crack Growth Model for Life and Reliability Prediction of Turbopropulsion Systems

NASA Astrophysics Data System (ADS)

Chan, Kwai S.; Enright, Michael P.; Moody, Jonathan; Fitch, Simeon H. K.

2014-01-01

The objective of this investigation was to develop an innovative methodology for life and reliability prediction of hot-section components in advanced turbopropulsion systems. A set of generic microstructure-based time-dependent crack growth (TDCG) models was developed and used to assess the sources of material variability due to microstructure and material parameters such as grain size, activation energy, and crack growth threshold for TDCG. A comparison of model predictions and experimental data obtained in air and in vacuum suggests that oxidation is responsible for higher crack growth rates at high temperatures, low frequencies, and long dwell times, but oxidation can also induce higher crack growth thresholds (Δ K th or K th) under certain conditions. Using the enhanced risk analysis tool and material constants calibrated to IN 718 data, the effect of TDCG on the risk of fracture in turboengine components was demonstrated for a generic rotor design and a realistic mission profile using the DARWIN® probabilistic life-prediction code. The results of this investigation confirmed that TDCG and cycle-dependent crack growth in IN 718 can be treated by a simple summation of the crack increments over a mission. For the temperatures considered, TDCG in IN 718 can be considered as a K-controlled or a diffusion-controlled oxidation-induced degradation process. This methodology provides a pathway for evaluating microstructural effects on multiple damage modes in hot-section components.

Uncertainty quantification methodologies development for stress corrosion cracking of canister welds

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dingreville, Remi Philippe Michel; Bryan, Charles R.

2016-09-30

This letter report presents a probabilistic performance assessment model to evaluate the probability of canister failure (through-wall penetration) by SCC. The model first assesses whether environmental conditions for SCC – the presence of an aqueous film – are present at canister weld locations (where tensile stresses are likely to occur) on the canister surface. Geometry-specific storage system thermal models and weather data sets representative of U.S. spent nuclear fuel (SNF) storage sites are implemented to evaluate location-specific canister surface temperature and relative humidity (RH). As the canister cools and aqueous conditions become possible, the occurrence of corrosion is evaluated. Corrosionmore » is modeled as a two-step process: first, pitting is initiated, and the extent and depth of pitting is a function of the chloride surface load and the environmental conditions (temperature and RH). Second, as corrosion penetration increases, the pit eventually transitions to a SCC crack, with crack initiation becoming more likely with increasing pit depth. Once pits convert to cracks, a crack growth model is implemented. The SCC growth model includes rate dependencies on both temperature and crack tip stress intensity factor, and crack growth only occurs in time steps when aqueous conditions are predicted. The model suggests that SCC is likely to occur over potential SNF interim storage intervals; however, this result is based on many modeling assumptions. Sensitivity analyses provide information on the model assumptions and parameter values that have the greatest impact on predicted storage canister performance, and provide guidance for further research to reduce uncertainties.« less

Reaction Control System Thruster Cracking Consultation: NASA Engineering and Safety Center (NESC) Materials Super Problem Resolution Team (SPRT) Findings

NASA Technical Reports Server (NTRS)

MacKay, Rebecca A.; Smith, Stephen W.; Shah, Sandeep R.; Piascik, Robert S.

2005-01-01

The shuttle orbiter s reaction control system (RCS) primary thruster serial number 120 was found to contain cracks in the counter bores and relief radius after a chamber repair and rejuvenation was performed in April 2004. Relief radius cracking had been observed in the 1970s and 1980s in seven thrusters prior to flight; however, counter bore cracking had never been seen previously in RCS thrusters. Members of the Materials Super Problem Resolution Team (SPRT) of the NASA Engineering and Safety Center (NESC) conducted a detailed review of the relevant literature and of the documentation from the previous RCS thruster failure analyses. It was concluded that the previous failure analyses lacked sufficient documentation to support the conclusions that stress corrosion cracking or hot-salt cracking was the root cause of the thruster cracking and lacked reliable inspection controls to prevent cracked thrusters from entering the fleet. The NESC team identified and performed new materials characterization and mechanical tests. It was determined that the thruster intergranular cracking was due to hydrogen embrittlement and that the cracking was produced during manufacturing as a result of processing the thrusters with fluoride-containing acids. Testing and characterization demonstrated that appreciable environmental crack propagation does not occur after manufacturing.

Automatic crack detection method for loaded coal in vibration failure process

PubMed Central

Li, Chengwu

2017-01-01

In the coal mining process, the destabilization of loaded coal mass is a prerequisite for coal and rock dynamic disaster, and surface cracks of the coal and rock mass are important indicators, reflecting the current state of the coal body. The detection of surface cracks in the coal body plays an important role in coal mine safety monitoring. In this paper, a method for detecting the surface cracks of loaded coal by a vibration failure process is proposed based on the characteristics of the surface cracks of coal and support vector machine (SVM). A large number of cracked images are obtained by establishing a vibration-induced failure test system and industrial camera. Histogram equalization and a hysteresis threshold algorithm were used to reduce the noise and emphasize the crack; then, 600 images and regions, including cracks and non-cracks, were manually labelled. In the crack feature extraction stage, eight features of the cracks are extracted to distinguish cracks from other objects. Finally, a crack identification model with an accuracy over 95% was trained by inputting the labelled sample images into the SVM classifier. The experimental results show that the proposed algorithm has a higher accuracy than the conventional algorithm and can effectively identify cracks on the surface of the coal and rock mass automatically. PMID:28973032

Automatic crack detection method for loaded coal in vibration failure process.

PubMed

Li, Chengwu; Ai, Dihao

2017-01-01

In the coal mining process, the destabilization of loaded coal mass is a prerequisite for coal and rock dynamic disaster, and surface cracks of the coal and rock mass are important indicators, reflecting the current state of the coal body. The detection of surface cracks in the coal body plays an important role in coal mine safety monitoring. In this paper, a method for detecting the surface cracks of loaded coal by a vibration failure process is proposed based on the characteristics of the surface cracks of coal and support vector machine (SVM). A large number of cracked images are obtained by establishing a vibration-induced failure test system and industrial camera. Histogram equalization and a hysteresis threshold algorithm were used to reduce the noise and emphasize the crack; then, 600 images and regions, including cracks and non-cracks, were manually labelled. In the crack feature extraction stage, eight features of the cracks are extracted to distinguish cracks from other objects. Finally, a crack identification model with an accuracy over 95% was trained by inputting the labelled sample images into the SVM classifier. The experimental results show that the proposed algorithm has a higher accuracy than the conventional algorithm and can effectively identify cracks on the surface of the coal and rock mass automatically.

Characteristics of lead induced stress corrosion cracking of alloy 690 in high temperature

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chung, K.K.; Lim, J.K.; Watanabe, Yutaka

1996-10-01

Slow strain rate tests (SSRT) were conducted on alloy 690 in various lead chloride solutions and metal lead added to 100 ppm chloride solution at 288 C. The corrosion potential (rest potential) for the alloy was measured with SSRT tests. The cracking was observed by metallographic examination and electron probe micro analyzer. Also, the corrosion behavior of the alloy was evaluated by anodic polarized measurement at 30 C. Resulting from the tests, cracking was characterized by cracking behavior, crack length and crack growth rate, and lead effects on cracking. The cracking was mainly intergranular in mode, approximately from 60 ummore » to 450 um in crack length, and approximately 10{sup {minus}6} to 10{sup {minus}7} mmS-1 in crack velocity. The cracking was evaluated through the variation the corrosion potential in potential-time and lead behavior during SSRTs. The lead effect in corrosion was evaluated through active to passive transition behavior in anodic polarized curves. The corrosion reactions in the cracking region were confirmed by electron probe microanalysis. Alloy 690 is used for steam generation tubes in pressurized water reactors.« less

First environmental data from the EUV engineering test stand

NASA Astrophysics Data System (ADS)

Klebanoff, Leonard E.; Malinowski, Michael E.; Grunow, Philip A.; Clift, W. Miles; Steinhaus, Chip; Leung, Alvin H.; Haney, Steven J.

2001-08-01

The first environmental data from the Engineering Test Stand (ETS) has been collected. Excellent control of high-mass hydrocarbons has been observed. This control is a result of extensive outgas testing of components and materials, vacuum compatible design of the ETS, careful cleaning of parts and pre-baking of cables and sub assemblies where possible, and clean assembly procedures. As a result of the hydrocarbon control, the residual ETS vacuum environment is rich in water vapor. Analysis of witness plate data indicates that the ETS environment does not pose a contamination risk to the optics in the absence of EUV irradiation. However, with EUV exposure, the water rich environment can lead to EUV- induced water oxidation of the Si-terminated Mo/Si optics. Added ethanol can prevent optic oxidation, allowing carbon growth via EUV cracking of low-level residual hydrocarbons to occur. The EUV environmental issues are understood, mitigation approaches have been validated, and EUV optic contamination appears to be manageable.

Bending fracture in carbon nanotubes.

PubMed

Kuo, Wen-Shyong; Lu, Hsin-Fang

2008-12-10

A novel approach was adopted to incur bending fracture in carbon nanotubes (CNTs). Expanded graphite (EG) was made by intercalating and exfoliating natural graphite flakes. The EG was deposited with nickel particles, from which CNTs were grown by chemical vapor deposition. The CNTs were tip-grown, and their roots were fixed on the EG flakes. The EG flakes were compressed, and many CNTs on the surface were fragmented due to the compression-induced bending. Two major modes of the bending fracture were observed: cone-shaped and shear-cut. High-resolution scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to examine the crack growth within the graphene layers. The bending fracture is characterized by two-region crack growth. An opening crack first appears around the outer-tube due to the bending-induced tensile stress. The crack then branches to grow along an inclined direction toward the inner-tube due to the presence of the shear stress in between graphene layers. An inner-tube pullout with inclined side surface is formed. The onset and development of the crack in these two regions are discussed.

Characteristics of solder joints under fatigue loads using piezomechanical actuation

NASA Astrophysics Data System (ADS)

Shim, Dong-Jin; Spearing, S. Mark

2003-07-01

Crack initiation and growth characteristics of solder joints under fatigue loads are investigated using piezomechanical actuation. Cracks in solder joints, which can cause failure in microelectronics components, are induced via piezoelectricity in piezo-ceramic bonded joints. Lead-zirconate-titanate ceramic plates and eutectic Sn-Pb solder bonded in a double-lap shear configuration are used in the investigation. Electric field across each piezo-ceramic plate is applied such that shear stresses/strains are induced in the solder joints. The experiments show that cracks initiate in the solder joints around defects such as voids and grow in length until they coalesce with other cracks from adjacent voids. These observations are compared with the similar thermal cycling tests from the literature to show feasibility and validity of the current method in investigating the fatigue characteristics of solder joints. In some specimens, cracks in the piezo-ceramic plates are observed, and failure in the specimens generally occurred due to piezo-ceramic plate fracture. The issues encountered in implementing this methodology such as low actuation and high processing temperatures are further discussed.

Crack deflection: Implications for the growth of long and short fatigue cracks

NASA Astrophysics Data System (ADS)

Suresh, S.

1983-11-01

The influences of crack deflection on the growth rates of nominally Mode I fatigue cracks are examined. Previous theoretical analyses of stress intensity solutions for kinked elastic cracks are reviewed. Simple elastic deflection models are developed to estimate the growth rates of nonlinear fatigue cracks subjected to various degrees of deflection, by incorporating changes in the effective driving force and in the apparent propagation rates. Experimental data are presented for intermediate-quenched and step-quenched conditions of Fe/2Si/0.1C ferrite-martensite dual phase steel, where variations in crack morphology alone influence considerably the fatigue crack propagation rates and threshold stress intensity range values. Such results are found to be in good quantitative agreement with the deflection model predictions of propagation rates for nonlinear cracks. Experimental information on crack deflection, induced by variable amplitude loading, is also provided for 2020-T651 aluminum alloy. It is demonstrated with the aid of elastic analyses and experiments that crack deflection models offer a physically-appealing rationale for the apparently slower growth rates of long fatigue cracks subjected to constant and variable amplitude loading and for the apparent deceleration and/or arrest of short cracks. The changes in the propagation rates of deflected fatigue cracks are discussed in terms of the local mode of crack advance, microstructure, effective driving force, growth mechanisms, mean stress, slip characteristics, and crack closure.

Unidirectionally oriented nanocracks on metal surfaces irradiated by low-fluence femtosecond laser pulses

NASA Astrophysics Data System (ADS)

Shimizu, Masahiro; Hashida, Masaki; Miyasaka, Yasuhiro; Tokita, Shigeki; Sakabe, Shuji

2013-10-01

We have investigated the origin of nanostructures formed on metals by low-fluence femtosecond laser pulses. Nanoscale cracks oriented perpendicular to the incident laser polarization are induced on tungsten, molybdenum, and copper targets. The number density of the cracks increases with the number of pulses, but crack length plateaus. Electromagnetic field simulation by the finite-difference time-domain method indicates that electric field is locally enhanced along the direction perpendicular to the incident laser polarization around a nanoscale hole on the metal surface. Crack formation originates from the hole.

Modeling time-dependent corrosion fatigue crack propagation in 7000 series aluminum alloys

NASA Technical Reports Server (NTRS)

Mason, Mark E.; Gangloff, Richard P.

1994-01-01

Stress corrosion cracking and corrosion fatigue experiments were conducted with the susceptible S-L orientation of AA7075-T651, immersed in acidified and inhibited NaCl solution, to provide a basis for incorporating environmental effects into fatigue crack propagation life prediction codes such as NASA FLAGRO. This environment enhances da/dN by five to ten-fold compared to fatigue in moist air. Time-based crack growth rates from quasi-static load experiments are an order of magnitude too small for accurate linear superposition prediction of da/dN for loading frequencies above 0.001 Hz. Alternate methods of establishing da/dt, based on rising-load or ripple-load-enhanced crack tip strain rate, do not increase da/dt and do not improve linear superposition. Corrosion fatigue is characterized by two regimes of frequency dependence; da/dN is proportional to f(exp -1) below 0.001 Hz and to F(exp 0) to F(exp -0.1) for higher frequencies. Da/dN increases mildly both with increasing hold-time at K(sub max) and with increasing rise-time for a range of loading waveforms. The mild time-dependence is due to cycle-time-dependent corrosion fatigue growth. This behavior is identical for S-L nd L-T crack orientations. The frequency response of environmental fatigue in several 7000 series alloys is variable and depends on undefined compositional or microstructural variables. Speculative explanations are based on the effect of Mg on occluded crack chemistry and embritting hydrogen uptake, or on variable hydrogen diffusion in the crack tip process zone. Cracking in the 7075/NaCl system is adequately described for life prediction by linear superposition for prolonged load-cycle periods, and by a time-dependent upper bound relationship between da/dN and delta K for moderate loading times.

Fatigue Analyses Under Constant- and Variable-Amplitude Loading Using Small-Crack Theory

NASA Technical Reports Server (NTRS)

Newman, J. C., Jr.; Phillips, E. P.; Everett, R. A., Jr.

1999-01-01

Studies on the growth of small cracks have led to the observation that fatigue life of many engineering materials is primarily "crack growth" from micro-structural features, such as inclusion particles, voids, slip-bands or from manufacturing defects. This paper reviews the capabilities of a plasticity-induced crack-closure model to predict fatigue lives of metallic materials using "small-crack theory" under various loading conditions. Constraint factors, to account for three-dimensional effects, were selected to correlate large-crack growth rate data as a function of the effective stress-intensity factor range (delta-Keff) under constant-amplitude loading. Modifications to the delta-Keff-rate relations in the near-threshold regime were needed to fit measured small-crack growth rate behavior. The model was then used to calculate small-and large-crack growth rates, and to predict total fatigue lives, for notched and un-notched specimens under constant-amplitude and spectrum loading. Fatigue lives were predicted using crack-growth relations and micro-structural features like those that initiated cracks in the fatigue specimens for most of the materials analyzed. Results from the tests and analyses agreed well.

Main factors causing intergranular and quasi-cleavage fractures at hydrogen-induced cracking in tempered martensitic steels

NASA Astrophysics Data System (ADS)

Kurokawa, Ami; Doshida, Tomoki; Hagihara, Yukito; Suzuki, Hiroshi; Takai, Kenichi

2018-05-01

Though intergranular (IG) and quasi-cleavage (QC) fractures have been widely recognized as typical fracture modes of the hydrogen-induced cracking in high-strength steels, the main factor has been unclarified yet. In the present study, the hydrogen content dependence on the main factor causing hydrogen-induced cracking has been examined through the fracture mode transition from QC to IG at the crack initiation site in the tempered martensitic steels. Two kinds of tempered martensitic steels were prepared to change the cohesive force due to the different precipitation states of Fe3C on the prior γ grain boundaries. A high amount of Si (H-Si) steel has a small amount of Fe3C on the prior austenite grain boundaries. Whereas, a low amount of Si (L-Si) steel has a large amount of Fe3C sheets on the grain boundaries. The fracture modes and initiations were observed using FE-SEM (Field Emission-Scanning Electron Microscope). The crack initiation sites of the H-Si steel were QC fracture at the notch tip under various hydrogen contents. While the crack initiation of the L-Si steel change from QC fracture at the notch tip to QC and IG fractures from approximately 10 µm ahead of the notch tip as increasing in hydrogen content. For L-Si steels, two possibilities are considered that the QC or IG fracture occurred firstly, or the QC and IG fractures occurred simultaneously. Furthermore, the principal stress and equivalent plastic strain distributions near the notch tip were calculated with FEM (Finite Element Method) analysis. The plastic strain was the maximum at the notch tip and the principle stress was the maximum at approximately 10 µm from the notch tip. The position of the initiation of QC and IG fracture observed using FE-SEM corresponds to the position of maximum strain and stress obtained with FEM, respectively. These findings indicate that the main factors causing hydrogen-induced cracking are different between QC and IG fractures.

Impairment of spatial working memory and oxidative stress induced by repeated crack cocaine inhalation in rats.

PubMed

Lipaus, Ingryd Fortes Souza; Gomes, Elisa Fraga; Martins, Cleciane Waldetário; E Silva, Cristina Martins; Pires, Rita Gomes Wanderley; Malgarin, Fernanda; Schuck, Patrícia Fernanda; Palacios, Ester Miyuki Nakamura; de Melo Rodrigues, Lívia Carla

2018-06-20

Crack cocaine is a highly toxic drug with great potential to induce addiction. It produces more intense effects than cocaine powder, with its use having grown worldwide. However, few studies have focused on the cognitive and biochemical consequences that result from crack cocaine inhalation. This study examined the effects of direct crack cocaine inhalation on spatial working memory and brain oxidative stress parameters in rats. Male adult Wistar rats, well-trained in an eight-arm radial maze (8-RM), underwent five sessions of crack cocaine inhalation (crack cocaine group) once a day or inhalation simulation (sham group), being tested in 1-h delayed tasks 24 h after the last inhalation. An additional inhalation session was carried out the following day, with the prefrontal cortex, hippocampus and striatum being removed five minutes afterwards in order to assess oxidative damage such as lipid peroxidation, thiobarbituric acid-reactive species (TBARS) levels, and advanced oxidation protein products (AOPP), as well as the activity of antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx). Animals from the crack cocaine group showed more errors (p <  0.01) in the 1-h post-delay performance in the 8-RM when compared to the sham group. The crack cocaine group showed decreased (p <  0.05) lipid peroxidation in the hippocampus and increased (p <  0.001) levels of AOPP and SOD activity (p < 0.05) in the striatum when compared to the sham group. Therefore, the repeated inhalation of crack cocaine impaired long-term spatial working memory and elicited oxidative stress in the hippocampus and striatum of rats. Copyright © 2018. Published by Elsevier B.V.

Crack cocaine inhalation induces schizophrenia-like symptoms and molecular alterations in mice prefrontal cortex.

PubMed

Areal, Lorena Bianchine; Herlinger, Alice Laschuk; Pelição, Fabrício Souza; Martins-Silva, Cristina; Pires, Rita Gomes Wanderley

2017-08-01

Crack cocaine (crack) addiction represents a major social and health burden, especially seeing as users are more prone to engage in criminal and violent acts. Crack users show a higher prevalence of psychiatric comorbidities - particularly antisocial personality disorders - when compared to powder cocaine users. They also develop cognitive deficits related mainly to executive functions, including working memory. It is noteworthy that stimulant drugs can induce psychotic states, which appear to mimic some symptoms of schizophrenia among users. Social withdraw and executive function deficits are, respectively, negative and cognitive symptoms of schizophrenia mediated by reduced dopamine (DA) tone in the prefrontal cortex (PFC) of patients. That could be explained by an increased expression of D2R short isoform (D2S) in the PFC of such patients and/or by hypofunctioning NMDA receptors in this region. Reduced DA tone has already been described in the PFC of mice exposed to crack smoke. Therefore, it is possible that behavioral alterations presented by crack users result from molecular and biochemical neuronal alterations akin to schizophrenia. Accordingly, we found that upon crack inhalation mice have shown decreased social interaction and working memory deficits analogous to schizophrenia's symptoms, along with increased D2S/D2L expression ratio and decreased expression of NR1, NR2A and NR2B NMDA receptor subunits in the PFC. Herein we propose two possible mechanisms to explain the reduced DA tone in the PFC elicited by crack consumption in mice, bringing also the first direct evidence that crack use may result in schizophrenia-like neurochemical, molecular and behavioral alterations. Copyright © 2017 Elsevier Ltd. All rights reserved.

Internal hydrogen-induced subcritical crack growth in austenitic stainless steels

NASA Astrophysics Data System (ADS)

Huang, J. H.; Altstetter, C. J.

1991-11-01

The effects of small amounts of dissolved hydrogen on crack propagation were determined for two austenitic stainless steel alloys, AISI 301 and 310S. In order to have a uniform distribution of hydrogen in the alloys, they were cathodically charged at high temperature in a molten salt electrolyte. Sustained load tests were performed on fatigue precracked specimens in air at 0 ‡C, 25 ‡C, and 50 ‡C with hydrogen contents up to 41 wt ppm. The electrical potential drop method with optical calibration was used to continuously monitor the crack position. Log crack velocity vs stress intensity curves had definite thresholds for subcritical crack growth (SCG), but stage II was not always clearly delineated. In the unstable austenitic steel, AISI 301, the threshold stress intensity decreased with increasing hydrogen content or increasing temperature, but beyond about 10 wt ppm, it became insensitive to hydrogen concentration. At higher concentrations, stage II became less distinct. In the stable stainless steel, subcritical crack growth was observed only for a specimen containing 41 wt ppm hydrogen. Fractographic features were correlated with stress intensity, hydrogen content, and temperature. The fracture mode changed with temperature and hydrogen content. For unstable austenitic steel, low temperature and high hydrogen content favored intergranular fracture while microvoid coalescence dominated at a low hydrogen content. The interpretation of these phenomena is based on the tendency for stress-induced phase transformation, the different hydrogen diffusivity and solubility in ferrite and austenite, and outgassing from the crack tip. After comparing the embrittlement due to internal hydrogen with that in external hydrogen, it is concluded that the critical hydrogen distribution for the onset of subcritical crack growth is reached at a location that is very near the crack tip.

Failure Mechanisms and Life Prediction of Thermal and Environmental Barrier Coatings under Thermal Gradients

NASA Technical Reports Server (NTRS)

Zju, Dongming; Ghosn, Louis J.; Miller, Robert A.

2008-01-01

Ceramic thermal and environmental barrier coatings (TEBCs) will play an increasingly important role in gas turbine engines because of their ability to further raise engine temperatures. However, the issue of coating durability is of major concern under high-heat-flux conditions. In particular, the accelerated coating delamination crack growth under the engine high heat-flux conditions is not well understood. In this paper, a laser heat flux technique is used to investigate the coating delamination crack propagation under realistic temperature-stress gradients and thermal cyclic conditions. The coating delamination mechanisms are investigated under various thermal loading conditions, and are correlated with coating dynamic fatigue, sintering and interfacial adhesion test results. A coating life prediction framework may be realized by examining the crack initiation and propagation driving forces for coating failure under high-heat-flux test conditions.

Liquid metal embrittlement of T91 and 316L steels by heavy liquid metals: A fracture mechanics assessment

NASA Astrophysics Data System (ADS)

Auger, T.; Hamouche, Z.; Medina-Almazàn, L.; Gorse, D.

2008-06-01

LME of the martensitic T91 and the austenitic 316L steels have been investigated in the CCT geometry in the plane-stress condition. Using such a geometry, premature cracking induced by a liquid metal (PbBi and Hg) can be studied using a fracture mechanics approach based on CTOD, J-Δ a and fracture assessment diagram. One is able to measure a reduction of the crack tip blunting and a reduction of the energy required for crack propagation induced by the liquid metal. In spite of some limitations, this qualitative evaluation shows that liquid metals do not induce strong embrittlement on steels in plane-stress condition. Rather, the effect of the liquid metal seems to promote a fracture mode by plastic collapse linked with strain localization. It indicates that the materials, in spite of a potential embrittlement, should still be acceptable in terms of safety criteria.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lee, S. Y.; Choo, Hahn; Liaw, Peter K

The combined effects of overload-induced enlarged compressive residual stresses and crack tip blunting with secondary cracks are suggested to be responsible for the observed changes in the crack opening load and resultant post-overload transient crack growth behavior [Lee SY, Liaw PK, Choo H, Rogge RB, Acta Mater 2010;59:485-94]. In this article, in situ neutron diffraction experiments were performed to quantify the influence of the combined effects by investigating the internal-stress evolution at various locations away from the crack tip. In the overload-retardation period, stress concentration occurs in the crack blunting region (an overload point) until a maximum crack arrest loadmore » is reached. The stress concentration is then transferred from the blunting region to the propagating crack tip (following the overload), requiring a higher applied load, as the closed crack is gradually opened. The transfer phenomena of the stress concentration associated with a crack opening process account for the nonlinearity of strain response in the vicinity of the crack tip. The delaying action of stress concentration at the crack tip is understood in conjunction with the concept of a critical stress (i.e. the stress required to open the closed crack behind the crack tip). A linear relationship between {Delta}{var_epsilon}{sub eff} and {Delta}K{sub eff} provides experimental support for the hypothesis that {Delta}K{sub eff} can be considered as the fatigue crack tip driving force.« less

Experimental Study on the Growth, Coalescence and Wrapping Behaviors of 3D Cross-Embedded Flaws Under Uniaxial Compression

NASA Astrophysics Data System (ADS)

Zhou, Xiao-Ping; Zhang, Jian-Zhi; Wong, Louis Ngai Yuen

2018-05-01

The crack initiation, growth, wrapping and coalescence of two 3D pre-existing cross-embedded flaws in PMMA specimens under uniaxial compression are investigated. The stress-strain curves of PMMA specimens with 3D cross-embedded flaws are obtained. The tested PMMA specimens exhibit dominant elastic deformation and eventual brittle failure. The experimental results show that four modes of crack initiation and five modes of crack coalescence are observed. The initiations of oblique secondary crack and anti-wing crack in 3D cracking behaviors are first reported as well as the coalescence of anti-wing cracks. Moreover, two types of crack wrapping are found. Substantial wrapping of petal cracks, which includes open and closed modes of wrapping, appears to be the major difference between 2D and 3D cracking behaviors of pre-existing flaws, which are also first reported. Petal crack wraps symmetrically from either the propagated wing cracks or the coalesced wing cracks. Besides, only limited growth of petal cracks is observed, and ultimate failure of specimens is induced by the further growth of the propagated wing crack. The fracture mechanism of the tested PMMA specimens is finally revealed. In addition, the initiation stress and the peak stress versus the geometry of two 3D pre-existing cross-embedded flaws are also investigated in detail.

Microstructure and yield strength effects on hydrogen and tritium induced cracking in HERF (high-energy-rate-forged) stainless steel

DOE Office of Scientific and Technical Information (OSTI.GOV)

Morgan, M J; Tosten, M H

1989-01-01

Rising-load J-integral measurements and falling-load threshold stress intensity measurements were used to characterize hydrogen and tritium induced cracking in high-energy-rate-forged (HERF) 21-6-9 stainless steel. Samples having yield strengths in the range 517--930 MPa were thermally charged with either hydrogen or tritium and tested at room temperature in either air or high-pressure hydrogen gas. In general, the hydrogen isotopes reduced the fracture toughness by affecting the fracture process. Static recrystallization in the HERF microstructures affected the material's fracture toughness and its relative susceptibility to hydrogen and tritium induced fracture. In hydrogen-exposed samples, the reduction in fracture toughness was primarily dependent onmore » the susceptibility of the microstructure to intergranular fracture and only secondarily affected by strength in the range of 660 to 930 MPa. Transmission-electron microscopy observations revealed that the microstructures least susceptible to hydrogen-induced intergranular cracking contained patches of fully recrystallized grains. These grains are surrounded by highly deformed regions containing a high number density of dislocations. The microstructure can best be characterized as duplex'', with soft recrystallized grains embedded in a hard, deformed matrix. The microstructures most susceptible to hydrogen-induced intergranular fracture showed no well-developed recrystallized grains. The patches of recrystallized grains seemed to act as crack barriers to hydrogen-induced intergranular fracture. In tritium-exposed-and-aged samples, the amount of static recrystallization also affected the fracture toughness properties but to a lesser degree. 7 refs., 25 figs.« less

Concrete Behavior under Dynamic Tensile-Compressive Load.

DTIC Science & Technology

1984-01-01

be reviewed as well. Although structural concrete does not possess the thermal cracking problems during curing to the extent that mass concrete does...reasonable bounds for these unknown properties were assumed, suggests that the extent of cracking induced by seismic ground motion can be very...space. But an understanding of biaxial tension-compression be- havior is the foremost concern, since the stress state of a dam’s cracked regions occur in

Effect of water vapor on fatigue crack growth in 7475-T651 aluminum alloy plate. [for aerospace applications

NASA Technical Reports Server (NTRS)

Dicus, D. L.

1984-01-01

The effects of water vapor on fatigue crack growth in 7475-T651 aluminum alloy plate at frequencies of 1 Hz and 10 Hz were investigated. Twenty-five mm thick compact specimens were subjected to constant amplitude fatigue testing at a load ratio of 0.2. Fatigue crack growth rates were calculated from effective crack lengths determined using a compliance method. Tests were conducted in hard vacuum and at water vapor partial pressures ranging from 94 Pa to 3.8 kPa. Fatigue crack growth rates were frequency insensitive under all environment conditions tested. For constant stress intensity factor ranges crack growth rate transitions occurred at low and high water vapor pressures. Crack growth rates at intermediate pressures were relatively constant and showed reasonable agreement with published data for two Al-Cu-Mg alloys. The existence of two crack growth rate transitions suggests either a change in rate controlling kinetics or a change in corrosion fatigue mechanism as a function of water vapor pressure. Reduced residual deformation and transverse cracking specimens tested in water vapor versus vacuum may be evidence of embrittlement within the plastic zone due to environmental interaction.

The effect of water vapor on fatigue crack Growth in 7475-t651 aluminum alloy plate. [for aerospace applications

NASA Technical Reports Server (NTRS)

Dicus, D. L.

1982-01-01

The effects of water vapor on fatigue crack growth in 7475-T651 aluminum alloy plate at frequencies of 1 Hz and 10 Hz were investigated. Twenty-five mm thick compact specimens were subjected to constant amplitude fatigue testing at a load ratio of 0.2. Fatigue crack growth rates were calculated from effective crack lengths determined using a compliance method. Tests were conducted in hard vacuum and at water vapor partial pressures ranging from 94 Pa to 3.8 kPa. Fatigue crack growth rates were frequency insensitive under all environment conditions tested. For constant stress intensity factor ranges crack growth rate transitions occurred at low and high water vapor pressures. Crack growth rates at intermediate pressures were relatively constant and showed reasonable agreement with published data for two Al-Cu-Mg alloys. The existence of two crack growth rate transitions suggests either a change in rate controlling kinetics or a change in corrosion fatigue mechanism as a function of water vapor pressure. Reduced residual deformation and transverse cracking specimens tested in water vapor versus vacuum may be evidence of embrittlement within the plastic zone due to environmental interaction.

Stress Corrosion Cracking of Pipeline Steels in Fuel Grade Ethanol and Blends - Study to Evaluate Alternate Standard Tests and Phenomenological Understanding of SCC

DOT National Transportation Integrated Search

2011-10-30

Main aim of this project was to evaluate alternate standard test methods for stress corrosion cracking (SCC) and compare them with the results from slow strain rate test (SSRT) results under equivalent environmental conditions. Other important aim of...

Simulation Analysis of the Mutual Influence of the Stress Intensity Factor on the Multiple Blisters Caused by Hydrogen Induced Damage

NASA Astrophysics Data System (ADS)

Ji, Congwei; Zhang, Shaojie; Wang, Hehui

2018-03-01

Hydrogen blisters are taken as the research object by using the finite element software ABAQUS. The stress intensity factors of blister cracks are numerically calculated at varying depths and different edge distances for established three-dimensional finite element models of single-blister and double-blisters, respectively. The mutual influence of the stress intensity factors of the multiple blisters is obtained. It shows that the blister crack is easier to expand when the crack is closer to inner wall of the cylinder. What’s more, the crack growth rate increases firstly and then decreases as the increasing of the distance between two blisters cracks. The investigated result is of great reference value for predicting the trend of blister crack growth.

The release of trapped gases from amorphous solid water films. I. "Top-down" crystallization-induced crack propagation probed using the molecular volcano.

PubMed

May, R Alan; Smith, R Scott; Kay, Bruce D

2013-03-14

In this (Paper I) and the companion paper (Paper II; R. May, R. Smith, and B. Kay, J. Chem. Phys. 138, 104502 (2013)), we investigate the mechanisms for the release of trapped gases from underneath amorphous solid water (ASW) films. In prior work, we reported the episodic release of trapped gases in concert with the crystallization of ASW, a phenomenon that we termed the "molecular volcano." The observed abrupt desorption is due to the formation of cracks that span the film to form a connected pathway for release. In this paper, we utilize the "molecular volcano" desorption peak to characterize the formation of crystallization-induced cracks. We find that the crack length distribution is independent of the trapped gas (Ar, Kr, Xe, CH4, N2, O2, or CO). Selective placement of the inert gas layer is used to show that cracks form near the top of the film and propagate downward into the film. Isothermal experiments reveal that, after some induction time, cracks propagate linearly in time with an Arrhenius dependent velocity corresponding to an activation energy of 54 kJ∕mol. This value is consistent with the crystallization growth rates reported by others and establishes a direct connection between crystallization growth rate and the crack propagation rate. A two-step model in which nucleation and crystallization occurs in an induction zone near the top of the film followed by the propagation of a crystallization∕crack front into the film is in good agreement with the temperature programmed desorption results.

The Release of Trapped Gases from Amorphous Solid Water Films: I. “Top-Down” Crystallization-Induced Crack Propagation Probed using the Molecular Volcano

DOE Office of Scientific and Technical Information (OSTI.GOV)

May, Robert A.; Smith, R. Scott; Kay, Bruce D.

In this (Paper I) and the companion paper (Paper II) we investigate the mechanisms for the release of trapped gases from underneath of amorphous solid water (ASW) films. In prior work, we reported the episodic release of trapped gases in concert with the crystallization ASW, a phenomenon that we termed the "molecular volcano". The observed abrupt desorption is due to the formation of cracks that span the film to form a connected pathway for release. In this paper we utilize the "molecular volcano" desorption peak to characterize the formation of crystallization-induced cracks. We find that the crack length and distributionmore » are independent of the trapped gas (Ar, Kr, Xe, CH4, N2, O2 or CO). Selective placement of the inert gas layer is used to show that cracks form near the top of the film and propagate downward into the film. Isothermal experiments reveal that, after some induction time, cracks propagate linearly in time with an Arrhenius dependent velocity corresponding to an activation energy of 54 kJ/mol. This value is consistent with the crystallization growth rate reported by others and establishes a direct connection between crystallization growth rate and the crack propagation rate. A two-step model in which nucleation and crystallization occurs in an induction zone near the top of the film followed by the propagation of a crystallization/crack front into the film is in good agreement with the temperature programmed desorption results.« less

Path (un)predictability of two interacting cracks in polycarbonate sheets using Digital Image Correlation.

PubMed

Koivisto, J; Dalbe, M-J; Alava, M J; Santucci, S

2016-08-31

Crack propagation is tracked here with Digital Image Correlation analysis in the test case of two cracks propagating in opposite directions in polycarbonate, a material with high ductility and a large Fracture Process Zone (FPZ). Depending on the initial distances between the two crack tips, one may observe different complex crack paths with in particular a regime where the two cracks repel each other prior to being attracted. We show by strain field analysis how this can be understood according to the principle of local symmetry: the propagation is to the direction where the local shear - mode KII in fracture mechanics language - is zero. Thus the interactions exhibited by the cracks arise from symmetry, from the initial geometry, and from the material properties which induce the FPZ. This complexity makes any long-range prediction of the path(s) impossible.

Microwave detection of fatigue cracks in specially prepared steel specimens.

DOT National Transportation Integrated Search

1998-01-01

In the aging highway systems the problems of fatigue-induced damage and cracking in metal structures are very severe. Many such systems are operating even beyond their design lifetime, which requires more than the originally prescribed inspection cyc...

Biaxial fatigue crack propagation behavior of perfluorosulfonic-acid membranes

NASA Astrophysics Data System (ADS)

Lin, Qiang; Shi, Shouwen; Wang, Lei; Chen, Xu; Chen, Gang

2018-04-01

Perfluorosulfonic-acid membranes have long been used as the typical electrolyte for polymer-electrolyte fuel cells, which not only transport proton and water but also serve as barriers to prevent reactants mixing. However, too often the structural integrity of perfluorosulfonic-acid membranes is impaired by membrane thinning or cracks/pinholes formation induced by mechanical and chemical degradations. Despite the increasing number of studies that report crack formation, such as crack size and shape, the underlying mechanism and driving forces have not been well explored. In this paper, the fatigue crack propagation behaviors of Nafion membranes subjected to biaxial loading conditions have been investigated. In particular, the fatigue crack growth rates of flat cracks in responses to different loading conditions are compared, and the impact of transverse stress on fatigue crack growth rate is clarified. In addition, the crack paths for slant cracks under both uniaxial and biaxial loading conditions are discussed, which are similar in geometry to those found after accelerated stress testing of fuel cells. The directions of initial crack propagation are calculated theoretically and compared with experimental observations, which are in good agreement. The findings reported here lays the foundation for understanding of mechanical failure of membranes.

Expansive Soil Crack Depth under Cumulative Damage

PubMed Central

Shi, Bei-xiao; Chen, Sheng-shui; Han, Hua-qiang; Zheng, Cheng-feng

2014-01-01

The crack developing depth is a key problem to slope stability of the expansive soil and its project governance and the crack appears under the roles of dry-wet cycle and gradually develops. It is believed from the analysis that, because of its own cohesion, the expansive soil will have a certain amount of deformation under pulling stress but without cracks. The soil body will crack only when the deformation exceeds the ultimate tensile strain that causes cracks. And it is also believed that, due to the combined effect of various environmental factors, particularly changes of the internal water content, the inherent basic physical properties of expansive soil are weakened, and irreversible cumulative damages are eventually formed, resulting in the development of expansive soil cracks in depth. Starting from the perspective of volumetric strain that is caused by water loss, considering the influences of water loss rate and dry-wet cycle on crack developing depth, the crack developing depth calculation model which considers the water loss rate and the cumulative damages is established. Both the proposal of water loss rate and the application of cumulative damage theory to the expansive soil crack development problems try to avoid difficulties in matrix suction measurement, which will surely play a good role in promoting and improving the research of unsaturated expansive soil. PMID:24737974

Thermal behavior of crumb-rubber modified asphalt concrete mixtures

NASA Astrophysics Data System (ADS)

Epps, Amy Louise

Thermal cracking is one of the primary forms of distress in asphalt concrete pavements, resulting from either a single drop in temperature to an extreme low or from multiple temperature cycles above the fracture temperature of the asphalt-aggregate mixture. The first mode described is low temperature cracking; the second is thermal fatigue. The addition of crumb-rubber, manufactured from scrap tires, to the binder in asphalt concrete pavements has been suggested to minimize both types of thermal cracking. Four experiments were designed and completed to evaluate the thermal behavior of crumb-rubber modified (CRM) asphalt-aggregate mixtures. Modified and unmodified mixture response to thermal stresses was measured in four laboratory tests. The Thermal Stress Restrained Specimen Test (TSRST) and the Indirect Tensile Test (IDT) were used to compare mixture resistance to low temperature cracking. Modified mixtures showed improved performance, and cooling rate did not affect mixture resistance according to the statistical analysis. Therefore results from tests with faster rates can predict performance under slower field rates. In comparison, predicted fracture temperatures and stresses (IDT) were generally higher than measured values (TSRST). In addition, predicted fracture temperatures from binder test results demonstrated that binder testing alone is not sufficient to evaluate CRM mixtures. Thermal fatigue was explored in the third experiment using conventional load-induced fatigue tests with conditions selected to simulate daily temperature fluctuations. Test results indicated that thermal fatigue may contribute to transverse cracking in asphalt pavements. Both unmodified and modified mixtures had a finite capacity to withstand daily temperature fluctuations coupled with cold temperatures. Modified mixtures again exhibited improved performance. The fourth experiment examined fracture properties of modified and unmodified mixtures using a common fracture toughness test. Results showed no effect from modification, but the small experiment size may have masked this effect. Reliability concepts were introduced to include risk and uncertainty in a comparison of mixture response measured in the laboratory and estimated environmental conditions. This comparison provided evidence that CRM mixtures exhibit improved resistance to both types of thermal cracking at high levels of reliability. In conclusion, a mix design and analysis framework for evaluating thermal behavior was recommended.

Failure analysis of an aluminum alloy material framework component induced by casting defects

NASA Astrophysics Data System (ADS)

Li, Bo; Hu, Weiye

2017-09-01

Failure analysis on a fractured radome framework component was carried out through visual observations, metallographic examination using optical microscope, fractog-raphy inspections using scanning electron microscope and chemical composition analysis. The failed frame was made of casting Al-Si7-Mg0.4 aluminum alloy. It had suffered a former vi-bration performance tests. It was indicated that the fractures were attributed to fatigue cracks which were induced by casting porosities at the outer surfaces of frame. Failure analysis was carefully conducted for the semi-penetrating crack appearing on the framework. According to the fractography inspected by scanning electron microscope, it was indicated that numerous casting porosities at the outer surface of the framework played the role of multiple fracture sources due to some applied stresses. Optical microstructure observations suggested that the dendrite-shaped casting porosities largely contributed to the crack-initiation. The groove-shaped structure at roots of spatial convex-bodies on the edge of casting porosities supplied the preferred paths of the crack-propagation. Besides, the brittle silicon eutectic particles distrib-uting along grain boundaries induced the intergranular fracture mode in the region of the over-load final fracture surface.

Desiccation of a Sessile Drop of Blood: Cracks Formation and Delamination

NASA Astrophysics Data System (ADS)

Sobac, Benjamin; Brutin, David

2011-11-01

The evaporation of drops of biological fluids has been studied since few years du to several applications in medical fields such as medical tests, drug screening, biostabilization... The evaporation of a drop of whole blood leads to the formation of final typical pattern of cracks. Flow motion, adhesion, gelation and fracturation all occur during the evaporation of this complex matter. During the drying, a sol-gel transition develops. The drying kinetics is explained by a simple model of evaporation taking account of the evolution of the gelation front. The system solidifies and when stresses are too important, cracks nucleate. The cracks formation and the structure of the crack pattern are investigated. The initial crack spacing is found in good agreement with the implementation in open geometry of the model of cracks formation induced by evaporation proposed by Allain and Limat. Finally, the drop is still drying after the end of the formation of cracks which leads, like in the situation of colloid suspensions, to the observation of a delamination phenomenon.

Environmental controls on micro fracture processes in shelf ice

NASA Astrophysics Data System (ADS)

Sammonds, Peter

2013-04-01

The recent retreat and collapse of the ice shelves on the Antarctic Peninsula has been associated with regional atmospheric warming, oceanic warming, increased summer melt and shelf flexure. Although the cause of collapse is a matter of active discussion, the process is that of fracture of a creep-brittle material, close to its melting point. The environmental controls on how fracturing initiates, at a micro-scale, strongly determine the macroscopic disintegration of ice shelves. In particular the shelf temperature profile controls the plasticity of the ice shelf; the densification of shelf ice due to melting and re-freezing affects the crack tip stress intensity; the accretion of marine ice at the bottom of the shelf imposes a thermal/mechanical discontinuity; saline environments control crack tip stress corrosion; cyclic loading promotes sub-critical crack propagation. These strong environmental controls on shelf ice fracture means that assessing shelf stability is a non-deterministic problem. How these factors may be parameterized in ice shelf models, through the use of fracture mechanisms maps, is discussed. The findings are discussed in relation to the stability of Larsen C.

Computerized evaluation of holographic interferograms for fatigue crack detection in riveted lap joints

NASA Astrophysics Data System (ADS)

Zhou, Xiang

Using an innovative portable holographic inspection and testing system (PHITS) developed at the Australian Defence Force Academy, fatigue cracks in riveted lap joints can be detected by visually inspecting the abnormal fringe changes recorded on holographic interferograms. In this thesis, for automatic crack detection, some modern digital image processing techniques are investigated and applied to holographic interferogram evaluation. Fringe analysis algorithms are developed for identification of the crack-induced fringe changes. Theoretical analysis of PHITS and riveted lap joints and two typical experiments demonstrate that the fatigue cracks in lightly-clamped joints induce two characteristic fringe changes: local fringe discontinuities at the cracking sites; and the global crescent fringe distribution near to the edge of the rivet hole. Both of the fringe features are used for crack detection in this thesis. As a basis of the fringe feature extraction, an algorithm for local fringe orientation calculation is proposed. For high orientation accuracy and computational efficiency, Gaussian gradient filtering and neighboring direction averaging are used to minimize the effects of image background variations and random noise. The neighboring direction averaging is also used to approximate the fringe directions in centerlines of bright and dark fringes. Experimental results indicate that for high orientation accuracy the scales of the Gaussian filter and neighboring direction averaging should be chosen according to the local fringe spacings. The orientation histogram technique is applied to detect the local fringe discontinuity due to the fatigue cracks. The Fourier descriptor technique is used to characterize the global fringe distribution change from a circular to a crescent distribution with the fatigue crack growth. Experiments and computer simulations are conducted to analyze the detectability and reliability of crack detection using the two techniques. Results demonstrate that the Fourier descriptor technique is more promising in the detection of the short cracks near the edge of the rivet head. However, it is not as reliable as the fringe orientation technique for detection of the long through cracks. For reliability, both techniques should be used in practical crack detection. Neither the Fourier descriptor technique nor the orientation histogram technique have been previously applied to holographic interferometry. While this work related primarily to interferograms of cracked rivets, the techniques would be readily applied to other areas of fringe pattern analysis.

Peeling mechanism of tomato under infrared heating

USDA-ARS?s Scientific Manuscript database

Critical behaviors of peeling tomatoes using infrared heat are thermally induced peel loosening and subsequent cracking. However, the mechanism of peel loosening and cracking due to infrared heating remains unclear. This study aimed at investigating the mechanism of peeling tomatoes under infrared h...

Assessment of surface relief and short cracks under cyclic creep in a type 316LN austenitic stainless steel

NASA Astrophysics Data System (ADS)

Sarkar, Aritra; Nagesha, A.; Parameswaran, P.; Sandhya, R.; Laha, K.

2015-12-01

Formation of surface relief and short cracks under cyclic creep (stress-controlled fatigue) in type 316LN stainless steel was studied at temperatures ranging from ambient to 923 K using scanning electron microscopy technique. The surface topography and crack distribution behaviour under cyclic creep were found to be strong functions of testing temperature due to the difference in strain accumulation. At 823 K, surface relief mainly consisted of fine slip markings due to negligible accumulation of strain as a consequence of dynamic strain ageing (DSA) which led to an increase in the cyclic life. Persistent slip markings (PSM) with distinct extrusions containing minute cracks were seen to prevail in the temperature range 873-923 K, indicating a higher slip activity causing higher strain accumulation in the absence of DSA. Besides, a large number of secondary cracks (both transgranular and intergranular) which were partially accentuated by severe oxidation, were observed. Extensive cavitation-induced grain boundary cracking took place at 923 K, which coalesced with PSM-induced transgranular cracks resulting in failure dominated by creep that in turn led to a drastic reduction in cyclic life. Investigations on the influence of stress rate were also carried out which underlined the presence of DSA at 823 K. At 923 K, lowering the stress rate caused further strengthening of the contribution from creep damage marked by a shift in the damage mechanism from cyclic slip to diffusion.

Self-healing of cracks formed in Silicon-Aluminum anodes electrochemically cycled at high lithiation rates

NASA Astrophysics Data System (ADS)

Bhattacharya, Sandeep; Alpas, Ahmet T.

2016-10-01

Lithiation-induced volume changes in Si result in fracture and fragmentation of Si anodes in Li-ion batteries. This paper reports the self-healing behaviour of cracks observed in micron-sized Si particles dispersed in a ductile Al matrix of a Si-Al electrode electrochemically cycled vs. Li/Li+ using a high lithiation rate of 15.6 C. Cross-sectional high-resolution transmission electron microscopy and Raman spectroscopy revealed that an amorphous layer with a depth up to ∼100 nm was formed at the surface of Si particles. In-situ optical microscopy performed during electrochemical experiments revealed development of cracks in Si particles as the voltage decreased to 0.02 V during lithiation. Self-healing of cracks in Si particles occurred in two steps: i) arresting of the crack growth at the Si/Al interface as the surrounding Al matrix had a higher fracture toughness and thus acted as a barrier to crack propagation, and ii) closure of cracks due to compressive stresses applied to the crack faces by the amorphous zones formed on each side of the crack paths.

Fatigue-Life Prediction Methodology Using Small-Crack Theory

NASA Technical Reports Server (NTRS)

Newmann, James C., Jr.; Phillips, Edward P.; Swain, M. H.

1997-01-01

This paper reviews the capabilities of a plasticity-induced crack-closure model to predict fatigue lives of metallic materials using 'small-crack theory' for various materials and loading conditions. Crack-tip constraint factors, to account for three-dimensional state-of-stress effects, were selected to correlate large-crack growth rate data as a function of the effective-stress-intensity factor range (delta K(eff)) under constant-amplitude loading. Some modifications to the delta k(eff)-rate relations were needed in the near-threshold regime to fit measured small-crack growth rate behavior and fatigue endurance limits. The model was then used to calculate small- and large-crack growth rates, and to predict total fatigue lives, for notched and un-notched specimens made of two aluminum alloys and a steel under constant-amplitude and spectrum loading. Fatigue lives were calculated using the crack-growth relations and microstructural features like those that initiated cracks for the aluminum alloys and steel for edge-notched specimens. An equivalent-initial-flaw-size concept was used to calculate fatigue lives in other cases. Results from the tests and analyses agreed well.

Quantification of fatigue cracking in CT specimens with passive and active piezoelectric sensing

NASA Astrophysics Data System (ADS)

Yu, Jianguo; Ziehl, Paul; Zarate, Boris; Caicedo, Juan; Yu, Lingyu; Giurgiutiu, Victor; Metrovich, Brian; Matta, Fabio

2010-04-01

Monitoring of fatigue cracks in steel bridges is of interest to bridge owners and agencies. Monitoring of fatigue cracks has been attempted with acoustic emission using either resonant or broadband sensors. One drawback of passive sensing is that the data is limited to that caused by growing cracks. In this work, passive emission was complemented with active sensing (piezoelectric wafer active sensors) for enhanced detection capabilities. Passive and active sensing methods were described for fatigue crack monitoring on specialized compact tension specimens. The characteristics of acoustic emission were obtained to understand the correlation of acoustic emission behavior and crack growth. Crack and noise induced signals were interpreted through Swansong II Filter and waveform-based approaches, which are appropriate for data interpretation of field tests. Upon detection of crack extension, active sensing was activated to measure the crack size. Model updating techniques were employed to minimize the difference between the numerical results and experimental data. The long term objective of this research is to develop an in-service prognostic system to monitor structural health and to assess the remaining fatigue life.

Experimental simulation of frost wedging-induced crack propagation in alpine rockwall

NASA Astrophysics Data System (ADS)

Jia, Hailiang; Leith, Kerry; Krautblatter, Michael

2016-04-01

Frost wedging is widely presumed to be the principal mechanism responsible for shattering jointed low-porosity rocks in high alpine rockwalls. The interaction of ice and rock physics regulates the efficacy of frost wedging. In order to better understand temporal aspects of this interaction, we present results of a series of laboratory experiments monitoring crack widening as a result of ice formation in an artificial crack (4mm wide, 80mm deep) cut 20 mm from the end of a rectangular granite block. Our results indicate that i) freezing direction plays a key role in determining the magnitude of crack widening; in short-term (1 day) experiments, maximum crack widening during top-down freezing (associated with 'autumn' conditions) was around 0.11mm, while inside-out freezing (resulting from 'spring' conditions) produced only 0.02 mm of deformation; ii) neither ice, nor water pressure (direct tension and hydraulic fracturing respectively) caused measurable irreversible crack widening during short-term tests, as the calculated maximum stress intensity at the crack tip was less than the fracture toughness of our granite sample; iii) development of ice pressure is closely related to the mechanical properties of the fracture in which it forms, and as such, the interaction of ice and rock is intrinsically dynamic; iv) irreversible crack widening (about 0.03mm) was only observed following a long-term (53 day) experiment representing a simplified transition from autumn to winter conditions. We suggest this is the result of stress corrosion aided by strong opening during freezing, and to a lesser degree by ice segregation up to one week after the initial freezing period, and downward migration of liquid water during the remainder of the test. Our results suggest the fundamental assumption of frost wedging, that rapid freezing from open ends of cracks can seal water inside the crack and thus cause damage through excessive stresses induced by volumetric expansion seems questionable.

Understanding breakage in curly hair.

PubMed

Camacho-Bragado, G A; Balooch, G; Dixon-Parks, F; Porter, C; Bryant, H

2015-07-01

In 2005, the L'Oréal Institute for hair and skin research carried out a multiethnic study to investigate hair breakage in women residing in the U.S.A. In this study it was reported that a large percentage (96%) of the African-American respondents experience breakage. A combination of structural differences and grooming-induced stresses seem to contribute to the higher breakage incidence in the African-American group as the chemical composition of African-American hair is not significantly different from other ethnic groups. Some authors have proposed that the repeated elongation, torsion and flexion actions may affect the components of the hair fibre. However, considering the different properties of cuticle and cortex, one would expect a different wearing mechanism of each, leading to the ultimate failure of hair. Knowing in detail how each part of the structure fails can potentially lead to better ways to protect the hair from physical insults. To investigate crack propagation and fracture mechanisms in African-American hair. Virgin hair of excellent quality was collected, with informed consent, from a female African-American volunteer. A series of controlled mechanical stresses was applied to 10-mm hair sections using a high-resolution mechanical stage (20 mN) up to the fracture of the fibre. The surface was monitored using scanning electron microscopy imaging during the stress application. X-ray tomographic microscopy images were acquired and quantified to detect changes in energy absorption as a function of applied stress that could be linked to increase in crack density. Analysis of the mechanical response of hair combined with the two imaging techniques led us to propose the following mechanism of hair breakage: cuticle sliding; failure of the cuticle-cortex interface; nucleation of intercellular cracks and growth of cracks at the cuticle-cortex junction; and propagation of intercellular cracks towards the surface of the hair and final breakage when these cracks merge at the cuticular junction. The combination of scanning electron microscopy and X-ray tomography provided new information about the fracture of hair. Mechanical damage from grooming and some environmental factors accumulate in hair creating internal cracks that eventually result in breakage at unpredictable sites and therefore a continuous care regimen for the hair throughout the life cycle of the fibres is recommended. © 2015 The Authors BJD © 2015 British Association of Dermatologists.

Features of CO2 fracturing deduced from acoustic emission and microscopy in laboratory experiments

NASA Astrophysics Data System (ADS)

Ishida, Tsuyoshi; Chen, Youqing; Bennour, Ziad; Yamashita, Hiroto; Inui, Shuhei; Nagaya, Yuya; Naoi, Makoto; Chen, Qu; Nakayama, Yoshiki; Nagano, Yu

2016-11-01

We conducted hydraulic fracturing (HF) experiments on 170 mm cubic granite specimens with a 20 mm diameter central hole to investigate how fluid viscosity affects HF process and crack properties. In experiments using supercritical carbon dioxide (SC-CO2), liquid carbon dioxide (L-CO2), water, and viscous oil with viscosity of 0.051-336.6 mPa · s, we compared the results for breakdown pressure, the distribution and fracturing mechanism of acoustic emission, and the microstructure of induced cracks revealed by using an acrylic resin containing a fluorescent compound. Fracturing with low-viscosity fluid induced three-dimensionally sinuous cracks with many secondary branches, which seem to be desirable pathways for enhanced geothermal system, shale gas recovery, and other processes.

Study of Solidification Cracking in a Transformation-Induced Plasticity-Aided Steel

NASA Astrophysics Data System (ADS)

Agarwal, G.; Kumar, A.; Gao, H.; Amirthalingam, M.; Moon, S. C.; Dippenaar, R. J.; Richardson, I. M.; Hermans, M. J. M.

2018-04-01

In situ high-temperature laser scanning confocal microscopy is applied to study solidification cracking in a TRIP steel. Solidification cracking was observed in the interdendritic region during the last stage of solidification. Atom probe tomography revealed notable enrichment of phosphorus in the last remaining liquid. Phase field simulations also confirm phosphorus enrichment leading to severe undercooling of more than 160 K in the interdendritic region. In the presence of tensile stress, an opening at the interdendritic region is difficult to fill with the remaining liquid due to low permeability and high viscosity, resulting in solidification cracking.

Fatigue Crack Growth in Peened Friction Stir Welds

NASA Technical Reports Server (NTRS)

Forth, Scott C.; Hatamleh, Omar

2008-01-01

Friction stir welding induces residual stresses that accelerates fatigue crack growth in the weld nugget. Shot peening over the weld had little effect on growth rate. Laser peening over the weld retarded the growth rate: Final crack growth rate was comparable to the base, un-welded material. Crack tunneling evident from residual compressive stresses. 2195-T8 fracture surfaces were highly textured. Texturing makes comparisons difficult as the material system is affecting the data as much as the processing. Material usage becoming more common in space applications requiring additional work to develop useful datasets for damage tolerance analyses.

Fracture Characteristics of Monolayer CVD-Graphene

PubMed Central

Hwangbo, Yun; Lee, Choong-Kwang; Kim, Sang-Min; Kim, Jae-Hyun; Kim, Kwang-Seop; Jang, Bongkyun; Lee, Hak-Joo; Lee, Seoung-Ki; Kim, Seong-Su; Ahn, Jong-Hyun; Lee, Seung-Mo

2014-01-01

We have observed and analyzed the fracture characteristics of the monolayer CVD-graphene using pressure bulge testing setup. The monolayer CVD-graphene has appeared to undergo environmentally assisted subcritical crack growth in room condition, i.e. stress corrosion cracking arising from the adsorption of water vapor on the graphene and the subsequent chemical reactions. The crack propagation in graphene has appeared to be able to be reasonably tamed by adjusting applied humidity and stress. The fracture toughness, describing the ability of a material containing inherent flaws to resist catastrophic failure, of the CVD-graphene has turned out to be exceptionally high, as compared to other carbon based 3D materials. These results imply that the CVD-graphene could be an ideal candidate as a structural material notwithstanding environmental susceptibility. In addition, the measurements reported here suggest that specific non-continuum fracture behaviors occurring in 2D monoatomic structures can be macroscopically well visualized and characterized. PMID:24657996

Environmental crack-growth behavior of high strength pressure vessel alloys

NASA Technical Reports Server (NTRS)

Forman, R. G.

1975-01-01

Results of sustained-load environmental crack growth threshold tests performed on six spacecraft pressure vessel alloys are presented. The alloys were Inconel 718, 6Al-4V titanium, A-286 steel, AM-350 stainless steel, cryoformed AISI 301 stainless steel; and cryoformed AISI 304L steel. The test environments for the program were air, pressurized gases of hydrogen, oxygen, nitrogen, and carbon dioxide, and liquid environments of distilled water, sea water, nitrogen tetroxide, hydrazine, aerozine 50, monomethyl hydrazine, and hydrogen peroxide. Surface flaw type specimens were used with flaws located in both base metal and weld metal.

Current NRC Perspectives Concerning Primary Water Stress Corrosion Cracking

NASA Astrophysics Data System (ADS)

Alley, David; Dunn, Darrell

Materials currently used in nuclear power plants are reliable and are generally resistant to environmental degradation. However, occurrences of environmental degradation have been observed as the current fleet of reactors ages. Primary water stress corrosion cracking (PWSCC) is of particular interest to the US Nuclear Regulatory Commission (NRC). This paper provides a historical assessment of operating experience associated with PWSCC and welding issues associated with PWSCC resistant materials. The paper also considers the regulatory issues associated with PWSCC, especially those associated with gaps in the understanding of the behavior of PWSCC resistant material under actual reactor conditions.

Corrosion Effects on the Fatigue Crack Propagation of Giga-Grade Steel and its Heat Affected Zone in pH Buffer Solutions for Automotive Application

NASA Astrophysics Data System (ADS)

Lee, H. S.

2018-03-01

Corrosion fatigue crack propagation test was conducted of giga-grade steel and its heat affected zone in pH buffer solutions, and the results were compared with model predictions. Pure corrosion effect on fatigue crack propagation, particularly, in corrosive environment was evaluated by means of the modified Forman equation. As shown in results, the average corrosion rate determined from the ratio of pure corrosion induced crack length to entire crack length under a cycle load were 0.11 and 0.37 for base metal and heat affected zone, respectively, with load ratio of 0.5, frequency of 0.5 and pH 10.0 environment. These results demonstrate new interpretation methodology for corrosion fatigue crack propagation enabling the pure corrosion effects on the behavior to be determined.

A method for evaluating the fatigue crack growth in spiral notch torsion fracture toughness test

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wang, Jy -An John; Tan, Ting

The spiral notch torsion test (SNTT) has been a recent breakthrough in measuring fracture toughness for different materials, including metals, ceramics, concrete, and polymers composites. Due to its high geometry constraint and unique loading condition, SNTT can be used to measure the fracture toughness with smaller specimens without concern of size effects. The application of SNTT to brittle materials has been proved to be successful. The micro-cracks induced by original notches in brittle materials could ensure crack growth in SNTT samples. Therefore, no fatigue pre-cracks are needed. The application of SNTT to the ductile material to generate valid toughness datamore » will require a test sample with sufficient crack length. Fatigue pre-crack growth techniques are employed to introduce sharp crack front into the sample. Previously, only rough calculations were applied to estimate the compliance evolution in the SNTT crack growth process, while accurate quantitative descriptions have never been attempted. This generates an urgent need to understand the crack evolution during the SNTT fracture testing process of ductile materials. Here, the newly developed governing equations for SNTT crack growth estimate are discussed in the paper.« less

Analyses of Fatigue and Fatigue-Crack Growth under Constant- and Variable-Amplitude Loading

NASA Technical Reports Server (NTRS)

Newman, J. C., Jr.

1999-01-01

Studies on the growth of small cracks have led to the observation that fatigue life of many engineering materials is primarily crack growth from micro-structural features, such as inclusion particles, voids, slip-bands or from manufacturing defects. This paper reviews the capabilities of a plasticity-induced crack-closure model to predict fatigue lives of metallic materials using small-crack theory under various loading conditions. Constraint factors, to account for three-dimensional effects, were selected to correlate large-crack growth rate data as a function of the effective stress-intensity factor range (delta K(sub eff)) under constant-amplitude loading. Modifications to the delta K(sub eff)-rate relations in the near-threshold regime were needed to fit measured small-crack growth rate behavior. The model was then used to calculate small- and large-crack growth rates, and to predict total fatigue lives, for notched and un-notched specimens under constant-amplitude and spectrum loading. Fatigue lives were predicted using crack-growth relations and micro-structural features like those that initiated cracks in the fatigue specimens for most of the materials analyzed. Results from the tests and analyses agreed well.

A method for evaluating the fatigue crack growth in spiral notch torsion fracture toughness test

DOE PAGES

Wang, Jy -An John; Tan, Ting

2018-05-21

The spiral notch torsion test (SNTT) has been a recent breakthrough in measuring fracture toughness for different materials, including metals, ceramics, concrete, and polymers composites. Due to its high geometry constraint and unique loading condition, SNTT can be used to measure the fracture toughness with smaller specimens without concern of size effects. The application of SNTT to brittle materials has been proved to be successful. The micro-cracks induced by original notches in brittle materials could ensure crack growth in SNTT samples. Therefore, no fatigue pre-cracks are needed. The application of SNTT to the ductile material to generate valid toughness datamore » will require a test sample with sufficient crack length. Fatigue pre-crack growth techniques are employed to introduce sharp crack front into the sample. Previously, only rough calculations were applied to estimate the compliance evolution in the SNTT crack growth process, while accurate quantitative descriptions have never been attempted. This generates an urgent need to understand the crack evolution during the SNTT fracture testing process of ductile materials. Here, the newly developed governing equations for SNTT crack growth estimate are discussed in the paper.« less

Determining the limitations of warm mix asphalt by water injection in mix design, quality control and placement.

DOT National Transportation Integrated Search

2013-08-01

In this project, a comprehensive study was conducted to evaluate the laboratory performance of foamed WMA mixtures with regard to permanent deformation, moisture-induced damage, fatigue cracking, and low-temperature (thermal) cracking; and compare it...

Crack growth induced by thermal-mechanical loading

NASA Astrophysics Data System (ADS)

John, R.; Hartman, G. A.; Gallagher, J. P.

1992-06-01

Advanced aerospace structures are often subjected to combined thermal and mechanical loads. The fracture-mechanics behavior of the structures may be altered by the thermal state existing around the crack. Hence, design of critical structural elements requires the knowledge of stress-intensity factors under both thermal and mechanical loads. This paper describes the development of an experimental technique to verify the thermal-stress-intensity factor generated by a temperature gradient around the crack. Thin plate specimens of a model material (AISI-SAE 1095 steel) were used for the heat transfer and thermal-mechanical fracture tests. Rapid thermal loading was achieved using high-intensity focused infrared spot heaters. These heaters were also used to generate controlled temperature rates for heat-transfer verification tests. The experimental results indicate that thermal loads can generate stress-intensity factors large enough to induce crack growth. The proposed thermal-stress-intensity factors appear to have the same effect as the conventional mechanical-stress-intensity factors with respect to fracture.

Characterization of Residual Stress Effects on Fatigue Crack Growth of a Friction Stir Welded Aluminum Alloy

NASA Technical Reports Server (NTRS)

Newman, John A.; Smith, Stephen W.; Seshadri, Banavara R.; James, Mark A.; Brazill, Richard L.; Schultz, Robert W.; Donald, J. Keith; Blair, Amy

2015-01-01

An on-line compliance-based method to account for residual stress effects in stress-intensity factor and fatigue crack growth property determinations has been evaluated. Residual stress intensity factor results determined from specimens containing friction stir weld induced residual stresses are presented, and the on-line method results were found to be in excellent agreement with residual stress-intensity factor data obtained using the cut compliance method. Variable stress-intensity factor tests were designed to demonstrate that a simple superposition model, summing the applied stress-intensity factor with the residual stress-intensity factor, can be used to determine the total crack-tip stress-intensity factor. Finite element, VCCT (virtual crack closure technique), and J-integral analysis methods have been used to characterize weld-induced residual stress using thermal expansion/contraction in the form of an equivalent delta T (change in local temperature during welding) to simulate the welding process. This equivalent delta T was established and applied to analyze different specimen configurations to predict residual stress distributions and associated residual stress-intensity factor values. The predictions were found to agree well with experimental results obtained using the crack- and cut-compliance methods.

Giant electrocaloric effect in a cracked ferroelectrics

NASA Astrophysics Data System (ADS)

Huang, Cheng; Yang, Hai-Bing; Gao, Cun-Fa

2018-04-01

The electrocaloric effect (ECE) is the temperature change in a material induced by electrical field variation under adiabatic condition. Considering an external electric load applied on a cracked ferroelectric solid, a non-uniform electric field would be induced at the crack tip, and thus, incompatible strain field and local stress concentration would be generated around it. Furthermore, the enormous strain energy and the electrostatic energy would affect the polarization switching of the ferroelectric solid, important for the electrocaloric response. In this paper, the large negative and positive ECEs in a ferroelectric sheet with a conducting crack are investigated by the phase field method with the consideration of time-dependent Ginzburg-Landau equation. The numerical calculations indicated that the polarization field generates a sharp rise during the domain transition from polydomain to monodomain under a certain electric load. Large negative ECEs, about -10.21 K and -7.55 K, are obtained at 135 °C and 85 °C, respectively. The domain transition temperature is much lower than the Curie temperature, which enlarges the existence scope of the large ECE in ferroelectrics. The results also imply that the domain transition from a multi-domain state to a single domain takes place with the minimization of total free energy, which involves the courses of the electric field, stress field, temperature, and polarization interaction. Therefore, the non-uniform distributions of the stress-electric fields induced by the crack play an important role in ECE.

Recrystallization-Induced Surface Cracks of Carbon Ions Irradiated 6H-SiC after Annealing.

PubMed

Ye, Chao; Ran, Guang; Zhou, Wei; Shen, Qiang; Feng, Qijie; Lin, Jianxin

2017-10-25

Single crystal 6H-SiC wafers with 4° off-axis [0001] orientation were irradiated with carbon ions and then annealed at 900 °C for different time periods. The microstructure and surface morphology of these samples were investigated by grazing incidence X-ray diffraction (GIXRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Ion irradiation induced SiC amorphization, but the surface was smooth and did not have special structures. During the annealing process, the amorphous SiC was recrystallized to form columnar crystals that had a large amount of twin structures. The longer the annealing time was, the greater the amount of recrystallized SiC would be. The recrystallization volume fraction was accorded with the law of the Johnson-Mehl-Avrami equation. The surface morphology consisted of tiny pieces with an average width of approximately 30 nm in the annealed SiC. The volume shrinkage of irradiated SiC layer and the anisotropy of newly born crystals during annealing process produced internal stress and then induced not only a large number of dislocation walls in the non-irradiated layer but also the initiation and propagation of the cracks. The direction of dislocation walls was perpendicular to the growth direction of the columnar crystal. The longer the annealing time was, the larger the length and width of the formed crack would be. A quantitative model of the crack growth was provided to calculate the length and width of the cracks at a given annealing time.

A new thermo-mechanical coupled DEM model with non-spherical grains for thermally induced damage of rocks

NASA Astrophysics Data System (ADS)

Chen, Zhiqiang; Jin, Xu; Wang, Moran

2018-07-01

Thermally induced damage often occurs in rocks in geophysical systems. Discrete element method (DEM) is a useful tool to model this thermo-mechanical coupled process owing to its explicit representation of fracture initiation and propagation. However, the previous DEM models for this are mostly based on spherical discrete elements, which are not able to capture all consequences (e.g. high ratio of compressive to tensile strength) of real rocks (e.g. granite) composed of complex-geometry grains. In order to overcome this intrinsic limitation, we present a new model allowing to mimick thermally induced damage of brittle rock with non-spherical grains. After validations, the new model is used to study thermal gradient cracking with a special emphasis on the effects from rock heterogeneity. The obtained fracture initiation and propagation are consistent with experimental observations, which demonstrates the ability of current model to reproduce the thermally induced damage of rocks. Meanwhile, the results show that rock heterogeneity influences thermal gradient cracking significantly, and more micro cracks uniformly scattering around the borehole are induced in the heterogeneous sample, which is not good for applications such as nuclear waste disposal. The present model provides a promising approach at micro-scale to explore mechanisms of thermally induced damage of rocks in geological engineering.

Effects of Residual Impurities on Hydrogen Assisted Cracking in High Strength Steels. Part II.

DTIC Science & Technology

1982-06-01

source of hydrogen is the corrosion reaction of steel with aqueous hydrogen sulfide solutions encountered either in the production of crude oil and...autoradiography technique, it has been shown that in Armco iron and in maraging steel of hydrogen is trapped at prior austenite grain boundaries. Tritium...also play a deleterious role in hydrogen-induced cracking. In these ultra-high strength steels , the crack-tip stress level and the concomitant stress

Final Report: Multi-Scale Analysis of Deformation and Failure in Polycrystalline Titanium Alloys Under High Strain-Rates

DTIC Science & Technology

2015-12-28

Masoud Anahid, Mahendra K. Samal , and Somnath Ghosh. Dwell fatigue crack nucleation model based on crystal plasticity finite element simulations of...induced crack nucleation in polycrystals. Model. Simul. Mater. Sci. Eng., 17, 064009. 19. Anahid, M., Samal , M. K. & Ghosh, S. (2011). Dwell fatigue...Jour. Plas., 24:428–454, 2008. 4. M. Anahid, M. K. Samal , and S. Ghosh. Dwell fatigue crack nucleation model based on crystal plasticity finite

Laser cutting sandwich structure glass-silicon-glass wafer with laser induced thermal-crack propagation

NASA Astrophysics Data System (ADS)

Cai, Yecheng; Wang, Maolu; Zhang, Hongzhi; Yang, Lijun; Fu, Xihong; Wang, Yang

2017-08-01

Silicon-glass devices are widely used in IC industry, MEMS and solar energy system because of their reliability and simplicity of the manufacturing process. With the trend toward the wafer level chip scale package (WLCSP) technology, the suitable dicing method of silicon-glass bonded structure wafer has become necessary. In this paper, a combined experimental and computational approach is undertaken to investigate the feasibility of cutting the sandwich structure glass-silicon-glass (SGS) wafer with laser induced thermal-crack propagation (LITP) method. A 1064 nm semiconductor laser cutting system with double laser beams which could simultaneously irradiate on the top and bottom of the sandwich structure wafer has been designed. A mathematical model for describing the physical process of the interaction between laser and SGS wafer, which consists of two surface heating sources and two volumetric heating sources, has been established. The temperature stress distribution are simulated by using finite element method (FEM) analysis software ABAQUS. The crack propagation process is analyzed by using the J-integral method. In the FEM model, a stationary planar crack is embedded in the wafer and the J-integral values around the crack front edge are determined using the FEM. A verification experiment under typical parameters is conducted and the crack propagation profile on the fracture surface is examined by the optical microscope and explained from the stress distribution and J-integral value.

Sub-10-micrometer toughening and crack tip toughness of dental enamel.

PubMed

Ang, Siang Fung; Schulz, Anja; Pacher Fernandes, Rodrigo; Schneider, Gerold A

2011-04-01

In previous studies, enamel showed indications to occlude small cracks in-vivo and exhibited R-curve behaviors for bigger cracks ex-vivo. This study quantifies the crack tip's toughness (K(I0),K(III0)), the crack's closure stress and the cohesive zone size at the crack tip of enamel and investigates the toughening mechanisms near the crack tip down to the length scale of a single enamel crystallite. The crack-opening-displacement (COD) profile of cracks induced by Vickers indents on mature bovine enamel was studied using atomic force microscopy (AFM). The mode I crack tip toughness K(I0) of cracks along enamel rod boundaries and across enamel rods exhibit a similar range of values: K(I0,Ir)=0.5-1.6MPa m(0.5) (based on Irwin's 'near-field' solution) and K(I0,cz)=0.8-1.5MPa m(0.5) (based on the cohesive zone solution of the Dugdale-Muskhelishvili (DM) crack model). The mode III crack tip toughness K(III0,Ir) was computed as 0.02-0.15MPa m(0.5). The crack-closure stress at the crack tip was computed as 163-770 MPa with a cohesive zone length and width 1.6-10.1μm and 24-44 nm utilizing the cohesive zone solution. Toughening elements were observed under AFM and SEM: crack bridging due to protein ligament and hydroxyapatite fibres (micro- and nanometer scale) as well as microcracks were identified. Crown Copyright © 2010. Published by Elsevier Ltd. All rights reserved.

Fatigue crack growth behavior of pressure vessel steels and submerged arc weldments in a high-temperature pressurized water environment

NASA Astrophysics Data System (ADS)

Liaw, P. K.; Logsdon, W. A.; Begley, J. A.

1989-10-01

The fatigue crack growth rate (FCGR) properties of SA508 C1 2a and SA533 Gr A C1 2 pressure vessel steels and the corresponding automatic submerged are weldments were developed in a high-temperature pressurized water (HPW) environment at 288 °C (550°F) and 7.2 MPa (1044 psi) at load ratios of 0.02 and 0.50. The HPW enviromment FCGR properties of these pressure vessel steels and submerged arc weldments were generally conservative, compared with the approrpriate American Society of Mechanical Engineers (ASME) Section XI water environmental reference curve. The growth rate of fatigue cracks in the base materials, however, was considerably faster in the HPW environment than in a corresponding 288°C (550°F) base line air environment. The growth rate of fatigue cracks in the two submerged are weldments was also accelerated in the HPW environment but to a significantly lesser degree than that demonstrated by the corresponding base materials. In the air environment, fatigue striations were observed, independent of material and load ratio, while in the HPW environment, some intergranular facets were present. The greater environmental effect on crack growth rates displayed by the base materials, as compared with the weldments, was attributed to a different sulfide composition and morphology.

Spatially resolved, in-situ monitoring of crack growth via the coupling current in aluminum alloy 5083

NASA Astrophysics Data System (ADS)

Williams, Krystaufeux D.

The work discussed in this dissertation is an experimental validation of a body of research that was created to model stress corrosion cracking phenomenon for 304 stainless steels in boiling water reactors. This coupled environment fracture model (CEFM) incorporates the natural laws of the conservation of charge and the differential aeration hypothesis to predict the amount of stress corrosion crack growth as a function of many external environmental variables, including potential, stress intensity, solution conductivity, oxidizer concentrations, and various other environmental parameters. Out of this approach came the concept of the coupling current; a local corrosion current that flows from within cracks, crevices, pits, etc... of a metal or alloy to the external surface. Because of the deterministic approach taken in the mentioned research, the coupling current analysis and CEFM model can be applied to the specific problem of SCC in aluminum alloy 5083 (the alloy of interest for this dissertation that is highly sought after today because of its corrosion resistance and high strength to weight ratio). This dissertation research is specifically devoted to the experimental verification of the coupling current, which results from a coupling between the crack's internal and external environments, by spatially resolving them using the scanning vibrating probe (SVP) as a tool. Hence, through the use of a unique fracture mechanics setup, simultaneous mechanical and local electrochemical data may be obtained, in situ..

Fatigue crack monitoring with coupled piezoelectric film acoustic emission sensors

NASA Astrophysics Data System (ADS)

Zhou, Changjiang

Fatigue-induced cracking is a commonly seen problem in civil infrastructures reaching their original design life. A number of high-profile accidents have been reported in the past that involved fatigue damage in structures. Such incidences often happen without prior warnings due to lack of proper crack monitoring technique. In order to detect and monitor the fatigue crack, acoustic emission (AE) technique, has been receiving growing interests recently. AE can provide continuous and real-time monitoring data on damage progression in structures. Piezoelectric film AE sensor measures stress-wave induced strain in ultrasonic frequency range and its feasibility for AE signal monitoring has been demonstrated recently. However, extensive work in AE monitoring system development based on piezoelectric film AE sensor and sensor characterization on full-scale structures with fatigue cracks, have not been done. A lack of theoretical formulations for understanding the AE signals also hinders the use of piezoelectric film AE sensors. Additionally, crack detection and source localization with AE signals is a very important area yet to be explored for this new type of AE sensor. This dissertation presents the results of both analytical and experimental study on the signal characteristics of surface stress-wave induced AE strain signals measured by piezoelectric film AE sensors in near-field and an AE source localization method based on sensor couple theory. Based on moment tensor theory, generalized expression for AE strain signal is formulated. A special case involving the response of piezoelectric film AE sensor to surface load is also studied, which could potentially be used for sensor calibration of this type of sensor. A new concept of sensor couple theory based AE source localization technique is proposed and validated with both simulated and experimental data from fatigue test and field monitoring. Two series of fatigue tests were conducted to perform fatigue crack monitoring on large-scale steel test specimens using piezoelectric film AE sensors. Continuous monitoring of fatigue crack growth in steel structures is demonstrated in these fatigue test specimens. The use of piezoelectric film AE sensor for field monitoring of existing fatigue crack is also demonstrated in a real steel I-girder bridge located in Maryland. The sensor couple theory based AE source localization is validated using a limited number of piezoelectric film AE sensor data from both fatigue test specimens and field monitoring bridge. Through both laboratory fatigue test and field monitoring of steel structures with active fatigue cracks, the signal characteristics of piezoelectric film AE sensor have been studied in real-world environment.

Detection and sizing of cracks in structural steel using the eddy current method

DOT National Transportation Integrated Search

2000-11-01

This report summarizes research pertaining to the application of the Eddy Current method as a means of crack detection in structural steel members of highway bridges. Eddy currents are induced when an energized coil is placed near the surface of a co...

Characterization of Cracking and Crack Growth Properties of the C5A Aircraft Tie-Box Forging

NASA Technical Reports Server (NTRS)

Piascik, Robert S.; Smith, Stephen W.; Newman, John A.; Willard, Scott A.

2003-01-01

Detailed destructive examinations were conducted to characterize the integrity and material properties of two aluminum alloy (7075-T6) horizontal stabilizer tie box forgings removed.from US. Air Force C5A and C5B transport aircraft. The C5B tie box forging was,found to contain no evidence of cracking. Thirteen cracks were found in the CSA,forging. All but one of the cracks observed in the C5A component were located along the top cap region (one crack was located in the bottom cap region). The cracks in the C5A component initiated at fastener holes and propagated along a highly tunneled intergranular crack path. The tunneled crack growth configuration is a likelv result of surface compressive stress produced during peening of the .forging suijace. The tie box forging ,fatigue crack growth, fracture and stress corrosion cracking (SCC) properties were characterized. Reported herein are the results of laboratory air ,fatigue crack growth tests and 95% relative humidity SCC tests conducted using specimens machined from the C5A ,forging. SCC test results revealed that the C5A ,forging material was susceptible to intergranular environmental assisted cracking: the C5A forging material exhibited a SCC crack-tip stress-intensity factor threshold of less than 6 MPadn. Fracture toughness tests revealed that the C5A forging material exhibited a fracture toughness that was 25% less than the C5B forging. The C5A forging exhibited rapid laboratory air fatigue crack growth rates having a threshold crack-tip stress-intensity factor range of less than 0.8 MPa sup m. Detailed fractographic examinations revealed that the ,fatigue crack intergranular growth crack path was similar to the cracking observed in the C5A tie box forging. Because both fatigue crack propagation and SCC exhibit similar intergranular crack path behavior, the damage mechanism resulting in multi-site cracking of tie box forgings cannot be determined unless local cyclic stresses can be quantified.

Role of prism decussation on fatigue crack growth and fracture of human enamel.

PubMed

Bajaj, Devendra; Arola, Dwayne

2009-10-01

The role of prism decussation on the crack growth resistance of human enamel is evaluated. Miniature inset compact tension (CT) specimens embodying a section of cuspal enamel were subjected to Mode I cyclic or monotonic loads. Cracks were grown in either the forward (from outer enamel inwards) or reverse (from inner enamel outwards) direction and the responses were compared quantitatively. Results showed that the outer enamel exhibits lower resistance to the inception and growth of cracks. Regardless of the growth direction, the near-threshold region of cyclic extension was typical of "short crack" behavior (i.e. deceleration of growth with an increase in crack length). Cyclic crack growth was more stable in the forward direction and occurred over twice the spatial distance achieved in the reverse direction. In response to the monotonic loads, a rising R-curve response was exhibited by growth in the forward direction only. The total energy absorbed in fracture for the forward direction was more than three times that in the reverse. The rise in crack growth resistance was largely attributed to a combination of mechanisms that included crack bridging, crack bifurcation and crack curving, which were induced by decussation in the inner enamel. An analysis of the responses distinguished that the microstructure of enamel appears optimized for resisting crack growth initiating from damage at the tooth's surface.

Development and Characterization of Embedded Sensory Particles Using Multi-Scale 3D Digital Image Correlation

NASA Technical Reports Server (NTRS)

Cornell, Stephen R.; Leser, William P.; Hochhalter, Jacob D.; Newman, John A.; Hartl, Darren J.

2014-01-01

A method for detecting fatigue cracks has been explored at NASA Langley Research Center. Microscopic NiTi shape memory alloy (sensory) particles were embedded in a 7050 aluminum alloy matrix to detect the presence of fatigue cracks. Cracks exhibit an elevated stress field near their tip inducing a martensitic phase transformation in nearby sensory particles. Detectable levels of acoustic energy are emitted upon particle phase transformation such that the existence and location of fatigue cracks can be detected. To test this concept, a fatigue crack was grown in a mode-I single-edge notch fatigue crack growth specimen containing sensory particles. As the crack approached the sensory particles, measurements of particle strain, matrix-particle debonding, and phase transformation behavior of the sensory particles were performed. Full-field deformation measurements were performed using a novel multi-scale optical 3D digital image correlation (DIC) system. This information will be used in a finite element-based study to determine optimal sensory material behavior and density.

Electromagnetic pulsed thermography for natural cracks inspection

NASA Astrophysics Data System (ADS)

Gao, Yunlai; Tian, Gui Yun; Wang, Ping; Wang, Haitao; Gao, Bin; Woo, Wai Lok; Li, Kongjing

2017-02-01

Emerging integrated sensing and monitoring of material degradation and cracks are increasingly required for characterizing the structural integrity and safety of infrastructure. However, most conventional nondestructive evaluation (NDE) methods are based on single modality sensing which is not adequate to evaluate structural integrity and natural cracks. This paper proposed electromagnetic pulsed thermography for fast and comprehensive defect characterization. It hybrids multiple physical phenomena i.e. magnetic flux leakage, induced eddy current and induction heating linking to physics as well as signal processing algorithms to provide abundant information of material properties and defects. New features are proposed using 1st derivation that reflects multiphysics spatial and temporal behaviors to enhance the detection of cracks with different orientations. Promising results that robust to lift-off changes and invariant features for artificial and natural cracks detection have been demonstrated that the proposed method significantly improves defect detectability. It opens up multiphysics sensing and integrated NDE with potential impact for natural understanding and better quantitative evaluation of natural cracks including stress corrosion crack (SCC) and rolling contact fatigue (RCF).

Microbial healing of cracks in concrete: a review.

PubMed

Joshi, Sumit; Goyal, Shweta; Mukherjee, Abhijit; Reddy, M Sudhakara

2017-11-01

Concrete is the most widely used construction material of the world and maintaining concrete structures from premature deterioration is proving to be a great challenge. Early age formation of micro-cracking in concrete structure severely affects the serviceability leading to high cost of maintenance. Apart from conventional methods of repairing cracks with sealants or treating the concrete with adhesive chemicals to prevent the cracks from widening, a microbial crack-healing approach has shown promising results. The unique feature of the microbial system is that it enables self-healing of concrete. The effectiveness of microbially induced calcium carbonate precipitation (MICCP) in improving durability of cementitious building materials, restoration of stone monuments and soil bioclogging is discussed. Main emphasis has been laid on the potential of bacteria-based crack repair in concrete structure and the applications of different bacterial treatments to self-healing cracks. Furthermore, recommendations to employ the MICCP technology at commercial scale and reduction in the cost of application are provided in this review.

Effect of Grain Orientation and Boundary Distributions on Hydrogen-Induced Cracking in Low-Carbon-Content Steels

NASA Astrophysics Data System (ADS)

Masoumi, Mohammad; Coelho, Hana Livia Frota; Tavares, Sérgio Souto Maior; Silva, Cleiton Carvalho; de Abreu, Hamilton Ferreira Gomes

2017-08-01

Hydrogen-induced cracking (HIC) causes considerable economic losses in a wide range of steels exposed to corrosive environments. The effect of crystallographic texture and grain boundary distributions tailored by rolling at 850 °C in three different steels with a body-centered cube structure was investigated on HIC resistance. The x-ray and electron backscattered diffraction techniques were used to characterize texture evolutions during the rolling process. The findings revealed a significant improvement against HIC based on texture engineering. In addition, increasing the number of {111} and {110} grains, associated with minimizing the number of {001} grains in warm-rolled samples, reduced HIC susceptibility. Moreover, the results showed that boundaries associated with low {hkl} indexing and denser packing planes had more resistance against crack propagation.

Lattice Modeling of Early-Age Behavior of Structural Concrete.

PubMed

Pan, Yaming; Prado, Armando; Porras, Rocío; Hafez, Omar M; Bolander, John E

2017-02-25

The susceptibility of structural concrete to early-age cracking depends on material composition, methods of processing, structural boundary conditions, and a variety of environmental factors. Computational modeling offers a means for identifying primary factors and strategies for reducing cracking potential. Herein, lattice models are shown to be adept at simulating the thermal-hygral-mechanical phenomena that influence early-age cracking. In particular, this paper presents a lattice-based approach that utilizes a model of cementitious materials hydration to control the development of concrete properties, including stiffness, strength, and creep resistance. The approach is validated and used to simulate early-age cracking in concrete bridge decks. Structural configuration plays a key role in determining the magnitude and distribution of stresses caused by volume instabilities of the concrete material. Under restrained conditions, both thermal and hygral effects are found to be primary contributors to cracking potential.

Quantification of water penetration into concrete through cracks by neutron radiography

NASA Astrophysics Data System (ADS)

Kanematsu, M.; Maruyama, I.; Noguchi, T.; Iikura, H.; Tsuchiya, N.

2009-06-01

Improving the durability of concrete structures is one of the ways to contribute to the sustainable development of society, and it has also become a crucial issue from an environmental viewpoint. It is well known that moisture behavior in reinforced concrete is linked to phenomena such as cement hydration, volume change and cracking caused by drying shrinkage, rebar corrosion and water leakage that affect the durability of concrete. In this research, neutron radiography was applied for visualization and quantification of water penetration into concrete through cracks. It is clearly confirmed that TNR can make visible the water behavior in/near horizontal/vertical cracks and can quantify the rate of diffusion and concentration distribution of moisture with high spatial and time resolution. On detailed analysis, it is observed that water penetrates through the crack immediately after pouring and its migration speed and distribution depend on the moisture condition in the concrete.

Lattice Modeling of Early-Age Behavior of Structural Concrete

PubMed Central

Pan, Yaming; Prado, Armando; Porras, Rocío; Hafez, Omar M.; Bolander, John E.

2017-01-01

The susceptibility of structural concrete to early-age cracking depends on material composition, methods of processing, structural boundary conditions, and a variety of environmental factors. Computational modeling offers a means for identifying primary factors and strategies for reducing cracking potential. Herein, lattice models are shown to be adept at simulating the thermal-hygral-mechanical phenomena that influence early-age cracking. In particular, this paper presents a lattice-based approach that utilizes a model of cementitious materials hydration to control the development of concrete properties, including stiffness, strength, and creep resistance. The approach is validated and used to simulate early-age cracking in concrete bridge decks. Structural configuration plays a key role in determining the magnitude and distribution of stresses caused by volume instabilities of the concrete material. Under restrained conditions, both thermal and hygral effects are found to be primary contributors to cracking potential. PMID:28772590

Theoretical model of impact damage in structural ceramics

NASA Technical Reports Server (NTRS)

Liaw, B. M.; Kobayashi, A. S.; Emery, A. G.

1984-01-01

This paper presents a mechanistically consistent model of impact damage based on elastic failures due to tensile and shear overloading. An elastic axisymmetric finite element model is used to determine the dynamic stresses generated by a single particle impact. Local failures in a finite element are assumed to occur when the primary/secondary principal stresses or the maximum shear stress reach critical tensile or shear stresses, respectively. The succession of failed elements thus models macrocrack growth. Sliding motions of cracks, which closed during unloading, are resisted by friction and the unrecovered deformation represents the 'plastic deformation' reported in the literature. The predicted ring cracks on the contact surface, as well as the cone cracks, median cracks, radial cracks, lateral cracks, and damage-induced porous zones in the interior of hot-pressed silicon nitride plates, matched those observed experimentally. The finite element model also predicted the uplifting of the free surface surrounding the impact site.

Nonlinear ultrasonic fatigue crack detection using a single piezoelectric transducer

NASA Astrophysics Data System (ADS)

An, Yun-Kyu; Lee, Dong Jun

2016-04-01

This paper proposes a new nonlinear ultrasonic technique for fatigue crack detection using a single piezoelectric transducer (PZT). The proposed technique identifies a fatigue crack using linear (α) and nonlinear (β) parameters obtained from only a single PZT mounted on a target structure. Based on the different physical characteristics of α and β, a fatigue crack-induced feature is able to be effectively isolated from the inherent nonlinearity of a target structure and data acquisition system. The proposed technique requires much simpler test setup and less processing costs than the existing nonlinear ultrasonic techniques, but fast and powerful. To validate the proposed technique, a real fatigue crack is created in an aluminum plate, and then false positive and negative tests are carried out under varying temperature conditions. The experimental results reveal that the fatigue crack is successfully detected, and no positive false alarm is indicated.

Development of an Improved Crack Propagation Model for Corrosion-Induced Cover Cracking in RC Structures

NASA Astrophysics Data System (ADS)

Hilyati, S.; Nizam, Z. M.; Zurisman, M. A. A.; Azhar, A. T. S.

2017-06-01

During the last two decades, reinforced concrete (RC) has been extensively used in most of the world as one of the common construction material due to its advantages and durability. However, RC structures exposed to marine environments are subjected to chloride attack. Chlorides from seawater penetrate into RC structures are not only causing severe corrosion problems but also affect the durability and serviceability of such structures. This paper investigates the influence of transverse reinforcement and spacing of reinforcing bars on concrete cover cracking of two-way RC slab specimens using accelerated corrosion tests. The experimental program involved the testing of four RC slab specimens and was generally designed to observe the crack width and the time of crack to propagate. An improved model for predicting the timing of crack propagation based on the experimental data was then developed.

40 CFR Table 10 to Subpart Uuu of... - Continuous Monitoring Systems for Organic HAP Emissions From Catalytic Cracking Units

Code of Federal Regulations, 2014 CFR

2014-07-01

... 40 Protection of Environment 13 2014-07-01 2014-07-01 false Continuous Monitoring Systems for Organic HAP Emissions From Catalytic Cracking Units 10 Table 10 to Subpart UUU of Part 63 Protection of Environment ENVIRONMENTAL PROTECTION AGENCY (CONTINUED) AIR PROGRAMS (CONTINUED) NATIONAL EMISSION STANDARDS FOR HAZARDOUS AIR POLLUTANTS FOR SOURCE...

40 CFR Table 10 to Subpart Uuu of... - Continuous Monitoring Systems for Organic HAP Emissions From Catalytic Cracking Units

Code of Federal Regulations, 2013 CFR

2013-07-01

... 40 Protection of Environment 13 2013-07-01 2012-07-01 true Continuous Monitoring Systems for Organic HAP Emissions From Catalytic Cracking Units 10 Table 10 to Subpart UUU of Part 63 Protection of Environment ENVIRONMENTAL PROTECTION AGENCY (CONTINUED) AIR PROGRAMS (CONTINUED) NATIONAL EMISSION STANDARDS FOR HAZARDOUS AIR POLLUTANTS FOR SOURCE...

40 CFR Table 10 to Subpart Uuu of... - Continuous Monitoring Systems for Organic HAP Emissions From Catalytic Cracking Units

Code of Federal Regulations, 2010 CFR

2010-07-01

... 40 Protection of Environment 12 2010-07-01 2010-07-01 true Continuous Monitoring Systems for Organic HAP Emissions From Catalytic Cracking Units 10 Table 10 to Subpart UUU of Part 63 Protection of Environment ENVIRONMENTAL PROTECTION AGENCY (CONTINUED) AIR PROGRAMS (CONTINUED) NATIONAL EMISSION STANDARDS FOR HAZARDOUS AIR POLLUTANTS FOR SOURCE...

40 CFR Table 10 to Subpart Uuu of... - Continuous Monitoring Systems for Organic HAP Emissions From Catalytic Cracking Units

Code of Federal Regulations, 2012 CFR

2012-07-01

... 40 Protection of Environment 13 2012-07-01 2012-07-01 false Continuous Monitoring Systems for Organic HAP Emissions From Catalytic Cracking Units 10 Table 10 to Subpart UUU of Part 63 Protection of Environment ENVIRONMENTAL PROTECTION AGENCY (CONTINUED) AIR PROGRAMS (CONTINUED) NATIONAL EMISSION STANDARDS FOR HAZARDOUS AIR POLLUTANTS FOR SOURCE...

40 CFR Table 10 to Subpart Uuu of... - Continuous Monitoring Systems for Organic HAP Emissions From Catalytic Cracking Units

Code of Federal Regulations, 2011 CFR

2011-07-01

... 40 Protection of Environment 12 2011-07-01 2009-07-01 true Continuous Monitoring Systems for Organic HAP Emissions From Catalytic Cracking Units 10 Table 10 to Subpart UUU of Part 63 Protection of Environment ENVIRONMENTAL PROTECTION AGENCY (CONTINUED) AIR PROGRAMS (CONTINUED) NATIONAL EMISSION STANDARDS FOR HAZARDOUS AIR POLLUTANTS FOR SOURCE...

Prediction of microcracking in composite laminates under thermomechanical loading

NASA Technical Reports Server (NTRS)

Maddocks, Jason R.; Mcmanus, Hugh L.

1995-01-01

Composite laminates used in space structures are exposed to both thermal and mechanical loads. Cracks in the matrix form, changing the laminate thermoelastic properties. An analytical methodology is developed to predict microcrack density in a general laminate exposed to an arbitrary thermomechanical load history. The analysis uses a shear lag stress solution in conjunction with an energy-based cracking criterion. Experimental investigation was used to verify the analysis. Correlation between analysis and experiment is generally excellent. The analysis does not capture machining-induced cracking, or observed delayed crack initiation in a few ply groups, but these errors do not prevent the model from being a useful preliminary design tool.

Shield gas induced cracks during nanosecond-pulsed laser irradiation of Zr-based metallic glass

NASA Astrophysics Data System (ADS)

Huang, Hu; Noguchi, Jun; Yan, Jiwang

2016-10-01

Laser processing techniques have been given increasing attentions in the field of metallic glasses (MGs). In this work, effects of two kinds of shield gases, nitrogen and argon, on nanosecond-pulsed laser irradiation of Zr-based MG were comparatively investigated. Results showed that compared to argon gas, nitrogen gas remarkably promoted the formation of cracks during laser irradiation. Furthermore, crack formation in nitrogen gas was enhanced by increasing the peak laser power intensity or decreasing the laser scanning speed. X-ray diffraction and micro-Raman spectroscopy indicated that the reason for enhanced cracks in nitrogen gas was the formation of ZrN.

Interlocking-induced stiffness in stochastically microcracked materials beyond the transport percolation threshold

NASA Astrophysics Data System (ADS)

Picu, R. C.; Pal, A.; Lupulescu, M. V.

2016-04-01

We study the mechanical behavior of two-dimensional, stochastically microcracked continua in the range of crack densities close to, and above, the transport percolation threshold. We show that these materials retain stiffness up to crack densities much larger than the transport percolation threshold due to topological interlocking of sample subdomains. Even with a linear constitutive law for the continuum, the mechanical behavior becomes nonlinear in the range of crack densities bounded by the transport and stiffness percolation thresholds. The effect is due to the fractal nature of the fragmentation process and is not linked to the roughness of individual cracks.

Towards crack-free ablation cutting of thin glass sheets with picosecond pulsed lasers

NASA Astrophysics Data System (ADS)

Sun, Mingying; Eppelt, Urs; Hartmann, Claudia; Schulz, Wolfgang; Zhu, Jianqiang; Lin, Zunqi

2017-08-01

We investigated the morphology and mechanism of laser-induced damage in the ablation cutting of thin glass sheets with picosecond laser. Two kinds of damage morphologies observed on the cross-section of the cut channel, are caused by high-density free-electrons and the temperature accumulation, respectively. Notches and micro-cracks can be observed on the top surface of the sample near the cut edge. The surface micro-cracks were related to high energy free-electrons and also the heat-affected zone. Heat-affected-zone and visible-cracks free conditions of glass cutting were achieved by controlling the repetition rate and spatial overlap of laser pulses.

Influence of tempering and contraction mismatch on crack development in ceramic surfaces.

PubMed

Anusavice, K J; DeHoff, P H; Hojjatie, B; Gray, A

1989-07-01

Tempering of glass produces a state of compressive stress in surface regions which can enhance the resistance to crack initiation and growth. The objective of this study was to determine the influence of tempering on the sizes of surface cracks induced within the tempered surfaces of opaque porcelain-body porcelain discs, with contraction coefficient differences (alpha O-alpha B) of +3.2, +0.7, 0.0, -0.9, and -1.5 ppm/degrees C. We fired the discs to the maturing temperature (982 degrees C) of body porcelain and then subjected them to three cooling procedures: slow cooling in a furnace (SC), fast cooling in air (FC), and tempering (T) by blasting the body porcelain surface with compressed air for 90 s. We used body porcelain discs as the thermally compatible (delta alpha = 0) control specimens. We measured the diameters of cracks induced by a microhardness indenter at an applied load of 4.9 N at 80 points along diametral lines within the surface of body porcelain. The mean values of the crack diameters varied from 75.9 microns (delta alpha = -1.5 ppm/degrees C) to 103.3 microns (delta alpha = +3.2 ppm/degrees C). The results of ANOVA indicate that significant differences in crack dimensions were controlled by cooling rate, contraction mismatch, and their combined effect (p less than 0.0001). Multiple contrast analysis (Tukey's HSD Test) revealed significantly lower (p less than 0.05) crack sizes for tempered specimens compared with those of fast-cooled and slow-cooled specimens.(ABSTRACT TRUNCATED AT 250 WORDS)

The onset and evolution of fatigue-induced abnormal grain growth in nanocrystalline Ni–Fe

DOE Office of Scientific and Technical Information (OSTI.GOV)

Furnish, T. A.; Mehta, A.; Van Campen, D.

Conventional structural metals suffer from fatigue-crack initiation through dislocation activity which forms persistent slip bands leading to notch-like extrusions and intrusions. Ultrafine-grained and nanocrystalline metals can potentially exhibit superior fatigue-crack initiation resistance by suppressing these cumulative dislocation activities. Prior studies on these metals have confirmed improved high-cycle fatigue performance. In the case of nano-grained metals, analyses of subsurface crack initiation sites have indicated that the crack nucleation is associated with abnormally large grains. But, these post-mortem analyses have led to only speculation about when abnormal grain growth occurs (e.g., during fatigue, after crack initiation, or during crack growth). In thismore » study, a recently developed synchrotron X-ray diffraction technique was used to detect the onset and progression of abnormal grain growth during stress-controlled fatigue loading. Our study provides the first direct evidence that the grain coarsening is cyclically induced and occurs well before final fatigue failure—our results indicate that the first half of the fatigue life was spent prior to the detectable onset of abnormal grain growth, while the second half was spent coarsening the nanocrystalline structure and cyclically deforming the abnormally large grains until crack initiation. Post-mortem fractography, coupled with cycle-dependent diffraction data, provides the first details regarding the kinetics of this abnormal grain growth process during high-cycle fatigue testing. Finally, precession electron diffraction images collected in a transmission electron microscope after the in situ fatigue experiment also confirm the X-ray evidence that the abnormally large grains contain substantial misorientation gradients and sub-grain boundaries.« less

The onset and evolution of fatigue-induced abnormal grain growth in nanocrystalline Ni–Fe

DOE PAGES

Furnish, T. A.; Mehta, A.; Van Campen, D.; ...

2016-10-11

Conventional structural metals suffer from fatigue-crack initiation through dislocation activity which forms persistent slip bands leading to notch-like extrusions and intrusions. Ultrafine-grained and nanocrystalline metals can potentially exhibit superior fatigue-crack initiation resistance by suppressing these cumulative dislocation activities. Prior studies on these metals have confirmed improved high-cycle fatigue performance. In the case of nano-grained metals, analyses of subsurface crack initiation sites have indicated that the crack nucleation is associated with abnormally large grains. But, these post-mortem analyses have led to only speculation about when abnormal grain growth occurs (e.g., during fatigue, after crack initiation, or during crack growth). In thismore » study, a recently developed synchrotron X-ray diffraction technique was used to detect the onset and progression of abnormal grain growth during stress-controlled fatigue loading. Our study provides the first direct evidence that the grain coarsening is cyclically induced and occurs well before final fatigue failure—our results indicate that the first half of the fatigue life was spent prior to the detectable onset of abnormal grain growth, while the second half was spent coarsening the nanocrystalline structure and cyclically deforming the abnormally large grains until crack initiation. Post-mortem fractography, coupled with cycle-dependent diffraction data, provides the first details regarding the kinetics of this abnormal grain growth process during high-cycle fatigue testing. Finally, precession electron diffraction images collected in a transmission electron microscope after the in situ fatigue experiment also confirm the X-ray evidence that the abnormally large grains contain substantial misorientation gradients and sub-grain boundaries.« less

Infrared Thermography as a Non-destructive Testing Solution for Thermal Spray Metal Coatings

NASA Astrophysics Data System (ADS)

Santangelo, Paolo E.; Allesina, Giulio; Bolelli, Giovanni; Lusvarghi, Luca; Matikainen, Ville; Vuoristo, Petri

2017-12-01

In this work, an infrared (IR) thermographic procedure was evaluated as a non-destructive testing tool to detect damage in thermal spray metallic coatings. As model systems, polished HVOF- and HVAF-sprayed Fe-based layers deposited onto steel plates were employed. Damage by external-object impingement was simulated through a cyclic impact-test apparatus, which induced circumferential and radial cracks across all model systems, and interface cracks of different sizes in distinct samples. Damaged and undamaged plates were bulk-heated to above 100 °C using an IR lamp; their free-convection cooling was then recorded by an IR thermocamera. The intentionally induced defects were hardly detectable in IR thermograms, due to IR reflection and artificial "hot" spots induced by residuals of transfer material from the impacting counterbody. As a micrometer-thin layer of black paint was applied, surface emissivity got homogenized and any artifacts were effectively suppressed, so that failed coating areas clearly showed up as "cold spots." This effect was more apparent when large interface cracks occurred. Finite-element modeling proved the physical significance of the IR-thermography approach, showing that failed coating areas are cooled by surrounding air faster than they are heated by conduction from the hot substrate, which is due to the insulating effect of cracks.

Dynamics of diffusive bubble growth and pressure recovery in a bubbly rhyolitic melt embedded in an elastic solid

USGS Publications Warehouse

Chouet, Bernard A.; Dawson, Phillip B.; Nakano, Masaru

2006-01-01

We present a model of gas exsolution and bubble expansion in a melt supersaturated in response to a sudden pressure drop. In our model, the melt contains a suspension of gas bubbles of identical sizes and is encased in a penny-shaped crack embedded in an elastic solid. The suspension is modeled as a three-dimensional lattice of spherical cells with slight overlap, where each elementary cell consists of a gas bubble surrounded by a shell of volatile-rich melt. The melt is then subjected to a step drop in pressure, which induces gas exsolution and bubble expansion, resulting in the compression of the melt and volumetric expansion of the crack. The dynamics of diffusion-driven bubble growth and volumetric crack expansion span 9 decades in time. The model demonstrates that the speed of the crack response depends strongly on volatile diffusivity in the melt and bubble number density and is markedly sensitive to the ratio of crack thickness to crack radius and initial bubble radius but is relatively insensitive to melt viscosity. The net drop in gas concentration in the melt after pressure recovery represents only a small fraction of the initial concentration prior to the drop, suggesting the melt may undergo numerous pressure transients before becoming significantly depleted of gases. The magnitude of pressure and volume recovery in the crack depends sensitively on the size of the input-pressure transient, becoming relatively larger for smaller-size transients in a melt containing bubbles with initial radii less than 10-5 m. Amplification of the input transient may be large enough to disrupt the crack wall and induce brittle failure in the rock matrix surrounding the crack. Our results provide additional basis for the interpretation of volume changes in the magma conduit under Popocatépetl Volcano during Vulcanian degassing bursts in its eruptive activity in April–May 2000.

Prediction of reinforcement corrosion using corrosion induced cracks width in corroded reinforced concrete beams

DOE Office of Scientific and Technical Information (OSTI.GOV)

Khan, Inamullah; François, Raoul; Castel, Arnaud

2014-02-15

This paper studies the evolution of reinforcement corrosion in comparison to corrosion crack width in a highly corroded reinforced concrete beam. Cracking and corrosion maps of the beam were drawn and steel reinforcement was recovered from the beam to observe the corrosion pattern and to measure the loss of mass of steel reinforcement. Maximum steel cross-section loss of the main reinforcement and average steel cross-section loss between stirrups were plotted against the crack width. The experimental results were compared with existing models proposed by Rodriguez et al., Vidal et al. and Zhang et al. Time prediction models for a givenmore » opening threshold are also compared to experimental results. Steel cross-section loss for stirrups was also measured and was plotted against the crack width. It was observed that steel cross-section loss in the stirrups had no relationship with the crack width of longitudinal corrosion cracks. -- Highlights: •Relationship between crack and corrosion of reinforcement was investigated. •Corrosion results of natural process and then corresponds to in-situ conditions. •Comparison with time predicting model is provided. •Prediction of load-bearing capacity from crack pattern was studied.« less

Environment-Induced Cracking of Metals, Proceedings of the International Conference (1st), Held in Kohler, Wisconsin on October 2-7, 1988

DTIC Science & Technology

1990-01-01

considerable microplasticity associated with cracking. applications, Some of this deformation may be involved in initiating the dealloying appli tan or...brittle fracture, but is Lea and Hondros 3 have defined susceptibility in terms of a fraglt accompanied by microplastic behavior in the crack-tip...stress admonished us to look for microplasticity in the SEM at 10,OOOX, field around a dislocation can be reduced by an atmosphere, the much as Lynch has

Environmental Barrier Coating Fracture, Fatigue and High-Heat-Flux Durability Modeling and Stochastic Progressive Damage Simulation

NASA Technical Reports Server (NTRS)

Zhu, Dongming; Nemeth, Noel N.

2017-01-01

Advanced environmental barrier coatings will play an increasingly important role in future gas turbine engines because of their ability to protect emerging light-weight SiC/SiC ceramic matrix composite (CMC) engine components, further raising engine operating temperatures and performance. Because the environmental barrier coating systems are critical to the performance, reliability and durability of these hot-section ceramic engine components, a prime-reliant coating system along with established life design methodology are required for the hot-section ceramic component insertion into engine service. In this paper, we have first summarized some observations of high temperature, high-heat-flux environmental degradation and failure mechanisms of environmental barrier coating systems in laboratory simulated engine environment tests. In particular, the coating surface cracking morphologies and associated subsequent delamination mechanisms under the engine level high-heat-flux, combustion steam, and mechanical creep and fatigue loading conditions will be discussed. The EBC compostion and archtechture improvements based on advanced high heat flux environmental testing, and the modeling advances based on the integrated Finite Element Analysis Micromechanics Analysis Code/Ceramics Analysis and Reliability Evaluation of Structures (FEAMAC/CARES) program will also be highlighted. The stochastic progressive damage simulation successfully predicts mud flat damage pattern in EBCs on coated 3-D specimens, and a 2-D model of through-the-thickness cross-section. A 2-parameter Weibull distribution was assumed in characterizing the coating layer stochastic strength response and the formation of damage was therefore modeled. The damage initiation and coalescence into progressively smaller mudflat crack cells was demonstrated. A coating life prediction framework may be realized by examining the surface crack initiation and delamination propagation in conjunction with environmental degradation under high-heat-flux and environment load test conditions.

Dwell Notch Low Cycle Fatigue Behavior of a Powder Metallurgy Nickel Disk Alloy

NASA Technical Reports Server (NTRS)

Telesman, J.; Gabb, T. P.; Yamada, Y.; Ghosn, L. J.; Jayaraman, N.

2012-01-01

A study was conducted to determine the processes which govern dwell notch low cycle fatigue (NLCF) behavior of a powder metallurgy (P/M) ME3 disk superalloy. The emphasis was placed on the environmentally driven mechanisms which may embrittle the highly stressed notch surface regions and reduce NLCF life. In conjunction with the environmentally driven notch surface degradation processes, the visco-plastic driven mechanisms which can significantly change the notch root stresses were also considered. Dwell notch low cycle fatigue testing was performed in air and vacuum on a ME3 P/M disk alloy specimens heat treated using either a fast or a slow cooling rate from the solutioning treatment. It was shown that dwells at the minimum stress typically produced a greater life debit than the dwells applied at the maximum stress, especially for the slow cooled heat treatment. Two different environmentally driven failure mechanisms were identified as the root cause of early crack initiation in the min dwell tests. Both of these failure mechanisms produced mostly a transgranular crack initiation failure mode and yet still resulted in low NLCF fatigue lives. The lack of stress relaxation during the min dwell tests produced higher notch root stresses which caused early crack initiation and premature failure when combined with the environmentally driven surface degradation mechanisms. The importance of environmental degradation mechanisms was further highlighted by vacuum dwell NLCF tests which resulted in considerably longer NLCF lives, especially for the min dwell tests.

Crack propagation and the material removal mechanism of glass-ceramics by the scratch test.

PubMed

Qiu, Zhongjun; Liu, Congcong; Wang, Haorong; Yang, Xue; Fang, Fengzhou; Tang, Junjie

2016-12-01

To eliminate the negative effects of surface flaws and subsurface damage of glass-ceramics on clinical effectiveness, crack propagation and the material removal mechanism of glass-ceramics were studied by single and double scratch experiments conducted using an ultra-precision machine. A self-manufactured pyramid shaped single-grit tool with a small tip radius was used as the scratch tool. The surface and subsurface crack propagations and interactions, surface morphology and material removal mechanism were investigated. The experimental results showed that the propagation of lateral cracks to the surface and the interaction between the lateral cracks and radial cracks are the two main types of material peeling, and the increase of the scratch depth increases the propagation angle of the radial cracks and the interaction between the cracks. In the case of a double scratch, the propagation of lateral cracks and radial cracks between paired scratches results in material peeling. The interaction between adjacent scratches depends on the scratch depth and separation distance. There is a critical separation distance where the normalized material removal volume reaches its peak. These findings can help reduce surface flaws and subsurface damage induced by the grinding process and improve the clinical effectiveness of glass-ceramics used as biological substitute and repair materials. Copyright © 2016 Elsevier Ltd. All rights reserved.

Rolling contact fatigue strengths of shot-peened and crack-healed ceramics

NASA Astrophysics Data System (ADS)

Takahashi, K.; Oki, T.

2018-06-01

The effects of shot-peening (SP) and crack-healing on the rolling contact fatigue (RCF) strengths of Al2O3/SiC composite ceramics were investigated. Non-shot-peened, shot- peened, and shot-peened + crack-healed specimens were prepared. SP was performed using ZrO2 beads. The shot-peened + crack-healed specimen was crack-healed after SP. X-ray diffraction clearly showed that SP induced a compressive residual stress up to 300 MPa at the specimen surfaces. Furthermore, the shot-peened + crack-healed specimen retained a compressive residual stress of 200 MPa. The apparent surface fracture toughness of the shot- peened specimens increased owing to the positive effects of the compressive residual stress. RCF tests were performed using a thrust load-bearing test device. The RCF lives of the shot- peened specimens did not improve compared to that of the non-shot-peened specimen, because the numerous SP-introduced surface cracks could act as crack initiation sites during the RCF tests. However, the RCF life of the shot-peened + crack-healed specimen did improve compared to those of non-shot-peened and shot-peened specimens, implying that combining SP and crack-healing was an effective strategy for improving the RCF lives of Al2O3/SiC composite ceramics.

Recrystallization-Induced Surface Cracks of Carbon Ions Irradiated 6H-SiC after Annealing

PubMed Central

Ye, Chao; Ran, Guang; Zhou, Wei; Shen, Qiang; Feng, Qijie; Lin, Jianxin

2017-01-01

Single crystal 6H-SiC wafers with 4° off-axis [0001] orientation were irradiated with carbon ions and then annealed at 900 °C for different time periods. The microstructure and surface morphology of these samples were investigated by grazing incidence X-ray diffraction (GIXRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Ion irradiation induced SiC amorphization, but the surface was smooth and did not have special structures. During the annealing process, the amorphous SiC was recrystallized to form columnar crystals that had a large amount of twin structures. The longer the annealing time was, the greater the amount of recrystallized SiC would be. The recrystallization volume fraction was accorded with the law of the Johnson–Mehl–Avrami equation. The surface morphology consisted of tiny pieces with an average width of approximately 30 nm in the annealed SiC. The volume shrinkage of irradiated SiC layer and the anisotropy of newly born crystals during annealing process produced internal stress and then induced not only a large number of dislocation walls in the non-irradiated layer but also the initiation and propagation of the cracks. The direction of dislocation walls was perpendicular to the growth direction of the columnar crystal. The longer the annealing time was, the larger the length and width of the formed crack would be. A quantitative model of the crack growth was provided to calculate the length and width of the cracks at a given annealing time. PMID:29068408

The effect of transverse crack upon parametric instability of a rotor-bearing system with an asymmetric disk

NASA Astrophysics Data System (ADS)

Han, Qinkai; Chu, Fulei

2012-12-01

It is well known that either the asymmetric disk or transverse crack brings parametric inertia (or stiffness) excitation to the rotor-bearing system. When both of them appear in a rotor system, the parametric instability behaviors have not gained sufficient attentions. Thus, the effect of transverse crack upon parametric instability of a rotor-bearing system with an asymmetric disk is studied. First, the finite element equations of motion are established for the asymmetric rotor system. Both the open and breathing transverse cracks are taken into account in the model. Then, the discrete state transition matrix (DSTM) method is introduced for numerically acquiring the instability regions. Based upon these, some computations for a practical asymmetric rotor system with open or breathing transverse crack are conducted, respectively. Variations of the primary and combination instability regions induced by the asymmetric disk with the crack depth are observed, and the effect of the orientation angle between the crack and asymmetric disk on various instability regions are discussed in detail. It is shown that for the asymmetric angle around 0, the existence of transverse (either open or breathing) crack has attenuation effect upon the instability regions. Under certain crack depth, the instability regions could be vanished by the transverse crack. When the asymmetric angle is around π/2, increasing the crack depth would enhance the instability regions.

Role of Prism Decussation on Fatigue Crack Growth and Fracture of Human Enamel

PubMed Central

Bajaj, Devendra; Arola, Dwayne

2009-01-01

The role of prism decussation on the crack growth resistance of human enamel is evaluated. Miniature inset Compact Tension (CT) specimens embodying a section of cuspal enamel were subjected to Mode I cyclic or monotonic loads. Cracks were grown in either the forward (from outer enamel inwards) or reverse (from inner enamel outwards) direction and the responses were compared quantitatively. Results showed that the outer enamel exhibits lower resistance to the inception and growth of cracks. Regardless of the growth direction, the near threshold region of cyclic extension was typical of ‘short crack’ behavior (i.e. deceleration of growth with an increase in crack length). Cyclic crack growth was more stable in the forward direction and occurred over twice the spatial distance achieved in the reverse direction. In response to the monotonic loads, a rising R-curve response was exhibited by growth in the forward direction only. The total energy absorbed in fracture for the forward direction was more than three times that in the reverse. The rise in crack growth resistance was largely attributed to a combination of mechanisms that included crack bridging, crack bifurcation and crack curving, which were induced by decussation in the inner enamel. An analysis of the responses distinguished that the microstructure of enamel appears optimized for resisting crack growth initiating from damage at the tooth’s surface. PMID:19433137

The effect of cathodic polarization on the corrosion fatigue behavior of a precipitation hardened aluminum alloy

NASA Astrophysics Data System (ADS)

Smith, E. F.; Duquette, D. J.

1986-02-01

Fatigue experiments were conducted on polycrystalline and monocrystalline samples of a high purity Al, 5.5 wt pct Zn, 2.5 wt pct Mg, 1.5 wt pct Cu alloy in the peak-hardened heat treatment condition. These experiments were conducted in dry laboratory air and in 0.5 N NaCl solutions at the corrosion potential and at applied potentials cathodic to the corrosion potential. It has been shown that saline solutions severely reduce the fatigue resistance of the alloy, resulting in considerable amounts of intergranular crack initiation and propagation under freely corroding conditions for polycrystalline samples. Applied cathodic potentials resulted in still larger decreases in fatigue resistance and, for poly crystals, increases in the degree of transgranular crack initiation and propagation. Increasing amounts of intergranular cracking were observed when applied cyclic stresses were reduced (longer test times). The characteristics of cracking, combined with results obtained on tensile tests of deformed and hydrogen charged samples, suggest that environmental cracking of these alloys is associated with a form of hydrogen embrittlement of the process zones of growing cracks. Further, it is suggested that stress corrosion cracking and corrosion fatigue of these alloys occurs by essentially the same mechanism, but that the often observed transgranular cracking under cyclic loading conditions occurs due to enhanced hydrogen transport and/or concentrations associated with mobile dislocations at growing crack tips.

Very High Cycle Fatigue Failure Analysis and Life Prediction of Cr-Ni-W Gear Steel Based on Crack Initiation and Growth Behaviors.

PubMed

Deng, Hailong; Li, Wei; Sakai, Tatsuo; Sun, Zhenduo

2015-12-02

The unexpected failures of structural materials in very high cycle fatigue (VHCF) regime have been a critical issue in modern engineering design. In this study, the VHCF property of a Cr-Ni-W gear steel was experimentally investigated under axial loading with the stress ratio of R = -1, and a life prediction model associated with crack initiation and growth behaviors was proposed. Results show that the Cr-Ni-W gear steel exhibits the constantly decreasing S-N property without traditional fatigue limit, and the fatigue strength corresponding to 10⁸ cycles is around 485 MPa. The inclusion-fine granular area (FGA)-fisheye induced failure becomes the main failure mechanism in the VHCF regime, and the local stress around the inclusion play a key role. By using the finite element analysis of representative volume element, the local stress tends to increase with the increase of elastic modulus difference between inclusion and matrix. The predicted crack initiation life occupies the majority of total fatigue life, while the predicted crack growth life is only accounts for a tiny fraction. In view of the good agreement between the predicted and experimental results, the proposed VHCF life prediction model involving crack initiation and growth can be acceptable for inclusion-FGA-fisheye induced failure.

Characterization of ion irradiation effects on the microstructure, hardness, deformation and crack initiation behavior of austenitic stainless steel:Heavy ions vs protons

NASA Astrophysics Data System (ADS)

Gupta, J.; Hure, J.; Tanguy, B.; Laffont, L.; Lafont, M.-C.; Andrieu, E.

2018-04-01

Irradiation Assisted Stress Corrosion Cracking (IASCC) is a complex phenomenon of degradation which can have a significant influence on maintenance time and cost of core internals of a Pressurized Water Reactor (PWR). Hence, it is an issue of concern, especially in the context of lifetime extension of PWRs. Proton irradiation is generally used as a representative alternative of neutron irradiation to improve the current understanding of the mechanisms involved in IASCC. This study assesses the possibility of using heavy ions irradiation to evaluate IASCC mechanisms by comparing the irradiation induced modifications (in microstructure and mechanical properties) and cracking susceptibility of SA 304 L after both type of irradiations: Fe irradiation at 450 °C and proton irradiation at 350 °C. Irradiation-induced defects are characterized and quantified along with nano-hardness measurements, showing a correlation between irradiation hardening and density of Frank loops that is well captured by Orowan's formula. Both irradiations (iron and proton) increase the susceptibility of SA 304 L to intergranular cracking on subjection to Constant Extension Rate Tensile tests (CERT) in simulated nominal PWR primary water environment at 340 °C. For these conditions, cracking susceptibility is found to be quantitatively similar for both irradiations, despite significant differences in hardening and degree of localization.

Effects of Aging and Environmental Conditions on Ammunition/Explosives Storage Magazines - Paper 2

DTIC Science & Technology

2010-07-01

characterized as destructive. The destructive category is apparently limited to reactions with impure dolomitic aggregates and are a result of either...extreme pressures that eventually overcome the tensile strength of the structure. These pressures will cause spalling, map cracking, discoloration, or...fill with this gel and expand to create extreme tensile pressures . These pressures cause micro-scale cracking and eventually develop into

40 CFR Table 11 to Subpart Uuu of... - Requirements for Performance Tests for Organic HAP Emissions From Catalytic Cracking Units Not...

Code of Federal Regulations, 2010 CFR

2010-07-01

...) for Carbon Monoxide (CO) 11 Table 11 to Subpart UUU of Part 63 Protection of Environment ENVIRONMENTAL... Refineries: Catalytic Cracking Units, Catalytic Reforming Units, and Sulfur Recovery Units Pt. 63, Subpt. UUU, Table 11 Table 11 to Subpart UUU of Part 63—Requirements for Performance Tests for Organic HAP Emissions...

40 CFR Table 11 to Subpart Uuu of... - Requirements for Performance Tests for Organic HAP Emissions From Catalytic Cracking Units Not...

Code of Federal Regulations, 2011 CFR

2011-07-01

...) for Carbon Monoxide (CO) 11 Table 11 to Subpart UUU of Part 63 Protection of Environment ENVIRONMENTAL... Refineries: Catalytic Cracking Units, Catalytic Reforming Units, and Sulfur Recovery Units Pt. 63, Subpt. UUU, Table 11 Table 11 to Subpart UUU of Part 63—Requirements for Performance Tests for Organic HAP Emissions...

Enhancement of crack healing efficiency and performance of SAP in biocrete

NASA Astrophysics Data System (ADS)

Giriselvam, M. G.; Poornima, V.; Venkatasubramani, R.; Sreevidya, V.

2018-02-01

Concrete usage in Construction becomes more common in this speedy world. Despite its benefits, concrete often exhibits crack which appear due to stresses. Larger cracks cause Structural integrity problems and smaller cracks may result in durability issues. A novel environmental friendly strategy to restore or remediate cracks formed in the structures is bio-mineralization of calcium carbonate using microbes such as Bacillus Subtilis (used in this study), as manual repair and maintenance is costly. In this Paper, an idea of using Super Absorbent Polymer in Bacterial Concrete was analysed which increases the strength and durability properties of concrete and also which acts as a protection to bacteria, where Self-Healing nature is viewed. In the span of 90 days, the results of Bacterial concrete cured under normal water providing nutrients inside with SAP shows healing up to 74 % and without SAP displays 49 % and when it is cured under nutrient medium, Bacterial Concrete having SAP displays healing up to 66 %, whereas without SAP it displays 57.4% of healing. During the observation it is discernible that the crack width ranging from 0.10 mm near 0.45 mm show better self-healing capacity. XRD analysis displays the presence of Calcium carbonate precipitation in cracks.

ENVIRONMENTAL ANALYSIS OF GASOLINE BLENDING COMPONENTS THROUGH THEIR LIFE CYCLE

EPA Science Inventory

The contributions of three major gasoline blending components (reformate, alkylate and cracked gasoline) to potential environmental impacts are assessed. This study estimates losses of the gasoline blending components due to evaporation and leaks through their life cycle, from pe...

Non-destructive inspection using HTS SQUID on aluminum liner covered by CFRP

NASA Astrophysics Data System (ADS)

Hatsukade, Y.; Yotsugi, K.; Sakaguchi, Y.; Tanaka, S.

2007-10-01

An eddy-current-based SQUID non-destructive inspection (NDI) system to detect deep-lying cracks in multi-layer composite-Al vessels was developed taking advantage of the uncontested sensitivity of HTS-SQUID in low-frequency range. An HTS-SQUID gradiometer was mounted in a pulse tube cryocooler. A pair of differential coils with C-shaped ferrite cores was employed to induce an enhanced eddy current in an Al vessel wrapped in a carbon fiber reinforced plastic (CFRP) cover. Ellipsoidal dome-shaped Al liners containing through cracks, which were made by pressure cycle tests, in the CFRP covers with total thickness of 6 mm (CFPR 3 mm, and Al 3 mm) were inspected by the system. While inducing eddy currents in the vessels with excitation fields at 100 Hz or 7 kHz, the vessels were rotated under the HTS-SQUID. Above the cracks, anomalous signals due to the cracks were clearly detected at both frequencies. These results suggested the SQUID-NDI technique would be a possible candidate for inspection of high-pressure multi-layer composite-Al vessels.

Nonlinear Lamb waves for fatigue damage identification in FRP-reinforced steel plates.

PubMed

Wang, Yikuan; Guan, Ruiqi; Lu, Ye

2017-09-01

A nonlinear Lamb-wave-based method for fatigue crack detection in steel plates with and without carbon fibre reinforcement polymer (CFRP) reinforcement is presented in this study. Both numerical simulation and experimental evaluation were performed for Lamb wave propagation and its interaction with a fatigue crack on these two steel plate types. With the generation of the second harmonic, the damage-induced wave nonlinearities were identified by surface-bonded piezoelectric sensors. Numerical simulation revealed that the damage-induced wave component at the second harmonic was slightly affected by the existence of CFRP laminate, although the total wave energy was decreased because of wave leakage into the CFRP laminate. Due to unavoidable nonlinearity from the experimental environments, it was impractical to directly extract the time-of-flight of the second harmonic for locating the crack. To this end, the correlation coefficient of benchmark and signal with damage at double frequency in the time domain was calculated, based on which an imaging method was introduced to locate the fatigue crack in steel plates with and without CFRP laminates. Copyright © 2017 Elsevier B.V. All rights reserved.

The static breaking technique for sustainable and eco-environmental coal mining.

PubMed

Bing-yuan, Hao; Hui, Huang; Zi-jun, Feng; Kai, Wang

2014-01-01

The initiating explosive devices are prohibited in rock breaking near the goaf of the highly gassy mine. It is effective and applicable to cracking the hard roof with static cracking agent. By testing the static cracking of cubic limestone (size: 200 × 200 × 200 mm) with true triaxial rock mechanics testing machine under the effect of bidirectional stress and by monitoring the evolution process of the cracks generated during the acoustic emission experiment of static cracking, we conclude the following: the experiment results of the acoustic emission show that the cracks start from the lower part of the hole wall until they spread all over the sample. The crack growth rate follows a trend of "from rapidness to slowness." The expansion time is different for the two bunches of cracks. The growth rates can be divided into the rapid increasing period and the rapid declining period, of which the growth rate in declining period is less than that in the increasing period. Also, the growth rate along the vertical direction is greater than that of the horizontal direction. Then the extended model for the static cracking is built according to the theories of elastic mechanics and fracture mechanics. Thus the relation formula between the applied forces of cracks and crack expansion radius is obtained. By comparison with the test results, the model proves to be applicable. In accordance with the actual geological situation of Yangquan No. 3 Mine, the basic parameters of manpower manipulated caving breaking with static crushing are settled, which reaps bumper industrial effects.

Fatigue and Creep Crack Propagation behaviour of Alloy 617 in the Annealed and Aged Conditions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Julian K. Benz; Richard N. Wright

2013-10-01

The crack propagation behaviour of Alloy 617 was studied under various conditions. Elevated temperature fatigue and creep-fatigue crack growth experiments were conducted at 650 and 800 degrees C under constant stress intensity (triangle K) conditions and triangular or trapezoidal waveforms at various frequencies on as-received, aged, and carburized material. Environmental conditions included both laboratory air and characteristic VHTR impure helium. As-received Alloy 617 displayed an increase in the crack growth rate (da/dN) as the frequency was decreased in air which indicated a time-dependent contribution component in fatigue crack propagation. Material aged at 650°C did not display any influence on themore » fatigue crack growth rates nor the increasing trend of crack growth rate with decreasing frequency even though significant microstructural evolution, including y’ (Ni3Al) after short times, occurred during aging. In contrast, carburized Alloy 617 showed an increase in crack growth rates at all frequencies tested compared to the material in the standard annealed condition. Crack growth studies under quasi-constant K (i.e. creep) conditions were also completed at 650 degrees C and a stress intensity of K = 40 MPa9 (square root)m. The results indicate that crack growth is primarily intergranular and increased creep crack growth rates exist in the impure helium environment when compared to the results in laboratory air. Furthermore, the propagation rates (da/dt) continually increased for the duration of the creep crack growth either due to material aging or evolution of a crack tip creep zone. Finally, fatigue crack propagation tests at 800 degrees C on annealed Alloy 617 indicated that crack propagation rates were higher in air than impure helium at the largest frequencies and lowest stress intensities. The rates in helium, however, eventually surpass the rates in air as the frequency is reduced and the stress intensity is decreased which was not observed at 650 degrees C.« less

Sub-grain induced crack deviation in multi-crystalline silicon

NASA Astrophysics Data System (ADS)

Zhao, Lv; Nelias, Daniel; Bardel, Didier; Wang, Meng; Marie, Benoit

2017-06-01

The fracture process in crystalline silicon is dictated by energy dissipation. Here, we show that sub-grains can deviate the crack path from the most energetically favorable ( 111) plane. Albeit a small misorientation across the sub-grain boundary is identified, upon entering into the sub-grain region, the crack either slightly deviates from the ideal ( 111) plane or directly chooses the secondly most favorable ( 110) one. We propose that the deviation is related to the dislocation core in the ( 111) crystal plane, which leads to a discontinuous atom debonding process and consequently a pronounced lattice trapping. In this circumstance, localized crystal defects prevail in the fracture process of silicon, while energetical criterion fails to interpret the crack path.

Hydrogen-induced slow crack growth of a plain carbon pipeline steel under conditions of cyclic loading

NASA Technical Reports Server (NTRS)

Nelson, H. G.

1976-01-01

The investigation described was aimed at establishing the degree of compatibility between a plain carbon pipeline-type steel and hydrogen and also hydrogen-rich environments containing small additions of H2S, O2, H2O, CO, CO2, CH4, and natural gas at pressures near 1 atm. Test were carried out under conditions of static and cyclic loading; the subcritical crack growth was monitored. The rates of crack growth observed in the hydrogen and hydrogen-rich environments are compared with the crack rate observed in a natural gas environment to determine the compatibility of the present natural gas transmission system with gaseous hydrogen transport.

Mechanics of tidally driven fractures in Europa's ice shell and implications for seismic and radar profiling

NASA Astrophysics Data System (ADS)

Lee, S.; Pappalardo, R. T.; Makris, N. C.

2005-12-01

Among Europa's surface features, cycloidal cracks are probably the most important for proving the existence of a subsurface liquid ocean. This is because (1) there is strong evidence that they are caused by tidally induced stress [1], and (2) this stress likely only approaches the ice failure strength if an ocean is present. There are a number of outstanding issues, however, in quantitatively explaining cycloidal cracks. First, current estimates of the pure diurnal tidal stress necessary to cause cycloidal cracks even in the presence of an ocean [1,2] is well below the typical stress known to cause tensile failure in natural terrestrial ice [3]. Second, models of ridge formation suggest that cycloidal cracks penetrate through the entire brittle-ice layer [1,4], but current models limit the depth of tidally induced surface cracks to be less than 100 m even in the presence of an ocean [1,5]. Third, the 3-km/h crack propagation speed determined by [1] is three orders of magnitude lower than the roughly 2-km/s speed at which cracks are known to propagate in ice. Our goal is to quantitatively address these issues in a unified manner. To do this, a fracture mechanics model is developed for the initiation and propagation of a crack through an ice layer of finite thickness in the presence of gravitational overburden and porosity. It is shown that Europa's ice shell may be highly porous and salt-rich. This implies that the strength of Europa's outer ice shell may be sufficiently low to make the crack initiation strengths arrived at by current kinematic models [1,2] highly plausible, even though they are much lower than those typically measured for terrestrial ice. A model is developed for the stress intensity factor at a crack tip in an ice shell with finite thickness, gravitational overburden, and depth-dependent porosity. This leads to the conclusion that cycloids are generated as a sequence of discrete and near instantaneous fracture events, each of which penetrates through the entire brittle layer with horizontal length on the order of the brittle layer thickness. This mechanism yields an apparent propagation speed that is consistent with the 3 km/h crack propagation speed necessary to generate cycloids in current kinematic models [1,2]. An implication of this model is that the level of seismic activity should be higher by orders of magnitude in the presence of an ocean. High correlation is then expected between the level of seismic activity and the tidal period in the presence but not in the absence of an ocean. The cracks associated with cycloids that fully penetrate the brittle layer should be at least 106 times more energetic than the shallow, roughly 100-m deep, surface cracks. We show that this greatly improves the signal-to-noise ratio for the type of seismic profiling discussed in [6] if fully penetrating cracks are used as sources of opportunity. Although Europa's ice is likely highly porous, the size of vacuous pores is likely on the order of a millimeter. Since the pore size is at least three orders of magnitude smaller than the ice-penetrating radar wavelength, our calculations show that porosity-induced scattering should not be significant. [1] Hoppa et al. 1999, Science 285. [2] Crawford et al. 2005, LPSC XXXVI #2042. [3] Weeks and Cox 1984, Ocean Sci. Eng. 9. [4] Pappalardo et al. 1999, J. Geophys. Res. 97. [5] Crawford and Stevenson 1988, Icarus 73. [6] Lee et al. 2003, Icarus 165.

Stress corrosion cracking of Zircaloys in unirradiated and irradiated CsI

NASA Astrophysics Data System (ADS)

Cox, B.; Surette, B. A.; Wood, J. C.

1986-03-01

Unirradiated split-ring specimens of Zircaloy fuel cladding, coated with CsI, cracked when stressed at elevated temperatures. The specimens have been reexamined fractographically and metallographically in order to confirm that the cause of cracking was stress corrosion (SCC) and not delayed hydride cracking (DHC). Further specimens have been cracked at 350°C by a solution of CsI in a fused mixture of nitrates of rubidium, cesium, strontium and barium, by a similar mechanism. CsI dissolved in a fused molybdate melt was not stable at 400°C, and rapidly evolved iodine, leaving a melt that was incapable of causing SCC. Irradiation of stressed split-ring specimens of Zircaloy fuel cladding in a γ-irradiator of 10 6 R/h and in the U-5 loop in the NRU reactor at an estimated 10 9 R/h caused SCC when the specimens were packed in dry CsI powder. Care had to be taken to dry the CsI, otherwise cracking occurred by a DHC mechanism from hydrogen absorbed from residual moisture in the CsI. Fractography showed that the crack surfaces obtained with dry CsI were typical of iodine-induced SCC rather than cesium-induced metal vapour embrittlement. Thus, if a transport process is provided for the iodide to obtain access to the zirconium surface, CsI is capable of causing SCC of Zircaloy. This transport process might be ionic diffusion in a fission product oxide melt in the fuel-clad gap, however, radiolysis of CsI to form a volatile iodine species in a radiation field is the more probable explanation of PCI failures.

Seismic velocities in fractured rocks: An experimental verification of Hudson`s theory

DOE Office of Scientific and Technical Information (OSTI.GOV)

Peacock, S.; McCann, C.; Sothcott, J.

1994-01-01

Flow of fluids in many hydrocarbon reservoirs aquifers is enhanced by the presence of cracks and fractures. These cracks could be detected by their effects on propagation of compressional and shear waves through the reservoir: several theories, including Hudson`s, claim to predict the seismic effects of cracks. Although Hudson`s theory has already been used to calculate crack densities from seismic survey`s, the predictions of the theory have not yet been tested experimentally on rocks containing a known crack distribution. This paper describes an experimental verification of the theory. The rock used, Carrara marble, was chosen for its uniformity and lowmore » porosity, so that the effect of cracks would not be obscured by other influences. Cracks were induced by loading of laboratory specimens. Velocities of compressional and shear waves were measured by ultrasound at 0.85 MHz in dry and water-saturated specimens at high and low effective pressures.The cracks were then counted in polished sections of the specimens. In ``dry`` specimens with both dry and saturated cracks, Hudson`s theory overpredicted observed crack densities by a constant amount that is attributed to the observed value being systematically underestimated. The theory made poor predictions for fully saturated specimens. Shear-wave splitting, caused by anisotropy due to both crystal and crack alignment, was observed. Cracks were seen to follow grain boundaries rather than the direction of maximum compression due to loading. The results demonstrate that Hudson`s theory may be used in some cases to determine crack and fracture densities from compressional- and shear-wave velocity data.« less

Advances in Structural Integrity Analysis Methods for Aging Metallic Airframe Structures with Local Damage

NASA Technical Reports Server (NTRS)

Starnes, James H., Jr.; Newman, James C., Jr.; Harris, Charles E.; Piascik, Robert S.; Young, Richard D.; Rose, Cheryl A.

2003-01-01

Analysis methodologies for predicting fatigue-crack growth from rivet holes in panels subjected to cyclic loads and for predicting the residual strength of aluminum fuselage structures with cracks and subjected to combined internal pressure and mechanical loads are described. The fatigue-crack growth analysis methodology is based on small-crack theory and a plasticity induced crack-closure model, and the effect of a corrosive environment on crack-growth rate is included. The residual strength analysis methodology is based on the critical crack-tip-opening-angle fracture criterion that characterizes the fracture behavior of a material of interest, and a geometric and material nonlinear finite element shell analysis code that performs the structural analysis of the fuselage structure of interest. The methodologies have been verified experimentally for structures ranging from laboratory coupons to full-scale structural components. Analytical and experimental results based on these methodologies are described and compared for laboratory coupons and flat panels, small-scale pressurized shells, and full-scale curved stiffened panels. The residual strength analysis methodology is sufficiently general to include the effects of multiple-site damage on structural behavior.

Fracture of concrete caused by the reinforcement corrosion products

NASA Astrophysics Data System (ADS)

Nguyen, Q. T.; Millard, A.; Caré, S.; L'Hostis, V.; Berthaud, Y.

2006-11-01

One of the most current degradations in reinforced concrete structures is related to the corrosion of the reinforcements. The corrosion products during active corrosion induce a mechanical pressure on the surrounding concrete that leads to cover cracking along the rebar. The objective of this work is to study the cracking of concrete due to the corrosion of the reinforcements. The phenomenon of corrosion/cracking is studied in experiments through tests of accelerated corrosion on plate and cylindrical specimens. A CCD camera is used to take images every hour and the pictures are analyzed by using the intercorrelation image technique (Correli^LMT) to derive the displacement and strain field. Thus the date of appearance of the first through crack is detected and the cinematic crack initiations are observed during the test. A finite element model that allows prediction of the mechanical consequences of the corrosion of steel in reinforced concrete structures is proposed. From the comparison between the test results and numerical simulations, it may be concluded that the model is validated in term of strains up to the moment when the crack becomes visible, and in terms of crack pattern.

Crack closure on rehydration of glass-ionomer materials.

PubMed

Sidhu, Sharanbir K; Pilecki, Peter; Sherriff, Martyn; Watson, Timothy F

2004-10-01

Moisture-sensitivity of immature glass-ionomer cements suggests that hydration-induced volumetric expansion might close and potentially heal established cracks. Crack closure in glass-ionomer cements (GICs) was observed following rehydration. Circular cavities were prepared in 15 teeth: 10 were restored with resin-modified GICs (5 with Fuji II LC and 5 with Photac-Fil) and 5 were restored with a conventional GIC (Fuji IX); all were dehydrated for 1 min with air and imaged immediately by confocal microscopy. Crack formation in each was located, after which water was placed on the surface and observed for 15 min via a CCD camera. Dehydration caused cracks with measurable gaps, while rehydration resulted in varying degrees of closure: closure was limited in the conventional GIC, and complete or near complete along part/s of the crack in the resin-modified GICs. In all, closure movement became imperceptible after the first 10 min. Statistical analysis indicated no significant difference between the closure behavior of all materials. However, the resin-modified GICs appeared to show a greater potential for closure of established cracks than the conventional GIC upon rehydration.

Incorporation of Half-Cycle Theory Into Ko Aging Theory for Aerostructural Flight-Life Predictions

NASA Technical Reports Server (NTRS)

Ko, William L.; Tran, Van T.; Chen, Tony

2007-01-01

The half-cycle crack growth theory was incorporated into the Ko closed-form aging theory to improve accuracy in the predictions of operational flight life of failure-critical aerostructural components. A new crack growth computer program was written for reading the maximum and minimum loads of each half-cycle from the random loading spectra for crack growth calculations and generation of in-flight crack growth curves. The unified theories were then applied to calculate the number of flights (operational life) permitted for B-52B pylon hooks and Pegasus adapter pylon hooks to carry the Hyper-X launching vehicle that air launches the X-43 Hyper-X research vehicle. A crack growth curve for each hook was generated for visual observation of the crack growth behavior during the entire air-launching or captive flight. It was found that taxiing and the takeoff run induced a major portion of the total crack growth per flight. The operational life theory presented can be applied to estimate the service life of any failure-critical structural components.

Electromagnetic pulsed thermography for natural cracks inspection

PubMed Central

Gao, Yunlai; Tian, Gui Yun; Wang, Ping; Wang, Haitao; Gao, Bin; Woo, Wai Lok; Li, Kongjing

2017-01-01

Emerging integrated sensing and monitoring of material degradation and cracks are increasingly required for characterizing the structural integrity and safety of infrastructure. However, most conventional nondestructive evaluation (NDE) methods are based on single modality sensing which is not adequate to evaluate structural integrity and natural cracks. This paper proposed electromagnetic pulsed thermography for fast and comprehensive defect characterization. It hybrids multiple physical phenomena i.e. magnetic flux leakage, induced eddy current and induction heating linking to physics as well as signal processing algorithms to provide abundant information of material properties and defects. New features are proposed using 1st derivation that reflects multiphysics spatial and temporal behaviors to enhance the detection of cracks with different orientations. Promising results that robust to lift-off changes and invariant features for artificial and natural cracks detection have been demonstrated that the proposed method significantly improves defect detectability. It opens up multiphysics sensing and integrated NDE with potential impact for natural understanding and better quantitative evaluation of natural cracks including stress corrosion crack (SCC) and rolling contact fatigue (RCF). PMID:28169361

Analysis of the Influence of Cracked Sleepers under Static Loading on Ballasted Railway Tracks

PubMed Central

Montalbán Domingo, Laura; Zamorano Martín, Clara; Palenzuela Avilés, Cristina; Real Herráiz, Julia I.

2014-01-01

The principal causes of cracking in prestressed concrete sleepers are the dynamic loads induced by track irregularities and imperfections in the wheel-rail contact and the in-phase and out-of-phase track resonances. The most affected points are the mid-span and rail-seat sections of the sleepers. Central and rail-seat crack detection require visual inspections, as legislation establishes, and involve sleepers' renewal even though European Normative considers that thicknesses up to 0.5 mm do not imply an inadequate behaviour of the sleepers. For a better understanding of the phenomenon, the finite element method constitutes a useful tool to assess the effects of cracking from the point of view of structural behaviour in railway track structures. This paper intends to study how the cracks at central or rail-seat section in prestressed concrete sleepers influence the track behaviour under static loading. The track model considers three different sleeper models: uncracked, cracked at central section, and cracked at rail-seat section. These models were calibrated and validated using the frequencies of vibration of the first three bending modes obtained from an experimental modal analysis. The results show the insignificant influence of the central cracks and the notable effects of the rail-seat cracks regarding deflections and stresses. PMID:25530998

Review on stress corrosion and corrosion fatigue failure of centrifugal compressor impeller

NASA Astrophysics Data System (ADS)

Sun, Jiao; Chen, Songying; Qu, Yanpeng; Li, Jianfeng

2015-03-01

Corrosion failure, especially stress corrosion cracking and corrosion fatigue, is the main cause of centrifugal compressor impeller failure. And it is concealed and destructive. This paper summarizes the main theories of stress corrosion cracking and corrosion fatigue and its latest developments, and it also points out that existing stress corrosion cracking theories can be reduced to the anodic dissolution (AD), the hydrogen-induced cracking (HIC), and the combined AD and HIC mechanisms. The corrosion behavior and the mechanism of corrosion fatigue in the crack propagation stage are similar to stress corrosion cracking. The effects of stress ratio, loading frequency, and corrosive medium on the corrosion fatigue crack propagation rate are analyzed and summarized. The corrosion behavior and the mechanism of stress corrosion cracking and corrosion fatigue in corrosive environments, which contain sulfide, chlorides, and carbonate, are analyzed. The working environments of the centrifugal compressor impeller show the behavior and the mechanism of stress corrosion cracking and corrosion fatigue in different corrosive environments. The current research methods for centrifugal compressor impeller corrosion failure are analyzed. Physical analysis, numerical simulation, and the fluid-structure interaction method play an increasingly important role in the research on impeller deformation and stress distribution caused by the joint action of aerodynamic load and centrifugal load.

Experimental and numerical investigation of crack initiation and propagation in silicon nitride ceramic under rolling and cyclic contact

NASA Astrophysics Data System (ADS)

Raga, Rahul; Khader, Iyas; Zdeněk, Chlup; Kailer, Andreas

2017-05-01

The focus of the work was to investigate crack initiation and propagation mechanisms in silicon nitride undergoing non-conforming hybrid contact under various tribological conditions. In order to understand the prevailing modes of damage in silicon nitride, two distinct model experiments were proposed, namely, rolling contact and cyclic contact experiments. The rolling contact experiment was designed in order to mimic the contact conditions appearing in hybrid bearings at contact pressures ranging from 3 to 6 GPa. On the other hand, cyclic contact experiments with stresses ranging from 4 to 15 GPa under different media were carried out to study damage under localised stresses. In addition, the experimentally observed cracks were implemented in a finite element model to study the stress redistribution and correlate the generated stresses with the corresponding mechanisms. Crack propagation under rolling contact was attributed to two different mechanisms, namely, fatigue induced fracture and lubricant driven crack propagation. The numerical simulations shed light on the tensile stress driven surface and subsurface crack propagation mechanisms. On the other hand, the cyclic contact experiments showed delayed crack formation for lubricated cyclic contact. Ceramographic cross-sectional analysis showed crack patterns similar to Hertzian crack propagation under cyclic contact load.

Environmental aging and degradation of multiwalled carbon nanotube reinforced polypropylene

EPA Science Inventory

The degradation of polypropylene (PP) and PP-multiwalled carbon nanotube (PP-MWCNT) panels during environmental weathering resulted in an increased degree of crystallinity, making them brittle, and creating surface cracks. The degradation led to a breakdown of the panels and incr...

Thermodynamic analysis on the role of hydrogen in anodic stress corrosion cracking

DOE Office of Scientific and Technical Information (OSTI.GOV)

Qiao, L.; Mao, X.

1995-11-01

A synergistic effect of hydrogen and stress on a corrosion rate was analyzed with thermodynamics. The results showed that an interaction of stress and hydrogen could increase the corrosion rate remarkably. Stress corrosion cracking (SCC) of austenitic stainless steel (ASS) was investigated in boiling chloride solution to confirm the analysis. Hydrogen could be introduced into the specimen concentrated at the crack tip during SCC in boiling LiCl solution (143 C). The concentrating factor is about 3 which is consistent with calculated results according to stress induced diffusion.

Analytical and Experimental Study of Near-Threshold Interactions Between Crack Closure Mechanisms

NASA Technical Reports Server (NTRS)

Newman, John A.; Riddell, William T.; Piascik, Robert S.

2003-01-01

The results of an analytical closure model that considers contributions and interactions between plasticity-, roughness-, and oxide-induced crack closure mechanisms are presented and compared with experimental data. The analytical model is shown to provide a good description of the combined influences of crack roughness, oxide debris, and plasticity in the near-threshold regime. Furthermore, analytical results indicate that closure mechanisms interact in a non-linear manner such that the total amount of closure is not the sum of closure contributions for each mechanism.

The role of magnetic iron oxide nanoparticles in the bacterially induced calcium carbonate precipitation.

PubMed

Seifan, Mostafa; Ebrahiminezhad, Alireza; Ghasemi, Younes; Samani, Ali Khajeh; Berenjian, Aydin

2018-04-01

Recently, magnetic iron oxide nanoparticles (IONs) have been used to control and modify the characteristics of concrete and mortar. Concrete is one of the most used materials in the world; however, it is susceptible to cracking. Over recent years, a sustainable biotechnological approach has emerged as an alternative approach to conventional techniques to heal the concrete cracks by the incorporation of bacterial cells and nutrients into the concrete matrix. Once cracking occurs, CaCO 3 is induced and the crack is healed. Considering the positive effects of IONs on the concrete properties, the effect of these nanoparticles on bacterial growth and CaCO 3 biosynthesis needs to be evaluated for their possible application in bio self-healing concrete. In the present work, IONs were successfully synthesized and characterized using various techniques. The presence of IONs showed a significant effect on both bacterial growth and CaCO 3 precipitation. The highest bacterial growth was observed in the presence of 150 μg/mL IONs. The highest concentration of induced CaCO 3 (34.54 g/L) was achieved when the bacterial cells were immobilized with 300 μg/mL of IONs. This study provides new data and supports the possibility of using IONs as a new tool in designing the next generation of bio self-healing concrete.

The effect of thermal processing on microstructure and mechanical properties in a nickel-iron alloy

NASA Astrophysics Data System (ADS)

Yang, Ling

The correlation between processing conditions, resulted microstructure and mechanical properties is of interest in the field of metallurgy for centuries. In this work, we investigated the effect of thermal processing parameters on microstructure, and key mechanical properties to turbine rotor design: tensile yield strength and crack growth resistance, for a nickel-iron based superalloy Inconel 706. The first step of the designing of experiments is to find parameter ranges for thermal processing. Physical metallurgy on superalloys was combined with finite element analysis to estimate variations in thermal histories for a large Alloy 706 forging, and the results were adopted for designing of experiments. Through the systematic study, correlation was found between the processing parameters and the microstructure. Five different types of grain boundaries were identified by optical metallography, fractography, and transmission electron microscopy, and they were found to be associated with eta precipitation at the grain boundaries. Proportions of types of boundaries, eta size, spacing and angle respect to the grain boundary were found to be dependent on processing parameters. Differences in grain interior precipitates were also identified, and correlated with processing conditions. Further, a strong correlation between microstructure and mechanical properties was identified. The grain boundary precipitates affect the time dependent crack propagation resistance, and different types of boundaries have different levels of resistance. Grain interior precipitates were correlated with tensile yield strength. It was also found that there is a strong environmental effect on time dependent crack propagation resistance, and the sensitivity to environmental damage is microstructure dependent. The microstructure with eta decorated on grain boundaries by controlled processing parameters is more resistant to environmental damage through oxygen embrittlement than material without eta phase on grain boundaries. Effort was made to explore the mechanisms of improving the time dependent crack propagation resistance through thermal processing, several mechanisms were identified in both environment dependent and environment independent category, and they were ranked based on their contributions in affecting crack propagation.

Combined effect of high curing temperature and crack width on chloride migration in reinforced concrete beams

NASA Astrophysics Data System (ADS)

Elkedrouci, L.; Diao, B.; Pang, S.; Li, Y.

2018-03-01

Deterioration of reinforced concrete structures is a serious concern in the construction engineering, largely due to chloride induced corrosion of reinforcement. Chloride penetration is markedly influenced by one or several major factors at the same time such as cuing in combination with different crack widths which have spectacular effect on reinforced concrete structures. This research presents the results of an experimental investigation involving reinforced concrete beams with three different crack widths ranging from 0 to 0.2mm, curing temperatures of 20°C or 40°C and water-to-cement of 0.5. Chloride content profiles were determined under non-steady state diffusion at 20°C. Based on the obtained results, higher chloride content was obtained under condition of high curing temperature in combination with large crack more than 0.1mm and there are no significant differences between narrow crack width (less than 0.1 mm) and beams without crack (0 mm).

Topography, surface features, and flooding of Rogers Lake playa, California

USGS Publications Warehouse

Dinehart, Randal L.; McPherson, Kelly R.

1998-01-01

Rogers Lake is a desert playa used as a military airport for Edwards Air Force Base in the Antelope Valley of southern California. Previous measurements of land subsidence and ground-water levels in the study area indicated that ground-water pumping induced tensional stresses in the playa, which were sporadically relieved through the formation of long cracks. Drying of the sediments beneath the playa also may have accelerated the natural formation of giant desiccation polygons. When water flows across the playa, the cracks erode into fissures of sufficient width and depth to endanger traffic on the playa. Topographic surveys of the playa were made to derive a contour map that would allow examination of erosive flow paths. Crack networks were surveyed in selected areas during 1995 and compared with cracks visible in aerial photographs taken in 1990. Crack networks remained visible in their positions following several inundations of the playa. The density of the crack networks increased in all of the selected areas.

Electromagnetic modulation of the ultrasonic signal for nondestructive detection of small defects and ferromagnetic inclusions in thin wall structures

NASA Astrophysics Data System (ADS)

Finkel, Peter

2008-03-01

We report on new nondestructive evaluation technique based on electromagnetic modulation of ultrasonic signal for detection of the small crack, flaws and inclusions in thin-walled parts. The electromagnetically induced high density current pulse produces stresses which alter the ultrasonic waves scanning the part with the defect and modulate ultrasonic signal. The excited electromagnetic field can produces crack-opening due to Lorentz forces that increase the ultrasonic reflection. The Joule heating associated with the high density current, and consequent thermal stresses may cause both crack-closure, as well as crack-opening, depending on various factors. Experimental data is presented here for the case of a small crack near holes in thin-walled structures. The measurements were taken at 2-10 MHz with a Lamb wave wedge transducer. It is shown that electromagnetic transient modulation of the ultrasonic echo pulse tone-burst suggest that this method could be used to enhance detection of small cracks and ferromagnetic inclusions in thin walled metallic structures.

Fracture mechanics and corrosion fatigue.

NASA Technical Reports Server (NTRS)

Mcevily, A. J.; Wei, R. P.

1972-01-01

Review of the current state-of-the-art in fracture mechanics, particularly in relation to the study of problems in environment-enhanced fatigue crack growth. The usefulness of this approach in developing understanding of the mechanisms for environmental embrittlement and its engineering utility are discussed. After a brief review of the evolution of the fracture mechanics approach and the study of environmental effects on the fatigue behavior of materials, a study is made of the response of materials to fatigue and corrosion fatigue, the modeling of the mechanisms of the fatigue process is considered, and the application of knowledge of fatigue crack growth to the prediction of the high cycle life of unnotched specimens is illustrated.

Stripe-PZT Sensor-Based Baseline-Free Crack Diagnosis in a Structure with a Welded Stiffener.

PubMed

An, Yun-Kyu; Shen, Zhiqi; Wu, Zhishen

2016-09-16

This paper proposes a stripe-PZT sensor-based baseline-free crack diagnosis technique in the heat affected zone (HAZ) of a structure with a welded stiffener. The proposed technique enables one to identify and localize a crack in the HAZ using only current data measured using a stripe-PZT sensor. The use of the stripe-PZT sensor makes it possible to significantly improve the applicability to real structures and minimize man-made errors associated with the installation process by embedding multiple piezoelectric sensors onto a printed circuit board. Moreover, a new frequency-wavenumber analysis-based baseline-free crack diagnosis algorithm minimizes false alarms caused by environmental variations by avoiding simple comparison with the baseline data accumulated from the pristine condition of a target structure. The proposed technique is numerically as well as experimentally validated using a plate-like structure with a welded stiffener, reveling that it successfully identifies and localizes a crack in HAZ.

Stripe-PZT Sensor-Based Baseline-Free Crack Diagnosis in a Structure with a Welded Stiffener

PubMed Central

An, Yun-Kyu; Shen, Zhiqi; Wu, Zhishen

2016-01-01

This paper proposes a stripe-PZT sensor-based baseline-free crack diagnosis technique in the heat affected zone (HAZ) of a structure with a welded stiffener. The proposed technique enables one to identify and localize a crack in the HAZ using only current data measured using a stripe-PZT sensor. The use of the stripe-PZT sensor makes it possible to significantly improve the applicability to real structures and minimize man-made errors associated with the installation process by embedding multiple piezoelectric sensors onto a printed circuit board. Moreover, a new frequency-wavenumber analysis-based baseline-free crack diagnosis algorithm minimizes false alarms caused by environmental variations by avoiding simple comparison with the baseline data accumulated from the pristine condition of a target structure. The proposed technique is numerically as well as experimentally validated using a plate-like structure with a welded stiffener, reveling that it successfully identifies and localizes a crack in HAZ. PMID:27649200

Corrosion fatigue characterization of reactor pressure vessel steels. [PWR; BWR

DOE Office of Scientific and Technical Information (OSTI.GOV)

Van Der Sluys, W.A.

1982-12-01

During routine operation, light water reactor (LWR) pressure vessels are subjected to a variety of transients that result in time-varying stresses. Consequently, fatigue and environmentally-assisted fatigue are mechanisms of growth relevant to flaws in these pressure vessels. To provide a better understanding of the resistance of nuclear pressure vessel steels to these flaw growth processes, fracture mechanics data were generated on the rates of fatigue crack growth for SA508-2 and SA533B-1 steels in both room temperature air and 288/sup 0/C water. Areas investigated were: the relationship of crack growth rate to prior loading history; the effects of loading frequency andmore » R ratio (K/sub min//K/sub max/) on crack growth rate as a function of the stress intensity factor range (..delta..K); transient aspects of the fatigue crack growth behavior; the effect of material chemistry (sulphur content) on fatigue crack; and growth rate; water chemistry effects (high-purity water versus simulated pressurized water reactotr (PWR) primary coolant).« less

Neural-Fuzzy model Based Steel Pipeline Multiple Cracks Classification

NASA Astrophysics Data System (ADS)

Elwalwal, Hatem Mostafa; Mahzan, Shahruddin Bin Hj.; Abdalla, Ahmed N.

2017-10-01

While pipes are cheaper than other means of transportation, this cost saving comes with a major price: pipes are subject to cracks, corrosion etc., which in turn can cause leakage and environmental damage. In this paper, Neural-Fuzzy model for multiple cracks classification based on Lamb Guide Wave. Simulation results for 42 sample were collected using ANSYS software. The current research object to carry on the numerical simulation and experimental study, aiming at finding an effective way to detection and the localization of cracks and holes defects in the main body of pipeline. Considering the damage form of multiple cracks and holes which may exist in pipeline, to determine the respective position in the steel pipe. In addition, the technique used in this research a guided lamb wave based structural health monitoring method whereas piezoelectric transducers will use as exciting and receiving sensors by Pitch-Catch method. Implementation of simple learning mechanism has been developed specially for the ANN for fuzzy the system represented.

The role of grain boundary chemistry and structure in the environmentally-assisted intergranular cracking of nickel-base alloys

DOE Office of Scientific and Technical Information (OSTI.GOV)

Was, G.S.

1992-07-01

Stress corrosion cracking tests in constant extension rate tensile (CERT) and constant load tensile (CLT) tests were conducted on Ni-xCr- 9Fe-yC in Ar, water, and a LiOH-boric acid solution. Cr and C improve the resistance of Ni-base alloys to IG cracking in both Ar and water at 360C. Since creep plays a role in IG cracking, one possible explanation for the role of the environment involves its effect on the creep. Experiments were conducted on the role of C in the deformation behavior and failure mode of Ni-16Cr-9Fe. Constant load experiments were conducted on Ni-16Cr-9Fe to determine if the CLTmore » test is more aggressive than CERT. The electron backscattering technique in a SEM is being developed in order to extend the IG cracking studies to grain sizes typical of commercial alloys, 20-30 microns.« less

The role of grain boundary chemistry and structure in the environmentally-assisted intergranular cracking of nickel-base alloys. Progress report, August 1, 1991--July 31, 1992

DOE Office of Scientific and Technical Information (OSTI.GOV)

Was, G.S.

1992-07-01

Stress corrosion cracking tests in constant extension rate tensile (CERT) and constant load tensile (CLT) tests were conducted on Ni-xCr- 9Fe-yC in Ar, water, and a LiOH-boric acid solution. Cr and C improve the resistance of Ni-base alloys to IG cracking in both Ar and water at 360C. Since creep plays a role in IG cracking, one possible explanation for the role of the environment involves its effect on the creep. Experiments were conducted on the role of C in the deformation behavior and failure mode of Ni-16Cr-9Fe. Constant load experiments were conducted on Ni-16Cr-9Fe to determine if the CLTmore » test is more aggressive than CERT. The electron backscattering technique in a SEM is being developed in order to extend the IG cracking studies to grain sizes typical of commercial alloys, 20-30 microns.« less

Fatigue resistance and crack propensity of novel "super-closed" sandwich composite resin restorations in large MOD defects.

PubMed

Magne, Pascal; Silva, Silvana; Andrada, Mauro de; Maia, Hamilton

2016-01-01

To assess the influence of conventional glass ionomer cement (GIC) vs resin-modified GIC (RMGIC) as a base material for novel, super-closed sandwich restorations (SCSR) and its effect on shrinkage-induced crack propensity and in vitro accelerated fatigue resistance. A standardized MOD slottype tooth preparation was applied to 30 extracted maxillary molars (5 mm depth/5 mm buccolingual width). A modified sandwich restoration was used, in which the enamel/dentin bonding agent was applied first (Optibond FL, Kerr), followed by a Ketac Molar (3M ESPE)(group KM, n = 15) or Fuji II LC (GC) (group FJ, n = 15) base, leaving 2 mm for composite resin material (Miris 2, Coltène-Whaledent). Shrinkageinduced enamel cracks were tracked with photography and transillumination. Samples were loaded until fracture or to a maximum of 185,000 cycles under isometric chewing (5 H z), starting with a load of 200 N (5,000 X), followed by stages of 400, 600, 800, 1,000, 1,200, and 1,400 N at a maximum of 30,000 X each. Groups were compared using the life table survival analysis (α = .008, Bonferroni method). Group FJ showed the highest survival rate (40% intact specimens) but did not differ from group KM (20%) or traditional direct restorations (13%, previous data). SCSR generated less shrinkage-induced cracks. Most failures were re-restorable (above the cementoenamel junction [CEJ]). Inclusion of GIC/RMGIC bases under large direct SCSRs does not affect their fatigue strength but tends to decrease the shrinkage-induced crack propensity. The use of GIC/ RMGIC bases and the SCSR is an easy way to minimize polymerization shrinkage stress in large MOD defects without weakening the restoration.

ENVIRONMENTAL ANALYSIS OF GASOLINE BLENDING COMPONENTS THROUGH THEIR LIFE CYCLE

EPA Science Inventory

The purpose of this study is to access the contribution of the three major gasoline blending components to the potential environmental impacts (PEI), which are the reformate, alkylate and cracked gasoline. This study accounts for losses of the gasoline blending components due to...

ENVIRONMENTAL ANALYSIS OF GASOLINE BLENDING COMPONENTS THROUGH THEIR LIFE CYCLE

EPA Science Inventory

The purpose of this study is to assess the contribution of the three major gasoline blending components to the potential environmental impacts (PEI), which are the reformate, alkylate and cracked gasoline. This study accounts for losses of the gasoline blending components due to ...

Psychological and environmental determinants of relapse in crack cocaine smokers.

PubMed

Wallace, B C

1989-01-01

The paper reviews approaches to relapse in the treatment of cocaine abusers. Approaches reveal a common mechanism underlying relapse that involves drug craving, recall of euphoria, environmental cues, denial, myths of being able to sell or use drugs, and painful affect states necessitating use of a multifaceted clinical technique. Empirical validation of a common mechanism underlying relapse establishes a typology of psychological and environmental determinants of relapse for crack cocaine smokers (N = 35) who relapse after hospital detoxification and return a second time. Major findings are that relapse follows a painful emotional state (40%), failure to enter arranged aftercare treatment (37%), or encounters with conditioned environmental stimuli (34%), and involves narcissistic psychopathology and denial (28.5%) and interpersonal stress (24%); 85.7% involve multideterminants. Case examples illustrate the role of multideterminants in relapse. The paper educates clinicians to the integrated theory and multifaceted clinical technique necessary for efficacious treatment of cocaine patients, while the typology predicts probable relapse situations.

Experimental Characterization and Simulation of Slip Transfer at Grain Boundaries and Microstructurally-Sensitive Crack Propagation

NASA Technical Reports Server (NTRS)

Gupta, Vipul; Hochhalter, Jacob; Yamakov, Vesselin; Scott, Willard; Spear, Ashley; Smith, Stephen; Glaessgen, Edward

2013-01-01

A systematic study of crack tip interaction with grain boundaries is critical for improvement of multiscale modeling of microstructurally-sensitive fatigue crack propagation and for the computationally-assisted design of more durable materials. In this study, single, bi- and large-grain multi-crystal specimens of an aluminum-copper alloy are fabricated, characterized using electron backscattered diffraction (EBSD), and deformed under tensile loading and nano-indentation. 2D image correlation (IC) in an environmental scanning electron microscope (ESEM) is used to measure displacements near crack tips, grain boundaries and within grain interiors. The role of grain boundaries on slip transfer is examined using nano-indentation in combination with high-resolution EBSD. The use of detailed IC and EBSD-based experiments are discussed as they relate to crystal-plasticity finite element (CPFE) model calibration and validation.

Changes in crack shape and saturation during water penetration into stressed rock

NASA Astrophysics Data System (ADS)

Masuda, K.; Nishizawa, O.

2012-12-01

Open cracks and cavities in rocks play important roles in fluid transport. Water penetration induced microcrack activities and caused the failure of rocks. Fluids in cracks affect earthquake generation mechanism through physical and physicochemical effects. Methods of characterizing crack shape and water saturation of rocks underground are needed for many scientific and industrial applications. It would be desirable to estimate the status of cracks using readily observable data such as elastic-wave velocities. We demonstrate a laboratory method for estimating crack status inside a cylindrical rock sample based on least-squares fitting of a cracked solid model to measured P- and S-wave velocities, and porosity derived from strain data. We used a cylinder (50 mm in diameter and 100 mm in length) of medium-grained granite. We applied a differential stress of 370 MPa, which corresponds to about 70% of fracture strength, to the rock sample under 30 MPa confining pressure and held it constant throughout the experiment. When the primary creep stage and acoustic emission (AE) caused by the initial loading had ceased, we injected distilled water into the bottom end of the sample at a constant pressure of 25 MPa until macroscopic fracture occurred. During water migration, we measured P waves and S waves (Sv and Sh), in five directions parallel to the top and bottom surfaces of the sample. We also measured strains of the sample surface and monitored AE. We created X-ray computer tomography (CT) images of the rock sample after the experiment in order to recognize the location and shape of fractured surfaces. We observed the different patterns of velocity changes in the upper and lower portions of the rock sample. Changes in P-wave velocities can be interpreted based on the crack density. S-waves showed the splitting with Vsv being faster than Vsh, corresponding to the second kind of anisotropy. We estimated two crack characteristics, crack shape and the degree of water saturation, and their changes during the loading and water migration into a granitic rock subjected to confining pressure and differential stress. We found that during injection of water to induce failure of a stressed rock sample, the aspect ratio of cracks increased and the degree of water saturation increased to about 70%. Laboratory derived method can be applicable for the well-planned observation in the field experiments. Monitoring in situ crack situations with seismic waves are useful for industrial and scientific applications such as sequestrations of carbon dioxide and waste, and measuring the regional stress field.

A methodology for the investigation of toughness and crack propagation in mouse bone.

PubMed

Carriero, Alessandra; Zimmermann, Elizabeth A; Shefelbine, Sandra J; Ritchie, Robert O

2014-11-01

Bone fracture is a health concern for those with aged bone and brittle bone diseases. Mouse bone is widely used as a model of human bone, especially to investigate preclinical treatment strategies. However, little is known about the mechanisms of mouse bone fracture and its similarities and differences from fracture in human bone. In this work we present a methodology to investigate the fracture toughness during crack initiation and crack propagation for mouse bone. Mouse femora were dissected, polished on their periosteal surface, notched on the posterior surface at their mid-diaphysis, and tested in three-point bending under displacement control at a rate of 0.1mm/min using an in situ loading stage within an environmental scanning electron microscope. We obtained high-resolution real-time imaging of the crack initiation and propagation in mouse bone. From the images we can measure the crack extension at each step of the crack growth and calculate the toughness of the bone (in terms of stress intensity factor (K) and work to fracture (Wf)) as a function of stable crack length (Δa), thus generating a resistance curve for the mouse bone. The technique presented here provides insight into the evolution of microdamage and the toughening mechanisms that resist crack propagation, which are essential for preclinical development of treatments to enhance bone quality and combat fracture risk. Copyright © 2014 Elsevier Ltd. All rights reserved.

Effectiveness of secondary prevention and treatment interventions for crack-cocaine abuse: a comprehensive narrative overview of English-language studies.

PubMed

Fischer, Benedikt; Blanken, Peter; Da Silveira, Dartiu; Gallassi, Andrea; Goldner, Elliot M; Rehm, Jürgen; Tyndall, Mark; Wood, Evan

2015-04-01

There are an estimated several million crack-cocaine users globally; use is highest in the Americas. Most crack users are socio-economically marginalized (e.g., homeless), and feature elevated risks for morbidity (e.g., blood-borne viruses), mortality and crime/violence involvement, resulting in extensive burdens. No comprehensive reviews of evidence-based prevention and/or treatment interventions specifically for crack use exist. We conducted a comprehensive narrative overview of English-language studies on the efficacy of secondary prevention and treatment interventions for crack (cocaine) abuse/dependence. Literature searches (1990-2014) using pertinent keywords were conducted in main scientific databases. Titles/abstracts were reviewed for relevance, and full studies were included in the review if involving a primary prevention/treatment intervention study comprising a substantive crack user sample. Intervention outcomes considered included drug use, health risks/status (e.g., HIV or sexual risks) and select social outcome indicators. Targeted (e.g., behavioral/community-based) prevention measures show mixed and short-term effects on crack use/HIV risk outcomes. Material (e.g., safer crack use kit distribution) interventions also document modest efficacy in risk reduction; empirical assessments of environmental (e.g., drug consumption facilities) for crack smokers are not available. Diverse psycho-social treatment (including contingency management) interventions for crack abuse/dependence show some positive but also limited/short-term efficacy, yet likely constitute best currently available treatment options. Ancillary treatments show little effects but are understudied. Despite ample studies, pharmaco-therapeutic/immunotherapy treatment agents have not produced convincing evidence; select agents may hold potential combined with personalized approaches and/or psycho-social strategies. No comprehensively effective 'gold-standard' prevention/treatment interventions for crack abuse exist; concerted research towards improved interventions is urgently needed. Copyright © 2015 Elsevier B.V. All rights reserved.

Fracture study of windshield glass panes

NASA Technical Reports Server (NTRS)

Yeh, H. Y.

1987-01-01

The major stresses which cause crack propagation in windshield glass panes are induced by bending moments which result from the pressure differentials across the panes. Hence the stress intensity factors for the finite plate with the semi-elliptical surface flaw and edge crack under the bending moments are examined. The results show that the crack growth will be upperbound if it is computed by using the stress intensity factor for the finite plate with the edge crack subjected to pure bending moments. Furthermore, if the ratio of crack depth to plate thickness, a/t, is within 0.3, the stress intensity factor can be conservatively assumed to be constant at the value of a/t equal to zero. A simplified equation to predict the structural life of glass panes is derived based on constant stress intensity factor. The accuracy of structural life is mainly dependent on how close the empirical parameter, m, can be estimated.

EPRI-NASA Cooperative Project on Stress Corrosion Cracking of Zircaloys. [nuclear fuel failures

NASA Technical Reports Server (NTRS)

Cubicciotti, D.; Jones, R. L.

1978-01-01

Examinations of the inside surface of irradiated fuel cladding from two reactors show the Zircaloy cladding is exposed to a number of aggressive substances, among them iodine, cadmium, and iron-contaminated cesium. Iodine-induced stress corrosion cracking (SCC) of well characterized samples of Zircaloy sheet and tubing was studied. Results indicate that a threshold stress must be exceeded for iodine SCC to occur. The existence of a threshold stress indicates that crack formation probably is the key step in iodine SCC. Investigation of the crack formation process showed that the cracks responsible for SCC failure nucleated at locations in the metal surface that contained higher than average concentrations of alloying elements and impurities. A four-stage model of iodine SCC is proposed based on the experimental results and the relevance of the observations to pellet cladding interaction failures is discussed.

Reference-free fatigue crack detection using nonlinear ultrasonic modulation under various temperature and loading conditions

NASA Astrophysics Data System (ADS)

Lim, Hyung Jin; Sohn, Hoon; DeSimio, Martin P.; Brown, Kevin

2014-04-01

This study presents a reference-free fatigue crack detection technique using nonlinear ultrasonic modulation. When low frequency (LF) and high frequency (HF) inputs generated by two surface-mounted lead zirconate titanate (PZT) transducers are applied to a structure, the presence of a fatigue crack can provide a mechanism for nonlinear ultrasonic modulation and create spectral sidebands around the frequency of the HF signal. The crack-induced spectral sidebands are isolated using a combination of linear response subtraction (LRS), synchronous demodulation (SD) and continuous wavelet transform (CWT) filtering. Then, a sequential outlier analysis is performed on the extracted sidebands to identify the crack presence without referring any baseline data obtained from the intact condition of the structure. Finally, the robustness of the proposed technique is demonstrated using actual test data obtained from simple aluminum plate and complex aircraft fitting-lug specimens under varying temperature and loading variations.

Capillary-induced crack healing between surfaces of nanoscale roughness.

PubMed

Soylemez, Emrecan; de Boer, Maarten P

2014-10-07

Capillary forces are important in nature (granular materials, insect locomotion) and in technology (disk drives, adhesion). Although well studied in equilibrium state, the dynamics of capillary formation merit further investigation. Here, we show that microcantilever crack healing experiments are a viable experimental technique for investigating the influence of capillary nucleation on crack healing between rough surfaces. The average crack healing velocity, v̅, between clean hydrophilic polycrystalline silicon surfaces of nanoscale roughness is measured. A plot of v̅ versus energy release rate, G, reveals log-linear behavior, while the slope |d[log(v̅)]/dG| decreases with increasing relative humidity. A simplified interface model that accounts for the nucleation time of water bridges by an activated process is developed to gain insight into the crack healing trends. This methodology enables us to gain insight into capillary bridge dynamics, with a goal of attaining a predictive capability for this important microelectromechanical systems (MEMS) reliability failure mechanism.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Preston, Daniel N; Peterson, Paul D; Kien - Yin, Lee

Structural damage in energetic materials plays a significant role in the probability of nonshock initiation events. Damage may occur in the form of voids or cracks either within crystals or in binder-rich regions between crystals. These cracks affect whether hotspots generated by impact will quench or propagate under non-shock insult. For this study, we have separately engineered intracrystalline and inter-crystalline cracks in to the HMX-based PBX 9501. Intra-crystalline cracks were created by subjecting HMX to forward and reverse solid-to-solid phase transformations prior to formulation. Inter-crystalline cracks were induced by compressing formulated samples of PBX 9501 at an average strain ratemore » of 0.00285 S{sup -1}. Both sets of pre-damaged explosives were then impact tested using the LANL Type 12 Drop Weight-Impact Machine and their sensitivities compared to nondamaged PBX 9501. Results of these tests clearly show significant differences in sensitivity between damaged and non-damaged PBX 9501.« less

A computerized test system for thermal-mechanical fatigue crack growth

NASA Technical Reports Server (NTRS)

Marchand, N.; Pelloux, R. M.

1986-01-01

A computerized testing system to measure fatigue crack growth under thermal-mechanical fatigue conditions is described. Built around a servohydraulic machine, the system is capable of a push-pull test under stress-controlled or strain-controlled conditions in the temperature range of 25 to 1050 C. Temperature and mechanical strain are independently controlled by the closed-loop system to simulate the complex inservice strain-temperature relationship. A d-c electrical potential method is used to measure crack growth rates. The correction procedure of the potential signal to take into account powerline and RF-induced noises and thermal changes is described. It is shown that the potential drop technique can be used for physical mechanism studies and for modelling crack tip processes.

Factors that lead to the use of crack cocaine in combination with marijuana in Brazil: a qualitative study.

PubMed

Gonçalves, Janaina R; Nappo, Solange A

2015-07-25

In Brazil, crack cocaine use remains a healthcare challenge due to the rapid onset of its pleasurable effects, its ability to induce craving and addiction, and the fact that it is easily accessible. Delayed action on the part of the Brazilian Government in addressing the drug problem has led users to develop their own strategies for surviving the effects of crack cocaine use, particularly the drug craving and psychosis. In this context, users have sought the benefits of combining crack cocaine with marijuana. Our aim was to identify the reasons why users combine crack cocaine with marijuana and the health implications of doing so. The present study is a qualitative study, using in-depth interviews and criteria-based sampling, following 27 crack cocaine users who combined its use with marijuana. Participants were recruited using the snowball sampling technique, and the point of theoretical saturation was used to define the sample size. Data were analyzed using the content analysis technique. The interviewees reported that the combination of crack cocaine use with marijuana provided "protection" (reduced undesirable effects, improved sleep and appetite, reduced craving for crack cocaine, and allowed the patients to recover some quality of life). Combined use of cannabis as a strategy to reduce the effects of crack exhibited several significant advantages, particularly an improved quality of life, which "protected" users from the violence typical of the crack culture. Crack use is considered a serious public health problem in Brazil, and there are few solution strategies. Within that limited context, the combination of cannabis and crack deserves more thorough clinical investigation to assess its potential use as a strategy to reduce the damage associated with crack use.

Progress of a Cross-Correlation Based Optical Strain Measurement Technique for Detecting Radial Growth on a Rotating Disk

NASA Technical Reports Server (NTRS)

Clem, Michelle M.; Woike, Mark R.; Abdul-Aziz, Ali

2014-01-01

The Aeronautical Sciences Project under NASA's Fundamental Aeronautics Program is interested in the development of novel measurement technologies, such as optical surface measurements for the in situ health monitoring of critical constituents of the internal flow path. In situ health monitoring has the potential to detect flaws, i.e. cracks in key components, such as engine turbine disks, before the flaws lead to catastrophic failure. The present study, aims to further validate and develop an optical strain measurement technique to measure the radial growth and strain field of an already cracked disk, mimicking the geometry of a sub-scale turbine engine disk, under loaded conditions in the NASA Glenn Research Center's High Precision Rotordynamics Laboratory. The technique offers potential fault detection by imaging an applied high-contrast random speckle pattern under unloaded and loaded conditions with a CCD camera. Spinning the cracked disk at high speeds (loaded conditions) induces an external load, resulting in a radial growth of the disk of approximately 50.0-µm in the flawed region and hence, a localized strain field. When imaging the cracked disk under static conditions, the disk will be undistorted; however, during rotation the cracked region will grow radially, thus causing the applied particle pattern to be 'shifted'. The resulting particle displacements between the two images is measured using the two-dimensional cross-correlation algorithms implemented in standard Particle Image Velocimetry (PIV) software to track the disk growth, which facilitates calculation of the localized strain field. A random particle distribution is adhered onto the surface of the cracked disk and two bench top experiments are carried out to evaluate the technique's ability to measure the induced particle displacements. The disk is shifted manually using a translation stage equipped with a fine micrometer and a hotplate is used to induce thermal growth of the disk, causing the particles to become shifted. For both experiments, reference and test images are acquired before and after the induced shifts, respectively, and then processed using PIV software. The controlled manual translation of the disk resulted in detection of the particle displacements accurate to 1.75% of full scale and the thermal expansion experiment resulted in successful detection of the disk's thermal growth as compared to the calculated thermal expansion results. After validation of the technique through the induced shift experiments, the technique is implemented in the Rotordynamics Lab for preliminary assessment in a simulated engine environment. The discussion of the findings and plans for future work to improve upon the results are addressed in the paper.

Microcracking in Composite Laminates: Simulation of Crack-Induced Ultrasound Attenuation

NASA Technical Reports Server (NTRS)

Leckey, C. A. C.; Rogge, M. D.; Parker, F. R.

2012-01-01

Microcracking in composite laminates is a known precursor to the growth of inter-ply delaminations and larger scale damage. Microcracking can lead to the attenuation of ultrasonic waves due to the crack-induced scattering. 3D elastodynamic finite integration technique (EFIT) has been implemented to explore the scattering of ultrasonic waves due to microcracks in anisotropic composite laminates. X-ray microfocus computed tomography data was directly input into the EFIT simulation for these purposes. The validated anisotropic 3D EFIT code is shown to be a useful tool for exploring the complex multiple-scattering which arises from extensive microcracking.

Fatigue crack growth in 7475-T651 aluminum alloy plate in hard vacuum and water vapor. M.S. Thesis - George Washington Univ.

NASA Technical Reports Server (NTRS)

Dicus, D. L.

1981-01-01

Compact specimens of 25 mm thick aluminum alloy plate were subjected to constant amplitude fatigue testing at a load ratio of 0.2. Crack growth rates were determined at frequencies of 1 Hz and 10 Hz in hard vacuum and laboratory air, and in mixtures of water vapor and nitrogen at water vapor partial pressures ranging from 94 Pa to 3.8 kPa. A significant effect of water vapor on fatigue crack growth rates was observed at the lowest water vapor pressure tested. Crack rates changed little for pressures up to 1.03 kPa, but abruptly accelerated at higher pressures. At low stress intensity factor ranges, cracking rates at the lowest and highest water vapor pressure tested were, respectively, two and five times higher than rates in vacuum. Although a frequency was observed in laboratory air, cracking rates in water vapor and vacuum are insensitive to a ten-fold change in frequency. Surfaces of specimens tested in water vapor and vacuum exhibited different amounts of residual deformation. Reduced deformation on the fracture surfaces of the specimens tested in water vapor suggests embrittlement of the plastic zone ahead of the crack tip as a result of environmental interaction.

Experimental and Finite Element Modeling of Near-Threshold Fatigue Crack Growth for the K-Decreasing Test Method

NASA Technical Reports Server (NTRS)

Smith, Stephen W.; Seshadri, Banavara R.; Newman, John A.

2015-01-01

The experimental methods to determine near-threshold fatigue crack growth rate data are prescribed in ASTM standard E647. To produce near-threshold data at a constant stress ratio (R), the applied stress-intensity factor (K) is decreased as the crack grows based on a specified K-gradient. Consequently, as the fatigue crack growth rate threshold is approached and the crack tip opening displacement decreases, remote crack wake contact may occur due to the plastically deformed crack wake surfaces and shield the growing crack tip resulting in a reduced crack tip driving force and non-representative crack growth rate data. If such data are used to life a component, the evaluation could yield highly non-conservative predictions. Although this anomalous behavior has been shown to be affected by K-gradient, starting K level, residual stresses, environmental assisted cracking, specimen geometry, and material type, the specifications within the standard to avoid this effect are limited to a maximum fatigue crack growth rate and a suggestion for the K-gradient value. This paper provides parallel experimental and computational simulations for the K-decreasing method for two materials (an aluminum alloy, AA 2024-T3 and a titanium alloy, Ti 6-2-2-2-2) to aid in establishing clear understanding of appropriate testing requirements. These simulations investigate the effect of K-gradient, the maximum value of stress-intensity factor applied, and material type. A material independent term is developed to guide in the selection of appropriate test conditions for most engineering alloys. With the use of such a term, near-threshold fatigue crack growth rate tests can be performed at accelerated rates, near-threshold data can be acquired in days instead of weeks without having to establish testing criteria through trial and error, and these data can be acquired for most engineering materials, even those that are produced in relatively small product forms.

Environmental testing of block 3 solar cell modules. Part 1: Qualification testing of standard production modules

NASA Technical Reports Server (NTRS)

Griffith, J. S.

1979-01-01

Qualification tests of solar cell modules are described. These modules continue to show improvement over earlier type modules tested. Cell cracking and delamination are less prevalent, and interconnect problems and electrical degradation from environmental testing are now rare.

Analyses of Fatigue Crack Growth and Closure Near Threshold Conditions for Large-Crack Behavior

NASA Technical Reports Server (NTRS)

Newman, J. C., Jr.

1999-01-01

A plasticity-induced crack-closure model was used to study fatigue crack growth and closure in thin 2024-T3 aluminum alloy under constant-R and constant-K(sub max) threshold testing procedures. Two methods of calculating crack-opening stresses were compared. One method was based on a contact-K analyses and the other on crack-opening-displacement (COD) analyses. These methods gave nearly identical results under constant-amplitude loading, but under threshold simulations the contact-K analyses gave lower opening stresses than the contact COD method. Crack-growth predictions tend to support the use of contact-K analyses. Crack-growth simulations showed that remote closure can cause a rapid rise in opening stresses in the near threshold regime for low-constraint and high applied stress levels. Under low applied stress levels and high constraint, a rise in opening stresses was not observed near threshold conditions. But crack-tip-opening displacement (CTOD) were of the order of measured oxide thicknesses in the 2024 alloy under constant-R simulations. In contrast, under constant-K(sub max) testing the CTOD near threshold conditions were an order-of-magnitude larger than measured oxide thicknesses. Residual-plastic deformations under both constant-R and constant-K(sub max) threshold simulations were several times larger than the expected oxide thicknesses. Thus, residual-plastic deformations, in addition to oxide and roughness, play an integral part in threshold development.

Chloride Permeability of Damaged High-Performance Fiber-Reinforced Cement Composite by Repeated Compressive Loads.

PubMed

Lee, Byung Jae; Hyun, Jung Hwan; Kim, Yun Yong; Shin, Kyung Joon

2014-08-11

The development of cracking in concrete structures leads to significant permeability and to durability problems as a result. Approaches to controlling crack development and crack width in concrete structures have been widely debated. Recently, it was recognized that a high-performance fiber-reinforced cement composite (HPFRCC) provides a possible solution to this inherent problem of cracking by smearing one or several dominant cracks into many distributed microcracks under tensile loading conditions. However, the chloride permeability of HPFRCC under compressive loading conditions is not yet fully understood. Therefore, the goal of the present study is to explore the chloride diffusion characteristics of HPFRCC damaged by compressive loads. The chloride diffusivity of HPFRCC is measured after being subjected to various repeated loads. The results show that the residual axial strain, lateral strain and specific crack area of HPFRCC specimens increase with an increase in the damage induced by repeated loads. However, the chloride diffusion coefficient increases only up to 1.5-times, whereas the specific crack area increases up to 3-times with an increase in damage. Although HPFRCC shows smeared distributed cracks in tensile loads, a significant reduction in the diffusion coefficient of HPFRCC is not obtained compared to plain concrete when the cyclic compressive load is applied below 85% of the strength.

Nonlinear damage identification of breathing cracks in Truss system

NASA Astrophysics Data System (ADS)

Zhao, Jie; DeSmidt, Hans

2014-03-01

The breathing cracks in truss system are detected by Frequency Response Function (FRF) based damage identification method. This method utilizes damage-induced changes of frequency response functions to estimate the severity and location of structural damage. This approach enables the possibility of arbitrary interrogation frequency and multiple inputs/outputs which greatly enrich the dataset for damage identification. The dynamical model of truss system is built using the finite element method and the crack model is based on fracture mechanics. Since the crack is driven by tensional and compressive forces of truss member, only one damage parameter is needed to represent the stiffness reduction of each truss member. Assuming that the crack constantly breathes with the exciting frequency, the linear damage detection algorithm is developed in frequency/time domain using Least Square and Newton Raphson methods. Then, the dynamic response of the truss system with breathing cracks is simulated in the time domain and meanwhile the crack breathing status for each member is determined by the feedback from real-time displacements of member's nodes. Harmonic Fourier Coefficients (HFCs) of dynamical response are computed by processing the data through convolution and moving average filters. Finally, the results show the effectiveness of linear damage detection algorithm in identifying the nonlinear breathing cracks using different combinations of HFCs and sensors.

Fatigue Crack Growth and Crack Bridging in SCS-6/Ti-24-11

NASA Technical Reports Server (NTRS)

Ghosn, Louis J.; Kantzos, Pete; Telesman, Jack

1995-01-01

Interfacial damage induced by relative fiber/matrix sliding was found to occur in the bridged zone of unidirectional SCS-6/Ti-24Al-11Nb intermetallic matrix composite specimens subjected to fatigue crack growth conditions. The degree of interfacial damage was not uniform along the bridged crack wake. Higher damage zones were observed near the machined notch in comparison to the crack tip. The interfacial friction shear strength tau(sub f) measured in the crack wake using pushout testing revealed lower values than the as-received interface. Interfacial wear also reduced the strength of the bridging fibers. The reduction in fiber strength is thought to be a function of the magnitude of relative fiber/matrix displacements ind the degree of interfacial damage. Furthermore, two different fiber bridging models were used to predict the influence of bridging on the fatigue crack driving force. The shear lag model required a variable tau(sub f) in the crack wake (reflecting the degradation of the interface) before its predictions agreed with trends exhibited by the experimental data. The fiber pressure model did an excellent job in predicting both the FCG data and the DeltaCOD in the bridged zone even though it does not require a knowledge of tau(sub f).

Remote monitoring and prognosis of fatigue cracking in steel bridges with acoustic emission

NASA Astrophysics Data System (ADS)

Yu, Jianguo Peter; Ziehl, Paul; Pollock, Adrian

2011-04-01

Acoustic emission (AE) monitoring is desirable to nondestructively detect fatigue damage in steel bridges. Investigations of the relationship between AE signals and crack growth behavior are of paramount importance prior to the widespread application of passive piezoelectric sensing for monitoring of fatigue crack propagation in steel bridges. Tests have been performed to detect AE from fatigue cracks in A572G50 steel. Noise induced AE signals were filtered based on friction emission tests, loading pattern, and a combined approach involving Swansong II filters and investigation of waveforms. The filtering methods based on friction emission tests and load pattern are of interest to the field evaluation using sparse datasets. The combined approach is suitable for data filtering and interpretation of actual field tests. The pattern recognition program NOESIS (Envirocoustics) was utilized for the evaluation of AE data quality. AE parameters are associated with crack length, crack growth rate, maximum stress intensity and stress intensity range. It is shown that AE hits, counts, absolute energy, and signal strength are able to provide warnings at the critical cracking level where cracking progresses from stage II (stable propagation) to stage III (unstable propagation which may result in failure). Absolute energy rate and signal strength rate may be better than count rate to assess the remaining fatigue life of inservice steel bridges.

Restrained shrinkage cracking of cementitious composites containing soft PCM inclusions: A paste (matrix) controlled response

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wei, Zhenhua; Falzone, Gabriel; Das, Sumanta

The addition of phase change materials (PCMs) has been proposed as a way to mitigate thermal cracking in cementitious materials. However, the addition of PCMs, i.e., soft inclusions, degrades the compressive strength of cementitious composites. From a strength-of-materials viewpoint, such reductions in strength are suspected to increase the tendency of cementitious materials containing PCMs to crack under load (e.g., volume instability-induced stresses resulting from thermal and/or hygral deformations). Based on detailed assessments of free and restrained shrinkage, elastic modulus, and tensile strength, this study shows that the addition of PCMs does not alter the cracking sensitivity of the material. Inmore » fact, the addition of PCMs (or other soft inclusions) enhances the cracking resistance as compared to a plain cement paste or composites containing equivalent dosages of (stiff) quartz inclusions. This is because composites containing soft inclusions demonstrate benefits resulting from crack blunting and deflection, and improved stress relaxation. As a result, although the tensile stress at failure remains similar, the time to failure (i.e., macroscopic cracking) of PCM-containing composites is considerably extended. More generally, the outcomes indicate that dosages of soft(er) inclusions, and the resulting decrease in compressive strength does not amplify the cracking risk of cementitious composites.« less

Effect of Low-Temperature Environment on Stress Corrosion Cracking Behavior of X80 Pipeline Steel in Simulated Alkaline Soil Solution

NASA Astrophysics Data System (ADS)

Xie, Fei; Wang, Dan; Wu, Ming; Yu, Chengxiang; Sun, Dongxu; Yang, Xu; Xu, Changhao

2018-04-01

The stress corrosion cracking (SCC) of X80 pipeline steel in simulated alkaline soil solution under different temperatures was investigated by slow-strain-rate testing, scanning electron microscopy and energy-dispersive spectroscopy. Results showed that the fracture was transgranular and brittle at 273 K to 278 K (0 °C to 5 °C), and the metal surface was dissolved by a large number of chloride ions. Furthermore, hydrogen embrittlement was caused by the hydrogen atom extended to the high-stress region. The fracture process was controlled by hydrogen-induced cracking, and SCC was highly sensitive at this stage. At 288 K to 298 K (15 °C to 25 °C), the fracture morphology was attributed to the mixed mode of ductile and brittle fractures, the fracture process was controlled by the mechanism of hydrogen-induced cracking and anodic dissolution, and the susceptibility to SCC decreased. When the temperature reached 308 K to 318 K (35 °C to 45 °C), the fracture was mainly intergranular and ductile, the fracture process was controlled by anodic dissolution, and SCC sensitivity was the smallest in this temperature range.

Relationship Between Unusual High-Temperature Fatigue Crack Growth Threshold Behavior in Superalloys and Sudden Failure Mode Transitions

NASA Technical Reports Server (NTRS)

Telesman, J.; Smith, T. M.; Gabb, T. P.; Ring, A. J.

2017-01-01

An investigation of high temperature cyclic fatigue crack growth (FCG) threshold behavior of two advanced nickel disk alloys was conducted. The focus of the study was the unusual crossover effect in the near-threshold region of these type of alloys where conditions which produce higher crack growth rates in the Paris regime, produce higher resistance to crack growth in the near threshold regime. It was shown that this crossover effect is associated with a sudden change in the fatigue failure mode from a predominant transgranular mode in the Paris regime to fully intergranular mode in the threshold fatigue crack growth region. This type of a sudden change in the fracture mechanisms has not been previously reported and is surprising considering that intergranular failure is typically associated with faster crack growth rates and not the slow FCG rates of the near-threshold regime. By characterizing this behavior as a function of test temperature, environment and cyclic frequency, it was determined that both the crossover effect and the onset of intergranular failure are caused by environmentally driven mechanisms which have not as yet been fully identified. A plausible explanation for the observed behavior is proposed.

Identification and prioritization of rail squat defects in the field using rail magnetisation technology

NASA Astrophysics Data System (ADS)

Kaewunruen, Sakdirat

2015-04-01

Inevitably, rail squats and studs are continuing to be a serious problem for railway organisations around the world in the 21st century. They are typically classified as the growth of any cracks that have grown longitudinally through the subsurface and some of the cracks propagating to the bottom of rails transversely, and have branched from initial longitudinal cracks with a depression of rail surface. The horizontal crack, which results in a depression of rail surface, induces increased maintenance level, more frequent monitoring, compromised rail testing (as the crack shields the signal echoes), and possible broken rails. This paper presents field investigations using a magnetised-rail testing device developed by MRX Technologies to identify and prioritise the rail squats. Most of the in situ squats were found on the high rail of the transition (variable-radius curved track), which is associated with rolling contact fatigue (RCF). This investigation highlights the field performance of the MRX's surface crack detection technology in comparison with the traditional ultrasonic method and detailed walking inspection. Visually, it was found in the field that the size of the RCF squats varies from very small to moderate. The predicted crack data were obtained by scanning the magnitised rails. The comparison of the actual crack depths (ultrasonic) and the predicted crack depths (MRX device) shows: • A possible correlation for small RCF/ squat cracks. • Poor interpretation of larger defects and welds. The field assessment also suggests some practical issues required for further development, including the detection of rail spalling, deep transverse crack, welding, and so on.

Versatility of Evaporation-Induced Self-Assembly (EISA) Method for Preparation of Mesoporous TiO2 for Energy and Environmental Applications

PubMed Central

Mahoney, Luther; Koodali, Ranjit T.

2014-01-01

Evaporation-Induced Self-Assembly (EISA) method for the preparation of mesoporous titanium dioxide materials is reviewed. The versatility of EISA method for the rapid and facile synthesis of TiO2 thin films and powders is highlighted. Non-ionic surfactants such as Pluronic P123, F127 and cationic surfactants such as cetyltrimethylammonium bromide have been extensively employed for the preparation of mesoporous TiO2. In particular, EISA method allows for fabrication of highly uniform, robust, crack-free films with controllable thickness. Eleven characterization techniques for elucidating the structure of the EISA prepared mesoporous TiO2 are discussed in this paper. These many characterization methods provide a holistic picture of the structure of mesoporous TiO2. Mesoporous titanium dioxide materials have been employed in several applications that include Dye Sensitized Solar Cells (DSSCs), photocatalytic degradation of organics and splitting of water, and batteries. PMID:28788590

Analysis of delamination in cross ply laminates initiating from impact induced matrix cracking

NASA Technical Reports Server (NTRS)

Salpekar, S. A.

1991-01-01

Several two dimensional finite element analyses of (0 sub 2/90 sub 8/0 sub 2) glass/epoxy and graphite-epoxy composite laminates were performed to study some of the characteristics of damage development due to an impact load. A cross section through the thickness of the laminate with fixed ends, and carrying a transverse load in the center was analyzed. Inclined matrix cracks such as those produced by low velocity impact were modeled in the 90 deg ply group. The introduction of the matrix cracks caused large interlaminar tension and shear stresses in the vicinity of both crack tips in the 0/90 and 90/0 interfaces. The large interlaminar stresses at the ends of the matrix cracks indicate that matrix cracking may give rise to delamination. The ratio of mode I to total strain energy release rate at the beginning of delamination calculated at the two matrix crack tips was 60 and 28 pct., respectively, in the glass/epoxy laminate. The corresponding ratio was 97 and 77 pct. in the graphite-epoxy laminate. Thus, a significant mode I component of strain energy release rate may be present at the delamination initiation due to an impact load.

Stable Boron Nitride Interphases for Ceramic Matrix Composites

NASA Technical Reports Server (NTRS)

Morscher, Gregory N.

1999-01-01

Ceramic matrix composites (CMC's) require strong fibers for good toughness and weak interphases so that cracks which are formed in the matrix debond and deflect around the fibers. If the fibers are strongly bonded to the matrix, CMC's behave like monolithic ceramics (e.g., a ceramic coffee cup), and when subjected to mechanical loads that induce cracking, such CMC's fail catastrophically. Since CMC's are being developed for high temperature corrosive environments such as the combustor liner for advanced High Speed Civil Transport aircraft, the interphases need to be able to withstand the environment when the matrix cracks.

Investigation of laser irradiation of WC-Co cemented carbides inside a scanning electron microscope (LASEM)

NASA Astrophysics Data System (ADS)

Schultrich, B.; Wetzig, K.

1987-09-01

A combination of SEM and laser enables direct observation of structural modifications by a high-energy input. With this new device, melting phenomena and fracture processes in a WC-6 percent Co hard metal were investigated. The first laser pulse leads to melting of a thin surface layer with the formation of blisters and craters. Cracking is induced by the relaxation of compressive surface stresses during the high-temperature stage and the appearance of tensile stresses during cooling. Besides crack formation and extension, complete welding of crack surfaces was observed after repeated laser irradiation.

Stressed Oxidation Life Prediction for C/SiC Composites

NASA Technical Reports Server (NTRS)

Levine, Stanley R.

2004-01-01

The residual strength and life of C/SiC is dominated by carbon interface and fiber oxidation if seal coat and matrix cracks are open to allow oxygen ingress. Crack opening is determined by the combination of thermal, mechanical and thermal expansion mismatch induced stresses. When cracks are open, life can be predicted by simple oxidation based models with reaction controlled kinetics at low temperature, and by gas phase diffusion controlled kinetics at high temperatures. Key life governing variables in these models include temperature, stress, initial strength, oxygen partial pressure, and total pressure. These models are described in this paper.

The influence of shrinkage-cracking on the drying behaviour of White Portland cement using Single-Point Imaging (SPI).

PubMed

Beyea, S D; Balcom, B J; Bremner, T W; Prado, P J; Cross, A R; Armstrong, R L; Grattan-Bellew, P E

1998-11-01

The removal of water from pores in hardened cement paste smaller than 50 nm results in cracking of the cement matrix due to the tensile stresses induced by drying shrinkage. Cracks in the matrix fundamentally alter the permeability of the material, and therefore directly affect the drying behaviour. Using Single-Point Imaging (SPI), we obtain one-dimensional moisture profiles of hydrated White Portland cement cylinders as a function of drying time. The drying behaviour of White Portland cement, is distinctly different from the drying behaviour of related concrete materials containing aggregates.

Railroad Rails Containing Electrode-Induced Pitting from Pressure Electric Welding

DOT National Transportation Integrated Search

2018-04-18

This paper describes the forensic evaluations of three railroad rails containing electrode-induced pitting. These evaluations include: magnetic particle inspection to nondestructively detect cracks emanating from the pitting; fractography to study th...

Surface aspects of pitting and stress corrosion cracking

NASA Technical Reports Server (NTRS)

Truhan, J. S., Jr.; Hehemann, R. F.

1977-01-01

The pitting and stress corrosion cracking of a stable austenitic stainless steel in aqueous chloride environments were investigated using a secondary ion mass spectrometer as the primary experimental technique. The surface concentration of hydrogen, oxygen, the hydroxide, and chloride ion, magnesium or sodium, chromium and nickel were measured as a function of potential in both aqueous sodium chloride and magnesium chloride environments at room temperature and boiling temperatures. It was found that, under anodic conditions, a sharp increase in the chloride concentration was observed to occur for all environmental conditions. The increase may be associated with the formation of an iron chloride complex. Higher localized chloride concentrations at pits and cracks were also detected with an electron microprobe.

Microstructural characterization of Charpy-impact-tested nanostructured bainite

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tsai, Y.T.; Chang, H.T.; Huang, B.M.

2015-09-15

In this work, a possible cause of the extraordinary low impact toughness of nanostructured bainite has been investigated. The microstructure of nanostructured bainite consisted chiefly of carbide-free bainitic ferrite with retained austenite films. X-ray diffractometry (XRD) measurement indicated that no retained austenite existed in the fractured surface of the Charpy-impact-tested specimens. Fractographs showed that cracks propagated mainly along bainitic ferrite platelet boundaries. The change in microstructure after impact loading was verified by transmission electron microscopy (TEM) observations, confirming that retained austenite was completely transformed to strain-induced martensite during the Charpy impact test. However, the zone affected by strained-induced martensite wasmore » found to be extremely shallow, only to a depth of several micrometers from the fracture surface. It is appropriately concluded that upon impact, as the crack forms and propagates, strain-induced martensitic transformation immediately occurs ahead of the advancing crack tip. The successive martensitic transformation profoundly facilitates the crack propagation, resulting in the extremely low impact toughness of nanostructured bainite. Retained austenite, in contrast to its well-known beneficial role, has a deteriorating effect on toughness during the course of Charpy impact. - Highlights: • The microstructure of nanostructured bainite consisted of nano-sized bainitic ferrite subunits with retained austenite films. • Special sample preparations for SEM, XRD and TEM were made, and the strain-affected structures have been explored. • Retained austenite films were found to transform into martensite after impact loading, as evidenced by XRD and TEM results. • The zone of strain-induced martensite was found to extend to only several micrometers from the fracture surface. • The poor Charpy impact toughness is associated with the fracture of martensite at a high strain rate during impact loading.« less

Crack growth through the thickness of thin-sheet Hydrided Zircaloy-4

NASA Astrophysics Data System (ADS)

Raynaud, Patrick A. C.

In recent years, the limits on fuel burnup have been increased to allow an increase in the amount of energy produced by a nuclear fuel assembly thus reducing waste volume and allowing greater capacity factors. As a result, it is paramount to ensure safety after longer reactor exposure times in the case of design-basis accidents, such as reactivity-initiated accidents (RIA). Previously proposed failure criteria do not directly address the particular cladding failure mechanism during a RIA, in which crack initiation in brittle outer-layers is immediately followed by crack growth through the thickness of the thin-wall tubing. In such a case, the fracture toughness of hydrided thin-wall cladding material must be known for the conditions of through-thickness crack growth in order to predict the failure of high-burnup cladding. The fracture toughness of hydrided Zircaloy-4 in the form of thin-sheet has been examined for the condition of through-thickness crack growth as a function of hydride content and distribution at 25°C, 300°C, and 375°C. To achieve this goal, an experimental procedure was developed in which a linear hydride blister formed across the width of a four-point bend specimen was used to inject a sharp crack that was subsequently extended by fatigue pre-cracking. The electrical potential drop method was used to monitor the crack length during fracture toughness testing, thus allowing for correlation of the load-displacement record with the crack length. Elastic-plastic fracture mechanics were used to interpret the experimental test results in terms of fracture toughness, and J-R crack growth resistance curves were generated. Finite element modeling was performed to adapt the classic theories of fracture mechanics applicable to thick-plate specimens to the case of through-thickness crack growth in thin-sheet materials, and to account for non-uniform crack fronts. Finally, the hydride microstructure was characterized in the vicinity of the crack tip by means of digital image processing, so as to understand the influence of the hydride microstructure on fracture toughness, at the various test temperatures. Crack growth occurred through a microstructure which varied within the thickness of the thin-sheet Zircaloy-4 such that the hydrogen concentration and the radial hydride content decreased with increasing distance from the hydride blister. At 25°C, the fracture toughness was sensitive to the changes in hydride microstructure, such that the toughness KJi decreased from 39 MPa√m to 24 MPa√m with increasing hydrogen content and increasing the fraction of radial hydrides. The hydride particles present in the Zircaloy-4 substrate fractured ahead of the crack tip, and crack growth occurred by linking the crack-tip with the next hydride-induced primary void ahead of it. Unstable crack growth was observed at 25°C prior to any stable crack growth in the specimens where the hydrogen content was the highest. At 375°C as well as in most cases at 300°C, the hydride particles were resistant to cracking and the resistance to crack-growth initiation was very high. As a result, for this bend test procedure, crack extension was solely due to crack-tip blunting instead of crack growth in all tests at 375°C and in most cases at 300°C. The lower bound for fracture toughness at these temperatures, the parameter KJPmax, had values of K JPmax˜54MPa√m at both 300°C and 375°C. For cases where stable crack growth occurred at 300°C, the fracture toughness was K Ji˜58MPa√m and the tearing modulus was twice as high as that at 25°C. It is believed that the failure of hydrided Zircaloy-4 thin-wall cladding can be predicted using fracture mechanics analyses when failure occurs by crack growth. This failure mechanism was observed to occur in all cases at 25°C and in some cases at 300°C. However, at more elevated temperatures, such as 375°C, failure will likely occur by a mechanism other than crack growth, possibly by an imperfection-induced shear instability.

Multiaxial and thermomechanical fatigue considerations in damage tolerant design

NASA Technical Reports Server (NTRS)

Leese, G. E.; Bill, R. C.

1985-01-01

In considering damage tolerant design concepts for gas turbine hot section components, several challenging concerns arise: Complex multiaxial loading situations are encountered; Thermomechanical fatigue loading involving very wide temperature ranges is imposed on components; Some hot section materials are extremely anisotropic; and coatings and environmental interactions play an important role in crack propagation. The effects of multiaxiality and thermomechanical fatigue are considered from the standpoint of their impact on damage tolerant design concepts. Recently obtained research results as well as results from the open literature are examined and their implications for damage tolerant design are discussed. Three important needs required to advance analytical capabilities in support of damage tolerant design become readily apparent: (1) a theoretical basis to account for the effect of nonproportional loading (mechanical and mechanical/thermal); (2) the development of practical crack growth parameters that are applicable to thermomechanical fatigue situations; and (3) the development of crack growth models that address multiple crack failures.

Influence of precracked specimen configuration and starting stress intensity on the stress corrosion cracking of 4340 steel

NASA Technical Reports Server (NTRS)

Lisagor, W. B.

1984-01-01

Since the pioneer work of Brown (1966), precracked specimens and related fracture mechanics analyses have been extensively used to study stress corrosion cracking. Certain questions arose in connection with initial attempts to prepare standardized recommended practices by ASTM Committee G-1 on Corrosion of Metals. These questions were related to adequacy of test control as it pertains to acceptable limits of variability, and to validity of expressions for stress intensity and crack-surface displacements for both specimen configurations. An interlaboratory test program, was, therefore, planned with the objective to examine the validity of KIscc testing for selected specimen configurations, materials,and environmental systems. The results reported in the present paper include details of a single laboratory test program. The program was conducted to determine if the threshold value of stress intensity for onset and arrest of stress corrosion cracking was independent for the two specimen configurations examined.

Diatomaceous earth as a protective vehicle for bacteria applied for self-healing concrete.

PubMed

Wang, J Y; Belie, N De; Verstraete, W

2012-04-01

Crack repair is crucial since cracks are the main cause for the decreased service life of concrete structures. An original and promising way to repair cracks is to pre-incorporate healing agents inside the concrete matrix to heal cracks the moment they appear. Thus, the concrete obtains self-healing properties. The goal of our research is to apply bacterially precipitated CaCO₃ to heal cracks in concrete since the microbial calcium carbonate is more compatible with the concrete matrix and more environmentally friendly relative to the normally used polymeric materials. Diatomaceous earth (DE) was used in this study to protect bacteria from the high-pH environment of concrete. The experimental results showed that DE had a very good protective effect for bacteria. DE immobilized bacteria had much higher ureolytic activity (12-17 g/l urea was decomposed within 3 days) than that of un-immobilized bacteria (less than 1 g/l urea was decomposed within the same time span) in cement slurry. The optimal concentration of DE for immobilization was 60% (w/v, weight of DE/volume of bacterial suspension). Self-healing in cracked specimens was visualized under light microscopy. The images showed that cracks with a width ranging from 0.15 to 0.17 mm in the specimens containing DE immobilized bacteria were completely filled by the precipitation. Scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) were used to characterize the precipitation around the crack wall, which was confirmed to be calcium carbonate. The result from a capillary water absorption test showed that the specimens with DE immobilized bacteria had the lowest water absorption (30% of the reference ones), which indicated that the precipitation inside the cracks increased the water penetration resistance of the cracked specimens.

"We need somewhere to smoke crack": An ethnographic study of an unsanctioned safer smoking room in Vancouver, Canada.

PubMed

McNeil, Ryan; Kerr, Thomas; Lampkin, Hugh; Small, Will

2015-07-01

Many cities around the globe have experienced substantial increases in crack cocaine use. Public health programmes have begun to address crack smoking, primarily through the distribution of safer crack use equipment, but their impacts have been limited. More comprehensive safer environmental interventions, specifically safer smoking rooms (SSR), have been implemented only in select European cities. However, none have been subjected to rigorous evaluation. This ethnographic study was undertaken at an 'unsanctioned' SSR operated by a drug user-led organization in Vancouver, Canada, to explore how this intervention shaped crack smoking practices, public crack smoking, and related harms. Ethnographic fieldwork was undertaken at this SSR from September to December 2011, and included approximately 50 hours of ethnographic observation and 23 in-depth interviews with people who smoke crack. Data were analyzed by drawing on the 'Risk Environment' framework and concepts of 'symbolic', 'everyday', and 'structural' violence. Our findings illustrate how a high demand for SSRs was driven by the need to minimize exposure to policing (structural violence), drug scene violence (everyday violence), and stigma (symbolic violence) that characterized unregulated drug use settings (e.g., public spaces). Although resource scarcity and social norms operating within the local drug scene (e.g., gendered power relations) perpetuated crack pipe-sharing within unregulated drug use settings, the SSR fostered harm reduction practices by reshaping the social-structural context of crack smoking and reduced the potential for health harms. Given the significant potential of SSRs in reducing health and social harms, there is an urgent need to scale up these interventions. Integrating SSRs into public health systems, and supplementing these interventions with health and social supports, has potential to improve the health and safety of crack-smoking populations. Copyright © 2015 Elsevier B.V. All rights reserved.

Thermal-Mechanical Stress Analysis of PWR Pressure Vessel and Nozzles under Grid Load-Following Mode: Interim Report on the Effect of Cyclic Hardening Material Properties and Pre-existing Cracks on Stress Analysis Results

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mohanty, Subhasish; Soppet, William; Majumdar, Saurin

This report provides an update on an assessment of environmentally assisted fatigue for light water reactor components under extended service conditions. This report is a deliverable under the work package for environmentally assisted fatigue as part of DOE’s Light Water Reactor Sustainability Program. In a previous report (September 2015), we presented tensile and fatigue test data and related hardening material properties for 508 low-alloys steel base metal and other reactor metals. In this report, we present thermal-mechanical stress analysis of the reactor pressure vessel and its hot-leg and cold-leg nozzles based on estimated material properties. We also present results frommore » thermal and thermal-mechanical stress analysis under reactor heat-up, cool-down, and grid load-following conditions. Analysis results are given with and without the presence of preexisting cracks in the reactor nozzles (axial or circumferential crack). In addition, results from validation stress analysis based on tensile and fatigue experiments are reported.« less

Development of an expert system for fractography of environmentally assisted cracking

DOE Office of Scientific and Technical Information (OSTI.GOV)

Minoshima, Kohji; Komai, Kenjiro; Yamasaki, Norimasa

1997-12-31

An expert system that diagnoses the causes of failure of environmentally assisted cracking (EAC) based upon fractography has been developed. The system uses the OPS83 programming language, expressing rules in the manner of production rules, and is composed of three independent subsystems, which respectively deal with EACs of high-strength or high-tensile-strength steel, aluminum alloy, and stainless steel in dry and humidified air, water, and aqueous solutions containing Cl, Br, or I ions. The concerned EAC issues cover stress corrosion cracking (SCC), hydrogen embrittlement, cyclic SCC, dynamic SCC, and corrosion fatigue as well as fatigue and overload fracture. The knowledge basemore » covers the rules relating to not only environments, materials, and loading conditions, but also macroscopic and microscopic fracture surface morphology. In order to deal with vague expressions of fracture surface morphology, fuzzy set theory is used in the system, and the description of rules about vague fracture surface appearance is thereby possible. Applying the developed expert system to case histories, accurate diagnoses were made. The authors discuss the related diagnosis results and usefulness of the developed system.« less

Microstructural indicators of transition mechanisms in time-dependent fatigue crack growth in nickel base superalloys

NASA Astrophysics Data System (ADS)

Heeter, Ann E.

Gas turbine engines are an important part of power generation in modern society, especially in the field of aerospace. Aerospace engines are design to last approximately 30 years and the engine components must be designed to survive for the life of the engine or to be replaced at regular intervals to ensure consumer safety. Fatigue crack growth analysis is a vital component of design for an aerospace component. Crack growth modeling and design methods date back to an origin around 1950 with a high rate of accuracy. The new generation of aerospace engines is designed to be efficient as possible and require higher operating temperatures than ever seen before in previous generations. These higher temperatures place more stringent requirements on the material crack growth performance under creep and time dependent conditions. Typically the types of components which are subject to these requirements are rotating disk components which are made from advanced materials such as nickel base superalloys. Traditionally crack growth models have looked at high temperature crack growth purely as a function of temperature and assumed that all crack growth was either controlled by a cycle dependent or time dependent mechanism. This new analysis is trying to evaluate the transition between cycle-dependent and time-dependent mechanism and the microstructural markers that characterize this transitional behavior. The physical indications include both the fracture surface morphology as well as the shape of the crack front. The research will evaluate whether crack tunneling occurs and whether it consistently predicts a transition from cycle-dependent crack growth to time-dependent crack growth. The study is part of a larger research program trying to include the effects of geometry, mission profile and environmental effects, in addition to temperature effects, as a part of the overall crack growth system. The outcome will provide evidence for various transition types and correlate those physical attributes back to the material mechanisms to improve predictive modeling capability.

An electro-mechanical impedance model of a cracked composite beam with adhesively bonded piezoelectric patches

NASA Astrophysics Data System (ADS)

Yan, Wei; Cai, J. B.; Chen, W. Q.

2011-01-01

A model of a laminated composite beam including multiple non-propagating part-through surface cracks as well as installed PZT transducers is presented based on the method of reverberation-ray matrix (MRRM) in this paper. Toward determining the local flexibility characteristics induced by the individual cracks, the concept of the massless rotational spring is applied. A Timoshenko beam theory is then used to simulate the behavior of the composite beam with open cracks. As a result, transverse shear and rotatory inertia effects are included in the model. Only one-dimensional axial vibration of the PZT wafer is considered and the imperfect interfacial bonding between PZT patches and the host beam is further investigated based on a Kelvin-type viscoelastic model. Then, an accurate electro-mechanical impedance (EMI) model can be established for crack detection in laminated beams. In this model, the effects of various parameters such as the ply-angle, fibre volume fraction, crack depth and position on the EMI signatures are highlighted. Furthermore, comparison with existent numerical results is presented to validate the present analysis.

Nonlinear ultrasonic wave modulation for online fatigue crack detection

NASA Astrophysics Data System (ADS)

Sohn, Hoon; Lim, Hyung Jin; DeSimio, Martin P.; Brown, Kevin; Derriso, Mark

2014-02-01

This study presents a fatigue crack detection technique using nonlinear ultrasonic wave modulation. Ultrasonic waves at two distinctive driving frequencies are generated and corresponding ultrasonic responses are measured using permanently installed lead zirconate titanate (PZT) transducers with a potential for continuous monitoring. Here, the input signal at the lower driving frequency is often referred to as a 'pumping' signal, and the higher frequency input is referred to as a 'probing' signal. The presence of a system nonlinearity, such as a crack formation, can provide a mechanism for nonlinear wave modulation, and create spectral sidebands around the frequency of the probing signal. A signal processing technique combining linear response subtraction (LRS) and synchronous demodulation (SD) is developed specifically to extract the crack-induced spectral sidebands. The proposed crack detection method is successfully applied to identify actual fatigue cracks grown in metallic plate and complex fitting-lug specimens. Finally, the effect of pumping and probing frequencies on the amplitude of the first spectral sideband is investigated using the first sideband spectrogram (FSS) obtained by sweeping both pumping and probing signals over specified frequency ranges.

Isolated Mesoporous Microstructures Prepared by Stress Localization-Induced Crack Manipulation.

PubMed

Wooh, Sanghyuk; Lee, Soojin; Lee, Yunchan; Ryu, Ji Ho; Lee, Won Bo; Yoon, Hyunsik; Char, Kookheon

2016-09-22

Cracks observed in brittle materials are mostly regarded as defects or failures. However, they can be a valuable tool when implemented in a controlled way. Here, we introduce a strategy to control the crack propagation of mesoporous micropatterns (prisms and pyramids), which leads to the isolation of well-defined microstructures. Mesoporous micropatterns were fabricated by the soft imprinting technique with wet TiO 2 nanoparticle (NP) pastes, followed by sintering to remove organic components. Since the volume of the paste significantly shrinks during the sintering step, stress is localized at the edge of micropatterns, in good agreement with finite element method simulations, creating well-defined cracks and their propagation. It was demonstrated that the degree of stress localization is determined by the thickness of residual layers, NP size, and heating rate. After controlled crack propagation and delamination of microparticles from the substrates, mesoporous microwires and microparticles were successfully produced and functionalized from the isolated mesoporous prisms and pyramids. The method proposed in this study for controlled crack manipulation and delamination opens a door for straightforward and economical fabrication of well-defined mesoporous microparticles.

The effect of ion plated silver and sliding friction on tensile stress-induced cracking in aluminum oxide

NASA Technical Reports Server (NTRS)

Sliney, Harold E.; Spalvins, Talivaldis

1991-01-01

A Hertzian analysis of the effect of sliding friction on contact stresses in alumina is used to predict the critical load for crack generation. The results for uncoated alumina and alumina coated with ion plated silver are compared. Friction coefficient inputs to the analysis are determined experimentally with a scratch test instrument employing an 0.2 mm radius diamond stylus. A series of scratches were made at constant load increments on coated and uncoated flat alumina surfaces. Critical loads for cracking are detected by microscopic examination of cross sections of scratches made at various loads and friction coefficients. Acoustic emission (AE) and friction trends were also evaluated as experimental techniques for determining critical loads for cracking. Analytical predictions correlate well with micrographic evidence and with the lowest load at which AE is detected in multiple scratch tests. Friction/load trends are not good indicators of early crack formation. Lubrication with silver films reduced friction and thereby increased the critical load for crack initiation in agreement with analytical predictions.

Initiation and growth kinetics of solidification cracking during welding of steel

PubMed Central

Aucott, L.; Huang, D.; Dong, H. B.; Wen, S. W.; Marsden, J. A.; Rack, A.; Cocks, A. C. F.

2017-01-01

Solidification cracking is a key phenomenon associated with defect formation during welding. To elucidate the failure mechanisms, solidification cracking during arc welding of steel are investigated in situ with high-speed, high-energy synchrotron X-ray radiography. Damage initiates at relatively low true strain of about 3.1% in the form of micro-cavities at the weld subsurface where peak volumetric strain and triaxiality are localised. The initial micro-cavities, with sizes from 10 × 10−6 m to 27 × 10−6 m, are mostly formed in isolation as revealed by synchrotron X-ray micro-tomography. The growth of micro-cavities is driven by increasing strain induced to the solidifying steel. Cavities grow through coalescence of micro-cavities to form micro-cracks first and then through the propagation of micro-cracks. Cracks propagate from the core of the weld towards the free surface along the solidifying grain boundaries at a speed of 2–3 × 10−3 m s−1. PMID:28074852

Nondestructive estimation of depth of surface opening cracks in concrete beams

DOE Office of Scientific and Technical Information (OSTI.GOV)

Arne, Kevin; In, Chiwon; Kurtis, Kimberly

Concrete is one of the most widely used construction materials and thus assessment of damage in concrete structures is of the utmost importance from both a safety point of view and a financial point of view. Of particular interest are surface opening cracks that extend through the concrete cover, as this can expose the steel reinforcement bars underneath and induce corrosion in them. This corrosion can lead to significant subsequent damage in concrete such as cracking and delamination of the cover concrete as well as rust staining on the surface of concrete. Concrete beams are designed and constructed in suchmore » a way to provide crack depths up to around 13 cm. Two different types of measurements are made in-situ to estimate depths of real surface cracks (as opposed to saw-cut notches) after unloading: one based on the impact-echo method and the other one based on the diffuse ultrasonic method. These measurements are compared to the crack depth visually observed on the sides of the beams. Discussions are given as to the advantages and disadvantages of each method.« less

The effect of ion-plated silver and sliding friction on tensile stress-induced cracking in aluminum oxide

NASA Technical Reports Server (NTRS)

Sliney, Harold E.; Spalvins, Talivaldis

1993-01-01

A Hertzian analysis of the effect of sliding friction on contact stresses in alumina is used to predict the critical load for crack generation. The results for uncoated alumina and alumina coated with ion plated silver are compared. Friction coefficient inputs to the analysis are determined experimentally with a scratch test instrument employing an 0.2 mm radius diamond stylus. A series of scratches were made at constant load increments on coated and uncoated flat alumina surfaces. Critical loads for cracking are detected by microscopic examination of cross sections of scratches made at various loads and friction coefficients. Acoustic emission (AE) and friction trends were also evaluated as experimental techniques for determining critical loads for cracking. Analytical predictions correlate well with micrographic evidence and with the lowest load at which AE is detected in multiple scratch tests. Friction/load trends are not good indicators of early crack formation. Lubrication with silver films reduced friction and thereby increased the critical load for crack initiation in agreement with analytical predictions.

Experimental Investigation and Finite Element Analysis on Fatigue Behavior of Aluminum Alloy 7050 Single-Lap Joints

NASA Astrophysics Data System (ADS)

Zhou, Bing; Cui, Hao; Liu, Haibo; Li, Yang; Liu, Gaofeng; Li, Shujun; Zhang, Shangzhou

2018-03-01

The fatigue behavior of single-lap four-riveted aluminum alloy 7050 joints was investigated by using high-frequency fatigue test and scanning electron microscope (SEM). Stress distributions obtained by finite element (FE) analysis help explain the fatigue performance. The fatigue test results showed that the fatigue lives of the joints depend on cold expansion and applied cyclic loads. FE analysis and fractography indicated that the improved fatigue lives can be attributed to the reduction in maximum stress and evolution of fatigue damage at the critical location. The beneficial effects of strengthening techniques result in tearing ridges or lamellar structure on fracture surface, decrease in fatigue striations spacing, delay of fatigue crack initiation, crack deflection in fatigue crack propagation and plasticity-induced crack closure.

Environmental Assessment for Installation of a New Jet Engine Test Cell, Edwards Air Force Base, California

DTIC Science & Technology

2012-09-01

suspended in the runoff waters around the lakebeds, sealing lakebed surface cracks and filling fissures. Shallow flooding along with consistent winds are...of the recorded specimens consist of isolated fragments of tooth enamel or bone that are not securely dated. Irvingtonian fossil localities have...require frequent repair as well. Concrete floors are cracking throughout all four test cells. Entry doors sag and drag on buckled floors. One-half

Early detection of materials degradation

NASA Astrophysics Data System (ADS)

Meyendorf, Norbert

2017-02-01

Lightweight components for transportation and aerospace applications are designed for an estimated lifecycle, taking expected mechanical and environmental loads into account. The main reason for catastrophic failure of components within the expected lifecycle are material inhomogeneities, like pores and inclusions as origin for fatigue cracks, that have not been detected by NDE. However, material degradation by designed or unexpected loading conditions or environmental impacts can accelerate the crack initiation or growth. Conventional NDE methods are usually able to detect cracks that are formed at the end of the degradation process, but methods for early detection of fatigue, creep, and corrosion are still a matter of research. For conventional materials ultrasonic, electromagnetic, or thermographic methods have been demonstrated as promising. Other approaches are focused to surface damage by using optical methods or characterization of the residual surface stresses that can significantly affect the creation of fatigue cracks. For conventional metallic materials, material models for nucleation and propagation of damage have been successfully applied for several years. Material microstructure/property relations are well established and the effect of loading conditions on the component life can be simulated. For advanced materials, for example carbon matrix composites or ceramic matrix composites, the processes of nucleation and propagation of damage is still not fully understood. For these materials NDE methods can not only be used for the periodic inspections, but can significantly contribute to the material scientific knowledge to understand and model the behavior of composite materials.

About microcracking due to leaching in cementitious composites: X-ray microtomography description and numerical approach

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rougelot, Thomas; Burlion, Nicolas, E-mail: nicolas.burlion@polytech-lille.f; Bernard, Dominique

2010-02-15

Chemical shock of cement based materials leads to significant degradation of their physical properties. A typical scenario is a calcium leaching due to water (water with very low pH compared with that of pore fluid). The main objective of this paper is to evaluate the evolution of microstructure induced by leaching of a cementitious composite using synchrotron X-ray micro tomography, mainly from an experimental point of view. In this particular case, it was possible to identify cracking induced by leaching. After a description of the degradation mechanism and the X-ray synchrotron microtomographic analysis, numerical simulations are performed in order tomore » show that cracking is induced by an initial pre-stressing of the composite, coupled with decalcification shrinkage and dramatic decrease in tensile strength during leaching. X-ray microtomography analysis allowed to make evidence of an induced microcracking in cementitious material submitted to leaching.« less

Investigation of Teflon FEP Embrittlement on Spacecraft in Low Earth Orbit

NASA Technical Reports Server (NTRS)

deGroh, Kim K.; Smith, Daniela C.

1997-01-01

Teflon(registered trademark) FEP (fluorinated ethylene-propylene) is commonly used on exterior spacecraft surfaces in the low Earth orbit (LEO) environment for thermal control. Silverized or aluminized FEP is used for the outer layer of thermal control blankets because of its low solar absorptance and high thermal emittance. FEP is also preferred over other spacecraft polymers because of its relatively high resistance to atomic oxygen erosion. Because of its low atomic oxygen erosion yield, FEP has not been protected in the space environment. Recent, long term space exposures such as on the Long Duration Exposure Facility (LDEF, 5.8 years in space), and the Hubble Space Telescope (HST, after 3.6 years in space) have provided evidence of LEO environmental degradation of FEP. These exposures provide unique opportunities for studying environmental degradation because of the long durations and the different conditions (such as differences in altitude) of the exposures. Samples of FEP from LDEF and from HST (retrieved during its first servicing mission) have been evaluated for solar induced embrittlement and for synergistic effects of solar degradation and atomic oxygen. Micro-indenter results indicate that the surface hardness increased as the ratio of atomic oxygen fluence to solar fluence decreased for the LDEF samples. FEP multilayer insulation (MLI) retrieved from HST provided evidence of severe embrittlement on solar facing surfaces. Micro-indenter measurements indicated higher surface hardness values for these samples than LDEF samples, but the solar exposures were higher. Cracks induced during bend testing were significantly deeper for the HST samples with the highest solar exposure than for LDEF samples with similar atomic oxygen fluence to solar fluence ratios. If solar fluences are compared, the LDEF samples appear as damaged as HST samples, except that HST had deeper induced cracks. The results illustrate difficulties in comparing LEO exposed materials from different missions. Because the HST FEP appears more damaged than LDEF FEP based on depth of embrittlement, other causes for FEP embrittlement in addition to atomic oxygen and ultraviolet (UV) radiation, such as thermal effects and the possible role of soft x-ray radiation, need to be considered. FEP that was exposed to soft x-rays in a ground test facility, showed embrittlement similar to that witnessed in LEO, which indicates that the observed differences between LDEF and HST FEP might be attributed to the different soft x-ray fluences during these two missions.

Electromagnetically induced acoustic emission—novel NDT technique for damage evaluation

NASA Astrophysics Data System (ADS)

Finkel, P.; Godinez, V.; Miller, R.; Finlayson, R.

2001-04-01

A recently developed electromagnetically induced acoustic emission technique (EM AE) which can be used for damage assessment of thin walled conducting structures is described. This technique allows a structure to be loaded locally by applying an electromagnetic field in order to produce an AE response, which may be captured by conventional or fiber optic (FO) AE sensors. The advantage of this technique is that the localized dynamic stresses induced by a short current pulse in the presence of an external magnetic field aid in the detection of cracks. Also, it is shown that electromagnetic stimulation can be applied to enhance conventional ultrasonics by modulation of the scattered signal from the defect (EM UT). Experimental data is presented for the case of a fatigue crack near rivet holes in thin walled aluminum structures.

A numerical investigation of choked flow dynamics and its application to the triggering mechanism of long-period events at Redoubt Volcano, Alaska

USGS Publications Warehouse

Morrissey, M.M.; Chouet, B.A.

1997-01-01

We use numerical simulations of transonic flow through a crack to study the dynamics of the formation of shock waves downstream from a nozzle-like constriction inside the crack. The model solves the full set of Navier-Stokes equations in two dimensions via an explicit multifield finite difference representation. The crack walls are assumed to be perfectly rigid, and elastic coupling to the solid is not considered. The simulations demonstrate how the behavior of unsteady shock waves near the walls can produce recurring step-like pressure transients in the flow, which in turn induce resonance of the fluid-filled crack. The motion of the shock waves is governed primarily by smooth, low-amplitude pressure fluctuations at the outlet of the crack. The force induced on the walls scales with the amplitude of the shock, which is a function of the magnitude of the inlet pressure, aperture of the constriction, and thickness of the boundary layer. The applied force also scales in proportion to the spatial extent of the shock excursion, which depends on the fluctuation rate of outlet pressure. Using the source parameters of long-period (LP) events at Redoubt Volcano, Alaska, as a guide for our simulations, we infer that coupling of the shock to the walls occurs for crack inlet to outlet pressure ratios pipo > 2.31 and that the position of the shock front becomes most sensitive to outlet pressure fluctuations for flow regimes with pipo > 2.48. For such regimes, fluctuations of outlet pressure of up to ??0.5 MPa at rates up to 3 MPa/s are sufficient to induce pressure transients with magnitudes up to 12.5 MPa over 0.1-2.5 m of the walls within ???0.5 s. These flow parameters may be adequate for triggering the LP events in the precursory swarm to the December 14, 1989, eruption of Redoubt. According to the flow model the recurrence rate and amplitudes of LP events are inferred to be a manifestation of the response of a shallow hydrothermal reservoir to the sustained injection of superheated steam from a magma column roofing below this reservoir.

The fracture behaviour of dental enamel.

PubMed

Bechtle, Sabine; Habelitz, Stefan; Klocke, Arndt; Fett, Theo; Schneider, Gerold A

2010-01-01

Enamel is the hardest tissue in the human body covering the crowns of teeth. Whereas the underlying dental material dentin is very well characterized in terms of mechanical and fracture properties, available data for enamel are quite limited and are apart from the most recent investigation mainly based on indentation studies. Within the current study, stable crack-growth experiments in bovine enamel have been performed, to measure fracture resistance curves for enamel. Single edge notched bending specimens (SENB) prepared out of bovine incisors were tested in 3-point bending and subsequently analysed using optical and environmental scanning electron microscopy. Cracks propagated primarily within the protein-rich rod sheaths and crack propagation occurred under an inclined angle to initial notch direction not only due to enamel rod and hydroxyapatite crystallite orientation but potentially also due to protein shearing. Determined mode I fracture resistance curves ranged from 0.8-1.5 MPa*m(1/2) at the beginning of crack propagation up to 4.4 MPa*m(1/2) at 500 microm crack extension; corresponding mode II values ranged from 0.3 to 1.5 MPa*m(1/2).

Hydrogen-enhanced fatigue crack growth in steels and its frequency dependence

NASA Astrophysics Data System (ADS)

Matsunaga, Hisao; Takakuwa, Osamu; Yamabe, Junichiro; Matsuoka, Saburo

2017-06-01

In the context of the fatigue life design of components, particularly those destined for use in hydrogen refuelling stations and fuel cell vehicles, it is important to understand the hydrogen-induced, fatigue crack growth (FCG) acceleration in steels. As such, the mechanisms for acceleration and its influencing factors are reviewed and discussed in this paper, with a special focus on the peculiar frequency dependence of the hydrogen-induced FCG acceleration. Further, this frequency dependence is debated by introducing some potentially responsible elements, along with new experimental data obtained by the authors. This article is part of the themed issue 'The challenges of hydrogen and metals'.

Effects of ply thickness on thermal cycle induced damage and thermal strain

NASA Astrophysics Data System (ADS)

Tompkins, Stephen S.

1994-07-01

An experimental study was conducted to determine the effects of ply thickness in composite laminates on thermally induced cracking and changes in the coefficient of thermal expansion, CTE. A graphite-epoxy composite material, P75/ERL 1962, in thin (1 mil) and thick (5 mils) prepregs was used to make cross-ply laminates, ((0/90)(sub n))s, with equal total thickness (n=2, n=10) and cross-ply laminates with the same total number of plies (n=2). Specimens of each laminate configuration were cycled up to 1500 times between -250 and 250 F. Thermally induced microdamage was assessed as a function of the number of cycles as was the change in CTE. The results showed that laminates fabricated with thin-plies microcracked at significantly different rates and reached significantly different equilibrium crack densities than the laminate fabricated with thick-ply and n=2. The CTE of thin-ply laminates was less affected by thermal cycling and damage than the CTE of thick-ply laminates. These differences are attributed primarily to differences in interply constraints. Observed effects of ply thickness on crack density was qualitatively predicted by a combined shear-lag stress/energy method.

Effects of ply thickness on thermal cycle induced damage and thermal strain

NASA Technical Reports Server (NTRS)

Tompkins, Stephen S.

1994-01-01

An experimental study was conducted to determine the effects of ply thickness in composite laminates on thermally induced cracking and changes in the coefficient of thermal expansion, CTE. A graphite-epoxy composite material, P75/ERL 1962, in thin (1 mil) and thick (5 mils) prepregs was used to make cross-ply laminates, ((0/90)(sub n))s, with equal total thickness (n=2, n=10) and cross-ply laminates with the same total number of plies (n=2). Specimens of each laminate configuration were cycled up to 1500 times between -250 and 250 F. Thermally induced microdamage was assessed as a function of the number of cycles as was the change in CTE. The results showed that laminates fabricated with thin-plies microcracked at significantly different rates and reached significantly different equilibrium crack densities than the laminate fabricated with thick-ply and n=2. The CTE of thin-ply laminates was less affected by thermal cycling and damage than the CTE of thick-ply laminates. These differences are attributed primarily to differences in interply constraints. Observed effects of ply thickness on crack density was qualitatively predicted by a combined shear-lag stress/energy method.

A thermoplastic/thermoset blend exhibiting thermal mending and reversible adhesion.

PubMed

Luo, Xiaofan; Ou, Runqing; Eberly, Daniel E; Singhal, Amit; Viratyaporn, Wantinee; Mather, Patrick T

2009-03-01

In this paper, we report on the development of a new and broadly applicable strategy to produce thermally mendable polymeric materials, demonstrated with an epoxy/poly(-caprolactone) (PCL) phase-separated blend. The initially miscible blend composed of 15.5 wt % PCL undergoes polymerization-induced phase separation during cross-linking of the epoxy, yielding a "bricks and mortar" morphology wherein the epoxy phase exists as interconnected spheres (bricks) interpenetrated with a percolating PCL matrix (mortar). The fully cured material is stiff, strong, and durable. A heating-induced "bleeding" behavior was witnessed in the form of spontaneous wetting of all free surfaces by the molten PCL phase, and this bleeding is capable of repairing damage by crack-wicking and subsequent recrystallization with only minor concomitant softening during that process. The observed bleeding is attributed to volumetric thermal expansion of PCL above its melting point in excess of epoxy brick expansion, which we term differential expansive bleeding (DEB). In controlled thermal-mending experiments, heating of a cracked specimen led to PCL extrusion from the bulk to yield a liquid layer bridging the crack gap. Upon cooling, a "scar" composed of PCL crystals formed at the site of the crack, restoring a significant portion of the mechanical strength. When a moderate force was applied to assist crack closure, thermal-mending efficiencies exceeded 100%. We further observed that the DEB phenomenon enables strong and facile adhesion of the same material to itself and to a variety of materials, without any requirement for macroscopic softening or flow.

An Evaluation of Apical Cracks in Teeth Undergoing Orthograde Root Canal Instrumentation.

PubMed

Rose, Elizabeth; Svec, Timothy

2015-12-01

Dentinal damage and cracks induced by orthograde preparation methods have been reported in studies using extracted teeth. The purpose of this in situ investigation was to evaluate dentinal cracks in nonextracted teeth after final instrumentation. The null hypothesis is that orthograde root canal instrumentation will have no effect on crack initiation in teeth retained in the natural periodontium. Mandibular first and second premolars of pig jaws were selected. Forty single-rooted canals were divided into 5 groups (n = 8): (1) WaveOne (Dentsply Tulsa Dental Specialties, Tulsa, OK) 25/08; (2) ProTaper rotary S1, S2, F2 (25/08) (Dentsply Tulsa Dental Specialties); (3) crown-down GT hand files 20/12, 20/10, 20/08 (Dentsply Tulsa Dental Specialties); (4) positive control (purposefully cracked); and (5) negative control (uninstrumented teeth). After instrumentation, superficial soft tissue was removed, and bone was carefully peeled away with surgical burs to the level of the root apices. Roots were resected 1 mm coronal to the working length, stained with caries indicator dye, and transilluminated; images were captured and viewed at 30× magnification to determine the presence or absence of dentinal cracks. WaveOne, ProTaper rotary, and GT hand files produced no cracks. All positive controls had cracks; all negative controls had no cracks. Within the limits of this investigation, the presence of natural periodontal structures may prevent cracking or dentinal damage in teeth receiving orthograde root canal instrumentation. Copyright © 2015 American Association of Endodontists. Published by Elsevier Inc. All rights reserved.

Self-protected nitrate reducing culture for intrinsic repair of concrete cracks.

PubMed

Erşan, Yusuf Ç; Gruyaert, Elke; Louis, Ghislain; Lors, Christine; De Belie, Nele; Boon, Nico

2015-01-01

Attentive monitoring and regular repair of concrete cracks are necessary to avoid further durability problems. As an alternative to current maintenance methods, intrinsic repair systems which enable self-healing of cracks have been investigated. Exploiting microbial induced CaCO3 precipitation (MICP) using (protected) axenic cultures is one of the proposed methods. Yet, only a few of the suggested healing agents were economically feasible for in situ application. This study presents a [Formula: see text] reducing self-protected enrichment culture as a self-healing additive for concrete. Concrete admixtures Ca(NO3)2 and Ca(HCOO)2 were used as nutrients. The enrichment culture, grown as granules (0.5-2 mm) consisting of 70% biomass and 30% inorganic salts were added into mortar without any additional protection. Upon 28 days curing, mortar specimens were subjected to direct tensile load and multiple cracks (0.1-0.6 mm) were achieved. Cracked specimens were immersed in water for 28 days and effective crack closure up to 0.5 mm crack width was achieved through calcite precipitation. Microbial activity during crack healing was monitored through weekly NOx analysis which revealed that 92 ± 2% of the available [Formula: see text] was consumed. Another set of specimens were cracked after 6 months curing, thus the effect of curing time on healing efficiency was investigated, and mineral formation at the inner crack surfaces was observed, resulting in 70% less capillary water absorption compared to healed control specimens. In conclusion, enriched mixed denitrifying cultures structured in self-protecting granules are very promising strategies to enhance microbial self-healing.

Self-protected nitrate reducing culture for intrinsic repair of concrete cracks

PubMed Central

Erşan, Yusuf Ç.; Gruyaert, Elke; Louis, Ghislain; Lors, Christine; De Belie, Nele; Boon, Nico

2015-01-01

Attentive monitoring and regular repair of concrete cracks are necessary to avoid further durability problems. As an alternative to current maintenance methods, intrinsic repair systems which enable self-healing of cracks have been investigated. Exploiting microbial induced CaCO3 precipitation (MICP) using (protected) axenic cultures is one of the proposed methods. Yet, only a few of the suggested healing agents were economically feasible for in situ application. This study presents a NO3− reducing self-protected enrichment culture as a self-healing additive for concrete. Concrete admixtures Ca(NO3)2 and Ca(HCOO)2 were used as nutrients. The enrichment culture, grown as granules (0.5–2 mm) consisting of 70% biomass and 30% inorganic salts were added into mortar without any additional protection. Upon 28 days curing, mortar specimens were subjected to direct tensile load and multiple cracks (0.1–0.6 mm) were achieved. Cracked specimens were immersed in water for 28 days and effective crack closure up to 0.5 mm crack width was achieved through calcite precipitation. Microbial activity during crack healing was monitored through weekly NOx analysis which revealed that 92 ± 2% of the available NO3− was consumed. Another set of specimens were cracked after 6 months curing, thus the effect of curing time on healing efficiency was investigated, and mineral formation at the inner crack surfaces was observed, resulting in 70% less capillary water absorption compared to healed control specimens. In conclusion, enriched mixed denitrifying cultures structured in self-protecting granules are very promising strategies to enhance microbial self-healing. PMID:26583015

Vibration and stability of cracked hollow-sectional beams

NASA Astrophysics Data System (ADS)

Zheng, D. Y.; Fan, S. C.

2003-10-01

This paper presents simple tools for the vibration and stability analysis of cracked hollow-sectional beams. It comprises two parts. In the first, the influences of sectional cracks are expressed in terms of flexibility induced. Each crack is assigned with a local flexibility coefficient, which is derived by virtue of theories of fracture mechanics. The flexibility coefficient is a function of the depth of a crack. The general formulae are derived and expressed in integral form. It is then transformed to explicit form through 128-point Gauss quadrature. According to the depth of the crack, the formulae are derived under two scenarios. The first is for shallow cracks, of which the penetration depth is contained within the top solid-sectional region. The second is for deeper penetration, in which the crack goes into the middle hollow-sectional region. The explicit formulae are best-fitted equations generated by the least-squares method. The best-fitted curves are presented. From the curves, the flexibility coefficients can be read out easily, while the explicit expressions facilitate easy implementation in computer analysis. In the second part, the flexibility coefficients are employed in the vibration and stability analysis of hollow-sectional beams. The cracked beam is treated as an assembly of sub-segments linked up by rotational springs. Division of segments are made coincident with the location of cracks or any abrupt change of sectional property. The crack's flexibility coefficient then serves as that of the rotational spring. Application of the Hamilton's principle leads to the governing equations, which are subsequently solved through employment of a simple technique. It is a kind of modified Fourier series, which is able to represent any order of continuity of the vibration/buckling modes. Illustrative numerical examples are included.

Influence of High Cycle Thermal Loads on Thermal Fatigue Behavior of Thick Thermal Barrier Coatings

NASA Technical Reports Server (NTRS)

Zhu, Dongming; Miller, Robert A.

1997-01-01

Thick thermal barrier coating systems in a diesel engine experience severe thermal Low Cycle Fatigue (LCF) and High Cycle Fatigue (HCF) during engine operation. In the present study, the mechanisms of fatigue crack initiation and propagation, as well as of coating failure, under thermal loads which simulate engine conditions, are investigated using a high power CO2 laser. In general, surface vertical cracks initiate early and grow continuously under LCF and HCF cyclic stresses. It is found that in the absence of interfacial oxidation, the failure associated with LCF is closely related to coating sintering and creep at high temperatures, which induce tensile stresses in the coating after cooling. Experiments show that the HCF cycles are very damaging to the coating systems. The combined LCF and HCF tests produced more severe coating surface cracking, microspallation and accelerated crack growth, as compared to the pure LCF test. It is suggested that the HCF component cannot only accelerate the surface crack initiation, but also interact with the LCF by contributing to the crack growth at high temperatures. The increased LCF stress intensity at the crack tip due to the HCF component enhances the subsequent LCF crack growth. Conversely, since a faster HCF crack growth rate will be expected with lower effective compressive stresses in the coating, the LCF cycles also facilitate the HCF crack growth at high temperatures by stress relaxation process. A surface wedging model has been proposed to account for the HCF crack growth in the coating system. This mechanism predicts that HCF damage effect increases with increasing temperature swing, the thermal expansion coefficient and the elastic modulus of the ceramic coating, as well as the HCF interacting depth. A good agreement has been found between the analysis and experimental evidence.

A Predictive Model for Chemically-Induced Fracture

NASA Astrophysics Data System (ADS)

Carter, Emily

2004-03-01

Mechanical properties of bulk solids are affected not only by macroscopic external loads, but also by chemical reactions, typically at surfaces and interfaces. For example, impurities in metals often coalesce at grain boundaries, leading to weakening of the sample under stress. Atmospheric corrosion is another example that, when combined with external loads, leads to stress-corrosion cracking. These are inherently multiscale phenomena, where the chemistry occurring at the atomic scale profoundly affects the mechanical properties at the micron to millimeter scale. Here we discuss a multiscale model of environmentally-assisted fracture. This involves coupling periodic density functional theory (DFT) at the atomic scale to a finite element continuum mechanics description of the coarser scale. A key component is the cohesive law, which we have shown takes on a universal form distinct from the generally used UBER model. Further, we propose a scheme to calculate physically realistic cohesive laws in the presence of mobile impurities. This cohesive law is then used to in a continuum model that couples stress-assisted diffusion with cohesive zone models of fracture to describe hydrogen embrittlement in metals. We show that this model, with a first principles-based cohesive law, provides insight into the observed intermittent cracking in steel, as well as good quantitative agreement with experiment.

Changing the Environmental Behaviour of Small Business Owners: The Business Case

ERIC Educational Resources Information Center

Walker, Beth; Redmond, Janice

2014-01-01

The importance of the environment is something of a cracked record to many small business owners, as historically any calls to business to change or improve their practices or behaviours were from the "environmental" or "green" perspective, rather than from a business perspective. As a consequence, many small businesses have…

Brittle fracture in viscoelastic materials as a pattern-formation process

NASA Astrophysics Data System (ADS)

Fleck, M.; Pilipenko, D.; Spatschek, R.; Brener, E. A.

2011-04-01

A continuum model of crack propagation in brittle viscoelastic materials is presented and discussed. Thereby, the phenomenon of fracture is understood as an elastically induced nonequilibrium interfacial pattern formation process. In this spirit, a full description of a propagating crack provides the determination of the entire time dependent shape of the crack surface, which is assumed to be extended over a finite and self-consistently selected length scale. The mechanism of crack propagation, that is, the motion of the crack surface, is then determined through linear nonequilibrium transport equations. Here we consider two different mechanisms, a first-order phase transformation and surface diffusion. We give scaling arguments showing that steady-state solutions with a self-consistently selected propagation velocity and crack shape can exist provided that elastodynamic or viscoelastic effects are taken into account, whereas static elasticity alone is not sufficient. In this respect, inertial effects as well as viscous damping are identified to be sufficient crack tip selection mechanisms. Exploring the arising description of brittle fracture numerically, we study steady-state crack propagation in the viscoelastic and inertia limit as well as in an intermediate regime, where both effects are important. The arising free boundary problems are solved by phase field methods and a sharp interface approach using a multipole expansion technique. Different types of loading, mode I, mode III fracture, as well as mixtures of them, are discussed.

High cycle fatigue in the transmission electron microscope

DOE PAGES

Bufford, Daniel C.; Stauffer, Douglas; Mook, William M.; ...

2016-06-28

One of the most common causes of structural failure in metals is fatigue induced by cyclic loading. Historically, microstructure-level analysis of fatigue cracks has primarily been performed post mortem. However, such investigations do not directly reveal the internal structural processes at work near micro- and nanoscale fatigue cracks and thus do not provide direct evidence of active microstructural mechanisms. In this paper, the tension–tension fatigue behavior of nanocrystalline Cu was monitored in real time at the nanoscale by utilizing a new capability for quantitative cyclic mechanical loading performed in situ in a transmission electron microscope (TEM). Controllable loads were appliedmore » at frequencies from one to several hundred hertz, enabling accumulations of 10 6 cycles within 1 h. The nanometer-scale spatial resolution of the TEM allows quantitative fatigue crack growth studies at very slow crack growth rates, measured here at ~10 –12 m·cycle –1. This represents an incipient threshold regime that is well below the tensile yield stress and near the minimum conditions for fatigue crack growth. Evidence of localized deformation and grain growth within 150 nm of the crack tip was observed by both standard imaging and precession electron diffraction orientation mapping. Finally, these observations begin to reveal with unprecedented detail the local microstructural processes that govern damage accumulation, crack nucleation, and crack propagation during fatigue loading in nanocrystalline Cu.« less

High cycle fatigue in the transmission electron microscope

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bufford, Daniel C.; Stauffer, Douglas; Mook, William M.

One of the most common causes of structural failure in metals is fatigue induced by cyclic loading. Historically, microstructure-level analysis of fatigue cracks has primarily been performed post mortem. However, such investigations do not directly reveal the internal structural processes at work near micro- and nanoscale fatigue cracks and thus do not provide direct evidence of active microstructural mechanisms. In this paper, the tension–tension fatigue behavior of nanocrystalline Cu was monitored in real time at the nanoscale by utilizing a new capability for quantitative cyclic mechanical loading performed in situ in a transmission electron microscope (TEM). Controllable loads were appliedmore » at frequencies from one to several hundred hertz, enabling accumulations of 10 6 cycles within 1 h. The nanometer-scale spatial resolution of the TEM allows quantitative fatigue crack growth studies at very slow crack growth rates, measured here at ~10 –12 m·cycle –1. This represents an incipient threshold regime that is well below the tensile yield stress and near the minimum conditions for fatigue crack growth. Evidence of localized deformation and grain growth within 150 nm of the crack tip was observed by both standard imaging and precession electron diffraction orientation mapping. Finally, these observations begin to reveal with unprecedented detail the local microstructural processes that govern damage accumulation, crack nucleation, and crack propagation during fatigue loading in nanocrystalline Cu.« less

Anisotropic Poroelasticity in a Rock With Cracks

NASA Astrophysics Data System (ADS)

Wong, Teng-Fong

2017-10-01

Deformation of a saturated rock in the field and laboratory may occur in a broad range of conditions, ranging from undrained to drained. The poromechanical response is often anisotropic, and in a brittle rock, closely related to preexisting and stress-induced cracks. This can be modeled as a rock matrix embedded with an anisotropic system of cracks. Assuming microisotropy, expressions for three of the poroelastic coefficients of a transversely isotropic rock were derived in terms of the crack density tensor. Together with published results for the five effective elastic moduli, this provides a complete micromechanical description of the eight independent poroelastic coefficients of such a cracked rock. Relatively simple expressions were obtained for the Skempton pore pressure tensor, which allow one to infer the crack density tensor from undrained measurement in the laboratory, and also to infer the Biot-Willis effective stress coefficients. The model assumes a dilute concentration of noninteractive penny-shaped cracks, and it shows good agreement with experimental data for Berea sandstone, with crack density values up to 0.6. Whereas predictions on the storage coefficient and normal components of the elastic stiffness tensor also seem reasonable, significant discrepancy between model and measurement was observed regarding the off-diagonal and shear components of the stiffness. A plausible model had been proposed for development of very strong anisotropy in the undrained response of a fault zone, and the model here placed geometric constraints on the associated fracture system.

Analysis of delamination in cross-ply laminates initiating from impact induced matrix cracking

NASA Technical Reports Server (NTRS)

Salpekar, S. A.

1993-01-01

Two-dimensional finite element analyses of (02/90(8)/02) glass/epoxy and graphite/epoxy composite laminates were performed to investigate some of the characteristics of damage development due to an impact load. A cross section through the thickness of the laminate with fixed ends, and carrying a transverse load in the center, was analyzed. Inclined matrix cracks, such as those produced by a low-velocity impact, were modeled in the 90 deg ply group. The introduction of the matrix cracks caused large interlaminar tensile and shear stresses in the vicinity of both crack tips in the 0/90 and 90/0 interfaces, indicating that matrix cracking may give rise to delamination. The ratio of Mode I to total strain energy release rate, G(I)/G(total), at the beginning of delamination, calculated at the two (top and bottom) matrix crack tips was 60 and 28 percent, respectively, in the glass/epoxy laminate. The corresponding ratio was 97 and 77 percent in the graphite/epoxy laminate. Thus, a significant Mode I component of strain energy release rate may be present at the delamination initiation due to an impact load. The value of strain energy release rate at either crack tip increased due to an increase in the delamination length at the other crack tip and may give rise to an unstable delamination growth under constant load.

Use of Several Thermal Analysis Techniques to Study the Cracking of an Nitrile Butadiene Rubber (NBR) Insulator on the Booster Separation Motor (BSM) of the Space Shuttle

NASA Technical Reports Server (NTRS)

Wingard, Charles D.; Whitaker, Ann F. (Technical Monitor)

2000-01-01

Two different vendor rubber formulations have been used to produce the silica-filled NBR insulators for the BSM used on both of the Solid Rocket Boosters (SRBs) of the Space Shuttle. A number of lots of the BSM insulator in 1998-99 exhibited surface cracks and/or crazing. Each insulator is bonded to the BSM aluminum aft closure with an epoxy adhesive. Induced insulator stresses from adhesive cure are likely greatest where the insulator/adhesive contour is the greatest, thus showing increased insulator surface cracking in this area. Thermal analysis testing by Dynamic Mechanical Analyzer (DMA) and Thermomechanical Analysis (TMA) was performed on one each of the two vendor BSM insulators previously bonded that exhibited the surface cracking. The TMA data from the film/fiber technique yielded the most meaningful results, with thin insulator surface samples containing cracks having roughly the same modulus (stiffness) as thin insulator bulk samples just underneath.

Stress corrosion cracking of Alloy 600 and Alloy 690 in all volatile treated water at elevated temperatures. Final report. [PWR

DOE Office of Scientific and Technical Information (OSTI.GOV)

Theus, G.J.; Emanuelson, R.H.

1983-05-01

This report describes a continuing study of stress corrosion cracking (SCC) of Inconel alloys 600 and 690 in all-volatile treated (AVT) water. Specimens of alloys 600 and 690 are being exposed to AVT water at 288/sup 0/, 332/sup 0/, 343/sup 0/, and 360/sup 0/C. Alloy 600 generally resists SCC in high-purity water under normal service conditions but is susceptible under other specific conditions. In general, mill-annealed alloy 600 is more susceptible than stress-relieved material. Susceptibility to SCC increases rapidly with increasing exposure temperature. Very high stresses (near or above yield) are required to induce cracking in AVT or other high-puritymore » waters. Most of the data presented in this report are for alloy 600; alloy 690 has not yet cracked. However, the program is being continued and will subsequently characterize the high-purity water cracking behavior, if any, of alloy 690.« less

Formation and prevention of fractures in sol-gel-derived thin films.

PubMed

Kappert, Emiel J; Pavlenko, Denys; Malzbender, Jürgen; Nijmeijer, Arian; Benes, Nieck E; Tsai, Peichun Amy

2015-02-07

Sol-gel-derived thin films play an important role as the functional coatings for various applications that require crack-free films to fully function. However, the fast drying process of a standard sol-gel coating often induces mechanical stresses, which may fracture the thin films. An experimental study on the crack formation in sol-gel-derived silica and organosilica ultrathin (submicron) films is presented. The relationships among the crack density, inter-crack spacing, and film thickness were investigated by combining direct micrograph analysis with spectroscopic ellipsometry. It is found that silica thin films are more prone to fracturing than organosilica films and have a critical film thickness of 300 nm, above which the film fractures. In contrast, the organosilica films can be formed without cracks in the experimentally explored regime of film thickness up to at least 1250 nm. These results confirm that ultrathin organosilica coatings are a robust silica substitute for a wide range of applications.

Fracture analysis of a central crack in a long cylindrical superconductor with exponential model

NASA Astrophysics Data System (ADS)

Zhao, Yu Feng; Xu, Chi

2018-05-01

The fracture behavior of a long cylindrical superconductor is investigated by modeling a central crack that is induced by electromagnetic force. Based on the exponential model, the stress intensity factors (SIFs) with the dimensionless parameter p and the length of the crack a/R for the zero-field cooling (ZFC) and field-cooling (FC) processes are numerically simulated using the finite element method (FEM) and assuming a persistent current flow. As the applied field Ba decreases, the dependence of p and a/R on the SIFs in the ZFC process is exactly opposite to that observed in the FC process. Numerical results indicate that the exponential model exhibits different characteristics for the trend of the SIFs from the results obtained using the Bean and Kim models. This implies that the crack length and the trapped field have significant effects on the fracture behavior of bulk superconductors. The obtained results are useful for understanding the critical-state model of high-temperature superconductors in crack problem.

Normal compression wave scattering by a permeable crack in a fluid-saturated poroelastic solid

NASA Astrophysics Data System (ADS)

Song, Yongjia; Hu, Hengshan; Rudnicki, John W.

2017-04-01

A mathematical formulation is presented for the dynamic stress intensity factor (mode I) of a finite permeable crack subjected to a time-harmonic propagating longitudinal wave in an infinite poroelastic solid. In particular, the effect of the wave-induced fluid flow due to the presence of a liquid-saturated crack on the dynamic stress intensity factor is analyzed. Fourier sine and cosine integral transforms in conjunction with Helmholtz potential theory are used to formulate the mixed boundary-value problem as dual integral equations in the frequency domain. The dual integral equations are reduced to a Fredholm integral equation of the second kind. It is found that the stress intensity factor monotonically decreases with increasing frequency, decreasing the fastest when the crack width and the slow wave wavelength are of the same order. The characteristic frequency at which the stress intensity factor decays the fastest shifts to higher frequency values when the crack width decreases.

Understanding the crack formation of graphite particles in cycled commercial lithium-ion batteries by focused ion beam - scanning electron microscopy

NASA Astrophysics Data System (ADS)

Lin, Na; Jia, Zhe; Wang, Zhihui; Zhao, Hui; Ai, Guo; Song, Xiangyun; Bai, Ying; Battaglia, Vincent; Sun, Chengdong; Qiao, Juan; Wu, Kai; Liu, Gao

2017-10-01

The structure degradation of commercial Lithium-ion battery (LIB) graphite anodes with different cycling numbers and charge rates was investigated by focused ion beam (FIB) and scanning electron microscopy (SEM). The cross-section image of graphite anode by FIB milling shows that cracks, resulted in the volume expansion of graphite electrode during long-term cycling, were formed in parallel with the current collector. The crack occurs in the bulk of graphite particles near the lithium insertion surface, which might derive from the stress induced during lithiation and de-lithiation cycles. Subsequently, crack takes place along grain boundaries of the polycrystalline graphite, but only in the direction parallel with the current collector. Furthermore, fast charge graphite electrodes are more prone to form cracks since the tensile strength of graphite is more likely to be surpassed at higher charge rates. Therefore, for LIBs long-term or high charge rate applications, the tensile strength of graphite anode should be taken into account.

Flexoelectricity in Bones.

PubMed

Vasquez-Sancho, Fabian; Abdollahi, Amir; Damjanovic, Dragan; Catalan, Gustau

2018-03-01

Bones generate electricity under pressure, and this electromechanical behavior is thought to be essential for bone's self-repair and remodeling properties. The origin of this response is attributed to the piezoelectricity of collagen, which is the main structural protein of bones. In theory, however, any material can also generate voltages in response to strain gradients, thanks to the property known as flexoelectricity. In this work, the flexoelectricity of bone and pure bone mineral (hydroxyapatite) are measured and found to be of the same order of magnitude; the quantitative similarity suggests that hydroxyapatite flexoelectricity is the main source of bending-induced polarization in cortical bone. In addition, the measured flexoelectric coefficients are used to calculate the (flexo)electric fields generated by cracks in bone mineral. The results indicate that crack-generated flexoelectricity is theoretically large enough to induce osteocyte apoptosis and thus initiate the crack-healing process, suggesting a central role of flexoelectricity in bone repair and remodeling. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

An analytical approach for the calculation of stress-intensity factors in transformation-toughened ceramics

NASA Astrophysics Data System (ADS)

Müller, W. H.

1990-12-01

Stress-induced transformation toughening in Zirconia-containing ceramics is described analytically by means of a quantitative model: A Griffith crack which interacts with a transformed, circular Zirconia inclusion. Due to its volume expansion, a ZrO2-particle compresses its flanks, whereas a particle in front of the crack opens the flanks such that the crack will be attracted and finally absorbed. Erdogan's integral equation technique is applied to calculate the dislocation functions and the stress-intensity-factors which correspond to these situations. In order to derive analytical expressions, the elastic constants of the inclusion and the matrix are assumed to be equal.

A Survey of Corrosion and Conditions of Corrosion Protection Systems in Civil Works Structures of the U.S. Army Corps of Engineers

DTIC Science & Technology

2014-09-01

corrosion: coatings and cathodic protection (CP). Coatings consist of paints, epoxies, enamels , metalizing, and other coatings. CP is a chem- ical means...environmental factors such as water quality and resistivity. One of the major problems associated with lock gates is structural cracking in the...One of the problems described by Mr. Davis is fatigue crack growth resulting from the poor welding usually associated with stress risers and

Environmental Effects on the Incubation Time Characteristics in Stress-Corrosion Cracking

DTIC Science & Technology

2011-04-01

Corrosion Experimental Data for Specimen SL51 143 Table B2. Stress Corrosion Experimental Data for Specimen SL76 (3.5% NaCI, K , =6.8M/Wm) 144 Table B3...of action of the external load; - 143 - K =applied stress intensity factor; da/dt=crack growth rate. Table B2. Stress Corrosion Experimental Data for...Submitted to Dr. Asuri K . Vasudevan Scientific Officer Office of Naval Research, Code-332 875 North Randolph Street, Suite 1425, Room- 629

Neutron Radiography Based Visualization and Profiling of Water Uptake in (Un)cracked and Autonomously Healed Cementitious Materials

PubMed Central

Van den Heede, Philip; Van Belleghem, Bjorn; Alderete, Natalia; Van Tittelboom, Kim; De Belie, Nele

2016-01-01

Given their low tensile strength, cement-based materials are very susceptible to cracking. These cracks serve as preferential pathways for corrosion inducing substances. For large concrete infrastructure works, currently available time-consuming manual repair techniques are not always an option. Often, one simply cannot reach the damaged areas and when making those areas accessible anyway (e.g., by redirecting traffic), the economic impacts involved would be enormous. Under those circumstances, it might be useful to have concrete with an embedded autonomous healing mechanism. In this paper, the effectiveness of incorporating encapsulated high and low viscosity polyurethane-based healing agents to ensure (multiple) crack healing has been investigated by means of capillary absorption tests on mortar while monitoring the time-dependent water ingress with neutron radiography. Overall visual interpretation and water front/sample cross-section area ratios as well as water profiles representing the area around the crack and their integrals do not show a preference for the high or low viscosity healing agent. Another observation is that in presence of two cracks, only one is properly healed, especially when using the latter healing agent. Exposure to water immediately after release of the healing agent stimulates the foaming reaction of the polyurethane and ensures a better crack closure. PMID:28773436

Source process of a long-period event at Kilauea volcano, Hawaii

USGS Publications Warehouse

Kumagai, H.; Chouet, B.A.; Dawson, P.B.

2005-01-01

We analyse a long-period (LP) event observed by a dense seismic network temporarily operated at Kilauea volcano, Hawaii, in 1996. We systematically perform spectral analyses, waveform inversions and forward modeling of the LP event to quantify its source process. Spectral analyses identify two dominant spectral frequencies at 0.6 and 1.3 Hz with associated Q values in the range 10-20. Results from waveform inversions assuming six moment-tensor and three single-force components point to the resonance of a horizontal crack located at a depth of approximately 150 m near the northeastern rim of the Halemaumau pit crater. Waveform simulations based on a fluid-filled crack model suggest that the observed frequencies and Q values can be explained by a crack filled with a hydrothermal fluid in the form of either bubbly water or steam. The shallow hydrothermal crack located directly above the magma conduit may have been heated by volcanic gases leaking from the conduit. The enhanced flux of heat raised the overall pressure of the hydrothermal fluid in the crack and induced a rapid discharge of fluid from the crack, which triggered the acoustic vibrations of the resonator generating the LP waveform. The present study provides further support to the idea that LP events originate in the resonance of a crack. ?? 2005 RAS.

Environmental and economic assessment of a cracked ammonia fuelled alkaline fuel cell for off-grid power applications

NASA Astrophysics Data System (ADS)

Cox, Brian; Treyer, Karin

2015-02-01

Global mobile telecommunication is possible due to millions of Base Transceiver Stations (BTS). Nearly 1 million of these are operating off-grid, typically powered by diesel generators and therefore leading to significant CO2 emissions and other environmental burdens. A novel type of Alkaline Fuel Cell (AFC) powered by cracked ammonia is being developed for replacement of these generators. This study compares the environmental and economic performance of the two systems by means of Life Cycle Assessment (LCA) and Levelised Cost of Electricity (LCOE), respectively. Results show that the production of ammonia dominates the LCA results, and that renewable ammonia production pathways greatly improve environmental performance. Sensitivity analyses reveal that the fuel cell parameters that most affect system cost and environmental burdens are cell power density and lifetime and system efficiency. Recycling of anode catalyst and electrode substrate materials is found to have large impacts on environmental performance, though without large cost incentives. For a set of target parameter values and fossil sourced ammonia, the AFC is calculated to produce electricity with life cycle CO2 eq emissions of 1.08 kg kWh-1, which is 23% lower than a diesel generator with electricity costs that are 14% higher in the same application.

Defect characterization by inductive heated thermography

NASA Astrophysics Data System (ADS)

Noethen, Matthias; Meyendorf, Norbert

2012-05-01

During inductive-thermographic inspection, an eddy current of high intensity is induced into the inspected material and the thermal response is detected by an infrared camera. Anomalies in the surface temperature during and after inductive heating correspond to inhomogeneities in the material. A finite element simulation of the surface crack detection process using active thermography with inductive heating has been developed. The simulation model is based on the finite element software ANSYS. The simulation tool was tested and used for investigations on steel components with different longitudinal orientated cracks, varying in shape, width and height. This paper focuses on surface connected longitudinal orientated cracks in austenitic steel. The results show that depending on the excitation frequency the temperature distribution of the material under test are different and a possible way to measure the depth of the crack will be discussed.

Development of Friction Stir Processing for Repair of Nuclear Dry Cask Storage System Canisters

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ross, Kenneth A.; Sutton, Ben; Grant, Glenn J.

The Nuclear Regulatory Commission has identified chloride-induced stress corrosion cracking (CISCC) of austenitic stainless steel dry cask storage systems (DCSS) as an area of great concern. Friction Stir Processing (FSP) was used to repair laboratory-generated stress corrosion cracking (SCC) in representative stainless steel 304 coupons. Results of this study show FSP is a viable method for repair and mitigation CISCC. This paper highlights lessons learned and developed techniques relative to FSP development for crack repair in sensitized thick section stainless steel 304. These include: development of process parameters, welding at low spindle speed, use of weld power and temperature controlmore » and optimization of these controls. NDE and destructive analysis are also presented to demonstrate effectiveness of the developed methods for SCC crack repair.« less

Quantitative image analysis of WE43-T6 cracking behavior

NASA Astrophysics Data System (ADS)

Ahmad, A.; Yahya, Z.

2013-06-01

Environment-assisted cracking of WE43 cast magnesium (4.2 wt.% Yt, 2.3 wt.% Nd, 0.7% Zr, 0.8% HRE) in the T6 peak-aged condition was induced in ambient air in notched specimens. The mechanism of fracture was studied using electron backscatter diffraction, serial sectioning and in situ observations of crack propagation. The intermetallic (rare earthed-enriched divorced intermetallic retained at grain boundaries and predominantly at triple points) material was found to play a significant role in initiating cracks which leads to failure of this material. Quantitative measurements were required for this project. The populations of the intermetallic and clusters of intermetallic particles were analyzed using image analysis of metallographic images. This is part of the work to generate a theoretical model of the effect of notch geometry on the static fatigue strength of this material.

Passive wireless antenna sensor for strain and crack sensing—electromagnetic modeling, simulation, and testing

NASA Astrophysics Data System (ADS)

Yi, Xiaohua; Cho, Chunhee; Cooper, James; Wang, Yang; Tentzeris, Manos M.; Leon, Roberto T.

2013-08-01

This research investigates a passive wireless antenna sensor designed for strain and crack sensing. When the antenna experiences deformation, the antenna shape changes, causing a shift in the electromagnetic resonance frequency of the antenna. A radio frequency identification (RFID) chip is adopted for antenna signal modulation, so that a wireless reader can easily distinguish the backscattered sensor signal from unwanted environmental reflections. The RFID chip captures its operating power from an interrogation electromagnetic wave emitted by the reader, which allows the antenna sensor to be passive (battery-free). This paper first reports the latest simulation results on radiation patterns, surface current density, and electromagnetic field distribution. The simulation results are followed with experimental results on the strain and crack sensing performance of the antenna sensor. Tensile tests show that the wireless antenna sensor can detect small strain changes lower than 20 με, and can perform well at large strains higher than 10 000 με. With a high-gain reader antenna, the wireless interrogation distance can be increased up to 2.1 m. Furthermore, an array of antenna sensors is capable of measuring the strain distribution in close proximity. During emulated crack and fatigue crack tests, the antenna sensor is able to detect the growth of a small crack.

Fatigue crack growth rates in a pressure vessel steel under various conditions of loading and the environment

NASA Astrophysics Data System (ADS)

Hicks, P. D.; Robinson, F. P. A.

1986-10-01

Corrosion fatigue (CF) tests have been carried out on SA508 Cl 3 pressure vessel steel, in simulated P.W.R. environments. The test variables investigated included air and P.W.R. water environments, frequency variation over the range 1 Hz to 10 Hz, transverse and longitudinal crack growth directions, temperatures of 20 °C and 50 °C, and R-ratios of 0.2 and 0.7. It was found that decreasing the test frequency increased fatigue crack growth rates (FCGR) in P.W.R. environments, P.W.R. environment testing gave enhanced crack growth (vs air tests), FCGRs were greater for cracks growing in the longitudinal direction, slight increases in temperature gave noticeable accelerations in FCGR, and several air tests gave FCGR greater than those predicted by the existing ASME codes. Fractographic evidence indicates that FCGRs were accelerated by a hydrogen embrittlement mechanism. The presence of elongated MnS inclusions aided both mechanical fatigue and hydrogen embrittlement processes, thus producing synergistically fast FCGRs. Both anodic dissolution and hydrogen embrittlement mechanisms have been proposed for the environmental enhancement of crack growth rates. Electrochemical potential measurements and potentiostatic tests have shown that sample isolation of the test specimens from the clevises in the apparatus is not essential during low temperature corrosion fatigue testing.

Environmental stress cracking of polymers

NASA Technical Reports Server (NTRS)

Mahan, K. I.

1980-01-01

A two point bending method for use in studying the environmental stress cracking and crazing phenomena is described and demonstrated for a variety of polymer/solvent systems. Critical strain values obtained from these curves are reported for various polymer/solvent systems including a considerable number of systems for which critical strain values have not been previously reported. Polymers studied using this technique include polycarbonate (PC), ABS, high impact styrene (HIS), polyphenylene oxide (PPO), and polymethyl methacrylate (PMMA). Critical strain values obtained using this method compared favorably with available existing data. The major advantage of the technique is the ability to obtain time vs. strain curves over a short period of time. The data obtained suggests that over a short period of time the transition in most of the polymer solvent systems is more gradual than previously believed.

Hydrogen transport and hydrogen embrittlement in stainless steels

DOE Office of Scientific and Technical Information (OSTI.GOV)

Perng, T.P.

1985-01-01

In order to understand the kinetics of gaseous hydrogen-induced slow crack growth (SCG) in metastable austenitic stainless steels, hydrogen permeation and/or cracking velocity were measured and compared for three types of stainless steels. These included austenitic, ferritic, and duplex (..gamma../..cap alpha..) alloys. Deformation in AISI 301 resulted in various amounts of ..cap alpha..' martensite, which enhanced the effective hydrogen diffusivity and permeability. No phase transformation was observed in deformed AISI 310. The effective hydrogen diffusivity in this alloy was slightly reduced after plastic deformation, presumably by dislocation trapping. In either the dynamic or static tensile test, AISI 301 exhibited themore » greatest hydrogen embrittlement and therefore the highest SCG velocity among all the alloys tested in this work. The SCG velocity was believed to be controlled by the rate of accumulation of hydrogen in the embrittlement region ahead of the crack tip and therefore could be explained with the hydrogen transport parameters measured from the permeation experiments. The relatively high SCG velocity in AISI 301 was probably due to the fast transport of hydrogen through the primarily stress-induced ..cap alpha..' phase around the crack. No SCG was observed in AISI 310. The presence of H/sub 2/O vapor was found to reduce both the hydrogen permeation and SCG velocity.« less

Intergranular stress corrosion cracking and selective internal oxidation of nickel-chromium-iron alloys in hydrogenated steam

NASA Astrophysics Data System (ADS)

Capell, Brent M.

2005-07-01

Selective internal oxidation (SIO) is a mechanism of grain boundary embrittlement through the formation of intergranular oxides of Cr2O3. SIO is proposed as a mechanism to explain intergranular stress corrosion cracking (IGSCC) of Ni-base alloys in pressurized water reactor environments. The purpose of this work is to investigate SIO through a series of experiments using controlled-purity alloys in a controlled, low-pressure steam environment in which the oxygen potential is varied. Five alloys; Ni-9Fe, Ni-5Cr, LCr (Ni-5Cr-9Fe), CD85 (Ni-16Cr-9Fe) and HCr (Ni-30Cr-9Fe), were used in corrosion coupon exposure tests and constant extension rate tensile (CERT) tests at 550°C and 400°C in an environment consisting of a controlled mixture of hydrogen, water vapor and argon. The hydrogen-to-water vapor partial pressure ratio (PPR) was varied between 0.001 and 0.9 to control the oxygen partial pressure. The Ni-9Fe, Ni-5Cr and LCr alloys formed a uniform Ni(OH)2 film at PPR values less than 0.09 while CD85 and HCr formed Cr2O 3 oxide films over the entire PPR range. Corrosion coupon results also show the formation of highly localized oxide particles at grain boundaries. Focused ion beam analysis revealed that intergranular oxides were observed at significant depths (>150 nm) down grain boundaries and the oxide morphology depended on the alloy composition and PPR value. Diffusion of oxygen along the grain boundary accounted for the growth of intergranular oxides. CERT test results showed that intergranular cracking was caused by creep-induced microvoid coalescence only at 550°C and did not depend on PPR. At 400°C, the cracking behavior depended on the PPR and resulted in a mixture of creep-induced microvoid coalescence and brittle intergranular failure. The cracked boundary fraction was higher at a PPR value where a Ni(OH)2 surface film formed. Alloy composition influenced cracking and the cracked boundary fraction decreased as the alloy chromium content increased. The compositions of oxides at intergranular crack tips were similar to intergranular oxides in corrosion coupons and demonstrated a link between intergranular oxide formation and intergranular cracking. The observation of intergranular oxides as well as the influence of alloy content on IG cracking was found to support SIO as a mechanism for IGSCC.

High-Heat-Flux Cyclic Durability of Thermal and Environmental Barrier Coatings

NASA Technical Reports Server (NTRS)

Zhu, Dongming; Ghosn, Louis L.; Miller, Robert A.

2007-01-01

Advanced ceramic thermal and environmental barrier coatings will play an increasingly important role in future gas turbine engines because of their ability to protect the engine components and further raise engine temperatures. For the supersonic vehicles currently envisioned in the NASA fundamental aeronautics program, advanced gas turbine engines will be used to provide high power density thrust during the extended supersonic flight of the aircraft, while meeting stringent low emission requirements. Advanced ceramic coating systems are critical to the performance, life and durability of the hot-section components of the engine systems. In this work, the laser and burner rig based high-heat-flux testing approaches were developed to investigate the coating cyclic response and failure mechanisms under simulated supersonic long-duration cruise mission. The accelerated coating cracking and delamination mechanism under the engine high-heat-flux, and extended supersonic cruise time conditions will be addressed. A coating life prediction framework may be realized by examining the crack initiation and propagation in conjunction with environmental degradation under high-heat-flux test conditions.

Transient cracks and triple junctions induced by Cocos-Nazca propagating rift

NASA Astrophysics Data System (ADS)

Schouten, H.; Smith, D. K.; Zhu, W.; Montesi, L. G.; Mitchell, G. A.; Cann, J. R.

2009-12-01

The Galapagos triple junction is a ridge-ridge-ridge triple junction where the Cocos, Nazca, and Pacific plates meet around the Galapagos microplate (GMP). On the Cocos plate, north of the large gore that marks the propagating Cocos-Nazca (C-N) Rift, a 250-km-long and 50-km-wide band of NW-SE-trending cracks crosscuts the N-S-trending abyssal hills of the East Pacific Rise (EPR). These appear as a succession of minor rifts, accommodating some NE-SW extension of EPR-generated seafloor. The rifts successively intersected the EPR in triple junctions at distances of 50-100 km north of the tip of the C-N Rift. We proposed a simple crack interaction model to explain the location of the transient rifts and their junction with the EPR. The model predicts that crack locations are controlled by the stress perturbation along the EPR, induced by the dominant C-N Rift, and scaled by the distance of its tip to the EPR (Schouten et al., 2008). The model also predicts that tensile stresses are symmetric about the C-N Rift and thus, similar cracks should have occurred south of the C-N Rift prior to formation of the GMP about 1 Ma. There were no data at the time to test this prediction. In early 2009 (AT 15-41), we mapped an area on the Nazca plate south of the C-N rift out to 4 Ma. The new bathymetric data confirm the existence of a distinctive pattern of cracks south of the southern C-N gore that mirrors the pattern on the Cocos plate until about 1 Ma, and lends support to the crack interaction model. The envelope of the symmetric cracking pattern indicates that the distance between the C-N Rift tip and the EPR varied between 40 and 65 km during this time (1-4 Ma). The breakdown of the symmetry at 1 Ma accurately dates the onset of a southern plate boundary of the GMP, now Dietz Deep Rift. At present, the southern rift boundary of the GMP joins the EPR with a steep-sided, 80 km long ridge. This ridge releases the stress perturbation otherwise induced along the EPR by elastic interaction with the C-N Rift and prevents the formation of minor rifts of the type in the North of the C-N Rift. However, the seafloor displays traces of rifts formed as the Dietz Deep Rift was approaching the EPR. In fact, the present day ridge appears to have originated as one of these minor rifts, probably stabilized by enhanced magma supply from a nearby volcano at the southwestern end of Dietz Deep.

Electromagnetic stimulation of the ultrasonic signal for nondestructive detection of the ferromagnetic inclusions and flaws

NASA Astrophysics Data System (ADS)

Finkel, Peter

2007-03-01

It was recently shown that thermal or optical stimulation can be used to increase sensitivity of the conventional nondestructive ultrasonic detection of the small crack, flaws and inclusions in a ferromagnetic thin-walled parts. We proposed another method based on electromagnetic modulation of the ultrasonic scattered signal from the inclusions or defects. The electromagnetically induced high density current pulse produces stresses which alter the ultrasonic waves scanning the part with the defect and modulate ultrasonic signal. The excited electromagnetic field can produces crack-opening due to Lorentz forces that increase the ultrasonic reflection. The Joule heating associated with the high density current, and consequent thermal stresses may cause both crack-closure, as well as crack-opening, depending on various factors. Experimental data is presented here for the case of a small cracks near small holes in thin-walled structures. The measurements were taken at 2-10 MHz with a Lamb wave wedge transducer. It is shown that electromagnetic transient modulation of the ultrasonic echo pulse tone-burst suggest that this method could be used to enhance detection of small cracks and ferromagnetic inclusions in thin walled metallic structures.

Indentation induced mechanical and electrical response in ferroelectric crystal investigated by acoustic mode AFM

NASA Astrophysics Data System (ADS)

Yu, H. F.; Zeng, H. R.; Ma, X. D.; Chu, R. Q.; Li, G. R.; Luo, H. S.; Yin, Q. R.

2005-01-01

The mechanical and electrical response of Pb (Mg1/3Nb2/3)- O3-PbTiO3 single crystals to micro-indentation are investigated using the newly developed low frequency scanning probe acoustic microscopy which is based on the atomic force microscope. There are three ways to release the stress produced by indentation. Plastic deformation emerged directly underneath the indentor and along the indentation diagonals. In addition, indentation-induced micro-cracks and new non-180° domain structures which are perpendicular to each other are also observed in the indented surface. Based on the experimental results, the relationship between the cracks and the domain patterns was discussed.

Hydrogen-enhanced fatigue crack growth in steels and its frequency dependence.

PubMed

Matsunaga, Hisao; Takakuwa, Osamu; Yamabe, Junichiro; Matsuoka, Saburo

2017-07-28

In the context of the fatigue life design of components, particularly those destined for use in hydrogen refuelling stations and fuel cell vehicles, it is important to understand the hydrogen-induced, fatigue crack growth (FCG) acceleration in steels. As such, the mechanisms for acceleration and its influencing factors are reviewed and discussed in this paper, with a special focus on the peculiar frequency dependence of the hydrogen-induced FCG acceleration. Further, this frequency dependence is debated by introducing some potentially responsible elements, along with new experimental data obtained by the authors.This article is part of the themed issue 'The challenges of hydrogen and metals'. © 2017 The Author(s).

InSitu SEM Investigation of Microstructural Damage Evolution and Strain Relaxation in a Melt Infiltrated SiC/SiC Composite

NASA Technical Reports Server (NTRS)

Sevener, Kathy; Chen, Zhe; Daly, Sam; Tracy, Jared; Kiser, Doug

2016-01-01

With CMC components poised to complete flight certification in turbine engines on commercial aircraft within the near future, there are many efforts within the aerospace community to model the mechanical and environmental degradation of CMCs. Direct observations of damage evolution are needed to support these modeling efforts and provide quantitative measures of damage parameters used in the various models. This study was performed to characterize the damage evolution during tensile loading of a melt infiltrated (MI) silicon carbide reinforced silicon carbide (SiC/SiC) composite. A SiC/SiC tensile coupon was loaded to a maximum global stress of 30 ksi in a tensile fixture within an SEM while observations were made at 5 ksi increments. Both traditional image analysis and DIC (digital image correlation) were used to quantify damage evolution. With the DIC analysis, microscale damage was observed at the fiber-matrix interfaces at stresses as low as 5 ksi. First matrix cracking took place between 20 and 25 ksi, accompanied by an observable relaxation in strain near matrix cracks. Matrix crack opening measurements at the maximum load ranged from 200 nm to 1.5 m. Crack opening along the fiber-matrix interface was also characterized as a function of load and angular position relative to the loading axis. This characterization was funded by NASA GRC and was performed to support NASA GRC modeling of SiC/SiC environmental degradation

Quantification of vapor intrusion pathways into a slab-on-ground building under varying environmental conditions.

PubMed

Patterson, Bradley M; Davis, Greg B

2009-02-01

Potential hydrocarbon-vapor intrusion pathways into a building through a concrete slab-on-ground were investigated and quantified under a variety of environmental conditions to elucidate the potential mechanisms for indoor air contamination. Vapor discharge from the uncovered open ground soil adjacent to the building and subsequent advection into the building was unlikely due to the low soil-gas concentrations at the edge of the building as a result of aerobic biodegradation of hydrocarbon vapors. When the building's interior was under ambient pressure, a flux of vapors into the building due to molecular diffusion of vapors through the building's concrete slab (cyclohexane 11 and methylcyclohexane 31 mg m(-2) concrete slab day(-1)) and short-term (up to 8 h) cyclical pressure-driven advection of vapors through an artificial crack (cyclohexane 4.2 x 10(3) and methylcyclohexane 1.2 x 10(4) mg m(-2) cracks day(-1)) was observed. The average subslab vapor concentration under the center of the building was 25,000 microg L(-1). Based on the measured building's interiorvapor concentrations and the building's air exchange rate of 0.66 h(-1), diffusion of vapors through the concrete slab was the dominantvapor intrusion pathway and cyclical pressure exchanges resulted in a near zero advective flux. When the building's interior was under a reduced pressure (-12 Pa), advective transport through cracks or gaps in the concrete slab (cyclohexane 340 and methylcyclohexane 1100 mg m(-2) cracks day(-1)) was the dominant vapor intrusion pathway.

The Influence of Hydrogen on the Evolving Microstructure During Fatigue Crack Growth in Metastable and Stable Austenitic Stainless Steels

NASA Astrophysics Data System (ADS)

Nygren, Kelly Elizabeth

The effect of high levels of internal hydrogen on the microstructure evolving during stage II fatigue crack growth was investigated through a series of tensile and fatigue studies in metastable (304) and stable (316, 316L) stainless steels. The first, a tensile study in 304 stainless steel, identified the underlying microstructure which resulted in the flat and quasi-cleavage features on the fracture surface of a hydrogen-charged tensile bar. The second study utilized single-edge notched tensile specimens loaded in fatigue, and compared the evolving microstructure ahead of a fatigue crack for cases of an uncharged, 10 wppm hydrogen-charged, and 104 wppm hydrogen-charged 304 and 316L alloy. The final fatigue study, a small fatigue crack growth study in round bars of 304 and 316, provided a contextual comparison of microstructures to previous results in literature. In the metastable 304 stainless steel, hydrogen is found to change the nature of the martensitic transformation and subsequent fracture path. This transformation is attributed to enhanced plasticity and hydrogen-dislocation interactions stabilizing the austenitic matrix and confining slip to particular close-packed planes. The martensite acts as a fast diffusion pathway for hydrogen, leading to final fracture along martensitic laths or cleavage planes. In 316L, the material deforms via slip and twinning and hydrogen does not induce a change in deformation mechanisms. Instead, the enhanced plasticity and hydrogen-dislocation interactions increase the degree of plasticity, leading to smaller dislocation cell sizes with thicker walls before the onset of twinning. The crack interacts with a heavily twinned structure superimposed on dislocation cells, resulting in a curvature of one twin-variant in the direction of crack growth and the formation of a refined region at the fracture surface. These structures are localized to the crack surface and limited in rotation in the presence of hydrogen. The presence of a hydrogen content above the solution-limit led to a strengthening effect over all other samples in the round bar samples for both 304 and 316. In the SENT studies, 304 stainless steel charged to these levels showed the greatest degree of hydrogen embrittlement, while in 316L stainless steel, the samples containing these levels of hydrogen were found to be stronger in fatigue life than the 10 wppm hydrogen-charged sample. Both of the 316L hydrogen-charged samples were weaker than their uncharged counterpart. Differences in the outcomes for the two studies was attributed to the geometry and testing condition employed. When strengthening was observed, the role of hydrogen was resolved to be two-fold: hydrogen hardens the matrix in the lattice, suppressing the process zone at the crack; and hydrogen at the dislocations enhances the plasticity experienced in the region where the material is yielding ahead of crack advance in the strain lobe. Irrespective of material class, increasing stress-state on the crack within the same stage of fatigue life leads to an increase in depth of the refinement layer and level of plasticity. The refinement at the surface can be attributed to the dislocation emission created during crack advance and the deformation induced in the crack wake. The former is the primary contributor to the SENT specimen studies, while the latter is responsible for the differences observed in the round-bar studies. In 316L, when the twin structure was observed to extend to the surface, regions of light and dark contrast were observed between the twinning bundle variants which corresponded to the peaks and troughs of the striations. This observation is counter to the current understanding that striations form during the crack advance process, independently of the evolved microstructure. The resulting microstructures in all hydrogen-induced studies are discussed in terms of the impact of hydrogen-dislocation interactions and enhanced plasticity. The evolving microstructure and unique observation of striations conforming to the microstructure developed ahead of crack advance emphasize the need to revisit how fatigue models are approached. Although a challenge, models must be informed by plasticity based arguments incorporating the deformation mechanisms observed in experiments to accurately reflect fatigue crack growth.

Noncontact measurement of guided ultrasonic wave scattering for fatigue crack characterization

NASA Astrophysics Data System (ADS)

Fromme, P.

2013-04-01

Fatigue cracks can develop in aerospace structures at locations of stress concentration such as fasteners. For the safe operation of the aircraft fatigue cracks need to be detected before reaching a critical length. Guided ultrasonic waves offer an efficient method for the detection and characterization of fatigue cracks in large aerospace structures. Noncontact excitation of guided waves was achieved using electromagnetic acoustic transducers (EMAT). The transducers were developed for the specific excitation of the A0 Lamb mode. Based on the induced eddy currents in the plate a simple theoretical model was developed and reasonably good agreement with the measurements was achieved. However, the detection sensitivity for fatigue cracks depends on the location and orientation of the crack relative to the measurement locations. Crack-like defects have a directionality pattern of the scattered field depending on the angle of the incident wave relative to the defect orientation and on the ratio of the characteristic defect size to wavelength. The detailed angular dependency of the guided wave field scattered at crack-like defects in plate structures has been measured using a noncontact laser interferometer. Good agreement with 3D Finite Element simulation predictions was achieved for machined part-through and through-thickness notches. The amplitude of the scattered wave was quantified for a variation of angle of the incident wave relative to the defect orientation and the defect depth. These results provide the basis for the defect characterization in aerospace structures using guided wave sensors.

Weibull crack density coefficient for polydimensional stress states

NASA Technical Reports Server (NTRS)

Gross, Bernard; Gyekenyesi, John P.

1989-01-01

A structural ceramic analysis and reliability evaluation code has recently been developed encompassing volume and surface flaw induced fracture, modeled by the two-parameter Weibull probability density function. A segment of the software involves computing the Weibull polydimensional stress state crack density coefficient from uniaxial stress experimental fracture data. The relationship of the polydimensional stress coefficient to the uniaxial stress coefficient is derived for a shear-insensitive material with a random surface flaw population.

Stress Intensity Solutions of Thermally Induced Cracks in Combustor Liner Hot Spots Using Finite Element Analysis (FEA)

DTIC Science & Technology

2005-12-01

EPFM elastic-plastic fracture mechanics FCG fatigue crack growth FEA finite element analysis FKN ANSYS FEA command for contact pair stiffness FTOLN...current TMF research is too general for thermal gradient applications . Moreover, the nature of a cyclically heated, localized region of higher...when separating this problem into the general engineering issues that are germane to the application , one can find much published research that is

Effect of hydrogen on fatigue crack propagation in vanadium

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chung, D.W.; Stoloff, N.S.

The influence of hydrogen on fatigue crack propagation in unalloyed vanadium and several hydrogen-charged vanadium alloys has been investigated. The Paris--Erdogan equation, da/dN = C(..delta..K)/sup m/, was approximately obeyed for all alloys. Crack growth rates were lowest in vanadium and dilute vanadium-hydrogen alloys, and were not very sensitive to volume fraction of hydrides in more concentrated alloys. The crack growth exponent, m, is inversely proportional to the cyclic strain hardening rate, n', and the rate constant C is inversely proportional to the square of the ultimate tensile stress, sigma/sub UTS/: metallographic examination showed hydride reorientation and growth in the originallymore » hydrided alloys. No stress-induced hydrides were observed in V-H solid-solution alloys. Fractures in hydrided materials exhibited cleavage-like features, while striations were noted in unalloyed vanadium and dilute solid-solution alloys.« less

Effect of hydrogen on fatigue crack propagation in vanadium

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chung, D.W.; Stoloff, N.S.

The influence of hydrogen on fatigue crack propagation in unalloyed vanadium and several hydrogen-charged vanadium alloys has been investigated. The Paris--Erdogan equation, da/dN = C(..delta..K)/sup m/, was approximately obeyed for all alloys. Crack growth rates were lowest in vanadium and dilute vanadium--hydrogen alloys, and were not very sensitive to volume fraction of hydrides in more concentrated alloys. The crack growth exponent, m, is inversely proportional to the cyclic strain hardening rate, n', and the rate constant C is inversely proportional to the square of the ultimate tensile stress, sigma/sub UTS/: metallographic examination showed hydride reorientation and growth in the originallymore » hydrided alloys. No stress-induced hydrides were observed in V--H solid-solution alloys. Fractures in hydrided materials exhibited cleavage-like features, while striations were noted in unalloyed vanadium and dilute solid-solution alloys.« less

High-Cycle, Push-Pull Fatigue Fracture Behavior of High-C, Si-Al-Rich Nanostructured Bainite Steel.

PubMed

Zhao, Jing; Ji, Honghong; Wang, Tiansheng

2017-12-29

The high-cycle, push-pull fatigue fracture behavior of high-C, Si-Al-rich nanostructured bainitic steel was studied through the measurement of fatigue limits, a morphology examination and phase composition analysis of the fatigue fracture surface, as well as fractography of the fatigue crack propagation. The results demonstrated that the push-pull fatigue limits at 10⁷ cycles were estimated as 710-889 MPa, for the samples isothermally transformed at the temperature range of 220-260 °C through data extrapolation, measured under the maximum cycle number of 10⁵. Both the interior inclusion and the sample surface constituted the fatigue crack origins. During the fatigue crack propagation, a high amount of secondary cracks were formed in almost parallel arrangements. The apparent plastic deformation occurred in the fracture surface layer, which induced approximately all retained austenite to transform into martensite.

High-Cycle, Push–Pull Fatigue Fracture Behavior of High-C, Si–Al-Rich Nanostructured Bainite Steel

PubMed Central

Zhao, Jing; Ji, Honghong

2017-01-01

The high-cycle, push–pull fatigue fracture behavior of high-C, Si–Al-rich nanostructured bainitic steel was studied through the measurement of fatigue limits, a morphology examination and phase composition analysis of the fatigue fracture surface, as well as fractography of the fatigue crack propagation. The results demonstrated that the push–pull fatigue limits at 107 cycles were estimated as 710–889 MPa, for the samples isothermally transformed at the temperature range of 220–260 °C through data extrapolation, measured under the maximum cycle number of 105. Both the interior inclusion and the sample surface constituted the fatigue crack origins. During the fatigue crack propagation, a high amount of secondary cracks were formed in almost parallel arrangements. The apparent plastic deformation occurred in the fracture surface layer, which induced approximately all retained austenite to transform into martensite. PMID:29286325

The Effect of Crack Orientation on the Nonlinear Interaction of a P-wave with an S-wave

DOE PAGES

TenCate, J. A.; Malcolm, A. E.; Feng, X.; ...

2016-06-06

Cracks, joints, fluids, and other pore-scale structures have long been hypothesized to be the cause of the large elastic nonlinearity observed in rocks. It is difficult to definitively say which pore-scale features are most important, however, because of the difficulty in isolating the source of the nonlinear interaction. In this work, we focus on the influence of cracks on the recorded nonlinear signal and in particular on how the orientation of microcracks changes the strength of the nonlinear interaction. We do this by studying the effect of orientation on the measurements in a rock with anisotropy correlated with the presencemore » and alignment of microcracks. We measure the nonlinear response via the traveltime delay induced in a low-amplitude P wave probe by a high-amplitude S wave pump. We find evidence that crack orientation has a significant effect on the nonlinear signal.« less

Fatigue crack growth behavior of pressure vessel steels and submerged arc weldments in a high-temperature pressurized water environment

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liaw, P.K.; Logsdon, W.A.; Begley, J.A.

1989-10-01

The fatigue crack growth rate (FCGR) properties of SA508 Cl 2a and SA533 Gr A Cl 2 pressure vessel steels and the corresponding automatic submerged arc weldments were developed in a high-temperature pressurized water (HPW) environment at 288{degrees} C (550{degrees} F) and 7.2 MPa (1044 psi) at load ratios of 0.20 and 0.50. The properties were generally conservative compared to American Society of Mechanical Engineers Section XI water environment reference curve. The growth rate of fatigue cracks in the base materials, however, was faster in the HPW environment than in a 288{degrees} C (550{degrees} F) base line air environment. Themore » growth rate of fatigue cracks in the two submerged arc weldments was also accelerated in the HPW environment but to a lesser degree than that demonstrated by the base materials. In the air environment, fatigue striations were observed, independent of material and load ratio, while in the HPW environment, some intergranular facets were present. The greater environmental effect on crack growth rates displayed by the base materials compared the weldments attributed to a different sulfide composition and morphology.« less

Mechanical Properties of Shock-Damaged Rocks

NASA Technical Reports Server (NTRS)

He, Hongliang; Ahrens, T. J.

1994-01-01

Stress-strain tests were performed both on shock-damaged gabbro and limestone. The effective Young's modulus decreases with increasing initial damage parameter value, and an apparent work-softening process occurs prior to failure. To further characterize shock-induced microcracks, the longitudinal elastic wave velocity behavior of shock-damaged gabbro in the direction of compression up to failure was measured using an acoustic transmission technique under uniaxial loading. A dramatic increase in velocity was observed for the static compressive stress range of 0-50 MPa. Above that stress range, the velocity behavior of lightly damaged (D(sub 0) less than 0.1) gabbro is almost equal to unshocked gabbro. The failure strength of heavily-damaged (D(sub 0) greater than 0.1) gabbro is approx. 100-150 MPa, much lower than that of lightly damaged and unshocked gabbros (approx. 230-260 MPa). Following Nur's theory, the crack shape distribution was analyzed. The shock-induced cracks in gabbro appear to be largely thin penny-shaped cracks with c/a values below 5 x 10(exp -4). Moreover, the applicability of Ashby and Sammis's theory relating failure strength and damage parameter of shock-damaged rocks was examined and was found to yield a good estimate of the relation of shock-induced deficit in elastic modulus with the deficit in compressive strength.

Locating and characterizing a crack in concrete with diffuse ultrasound: A four-point bending test.

PubMed

Larose, Eric; Obermann, Anne; Digulescu, Angela; Planès, Thomas; Chaix, Jean-Francois; Mazerolle, Frédéric; Moreau, Gautier

2015-07-01

This paper describes an original imaging technique, named Locadiff, that benefits from the diffuse effect of ultrasound waves in concrete to detect and locate mechanical changes associated with the opening of pre-existing cracks, and/or to the development of diffuse damage at the tip of the crack. After giving a brief overview of the theoretical model to describe the decorrelation of diffuse waveforms induced by a local change, the article introduces the inversion procedure that produces the three dimensional maps of density of changes. These maps are interpreted in terms of mechanical changes, fracture opening, and damage development. In addition, each fracture is characterized by its effective scattering cross section.

Cleavage crystallography of liquid metal embrittled aluminum alloys

NASA Technical Reports Server (NTRS)

Reynolds, A. P.; Stoner, G. E.

1991-01-01

The crystallography of liquid metal-induced transgranular cleavage in six aluminum alloys having a variety of microstructures has been determined via Laue X-ray back reflection. The cleavage crystallography was independent of alloy microstructure, and the cleavage plane was 100-plane oriented in all cases. It was further determined that the cleavage crystallography was not influenced by alloy texture. Examination of the fracture surface indicated that there was not a unique direction of crack propagation. In addition, the existence of 100-plane cleavage on alloy 2024 fracture surfaces was inferred by comparison of secondary cleavage crack intersection geometry on the 2024 surfaces with the geometry of secondary cleavage crack intersections on the test alloys.

Evaluating the irradiation effects on the elastic properties of miniature monolithic SiC tubular specimens

NASA Astrophysics Data System (ADS)

Singh, G.; Koyanagi, T.; Petrie, C.; Terrani, K.; Katoh, Y.

2018-02-01

The initial results of a post-irradiation examination study conducted on CVD SiC tubular specimens irradiated under a high radial heat flux are presented herein. The elastic moduli were found to decrease more than that estimated based on previous studies. The significant decreases in modulus are attributed to the cracks present in the specimens. The stresses in the specimens, calculated through finite element analyses, were found to be greater than the expected strength of irradiated specimens, indicating that the irradiation-induced stresses caused these cracks. The optical microscopy images and predicted stress distributions indicate that the cracks initiated at the inner surface and propagated outward.

FAA/NASA International Symposium on Advanced Structural Integrity Methods for Airframe Durability and Damage Tolerance

NASA Technical Reports Server (NTRS)

Harris, Charles E. (Editor)

1994-01-01

International technical experts in durability and damage tolerance of metallic airframe structures were assembled to present and discuss recent research findings and the development of advanced design and analysis methods, structural concepts, and advanced materials. The symposium focused on the dissemination of new knowledge and the peer-review of progress on the development of advanced methodologies. Papers were presented on: structural concepts for enhanced durability, damage tolerance, and maintainability; new metallic alloys and processing technology; fatigue crack initiation and small crack effects; fatigue crack growth models; fracture mechanics failure, criteria for ductile materials; structural mechanics methodology for residual strength and life prediction; development of flight load spectra for design and testing; and advanced approaches to resist corrosion and environmentally assisted fatigue.

Aqueous chloride stress corrosion cracking of titanium - A comparison with environmental hydrogen embrittlement

NASA Technical Reports Server (NTRS)

Nelson, H. G.

1974-01-01

The physical characteristics of stress corrosion cracking of titanium in an aqueous chloride environment are compared with those of embrittlement of titanium by a gaseous hydrogen environment in an effort to help contribute to the understanding of the possible role of hydrogen in the complex stress corrosion cracking process. Based on previous studies, the two forms of embrittlement are shown to be similar at low hydrogen pressures (100 N/sq m) but dissimilar at higher hydrogen pressures. In an effort to quantify this comparison, tests were conducted in an aqueous chloride solution using the same material and test techniques as had previously been employed in a gaseous hydrogen environment. The results of these tests strongly support models based on hydrogen as the embrittling species in an aqueous chloride environment.

Natural and human-induced sinkholes in gypsum terrain and associated environmental problems in NE Spain

NASA Astrophysics Data System (ADS)

Benito, G.; Del Campo, P. Pérez; Gutiérrez-Elorza, M.; Sancho, C.

1995-04-01

The central Ebro Basin comprises thick evaporite materials whose high solubility produces typically karstic landforms. The sinkhole morphology developed in the overlying alluvium has been studied using gravimetry and ground-penetrating radar (GPR) on stream terraces, as well as analyzing the evolution of sinkhole morphologies observed in aerial photographs taken in 1928, 1957, and 1985. The sinkhole morphologies give some idea of possible subsurface processes as well as an indication of the final mechanisms involve in sinkhole development. On stream terraces and cover pediments the most commonly encountered dolines are bowl-shaped in their morphology with both diffuse and scarped edges. In contrast, dolines developed in the gypsiferous silt infilled valleys have a funnel and well-shaped morphology. The diffuse-edged bowl-shaped dolines are developed through the progressive subsidence of the alluvial cover, due to washing down of alluvial particles through small voids and cracks into deeper subsurface caves, resulting in a decrease alluvial density. Future compaction of the alluvial cover will produce surface subsidences. This type of dolines are associated with negative gravity anomalies. In contrast, the scarped-edge dolines are formed by the sudden collapse of a cavity roof. The cavities and cracks formed in the gypsum karst may migrate to the surface through the alluvial deposits by piping, and they may subsequently collapse. In this instance, the cavities can be detected by both gravity and GPR anomalies where the voids are not deeper than 4 5 m from the surface. These processes forming sinkholes can be enhanced by man-induced changes in the groundwater hydrologic regime by both inflows, due to irrigation, ditch losses, or pipe leakages, and by outflows from pumping activities.

Seismological evidence for monsoon induced micro to moderate earthquake sequence beneath the 2011 Talala, Saurashtra earthquake, Gujarat, India

NASA Astrophysics Data System (ADS)

Singh, A. P.; Mishra, O. P.

2015-10-01

In order to understand the processes involved in the genesis of monsoon induced micro to moderate earthquakes after heavy rainfall during the Indian summer monsoon period beneath the 2011 Talala, Saurashtra earthquake (Mw 5.1) source zone, we assimilated 3-D microstructures of the sub-surface rock materials using a data set recorded by the Seismic Network of Gujarat (SeisNetG), India. Crack attributes in terms of crack density (ε), the saturation rate (ξ) and porosity parameter (ψ) were determined from the estimated 3-D sub-surface velocities (Vp, Vs) and Poisson's ratio (σ) structures of the area at varying depths. We distinctly imaged high-ε, high-ξ and low-ψ anomalies at shallow depths, extending up to 9-15 km. We infer that the existence of sub-surface fractured rock matrix connected to the surface from the source zone may have contributed to the changes in differential strain deep down to the crust due to the infiltration of rainwater, which in turn induced micro to moderate earthquake sequence beneath Talala source zone. Infiltration of rainwater during the Indian summer monsoon might have hastened the failure of the rock by perturbing the crustal volume strain of the causative source rock matrix associated with the changes in the seismic moment release beneath the surface. Analyses of crack attributes suggest that the fractured volume of the rock matrix with high porosity and lowered seismic strength beneath the source zone might have considerable influence on the style of fault displacements due to seismo-hydraulic fluid flows. Localized zone of micro-cracks diagnosed within the causative rock matrix connected to the water table and their association with shallow crustal faults might have acted as a conduit for infiltrating the precipitation down to the shallow crustal layers following the fault suction mechanism of pore pressure diffusion, triggering the monsoon induced earthquake sequence beneath the source zone.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Huang, J. Y.; E, J. C.; Huang, J. W.

Impact fracture of single-crystal Si is critical to long-term reliability of electronic devices and solar cells for its wide use as components or substrates in semiconductor industry. Single-crystal Si is loaded along two different crystallographic directions with a split Hopkinson pressure bar integrated with an in situ x-ray imaging and diffraction system. Bulk stress histories are measured, simultaneously with x-ray phase contrast imaging (XPCI) and Laue diffraction. Damage evolution is quantified with grayscale maps from XPCI. Single-crystal Si exhibits pronounced anisotropy in fracture modes, and thus fracture strengths and damage evolution. For loading along [11¯ 0] and viewing along [001],more » (1¯1¯0)[11¯ 0] cleavage is activated and induces horizontal primary cracks followed by perpendicular wing cracks. However, for loading along [011¯] and viewing along [111], random nucleation and growth of shear and tensile-splitting crack networks lead to catastrophic failure of materials with no cleavage. The primary-wing crack mode leads to a lower characteristic fracture strength due to predamage, but a more concentrated strength distribution, i.e., a higher Weibull modulus, compared to the second loading case. Furthermore, the sequential primary cracking, wing cracking and wing-crack coalescence processes result in a gradual increase of damage with time, deviating from theoretical predictions. Particle size and aspect ratios of fragments are discussed with postmortem fragment analysis, which verifies fracture modes observed in XPCI.« less

Statistical study of ductility-dip cracking induced plastic deformation in polycrystalline laser 3D printed Ni-based superalloy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Qian, Dan; Xue, Jiawei; Zhang, Anfeng

Ductility-dip cracking in Ni-based superalloy, resulting from heat treatment, is known to cause disastrous failure, but its mechanism is still not completely clear. A statistical study of the cracking behavior as a function of crystal orientation in a laser 3D-printed DL125L Ni-based superalloy polycrystal is investigated here using the synchrotron X-ray microdiffraction. The dislocation slip system in each of the forty crystal grains adjacent to the 300 μm long crack has been analyzed through Laue diffraction peak shapes. In all these grains, edge-type geometrically necessary dislocations (GNDs) dominate, and their dislocation line directions are almost parallel to the crack plane.more » Based on Schmid's law, the equivalent uniaxial tensile force direction is revealed normal to the trace of the crack. A qualitative mechanism is thus proposed. Thermal tensile stress perpendicular to the laser scanning direction is elevated due to a significant temperature gradient, and thus locations in the materials where the thermal stress exceeds the yield stress undergo plastic deformation mediated by GND activations. As the dislocations slip inside the crystal grains and pile up at the grain boundaries, local strain/stress keeps increasing, until the materials in these regions fail to sustain further deformation, leading to voids formation and cracks propagation.« less

Statistical study of ductility-dip cracking induced plastic deformation in polycrystalline laser 3D printed Ni-based superalloy

DOE PAGES

Qian, Dan; Xue, Jiawei; Zhang, Anfeng; ...

2017-06-06

Ductility-dip cracking in Ni-based superalloy, resulting from heat treatment, is known to cause disastrous failure, but its mechanism is still not completely clear. A statistical study of the cracking behavior as a function of crystal orientation in a laser 3D-printed DL125L Ni-based superalloy polycrystal is investigated here using the synchrotron X-ray microdiffraction. The dislocation slip system in each of the forty crystal grains adjacent to the 300 μm long crack has been analyzed through Laue diffraction peak shapes. In all these grains, edge-type geometrically necessary dislocations (GNDs) dominate, and their dislocation line directions are almost parallel to the crack plane.more » Based on Schmid's law, the equivalent uniaxial tensile force direction is revealed normal to the trace of the crack. A qualitative mechanism is thus proposed. Thermal tensile stress perpendicular to the laser scanning direction is elevated due to a significant temperature gradient, and thus locations in the materials where the thermal stress exceeds the yield stress undergo plastic deformation mediated by GND activations. As the dislocations slip inside the crystal grains and pile up at the grain boundaries, local strain/stress keeps increasing, until the materials in these regions fail to sustain further deformation, leading to voids formation and cracks propagation.« less

Estimate feedstock processability

DOE Office of Scientific and Technical Information (OSTI.GOV)

Amorelli, A.; Amos, Y.D.; Halsig, C.P.

1992-06-01

Currently, one of the major environmental pressures is to further reduce sulfur levels in middle distillate products. This paper reports that the key to this is understanding reactivities of individual sulfur components in the feedstocks to be treated. The major sulfur species in middle distillates is aromatic compounds, predominantly benzothiophenes and dibenzothiophenes. However, in straight run materials, significant quantities of aliphatic sulfur compounds and further higher boiling benzothiophenes are also expected. Simultaneous simulated distillation with a gas chromatograph microwave-induced plasma atomic emission detector (SIMDIS/AED) is used for middle distillate characterization of sulfur distribution as a function of boiling point. Itmore » is able to discriminate between middle distillate feed types such as cracked and straight run gas oils, and has shown that similar feeds, with different total sulfur contents (unevenly distributed throughout a feedstock), have the same normalized sulfur distribution.« less

The global mechanical properties and multi-scale failure mechanics of heterogeneous human stratum corneum.

PubMed

Liu, X; Cleary, J; German, G K

2016-10-01

The outermost layer of skin, or stratum corneum, regulates water loss and protects underlying living tissue from environmental pathogens and insults. With cracking, chapping or the formation of exudative lesions, this functionality is lost. While stratum corneum exhibits well defined global mechanical properties, macroscopic mechanical testing techniques used to measure them ignore the structural heterogeneity of the tissue and cannot provide any mechanistic insight into tissue fracture. As such, a mechanistic understanding of failure in this soft tissue is lacking. This insight is critical to predicting fracture risk associated with age or disease. In this study, we first quantify previously unreported global mechanical properties of isolated stratum corneum including the Poisson's ratio and mechanical toughness. African American breast stratum corneum is used for all assessments. We show these parameters are highly dependent on the ambient humidity to which samples are equilibrated. A multi-scale investigation assessing the influence of structural heterogeneities on the microscale nucleation and propagation of cracks is then performed. At the mesoscale, spatially resolved equivalent strain fields within uniaxially stretched stratum corneum samples exhibit a striking heterogeneity, with localized peaks correlating closely with crack nucleation sites. Subsequent crack propagation pathways follow inherent topographical features in the tissue and lengthen with increased tissue hydration. At the microscale, intact corneocytes and polygonal shaped voids at crack interfaces highlight that cracks propagate in superficial cell layers primarily along intercellular junctions. Cellular fracture does occur however, but is uncommon. Human stratum corneum protects the body against harmful environmental pathogens and insults. Upon mechanical failure, this barrier function is lost. Previous studies characterizing the mechanics of stratum corneum have used macroscopic testing equipment designed for homogenous materials. Such measurements ignore the tissue's rich topography and heterogeneous structure, and cannot describe the underlying mechanistic process of tissue failure. For the first time, we establish a mechanistic insight into the failure mechanics of soft heterogeneous tissues by investigating how cracks nucleate and propagate in stratum corneum. We further quantify previously unreported values of the tissue's Poisson's ratio and toughness, and their dramatic variation with ambient humidity. To date, skin models examining drug delivery, wound healing, and ageing continue to estimate these parameters. Copyright © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Nuclear changes in oral mucosa of alcoholics and crack cocaine users.

PubMed

Webber, L P; Pellicioli, A C A; Magnusson, A S; Danilevicz, C K; Bueno, C C; Sant'Ana Filho, M; Rados, P V; Carrard, V C

2016-02-01

The effects of drugs of abuse on oral mucosa are only partly understood. The aims of the present study were to: (1) evaluate the frequency of nuclear changes in normal-appearing oral mucosa of alcoholics and crack cocaine users and (2) assess their association with cell proliferation rate. Oral smears were obtained from the border of the tongue and floor of the mouth of 26 crack cocaine users (24 males and 2 females), 29 alcoholics (17 males and 12 females), and 35 controls (17 males and 18 females). Histological slides were submitted to Feulgen staining to assess the frequency of micronuclei (MN), binucleated cells (BN), broken eggs (BE), and karyorrhexis (KR). A significant increase in the frequency of MN was observed in cells exfoliated from the tongue of crack cocaine users (p = 0.01), and alcoholics showed a higher frequency of KR in cells obtained from the floor of the mouth (p = 0.01). Our findings suggest that the use of crack cocaine induces clastogenic effects, whereas alcoholism is associated with higher degrees of keratinization in the floor of the mouth. © The Author(s) 2015.

Cracking Problems and Mechanical Characteristics of PME and BME Ceramic Capacitors

NASA Technical Reports Server (NTRS)

Teverovsky, Alexander

2018-01-01

Most failures in MLCCs are caused by cracking that create shorts between opposite electrodes of the parts. A use of manual soldering makes this problem especially serious for space industry. Experience shows that different lots of ceramic capacitors might have different susceptibility to cracking under manual soldering conditions. This simulates a search of techniques that would allow revealing capacitors that are most robust to soldering-induced stresses. Currently, base metal electrode (BME) capacitors are introduced to high-reliability applications as a replacement of precious metal electrode (PME) parts. Understanding the difference in the susceptibility to cracking between PME and BME capacitors would facilitate this process. This presentation gives a review of mechanical characteristics measured in-situ on MLCCs that includes flexural strength, Vickers hardness, indentation fracture toughness, and the board flex testing and compare characteristics of BME and PME capacitors. A history case related to cracking in PME capacitors that caused flight system malfunctions and mechanisms of failure are considered. Possible qualification tests that would allow evaluation of the resistance of MLCCs to manual soldering are suggested and perspectives related to introduction of BME capacitors discussed.

The Molecular Volcano Revisited: Determination of Crack Propagation and Distribution During the Crystallization of Nanoscale Amorphous Solid Water Films.

DOE Office of Scientific and Technical Information (OSTI.GOV)

May, Robert A.; Smith, R. Scott; Kay, Bruce D.

2012-02-02

Temperature programmed desorption (TPD) is utilized to determine the length distribution of cracks formed through amorphous solid water (ASW) during crystallization. This distribution is determined by monitoring how the thickness of an ASW overlayer alters desorption of an underlayer of O2. As deposited the ASW overlayer prevents desorption of O2. During crystallization, cracks form through the ASW overlayer and open a path to vacuum which allows O2 to escape in a rapid episodic release known as the 'molecular volcano'. Sufficiently thick ASW overlayers further trap O2 resulting in a second O2 desorption peak commensurate with desorption of the last ofmore » the ASW overlayer. The evolution of this trapping peak with overlayer thickness is the basis for determining the distribution of crystallization induced cracks through the ASW. Reflection adsorption infrared spectroscopy (RAIRS) and TPD of multicomponent parfait structures of ASW, O2 and Kr indicate that a preponderance of these cracks propagate down from the outer surface of the ASW.« less

Understanding the crack formation of graphite particles in cycled commercial lithium-ion batteries by focused ion beam - scanning electron microscopy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lin, Na; Jia, Zhe; Wang, Zhihui

Here in this paper, the structure degradation of commercial Lithium-ion battery (LIB) graphite anodes with different cycling numbers and charge rates was investigated by focused ion beam (FIB) and scanning electron microscopy (SEM). The cross-section image of graphite anode by FIB milling shows that cracks, resulted in the volume expansion of graphite electrode during long-term cycling, were formed in parallel with the current collector. The crack occurs in the bulk of graphite particles near the lithium insertion surface, which might derive from the stress induced during lithiation and de-lithiation cycles. Subsequently, crack takes place along grain boundaries of the polycrystallinemore » graphite, but only in the direction parallel with the current collector. Furthermore, fast charge graphite electrodes are more prone to form cracks since the tensile strength of graphite is more likely to be surpassed at higher charge rates. Therefore, for LIBs long-term or high charge rate applications, the tensile strength of graphite anode should be taken into account.« less

Understanding the crack formation of graphite particles in cycled commercial lithium-ion batteries by focused ion beam - scanning electron microscopy

DOE PAGES

Lin, Na; Jia, Zhe; Wang, Zhihui; ...

2017-10-01

Here in this paper, the structure degradation of commercial Lithium-ion battery (LIB) graphite anodes with different cycling numbers and charge rates was investigated by focused ion beam (FIB) and scanning electron microscopy (SEM). The cross-section image of graphite anode by FIB milling shows that cracks, resulted in the volume expansion of graphite electrode during long-term cycling, were formed in parallel with the current collector. The crack occurs in the bulk of graphite particles near the lithium insertion surface, which might derive from the stress induced during lithiation and de-lithiation cycles. Subsequently, crack takes place along grain boundaries of the polycrystallinemore » graphite, but only in the direction parallel with the current collector. Furthermore, fast charge graphite electrodes are more prone to form cracks since the tensile strength of graphite is more likely to be surpassed at higher charge rates. Therefore, for LIBs long-term or high charge rate applications, the tensile strength of graphite anode should be taken into account.« less

Direct observation of the residual plastic deformation caused by a single tensile overload

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bichler, C.; Pippan, R.

1999-07-01

The fatigue crack growth behavior following single tensile overloads at high stress intensity ranges in a cold-rolled austenitic steel has been studied experimentally. After tensile overloads, fatigue cracks initially accelerate, followed by significant retardation, before the growth rates return to their baseline level. The initial acceleration was attributed to an immediate reduction in near-tip closure. Scanning electron micrography and stereophotogrammetric reconstruction of the fracture surface were applied to study the residual plastic deformation caused by a single tensile overload in the mid-thickness of the specimen. The measured residual opening displacement of the crack as a function of the overload ismore » presented and compared with simple estimations. Also, free specimen surface observations of the residual plastic deformation and crack growth rate were performed. In the midsection of the specimens the striation spacing-length, i.e., the microscopic growth rates, were measured before and after the applied overload. It will be shown that the measured plasticity-induced wedges from the single overload and the observed propagation behavior support the significance of the concept of crack closure.« less

Suppression of imaging crack caused by the gap between micromirrors in maskless lithography

NASA Astrophysics Data System (ADS)

Liang, Liwen; Zhou, Jinyun; Lei, Liang; Wang, Bo; Wang, Qu; Wen, Kunhua

2017-10-01

The digital micromirror device (DMD) is the key device in maskless lithography. However, because of the machinery manufacturing limit of DMDs, the gap between the micromirrors may destroy the continuity of the graphic. This work presents a simple way to fill the imaging crack by controlling the partial coherence factor σ of the light source. A crack can be regarded as the image of a dark space. By considering the resolving power for such cracks under partially coherent illumination, the images of such dark spaces can be covered, preventing them from being imaged on the substrate. By using mathematical derivations of the light intensity distribution exposed to the substrate, and by utilizing the diffraction effect induced by the finite aperture of the optical projection system, an appropriate σ value can be determined for eliminating the image of the crack in an actual scene. The numerical simulation results demonstrate that this method can ensure the continuity of the graphic at the critical partial coherence factor σc regardless of the shape of the target graphic.

Fracture Toughness of Advanced Ceramics at Room Temperature

PubMed Central

Quinn, George D.; Salem, Jonathan; Bar-on, Isa; Cho, Kyu; Foley, Michael; Fang, Ho

1992-01-01

This report presents the results obtained by the five U.S. participating laboratories in the Versailles Advanced Materials and Standards (VAMAS) round-robin for fracture toughness of advanced ceramics. Three test methods were used: indentation fracture, indentation strength, and single-edge pre-cracked beam. Two materials were tested: a gas-pressure sintered silicon nitride and a zirconia toughened alumina. Consistent results were obtained with the latter two test methods. Interpretation of fracture toughness in the zirconia alumina composite was complicated by R-curve and environmentally-assisted crack growth phenomena. PMID:28053447

Chemical Degradation and Stress Cracking of Polycarbonate in DS2.

DTIC Science & Technology

1987-09-01

materials are used. For instance, polycarbonate, used widely for air- craft windows, helicopter canopies and transparent armor because of its good impact ...predicting environmental stress cracking of the polymer from solubility con- siderations. The concept has been extended to include a hydrogen bonding...IML Authors . 04 * ! 9 00 9 A S . - . . . q w * . - .* *,.*A CC AX -4-’-~~~~ U--’- ; IO i- A - -C4 = tO -~’ 1 .’ . M0 C , W V E WE1 C ~ 0.0.’un WEC

Fused Silica and Other Transparent Window Materials

NASA Technical Reports Server (NTRS)

Salem, Jon

2016-01-01

Several transparent ceramics, such as spinel and AlONs are now being produced in sufficient large areas to be used in space craft window applications. The work horse transparent material for space missions from Apollo to the International Space Station has been fused silica due in part to its low coefficient of expansion and optical quality. Despite its successful use, fused silica exhibits anomalies in its crack growth behavior, depending on environmental preconditioning and surface damage. This presentation will compare recent optical ceramics to fused silica and discuss sources of variation in slow crack growth behavior.

Durability and Design Issues of Thermal/environmental Barrier Coatings on Sic/sic Ceramic Matrix Composites Under 1650 C Test Conditions

NASA Technical Reports Server (NTRS)

Zhu, Dong-Ming; Choi, Sung R.; Ghosn, Louis J.; Miller, Robert A.

2004-01-01

Ceramic thermal/environmental barrier coatings for SiC-based ceramics will play an increasingly important role in future gas turbine engines because of their ability to effectively protect the engine components and further raise engine temperatures. However, the coating durability remains a major concern with the ever-increasing temperature requirements. Currently, advanced T/EBC systems, which typically include a high temperature capable zirconia- (or hahia-) based oxide top coat (thermal barrier) on a less temperature capable mullite/barium-strontium-aluminosilicate (BSAS)/Si inner coat (environmental barrier), are being developed and tested for higher temperature capability Sic combustor applications. In this paper, durability of several thermal/environmental barrier coating systems on SiC/SiC ceramic matrix composites was investigated under laser simulated engine thermal gradient cyclic, and 1650 C (3000 F) test conditions. The coating cracking and delamination processes were monitored and evaluated. The effects of temperature gradients and coating configurations on the ceramic coating crack initiation and propagation were analyzed using finite element analysis (FEA) models based on the observed failure mechanisms, in conjunction with mechanical testing results. The environmental effects on the coating durability will be discussed. The coating design approach will also be presented.

Detection of cracks on concrete surfaces by hyperspectral image processing

NASA Astrophysics Data System (ADS)

Santos, Bruno O.; Valença, Jonatas; Júlio, Eduardo

2017-06-01

All large infrastructures worldwide must have a suitable monitoring and maintenance plan, aiming to evaluate their behaviour and predict timely interventions. In the particular case of concrete infrastructures, the detection and characterization of crack patterns is a major indicator of their structural response. In this scope, methods based on image processing have been applied and presented. Usually, methods focus on image binarization followed by applications of mathematical morphology to identify cracks on concrete surface. In most cases, publications are focused on restricted areas of concrete surfaces and in a single crack. On-site, the methods and algorithms have to deal with several factors that interfere with the results, namely dirt and biological colonization. Thus, the automation of a procedure for on-site characterization of crack patterns is of great interest. This advance may result in an effective tool to support maintenance strategies and interventions planning. This paper presents a research based on the analysis and processing of hyper-spectral images for detection and classification of cracks on concrete structures. The objective of the study is to evaluate the applicability of several wavelengths of the electromagnetic spectrum for classification of cracks in concrete surfaces. An image survey considering highly discretized wavelengths between 425 nm and 950 nm was performed on concrete specimens, with bandwidths of 25 nm. The concrete specimens were produced with a crack pattern induced by applying a load with displacement control. The tests were conducted to simulate usual on-site drawbacks. In this context, the surface of the specimen was subjected to biological colonization (leaves and moss). To evaluate the results and enhance crack patterns a clustering method, namely k-means algorithm, is being applied. The research conducted allows to define the suitability of using clustering k-means algorithm combined with hyper-spectral images highly discretized for crack detection on concrete surfaces, considering cracking combined with the most usual concrete anomalies, namely biological colonization.

Modeling of slow crack propagation in heterogeneous rocks

NASA Astrophysics Data System (ADS)

Lengliné, Olivier; Stormo, Arne; Hansen, Alex; Schmittbuhl, Jean

2015-04-01

Crack propagation in heterogeneous media is a rich problem which involves the interplay of various physical processes. The problem has been intensively investigated theoretically, numerically, and experimentally, but a unifying model capturing all the experimental features has not been entirely achieved despite its broad range of implications in Earth sciences problems. The slow propagation of a crack front where long range elastic interactions are dominant, is of crucial importance to fill the gap between experiments and models. Several theoretical and numerical works have been devoted to quasi-static models. Such models give rise to an intermittent local activity characterized by a depinning transition and can be viewed as a critical phenomenon. However these models fail to reproduce all experimental conditions, notably the front morphology does not display any cross-over length with two different roughness exponents above and below the cross-over as observed experimentally. Here, we compare experimental observations of a slow interfacial crack propagation along an heterogeneous interface to numerical simulations from a cantilever fiber bundle model. The model consists of a planar set of brittle fibers between an elastic half-space and a rigid square root shaped plate which loads the system in a cantilever configuration. The latter is shown to provide an improved opening and stress field in the process zone around the crack tip. The model shares a similar scale invariant roughening of the crack front both at small and large scales and a similar power law distribution of the local velocity of the crack front to experiments. Implications for induced seismicity at the brittle-creep transition are discussed. We show that a creep route for induced seismicity is possible when heterogeneities exist along the fault. Indeed, seismic event occurrences in time and space are in strong relation with the development of the aseismic motion recorded during the experiment and the model. We also infer the statistical properties of the organization of the seismicity that shows strong space-time clustering. We conclude that aseismic processes might drive seismicity in the brittle-creep regime.

Factors affecting stress assisted corrosion cracking of carbon steel under industrial boiler conditions

NASA Astrophysics Data System (ADS)

Yang, Dong

Failure of carbon steel boiler tubes from waterside has been reported in the utility boilers and industrial boilers for a long time. In industrial boilers, most waterside tube cracks are found near heavy attachment welds on the outer surface and are typically blunt, with multiple bulbous features indicating a discontinuous growth. These types of tube failures are typically referred to as stress assisted corrosion (SAC). For recovery boilers in the pulp and paper industry, these failures are particularly important as any water leak inside the furnace can potentially lead to smelt-water explosion. Metal properties, environmental variables, and stress conditions are the major factors influencing SAC crack initation and propagation in carbon steel boiler tubes. Slow strain rate tests (SSRT) were conducted under boiler water conditions to study the effect of temperature, oxygen level, and stress conditions on crack initation and propagation on SA-210 carbon steel samples machined out of boiler tubes. Heat treatments were also performed to develop various grain size and carbon content on carbon steel samples, and SSRTs were conducted on these samples to examine the effect of microstructure features on SAC cracking. Mechanisms of SAC crack initation and propagation were proposed and validated based on interrupted slow strain tests (ISSRT). Water chemistry guidelines are provided to prevent SAC and fracture mechanics model is developed to predict SAC failure on industrial boiler tubes.

Catalytic and thermal cracking processes of waste cooking oil for bio-gasoline synthesis

NASA Astrophysics Data System (ADS)

Dewanto, Muhammad Andry Rizki; Januartrika, Aulia Azka; Dewajani, Heny; Budiman, Arief

2017-03-01

Non-renewable energy resources such as fossil fuels, and coal were depleted as the increase of global energy demand. Moreover, environmental aspect becomes a major concern which recommends people to utilize bio-based resources. Waste cooking oil is one of the economical sources for biofuel production and become the most used raw material for biodiesel production. However, the products formed during frying, can affect the trans-esterification reaction and the biodiesel properties. Therefore, it needs to convert low-quality cooking oil directly into biofuel by both thermal and catalytic cracking processes. Thermal and catalytic cracking sometimes are regarded as prospective bio-energy conversion processes. This research was carried out in the packed bed reactor equipped with 2 stages preheater with temperature of reactor was variated in the range of 450-550°C. At the same temperature, catalytic cracking had been involved in this experiment, using activated ZSM-5 catalyst with 1 cm in length. The organic liquid product was recovered by three stages of double pipe condensers. The composition of cracking products were analyzed using GC-MS instrument and the caloric contents were analyzed using Bomb calorimeter. The results reveal that ZSM-5 was highly selective toward aromatic and long aliphatic compounds formation. The percentage recovery of organic liquid product from the cracking process varies start from 8.31% and the optimal results was 54.08%. The highest heating value of liquid product was resulted from catalytic cracking process at temperature of 450°C with value of 10880.48 cal/gr and the highest product yield with 54.08% recovery was achieved from thermal cracking process with temperature of 450°C.

Non-double-couple mechanisms of microearthquakes induced by hydraulic fracturing

USGS Publications Warehouse

Sileny, J.; Hill, D.P.; Eisner, Leo; Cornet, F.H.

2009-01-01

We have inverted polarity and amplitude information of representative microearthquakes to investigate source mechanisms of seismicity induced by hydraulic fracturing in the Carthage Cotton Valley, east Texas, gas field. With vertical arrays of four and eight three-component geophones in two monitoring wells, respectively, we were able to reliably determine source mechanisms of the strongest events with the best signal-to-noise ratio. Our analysis indicates predominantly non-double-couple source mechanisms with positive volumetric component consistent with opening cracks oriented close to expected hydraulic fracture orientation. Our observations suggest the induced events are directly the result of opening cracks by fluid injection, in contrast to many previous studies where the seismicity is interpreted to be primarily shearing caused by pore pressure diffusion into the surrounding rock or associated with shear stresses created at the hydraulic fracture tip. Copyright 2009 by the American Geophysical Union.

Searching for a Crack to Let Environment Light In: Ecological Biopolitics and Education for Sustainable Development Discourses

ERIC Educational Resources Information Center

Gough, Annette

2017-01-01

This article traces the shifts in environmental education discourses from the 1972 UN Conference on the Human Environment, to the 2012 UN Rio+20 Conference on Sustainable Development, and beyond through a biopolitical lens. Each of the earlier shifts is reflected in environmental, sustainability and science education policies and curricula--but…

The effect of plasma electrolytic oxidation on the mean stress sensitivity of the fatigue life of the 6082 aluminum alloy

NASA Astrophysics Data System (ADS)

Winter, L.; Morgenstern, R.; Hockauf, K.; Lampke, T.

2016-03-01

In this work the mean stress influence on the high cycle fatigue behavior of the plasma electrolytic oxidized (PEO) 6082 aluminum alloy (AlSi1MgMn) is investigated. The present study is focused on the fatigue life time and the susceptibility of fatigue-induced cracking of the oxide coating and their dependence on the applied mean stress. Systematic work is done comparing conditions with and without PEO treatment, which have been tested using three different load ratios. For the uncoated substrate the cycles to failure show a significant dependence on the mean stress, which is typical for aluminum alloys. With increased load ratio and therefore increased mean stress, the fatigue strength decreases. The investigation confirms the well-known effect of PEO treatment on the fatigue life: The fatigue strength is significantly reduced by the PEO process, compared to the uncoated substrate. However, also the mean stress sensitivity of the fatigue performance is reduced. The fatigue limit is not influenced by an increasing mean stress for the PEO treated conditions. This effect is firstly shown in these findings and no explanation for this effect can be found in literature. Supposedly the internal compressive stresses and the micro-cracks in the oxide film have a direct influence on the crack initiation and growth from the oxide film through the interface and in the substrate. Contrary to these findings, the susceptibility of fatigue-induced cracking of the oxide coating is influenced by the load ratio. At tension-tension loading a large number of cracks, which grow partially just in the aluminum substrate, are present. With decreasing load ratio to alternating tension-compression stresses, the crack number and length increases and shattering of the oxide film is more pronounced due to the additional effective compressive part of the load cycle.

Environmental effects on natural frequencies of the San Pietro bell tower in Perugia, Italy, and their removal for structural performance assessment

NASA Astrophysics Data System (ADS)

Ubertini, Filippo; Comanducci, Gabriele; Cavalagli, Nicola; Laura Pisello, Anna; Luigi Materazzi, Annibale; Cotana, Franco

2017-01-01

Continuously identified natural frequencies of vibration can provide unique information for low-cost automated condition assessment of civil constructions and infrastructures. However, the effects of changes in environmental parameters, such as temperature and humidity, need to be effectively investigated and accurately removed from identified frequency data for an effective performance assessment. This task is particularly challenging in the case of historical constructions that are typically massive and heterogeneous masonry structures characterized by complex variations of materials' properties with varying environmental parameters and by a differential heat conduction process where thermal capacity plays a major role. While there is abundance of documented monitoring data highlighting correlations between environmental parameters and natural frequencies in the case of new structures, such as long-span bridges, similar studies for historical constructions are still missing, with only a few literature works occasionally reporting increments in natural frequencies with increasing temperature of construction materials due to the closure of internal micro-cracks in the mortar layers caused by thermal expansion. In order to gain some knowledge on the effects of changes in temperature and humidity on the natural frequencies of slender masonry buildings, the paper focuses on the case study of an Italian monumental bell tower that has been monitored by the authors for more than nine months. Correlations between natural frequencies and environmental parameters are investigated in detail and the predictive capabilities of linear statistical regressive models based on the use of several environmental continuous monitoring sensors are assessed. At the end, three basic mechanisms governing environmentally-induced changes in the dynamic behavior of the tower are identified and essential information is achieved on the optimal location and minimum number of environmental sensors that are necessary in a structural health monitoring perspective.