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Sample records for fatigue cracking research

  1. Mode II fatigue crack propagation.

    NASA Technical Reports Server (NTRS)

    Roberts, R.; Kibler, J. J.

    1971-01-01

    Fatigue crack propagation rates were obtained for 2024-T3 bare aluminum plates subjected to in-plane, mode I, extensional loads and transverse, mode II, bending loads. These results were compared to the results of Iida and Kobayashi for in-plane mode I-mode II extensional loads. The engineering significance of mode I-mode II fatigue crack growth is considered in view of the present results. A fatigue crack growth equation for handling mode I-mode II fatigue crack growth rates from existing mode I data is also discussed.

  2. 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.

  3. Intermittent crack growth in fatigue

    NASA Astrophysics Data System (ADS)

    Kokkoniemi, R.; Miksic, A.; Ovaska, M.; Laurson, L.; Alava, M. J.

    2017-07-01

    Fatigue occurs under cyclic loading at stresses below a material’s static strength limit. We consider fatigue crack growth as a stochastic process and perform crack growth experiments in a metal (copper). We follow optically cracks propagating from initial edge notches. The main interest is in the dynamics of the crack growth—the Paris’ law and the initiation phase prior to that—and especially the intermittency this is discovered to display. How the sampling of the crack advancement, performed at regular intervals, influences such measurement results is analysed by the analogy of planar crack dynamics in slow, driven growth.

  4. Fatigue crack layer propagation in silicon-iron

    NASA Technical Reports Server (NTRS)

    Birol, Y.; Welsch, G.; Chudnovsky, A.

    1986-01-01

    Fatigue crack propagation in metal is almost always accompanied by plastic deformation unless conditions strongly favor brittle fracture. The analysis of the plastic zone is crucial to the understanding of crack propagation behavior as it governs the crack growth kinetics. This research was undertaken to study the fatigue crack propagation in a silicon iron alloy. Kinetic and plasticity aspects of fatigue crack propagation in the alloy were obtained, including the characterization of damage evolution.

  5. Peridynamic model for fatigue cracking.

    SciTech Connect

    Silling, Stewart Andrew; Abe Askari

    2014-10-01

    The peridynamic theory is an extension of traditional solid mechanics in which the field equations can be applied on discontinuities, such as growing cracks. This paper proposes a bond damage model within peridynamics to treat the nucleation and growth of cracks due to cyclic loading. Bond damage occurs according to the evolution of a variable called the "remaining life" of each bond that changes over time according to the cyclic strain in the bond. It is shown that the model reproduces the main features of S-N data for typical materials and also reproduces the Paris law for fatigue crack growth. Extensions of the model account for the effects of loading spectrum, fatigue limit, and variable load ratio. A three-dimensional example illustrates the nucleation and growth of a helical fatigue crack in the torsion of an aluminum alloy rod.

  6. TV fatigue crack monitoring system

    NASA Technical Reports Server (NTRS)

    Exton, R. J. (Inventor)

    1977-01-01

    An apparatus is disclosed for monitoring the development and growth of fatigue cracks in a test specimen subjected to a pulsating tensile load. A plurality of television cameras photograph a test specimen which is illuminated at the point of maximum tensile stress. The television cameras have a modified vidicon tube which has an increased persistence time thereby eliminating flicker in the displayed images.

  7. Biaxial Fatigue Cracking from Notch

    DTIC Science & Technology

    2013-03-04

    UNCLASSIFIED UNCLASSIFIED NAVAL AIR WARFARE CENTER AIRCRAFT DIVISION PATUXENT RIVER, MARYLAND TECHNICAL REPORT REPORT NO... AIRCRAFT DIVISION PATUXENT RIVER, MARYLAND NAWCADPAX/TR-2013/32 4 March 2013 BIAXIAL FATIGUE CRACKING FROM NOTCH by Eun U. Lee...Materials Engineering Division Naval Air Warfare Center Aircraft Division NAWCADPAX/TR-2013/32 i REPORT DOCUMENTATION PAGE Form Approved OMB

  8. Shear fatigue crack growth - A literature survey

    NASA Technical Reports Server (NTRS)

    Liu, H. W.

    1985-01-01

    Recent studies of shear crack growth are reviewed, emphasizing test methods and data analyses. The combined mode I and mode II elastic crack tip stress fields are considered. The development and design of the compact shear specimen are described, and the results of fatigue crack growth tests using compact shear specimens are reviewed. The fatigue crack growth tests are discussed and the results of inclined cracks in tensile panels, center cracks in plates under biaxial loading, cracked beam specimens with combined bending and shear loading, center-cracked panels and double edge-cracked plates under cyclic shear loading are examined and analyzed in detail.

  9. Corrosion on Fatigue and Fatigue Crack Growth in Aircraft Structural Materials

    DTIC Science & Technology

    1979-06-01

    fatigue initiated by stress corrosion cracking and thus is not directly relevant o this work except that conceptually iL has similarities. The other paper...34 Corrosion Fatigue Initiated by Stress Corrosion Cracking ", personal coi’iuni cation. 12. II. Kitagaw,.:a, T. Fugita, K. Miyazawa, "Sniall Randomly...a result, extensive research on corrosion fatigue has led to the traditional model which superimposes the stress corrosion cracking phenomenon upon

  10. Detecting Gear Tooth Fatigue Cracks in Advance of Complete Fracture

    NASA Technical Reports Server (NTRS)

    Zakrajsek, James J.; Lewicki, David G.

    1996-01-01

    Results of using vibration-based methods to detect gear tooth fatigue cracks are presented. An experimental test rig was used to fail a number of spur gear specimens through bending fatigue. The gear tooth fatigue crack in each test was initiated through a small notch in the fillet area of a tooth on the gear. The primary purpose of these tests was to verify analytical predictions of fatigue crack propagation direction and rate as a function of gear rim thickness. The vibration signal from a total of three tests was monitored and recorded for gear fault detection research. The damage consisted of complete rim fracture on the two thin rim gears and single tooth fracture on the standard full rim test gear. Vibration-based fault detection methods were applied to the vibration signal both on-line and after the tests were completed. The objectives of this effort were to identify methods capable of detecting the fatigue crack and to determine how far in advance of total failure positive detection was given. Results show that the fault detection methods failed to respond to the fatigue crack prior to complete rim fracture in the thin rim gear tests. In the standard full rim gear test all of the methods responded to the fatigue crack in advance of tooth fracture; however, only three of the methods responded to the fatigue crack in the early stages of crack propagation.

  11. Thermal-mechanical fatigue crack growth in aircraft engine materials

    NASA Astrophysics Data System (ADS)

    Dai, Yi

    1993-08-01

    This thesis summarizes the major technical achievements obtained as a part of a collaborative research and development project between Ecole Polytechnique and Pratt & Whitney Canada. These achievements include: (1) a thermal-mechanical fatigue (TMF) testing rig which is capable of studying the fatigue behaviors of gas turbine materials under simultaneous changes of temperatures and strains or stress; (2) an advanced alternative current potential drop (ACPD) measurement system which is capable of performing on-line monitoring of fatigue crack initiation and growth in specimen testing under isothermal and TMF conditions; (3) fatigue crack initiation and short crack growth data for the titanium specimens designed with notch features associated with bolt holes of compressor discs; (4) thermal-mechanical fatigue crack growth data for two titanium alloys being used in PWC engine components, which explained the material fatigue behavior encountered in full-scale component testing; (5) a complete fractographic analysis for the tested specimens which enhanced the understanding of the fatigue crack growth mechanisms and helped to establish an analytical crack growth model; and (6) application of the ACPD fatigue crack monitoring technique to single tooth firtree specimen (STFT) LCF testing of PWA 1480 single crystal alloy. Finally, a comprehensive discussion concerning the results pertaining to this research project is presented.

  12. Visual simulation of fatigue crack growth

    SciTech Connect

    Wang, S.; Margolin, H.; Lin, F.B.

    1998-07-01

    An attempt has been made to visually simulate fatigue crack propagation from a precrack. An integrated program was developed for this purpose. The crack-tip shape was determined at four load positions in the first load cycle. The final shape was a blunt front with an ear profile at the precrack tip. A more general model, schematically illustrating the mechanism of fatigue crack growth and striation formation in a ductile material, was proposed based on this simulation. According to the present model, fatigue crack growth is an intermittent process; cyclic plastic shear strain is the driving force applied to both state 1 and 2 crack growth. No fracture mode transition occurs between the two stages in the present study. The crack growth direction alternates, moving up and down successively, producing fatigue striations. A brief examination has been made of the crack growth path in a ductile two-phase material.

  13. Fatigue Growth and Closure of Short Cracks

    DTIC Science & Technology

    1989-06-03

    TESTS 87 4.5 SHORT CRACK FATIGUE TESTS IN NOTCHED SPECIMENS 101 5. DISCUSSION 5.1 DURABILITY ANALYSIS - EQUIVALENT INITIAL FLAW SIZE 232 5.2 SHORT... equivalent initial flaw size approach, (2) effects of plasticity, (3) crack closure response of long cracks and (4) crack closure response of short...cracks. 5.1 EQUIVALENT INITIAL FLAW SIZE - DURABILITY ANALYSIS Aerospace structures were Initially designed on a safe-life approach. The underlying

  14. Jumplike fatigue crack growth in compressor blades

    NASA Astrophysics Data System (ADS)

    Limar', L. V.; Demina, Yu. A.; Botvina, L. R.

    2014-04-01

    It is shown that power relations between the two main fractographic characteristics of fracture surfaces forming during jumplike fatigue crack growth, namely, the crack depth and the corresponding crack front length, can be used to estimate the fracture stress during vibration tests of the compressor blades of an aviation gas turbine engine, which are made of VT3-1 titanium alloy.

  15. 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.

  16. Competition between fatigue crack propagation and wear

    SciTech Connect

    Fan, H.; Keer, L.M.; Cheng, W.; Cheng, H.S. )

    1993-01-01

    Based on a semi-empirical derivation of the Paris fatigue law, the fatigue crack length a is related to the yield limit or flow stress, which ultimately is related to the hardness of the material. The analysis considers together the cyclic loading, which tends to increase the surface crack length, and the wear, which tends to decrease the crack length at the surface, and shows that under certain conditions a stable crack length may be developed. Experiments conducted on two test groups (Rc = 58.5 and Rc = 62.7) tend to support the present analysis. 10 refs.

  17. Fatigue crack propagation analysis of plaque rupture.

    PubMed

    Pei, Xuan; Wu, Baijian; Li, Zhi-Yong

    2013-10-01

    Rupture of atheromatous plaque is the major cause of stroke or heart attack. Considering that the cardiovascular system is a classic fatigue environment, plaque rupture was treated as a chronic fatigue crack growth process in this study. Fracture mechanics theory was introduced to describe the stress status at the crack tip and Paris' law was used to calculate the crack growth rate. The effect of anatomical variation of an idealized plaque cross-section model was investigated. The crack initiation was considered to be either at the maximum circumferential stress location or at any other possible locations around the lumen. Although the crack automatically initialized at the maximum circumferential stress location usually propagated faster than others, it was not necessarily the most critical location where the fatigue life reached its minimum. We found that the fatigue life was minimum for cracks initialized in the following three regions: the midcap zone, the shoulder zone, and the backside zone. The anatomical variation has a significant influence on the fatigue life. Either a decrease in cap thickness or an increase in lipid pool size resulted in a significant decrease in fatigue life. Comparing to the previously used stress analysis, this fatigue model provides some possible explanations of plaque rupture at a low stress level in a pulsatile cardiovascular environment, and the method proposed here may be useful for further investigation of the mechanism of plaque rupture based on in vivo patient data.

  18. Determining fatigue crack opening loads from near-crack-tip displacement measurements

    SciTech Connect

    Riddell, W.T.; Piascik, R.S.; Sutton, M.A.; Zhao, W.; McNeill, S.R.; Helm, J.D.

    1999-07-01

    The aim of this research was to develop a near-crack-tip measurement method that quantifies crack closure levels in the near-threshold fatigue crack growth regime--a regime where crack closure is not well characterized by remote compliance methods. Further understanding of crack closure mechanics was gained by performing novel crack growth experiments in conjunction with numerical simulations of three-dimensional crack-front propagation. Steady-state (i.e., constant growth rate) fatigue crack growth rates were characterized by performing constant cyclic stress intensity range ({Delta}K) experiments over a wide range of stress ratios (R). Near-crack-tip (less than 0.3 mm behind) load-versus-displacement measurements were conducted on the specimen surface using a novel noncontact experimental technique (Digital Imaging Displacement System--DIDS). The experiments and simulations revealed that the three-dimensional aspects of fatigue crack closure must be considered to determine correct opening load levels from near-crack-tip load-versus-displacement data. It was shown that near-crack-front, but increase near the free surface. The interior opening load was found to collapse closure-affected data to intrinsic rates, and thus shown to relate to the true crack-front driving force parameter. Surface opening load DIDS measurements made at an optimal distance behind the crack tip were used to correlate da/dN with {Delta}K{sub eff}. Opening load determinations made less than the optimal distance behind the crack tip were shown to be too high to correlate fatigue crack growth rates.

  19. A study of crack closure in fatigue

    NASA Technical Reports Server (NTRS)

    Shih, T. T.; Wei, R. P.

    1973-01-01

    Crack closure phenomenon in fatigue was studied by using a Ti-6Al-4V titanium alloy. The occurrence of crack closure was directly measured by an electrical-potential method, and indirectly by load-strain measurement. The experimental results showed that the onset of crack closure depends on both the stress ratio, and the maximum stress intensity factor. No crack closure was observed for stress ratio, greater than 0.3 in this alloy. A two-dimensional elastic model was used to explain the behavior of the recorded load-strain curves. Closure force was estimated by using this model. Yield level stress was found near the crack tip. Based on this estimated closure force, the crack opening displacement was calculated. This result showed that onset of crack closure detected by electrical-potential measurement and crack-opening-displacement measurement is the same. The implications of crack closure on fatigue crack are considered. The experimental results show that crack closure cannot fully account for the effect of stress ratio, on crack growth, and that it cannot be regarded as the sole cause for delay.

  20. Analysis of acoustic emission waveforms from fatigue cracks

    NASA Astrophysics Data System (ADS)

    Bhuiyan, Md. Yeasin; Bao, Jingjing; Poddar, Banibrata; Giurgiutiu, Victor

    2017-04-01

    Acoustic emission (AE) monitoring technique is a well-known approach in the field of NDE/SHM. AE monitoring from the defect formation and failure in the materials were well studied by the researchers. However, conventional AE monitoring techniques are predominantly based on statistical analysis. In this study we focus on understanding the AE waveforms from the fatigue crack growth using physics based approach. The growth of the fatigue crack causes the acoustic emission in the material that propagates in the structure. One of the main challenges of this approach is to develop the physics based understanding of the AE source itself. The acoustic emission happens not only from the crack growth but also from the interaction of the crack lips during fatigue loading of the materials. As the waveforms are generated from the AE event, they propagate and create local vibration modes along the crack faces. Fatigue experiments were performed to generate the fatigue cracks. Several test specimens were used in the fatigue experiments and corresponding AE waveforms were captured. The AE waveforms were analyzed and distinguished into different groups based on the similar nature on both time domain and frequency domain. The experimental results are explained based on the physical observation of the specimen.

  1. 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.

  2. Grain boundary resistance to fatigue crack growth

    NASA Technical Reports Server (NTRS)

    Chen, QI; Liu, H. W.

    1993-01-01

    Results of an experimental study tracing the grain boundary effect on the fatigue crack growth rate are reported. Direct experimental evidence for the grain boundary blockage mechanism is presented. The orientation difference between two neighboring grains directly contributed to the extent of crack growth retardation.

  3. Reliability of welded structures containing fatigue cracks

    SciTech Connect

    Lanning, D.; Shen, M.H.H.

    1996-11-01

    This study investigates the reliability of a cracked fillet welded T-joint typically found in offshore structures. A formulation for the aspect ratio (a/c) of a propagating semi-elliptical fatigue crack located at the toe of the weld is developed using Newman and Raju`s stress intensity factor for a cracked flat plate in conjunction with a weld magnification factor. The reliability in terms of fatigue lifetime is then calculated using the aspect ratio and Paris`s law of crack propagation with both fracture toughness and elastic-plastic failure criteria. The variation in crack aspect ratio in the T-joint is compared to that in a cracked flat plate, and examples are provided of reliability calculations for tension and bending loads.

  4. Engineering Aspects of Fatigue Crack Propagation

    DTIC Science & Technology

    1962-01-01

    Estimating Notch-Size Effect in Fatigue Tests on Steel. NACA TN 2805, 1952. - 37 - 19. Landers, Charles B., and Hardrath, Herbert F.: Results of Axial- Load... Charles B., and Howell, F. M.: Axial-Load Fatigue Properties of 24S-T and 75S-T Aluminum Alloy as Determined in Several Laboratories. NACA TR 1190, 1954...Hardrath, Herbert F., Leybold, Herbert A., Landers, Charles B., and Hauschild, Louis W.: Fatigue-Crack Propagation in Aluminum- Alloy Box Beams. NACA

  5. 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

  6. Fracture mechanics parameters for small fatigue cracks

    NASA Technical Reports Server (NTRS)

    Newman, J. C., Jr.

    1992-01-01

    This paper presents a review of some common small-crack test specimens, the underlying causes of the small-crack effect, and the fracture-mechanics parameters that have been used to correlate or predict their growth behavior. This review concentrates on continuum mechanics concepts and on the nonlinear behavior of small cracks. The paper reviews some stress-intensity factor solutions for small-crack test specimens and develops some simple elastic-plastic J integral and cyclic J integral expressions that include the influence of crack-closure. These parameters were applied to small-crack growth data on two aluminum alloys, and a fatigue life prediction methodology is demonstrated. For these materials, the crack-closure transient from the plastic wake was found to be the major factor in causing the small-crack effect.

  7. Three-dimensional measurements of fatigue crack closure

    NASA Technical Reports Server (NTRS)

    Grandt, A. F., Jr.

    1984-01-01

    Three dimensional fatigue crack opening profiles in transparent polymer test specimens were determined. The load required to separate crack faces was measured along the crack profile at various positions through the specimens thickness. Crack opening loads at the specimen surface (under plane stress conditions) were compared with measurements made under plane strain conditions the specimen interior. The fatigue crack opening load was correlated with fatigue crack retardation behavior caused by peak overloads, and the results discussed in terms of three dimensional aspects of the fatigue crack closure mechanism for fatigue crack retardation.

  8. Three-dimensional measurements of fatigue crack closure

    NASA Technical Reports Server (NTRS)

    Ray, S. K.; Grandt, A. F., Jr.

    1984-01-01

    Fatigue crack growth and retardation experiments conducted in polycarbonate test specimen are described. The transparent test material allows optical interferometry measurements of the fatigue crack opening (and closing) profiles. Crack surface displacements are obtained through the specimen thickness and three dimensional aspects of fatigue crack closure are discussed.

  9. Fatigue crack growth in metastable austenitic stainless steels

    SciTech Connect

    Mei, Z.; Chang, G.; Morris, J.W. Jr.

    1988-06-01

    The research reported here is an investigation of the influence of the mechanically induced martensitic transformation on the fatigue crack growth rate in 304-type steels. The alloys 304L and 304LN were used to test the influence of composition, the testing temperatures 298 K and 77 K were used to study the influence of test temperature, and various load ratios (R) were used to determine the influence of the load ratio. It was found that decreasing the mechanical stability of the austenite by changing composition or lowering temperature decreases the fatigue crack growth rate. The R-ratio effect is more subtle. The fatigue crack growth rate increases with increasing R-ratio, even though this change increases the martensite transformation. Transformation-induced crack closure can explain the results in the threshold regime, but cannot explain the R-ratio effect at higher cyclic stress intensities. 26 refs., 6 figs.

  10. Thermally activated processes of fatigue crack growth in steels

    NASA Astrophysics Data System (ADS)

    Tanaka, Masaki; Fujii, Atsushi; Noguchi, Hiroshi; Higashida, Kenji

    2014-02-01

    Fatigue crack growth rates in steels at high and low temperatures have been investigated using Paris curves. The fatigue crack growth rates at high temperatures are quite different from those at low temperatures. Arrhenius plots between fatigue crack growth rate (da/dN) and test temperatures at constant stress intensity factor range (ΔKI) indicate a difference of the rate-controlling process for fatigue crack growth with temperature. Slip deformation at the crack tip governs fatigue crack growth at high temperatures, while hydrogen diffusion is associated with crack growth at low temperatures.

  11. Fatigue crack growth in lithium hydride

    SciTech Connect

    Healy, T.E.

    1993-09-01

    Subcritical fatigue crack growth, from cyclic tensile loading, was demonstrated in warm pressed Polycrystalline lithium hydride. Experiments were performed with cyclic tension-tension crack opening (mode I) loads applied to a pre-cracked compact type specimen in an argon environment at a temperature of 21C (70F). The fatigue crack growth was found to occur between 7.56 {times} 10{sup {minus}ll} M/cycle (2.98 {times} l0{sup {minus}9} in/cycle) and 2.35 {times} l0{sup {minus}8} m/cycle (9.24{times}10{sup {minus}7} in/cycle) for a range of stress intensity factors between 1.04 MPa{center_dot}{radical}m (0.95 ksi{center_dot}{radical}in) and 1.49 MPa{center_dot}{radical}m (1.36 ksi{center_dot}{radical}in). The rate of fatigue crack growth from cyclic tensile loading was found to be in excess of crack growth from sustained loading at an equivalent stress intensity factor. Furthermore, a fatigue threshold was not evident from the acquired data.

  12. Fatigue Crack Growth Threshold Testing of Metallic Rotorcraft Materials

    NASA Technical Reports Server (NTRS)

    Newman, John A.; James, Mark A.; Johnson, William M.; Le, Dy D.

    2008-01-01

    Results are presented for a program to determine the near-threshold fatigue crack growth behavior appropriate for metallic rotorcraft alloys. Four alloys, all commonly used in the manufacture of rotorcraft, were selected for study: Aluminum alloy 7050, 4340 steel, AZ91E Magnesium, and Titanium alloy Ti-6Al-4V (beta-STOA). The Federal Aviation Administration (FAA) sponsored this research to advance efforts to incorporate damage tolerance design and analysis as requirements for rotorcraft certification. Rotorcraft components are subjected to high cycle fatigue and are typically subjected to higher stresses and more stress cycles per flight hour than fixed-wing aircraft components. Fatigue lives of rotorcraft components are generally spent initiating small fatigue cracks that propagate slowly under near-threshold cracktip loading conditions. For these components, the fatigue life is very sensitive to the near-threshold characteristics of the material.

  13. Corrosion-Fatigue Cracking in Al 7075 Alloys

    DTIC Science & Technology

    2014-12-09

    Naval Research Laboratory Washington, DC 20375-5320 NRL/MR/6355--14-9582 Corrosion-Fatigue Cracking in Al 7075 Alloys December 9, 2014 P.S. Pao...7075 Alloys P.S. Pao and R.L. Holtz Naval Research Laboratory 4555 Overlook Avenue, SW Washington, DC 20375-5328 Office of Naval Research One Liberty...NaCl] concentration), and load ratio on fatigue crack growth kinetics of Al 7075 alloy . The materials investigated were peakaged Al 7075-T651 and

  14. Evaluation of the effect of crack closure on fatigue crack growth of simulated short cracks

    NASA Technical Reports Server (NTRS)

    Telesman, J.; Fisher, D. M.

    1984-01-01

    A test program was performed to determine the influence of crack closure on fatigue crack growth (FCG) rates of short cracks. By use of the standard compact tension specimen, test procedures were devised to evaluate closure loads in the wake of the crack behind its tip. The first procedure determined the magnitude of crack closure as a function of the fatigued crack wave by incrementally removing the contacting wake surfaces and measuring closure load at each increment. The second procedure used a low-high loading sequence to simulate short crack behavior. Based on the results, it was concluded that crack closure is not the major reason for the more rapid growth of short cracks as compared to long crack growth.

  15. Ultrasonic characterization of fatigue crack closure

    SciTech Connect

    Thompson, R.B.; Buck, O.; Rehbein, D.K.

    1991-01-01

    The characterization of fatigue crack closure is an important objective because of its influence on fatigue crack propagation, particularly under conditions of variable amplitude loading. This paper describes a nontraditional technique for characterizing closure, in which ultrasonic scattering measurements are used to obtain estimates of the number density and size of asperities bridging the crack faces, with subsequent estimates of the crack tip shielding being based on those geometrical parameters. The paper first reviews the experimental configuration and the basic elasto-dynamic theory underlying the technique. It then presents recent results obtained in studies of the influence of block overloads and load shedding on the growth of fatigue cracks in aluminum alloys. In both cases, the change in the closure state after the overload can be unambiguously seen even in the raw data. Moreover, data analysis suggests that it may be possible to predict when the crack will reinitiate based on more subtle changes in the ultrasonically inferred closure state. In the case of load shedding, a massive closure region is observed, whose characteristics appear consistent with the notion that threshold phenomena can be explained in terms of crack closure. 20 refs., 10 figs.

  16. Experimental characterization of fatigue crack tip processes

    NASA Technical Reports Server (NTRS)

    Lankford, J.; Leverant, G. R.

    1985-01-01

    Many analytical models have been proposed to describe the physical processes attendant to a fatigue crack tip, as well as the rate at which fatigue cracks grow. By applying advanced experimental methods (such as electron channeling, stereoimaging, and in-situ cyclic loading in an SEM) to a broad range of structural materials, it has been shown that it is possible to critically assess the physical assumptions incorporated into the models. Refinements in existing models and the development of new ones have resulted. New insights into materials' behavior are providing guidelines for improving the fatigue resistance of structural alloys. In the near future, even more advanced experimental methods, such as high temperature SEM stages and small angle neutron scattering, will be available to study creep/fatigue interactions in metals and ceramics.

  17. Fatigue crack growth in aluminum laminate composites

    SciTech Connect

    Hoffman, P.B.; Carpenter, R.D.; Gibeling, J.C.

    1996-12-31

    Fatigue crack growth has been measured in a laminated metal composite (LMC) consisting of alternating layers of AA6090/SiC/25p metal matrix composite (MMC) and AA5182 alloy. This material was tested in both as-pressed (F temper) and aged (T6 temper) conditions. Corresponding crack growth measurements were made in self-laminates of both the MMC and AA5182 materials to examine the role of the interfaces.

  18. Fatigue Crack Closure Analysis Using Digital Image Correlation

    NASA Technical Reports Server (NTRS)

    Leser, William P.; Newman, John A.; Johnston, William M.

    2010-01-01

    Fatigue crack closure during crack growth testing is analyzed in order to evaluate the critieria of ASTM Standard E647 for measurement of fatigue crack growth rates. Of specific concern is remote closure, which occurs away from the crack tip and is a product of the load history during crack-driving-force-reduction fatigue crack growth testing. Crack closure behavior is characterized using relative displacements determined from a series of high-magnification digital images acquired as the crack is loaded. Changes in the relative displacements of features on opposite sides of the crack are used to generate crack closure data as a function of crack wake position. For the results presented in this paper, remote closure did not affect fatigue crack growth rate measurements when ASTM Standard E647 was strictly followed and only became a problem when testing parameters (e.g., load shed rate, initial crack driving force, etc.) greatly exceeded the guidelines of the accepted standard.

  19. Fatigue Crack Growth of Gun Tube Steel under Spectrum Loading

    DTIC Science & Technology

    1986-09-01

    current research. Early attempts at accounting for variable load effects ignored sequence effects. Palmgren (8 ) and Miner(9 ) (1945) used the concept of...decreased to conform to the requirements of ASTM Standard E647( 27) on fatigue crack growth. B. Test Equipment All fatigue and tensile tests were...94, ASME 4th National Congress on Pressure Vessel and Piping Technology, June 19-24, 1983, Portland, Oregon. 8. Palmgren , A., "Durability of Ball

  20. Prediction of fatigue crack-growth patterns and lives in three-dimensional cracked bodies

    NASA Technical Reports Server (NTRS)

    Newman, J. C., Jr.; Raju, I. S.

    1986-01-01

    Fatigue crack growth patterns and lives for surface cracks, surface cracks at holes, and corner cracks at holes in three dimensional bodies were predicted using linear-elastic fracture mechanics concepts that were modified to account for crack-closure behavior. The predictions were made by using stress intensity factor equations for these crack configurations and the fatigue crack-growth (delta K against rate) relationship for the material of interest. The crack configurations were subjected to constant-amplitude fatigue loading under either remote tension or bending loads. The predicted crack growth patterns and crack growth lives for aluminum alloys agreed well with test data from the literature.

  1. Prediction of fatigue crack-growth patterns and lives in three-dimensional cracked bodies

    NASA Technical Reports Server (NTRS)

    Newman, J. C., Jr.; Raju, I. S.

    1984-01-01

    Fatigue crack growth patterns and lives for surface cracks, surface cracks at holes, and corner cracks at holes in three dimensional bodies were predicted using linear-elastic fracture mechanics concepts that were modified to account for crack-closure behavior. The predictions were made by using stress intensity factor equations for these crack configurations and the fatigue crack-growth (delta K against rate) relationship for the material of interest. The crack configurations were subjected to constant-amplitude fatigue loading under either remote tension or bending loads. The predicted crack growth patterns and crack growth lives for aluminum alloys agreed well with test data from the literature.

  2. Fatigue-Crack-Growth Computer Program

    NASA Technical Reports Server (NTRS)

    Forman, Royce G.; Shivakumar, V.; Newman, James C., Jr.

    1991-01-01

    Fatigue Crack Growth (NASA/FLAGRO) computer program developed as aid in predicting growth of preexisting flaws and cracks in structural components of space systems. Is enhanced version of FLAGRO4 and incorporates state-of-the-art improvements in both fracture mechanics and computer technology. Provides fracture-mechanics analyst with computerized method of evaluating "safe-crack-growth-life" capabilities of structural components. Also used to evaluate tolerance to damage of structure of given design. Designed modular to facilitate revisions and operation on minicomputers. Written in FORTRAN 77.

  3. Stochastic modeling of crack initiation and short-crack growth under creep and creep-fatigue conditions

    NASA Technical Reports Server (NTRS)

    Kitamura, Takayuki; Ghosn, Louis J.; Ohtani, Ryuichi

    1989-01-01

    A simplified stochastic model is proposed for crack initiation and short-crack growth under creep and creep-fatigue conditions. Material inhomogeneity provides the random nature of crack initiation and early growth. In the model, the influence of microstructure is introduced by the variability of: (1) damage accumulation along grain boundaries, (2) critical damage required for crack initiation or growth, and (3) the grain-boundary length. The probabilities of crack initiation and growth are derived by using convolution integrals. The model is calibrated and used to predict the crack density and crack-growth rate of short cracks of 304 stainless steel under creep and creep-fatigue conditions. The mean-crack initiation lives are predicted to be within an average deviation of about 10 percent from the experimental results. The predicted cumulative distributions of crack-growth rate follow the experimental data closely. The applicability of the simplified stochastic model is discussed and the future research direction is outlined.

  4. Fatigue cracks in Eurofer 97 steel: Part II. Comparison of small and long fatigue crack growth

    NASA Astrophysics Data System (ADS)

    Kruml, T.; Hutař, P.; Náhlík, L.; Seitl, S.; Polák, J.

    2011-05-01

    The fatigue crack growth rate in the Eurofer 97 steel at room temperature was measured by two different methodologies. Small crack growth data were obtained using cylindrical specimens with a shallow notch and no artificial crack starters. The growth of semicircular cracks of length between 10-2000 μm was followed in symmetrical cycling with constant strain amplitude ( R ɛ = -1). Long crack data were measured using standard CT specimen and ASTM methodology, i.e. R = 0.1. The growth of cracks having the length in the range of 10-30 mm was measured. It is shown that the crack growth rates of both types of cracks are in a very good agreement if J-integral representation is used and usual assumptions of the crack closure effects are taken into account.

  5. Life prediction for bridged fatigue cracks

    SciTech Connect

    Cox, B.N.

    1994-08-01

    One of the more promising classes of composites touted for high temperature applications, and certainly the most available, is that of relatively brittle matrices, either ceramic or intermetallic, reinforced by strong, aligned, continuous fibers. Under cyclic loading in the fiber direction, these materials develop matrix cracks that often run perpendicular to the fibers, while the fibers remain intact in the crack wake, supplying bridging tractions across the fracture surfaces. The bridging tractions shield the crack tip from the applied load, dramatically reducing the crack velocity from that expected in an unreinforced material subjected to the same value, {Delta}K{sub a}, of the cyclic applied stress intensity factor. An important issue in reliability is the prediction of the growth rates of the bridged cracks. The growth rates of matrix fatigue cracks bridged by sliding fibers are now commonly predicted by models based on the micromechanics of frictional interfaces. However, there exist many reasons, both theoretical and experimental, for suspecting that the most popular micromechanical models are probably wrong in detail in the context of fatigue cracks. Furthermore, a review of crack growth data reveals that the validity of the micromechanics-based predictive model has never been tested and may never be tested. In this paper, two alternative approaches are suggested to the engineering problem of predicting the growth rates of bridged cracks without explicit recourse to micromechanics. Instead, it is shown that the material properties required to analyze bridging effects can be deduced directly from crack growth data. Some experiments are proposed to test the validity of the proposals.

  6. The Growth of Small Corrosion Fatigue Cracks in Alloy 7075

    NASA Technical Reports Server (NTRS)

    Piascik, Robert S.

    2015-01-01

    The corrosion fatigue crack growth characteristics of small (greater than 35 micrometers) surface and corner cracks in aluminum alloy 7075 is established. The early stage of crack growth is studied by performing in situ long focal length microscope (500×) crack length measurements in laboratory air and 1% sodium chloride (NaCl) environments. To quantify the "small crack effect" in the corrosive environment, the corrosion fatigue crack propagation behavior of small cracks is compared to long through-the-thickness cracks grown under identical experimental conditions. In salt water, long crack constant K(sub max) growth rates are similar to small crack da/dN.

  7. The Growth of Small Corrosion Fatigue Cracks in Alloy 7075

    NASA Technical Reports Server (NTRS)

    Piascik, R. S.

    2001-01-01

    The corrosion fatigue crack growth characteristics of small (less than 35 microns) surface and corner cracks in aluminum alloy 7075 is established. The early stage of crack growth is studied by performing in situ long focal length microscope (500X) crack length measurements in laboratory air and 1% NaCl environments. To quantify the "small crack effect" in the corrosive environment, the corrosion fatigue crack propagation behavior of small cracks is compared to long through-the-thickness cracks grown under identical experimental conditions. In salt water, long crack constant K(sub max) growth rates are similar to small crack da/dN.

  8. The growth of small corrosion fatigue cracks in alloy 2024

    NASA Technical Reports Server (NTRS)

    Piascik, Robert S.; Willard, Scott A.

    1993-01-01

    The corrosion fatigue crack growth characteristics of small surface and corner cracks in aluminum alloy 2024 is established. The damaging effect of salt water on the early stages of small crack growth is characterized by crack initiation at constituent particle pits, intergranular microcracking for a less than 100 micrometers, and transgranular small crack growth for a micrometer. In aqueous 1 percent NaCl and at a constant anodic potential of -700 mV(sub SCE), small cracks exhibit a factor of three increase in fatigue crack growth rates compared to laboratory air. Small cracks exhibit accelerated corrosion fatigue crack growth rates at low levels of delta-K (less than 1 MPa square root of m) below long crack delta-K (sub th). When exposed to Paris regime levels of crack tip stress intensity, small corrosion fatigue cracks exhibit growth rates similar to that observed for long cracks. Results suggest that crack closure effects influence the corrosion fatigue crack growth rates of small cracks (a less than or equal to 100 micrometers). This is evidenced by similar small and long crack growth behavior at various levels of R. Contrary to the corrosion fatigue characteristics of small cracks in high strength steels, no pronounced chemical crack length effect is observed for Al by 2024 exposed to salt water.

  9. Fatigue and fracture research in metals

    NASA Technical Reports Server (NTRS)

    Newman, J. C., Jr.; Davidson, J. R.

    1982-01-01

    Fatigue and fracture research on monolithic and laminated metals is discussed. The research concentrated on three areas: stress analyses of two and three dimensional cracked bodies, fatigue crack growth, and fracture toughness. Analytical methods were developed to predict fatigue crack growth and fracture strengths of cracked specimens. Such specimens represent typical aircraft structural details (such as cracks from holes). These specimens were subjected to simple constant amplitude loading and to more complex flight load histories. Test data from both in house tests and from the literature are used to substantiate the analytical methods. These analyses extended the theory of fracture mechanics to deal with fatigue crack growth and fracture of complex crack configurations that are typical of aircraft materials and structural details.

  10. Fatigue crack growth under variable amplitude loading

    NASA Astrophysics Data System (ADS)

    Sidawi, Jihad A.

    1994-09-01

    Fatigue crack growth tests were conducted on an Fe 510 E C-Mn steel and a submerged arc welded joint from the same material under constant, variable, and random loading amplitudes. Paris-Erdogan's crack growth rate law was tested for the evaluation of m and C using the stress intensity factor K, the J-integral, the effective stress intensity factor K(sub eff), and the root mean square stress intensity factor K(sub rms) fracture mechanics concepts. The effect of retardation and residual stresses resulting from welding was also considered. It was found that all concepts gave good life predictions in all cases.

  11. Fatigue crack growth under variable amplitude loading

    NASA Technical Reports Server (NTRS)

    Sidawi, Jihad A.

    1994-01-01

    Fatigue crack growth tests were conducted on an Fe 510 E C-Mn steel and a submerged arc welded joint from the same material under constant, variable, and random loading amplitudes. Paris-Erdogan's crack growth rate law was tested for the evaluation of m and C using the stress intensity factor K, the J-integral, the effective stress intensity factor K(sub eff), and the root mean square stress intensity factor K(sub rms) fracture mechanics concepts. The effect of retardation and residual stresses resulting from welding was also considered. It was found that all concepts gave good life predictions in all cases.

  12. Fatigue Crack Closure - A Review

    DTIC Science & Technology

    1990-09-01

    gauge along the crack line. They used CCT speci- mens of high tensile strength steel ( HY80 ). The measured value of U was found to be a minimum at the...ultrasonic surface wave technique on 12.5mm thick specimens of 2024-T851, 2024-T351, Al 2219, Ti-6AI-4V and 17-4 PH steel . Most of the results were...medium and high strength steels . Exami- nation of the fracture surfaces suggested that raising the mean stress in low fracture toughness steels could

  13. Metallurgical control of fatigue crack propagation in superalloys

    NASA Astrophysics Data System (ADS)

    Chang, K.-M.; Henry, M. F.; Benz, M. G.

    1990-12-01

    Low-cycle fatigue life of turbine engine disk alloys is determined by the initiation and propagation of fatigue cracks. Performance improvements can be achieved through the combination of clean melting technology, to reduce the defect size, and a new generation of high-strength superalloys with fatigue cracking resistance. Metallurgical control of fatigue crack propagation in high-strength superalloys becomes feasible only through a clear understanding of the fatigue cracking mechanism, as well as the micro-structure/property relationships. Many metallurgical parameters have been identified to control the fatigue cracking resistance at high temperatures. One of the most effective methods, applicable to all high γ' content superalloys, is to modify the grain boundary structure by means of a controlled cooling from a supersolvus solutioning. The precipitation reaction occurring on the grain boundaries during cooling generates a serrated structure that exhibits a good stress oxidation resistance for fatigue cracking.

  14. Progress in understanding corrosion fatigue crack growth

    SciTech Connect

    Wei, R.P.

    1997-12-31

    The paper reviews key developments since the early 1960s in the understanding of corrosion fatigue crack growth in metallic materials. Microstructural response of a metal to the mechanical and chemical driving forces is discussed. The author believes that greater emphasis needs to be placed on the development and utilization of mechanistically based probability methods of analyses to better estimate the life cycle costs and manage new and aging mechanical structures.

  15. Fatigue crack propagation at polymer adhesive interfaces

    SciTech Connect

    Ritter, J.E.

    1996-12-31

    Delamination of polymer adhesive interfaces often occurs due to slow crack growth under either monotonic or cyclic loading. The author`s previous research showed that moisture-assisted crack growth at epoxy/glass and epoxy acrylate/glass interfaces under monotonic loading was directly related to the applied energy release rate and relative humidity and that cyclic loading could enhance crack growth. The purpose of the present research is to compare crack growth along epoxy acrylate/glass and epoxy/PMMA interfaces under monotonic and cyclic loading.

  16. Fatigue crack growth reliability by probabilistic finite elements

    NASA Technical Reports Server (NTRS)

    Besterfield, Glen H.; Liu, Wing K.; Lawrence, Mark A.; Belytschko, Ted

    1991-01-01

    Fusion of the probabilistic finite-element method and reliability analysis for probabilistic fatigue-crack growth is presented. A comprehensive method for determining the probability of fatigue failure for mixed-mode cyclic loading is also presented. The loading is mixed-mode with randomness in the initial and final crack lengths, initial crack angle and position, material properties, crack-growth law, crack-direction law, and loading. The methodology consists of calculating the reliability index via an optimization procedure which is used to calculate the probability of fatigue failure. Performance of the methodology presented is demonstrated on a classical mode-I fatigue problem.

  17. Fatigue crack growth in an aluminum alloy-fractographic study

    NASA Astrophysics Data System (ADS)

    Salam, I.; Muhammad, W.; Ejaz, N.

    2016-08-01

    A two-fold approach was adopted to understand the fatigue crack growth process in an Aluminum alloy; fatigue crack growth test of samples and analysis of fractured surfaces. Fatigue crack growth tests were conducted on middle tension M(T) samples prepared from an Aluminum alloy cylinder. The tests were conducted under constant amplitude loading at R ratio 0.1. The stress applied was from 20,30 and 40 per cent of the yield stress of the material. The fatigue crack growth data was recorded. After fatigue testing, the samples were subjected to detailed scanning electron microscopic (SEM) analysis. The resulting fracture surfaces were subjected to qualitative and quantitative fractographic examinations. Quantitative fracture analysis included an estimation of crack growth rate (CGR) in different regions. The effect of the microstructural features on fatigue crack growth was examined. It was observed that in stage II (crack growth region), the failure mode changes from intergranular to transgranular as the stress level increases. In the region of intergranular failure the localized brittle failure was observed and fatigue striations are difficult to reveal. However, in the region of transgranular failure the crack path is independent of the microstructural features. In this region, localized ductile failure mode was observed and well defined fatigue striations were present in the wake of fatigue crack. The effect of interaction of growing fatigue crack with microstructural features was not substantial. The final fracture (stage III) was ductile in all the cases.

  18. The analysis of fatigue crack growth mechanism and oxidation and fatigue life at elevated temperatures

    NASA Technical Reports Server (NTRS)

    Liu, H. W.

    1988-01-01

    Two quantitative models based on experimentally observed fatigue damage processes have been made: (1) a model of low cycle fatigue life based on fatigue crack growth under general-yielding cyclic loading; and (2) a model of accelerated fatigue crack growth at elevated temperatures based on grain boundary oxidation. These two quantitative models agree very well with the experimental observations.

  19. Fatigue crack arrest in a self-healing polymer composite

    SciTech Connect

    Brown, E. N.; White, S. R.; Sottos, Nancy R.

    2004-01-01

    A comprehensive experimental program is performed to assess the in situ fatigue behavior of a self-healing polymer. A fatigue-life-extension protocol is established for characterizing healing efficiency of the self-healing epoxy under cyclic loading. At moderate {Delta}K{sub I} and at high {Delta}K{sub I}, when a rest period is employed, in situ healing extends fatigue life though temporary crack arrest and retardation. In situ self-healing permanently arrests crack growth at low {delta}K{sub I} and at moderate {Delta}K{sub I}, when a rest period is employed. Fatigue crack retardation and arrest result from two primary crack-tip shielding mechanisms: hydrodynamic pressure in the viscous healing agent and artificial crack closure. Application of self-healing functionality to fatigue slows the crack growth rate and increases the fatigue threshold.

  20. Fatigue and environmentally assisted cracking in light water reactors

    SciTech Connect

    Kassner, T.F.; Ruther, W.E.; Chung, H.M.; Hicks, P.D.; Hins, A.G.; Park, J.Y.; Shack, W.J.

    1991-12-01

    Fatigue and environmentally assisted cracking of piping, pressure vessels, and core components in light water reactors (LWRs) are important concerns as extended reactor lifetimes are envisaged. The degradation processes include intergranular stress corrosion cracking (IGSCC) of austenitic stainless steel (SS) piping in boiling water reactors (BWRs), and propagation of fatigue or SCC cracks (which initiate in sensitized SS cladding) into low-alloy ferritic steels in BWR pressure vessels. Similar cracking has also occurred in upper shell-to-transition cone girth welds in pressurized water reactor (PWR) steam generator vessels. Another concern is failure of reactor-core internal components after accumulation of relatively high fluence, which has occurred in both BWRs and PWRs. Research during the past year focused on (1) fatigue and SCC of ferritic steels used in piping and in steam generator and reactor pressure vessels, (2) role of chromate and sulfate in simulated BWR water in SCC of sensitized Type 304 SS, and (3) irradiation-assisted SCC in high- and commercial-purity Type 304 SS specimens from control-blade absorber tubes used in two operating BWRs. Failure after accumulation of relatively high fluence has been attributed to radiation-induced segregation (RIS) of elements such as Si, P, Ni, and Cr. This document provides a summary of research progress in these areas.

  1. Low-Cycle Fatigue Life and Fatigue Crack Propagation of Sintered Ag Nanoparticles

    NASA Astrophysics Data System (ADS)

    Shioda, Ryutaro; Kariya, Yoshiharu; Mizumura, Noritsuka; Sasaki, Koji

    2017-02-01

    The low-cycle fatigue life and fatigue crack propagation behavior of sintered silver nanoparticles were investigated using miniature specimens sintered at two different temperatures. The fatigue crack initiation life and fatigue crack propagation rate of sintered Ag nanoparticles were extremely sensitive to changes in the range of inelastic energy density and the cyclic J integral, exhibiting brittle characteristics, in contrast to tin-based lead-free solder alloys. With increasing sintering temperature, the fatigue crack propagation rate decreased. On the other hand, the effect of sintering temperature on the fatigue crack initiation life differed depending on the use of either a smooth specimen (low-cycle fatigue test) or notched specimen (fatigue crack propagation test). For the notched specimens, the probability of grain boundaries around the notch decreased due to increased sintering temperature. Therefore, the fatigue crack initiation life was increased with an increase in sintering temperature in the fatigue crack propagation test. In the smooth specimen, however, the fatigue life decreased with an increase in sintering temperature, as the elastic modulus of the specimen increased with increasing sintering temperature. In the low-cycle fatigue test, the specimen sintered with high internal stress started to develop crack initiation early, causing a decrease in the crack initiation life.

  2. Fatigue crack growth behavior in equine cortical bone

    NASA Astrophysics Data System (ADS)

    Shelton, Debbie Renee

    2001-07-01

    Objectives for this research were to experimentally determine crack growth rates, da/dN, as a function of alternating stress intensity factor, DeltaK, for specimens from lateral and dorsal regions of equine third metacarpal cortical bone tissue, and to determine if the results were described by the Paris law. In one set of experiments, specimens were oriented for crack propagation in the circumferential direction with the crack plane transverse to the long axis of the bone. In the second set of experiments, specimens were oriented for radial crack growth with the crack plane parallel to the long axis of the bone. Results of fatigue tests from the latter specimens were used to evaluate the hypothesis that crack growth rates differ regionally. The final experiments were designed to determine if crack resistance was dependent on region, proportion of hooped osteons (those with circumferentially oriented collagen fibers in the outer lamellae) or number of osteons penetrated by the crack, and to address the hypothesis that hooped osteons resist invasion by cracks better than other osteonal types. The transverse crack growth data for dorsal specimens were described by the Paris law with an exponent of 10.4 and suggested a threshold stress intensity factor, DeltaKth, of 2.0 MPa·m1/2 and fracture toughness of 4.38 MPa·m 1/2. Similar results were not obtained for lateral specimens because the crack always deviated from the intended path and ran parallel to the loading direction. Crack growth for the dorsal and lateral specimens in the radial orientation was described by the Paris law with exponents of 8.7 and 10.2, respectively, and there were no regional differences in the apparent DeltaK th (0.5 MPa·m1/2) or fracture toughness (1.2 MPa·m 1/2). Crack resistance was not associated with cortical region, proportion of hooped osteons or the number of osteons penetrated by the crack. The extent to which cracks penetrate osteons was influenced by whether the collagen fiber

  3. Grain boundary oxidation and fatigue crack growth at elevated temperatures

    NASA Technical Reports Server (NTRS)

    Liu, H. W.; Oshida, Y.

    1986-01-01

    Fatigue crack growth rate at elevated temperatures can be accelerated by grain boundary oxidation. Grain boundary oxidation kinetics and the statistical distribution of grain boundary oxide penetration depth were studied. At a constant delta K-level and at a constant test temperature, fatigue crack growth rate, da/dN, is a function of cyclic frequency, nu. A fatigue crack growth model of intermittent micro-ruptures of grain boundary oxide is constructed. The model is consistent with the experimental observations that, in the low frequency region, da/dN is inversely proportional to nu, and fatigue crack growth is intergranular.

  4. Fatigue Crack Propagation Behavior According Tofiber Arraying Direction for Load Direction Inwoven CFRP Composite

    NASA Astrophysics Data System (ADS)

    Choi, Jung-Hun; Kang, Min-Sung; Koo, Jae-Mean; Seok, Chang-Sung; Kim, Hyung-Ick

    The fatigue crack propagation of CFRP (carbon fiber reinforced composite material) laminates is of current interest, particularly with regard to their durability under fatigue loading. Recently, carbon fiber reinforced composite materials (Woven fabric) are widely used in various fields of engineering because of its advanced properties. Then, many researchers have studied woven fabric CFRP materials but fatigue crack propagation behaviors for composites have not been still standardized . It shows the different crack propagation behavior according to load and fiber direction. Therefore, there is a need to consider fatigue crack propagation behavior in conformity with fiber arraying direction to load direction at designing structure using woven CFRP materials. In this study, therefore, the fatigue crack propagation for plain woven CFRP composite materials was investigated under two different fiber array direction (fiber arraying direction to load : 0°, 45°). Fatigue crack propagation tests of the woven CFRP composite were conducted under sinusoidal wave-form with stress ratios of 0.3 at a frequency of 10Hz, respectively. As a result of test, fatigue crack propagation rates (da/dN) were plotted against the stress-intensity factor amplitude (ΔK) and other factor. Also we compared ΔK with other factor that considering in-plain anisotropy. All of tests of fatigue crack propagation were carried out under mode I opening loading by using compact tension specimens.

  5. Fatigue Crack Growth Monitoring of AN Aluminum Joint Structure

    NASA Astrophysics Data System (ADS)

    Lissenden, C. J.; Cho, H.; Kim, C. S.

    2010-02-01

    The detection, location, and sizing of a fatigue crack emanating from a fastener hole in an aluminum plate is investigated. Two linear arrays of surface mounted piezoelectric disk transducers send and receive ultrasonic guided waves that are transmitted, reflected, and scattered by both the joint geometry and the fatigue crack. A tomography algorithm is used to detect and locate the crack. Amplitude ratio and signal difference coefficients are explored as candidate features to size the crack, which is necessary for reliability and remaining life calculations. Both of these features are quite sensitive to fatigue crack lengths as small as 0.13 of the hole diameter.

  6. The effect of fatigue cracks on fastener flexibility, load distribution, and fatigue crack growth

    NASA Astrophysics Data System (ADS)

    Whitman, Zachary Layne

    Fatigue cracks typically occur at stress risers such as geometry changes and holes. This type of failure has serious safety and economic repercussions affecting structures such as aircraft. The need to prevent catastrophic failure due to fatigue cracks and other discontinuities has led to durability and damage tolerant methodologies influencing the design of aircraft structures. Holes in a plate or sheet filled with a fastener are common fatigue critical locations in aircraft structure requiring damage tolerance analysis (DTA). Often, the fastener is transferring load which leads to a loading condition involving both far-field stresses such as tension and bending, and localized bearing at the hole. The difference between the bearing stress and the tensile field at the hole is known as load transfer. The ratio of load transfer as well as the magnitude of the stresses plays a significant part in how quickly a crack will progress to failure. Unfortunately, the determination of load transfer in a complex joint is far from trivial. Many methods exist in the open literature regarding the analysis of splices, doublers and attachment joints to determine individual fastener loads. These methods work well for static analyses but greater refinement is needed for crack growth analysis. The first fastener in a splice or joint is typically the most critical but different fastener flexibility equations will all give different results. The constraint of the fastener head and shop end, along with the type of fastener, affects the stiffness or flexibility of the fastener. This in turn will determine the load that the fastener will transfer within a given fastener pattern. However, current methods do not account for the change in flexibility at a fastener as the crack develops. It is put forth that a crack does indeed reduce the stiffness of a fastener by changing its constraint, thus lessening the load transfer. A crack growth analysis utilizing reduced load transfer will result in

  7. Mechanism of corrosion fatigue cracking of automotive coil spring steel

    NASA Astrophysics Data System (ADS)

    Nam, Tae-Heum; Kwon, Min-Seok; Kim, Jung-Gu

    2015-11-01

    The AISI 300M ultra-high strength steel was applied for the automotive suspension coil spring. Recently, some premature failures were reported, which caused by synergistic effect of cyclic mechanical stress and corrosion, namely corrosion fatigue cracking. In this study, the accurate mechanism of corrosion fatigue cracking for coil spring steel was studied for the proper prevention method against the catastrophic failure. Fatigue life was evaluated in 5 wt% NaCl solution under the anodic dissolution and hydrogen embrittlement conditions, which is simulated by applying constant potentials. Scanning electron microscopy and energy dispersive X-ray spectroscopy analysis indicated that the corrosion fatigue cracking was initiated at the MnS inclusion of the pit initiation site. The calculation of hydrogen production corresponding to each corrosion fatigue test condition revealed the two operating mechanisms of the cracking process. The corrosion fatigue cracking failure of coil spring steel was mainly caused by the anodic dissolution combined with hydrogen embrittlement.

  8. Mesoscopic approach to subcritical fatigue crack growth

    NASA Astrophysics Data System (ADS)

    Araújo, Maycon S.; Vieira, André P.; Andrade, José S.; Herrmann, Hans J.

    2016-10-01

    We investigate a model for fatigue crack growth in which damage accumulation is assumed to follow a power law of the local stress amplitude, a form that can be generically justified on the grounds of the approximately self-similar aspect of microcrack distributions. Our aim is to determine the relation between model ingredients and the Paris exponent governing subcritical crack-growth dynamics at the macroscopic scale, starting from a single small notch propagating along a fixed line. By a series of analytical and numerical calculations, we show that, in the absence of disorder, there is a critical damage-accumulation exponent γ , namely γc=2 , separating two distinct regimes of behavior for the Paris exponent m . For γ >γc , the Paris exponent is shown to assume the value m =γ , a result that proves robust against the separate introduction of various modifying ingredients. Explicitly, we deal here with (i) the requirement of a minimum stress for damage to occur, (ii) the presence of disorder in local damage thresholds, and (iii) the possibility of crack healing. On the other hand, in the regime γ <γc , the Paris exponent is seen to be sensitive to the different ingredients added to the model, with rapid healing or a high minimum stress for damage leading to m =2 for all γ <γc , in contrast with the linear dependence m =6 -2 γ observed for very long characteristic healing times in the absence of a minimum stress for damage. Upon the introduction of disorder on the local fatigue thresholds, which leads to the possible appearance of multiple cracks along the propagation line, the Paris exponent tends to m ≈4 for γ ≲2 while retaining the behavior m =γ for γ ≳4 .

  9. Fatigue crack growth in ferroelectrics driven by cyclic electric loading

    NASA Astrophysics Data System (ADS)

    Zhu, Ting; Yang, Wei

    1998-12-01

    Fatigue crack growth has been observed recently in ferroelectrics under cyclic electric loading. Does the crack grow by electric breakdown, or by the stress field near the crack tip? The present paper provides a mechanistic explanation for the electric-field-induced fatigue crack growth. The non-uniform electric field near an insulated crack tip might cause domain switching which in turn produces a concentrated stress field characterized by a stress intensity factor. For ferroelectrics poled along a direction perpendicular to the crack, we are able to show quantitatively that: (1) the stress intensity factor under a negative electric field is nine times as large as the stress intensity factor under a positive electric field; (2) the crack starts to grow if the stress intensity factor is higher than the fracture toughness of the material, but the stress intensity factor decreases as the crack extends and eventually results in crack arrest; (3) by reversing the electric field, the stress intensity factor is increased and crack growth resumes; and (4) this model can predict the extent of fatigue crack growth. In contrast to the conventional perception of (mechanical) fatigue, the fatigue crack growth in ferroelectrics under cyclic electric loading is a step by step cleavage process caused by a domain switching sequence that generates a cyclic driving stress field near the crack tip.

  10. Fatigue Crack Growth Analysis Models for Functionally Graded Materials

    SciTech Connect

    Dag, Serkan; Yildirim, Bora; Sabuncuoglu, Baris

    2008-02-15

    The objective of this study is to develop crack growth analysis methods for functionally graded materials (FGMs) subjected to mode I cyclic loading. The study presents finite elements based computational procedures for both two and three dimensional problems to examine fatigue crack growth in functionally graded materials. Developed methods allow the computation of crack length and generation of crack front profile for a graded medium subjected to fluctuating stresses. The results presented for an elliptical crack embedded in a functionally graded medium, illustrate the competing effects of ellipse aspect ratio and material property gradation on the fatigue crack growth behavior.

  11. Effect of Microstructural Parameters on Fatigue Crack Propagation in an API X65 Pipeline Steel

    NASA Astrophysics Data System (ADS)

    Mohtadi-Bonab, M. A.; Eskandari, M.; Ghaednia, H.; Das, S.

    2016-11-01

    In the current research, we investigate fatigue crack growth in an API X65 pipeline steel by using an Instron fatigue testing machine. To this, first the microstructure of steel was accurately investigated using scanning electron microscope. Since nonmetallic inclusions play a key role during crack propagation, the type and distribution of such inclusions were studied through the thickness of as-received X65 steel using energy-dispersive spectroscopy technique. It was found that the accumulation of such defects at the center of thickness of the pipe body was higher than in other regions. Our results showed that there were very fine oxide inclusions (1-2 µm in length) appeared throughout the cross section of X65 steel. Such inclusions were observed not at the fatigue crack path nor on both sides of the fatigue crack. However, we found that large manganese sulfide inclusions (around 20 µm in length) were associated with fatigue crack propagation. Fatigue experiments on CT specimens showed that the crack nucleated when the number of fatigue cycles was higher than 340 × 103. On fracture surfaces, crack propagation also occurred by joining the microcracks at tip of the main crack.

  12. A survey of fatigue crack growth life estimation methodologies

    NASA Astrophysics Data System (ADS)

    Akyurek, T.; Bilir, O. G.

    1992-07-01

    In this study, three fatigue crack growth life estimation methodologies are reviewed and sample calculations are made using these methodologies. Comparisons of the results with respect to the methodologies are made. Three computer codes which represent these methodologies, CRACKS IV, FAST and FATIGUE, are selected for the analyses. The estimations are also correlated to the test results.

  13. FASTRAN II - FATIGUE CRACK GROWTH STRUCTURAL ANALYSIS (UNIX VERSION)

    NASA Technical Reports Server (NTRS)

    Newman, J. C.

    1994-01-01

    Predictions of fatigue crack growth behavior can be made with the Fatigue Crack Growth Structural Analysis (FASTRAN II) computer program. As cyclic loads are applied to a selected crack configuration with an initial crack size, FASTRAN II predicts crack growth as a function of cyclic load history until either a desired crack size is reached or failure occurs. FASTRAN II is based on plasticity-induced crack-closure behavior of cracks in metallic materials and accounts for load-interaction effects, such as retardation and acceleration, under variable-amplitude loading. The closure model is based on the Dugdale model with modifications to allow plastically deformed material to be left along the crack surfaces as the crack grows. Plane stress and plane strain conditions, as well as conditions between these two, can be simulated in FASTRAN II by using a constraint factor on tensile yielding at the crack front to approximately account for three-dimensional stress states. FASTRAN II contains seventeen predefined crack configurations (standard laboratory fatigue crack growth rate specimens and many common crack configurations found in structures); and the user can define one additional crack configuration. The baseline crack growth rate properties (effective stress-intensity factor against crack growth rate) may be given in either equation or tabular form. For three-dimensional crack configurations, such as surface cracks or corner cracks at holes or notches, the fatigue crack growth rate properties may be different in the crack depth and crack length directions. Final failure of the cracked structure can be modelled with fracture toughness properties using either linear-elastic fracture mechanics (brittle materials), a two-parameter fracture criterion (brittle to ductile materials), or plastic collapse (extremely ductile materials). The crack configurations in FASTRAN II can be subjected to either constant-amplitude, variable-amplitude or spectrum loading. The applied

  14. FASTRAN II - FATIGUE CRACK GROWTH STRUCTURAL ANALYSIS (IBM PC VERSION)

    NASA Technical Reports Server (NTRS)

    Newman, J. C.

    1994-01-01

    Predictions of fatigue crack growth behavior can be made with the Fatigue Crack Growth Structural Analysis (FASTRAN II) computer program. As cyclic loads are applied to a selected crack configuration with an initial crack size, FASTRAN II predicts crack growth as a function of cyclic load history until either a desired crack size is reached or failure occurs. FASTRAN II is based on plasticity-induced crack-closure behavior of cracks in metallic materials and accounts for load-interaction effects, such as retardation and acceleration, under variable-amplitude loading. The closure model is based on the Dugdale model with modifications to allow plastically deformed material to be left along the crack surfaces as the crack grows. Plane stress and plane strain conditions, as well as conditions between these two, can be simulated in FASTRAN II by using a constraint factor on tensile yielding at the crack front to approximately account for three-dimensional stress states. FASTRAN II contains seventeen predefined crack configurations (standard laboratory fatigue crack growth rate specimens and many common crack configurations found in structures); and the user can define one additional crack configuration. The baseline crack growth rate properties (effective stress-intensity factor against crack growth rate) may be given in either equation or tabular form. For three-dimensional crack configurations, such as surface cracks or corner cracks at holes or notches, the fatigue crack growth rate properties may be different in the crack depth and crack length directions. Final failure of the cracked structure can be modelled with fracture toughness properties using either linear-elastic fracture mechanics (brittle materials), a two-parameter fracture criterion (brittle to ductile materials), or plastic collapse (extremely ductile materials). The crack configurations in FASTRAN II can be subjected to either constant-amplitude, variable-amplitude or spectrum loading. The applied

  15. FASTRAN II - FATIGUE CRACK GROWTH STRUCTURAL ANALYSIS (UNIX VERSION)

    NASA Technical Reports Server (NTRS)

    Newman, J. C.

    1994-01-01

    Predictions of fatigue crack growth behavior can be made with the Fatigue Crack Growth Structural Analysis (FASTRAN II) computer program. As cyclic loads are applied to a selected crack configuration with an initial crack size, FASTRAN II predicts crack growth as a function of cyclic load history until either a desired crack size is reached or failure occurs. FASTRAN II is based on plasticity-induced crack-closure behavior of cracks in metallic materials and accounts for load-interaction effects, such as retardation and acceleration, under variable-amplitude loading. The closure model is based on the Dugdale model with modifications to allow plastically deformed material to be left along the crack surfaces as the crack grows. Plane stress and plane strain conditions, as well as conditions between these two, can be simulated in FASTRAN II by using a constraint factor on tensile yielding at the crack front to approximately account for three-dimensional stress states. FASTRAN II contains seventeen predefined crack configurations (standard laboratory fatigue crack growth rate specimens and many common crack configurations found in structures); and the user can define one additional crack configuration. The baseline crack growth rate properties (effective stress-intensity factor against crack growth rate) may be given in either equation or tabular form. For three-dimensional crack configurations, such as surface cracks or corner cracks at holes or notches, the fatigue crack growth rate properties may be different in the crack depth and crack length directions. Final failure of the cracked structure can be modelled with fracture toughness properties using either linear-elastic fracture mechanics (brittle materials), a two-parameter fracture criterion (brittle to ductile materials), or plastic collapse (extremely ductile materials). The crack configurations in FASTRAN II can be subjected to either constant-amplitude, variable-amplitude or spectrum loading. The applied

  16. FASTRAN II - FATIGUE CRACK GROWTH STRUCTURAL ANALYSIS (IBM PC VERSION)

    NASA Technical Reports Server (NTRS)

    Newman, J. C.

    1994-01-01

    Predictions of fatigue crack growth behavior can be made with the Fatigue Crack Growth Structural Analysis (FASTRAN II) computer program. As cyclic loads are applied to a selected crack configuration with an initial crack size, FASTRAN II predicts crack growth as a function of cyclic load history until either a desired crack size is reached or failure occurs. FASTRAN II is based on plasticity-induced crack-closure behavior of cracks in metallic materials and accounts for load-interaction effects, such as retardation and acceleration, under variable-amplitude loading. The closure model is based on the Dugdale model with modifications to allow plastically deformed material to be left along the crack surfaces as the crack grows. Plane stress and plane strain conditions, as well as conditions between these two, can be simulated in FASTRAN II by using a constraint factor on tensile yielding at the crack front to approximately account for three-dimensional stress states. FASTRAN II contains seventeen predefined crack configurations (standard laboratory fatigue crack growth rate specimens and many common crack configurations found in structures); and the user can define one additional crack configuration. The baseline crack growth rate properties (effective stress-intensity factor against crack growth rate) may be given in either equation or tabular form. For three-dimensional crack configurations, such as surface cracks or corner cracks at holes or notches, the fatigue crack growth rate properties may be different in the crack depth and crack length directions. Final failure of the cracked structure can be modelled with fracture toughness properties using either linear-elastic fracture mechanics (brittle materials), a two-parameter fracture criterion (brittle to ductile materials), or plastic collapse (extremely ductile materials). The crack configurations in FASTRAN II can be subjected to either constant-amplitude, variable-amplitude or spectrum loading. The applied

  17. 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.

  18. Fatigue crack identification method based on strain amplitude changing

    NASA Astrophysics Data System (ADS)

    Guo, Tiancai; Gao, Jun; Wang, Yonghong; Xu, Youliang

    2017-09-01

    Aiming at the difficulties in identifying the location and time of crack initiation in the castings of helicopter transmission system during fatigue tests, by introducing the classification diagnostic criteria of similar failure mode to find out the similarity of fatigue crack initiation among castings, an engineering method and quantitative criterion for detecting fatigue cracks based on strain amplitude changing is proposed. This method is applied on the fatigue test of a gearbox housing, whose results indicates: during the fatigue test, the system alarms when SC strain meter reaches the quantitative criterion. The afterwards check shows that a fatigue crack less than 5mm is found at the corresponding location of SC strain meter. The test result proves that the method can provide accurate test data for strength life analysis.

  19. Steel bridge fatigue crack detection with piezoelectric wafer active sensors

    NASA Astrophysics Data System (ADS)

    Yu, Lingyu; Giurgiutiu, Victor; Ziehl, Paul; Ozevin, Didem; Pollock, Patrick

    2010-04-01

    Piezoelectric wafer active sensors (PWAS) are well known for its dual capabilities in structural health monitoring, acting as either actuators or sensors. Due to the variety of deterioration sources and locations of bridge defects, there is currently no single method that can detect and address the potential sources globally. In our research, our use of the PWAS based sensing has the novelty of implementing both passive (as acoustic emission) and active (as ultrasonic transducers) sensing with a single PWAS network. The combined schematic is using acoustic emission to detect the presence of fatigue cracks in steel bridges in their early stage since methods such as ultrasonics are unable to quantify the initial condition of crack growth since most of the fatigue life for these details is consumed while the fatigue crack is too small to be detected. Hence, combing acoustic emission with ultrasonic active sensing will strengthen the damage detection process. The integration of passive acoustic emission detection with active sensing will be a technological leap forward from the current practice of periodic and subjective visual inspection, and bridge management based primarily on history of past performance. In this study, extensive laboratory investigation is performed supported by theoretical modeling analysis. A demonstration system will be presented to show how piezoelectric wafer active sensor is used for acoustic emission. Specimens representing complex structures are tested. The results will also be compared with traditional acoustic emission transducers to identify the application barriers.

  20. Environmental Effects on Fatigue Crack Growth in 7075 Aluminum Alloy

    NASA Astrophysics Data System (ADS)

    Bonakdar, A.; Wang, F.; Williams, J. J.; Chawla, N.

    2012-08-01

    The fatigue behavior of aluminum alloys is greatly influenced by the environmental conditions. In this article, fatigue crack growth rates were measured for 7075-T651 Al alloy under ultrahigh vacuum (UHV, ~10-10 Torr), dry air, and water vapor. Standard compact tension (CT) specimens were tested along the L-T orientation under various load ratios of 0.1, 0.5, and 0.8. Fracture surfaces and crack morphologies were studied using scanning electron microscopy and crack deflection analysis. The crack growth behavior under vacuum was affected by friction and possible rewelding of crack surfaces, causing an asymmetry in the crack growth behavior, from load shedding to constant load. The enhancement of crack growth at higher moisture levels was observed and is discussed in terms of moisture decreasing friction between the crack faces. The effect of crack deflection as a function of R ratio and environment is also presented.

  1. Designing of a Testing Machine for Shear-Mode Fatigue Crack Growth

    NASA Astrophysics Data System (ADS)

    Kusaba, A.; Okazaki, S.; Endo, M.; Yanase, K.

    As recognized, flaking-type failure is one of the serious problems for railroad tracks and bearings. In essence, flaking-type failure is closely related to the growth of the shear-mode (Mode-II and Mode-III) fatigue crack. In our research group, it is demonstrated that a shear-mode fatigue crack can be reproduced for cylindrical specimens by applying the cyclic torsion in the presence of the static axial compressive stress. However, a biaxial servo-hydraulic fatigue testing machine is quite expensive to purchase and costly to maintain. The low testing speed (about 10Hz) of the testing machine further aggravates the situation. As a result, study on shear-mode fatigue crack growth is still in the nascent stage. To overcome the difficulties mentioned above, in this research activity, we developed a high-performance and cost-effective testing machine to reproduce the shear-mode fatigue crack growth by improving the available resonance-type torsion fatigue testing machine. The primary advantage of using the resonance-type torsion fatigue testing machine is cost-efficiency. In addition, the testing speed effectively can be improved, in comparison with that of a biaxial servo-hydraulic fatigue testing machine. By utilizing the newly-designed testing machine, we have demonstrated that we can successfully reproduce the shear-mode fatigue crack.

  2. Fatigue crack growth with single overload - Measurement and modeling

    NASA Technical Reports Server (NTRS)

    Davidson, D. L.; Hudak, S. J., Jr.; Dexter, R. J.

    1987-01-01

    This paper compares experiments with an analytical model of fatigue crack growth under variable amplitude. The stereoimaging technique was used to measure displacements near the tips of fatigue cracks undergoing simple variations in load amplitude-single overloads and overload/underload combinations. Measured displacements were used to compute strains, and stresses were determined from the strains. Local values of crack driving force (Delta-K effective) were determined using both locally measured opening loads and crack tip opening displacements. Experimental results were compared with simulations made for the same load variation conditions using Newman's FAST-2 model. Residual stresses caused by overloads, crack opening loads, and growth retardation periods were compared.

  3. Intergranular Strain Evolution near Fatigue Crack Tips in Polycrystalline Metals

    SciTech Connect

    Zheng, Lili; Gao, Yanfei; Lee, Sooyeol; Barabash, Rozaliya; Lee, Jinhaeng; Liaw, Peter K

    2011-01-01

    The deformation field near a steady fatigue crack includes a plastic zone in front of the crack tip and a plastic wake behind it, and the magnitude, distribution, and history of the residual strain along the crack path depend on the stress multiaxiality, material properties, and history of stress intensity factor and crack growth rate. An in situ, full-field, non-destructive measurement of lattice strain (which relies on the intergranular interactions of the inhomogeneous deformation fields in neighboring grains) by neutron diffraction techniques has been performed for the fatigue test of a Ni-based superalloy compact tension specimen. These microscopic grain level measurements provided unprecedented information on the fatigue growth mechanisms. A two-scale model is developed to predict the lattice strain evolution near fatigue crack tips in polycrystalline materials. An irreversible, hysteretic cohesive interface model is adopted to simulate a steady fatigue crack, which allows us to generate the stress/strain distribution and history near the fatigue crack tip. The continuum deformation history is used as inputs for the micromechanical analysis of lattice strain evolution using the slip-based crystal plasticity model, thus making a mechanistic connection between macro- and micro-strains. Predictions from perfect grain-boundary simulations exhibit the same lattice strain distributions as in neutron diffraction measurements, except for discrepancies near the crack tip within about one-tenth of the plastic zone size. By considering the intergranular damage, which leads to vanishing intergranular strains as damage proceeds, we find a significantly improved agreement between predicted and measured lattice strains inside the fatigue process zone. Consequently, the intergranular damage near fatigue crack tip is concluded to be responsible for fatigue crack growth.

  4. Intergranular strain evolution near fatigue crack tips in polycrystalline metals

    NASA Astrophysics Data System (ADS)

    Zheng, L. L.; Gao, Y. F.; Lee, S. Y.; Barabash, R. I.; Lee, J. H.; Liaw, P. K.

    2011-11-01

    The deformation field near a steady fatigue crack includes a plastic zone in front of the crack tip and a plastic wake behind it, and the magnitude, distribution, and history of the residual strain along the crack path depend on the stress multiaxiality, material properties, and history of stress intensity factor and crack growth rate. An in situ, full-field, non-destructive measurement of lattice strain (which relies on the intergranular interactions of the inhomogeneous deformation fields in neighboring grains) by neutron diffraction techniques has been performed for the fatigue test of a Ni-based superalloy compact tension specimen. These microscopic grain level measurements provided unprecedented information on the fatigue growth mechanisms. A two-scale model is developed to predict the lattice strain evolution near fatigue crack tips in polycrystalline materials. An irreversible, hysteretic cohesive interface model is adopted to simulate a steady fatigue crack, which allows us to generate the stress/strain distribution and history near the fatigue crack tip. The continuum deformation history is used as inputs for the micromechanical analysis of lattice strain evolution using the slip-based crystal plasticity model, thus making a mechanistic connection between macro- and micro-strains. Predictions from perfect grain-boundary simulations exhibit the same lattice strain distributions as in neutron diffraction measurements, except for discrepancies near the crack tip within about one-tenth of the plastic zone size. By considering the intergranular damage, which leads to vanishing intergranular strains as damage proceeds, we find a significantly improved agreement between predicted and measured lattice strains inside the fatigue process zone. Consequently, the intergranular damage near fatigue crack tip is concluded to be responsible for fatigue crack growth.

  5. Measurement of fatigue crack closure for negative stress ratio

    SciTech Connect

    Romeiro, F.F.J.; Domingos, C.A.; Freitas, M.J.M. de

    1999-07-01

    The concept of fatigue crack closure, introduced in the 60's by Elber, has been used to explain a wide range of positive stress ratio crack propagation results (R {ge} 0). Less attention has been given to fatigue loading for negative stress ratios (R {lt} 0). In this work the results of crack propagation tests of middle-crack tension M(T) specimens of a normalized medium carbon steel DIN Ck 45 for a wide range of stress ratios from 0.7 {ge} R {ge} {minus}3 are presented. Crack closure loads were measured with the compliance technique at test frequency, using a data acquisition system. Negative crack closure loads were found for negative stress ratios R {le} {minus}1, which can explain higher crack propagation rates and accelerations in crack growth during variable-amplitude tests where compressive loads of different stress ratios are present.

  6. Fatigue Crack Growth Monitoring Using Rayleigh-Like Waves

    NASA Astrophysics Data System (ADS)

    Masserey, B.; Fromme, P.

    2010-02-01

    A common problem in aircraft maintenance is the development of fatigue cracks at fasteners due to stress concentration. The use of Rayleigh-like waves for the monitoring of fatigue crack growth at a fastener hole in tensile, aluminum specimens is investigated. Rayleigh-like waves can propagate along the structure and have good sensitivity for the detection of small defects. They are excited in the specimen during fatigue experiments using standard wedge transducers and measured using laser interferometry. Fatigue crack growth during cyclic loading is monitored optically and the changes in the ultrasonic signal caused by crack growth are quantified. The laser measurements show a good sensitivity for the early detection of fatigue damage.

  7. Experimental study of thermodynamics propagation fatigue crack in metals

    NASA Astrophysics Data System (ADS)

    Vshivkov, A.; Iziumova, A.; Plekhov, O.

    2015-10-01

    This work is devoted to the development of an experimental method for studying the energy balance during cyclic deformation and fracture. The studies were conducted on 304 stainless steel AISE samples. The investigation of the fatigue crack propagation was carried out on flat samples with stress concentrators. The stress concentrator was three central holes. The heat flux sensor was developed based on the Seebeck effect. This sensor was used for measuring the heat dissipation power in the examined samples during the fatigue tests. The measurements showed that the rate of fatigue crack growth depends on the heat flux at the crack tip and there are two propagation mode of fatigue crack with different link between the propagation mode and heat flux from crack tip.

  8. Monitoring fatigue crack growth using nonlinear ultrasonic phased array imaging

    NASA Astrophysics Data System (ADS)

    Cheng, Jingwei; Potter, Jack N.; Croxford, Anthony J.; Drinkwater, Bruce W.

    2017-05-01

    Nonlinear imaging techniques have recently emerged which have the potential to detect material degradation and challenging defects, such as closed cracks. This paper describes an investigation into the performance of nonlinear ultrasonic imaging (NUI) for the monitoring of the early stages of fatigue crack growth. This technique, in conjunction with conventional array imaging, is applied to the periodic monitoring of steel compact tension specimens subjected to high cycle fatigue loading. The detection limits of these techniques are investigated. Their abilities to localise and detect small cracks are further quantified with the aid of micrography. The results suggest that NUI is more sensitive than conventional ultrasonic imaging to the microscale changes occurring at the early stages of failure, i.e. detectability starts c. 15% of fatigue life. In addition to early detection, the potential for NUI to deliver accurate sizing of fatigue cracks and monitor crack propagation is also presented.

  9. Experimental study of thermodynamics propagation fatigue crack in metals

    SciTech Connect

    Vshivkov, A. Iziumova, A. Plekhov, O.

    2015-10-27

    This work is devoted to the development of an experimental method for studying the energy balance during cyclic deformation and fracture. The studies were conducted on 304 stainless steel AISE samples. The investigation of the fatigue crack propagation was carried out on flat samples with stress concentrators. The stress concentrator was three central holes. The heat flux sensor was developed based on the Seebeck effect. This sensor was used for measuring the heat dissipation power in the examined samples during the fatigue tests. The measurements showed that the rate of fatigue crack growth depends on the heat flux at the crack tip and there are two propagation mode of fatigue crack with different link between the propagation mode and heat flux from crack tip.

  10. Fatigue crack growth behavior of small and large cracks in titanium alloys and intermetallics. Final report, 21 January 1991-29 January 1992

    SciTech Connect

    Ravichandran, K.S.

    1993-03-01

    This report is a compilation of results of research performed on small fatigue cracks in titanium alloys and titanium aluminide intermetallics. The principal theme underlying this investigation is the measurement of surface crack lengths and aspect ratios during the growth of three-dimensional small surface cracks in fatigue using a laser interferometric and photomicroscopic system at the Materials Behavior Branch, Materials Directorate, Wright Laboratory. It has been shown that such measurements could be made accurately on a number of candidate alloys systems comprising titanium alloys and newly developed titanium aluminide intermetallics. Fatigue crack growth rates could be accurately calculated and were correlated to data obtained on large cracks in the corresponding materials. Specific test programs, which were designed to accomplish this task and the corresponding results of the study, are categorically discussed in the following. Measurements of shapes of three dimensional surface cracks continuously during fatigue crack growth were made in a near-alpha titanium alloy, Ti-6Al-2Sn-4Zr-6Mo. Crack aspect ratio measurements are demonstrated for cracks growing from electro-discharge-machined (EDM) notches of different geometries (shallow or deep). The experimentally determined aspect ratio variations during crack growth are shown to be in good agreement with the expected variations in aspect ratio. The fatigue crack growth rates of surface cracks, after incorporating the variations in aspect ratio in the calculations, agreed with the large-crack growth data.

  11. Microstructural mechanisms of cyclic deformation, fatigue crack initiation and early crack growth.

    PubMed

    Mughrabi, Haël

    2015-03-28

    In this survey, the origin of fatigue crack initiation and damage evolution in different metallic materials is discussed with emphasis on the responsible microstructural mechanisms. After a historical introduction, the stages of cyclic deformation which precede the onset of fatigue damage are reviewed. Different types of cyclic slip irreversibilities in the bulk that eventually lead to the initiation of fatigue cracks are discussed. Examples of trans- and intercrystalline fatigue damage evolution in the low cycle, high cycle and ultrahigh cycle fatigue regimes in mono- and polycrystalline face-centred cubic and body-centred cubic metals and alloys and in different engineering materials are presented, and some microstructural models of fatigue crack initiation and early crack growth are discussed. The basic difficulties in defining the transition from the initiation to the growth of fatigue cracks are emphasized. In ultrahigh cycle fatigue at very low loading amplitudes, the initiation of fatigue cracks generally occupies a major fraction of fatigue life and is hence life controlling.

  12. Localization of Dwell Fatigue Cracks in Ti-6242 Alloy Samples

    NASA Astrophysics Data System (ADS)

    Rokhlin, S. I.; Kim, J.-Y.; Xie, B.; Yakovlev, V. A.; Zoofan, B.

    2003-03-01

    An in-situ ultrasonic guided wave technique is employed for real-time monitoring of crack initiation and evolution during dwell, cyclic fatigue and creep tests of Ti-6242 alloy samples. Ultrasonic signals are acquired continuously during the test at different levels of fatigue load using a high-speed data acquisition system. The initiation time and growth history of primary and multiple secondary cracks are assessed. Localization of the secondary cracks is performed by both the in-situ ultrasonic method and an ultrasonic immersion scanning method which we call "vertical C-scan" (VC scan). The VC scan is developed for imaging small cracks aligned normal to the fatigue sample axis. The fusion of ultrasonic and microradiographic images exhibits good agreement in crack location. Joint use of the three techniques provides location, shape, and size of the secondary cracks.

  13. Monitoring Growth of Closed Fatigue Crack Using Subharmonic Phased Array

    NASA Astrophysics Data System (ADS)

    Ohara, Y.; Endo, H.; Hashimoto, M.; Shintaku, Y.; Yamanaka, K.

    2010-02-01

    To ensure the safety and reliability of atomic power plants and airplanes, the technique of monitoring closed fatigue cracks is requisite. Here we monitored the distribution of the crack depths and closure behavior in the length direction after 48000 and 87000 fatigue cycles using subharmonic phased array for crack evaluation (SPACE). The crack depths in the subharmonic images were larger than those in the fundamental images. Specifically, the difference was larger at near the side surface than at the center. The percentage of the closed part varied with the crack growth in the specimen. In addition, we fabricated shoe for SPACE to facilitate mechanical scanning. Thus, it was demonstrated that SPACE is useful in monitoring closed fatigue crack growth.

  14. Monitoring fatigue crack growth and opening using antenna sensors

    NASA Astrophysics Data System (ADS)

    Mohammad, I.; Huang, H.

    2010-05-01

    Fatigue cracking is one of the most common failure modes of various load-bearing structures. Even though sensors of many different types have been developed for crack detection, very few can monitor crack growth with a high sensitivity. This paper presents an antenna sensor that is capable of monitoring the growth of fatigue cracks with a sub-millimeter resolution. According to microstrip patch antenna theory, the resonant frequencies of a dual-frequency patch antenna are inversely proportional to the electrical lengths of the corresponding antenna radiation modes. The presence of a crack in the ground plane or the elongation of the antenna patch due to crack opening increases the electric length, thereby causing a shift in its corresponding resonant frequency. As a result, crack propagation and opening can be monitored from the resonant frequency shifts of the patch antenna. The patch antenna's capability of monitoring crack growth was validated using fatigue testing of a compact tension specimen. The specimen preparation, sensor fabrication, and experimental procedure are presented. The experimental results demonstrated that the corresponding resonant frequency of the antenna sensor shifted linearly with crack growth. On average, 1 mm crack growth caused the antenna frequency to shift by 22.1 MHz. The orientation of the crack and the effect of crack closure on the resonant frequencies of the antenna sensor are also discussed.

  15. Effect of Sensitization on Corrosion-Fatigue Cracking in Al 5083 Alloy

    DTIC Science & Technology

    2015-01-21

    Naval Research Laboratory Washington, DC 20375-5320 NRL/MR/6355--15-9581 Effect of Sensitization on Corrosion -Fatigue Cracking in Al 5083 Alloy... Corrosion Science and Engineering Chemistry Division Approved for public release; distribution is unlimited. i REPORT DOCUMENTATION PAGE Form ApprovedOMB...area code) b. ABSTRACT c. THIS PAGE 18. NUMBER OF PAGES 17. LIMITATION OF ABSTRACT Effect of Sensitization on Corrosion -Fatigue Cracking in Al 5083

  16. Accommodating and cracking mechanisms in low-cycle fatigue

    NASA Technical Reports Server (NTRS)

    Pineau, A.

    1978-01-01

    The three main stages of fatigue life (accommodation, crack initiation and crack growth) are briefly reviewed. The cyclic behavior of annealed or predeformed face-centered cubic metals is described. Moreover, two types of alloys (Al-4-Cu and WASPALOY) are examined regarding the influence of the interactions between the precipitates and the dislocations on the cyclic behavior. Data on the percent of life to crack initiation (for a microcrack smaller than about 100 microns) are also given. Finally, experimental and theoretical results on crack growth rates in lowcycle fatigue are described.

  17. Mechanisms of fatigue damage and crack growth in advanced materials

    NASA Astrophysics Data System (ADS)

    Ritchie, Robert O.

    2001-03-01

    In terms of in-service failures, cyclic fatigue is the most prevalent form of fracture. Despite the wealth of information on fatigue failures in traditional structural materials such as (ductile) metals and alloys, far less is understood about the susceptibility of the newer advanced materials, such as (brittle) intermetallics, ceramics and their composites. In this presentation, the mechanics and mechanisms of fatigue damage and crack propagation are examined with particular emphasis on the similarities and differences between cyclic crack growth in ductile metallic materials, and corresponding behavior in the more brittle advanced materials. This is achieved by considering the process of subcritical crack growth as a mutual competition between intrinsic mechanisms of microstructural damage ahead of the crack tip, which promote crack growth, and extrinsic mechanisms of crack-tip shielding behind the tip, which impede it. This approach is shown to be important for the understanding of the structural fatigue properties of advanced materials, such as monolithic and composite ceramics, and a range of intermetallics (e.g., TiAl, MoSi2, Nb3Al), as the mechanisms of fatigue in these brittle materials are conceptually distinct from that associated with the well known metal fatigue. Examples of the application and life-prediction methodologies for such materials in fatigue-critical situations will be given from the aerospace and bioengineering industries.

  18. Fatigue crack propagation in aluminum-lithium alloys

    NASA Technical Reports Server (NTRS)

    Rao, K. T. V.; Ritchie, R. O.; Piascik, R. S.; Gangloff, R. P.

    1989-01-01

    The principal mechanisms which govern the fatigue crack propagation resistance of aluminum-lithium alloys are investigated, with emphasis on their behavior in controlled gaseous and aqueous environments. Extensive data describe the growth kinetics of fatigue cracks in ingot metallurgy Al-Li alloys 2090, 2091, 8090, and 8091 and in powder metallurgy alloys exposed to moist air. Results are compared with data for traditional aluminum alloys 2024, 2124, 2618, 7075, and 7150. Crack growth is found to be dominated by shielding from tortuous crack paths and resultant asperity wedging. Beneficial shielding is minimized for small cracks, for high stress ratios, and for certain loading spectra. While water vapor and aqueous chloride environments enhance crack propagation, Al-Li-Cu alloys behave similarly to 2000-series aluminum alloys. Cracking in water vapor is controlled by hydrogen embrittlement, with surface films having little influence on cyclic plasticity.

  19. 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.

  20. Probabilistic Aspects of the Growth and Detection of Fatigue Cracks

    NASA Astrophysics Data System (ADS)

    Cohen, Moshe L.

    For engineering structures, fatigue, and more specifically fatigue life prediction, is a problem of great interest. The first part of this dissertation focuses on the importance of probabilistic methods in structural health management in general, and how they can be applied to fatigue failure prognosis. Paris' law, using measured values of the parameters in the law, has been used to forecast fatigue crack growth from an assumed initial probability distribution of crack lengths, represented by a truncated lognormal distribution. The evolution of this distribution with the number of cycles has been determined, and the probability of the existence of a crack larger than an undesirable value, but smaller than the value where failure may occur, has been calculated. In addition, inspections have been modeled using three typical probability of detection curves, and the effect of an inspection has been evaluated. The probability of detection concept has also been extended using a Bayesian approach to include the effect of the number of elapsed cycles and the stress range of each cycle. All statistics considered in this dissertation have been evaluated for a surface-breaking crack in a half-space and a cracked rivet hole in a lap joint, both under cyclic tensile loading. The second part of this dissertation focuses on an aspect of the diagnosis of fatigue cracks. The acoustic emission from fatigue crack growth has been calculated using the reciprocity relation, again for a surface-breaking crack in a half-space and a cracked rivet hole in a lap joint. This result, which is a stochastic quantity because the amount of crack growth is stochastic, is used to calculate the probability of detection of an acoustic wave emitted by the crack growth for these cases.

  1. Monitoring of fatigue crack under complex environment using guided waves

    NASA Astrophysics Data System (ADS)

    Tang, Jianfei; Yan, Gang; Xu, Xiwu

    2011-11-01

    This paper presents an experimental study on monitoring of fatigue crack under complex environment using guided waves. An experimental set-up consisting of an electrical oven, a MTS testing machine and a monitoring system is established to perform the study. First, the combined effects of temperature, load and vibration on the propagation of guided waves in metallic structure is studied. Then, a statistical approach is proposed to detect fatigue crack under these combined effects. Damage feature is extracted after the guided wave signals are processed by Fourier transform. A Monte Carlo procedure is employed to estimate the probability density functions of the feature before and after cracking, respectively. By comparing the probability density functions, the probability of existence of fatigue crack is determined. Experimental study on a fatigue coupon under combined effects of temperature, load and vibration is conducted to demonstrate the effectiveness of the proposed method.

  2. Monitoring of fatigue crack under complex environment using guided waves

    NASA Astrophysics Data System (ADS)

    Tang, Jianfei; Yan, Gang; Xu, Xiwu

    2012-04-01

    This paper presents an experimental study on monitoring of fatigue crack under complex environment using guided waves. An experimental set-up consisting of an electrical oven, a MTS testing machine and a monitoring system is established to perform the study. First, the combined effects of temperature, load and vibration on the propagation of guided waves in metallic structure is studied. Then, a statistical approach is proposed to detect fatigue crack under these combined effects. Damage feature is extracted after the guided wave signals are processed by Fourier transform. A Monte Carlo procedure is employed to estimate the probability density functions of the feature before and after cracking, respectively. By comparing the probability density functions, the probability of existence of fatigue crack is determined. Experimental study on a fatigue coupon under combined effects of temperature, load and vibration is conducted to demonstrate the effectiveness of the proposed method.

  3. High speed thin plate fatigue crack monitor

    NASA Technical Reports Server (NTRS)

    Wincheski, Buzz A. (Inventor); Heyman, Joseph S. (Inventor); Namkung, Min (Inventor); Fulton, James P. (Inventor)

    1996-01-01

    A device and method are provided which non-destructively detect crack length and crack geometry in thin metallic plates. A non-contacting vibration apparatus produces resonant vibrations without introducing extraneous noise. Resulting resonant vibration shifts in cracked plates are correlated to known crack length in plates with similar resonant vibration shifts. In addition, acoustic emissions of cracks at resonance frequencies are correlated to acoustic emissions from known crack geometries.

  4. Analysis of Fatigue Crack Paths in Cold Drawn Pearlitic Steel.

    PubMed

    Toribio, Jesús; González, Beatriz; Matos, Juan-Carlos

    2015-11-04

    In this paper, a fracto-metallographic analysis was performed on the cracked specimens of cold drawn pearlitic steel subjected to fatigue tests. Fatigue cracks are transcollonial and exhibit a preference for fracturing pearlitic lamellae, with non-uniform crack opening displacement values, micro-discontinuities, branchings, bifurcations and frequent local deflections that create microstructural roughness. At the micro-level, the cold drawn pearlitic steel exhibits higher micro-roughness than the hot rolled bar (this is a consequence of the manufacturing process by cold drawing), so that the actual fractured surface in the cold drawn wire is greater than that in the hot rolled bar, due to the fact that the crack deflection events are more frequent and with higher angle in the former (the heavily drawn prestressing steel wire). These findings show the relevant role on the manufacturing process by cold drawing in the fatigue crack propagation in pearlitic steel.

  5. 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

  6. Continuous fatigue crack monitoring of bridges: Long-Term Electrochemical Fatigue Sensor (LTEFS)

    NASA Astrophysics Data System (ADS)

    Moshier, Monty A.; Nelson, Levi; Brinkerhoff, Ryan; Miceli, Marybeth

    2016-04-01

    Fatigue cracks in steel bridges degrade the load-carrying capacity of these structures. Fatigue damage accumulation caused by the repetitive loading of everyday truck traffic can cause small fatigue cracks initiate. Understanding the growth of these fatigue cracks is critical to the safety and reliability of our transportation infrastructure. However, modeling fatigue in bridges is difficult due to the nature of the loading and variations in connection integrity. When fatigue cracks reach critical lengths failures occur causing partial or full closures, emergency repairs, and even full structural failure. Given the aging US highway and the trend towards asset management and life extension, the need for reliable, cost effective sensors and monitoring technologies to alert bridge owners when fatigue cracks are growing is higher than ever. In this study, an innovative Long-Term Electrochemical Fatigue Sensor (LTEFS) has been developed and introduced to meet the growing NDT marketplace demand for sensors that have the ability to continuously monitor fatigue cracks. The performance of the LTEFS has been studied in the laboratory and in the field. Data was collected using machined specimens with different lengths of naturally initiated fatigue cracks, applied stress levels, applied stress ratios, and for both sinusoidal and real-life bridge spectrum type loading. The laboratory data was evaluated and used to develop an empirically based algorithm used for crack detection. Additionally, beta-tests on a real bridge structure has been completed. These studies have conclusively demonstrated that LTEFS holds great potential for long-term monitoring of fatigue cracks in steel structures

  7. Monitoring of fatigue crack growth using guided ultrasonic waves

    NASA Astrophysics Data System (ADS)

    Masserey, B.; Kostson, E.; Fromme, P.

    2010-04-01

    Varying loading conditions of aircraft structures result in stress concentration at fastener holes, where multi layer components are connected, possibly leading to the development of fatigue cracks. Guided ultrasonic waves propagating along a structure allow in principle for the efficient non-destructive testing of large plate-like structures, such as aircraft wings. This contribution presents a study of the detection and monitoring of fatigue crack growth using both low frequency and higher frequency guided ultrasonic wave modes. Two types of structures were used, single layer aluminum tensile specimens, and multi layer structures consisting of two adhesively bonded aluminum plate-strips. Fatigue experiments were carried out and it was shown that fatigue crack detection and growth monitoring at a fastener hole during cyclic loading using both guided wave types is possible. The sensitivity and repeatability of the measurements were ascertained, having the potential for fatigue crack detection at critical and difficult to access fastener locations. Good agreement was observed between the experimental results and predictions from full three-dimensional numerical simulations of the scattering of the low frequency guided ultrasonic wave at the fastener hole and crack. The robustness of the methodology for practical in-situ ultrasonic monitoring of fatigue crack growth is discussed.

  8. Deformation fields near a steady fatigue crack with anisotropic plasticity

    SciTech Connect

    Gao, Yanfei

    2015-11-30

    In this work, from finite element simulations based on an irreversible, hysteretic cohesive interface model, a steady fatigue crack can be realized if the crack extension exceeds about twice the plastic zone size, and both the crack increment per loading cycle and the crack bridging zone size are smaller than the plastic zone size. The corresponding deformation fields develop a plastic wake behind the crack tip and a compressive residual stress field ahead of the crack tip. In addition, the Hill’s plasticity model is used to study the role of plastic anisotropy on the retardation of fatigue crack growth and the elastic strain fields. It is found that for Mode-I cyclic loading, an enhanced yield stress in directions that are inclined from the crack plane will lead to slower crack growth rate, but this retardation is insignificant for typical degrees of plastic anisotropy. Furthermore, these results provide key inputs for future comparisons to neutron and synchrotron diffraction measurements that provide full-field lattice strain mapping near fracture and fatigue crack tips, especially in textured materials such as wrought or rolled Mg alloys.

  9. Deformation fields near a steady fatigue crack with anisotropic plasticity

    DOE PAGES

    Gao, Yanfei

    2015-11-30

    In this work, from finite element simulations based on an irreversible, hysteretic cohesive interface model, a steady fatigue crack can be realized if the crack extension exceeds about twice the plastic zone size, and both the crack increment per loading cycle and the crack bridging zone size are smaller than the plastic zone size. The corresponding deformation fields develop a plastic wake behind the crack tip and a compressive residual stress field ahead of the crack tip. In addition, the Hill’s plasticity model is used to study the role of plastic anisotropy on the retardation of fatigue crack growth andmore » the elastic strain fields. It is found that for Mode-I cyclic loading, an enhanced yield stress in directions that are inclined from the crack plane will lead to slower crack growth rate, but this retardation is insignificant for typical degrees of plastic anisotropy. Furthermore, these results provide key inputs for future comparisons to neutron and synchrotron diffraction measurements that provide full-field lattice strain mapping near fracture and fatigue crack tips, especially in textured materials such as wrought or rolled Mg alloys.« less

  10. Smart sensing skin for detection and localization of fatigue cracks

    NASA Astrophysics Data System (ADS)

    Kharroub, Sari; Laflamme, Simon; Song, Chunhui; Qiao, Daji; Phares, Brent; Li, Jian

    2015-06-01

    Fatigue cracks on steel components may have strong consequences on the structure’s serviceability and strength. Their detection and localization is a difficult task. Existing technologies enabling structural health monitoring have a complex link signal-to-damage or have economic barriers impeding large-scale deployment. A solution is to develop sensing methods that are inexpensive, scalable, with signals that can directly relate to damage. The authors have recently proposed a smart sensing skin for structural health monitoring applications to mesosystems. The sensor is a thin film soft elastomeric capacitor (SEC) that transduces strain into a measurable change in capacitance. Arranged in a network configuration, the SEC would have the capacity to detect and localize damage by detecting local deformation over a global surface, analogous to biological skin. In this paper, the performance of the SEC at detecting and localizing fatigue cracks in steel structures is investigated. Fatigue cracks are induced in steel specimens equipped with SECs, and data measured continuously. Test results show that the fatigue crack can be detected at an early stage. The smallest detectable crack length and width are 27.2 and 0.254 mm, respectively, and the average detectable crack length and width are 29.8 and 0.432 mm, respectively. Results also show that, when used in a network configuration, only the sensor located over the formed fatigue crack detect the damage, thus validating the capacity of the SEC at damage localization.

  11. Effect of Crack Closure on Ultrasonic Detection of Fatigue Cracks at Fastener Holes

    NASA Astrophysics Data System (ADS)

    Bowles, S. J.; Harding, C. A.; Hugo, G. R.

    2009-03-01

    The ultrasonic response from closed fatigue cracks grown in aluminium alloy specimens using a representative aircraft spectrum loading has been characterised as a function of tensile applied load using pulse-echo 45° shear-wave ultrasonic C-scans with focused immersion transducers. Observed trends with crack size and applied load are described and compared to results for artificial machined defects. The results demonstrate that crack closure significantly reduces the ultrasonic response compared to open cracks or machined defects.

  12. Interfacial Crack Arrest in Sandwich Panels with Embedded Crack Stoppers Subjected to Fatigue Loading

    NASA Astrophysics Data System (ADS)

    Martakos, G.; Andreasen, J. H.; Berggreen, C.; Thomsen, O. T.

    2017-02-01

    A novel crack arresting device has been implemented in sandwich panels and tested using a special rig to apply out-of-plane loading on the sandwich panel face-sheets. Fatigue crack propagation was induced in the face-core interface of the sandwich panels which met the crack arrester. The effect of the embedded crack arresters was evaluated in terms of the achieved enhancement of the damage tolerance of the tested sandwich panels. A finite element (FE) model of the experimental setup was used for predicting propagation rates and direction of the crack growth. The FE simulation was based on the adoption of linear fracture mechanics and a fatigue propagation law (i.e. Paris law) to predict the residual fatigue life-time and behaviour of the test specimens. Finally, a comparison between the experimental results and the numerical simulations was made to validate the numerical predictions as well as the overall performance of the crack arresters.

  13. Microstructure-sensitive small fatigue crack growth assessment. Effect of strain ratio multiaxial strain state and geometric discontinuities

    SciTech Connect

    Castelluccio, Gustavo M.; McDowell, David L.

    2015-09-16

    Fatigue crack initiation in the high cycle fatigue regime is strongly influenced by microstructural features. Research efforts have usually focused on predicting fatigue resistance against crack incubation without considering the early fatigue crack growth after encountering the first grain boundary. However, a significant fraction of the variability of the total fatigue life can be attributed to growth of small cracks as they encounter the first few grain boundaries, rather than crack formation within the first grain. Our paper builds on the framework previously developed by the authors to assess microstructure-sensitive small fatigue crack formation and early growth under complex loading conditions. Moreover, the scheme employs finite element simulations that explicitly render grains and crystallographic directions along with simulation of microstructurally small fatigue crack growth from grain to grain. The methodology employs a crystal plasticity algorithm in ABAQUS that was previously calibrated to study fatigue crack initiation in RR1000 Ni-base superalloy. Our work present simulations with non-zero applied mean strains and geometric discontinuities that were not previously considered for calibration. Results exhibit trends similar to those found in experiments for multiple metallic materials, conveying a consistent physical description of fatigue damage phenomena.

  14. Microstructure-sensitive small fatigue crack growth assessment. Effect of strain ratio multiaxial strain state and geometric discontinuities

    DOE PAGES

    Castelluccio, Gustavo M.; McDowell, David L.

    2015-09-16

    Fatigue crack initiation in the high cycle fatigue regime is strongly influenced by microstructural features. Research efforts have usually focused on predicting fatigue resistance against crack incubation without considering the early fatigue crack growth after encountering the first grain boundary. However, a significant fraction of the variability of the total fatigue life can be attributed to growth of small cracks as they encounter the first few grain boundaries, rather than crack formation within the first grain. Our paper builds on the framework previously developed by the authors to assess microstructure-sensitive small fatigue crack formation and early growth under complex loadingmore » conditions. Moreover, the scheme employs finite element simulations that explicitly render grains and crystallographic directions along with simulation of microstructurally small fatigue crack growth from grain to grain. The methodology employs a crystal plasticity algorithm in ABAQUS that was previously calibrated to study fatigue crack initiation in RR1000 Ni-base superalloy. Our work present simulations with non-zero applied mean strains and geometric discontinuities that were not previously considered for calibration. Results exhibit trends similar to those found in experiments for multiple metallic materials, conveying a consistent physical description of fatigue damage phenomena.« less

  15. Fatigue and fatigue crack growth processes in hard tissues: The importance of age and surface integrity

    NASA Astrophysics Data System (ADS)

    Majd, Hessam

    With the progressive increase in partially and fully dentate seniors, fracture has become an increasingly common form of restored tooth failure. Dentin undergoes progressive changes in microstructure with patient age, and studies are now suggesting that there is a reduction in fatigue strength and fatigue crack growth resistance of this tissue. This dissertation explores aging of dentin, the influence of flaws that are introduced during restorative processes on the fatigue properties of dentin, and proposes models for characterizing the damage initiation and growth process during fatigue of dentin. Results from this investigation show that the fatigue crack growth properties (Paris Law parameters (C, m) andDeltaKth) of human dentin undergo the most significant changes at a patient age of 42 years. Based on the fatigue crack growth responses, three age groups were established including young (age≤33), aged (34≤age ≤49) and old (50≤age) patients for further analysis. There were significant differences in the initiation and growth behavior between the tissues of patients from the three age groups. With regards to the influence of restorative processes, there was no influence on the quasi-static responses of dentin. However, the endurance limit of dentin treated with the dental burs (28 MPa) and abrasive air jet (35 MPa) were approximately 36% and 20% lower than that of the control (44 MPa), respectively. Both cutting processes caused a significant reduction (p≤0.0001) in fatigue strength. An accumulative damage model was developed to characterize fatigue of the control and bur treated dentin as well as provide a model for fatigue life prediction. The damage models were derived as a function of number of loading cycles (N), and ratio of applied stress to ultimate strength (r). The developed models provide estimations for the initial state of damage, the state of damage during the life, as well as the damage accumulation rate for cyclic loading of dentin

  16. The algorithm of crack and crack tip coordinates detection in optical images during fatigue test

    NASA Astrophysics Data System (ADS)

    Panin, S. V.; Chemezov, V. O.; Lyubutin, P. S.; Titkov, V. V.

    2017-02-01

    An algorithm of crack detection during fatigue testing of materials, designed to automate the process of cyclic loading and tracking the crack tip, is proposed and tested. The ultimate goal of the study is aimed at controlling the displacements of the optical system with regard to the specimen under fatigue loading to ensure observation of the ‘area of interest’. It is shown that the image region that contains the crack may be detected and positioned with an average error of 1.93%. In terms of determining the crack tip position, the algorithm provides the accuracy of its localization with the average error value of 56 pixels.

  17. 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.

  18. Measurement and analysis of critical crack tip processes during fatigue crack growth

    NASA Technical Reports Server (NTRS)

    Davidson, D. L.; Hudak, S. J.; Dexter, R. J.

    1985-01-01

    The mechanics of fatigue crack growth under constant-amplitudes and variable-amplitude loading were examined. Critical loading histories involving relatively simple overload and overload/underload cycles were studied to provide a basic understanding of the underlying physical processes controlling crack growth. The material used for this study was 7091-T7E69, a powder metallurgy aluminum alloy. Local crack-tip parameters were measured at various times before, during, and after the overloads, these include crack-tip opening loads and displacements, and crack-tip strain fields. The latter were useed, in combination with the materials cyclic and monotonic stress-strain properties, to compute crack-tip residual stresses. The experimental results are also compared with analytical predictions obtained using the FAST-2 computer code. The sensitivity of the analytical model to constant-amplitude fatigue crack growth rate properties and to through-thickness constrain are studied.

  19. Nonlinear spectral correlation for fatigue crack detection under noisy environments

    NASA Astrophysics Data System (ADS)

    Liu, Peipei; Sohn, Hoon; Jeon, Ikgeun

    2017-07-01

    When ultrasonic waves at two distinct frequencies are applied to a structure with a fatigue crack, crack-induced nonlinearity creates nonlinear ultrasonic modulations at the sum and difference of the two input frequencies. The amplitude of the nonlinear modulation components is typically one or two orders of magnitude smaller than that of the primary linear components. Therefore, the modulation components can be easily buried under noise levels and it becomes difficult to extract the nonlinear modulation components under noisy environments using a conventional spectral density function. In this study, nonlinear spectral correlation, which calculates the spectral correlation between nonlinear modulation components, is proposed to isolate the nonlinear modulation components from noisy environments and used for fatigue crack detection. The proposed nonlinear spectral correlation offers the following benefits: (1) Stationary noises have little effect on nonlinear spectral correlation; (2) By using a wideband high-frequency input and a single low-frequency input, the contrast of nonlinear spectral correlation between damage and intact conditions can be enhanced; and (3) The test efficiency can be also improved via reducing the data collection time. Validation tests are performed on aluminum plates and scaled steel shafts with real fatigue cracks. The experimental results demonstrate that the proposed nonlinear spectral correlation owns a higher sensitivity to fatigue crack than the classical nonlinear coefficient estimated from the spectral density function, and the usage of nonlinear spectral correlation allows the detection of fatigue crack even using noncontact air-coupled transducers with a low signal-to-noise ratio.

  20. Modeling growth of fatigue cracks which originate at rivet holes

    NASA Technical Reports Server (NTRS)

    Mear, Mark E.

    1989-01-01

    When a structural component is subjected to repeated stress cycles, it can fail at stresses which are well below the tensile strength of the material. The processes leading to this failure are termed fatigue. Instances of fatigue failure in aircraft have become an increasing concern. The crack leading to failure often originate at rivet holes and then grow in response to stress cycles which occur during the operation of the aircraft. A necessary step to preventing failures in todays fleet of aging aircraft is to increase the frequency and quality of inspections; steps were already taken in this direction. There is also a need for modeling of fatigue crack growth in the aircraft structures so that improvements in design can be established and predictions of the life of the components can be made. The purpose is to provide a method to accurately predict the growth of fatigue cracks and to use this method to make predictions about the life of aircraft structural components. The method relies on the formulation and numerical solution of a singular integral equation(s) for an arbitrarily shaped crack(s) which propagate in response to the applied loading. Of special interest to the aging aircraft studies are cracks which originate at circular holes (i.e., rivet holes), but other crack geometries can be treated equally as well.

  1. Plane strain crack growth models for fatigue crack growth life predictions

    SciTech Connect

    Bloom, J.M.; Daniewicz, S.R.; Hechmer, J.L.

    1996-02-01

    Experimental data and analytical models have shown that a growing fatigue crack produces a plastic wake. This, in turn, leads to residual compressive stresses acting over the crack faces during the unloading portion of the fatigue cycle. This crack closure effect results in an applied stress intensity factor during unloading which is greater than that associated with the K{sub min}, thus producing a crack-driving force which is less than {Delta}K = K{sub max} {minus} K{sub min}. Life predictions which do not account for this crack closure effect give inaccurate life estimates, especially for fully reversed loadings. This paper discusses the development of a crack closure expression for the 4-point bend specimen using numerical results obtained from a modified strip-yield model. Data from tests of eight 4-point bend specimens were used to estimate the specimen constraint factor (stress triaxiality effect). The constraint factor was then used in the estimation of the crack opening stresses for each of the bend tests. The numerically estimated crack opening stresses were used to develop an effective stress intensity factor range, {Delta}K{sub eff}. The resulting crack growth rate data when plotted versus {Delta}K{sub eff} resulted in a material fatigue crack growth rate property curve independent of test specimen type, stress level, and R-ratio. Fatigue crack growth rate data from center-cracked panels using Newman`s crack closure model, from compact specimens using Eason`s R-ratio expression, and from bend specimens using the model discussed in this paper are all shown to fall along the same straight line (on log-log paper) when plotted versus {Delta}K{sub eff}, even though crack closure differs for each specimen type.

  2. Fatigue crack damage detection using subharmonic component with nonlinear boundary condition

    NASA Astrophysics Data System (ADS)

    Wu, Weiliang; Shen, Yanfeng; Qu, Wenzhong; Xiao, Li; Giurgiutiu, Victor

    2015-03-01

    In recent years, researchers have focused on structural health monitoring (SHM) and damage detection techniques using nonlinear vibration and nonlinear ultrasonic methods. Fatigue cracks may exhibit contact acoustic nonlinearity (CAN) with distinctive features such as superharmonics and subharmonics in the power spectrum of the sensing signals. However, challenges have been noticed in the practical applications of the harmonic methods. For instance, superharmonics can also be generated by the piezoelectric transducers and the electronic equipment; super/subharmonics may also stem from the nonlinear boundary conditions such as structural fixtures and joints. It is hard to tell whether the nonlinear features come from the structural damage or the intrinsic nonlinear boundary conditions. The objective of this paper is to demonstrate the application of nonlinear ultrasonic subharmonic method for detecting fatigue cracks with nonlinear boundary conditions. The fatigue crack was qualitatively modeled as a single-degree-of-freedom (SDOF) system with non-classical hysteretic nonlinear interface forces at both sides of the crack surfaces. The threshold of subharmonic generation was studied, and the influence of crack interface parameters on the subharmonic resonance condition was investigated. The different threshold behaviors between the nonlinear boundary condition and the fatigue crack was found, which can be used to distinguish the source of nonlinear subharmonic features. To evaluate the proposed method, experiments of an aluminum plate with a fatigue crack were conducted to quantitatively verify the subharmonic resonance range. Two surface-bonded piezoelectric transducers were used to generate and receive ultrasonic wave signals. The fatigue damage was characterized in terms of a subharmonic damage index. The experimental results demonstrated that the subharmonic component of the sensing signal can be used to detect the fatigue crack and further distinguish it from

  3. Fatigue crack damage detection using subharmonic component with nonlinear boundary condition

    SciTech Connect

    Wu, Weiliang Qu, Wenzhong E-mail: xiaoli6401@126.com; Xiao, Li E-mail: xiaoli6401@126.com; Shen, Yanfeng Giurgiutiu, Victor

    2015-03-31

    In recent years, researchers have focused on structural health monitoring (SHM) and damage detection techniques using nonlinear vibration and nonlinear ultrasonic methods. Fatigue cracks may exhibit contact acoustic nonlinearity (CAN) with distinctive features such as superharmonics and subharmonics in the power spectrum of the sensing signals. However, challenges have been noticed in the practical applications of the harmonic methods. For instance, superharmonics can also be generated by the piezoelectric transducers and the electronic equipment; super/subharmonics may also stem from the nonlinear boundary conditions such as structural fixtures and joints. It is hard to tell whether the nonlinear features come from the structural damage or the intrinsic nonlinear boundary conditions. The objective of this paper is to demonstrate the application of nonlinear ultrasonic subharmonic method for detecting fatigue cracks with nonlinear boundary conditions. The fatigue crack was qualitatively modeled as a single-degree-of-freedom (SDOF) system with non-classical hysteretic nonlinear interface forces at both sides of the crack surfaces. The threshold of subharmonic generation was studied, and the influence of crack interface parameters on the subharmonic resonance condition was investigated. The different threshold behaviors between the nonlinear boundary condition and the fatigue crack was found, which can be used to distinguish the source of nonlinear subharmonic features. To evaluate the proposed method, experiments of an aluminum plate with a fatigue crack were conducted to quantitatively verify the subharmonic resonance range. Two surface-bonded piezoelectric transducers were used to generate and receive ultrasonic wave signals. The fatigue damage was characterized in terms of a subharmonic damage index. The experimental results demonstrated that the subharmonic component of the sensing signal can be used to detect the fatigue crack and further distinguish it from

  4. 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.

  5. On the interaction of ultrasound with cracks: Applications to fatigue crack growth

    NASA Technical Reports Server (NTRS)

    Buck, O.; Thompson, R. B.; Rehbein, D. K.

    1986-01-01

    Partial contact of two rough fatigue crack surfaces leads to transmission and diffraction of an acoustic signal at those contacts. Recent experimental and theoretical efforts to understand and quantify such contact in greater detail are discussed. The objective is to develop an understanding of the closure phenomenon and its application to the interpretation of fatigue data, in particular the R-ratio, spike overload/underload and threshold effects on crack propagation.

  6. Simulating Fatigue Crack Growth in Spiral Bevel Pinion

    NASA Technical Reports Server (NTRS)

    Ural, Ani; Wawrzynek, Paul A.; Ingraffe, Anthony R.

    2003-01-01

    This project investigates computational modeling of fatigue crack growth in spiral bevel gears. Current work is a continuation of the previous efforts made to use the Boundary Element Method (BEM) to simulate tooth-bending fatigue failure in spiral bevel gears. This report summarizes new results predicting crack trajectory and fatigue life for a spiral bevel pinion using the Finite Element Method (FEM). Predicting crack trajectories is important in determining the failure mode of a gear. Cracks propagating through the rim may result in catastrophic failure, whereas the gear may remain intact if one tooth fails and this may allow for early detection of failure. Being able to predict crack trajectories is insightful for the designer. However, predicting growth of three-dimensional arbitrary cracks is complicated due to the difficulty of creating three-dimensional models, the computing power required, and absence of closed- form solutions of the problem. Another focus of this project was performing three-dimensional contact analysis of a spiral bevel gear set incorporating cracks. These analyses were significant in determining the influence of change of tooth flexibility due to crack growth on the magnitude and location of contact loads. This is an important concern since change in contact loads might lead to differences in SIFs and therefore result in alteration of the crack trajectory. Contact analyses performed in this report showed the expected trend of decreasing tooth loads carried by the cracked tooth with increasing crack length. Decrease in tooth loads lead to differences between SIFs extracted from finite element contact analysis and finite element analysis with Hertz contact loads. This effect became more pronounced as the crack grew.

  7. Fatigue crack growth in unidirectional metal matrix composite

    NASA Technical Reports Server (NTRS)

    Ghosn, Louis J.; Telesman, Jack; Kantzos, Peter

    1990-01-01

    The weight function method was used to determine the effective stress intensity factor and the crack opening profile for a fatigue tested composite which exhibited fiber bridging. The bridging mechanism was modeled using two approaches; the crack closure approach and the shear lag approach. The numerically determined stress intensity factor values from both methods were compared and correlated with the experimentally obtained crack growth rates for SiC/Ti-15-3 (0)(sub 8) oriented composites. The near crack tip opening profile was also determined for both methods and compared with the experimentally obtained measurements.

  8. Fatigue crack growth in unidirectional metal matrix composite

    NASA Technical Reports Server (NTRS)

    Ghosn, L. J.; Telesman, J.; Kantzos, P.

    1990-01-01

    The weight function method was used to determine the effective stress intensity factor and the crack opening profile for a fatigue tested composite which exhibited fiber bridging. The bridging mechanism was modeled using two approaches; the crack closure approach and the shear lag approach. The numerically determined stress intensity factor values from both methods were compared and correlated with the experimentally obtained crack growth rates for SiC/Ti-15-3 (0) (sub 8) oriented composites. The near crack tip opening profile was also determined for both methods and compared with the experimentally obtained measurements.

  9. A model for the formation of fatigue striations and its relationship with small fatigue crack growth in an aluminum alloy

    SciTech Connect

    Shyam, Amit; Lara-Curzio, Edgar

    2010-01-01

    The fatigue crack growth process involves damage accumulation and crack extension. The two sub-processes that lead to fatigue crack extension were quantified separately in a recent model for small fatigue crack growth applicable to engineering alloys. Here, we report the results of an experimental investigation to assess the assumptions of that model. The fatigue striation formation in an aluminum alloy is modeled and it is verified that the number of cycles required for striation formation is related to the cyclic crack tip opening displacement and that the striation spacing is related to the monotonic crack tip displacement. It is demonstrated that extensive cyclic crack tip plasticity in the aluminum alloy causes a reduction in the magnitude of the slope of the fatigue crack propagation curves. The implications of these results on the fatigue crack propagation lifetime calculations are identified.

  10. Simulating Fatigue Cracks in Healthy Beam Models for Improved Identification

    NASA Astrophysics Data System (ADS)

    Cooley, Phillip E.; Slater, Joseph C.; Shiryayev, Oleg V.

    There is need for an automated Structural Health Monitoring (SHM) system capable of fatigue crack detection in bladed disks as current methods are slow, costly and imperfect. Prerequisite for such a system is a fast method for producing the necessary data libraries. In an effort to develop such a method for simulating nonlinear structural response, fatigue cracks in beams have been modeled by modifying the inputs to the structure rather than the structure model itself to produce a closed-form solution for the total response. Although the time savings are enormous, and the method has proven capable of correctly identifying fatigue cracks over an effective region in data generated by a more traditional bilinear model, additional refinement is needed. The method for calibrating the signature profiles used to identify fatigue cracks between methods is revisited and successfully improved. Existing signature profiles for the bilinear model are reproduced with higher resolution, and new features are observed. Different boundary conditions are evaluated with the new method and compared to published results. Although similar, there are still discrepancies that remain and will need to be investigated. Overall, the proposed method for modeling and identifying fatigue cracks in beams has been improved, but will require validation against physical experiments before being used on more complicated structures such as bladed disks.

  11. Consolidation of fatigue and fatigue-crack-propagation data for design use

    NASA Technical Reports Server (NTRS)

    Rice, R. C.; Davies, K. B.; Jaske, C. E.; Feddersen, C. E.

    1975-01-01

    Analytical methods developed for consolidation of fatigue and fatigue-crack-propagation data for use in design of metallic aerospace structural components are evaluated. A comprehensive file of data on 2024 and 7075 aluminums, Ti-6Al-4V alloy, and 300M steel was established by obtaining information from both published literature and reports furnished by aerospace companies. Analyses are restricted to information obtained from constant-amplitude load or strain cycling of specimens in air at room temperature. Both fatigue and fatigue-crack-propagation data are analyzed on a statistical basis using a least-squares regression approach. For fatigue, an equivalent strain parameter is used to account for mean stress or stress ratio effects and is treated as the independent variable; cyclic fatigue life is considered to be the dependent variable. An effective stress-intensity factor is used to account for the effect of load ratio on fatigue-crack-propagation and treated as the independent variable. In this latter case, crack-growth rate is considered to be the dependent variable. A two term power function is used to relate equivalent strain to fatigue life, and an arc-hyperbolic-tangent function is used to relate effective stress intensity to crack-growth rate.

  12. Fatigue crack propagation in self-assembling nanocomposites

    SciTech Connect

    Klingler, Andreas; Wetzel, Bernd

    2016-05-18

    Self-assembling block-copolymers allow the easy manufacturing of nanocomposites due to the thermodynamically driven in situ formation of nanosized phases in thermosetting resins during the curing process. Complex mechanical dispersion processes can be avoided. The current study investigates the effect of a block-copolymer on the fatigue crack propagation resistance of a cycloaliphatic amine cured epoxy resin. It was found that a small amount of MAM triblock-copolymer significantly increases the resistance to fatigue crack propagation of epoxy. Crack growth rate and the Paris law exponent for fatigue-crack growth were considerably reduced from m=15.5 of the neat epoxy to m=8.1 of the nanocomposite. To identify the related reinforcing and fracture mechanisms structural analyses of the fractured surfaces were performed by scanning electron microscope. Characteristic features were identified to be deformation, debonding and fracture of the nano-phases as well as crack pinning. However, the highest resistance against fatigue crack propagation was achieved in a bi-continuous microstructure that consisted of an epoxy-rich phase with embedded submicron sized MAM inclusions, and which was surrounded by a block-copolymer-rich phase that showed rupture and plastic deformation.

  13. Anomolous Fatigue Crack Growth Phenomena in High-Strength Steel

    NASA Technical Reports Server (NTRS)

    Forth, Scott C.; James, Mark A.; Johnston, William M., Jr.; Newman, James C., Jr.

    2004-01-01

    The growth of a fatigue crack through a material is the result of a complex interaction between the applied loading, component geometry, three-dimensional constraint, load history, environment, material microstructure and several other factors. Previous studies have developed experimental and computational methods to relate the fatigue crack growth rate to many of the above conditions, with the intent of discovering some fundamental material response, i.e. crack growth rate as a function of something. Currently, the technical community uses the stress intensity factor solution as a simplistic means to relate fatigue crack growth rate to loading, geometry and all other variables. The stress intensity factor solution is a very simple linear-elastic representation of the continuum mechanics portion of crack growth. In this paper, the authors present fatigue crack growth rate data for two different high strength steel alloys generated using standard methods. The steels exhibit behaviour that appears unexplainable, compared to an aluminium alloy presented as a baseline for comparison, using the stress intensity factor solution.

  14. Fatigue crack propagation in self-assembling nanocomposites

    NASA Astrophysics Data System (ADS)

    Klingler, Andreas; Wetzel, Bernd

    2016-05-01

    Self-assembling block-copolymers allow the easy manufacturing of nanocomposites due to the thermodynamically driven in situ formation of nanosized phases in thermosetting resins during the curing process. Complex mechanical dispersion processes can be avoided. The current study investigates the effect of a block-copolymer on the fatigue crack propagation resistance of a cycloaliphatic amine cured epoxy resin. It was found that a small amount of MAM triblock-copolymer significantly increases the resistance to fatigue crack propagation of epoxy. Crack growth rate and the Paris law exponent for fatigue-crack growth were considerably reduced from m=15.5 of the neat epoxy to m=8.1 of the nanocomposite. To identify the related reinforcing and fracture mechanisms structural analyses of the fractured surfaces were performed by scanning electron microscope. Characteristic features were identified to be deformation, debonding and fracture of the nano-phases as well as crack pinning. However, the highest resistance against fatigue crack propagation was achieved in a bi-continuous microstructure that consisted of an epoxy-rich phase with embedded submicron sized MAM inclusions, and which was surrounded by a block-copolymer-rich phase that showed rupture and plastic deformation.

  15. Accelerated Near-Threshold Fatigue Crack Growth Behavior of an Aluminum Powder Metallurgy Alloy

    NASA Technical Reports Server (NTRS)

    Piascik, Robert S.; Newman, John A.

    2002-01-01

    Fatigue crack growth (FCG) research conducted in the near threshold regime has identified a room temperature creep crack growth damage mechanism for a fine grain powder metallurgy (PM) aluminum alloy (8009). At very low DK, an abrupt acceleration in room temperature FCG rate occurs at high stress ratio (R = Kmin/Kmax). The near threshold accelerated FCG rates are exacerbated by increased levels of Kmax (Kmax less than 0.4 KIC). Detailed fractographic analysis correlates accelerated FCG with the formation of crack-tip process zone micro-void damage. Experimental results show that the near threshold and Kmax influenced accelerated crack growth is time and temperature dependent.

  16. Healing of Fatigue Crack in 1045 Steel by Using Eddy Current Treatment

    PubMed Central

    Yang, Chuan; Xu, Wenchen; Guo, Bin; Shan, Debin; Zhang, Jian

    2016-01-01

    In order to investigate the methods to heal fatigue cracks in metals, tubular specimens of 1045 steel with axial and radial fatigue cracks were treated under the eddy current. The optical microscope was employed to examine the change of fatigue cracks of specimens before and after the eddy current treatment. The results show that the fatigue cracks along the axial direction of the specimen could be healed effectively in the fatigue crack initiation zone and the crack tip zone under the eddy current treatment, and the healing could occur within a very short time. The voltage breakdown and the transient thermal compressive stress caused by the detouring of eddy current around the fatigue crack were the main factors contributing to the healing in the fatigue crack initiation zone and the crack tip zone, respectively. Eddy current treatment may be a novel and effective method for crack healing. PMID:28773761

  17. An experimental investigation of fatigue crack growth in drillstring tubulars

    SciTech Connect

    Dale, B.A.

    1986-01-01

    Drill-string failures continue to plague the oil industry, often costing millions of dollars each year. This problem is frequently intensified with the drilling of deep deviated wellbores or ''hard rock'' drilling conditions. The drilling industry attempts to guard against these costly failures by performing periodic nondestructive inspections to remove damaged tubulars from service. This paper describes the results of full-scale fatigue crack growth tests of drill collars under rotating and bending loads. In addition, corrosion fatigue crack growth data are also presented for API drill-pipe steels in air and in three representative water-base drilling fluid environments. Based on this experimental investigation, the test data support the practical application of fatigue crack growth mechanics principles for the development of nondestructive inspection intervals to reduce drill-string failures.

  18. Effect of underloads on fatigue crack growth of titanium-17

    NASA Astrophysics Data System (ADS)

    Russ, Stephan M.

    An improved understanding of fatigue crack growth phenomenon applicable to titanium engine disks was developed through complimentary experimental and analytical investigations of Ti-17. Two significant findings resulted from this study. First, it was concluded that an R = 0.1 underload accelerated the fatigue crack growth rates of subsequent high- R cycles, even when closure was negligible and the traditional Delta Keff was identical to DeltaKapplied. Second, the strains ahead of the crack were determined to be higher following the underload and remained elevated for some time. A simplified variable-amplitude spectrum, consisting of high- R baseline cycles and periodic R = 0.1 underloads, was used to demonstrate a load-interaction effect which led to nonconservative life predictions using conventional fatigue crack growth predictive methodologies. A phenomenological model was formulated based on hypothesized changes in the propagation resistance, KPR, and fit to the test data. The results demonstrated that periodic R = 0.1 underloads increased fatigue crack growth rates of subsequent high-R cycles. When the number of baseline cycles was 100 or more, the higher fatigue crack growth rates led to significantly lower lifetimes than predicted using methods assuming no load-interaction effect. The model also predicted an observed decrease in threshold. A finite element model was developed to investigate what transpires in the wake and ahead of the crack tip. The results from finite element analyses compared favorably to experimental evidence acquired in the vicinity of a crack tip during a comparable test. The R = 0.1 underload cycle produced a subtle increase in the crack opening displacement profiles of the subsequent R = 0.7 cycles. More importantly, the simulations revealed an increase in the strains ahead of the crack tip after the underload, although the strain range was unchanged. It was concluded that the higher mean strains were an indication of increased damage

  19. Fatigue crack monitoring in aero-engines: simulation and experiments

    NASA Astrophysics Data System (ADS)

    Gelman, Leonid M.; Petrunin, Ivan V.; Thompson, Chris

    2004-03-01

    A new genetic approach to fatigue crack monitoring in aero-engine blades is presented. The approach consists of simultaneously using two diagnostic features: the real and imaginary parts of the Fourier transform of vibroacoustical signals. This approach is more fundamental than traditional approaches based on the power spectral density, phase spectrum and Hartley transform; each of these approaches is a special case of the proposed approach. Numerical examples are given based on the processing of signals generated using a nonlinear model of tested blades. The generated signals are the forced vibroacoustical oscillations of cracked and un-cracked blades. The numerical examples show that crack detection ismore effective when using the new approach than when u sing the power spectral density approach. The presented experimental results using un-cracked and cracked turbuine blades from an aero-engine are matched with numerical results. The proposed approach offers an effectiveness improvement over the traditional approach based on power spectral density.

  20. Electromagnetic Detection of Fatigue Cracks under Protruding Head Ferromagnetic Fasteners

    NASA Technical Reports Server (NTRS)

    Wincheski, Buzz; Namkung, Min

    2004-01-01

    The detection of fatigue cracks under installed fasteners has been a major goal of the aging aircraft NDE community. The Sliding Probe, Magneto-Optic Imager, Rotating Self-Nulling Probe, Low Frequency Eddy Current Array, and Eddyscan systems are among the instruments developed for this inspection. It has been verified that the detection of fatigue cracks under flush head aluminum and titanium fasteners can be accomplished with a high resolution by the above techniques. The detection of fatigue cracks under ferromagnetic and protruding head fasteners, however, has been found to be much more difficult. For the present work, the inspection for fatigue cracks under SAE 4340 Steel Hi-Lok fasteners is explored. Modifications to the Rotating Self-Nulling Eddy Current Probe System are presented which enable the detection of fatigue cracks hidden under the protruding head of the ferromagnetic fastener. Inspection results for samples with varying length EDM notches are shown, as well as a comparison between the signature from an EDM notch and an actual fatigue crack. Finite Element Modeling is used to investigate the effect of the ferromagnetic fastener on the induced eddy current distribution in order to help explain the detection characteristics of the system. This paper will also introduce a modification to the Rotating Probe System designed specifically for the detection of deeply buried flaws in multilayer conductors. The design change incorporates a giant magnetoresistive (GMR) sensor as the pickup device to improve the low frequency performance of the probe. The flaw detection capabilities of the GMR based Self- Nulling Probe are presented along with the status of the GMR based Rotating Probe System for detection of deeply buried flaws under installed fasteners.

  1. Fatigue Crack Detection Using Digital Image Correlation

    NASA Astrophysics Data System (ADS)

    Cawley, P.; Hutt, T. D.

    2009-03-01

    At present, detecting structural defects such as cracking and corrosion before they become critical is largely achieved by time consuming techniques such as eddy current and ultrasonic testing. These techniques require point-by-point scanning over the area to be tested. Digital Image Correlation could provide a cheaper and quicker testing technique. It works by correlating images of the structure surface in unloaded and loaded states taken with a standard digital camera, giving the displacement and strain fields. The specific case of a crack at a hole in an aluminium plate was investigated. It was found that the strain concentration around the crack tip is too localised to detect; however the displacement jump across the crack could be seen. This technique allows the cracks to be detected and would allow rapid testing of a structure if it can easily be loaded.

  2. Thermo-Mechanical Fatigue Crack Growth of RR1000.

    PubMed

    Pretty, Christopher John; Whitaker, Mark Thomas; Williams, Steve John

    2017-01-04

    Non-isothermal conditions during flight cycles have long led to the requirement for thermo-mechanical fatigue (TMF) evaluation of aerospace materials. However, the increased temperatures within the gas turbine engine have meant that the requirements for TMF testing now extend to disc alloys along with blade materials. As such, fatigue crack growth rates are required to be evaluated under non-isothermal conditions along with the development of a detailed understanding of related failure mechanisms. In the current work, a TMF crack growth testing method has been developed utilising induction heating and direct current potential drop techniques for polycrystalline nickel-based superalloys, such as RR1000. Results have shown that in-phase (IP) testing produces accelerated crack growth rates compared with out-of-phase (OOP) due to increased temperature at peak stress and therefore increased time dependent crack growth. The ordering of the crack growth rates is supported by detailed fractographic analysis which shows intergranular crack growth in IP test specimens, and transgranular crack growth in 90° OOP and 180° OOP tests. Isothermal tests have also been carried out for comparison of crack growth rates at the point of peak stress in the TMF cycles.

  3. Thermo-Mechanical Fatigue Crack Growth of RR1000

    PubMed Central

    Pretty, Christopher John; Whitaker, Mark Thomas; Williams, Steve John

    2017-01-01

    Non-isothermal conditions during flight cycles have long led to the requirement for thermo-mechanical fatigue (TMF) evaluation of aerospace materials. However, the increased temperatures within the gas turbine engine have meant that the requirements for TMF testing now extend to disc alloys along with blade materials. As such, fatigue crack growth rates are required to be evaluated under non-isothermal conditions along with the development of a detailed understanding of related failure mechanisms. In the current work, a TMF crack growth testing method has been developed utilising induction heating and direct current potential drop techniques for polycrystalline nickel-based superalloys, such as RR1000. Results have shown that in-phase (IP) testing produces accelerated crack growth rates compared with out-of-phase (OOP) due to increased temperature at peak stress and therefore increased time dependent crack growth. The ordering of the crack growth rates is supported by detailed fractographic analysis which shows intergranular crack growth in IP test specimens, and transgranular crack growth in 90° OOP and 180° OOP tests. Isothermal tests have also been carried out for comparison of crack growth rates at the point of peak stress in the TMF cycles. PMID:28772394

  4. Simulating Fatigue Crack Growth in Spiral Bevel Gears

    NASA Technical Reports Server (NTRS)

    Spievak, Lisa E.; Wawrzynek, Paul A.; Ingraffea, Anthony R.

    2000-01-01

    The majority of helicopter transmission systems utilize spiral bevel gears to convert the horizontal power from the engine into vertical power for the rotor. Due to the cyclical loading on a gear's tooth, fatigue crack propagation can occur. In rotorcraft applications, a crack's trajectory determines whether the gear failure will be benign or catastrophic for the aircraft. As a result, the capability to predict crack growth in gears is significant. A spiral bevel gear's complex shape requires a three dimensional model of the geometry and cracks. The boundary element method in conjunction with linear elastic fracture mechanics theories is used to predict arbitrarily shaped three dimensional fatigue crack trajectories in a spiral bevel pinion under moving load conditions. The predictions are validated by comparison to experimental results. The sensitivity of the predictions to variations in loading conditions and crack growth rate model parameters is explored. Critical areas that must be understood in greater detail prior to predicting more accurate crack trajectories and crack growth rates in three dimensions are identified.

  5. Fatigue crack monitoring via load-differential guided wave methods

    NASA Astrophysics Data System (ADS)

    Lee, Sang Jun; Michaels, Jennifer E.; Chen, Xin; Michaels, Thomas E.

    2012-05-01

    Detection and localization of fatigue cracks is an important application for inspection and monitoring of civil, mechanical and aerospace structures, but assessment of such damage via ultrasonic guided waves can be problematic when cracks are tightly closed in the absence of applied tensile loads. Proposed here are load-differential methods, which compare signals at one load to those at another load at the same damage state. The main advantage of such methods is that cracks can be detected and localized by analyzing current signals obtained from different loading conditions without using baseline data from the damage-free state. The efficacy of the proposed load-differential imaging method is examined using fatigue test data where multiple cracks grow from a single through-hole. Data were acquired with a spatially distributed array of piezoelectric discs by recording ultrasonic signals as a function of applied uniaxial load at intervals throughout the fatigue test. Load-differential guided wave images are generated from residual signals via delay-and-sum imaging methods, and these images are evaluated in terms of their ability to detect and localize fatigue cracks.

  6. A Nonarbitrary Fatigue Crack Size Concept to Predict Total Fatigue Lives

    DTIC Science & Technology

    1979-12-01

    fatigue life of fastener holes, Potter (13) postulated an equivalent initial flaw size . It is determined by calculating the initial flaw size that...would be required to give the total fatigue life of a laboratory specimen if the crack growth rate was integrated. The equivalent initial flaw size is not

  7. Crack Tip Plasticity Associated with Corrosion Assisted Fatigue.

    DTIC Science & Technology

    1982-11-15

    growing. The model presented is very similar to those previously developed by Antolovich , Saxena and I Chanani[83 and by Lanteigne and BailonE9] but...in crack tip plasticity associated with environment. The model used here is conceptually similar to those formulated by * Antolovich , et al,[ and...Lankford, J. ’Fatigue-Crack-Tip I Plastic Strains by the Stereoimaging Technique’ Exp. Mech. 1980 20, 3 134-139. 8. Antolovich , S. D., Saxena, A., and

  8. A new approach for the influence of residual stress on fatigue crack propagation

    NASA Astrophysics Data System (ADS)

    Zhu, Lin; Jia, Min-Ping

    Many manufacturing processes can induce residual stresses in produced components. These residual stresses influence the mean stress during cyclic loading. The initial residual stresses induced during manufacturing change during fatigue damage. This paper presents a research on the change of residual stress distribution during fatigue crack propagation; the research predicts crack propagation by considering residual stress. An analysis approach for the change in residual stress distribution is established according to the diffusion theory of cavity, which is also used to investigate cracks with different orientations. Experiments are conducted to verify the prediction results of residual stress. A probability density function based on Weibull distribution is established to evaluate the accuracy of predicted residual stress. The influence of residual stress on fatigue crack propagation is considered the effective stress intensity factor range, which is calculated under the combined stress field of applied stress and residual stress. An analysis model of crack propagation is established. Furthermore, the model of crack propagation is used to estimate the velocity of crack propagation for the cases, same as the cases of residual stress prediction. The case studies show that the results are basically identical with the experimental results, indicating that the proposed approach is acceptable.

  9. Closure of fatigue cracks at high strains

    NASA Technical Reports Server (NTRS)

    Iyyer, N. S.; Dowling, N. E.

    1985-01-01

    Experiments were conducted on smooth specimens to study the closure behavior of short cracks at high cyclic strains under completely reversed cycling. Testing procedures and methodology, and closure measurement techniques, are described in detail. The strain levels chosen for the study cover from predominantly elastic to grossly plastic strains. Crack closure measurements are made at different crack lengths. The study reveals that, at high strains, cracks close only as the lowest stress level in the cycle is approached. The crack opening is observed to occur in the compressive part of the loading cycle. The applied stress needed to open a short crack under high strain is found to be less than for cracks under small scale yielding. For increased plastic deformations, the value of sigma sub op/sigma sub max is observed to decrease and approaches the value of R. Comparison of the experimental results with existing analysis is made and indicates the limitations of the small scale yielding approach where gross plastic deformation behavior occurs.

  10. 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.

  11. Fatigue crack detection in a plate girder using Lamb waves

    NASA Astrophysics Data System (ADS)

    Greve, D. W.; Oppenheim, I. J.; Wu, Wei; Zheng, Peng

    2007-04-01

    We report on the application of wafer-type PZT transducers to the detection of flaws in steel plate girders. In these experiments one transducer is used to emit a pulse and the second receives the pulse and reflections from nearby boundaries, flaws, or discontinuities (pitch-catch mode). In this application there will typically be numerous reflections observed in the undamaged structure. A major challenge is to recognize new reflections caused by fatigue cracks in the presence of these background reflections. A laboratory specimen plate girder was fabricated at approximately half scale, 910 mm deep with an h/t ratio of 280 for the web and a b/t ratio of 16 for the flanges, and with transverse stiffeners fabricated with a web gap at the tension flange. Two wafer-type transducers were mounted on the web approximately 175 mm from the crack location, one on each side of the stiffener. The transducers were operated in pitch-catch mode, excited by a windowed sinusoid to create a narrowband transient excitation. The transducer location relative to the crack corresponded to a total included angle of roughly 30 degrees in the path reflecting from the crack. Cyclic loading was applied to develop a distortion-induced fatigue crack in the web at the web gap location. After appearance of the crack, ultrasonic measurements were performed at a range of center frequencies below the cutoff frequency of the A1 Lamb wave mode. Subsequently the crack was extended mechanically to simulate crack growth under primary longitudinal (bending) stress and the measurements were repeated. Direct differencing of the signals showed arrivals at times corresponding to reflection from the crack location, growing in amplitude as the crack was lengthened mechanically. These results demonstrate the utility of Lamb waves for crack detection even in the presence of numerous background reflections.

  12. Investigation of eddy current examination on OD fatigue crack for steam generator tubes

    NASA Astrophysics Data System (ADS)

    Kong, Yuying; Ding, Boyuan; Li, Ming; Liu, Jinhong; Chen, Huaidong; Meyendorf, Norbert G.

    2015-03-01

    The opening width of fatigue crack was very small, and conventional Bobbin probe was very difficult to detect it in steam generator tubes. Different sizes of 8 fatigue cracks were inspected using bobbin probe rotating probe. The analysis results showed that, bobbin probe was not sensitive for fatigue crack even for small through wall crack mixed with denting signal. On the other hand, the rotating probe was easily to detect all cracks. Finally, the OD phase to depth curve for fatigue crack using rotating probe was established and the results agreed very well with the true crack size.

  13. Damage Tolerance Assessment Handbook. Volume 1. Introduction Fracture Mechanics Fatigue Crack Propagation

    DTIC Science & Technology

    1993-10-01

    Volume ,: Introduction NJ 08405 Fracture Mechanics Fatigue Crack Propagation Research and Special Programs Administration John A. Volpe National...Load-displacement plot [Adapted from John M. Barson/Stanley T. Rolfe, Fracture and Fatigue Control in Structures. Applications of Fracture Mechanics...Methods ASTM STP 527, American Society for Testing and Materials, Philadelphia, PA, 1973. 2-19. Ratwani, M.M. and Wilhem , DP. Develonment and EvaluAtion of

  14. Load-Differential Imaging for Detection and Localization of Fatigue Cracks Using Lamb Waves (Preprint)

    DTIC Science & Technology

    2012-03-01

    are hard to detect with conventional ultrasonic testing methods because ultrasound can propagate through a tightly closed crack [16,17]. Research on...Michaels and T. E. Michaels, “An ultrasonic method for dynamic monitoring of fatigue crack initiation and growth,” Journal of the Acoustical Society of...America, 119(1), pp. 74-85, 2005. 20. Y. Ohara, S. Horinouchi, M. Hashimoto, Y. Shintaku, and K. Yamanaka, “Nonlinear ultrasonic imaging method for

  15. 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.

  16. The Statistical Nature of Fatigue Crack Propagation

    DTIC Science & Technology

    1977-03-01

    184 xtii LIST OF SYMBOLS A chi-square tail area a half crack length (in. or mm.). af final half crack length (in. or mn.). ai any discrete half...incremental polynomial method. u variable of integration in the chi-square tail area equation. c V covariance matrix. Sv inverse covariance matrix. X variable...calculated at two optimal locations and, based on these values, a certain area where the curvature minimum is know, not to exist is excluded from the

  17. Thermomechanical fatigue cracking in fiber reinforced metal-matrix composites

    NASA Astrophysics Data System (ADS)

    Bao, G.; McMeeking, R. M.

    1995-09-01

    A theoretical model is developed for thermomechanical fatigue cracking in fiber reinforced metal-matrix composites. Interfacial debonding is assumed to occur readily, allowing fibers to slide relative to the matrix resisted by a uniform shear stress. The fibers therefore bridge any matrix crack which develops. The crack bridging traction law is obtained, including the effect of thermal expansion mismatch between the fiber and the matrix and a temperature dependence of the frictional shear stress. Any combination of thermal and mechanical cycling is considered as long as the slip zone along the fiber increases in length monotonically during each increment of cycling. However, for clarity, the results are presented in terms of in-phase and out-of-phase cycling of the thermal and mechanical loads at the same frequency. For each case, the stress distributions in the bridging zone as well as the stress intensity factors at the crack tip are computed for relevant regimes of the thermal and mechanical loading conditions. Predictions are made of the matrix fatigue crack growth under combined thermal and mechanical loading conditions. It is found that when the thermal expansion coefficient of the fiber is less than that of the matrix, a significant increase in the crack growth rate results in out-of-phase thermomechanical fatigue. On the other hand, there is decreased tendency for fibers to fail in this case. For in-phase thermomechanical fatigue, the crack growth rate is reduced but the stress in the fiber is larger than that due to mechanical loading alone, resulting in an increased tendency for fiber failure. The implications for life prediction for fiber reinforced metal-matrix composites are discussed.

  18. Fatigue crack growth and low cycle fatigue of two nickel base superalloys

    NASA Technical Reports Server (NTRS)

    Stoloff, N. S.; Duquette, D. J.; Choe, S. J.; Golwalkar, S.

    1983-01-01

    The fatigue crack growth and low cycle fatigue behavior of two P/M superalloys, Rene 95 and Astroloy, in the hot isostatically pressed (HIP) condition, was determined. Test variables included frequency, temperature, environment, and hold times at peak tensile loads (or strains). Crack initiation sites were identified in both alloys. Crack growth rates were shown to increase in argon with decreasing frequency or with the imposition of hold times. This behavior was attributed to the effect of oxygen in the argon. Auger analyses were performed on oxide films formed in argon. Low cycle fatigue lives also were degraded by tensile hold, contrary to previous reports in the literature. The role of environment in low cycle fatigue behavior is discussed.

  19. Thermal-mechanical fatigue crack growth in Inconel X-750

    NASA Technical Reports Server (NTRS)

    Marchand, N.; Pelloux, R. M.

    1985-01-01

    Thermal-mechanical fatigue crack growth (TMFCG) was studied in a 'gamma-gamma' nickel base superalloy Inconel X-750 under controlled load amplitude in the temperature range from 300 to 650 C. In-phase (T sub max at sigma sub max), out-of-phase (T sub min at sigma sub max), and isothermal tests at 650 C were performed on single-edge notch bars under fully reversed cyclic conditions. A dc electrical potential method was used to measure crack length. The electrical potential response obtained for each cycle of a given wave form and R value yields information on crack closure and crack extension per cycle. The macroscopic crack growth rates are reported as a function of delta k and the relative magnitude of the TMFCG are discussed in the light of the potential drop information and of the fractographic observations.

  20. Thermal-mechanical fatigue crack growth in Inconel X-750

    NASA Technical Reports Server (NTRS)

    Marchand, N.; Pelloux, R. M.

    1984-01-01

    Thermal-mechanical fatigue crack growth (TMFCG) was studied in a gamma-gamma' nickel base superalloy Inconel X-750 under controlled load amplitude in the temperature range from 300 to 650 C. In-phase (T sub max at sigma sub max), out-of-phase (T sub min at sigma sub max), and isothermal tests at 650 C were performed on single-edge notch bars under fully reversed cyclic conditions. A dc electrical potential method was used to measure crack length. The electrical potential response obtained for each cycle of a given wave form and R value yields information on crack closure and crack extension per cycle. The macroscopic crack growth rates are reported as a function of delta k and the relative magnitude of the TMFCG are discussed in the light of the potential drop information and of the fractographic observations.

  1. 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

  2. Fatigue crack closure behavior at high stress ratios

    NASA Technical Reports Server (NTRS)

    Turner, C. Christopher; Carman, C. Davis; Hillberry, Ben M.

    1988-01-01

    Fatigue crack delay behavior at high stress ratio caused by single peak overloads was investigated in two thicknesses of 7475-T731 aluminum alloy. Closure measurements indicated no closure occurred before or throughout the overload plastic zones following the overload. This was further substantiated by comparing the specimen compliance following the overload with the compliance of a low R ratio test when the crack was fully open. Scanning electron microscope studies revealed that crack tunneling and possibly reinitiation of the crack occurred, most likely a result of crack-tip blunting. The number of delay cycles was greater for the thinner mixed mode stress state specimen than for the thicker plane strain stress state specimen, which is similar to low R ratio test results and may be due to a larger plastic zone for the mixed mode cased.

  3. Thermographic inspection of fatigue cracks in riveted plates

    NASA Astrophysics Data System (ADS)

    Welch, Christopher S.; Cramer, K. E.; Dawicke, David S.

    A theoretical and experimental analysis of the inspection of reiveted plates using the thermoelastic effect is presented. Theoretically, a model is employed to estimate the thermoelastic response expected from a cracked and uncracked rivet hole. The stress invariant, which causes the thermoelastic response, is calculated for the cracked case using an FEM model and for the uncracked case using a theoretical approximation. The differences in the two thermal responses are reduced when the effects of optical spatial averaging, thermal conduction, and a concentric blind area representing the rivet heat are considered. It is suggested that discrimination between the cracked and uncracked cases for short cracks will improve with increasing frequency of stress oscillation. Measurements are shown for aerospace aluminum samples with and without fatigue cracks propagating from holes in a plate. The effect of mean stress on the thermoelastic response is illustrated, and the results are qualitatively compared with the finite element analysis.

  4. Fatigue crack propagation behavior of ultrahigh molecular weight polyethylene.

    PubMed

    Connelly, G M; Rimnac, C M; Wright, T M; Hertzberg, R W; Manson, J A

    1984-01-01

    The relative fatigue crack propagation resistance of plain and carbon fiber-reinforced ultrahigh molecular weight polyethylene (UHMWPE) was determined from cyclic loading tests performed on compact tension specimens machined from the tibial components of total knee prostheses. Both materials were characterized by dynamic mechanical spectroscopy, X-ray diffraction, and differential scanning calorimetry. The cyclic tests used loading in laboratory air at 5 Hz using a sinusoidal wave form. Dynamic mechanical spectroscopy showed that the reinforced UHMWPE had a higher elastic storage modulus than the plain UHMWPE, whereas X-ray diffraction and differential scanning calorimetry showed that the percent crystallinity and degree of order in the crystalline regions were similar for the two materials. Fatigue crack propagation in both materials proved to be very sensitive to small changes in the applied cyclic stress intensity range. A 10% increase in stress intensity resulted in approximately an order of magnitude increase in fatigue crack growth rate. The fatigue crack propagation resistance of the reinforced UHMWPE was found to be significantly worse than that of the plain UHMWPE. This result was attributed to poor bonding between the carbon fibers and the UHMWPE matrix and the ductile nature of the matrix itself.

  5. Fatigue Crack Growth in Friction Stir Welded Ti-5111

    DTIC Science & Technology

    2009-02-01

    systematically investigated. In this paper , the microstructures, microhardness, and fatigue crack growth kinetics through FSW Ti-5111 weld region...free, radiographic examinations indicated the presence of wormholes in the 12.7 mm thick Ti-5111 Weld 1. Most of these wormholes were located near the

  6. Effect of CTE on Fatigue Cracking of Stainless Steel Vessels

    SciTech Connect

    Bird, E. L.; Mustaleski, T. M.

    2002-01-31

    Visual examination of lithium hydride reactor vessels revealed cracks that were adjacent to welds. Most cracks were parallel to the weld in the bottom portion of the vessel. Sections were cut out of the vessel containing these cracks and examined using the metallograph, scanning electron microscope, and microprobe to determine the cause of cracking. most of the cracks originated on the outer surface just outside the weld fusion line in the heat affected zone and propagated along grain boundaries. Crack depth of those sections examined ranged from about 300 to 500 {micro}m. Other cracks were reported to have reached a maximum depth of 0.32-cm (0.125-inch). The primary cause of cracking was the creation of high tensile stresses associated with the CTE differences between the filler metal and the base metal during operation of the vessel in a thermally cyclic environment. This failure mechanism could be described as creep-type fatigue whereby crack propagation might have been aided by the presence of brittle chromium carbides along the grain boundaries, which is indicative of a slightly sensitized microstructure.

  7. Simulation of fatigue crack growth under large scale yielding conditions

    NASA Astrophysics Data System (ADS)

    Schweizer, Christoph; Seifert, Thomas; Riedel, Hermann

    2010-07-01

    A simple mechanism based model for fatigue crack growth assumes a linear correlation between the cyclic crack-tip opening displacement (ΔCTOD) and the crack growth increment (da/dN). The objective of this work is to compare analytical estimates of ΔCTOD with results of numerical calculations under large scale yielding conditions and to verify the physical basis of the model by comparing the predicted and the measured evolution of the crack length in a 10%-chromium-steel. The material is described by a rate independent cyclic plasticity model with power-law hardening and Masing behavior. During the tension-going part of the cycle, nodes at the crack-tip are released such that the crack growth increment corresponds approximately to the crack-tip opening. The finite element analysis performed in ABAQUS is continued for so many cycles until a stabilized value of ΔCTOD is reached. The analytical model contains an interpolation formula for the J-integral, which is generalized to account for cyclic loading and crack closure. Both simulated and estimated ΔCTOD are reasonably consistent. The predicted crack length evolution is found to be in good agreement with the behavior of microcracks observed in a 10%-chromium steel.

  8. Quantitative investigation of surface and subsurface fatigue cracks near rivets in riveted joints using acoustic, electron and optical microscopy

    NASA Astrophysics Data System (ADS)

    Connor, Zayna Marie

    2000-10-01

    Using scanning acoustic microscopy, optical microscopy and scanning electron microscopy, in conjunction with fractography of fractured surfaces, the crack formation and growth kinetics of subsurface fatigue cracks and surface breaking fatigue cracks near rivets have been characterized in detail in this research. The scanning acoustic microscope was used to quantitatively investigate subsurface fatigue cracks (even when they were very small) at and near countersunk rivets in riveted lap joint specimens that are similar to the riveted lap joints found in the fuselages of many aircraft. It was found that the maximum nominal applied stress influences the fatigue crack initiation and propagation behavior. Eyebrow type cracks develop at lower stresses and centerline cracks develop at higher stresses. At low stress ranges, the fatigue cracks initiate a short distance from the rivet at or near the hidden surface of the chamfered panel. At higher stress amplitudes, the cracks initiate at the blunt knife edge. Residual compressive stresses and fretting are suggested to play more important roles at lower stress ranges. Both types of cracks initiate in a shear mode but transform to tensile, mode I, cracks as they grow. This transition occurs much more rapidly at the higher stress amplitude. At both high and low stresses, the cracks are longer on the fayed surface of the panel than elsewhere. In a comparison of Alclad 2024-T3 and Alclad 2524-T3, it was found that the high purity aluminum alloy 2524 nucleates cracks at a greater number of cycles than the less pure aluminum alloy 2024. At high stress, crack initiation plays less of a roll and the 2024 alloy has a longer life. The scanning acoustic microscope enabled us to study subsurface fatigue cracks. The understanding gained from the characterization of the subsurface fatigue cracks will help in the modeling of crack initiation and growth in the riveted lap joint and will also aid in the improvement of NDE techniques for the

  9. Fatigue Crack Growth Database for Damage Tolerance Analysis

    NASA Technical Reports Server (NTRS)

    Forman, R. G.; Shivakumar, V.; Cardinal, J. W.; Williams, L. C.; McKeighan, P. C.

    2005-01-01

    The objective of this project was to begin the process of developing a fatigue crack growth database (FCGD) of metallic materials for use in damage tolerance analysis of aircraft structure. For this initial effort, crack growth rate data in the NASGRO (Registered trademark) database, the United States Air Force Damage Tolerant Design Handbook, and other publicly available sources were examined and used to develop a database that characterizes crack growth behavior for specific applications (materials). The focus of this effort was on materials for general commercial aircraft applications, including large transport airplanes, small transport commuter airplanes, general aviation airplanes, and rotorcraft. The end products of this project are the FCGD software and this report. The specific goal of this effort was to present fatigue crack growth data in three usable formats: (1) NASGRO equation parameters, (2) Walker equation parameters, and (3) tabular data points. The development of this FCGD will begin the process of developing a consistent set of standard fatigue crack growth material properties. It is envisioned that the end product of the process will be a general repository for credible and well-documented fracture properties that may be used as a default standard in damage tolerance analyses.

  10. Fatigue Crack Growth Database for Damage Tolerance Analysis

    NASA Technical Reports Server (NTRS)

    Forman, R. G.; Shivakumar, V.; Cardinal, J. W.; Williams, L. C.; McKeighan, P. C.

    2005-01-01

    The objective of this project was to begin the process of developing a fatigue crack growth database (FCGD) of metallic materials for use in damage tolerance analysis of aircraft structure. For this initial effort, crack growth rate data in the NASGRO (Registered trademark) database, the United States Air Force Damage Tolerant Design Handbook, and other publicly available sources were examined and used to develop a database that characterizes crack growth behavior for specific applications (materials). The focus of this effort was on materials for general commercial aircraft applications, including large transport airplanes, small transport commuter airplanes, general aviation airplanes, and rotorcraft. The end products of this project are the FCGD software and this report. The specific goal of this effort was to present fatigue crack growth data in three usable formats: (1) NASGRO equation parameters, (2) Walker equation parameters, and (3) tabular data points. The development of this FCGD will begin the process of developing a consistent set of standard fatigue crack growth material properties. It is envisioned that the end product of the process will be a general repository for credible and well-documented fracture properties that may be used as a default standard in damage tolerance analyses.

  11. Distribution of Inclusion-Initiated Fatigue Cracking in Powder Metallurgy Udimet 720 Characterized

    NASA Technical Reports Server (NTRS)

    Bonacuse, Peter J.; Kantzos, Pete T.; Barrie, Robert; Telesman, Jack; Ghosn, Louis J.; Gabb, Timothy P.

    2004-01-01

    In the absence of extrinsic surface damage, the fatigue life of metals is often dictated by the distribution of intrinsic defects. In powder metallurgy (PM) alloys, relatively large defects occur rarely enough that a typical characterization with a limited number of small volume fatigue test specimens will not adequately sample inclusion-initiated damage. Counterintuitively, inclusion-initiated failure has a greater impact on the distribution in PM alloy fatigue lives because they tend to have fewer defects than their cast and wrought counterparts. Although the relative paucity of defects in PM alloys leads to higher mean fatigue lives, the distribution in observed lives tends to be broader. In order to study this important failure initiation mechanism without expending an inordinate number of specimens, a study was undertaken at the NASA Glenn Research Center where known populations of artificial inclusions (seeds) were introduced to production powder. Fatigue specimens were machined from forgings produced from the seeded powder. Considerable effort has been expended in characterizing the crack growth rate from inclusion-initiated cracks in seeded PM alloys. A rotating and translating positioning system, with associated software, was devised to map the surface inclusions in low-cycle fatigue (LCF) test bars and to monitor the crack growth from these inclusions. The preceding graph illustrates the measured extension in fatigue cracks from inclusions on a seeded LCF test bar subjected to cyclic loading at a strain range of 0.8 percent and a strain ratio (max/min) of zero. Notice that the observed inclusions fall into three categories: some do not propagate at all (arrest), some propagate with a decreasing crack growth rate, and a few propagate at increasing rates that can be modeled by fracture mechanics. The following graph shows the measured inclusion-initiated crack growth rates from 10 interrupted LCF tests plotted against stress intensities calculated for semi

  12. Fatigue crack initiation and damage evolution of unnotched titanium matrix composites

    NASA Astrophysics Data System (ADS)

    Her, Yung-Chiun

    Fatigue crack initiation, multiplication, matrix crack density evolution, and stiffness reduction of several unnotched SCS-6 silicon carbide fiber-reinforced titanium and titanium aluminide matrix composites have been investigated experimentally and analytically. The effects of the thickness of the interfacial reaction layer and fiber coating on fatigue crack initiation life, crack growth rate, and fatigue damage evolution of the composites were examined. Growth behavior of small fatigue cracks in TMCs was also studied carefully. It was found that fatigue crack initiation and multiplication of TMCs are strongly influenced by the thickness of the interfacial reaction layer. Fatigue crack will not develop from the micro-notches in the interfacial reaction layer until the thickness of the reaction layer exceeds a critical value. Matrix crack growth rate is affected by the applied stress level, however, it appears to be independent of the matrix material and heat treatment. The combined effects of fatigue crack multiplication and propagation result in stiffness degradation of the composites. The Ag/Ta duplex fiber coating significantly improves the transverse tensile and flexural creep resistance of the SCS-6/Ti-25-10 composite. However, the Ag/Ta-coated composite exhibits a shorter crack initiation life, higher number of matrix cracks, and higher crack growth rate than the uncoated composite. The embrittlement of the residual Ag/Ta layer suggests that Ag is not an effective diffusion barrier to prevent the interdiffusion of atomic species across the interface. The high interfacial cracking density and high interfacial bond strength in the Ag/Ta-coated SCS-6/Tisb3Al composite are believed to be responsible for its poor fatigue damage tolerance. For titanium alloys, the threshold intensity factor range, Delta Ksbth, for small fatigue cracks in the matrix alloys of TMCs has been determined to be between 0.9 ˜ 1.0 MPa*msp{1/2} which is much lower than that for long

  13. Interface effects on crack deflection and bridging during fatigue crack growth of titanium matrix composites

    SciTech Connect

    Warrier, S.G.; Majumdar, B.S.; Miracle, D.B.

    1997-12-01

    The effect of the interface on the crack deflection and crack bridging behavior of continuous fiber-reinforced titanium matrix composites has been investigated using three interfaces with significantly different mechanical characteristics. Each of these composites exhibited stress ranges in which fiber bridging was present and stress ranges in which stable fiber bridging was not present. The fatigue crack growth rate for all composites, even for the ones that did not exhibit fiber bridging, was significantly below that of the matrix. This phenomenon, believed to be an effect of elastic crack shielding, was most significant for composites with the strongest interfacial bond. Interface failure ahead of the crack tip and its influence on the local stress intensity factor is believed to be responsible for the decrease in the shielding effect of low strength interfaces. Interface debonding was observed in all three composites, and damage to the interface ahead of the crack tip was seen in two of the three composites. A stress-based criterion for predicting debonding appeared to effectively explain the crack deflection behavior for the three composites. Evidence of crack deflection even for the strongest interface suggests that there is scope to increase the interface bond strength in SiC/Ti-alloy system for improved transverse properties without compromising the fatigue life.

  14. Corrosion fatigue crack growth: The role of crack-tip deformation and film formation kinetics

    SciTech Connect

    Hudak, S.J. Jr.

    1988-01-01

    The objectives of this study was to elucidate the mechanical and electrochemical conditions at the crack-tip, then use this information to critically assess anodic dissolution versus hydrogen embrittlement mechanisms of crack growth. The system studied is sensitized 304 stainless steel in an aqueous 0.1M Na{sub 2}SO{sub 4} environment at temperatures of 343 to 363 K. The first-ever measurements of crack-tip strains at growing corrosion-fatigue cracks were obtained using the stereoimaging technique. Results showed that the crack-tip strains, and thus strain rates, were significantly less than those attending crack growth in an inert environment. The local environment was determined using a novel crack-simulation experiment involving cyclic straining of a creviced electrode. Cyclic straining was found to decrease the electrode potential and promote acidification of the occluded environment and resulted in the following steady-state electrochemical conditions: pH = 5.2 to 6.2 with local cell (mixed) potentials of 0 to {minus}200 mV (vs SHE) and bare surfaces potentials of approximately {minus}300 to {minus}500 mV (SHE). It is concluded that hydrogen embrittlement is the predominant mechanism of corrosion fatigue crack growth in the 304 stainless steel-aqueous environment system.

  15. Research on anti crack mechanism of bionic coupling brake disc

    NASA Astrophysics Data System (ADS)

    Shi, Lifeng; Yang, Xiao; Zheng, Lingnan; Wu, Can; Ni, Jing

    2017-09-01

    According to the biological function of fatigue resistance possessed by biology, this study designed a Bionic Coupling Brake Disc (BCBD) which can inhibit crack propagation as the result of improving fatigue property. Thermal stress field of brake disc was calculated under emergency working condition, and circumferential and radial stress field which lead to fatigue failure of brake disc were investigated simultaneously. Results showed that the maximum temperature of surface reached 890°C and the maximum residual tensile stress was 207 Mpa when the initial velocity of vehicle was 200 km/h. Based on the theory of elastic plastic fracture mechanics, the crack opening displacement and the crack front J integrals of the BCBD and traditional brake disc (TBD) with pre-cracking were calculated, and the strength of crack front was compared. Results revealed the growth behavior of fatigue crack located on surface of brake disc, and proved the anti fatigue resistance of BCBD was better, and the strength of crack resistance of BCBD was much stronger than that of TBD. This simulation research provided significant references for optimization and manufacturing of BCBD.

  16. Fatigue damage accumulation in nickel prior to crack initiation

    NASA Technical Reports Server (NTRS)

    Grobstein, T. L.; Sivashankaran, S.; Welsch, G.; Panigrahi, N.; Mcgervey, J. D.; Blue, J. W.

    1991-01-01

    The accumulation of lattice defects during fatigue cycling of nickel was investigated by electrical resistivity measurements, positron annihilation lifetime spectroscopy and transmission electron microscopy. Dislocations and vacancy clusters were found to be the main defect types. During cycling of axial and flexural samples at constant load amplitude, the dislocations form a saturated structure early in the fatigue life. This saturated structure consists of a cellular dislocation matrix, in which persistent slip bands (PSBs) begin to operate after the saturation has been established. Vacancies and vacancy clusters are formed during fatigue as a consequence of repetitive dislocation glide in the PSB structure. When PSBs operate, the matrix is assumed to be dormant, allowing vacancies to accumulate preferentially in the PSBs. The increase in vacancy concentration then accounts for the monotonic accumulation of fatigue damage, which points to the importance of vacancy accumulation as a precursor to crack nucleation.

  17. Fatigue Crack Growth Fundamentals in Shape Memory Alloys

    NASA Astrophysics Data System (ADS)

    Wu, Y.; Ojha, A.; Patriarca, L.; Sehitoglu, H.

    2015-03-01

    In this study, based on a regression of the crack tip displacements, the stress intensity range in fatigue is quantitatively determined for the shape memory alloy Ni2FeGa. The results are compared to the calculated stress intensity ranges with a micro-mechanical analysis accounting for the transformation-induced tractions. The effective stress intensity ranges obtained with both methods are in close agreement. Also, the fatigue crack closure levels were measured as 30 % of the maximum load using virtual extensometers along the crack flanks. This result is also in close agreement with the regression and micro-mechanical modeling findings. The current work pointed to the importance of elastic moduli changes and the residual transformation strains playing a role in the fatigue crack growth behavior. Additional simulations are conducted for two other important shape memory alloys, NiTi and CuZnAl, where the reductions in stress intensity range were found to be lower than Ni2FeGa.

  18. Working toward a three-dimensional fatigue closure model for surface cracks

    NASA Technical Reports Server (NTRS)

    Joseph, Paul F.

    1995-01-01

    The first reliable elastic fracture mechanics solutions for a surface crack in a plate were obtained by Newman and Raju. The authors, both from the Mechanics of Materials Branch at NASA-Langley, used a highly detailed finite element solution requiring substantial computational resources. Computers have since become more powerful and available; however, many important related problems remain computationally expensive. The problem of three-dimensional fatigue crack growth taking into account plasticity-induced crack closure is one such problem. It is the goal of this research to provide an efficient method to account for three-dimensional crack closure in fatigue. Newman developed a two-dimensional plasticity-induced crack closure model for center cracked specimens. This model requires iterations to determine both the contact solution at each growth step and the extent of the plastic zone at the crack tip. A three-dimensional version of this model would require obtaining these nonlinear variables all along the crack front. This model must be efficient so that repeated calculations can be performed during crack growth simulations. The highly versatile line spring model (LSM) with contact, fatigue, and plasticity will form the basis of the closure model. There are several required additions to past work to address the three-dimensional crack closure problem. Initially, these additions will include (1) an improved LSM to more accurately obtain the crack opening displacement, stress intensity factors, and elastic T-stress near the ends of the surface crack; (2) a method to determine the extent of the plastic zone all along the crack front; (3) a method to determine the contact zone given a perfectly plastic layer of material on the crack surfaces; (4) a method to determine the magnitude of the compressive contact stress; and (5) a way to implement the degree of constraint along the curved crack front. During the summer ASEE program an enhanced LSM was developed. A method

  19. A new testing method for investigating the shear-mode fatigue crack growth behavior in hydrogen environment

    NASA Astrophysics Data System (ADS)

    Akaki, Y.; Matsuo, T.; Nishimura, Y.; Miyakawa, S.; Endo, M.

    2017-05-01

    Ball bearing is widely used in a variety of machines including the transportation equipments of the automobiles and airplanes. Flaking failure is a common problem for ball bearing and it is caused by shear-mode fatigue crack growth under cyclic shear stress. Further, it is known that the premature flaking is attributed to the combined effect of hydrogen penetration into the material and cyclic shear stress during the operation. Therefore, in order to ensure the integrity of ball bearing, it is necessary to clarify the effect of hydrogen on shear-mode fatigue crack growth behavior, in particular, the threshold behavior. The evaluation of the shear-mode crack growth behavior is not easy because mode I crack branching occurs easily. Our previous studies revealed that it is required to apply static compression in the direction of specimen axis to attain a stable shear-mode fatigue crack growth. In addition, successive hydrogen supply to the specimen is essential for the evaluation of hydrogen effect on the fatigue threshold because hydrogen emits from the specimen during the fatigue test. In other words, the hydrogen-precharging method, commonly used for the research on hydrogen embrittlement, is not appropriate for the evaluation of fatigue threshold. In this study, to solve these problems, we have developed a novel, easy-to-use experimental method to evaluate the threshold behavior of shear-mode fatigue crack in the presence of hydrogen. The fundamental principle of the method is introduced in this paper.

  20. Effects of microstructure banding on hydrogen assisted fatigue crack growth in X65 pipeline steels

    SciTech Connect

    Ronevich, Joseph A.; Somerday, Brian P.; San Marchi, Chris W.

    2015-09-10

    Banded ferrite-pearlite X65 pipeline steel was tested in high pressure hydrogen gas to evaluate the effects of oriented pearlite on hydrogen assisted fatigue crack growth. Test specimens were oriented in the steel pipe such that cracks propagated either parallel or perpendicular to the banded pearlite. The ferrite-pearlite microstructure exhibited orientation dependent behavior in which fatigue crack growth rates were significantly lower for cracks oriented perpendicular to the banded pearlite compared to cracks oriented parallel to the bands. Thus the reduction of hydrogen assisted fatigue crack growth across the banded pearlite is attributed to a combination of crack-tip branching and impeded hydrogen diffusion across the banded pearlite.

  1. Crack Growth Behavior in the Threshold Region for High Cycle Fatigue Loading

    NASA Technical Reports Server (NTRS)

    Forman, Royce G.; Figert, J.; Beek, J.; Ventura, J.; Martinez, J.; Samonski, F.

    2011-01-01

    This presentation describes results obtained from a research project conducted at the NASA Johnson Space Center (JSC) that was jointly supported by the FAA Technical Center and JSC. The JSC effort was part of a multi-task FAA program involving several U.S. laboratories and initiated for the purpose of developing enhanced analysis tools to assess damage tolerance of rotorcraft and aircraft propeller systems. The research results to be covered in this presentation include a new understanding of the behavior of fatigue crack growth in the threshold region. This behavior is important for structural life analysis of aircraft propeller systems and certain rotorcraft structural components (e.g., the mast). These components are often designed to not allow fatigue crack propagation to exceed an experimentally determined fatigue crack growth threshold value. During the FAA review meetings for the program, disagreements occurred between the researchers regarding the observed fanning (spread between the da/dN curves of constant R) in the threshold region at low stress ratios, R. Some participants believed that the fanning was a result of the ASTM load shedding test method for threshold testing, and thus did not represent the true characteristics of the material. If the fanning portion of the threshold value is deleted or not included in a life analysis, a significant penalty in the calculated life and design of the component would occur. The crack growth threshold behavior was previously studied and reported by several research investigators in the time period: 1970-1980. Those investigators used electron microscopes to view the crack morphology of the fatigue fracture surfaces. Their results showed that just before reaching threshold, the crack morphology often changed from a striated to a faceted or cleavage-like morphology. This change was reported to have been caused by particular dislocation properties of the material. Based on the results of these early investigations, a

  2. Comparison of Fatigue Properties and Fatigue Crack Growth Rates of Various Implantable Metals

    PubMed Central

    Okazaki, Yoshimitsu

    2012-01-01

    The fatigue strength, effects of a notch on the fatigue strength, and fatigue crack growth rate of Ti-15Zr-4Nb-4Ta alloy were compared with those of other implantable metals. Zr, Nb, and Ta are important alloying elements for Ti alloys for attaining superior long-term corrosion resistance and biocompatibility. The highly biocompatible Ti-15Zr-4Nb-4Ta alloy exhibited an excellent balance between strength and ductility. Its notched tensile strength was much higher than that of a smooth specimen. The strength of 20% cold-worked commercially pure (C.P.) grade 4 Ti was close to that of Ti alloy. The tension-to-tension fatigue strength of an annealed Ti-15Zr-4Nb-4Ta rod at 107 cycles was approximately 740 MPa. The fatigue strength of this alloy was much improved by aging treatment after solution treatment. The fatigue strengths of C.P. grade 4 Ti and stainless steel were markedly improved by 20% cold working. The fatigue strength of Co-Cr-Mo alloy was markedly increased by hot forging. The notch fatigue strengths of 20% cold-worked C.P. grade 4 Ti, and annealed and aged Ti-15Zr-4Nb-4Ta, and annealed Ti-6Al-4V alloys were less than those of the smooth specimens. The fatigue crack growth rate of Ti-15Zr-4Nb-4Ta was the same as that of Ti-6Al-4V. The fatigue crack growth rate in 0.9% NaCl was the same as that in air. Stainless steel and Co-Cr-Mo-Ni-Fe alloy had a larger stress-intensity factor range (ΔK) than Ti alloy.

  3. The Effects of Slip Character and Crack Closure on the Growth of Small Fatigue Cracks in Titanium-Aluminium Alloys

    DTIC Science & Technology

    1990-02-01

    Measurement of Small Cracks 128 4.2.5.4 Propagation of Small Cracks 134 4.2.5.4.1 Effect of Stress Level 135 4.2.5.4.2 Effect of Stress Ratio 139...Initiation from Persistent Slip Bands 197 5.1.3.3 Crack -Shape Effects on the Stress Intensity Factor 198 5.2 Primary Experimental Variables 201 5.2.1 Effects...demonstrating the "anomalous" behavior of 6 small fatigue cracks [20,211. Fig. 2.2 Fatigue-limit stress predicted by combining crack initiation and 8 crack

  4. New specimen design for studying the growth of small fatigue cracks with surface acoustic waves

    NASA Astrophysics Data System (ADS)

    London, Blair

    1985-08-01

    The study of small surface fatigue cracks in AISI 4140 quenched and tempered steel by a nondestructive surface acoustic wave technique is summarized. A novel cantilevered bending, plate-type fatigue specimen is described that is compatible with the acoustic method. Small cracks are initiated from a 25-μm deep surface pit produced by an electrospark machine. The importance of studying these cracks which closely approximate naturally occurring fatigue cracks is briefly discussed.

  5. Empirical modeling of environment-enhanced fatigue crack propagation in structural alloys for component life prediction

    NASA Technical Reports Server (NTRS)

    Richey, Edward, III

    1995-01-01

    This research aims to develop the methods and understanding needed to incorporate time and loading variable dependent environmental effects on fatigue crack propagation (FCP) into computerized fatigue life prediction codes such as NASA FLAGRO (NASGRO). In particular, the effect of loading frequency on FCP rates in alpha + beta titanium alloys exposed to an aqueous chloride solution is investigated. The approach couples empirical modeling of environmental FCP with corrosion fatigue experiments. Three different computer models have been developed and incorporated in the DOS executable program. UVAFAS. A multiple power law model is available, and can fit a set of fatigue data to a multiple power law equation. A model has also been developed which implements the Wei and Landes linear superposition model, as well as an interpolative model which can be utilized to interpolate trends in fatigue behavior based on changes in loading characteristics (stress ratio, frequency, and hold times).

  6. Modeling fatigue crack growth in cross ply titanium matrix composites

    NASA Technical Reports Server (NTRS)

    Bakuckas, J. G., Jr.; Johnson, W. S.

    1993-01-01

    In this study, the fatigue crack growth behavior of fiber bridging matrix cracks in cross-ply SCS-6/Ti-15-3 and SCS-6/Timetal-21S laminates containing center holes was investigated. Experimental observations revealed that matrix cracking was far more extensive and wide spread in the SCS-6/Ti-15-3 laminates compared to that in the SCS-6/Timetal-21S laminates. In addition, the fatigue life of the SCS-6/Ti-15-3 laminates was significantly longer than that of the SCS-6/Timetal-21S laminates. The matrix cracking observed in both material systems was analyzed using a fiber bridging (FB) model which was formulated using the boundary correction factors and weight functions for center hole specimen configurations. A frictional shear stress is assumed in the FB model and was used as a curve fitting parameter to model matrix crack growth data. The higher frictional shear stresses calculated in the SCS-6/Timetal-21S laminates resulted in lower stress intensity factors in the matrix and higher axial stresses in the fibers compared to those in the SCS-6/Ti-15-3 laminates at the same applied stress levels.

  7. Analysis of fatigue crack growth in terms of crack closure and energy

    SciTech Connect

    Ranganathan, N.

    1999-07-01

    The fatigue crack growth behavior of the aluminum alloy 2024 is analyzed using the crack closure and an energy-based concept. The different test conditions studied include load ratio and environmental effects, crack growth retardation following a single overload, and crack propagation under block load tests. Crack opening loads using the compliance technique permit the effect of load ratio to be taken into account. After an overload, in the deceleration phase, the evolution of the crack opening load is not compatible with that of the crack growth rate. The measured crack opening levels under constant-amplitude loading conditions are comparable to those predicted under plane strain conditions for moderate {Delta}K levels. It is shown that most of the effects usually attributed to closure can be successfully explained using energy concepts. In particular, it is shown that there exists a linear relationship between the crack growth rate and the energy dissipated per cycle at high growth rates, which is valid for both the environments studied, and it corresponds to a crack growth mechanism characterized by striation formation during each cycle. For lower growth rates a power law relationship can be proposed between these two parameters. The above-mentioned linear relationship holds also for the block loading conditions based on total energy dissipated per block. Certain experimental facts bring out the effect of closure on the energy dissipated. It is further shown that the possible existence of a mixed (Mode I and Mode II) mode crack opening at the crack tip has to be taken into account to correctly evaluate the energy dissipated near the crack tip.

  8. Acoustic emission fatigue crack monitoring of a simulated aircraft fuselage structure

    NASA Astrophysics Data System (ADS)

    Lucas, Jeremy James

    The purpose of this research was to replicate the fatigue cracking that occurs in aircraft placed under loads from cyclical compression and decompression. As a fatigue crack grows, it releases energy in the form of acoustic emissions. These emissions are transmitted through the structure in waves, which can be recorded using acoustic emission (AE) transducers. This research employed a pressure vessel constructed out of aluminum and placed under cyclical loads at 1 Hz in order to simulate the loads placed on an aircraft fuselage in flight. The AE signals were recorded by four resonant AE transducers. These were placed on the pressure vessel such that it was possible to determine the location of each AE signal. These signals were then classified using a Kohonen self organizing map (SOM) neural network. By using proper data filtering before the SOM was run and using the correct classification parameters, it was shown that this is a highly accurate method of classifying AE waveforms from fatigue crack growth. This initial classification was done using AE waveform quantification parameters. The method was then validated by using both source location and then examining the waveforms in order to ensure that the waveforms classified into each category were the expected waveform types associated with each of the AE sources. Thus, acoustic emission nondestructive testing (NDT), in combination with a SOM neural network, proved to be an excellent means of fatigue crack growth monitoring in a simulated aluminum aircraft structure.

  9. Simultaneous Measurements of Harmonic Waves at Fatigue-Cracked Interfaces

    NASA Astrophysics Data System (ADS)

    Hyunjo, Jeong; Dan, Barnard

    2011-08-01

    Nonlinear harmonic waves generated at cracked interfaces are investigated theoretically and experimentally. A compact tension specimen is fabricated and the amplitude of the transmitted wave is analyzed as a function of position along the fatigued crack surface. In order to measure as many nonlinear harmonic components as possible, broadband lithium niobate (LiNbO3) transducers are employed together with a calibration technique for making absolute amplitude measurements with fluid-coupled receiving transducers. Cracked interfaces are shown to generate high acoustic nonlinearities, which are manifested as harmonics in the power spectrum of the received signal. The first subharmonic f/2 and the second harmonic 2f waves are found to be dominant nonlinear components for an incident toneburst signal of frequency f. To explain the observed nonlinear behavior, a partially closed crack is modeled by planar half interfaces that can account for crack parameters, such as crack opening displacement and crack surface conditions. The simulation results show reasonable agreement with the experimental results.

  10. Modeling fatigue crack growth for life-extending control

    NASA Astrophysics Data System (ADS)

    Patankar, Ravindra Prakash

    1999-12-01

    This dissertation presents a nonlinear dynamic model of fatigue crack growth in the state-space setting under variable amplitude cyclic load. The model is especially suited to the needs of real-time decision-making for life-extending control. The state variables are crack length and crack opening stress. The model is capable of capturing the effects of a single-cycle overload, block loads, random loads, and irregular sequences through a fading memory algorithm. Model predictions are in good agreement with experimental data on 7075-T6 and 2024-T3 aluminum alloys. Compiled results also demonstrate that the proposed model compares well with one of the most comprehensive models, FASTRAN-II that is used by the aircraft industry. Specifically, the state-space model recursively computes the crack opening stress via a simple functional relationship based on the principle of fading memory and does not require the storage of the stress history for its execution. Therefore, savings in both computation time and memory requirements are significant. The need for a reliable damage model for life-extending control is addressed with reference to the colossal inaccuracies that could occur in controller synthesis for a reusable rocket engine if a simplistic damage model is used under variable-amplitude load conditions. The seemingly counter-intuitive notion of overload injection could be gainfully utilized for life-extending optimization. The proof of this concept is demonstrated on a laboratory test apparatus by life-extension of test specimens with intentionally injected overload pulses at specific intervals. A stochastic model of fatigue crack growth under variable-amplitude load is proposed using the framework of the state-space model. The stochastic model is validated with four sets of constant-amplitude load test data and a set under variable-amplitude load test. The crack growth process is observed to be nearly deterministic for a cyclic load applied to a given specimen

  11. Measurement and analysis of critical crack tip processes associated with variable amplitude fatigue crack growth

    NASA Technical Reports Server (NTRS)

    Hudak, S. J., Jr.; Davidson, D. L.; Chan, K. S.

    1983-01-01

    Crack growth retardation following overloads can result in overly conservative life predictions in structures subjected to variable amplitude fatigue loading when linear damage accumulation procedures are employed. Crack closure is believed to control the crack growth retardation, although the specific closure mechanism is debatable. Information on the relative contributions to crack closure from: (1) plasticity left in the wake of the advancing crack and (2) crack tip residual stresses is provided. The delay period and corresponding crack growth rate transients following overloads are systematically measured as a function of load ratio (R) and overload magnitude. These responses are correlated in terms of the local 'driving force' for crack growth as measured by crack tip opening loads and delta K sub eff. The latter measurements are obtained using a scanning electron microscope equipped with a cyclic loading stage; measurements are quantified using a relatively new stereoimaging technique. Combining experimental results with analytical predictions suggests that both plastic wake and residual stress mechanism are operative, the latter becoming predominate as R increases.

  12. Study on fatigue crack propagation characteristics around welded joint interface in complexed conditions

    NASA Astrophysics Data System (ADS)

    Ota, Akihiko; Suzuki, Naoyuki; Maeda, Yoshio; Mawari, Toshio; Matsuoka, Saburo; Nishijima, Satoshi

    1993-01-01

    Marine structures are often constructed by welding, and they are subject to repeated loading such as waves and mechanical vibrations which can create fatigue cracks and consequently break the structures. Fatigue crack propagation properties of welded joints are studied under random loading in the air, synthetic sea water, and compressive cycling. It was found that the most crucial factor that controls fatigue crack propagation was high tensile residual stress fields of welded joints. This stress constantly kept the cracks open, simplifying fatigue crack propagation, and therefore, the rate of crack propagation could be assessed with high accuracy. In the transverse matching welded joints with cracks in the center, crack closure did not occur due to the tensile residual stress constantly induced at the crack ends in the center of the test samples. Fatigue crack propagation was accelerated both in artificial sea water and in compressive cycling compared to that in the air, and the fatigue lowest values were about half. Serious crack closures occurred in compressive cycling in which dry hours exceeded 45 minutes, and the fatigue crack propagation rate deteriorated remarkably. Mean fatigue crack propagation rate under the random loading is estimated precisely using equivalent stress intensity factor limit.

  13. NASA/FLAGRO - FATIGUE CRACK GROWTH COMPUTER PROGRAM

    NASA Technical Reports Server (NTRS)

    Forman, R. G.

    1994-01-01

    Structural flaws and cracks may grow under fatigue inducing loads and, upon reaching a critical size, cause structural failure to occur. The growth of these flaws and cracks may occur at load levels well below the ultimate load bearing capability of the structure. The Fatigue Crack Growth Computer Program, NASA/FLAGRO, was developed as an aid in predicting the growth of pre-existing flaws and cracks in structural components of space systems. The earlier version of the program, FLAGRO4, was the primary analysis tool used by Rockwell International and the Shuttle subcontractors for fracture control analysis on the Space Shuttle. NASA/FLAGRO is an enhanced version of the program and incorporates state-of-the-art improvements in both fracture mechanics and computer technology. NASA/FLAGRO provides the fracture mechanics analyst with a computerized method of evaluating the "safe crack growth life" capabilities of structural components. NASA/FLAGRO could also be used to evaluate the damage tolerance aspects of a given structural design. The propagation of an existing crack is governed by the stress field in the vicinity of the crack tip. The stress intensity factor is defined in terms of the relationship between the stress field magnitude and the crack size. The propagation of the crack becomes catastrophic when the local stress intensity factor reaches the fracture toughness of the material. NASA/FLAGRO predicts crack growth using a two-dimensional model which predicts growth independently in two directions based on the calculation of stress intensity factors. The analyst can choose to use either a crack growth rate equation or a nonlinear interpolation routine based on tabular data. The growth rate equation is a modified Forman equation which can be converted to a Paris or Walker equation by substituting different values into the exponent. This equation provides accuracy and versatility and can be fit to data using standard least squares methods. Stress

  14. NASA/FLAGRO - FATIGUE CRACK GROWTH COMPUTER PROGRAM

    NASA Technical Reports Server (NTRS)

    Forman, R. G.

    1994-01-01

    Structural flaws and cracks may grow under fatigue inducing loads and, upon reaching a critical size, cause structural failure to occur. The growth of these flaws and cracks may occur at load levels well below the ultimate load bearing capability of the structure. The Fatigue Crack Growth Computer Program, NASA/FLAGRO, was developed as an aid in predicting the growth of pre-existing flaws and cracks in structural components of space systems. The earlier version of the program, FLAGRO4, was the primary analysis tool used by Rockwell International and the Shuttle subcontractors for fracture control analysis on the Space Shuttle. NASA/FLAGRO is an enhanced version of the program and incorporates state-of-the-art improvements in both fracture mechanics and computer technology. NASA/FLAGRO provides the fracture mechanics analyst with a computerized method of evaluating the "safe crack growth life" capabilities of structural components. NASA/FLAGRO could also be used to evaluate the damage tolerance aspects of a given structural design. The propagation of an existing crack is governed by the stress field in the vicinity of the crack tip. The stress intensity factor is defined in terms of the relationship between the stress field magnitude and the crack size. The propagation of the crack becomes catastrophic when the local stress intensity factor reaches the fracture toughness of the material. NASA/FLAGRO predicts crack growth using a two-dimensional model which predicts growth independently in two directions based on the calculation of stress intensity factors. The analyst can choose to use either a crack growth rate equation or a nonlinear interpolation routine based on tabular data. The growth rate equation is a modified Forman equation which can be converted to a Paris or Walker equation by substituting different values into the exponent. This equation provides accuracy and versatility and can be fit to data using standard least squares methods. Stress

  15. A large-area strain sensing technology for monitoring fatigue cracks in steel bridges

    NASA Astrophysics Data System (ADS)

    Kong, Xiangxiong; Li, Jian; Collins, William; Bennett, Caroline; Laflamme, Simon; Jo, Hongki

    2017-08-01

    This paper presents a novel large-area strain sensing technology for monitoring fatigue cracks in steel bridges. The technology is based on a soft elastomeric capacitor (SEC), which serves as a flexible and large-area strain gauge. Previous experiments have verified the SEC’s capability to monitor low-cycle fatigue cracks experiencing large plastic deformation and large crack opening. Here an investigation into further extending the SEC’s capability for long-term monitoring of fatigue cracks in steel bridges subject to traffic loading, which experience smaller crack openings. It is proposed that the peak-to-peak amplitude (pk-pk amplitude) of the sensor’s capacitance measurement as the indicator of crack growth to achieve robustness against capacitance drift during long-term monitoring. Then a robust crack monitoring algorithm is developed to reliably identify the level of pk-pk amplitudes through frequency analysis, from which a crack growth index (CGI) is obtained for monitoring fatigue crack growth under various loading conditions. To generate representative fatigue cracks in a laboratory, loading protocols were designed based on constant ranges of stress intensity to limit plastic deformations at the crack tip. A series of small-scale fatigue tests were performed under the designed loading protocols with various stress intensity ratios. Test results under the realistic fatigue crack conditions demonstrated the proposed crack monitoring algorithm can generate robust CGIs which are positively correlated with crack lengths and independent from loading conditions.

  16. Investigation of the effects of manufacturing variations and materials on fatigue crack detection methods in gear teeth

    NASA Technical Reports Server (NTRS)

    Wheitner, Jeffrey A.; Houser, Donald R.

    1994-01-01

    The fatigue life of a gear tooth can be thought of as the sum of the number of cycles required to initiate a crack, N(sub i), plus the number of cycles required to propagate the crack to such a length that fracture occurs, N(sub p). The factors that govern crack initiation are thought to be related to localized stress or strain at a point, while propagation of a fatigue crack is a function of the crack tip parameters such as crack shape, stress state, and stress intensity factor. During a test there is no clear transition between initiation and propagation. The mechanisms of initiation and propagation are quite different and modeling them separately produces a higher degree of accuracy, but then the question that continually arises is 'what is a crack?' The total life prediction in a fracture mechanics model presently hinges on the assumption of an initial crack length, and this length can significantly affect the total life prediction. The size of the initial crack is generally taken to be in the range of 0.01 in. to 0.2 in. Several researchers have used various techniques to determine the beginning of the crack propagation stage. Barhorst showed the relationship between dynamic stiffness changes and crack propagation. Acoustic emissions, which are stress waves produced by the sudden movement of stressed materials, have also been successfully used to monitor the growth of cracks in tensile and fatigue specimens. The purpose of this research is to determine whether acoustic emissions can be used to define the beginning of crack propagation in a gear using a single-tooth bending fatigue test.

  17. 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.

  18. 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).

  19. Fatigue-Crack-Growth Behavior of Two Pipeline Steels

    DOE PAGES

    Chen, Bilin; Wang, Gongyao; Chen, Shuying; ...

    2016-10-17

    This paper focuses on studying the fatigue-crack-growth behavior of two types of pipeline steels, and investigating their microstructural differences, which could influence the fatigue behavior. For fatigue experiments, compact-tension (CT) specimens are employed. These two kinds of base pipeline steels are Alloy B [Fe-0.05C-1.52Mn-0.12Si-0.092Nb, weight percent (wt.%)] and Alloy C [(Fe- 0.04C-1.61Mn-0.14Si-0.096Nb, wt.%)]. They have been tested at various frequencies (10 Hz, 1 Hz, and 0.1 Hz) and different R ratios (0.1 and 0.5, R = Pmin./Pmax. where Pmin. is the minimum applied load, and Pmax. is the maximum applied load) in air. The effects of frequencies and R ratiosmore » on crackpropagation behavior are compared. The microstructures of fracture surfaces are investigated, using both scanning-electron microscopy (SEM) and transmission-electron microscopy (TEM). It is concluded that higher R ratios lead to faster crack-growth rates, while frequency does not have much influence on the fatigue-crack-growth rates. Moreover, Alloy B (Fe-0.05C-1.52Mn-0.12Si-0.092Nb, wt.%) tends to have better fatigue resistance than Alloy C (Fe-0.04C-1.61Mn-0.14Si-0.096Nb, wt.%) under various test conditions in air.« less

  20. Fatigue-Crack-Growth Behavior of Two Pipeline Steels

    SciTech Connect

    Chen, Bilin; Wang, Gongyao; Chen, Shuying; Muralidharan, Govindarajan; Stalheim, Doug; Sun, An-Cheng; Huang, E-Wen; Liaw, Peter K.

    2016-10-17

    This paper focuses on studying the fatigue-crack-growth behavior of two types of pipeline steels, and investigating their microstructural differences, which could influence the fatigue behavior. For fatigue experiments, compact-tension (CT) specimens are employed. These two kinds of base pipeline steels are Alloy B [Fe-0.05C-1.52Mn-0.12Si-0.092Nb, weight percent (wt.%)] and Alloy C [(Fe- 0.04C-1.61Mn-0.14Si-0.096Nb, wt.%)]. They have been tested at various frequencies (10 Hz, 1 Hz, and 0.1 Hz) and different R ratios (0.1 and 0.5, R = Pmin./Pmax. where Pmin. is the minimum applied load, and Pmax. is the maximum applied load) in air. The effects of frequencies and R ratios on crackpropagation behavior are compared. The microstructures of fracture surfaces are investigated, using both scanning-electron microscopy (SEM) and transmission-electron microscopy (TEM). It is concluded that higher R ratios lead to faster crack-growth rates, while frequency does not have much influence on the fatigue-crack-growth rates. Moreover, Alloy B (Fe-0.05C-1.52Mn-0.12Si-0.092Nb, wt.%) tends to have better fatigue resistance than Alloy C (Fe-0.04C-1.61Mn-0.14Si-0.096Nb, wt.%) under various test conditions in air.

  1. Fatigue crack propagation in additively manufactured porous biomaterials.

    PubMed

    Hedayati, R; Amin Yavari, S; Zadpoor, A A

    2017-07-01

    Additively manufactured porous titanium implants, in addition to preserving the excellent biocompatible properties of titanium, have very small stiffness values comparable to those of natural bones. Although usually loaded in compression, biomedical implants can also be under tensional, shear, and bending loads which leads to crack initiation and propagation in their critical points. In this study, the static and fatigue crack propagation in additively manufactured porous biomaterials with porosities between 66% and 84% is investigated using compact-tension (CT) samples. The samples were made using selective laser melting from Ti-6Al-4V and were loaded in tension (in static study) and tension-tension (in fatigue study) loadings. The results showed that displacement accumulation diagram obtained for different CT samples under cyclic loading had several similarities with the corresponding diagrams obtained for cylindrical samples under compression-compression cyclic loadings (in particular, it showed a two-stage behavior). For a load level equaling 50% of the yield load, both the CT specimens studied here and the cylindrical samples we had tested under compression-compression cyclic loading elsewhere exhibited similar fatigue lives of around 10(4) cycles. The test results also showed that for the same load level of 0.5Fy, the lower density porous structures demonstrate relatively longer lives than the higher-density ones. This is because the high bending stresses in high-density porous structures gives rise to local Mode-I crack opening in the rough external surface of the struts which leads to quicker formation and propagation of the cracks. Under both the static and cyclic loading, all the samples showed crack pathways which were not parallel to but made 45(°) angles with respect to the notch direction. This is due to the fact that in the rhombic dodecahedron unit cell, the weakest struts are located in 45(°) direction with respect to the notch direction

  2. Fatigue crack closure under triaxial stress constraint. 1: Experimental investigation

    SciTech Connect

    Wanlin, G.

    1994-09-01

    Experimental investigation has been done for fatigue crack closure in 7050-T7451 aluminum alloy and 30CrMnSiNi2A high strength steel. CT specimens with thicknesses of 2, 3, 4, and 8 mm are used and four techniques of closure measurement are adopted in the test, namely the COD-load compliance technique, back face strain-load compliance technique, near tip strain gauge technique, and the indirect technique based on fatigue crack propagation rate (FCPR) measurement. The uncertainties associated with different techniques of closure measurement as well as the thickness effects are analyzed. It is shown that the global-compliance based methods cannot reflect the shielding effect of closure upon the crack tip strain intensification sufficiently. The near tip strain gauge method is more sensitive and can give a better result if the gauge lies in an optimum position. For materials in which the striation mechanism is dominant the indirect methods based on FCPR or fatigue striation measurement can predict the closure level very well. 12 refs.

  3. Fatigue crack closure under triaxial stress constraint. 1: Experimental investigation

    NASA Astrophysics Data System (ADS)

    Wanlin, Guo

    1994-09-01

    Experimental investigation has been done for fatigue crack closure in 7050-T7451 aluminum alloy and 30CrMnSiNi2A high strength steel. CT specimens with thicknesses of 2, 3, 4, and 8 mm are used and four techniques of closure measurement are adopted in the test, namely the COD-load compliance technique, back face strain-load compliance technique, near tip strain gauge technique, and the indirect technique based on fatigue crack propagation rate (FCPR) measurement. The uncertainties associated with different techniques of closure measurement as well as the thickness effects are analyzed. It is shown that the global-compliance based methods cannot reflect the shielding effect of closure upon the crack tip strain intensification sufficiently. The near tip strain gauge method is more sensitive and can give a better result if the gauge lies in an optimum position. For materials in which the striation mechanism is dominant the indirect methods based on FCPR or fatigue striation measurement can predict the closure level very well.

  4. Fatigue crack growth behavior in niobium-hydrogen alloys

    NASA Astrophysics Data System (ADS)

    Lin, Mark Ching-Cheng; Salama, K.

    1997-10-01

    Near-threshold fatigue crack growth behavior has been investigated in niobium-hydrogen alloys. Compact tension specimens (CTS) with three hydrogen conditions are used: hydrogen-free, hydrogen in solid solution, and hydride alloy. The specimens are fatigued at a temperature of 296 K and load ratios of 0.05, 0.4, and 0.75. The results at load ratios of 0.05 and 0.4 show that the threshold stress intensity range (Δ K th ) decreases as hydrogen is added to niobium. It reaches a minimum at the critical hydrogen concentration ( C cr ), where maximum embrittlement occurs. The critical hydrogen concentration is approximately equal to the solubility limit of hydrogen in niobium. As the hydrogen concentration exceeds C cr , Δ K th increases slowly as more hydrogen is added to the specimen. At load ratio 0.75, Δ K th decreases continuously as the hydrogen concentration is increased. The results provide evidence that two mechanisms are responsible for fatigue crack growth behavior in niobium-hydrogen alloys. First, embrittlement is retarded by hydride transformation-induced and plasticity-induced crack closures. Second, embrittlement is enhanced by the presence of hydrogen and hydride.

  5. Vibration-based detection of fatigue cracks in structures

    NASA Astrophysics Data System (ADS)

    Razi, P.; Taheri, F.

    2011-04-01

    This paper presents the application of a novel vibration based technique for detecting fatigue cracks in structures. The method utilizes the empirical mode decomposition method (EMD) to establish an effective energy-based damage index. To investigate the feasibility of the method, fatigue cracks of different sizes were introduced in an aluminum beam subjected to a cyclic load under three point bending configuration. The vibration signals corresponding to the healthy and the damaged states of the beam were acquired via piezoceramic sensors. The signals were then processed by the proposed methodology to obtain the damage indices. In addition, for the sake of comparison, the natural frequencies of the healthy and damaged states of the beam were obtained through the Fast Fourier Transform (FFT). The results of this study concluded in two major observations. Firstly, the method was highly successful in not only predicting the presence of the fatigue crack, but also in quantifying its progression. Secondly, the proposed energy-based damage index was proved to be superior over the frequency-based method in terms of sensitivity to the damage detection and quantification. Moreover, this technique could be regarded as an efficient non-destructive tool, since it is simple, cost effective, and does not rely on analytical modelling of the structure.

  6. Detection of fatigue cracks by nondestructive testing methods

    NASA Technical Reports Server (NTRS)

    Anderson, R. T.; Delacy, T. J.; Stewart, R. C.

    1973-01-01

    The effectiveness was assessed of various NDT methods to detect small tight cracks by randomly introducing fatigue cracks into aluminum sheets. The study included optimizing NDT methods calibrating NDT equipment with fatigue cracked standards, and evaluating a number of cracked specimens by the optimized NDT methods. The evaluations were conducted by highly trained personnel, provided with detailed procedures, in order to minimize the effects of human variability. These personnel performed the NDT on the test specimens without knowledge of the flaw locations and reported on the flaws detected. The performance of these tests was measured by comparing the flaws detected against the flaws present. The principal NDT methods utilized were radiographic, ultrasonic, penetrant, and eddy current. Holographic interferometry, acoustic emission monitoring, and replication methods were also applied on a reduced number of specimens. Generally, the best performance was shown by eddy current, ultrasonic, penetrant and holographic tests. Etching provided no measurable improvement, while proof loading improved flaw detectability. Data are shown that quantify the performances of the NDT methods applied.

  7. Microstructural changes induced near crack tip during corrosion fatigue tests in austenitic-ferritic steel.

    PubMed

    Gołebiowski, B; Swiatnicki, W A; Gaspérini, M

    2010-03-01

    Microstructural changes occurring during fatigue tests of austenitic-ferritic duplex stainless steel (DSS) in air and in hydrogen-generating environment have been investigated. Hydrogen charging of steel samples during fatigue crack growth (FCG) tests was performed by cathodic polarization of specimens in 0.1M H(2)SO(4) aqueous solution. Microstructural investigations of specimens after FCG tests were carried out using transmission electron microscopy to reveal the density and arrangement of dislocations formed near crack tip. To determine the way of crack propagation in the microstructure, electron backscatter diffraction investigations were performed on fatigue-tested samples in both kinds of environment. To reveal hydrogen-induced phase transformations the atomic force microscopy was used. The above investigations allowed us to define the character of fatigue crack propagation and microstructural changes near the crack tip. It was found that crack propagation after fatigue tests in air is accompanied with plastic deformation; a high density of dislocations is observed at large distance from the crack. After fatigue tests performed during hydrogen charging the deformed zone containing high density of dislocations is narrow compared to that after fatigue tests in air. It means that hydrogenation leads to brittle character of fatigue crack propagation. In air, fatigue cracks propagate mostly transgranularly, whereas in hydrogen-generating environment the cracks have mixed transgranular/interfacial character.

  8. Small fatigue cracks; Proceedings of the Second International Conference/Workshop, Santa Barbara, CA, Jan. 5-10, 1986

    SciTech Connect

    Ritchie, R.O.; Lankford, J.

    1986-01-01

    Topics discussed in this volume include crack initiation and stage I growth, microstructure effects, crack closure, environment effects, the role of notches, analytical modeling, fracture mechanics characterization, experimental techniques, and engineering applications. Papers are presented on fatigue crack initiation along slip bands, the effect of microplastic surface deformation on the growth of small cracks, short fatigue crack behavior in relation to three-dimensional aspects and the crack closure effect, the influence of crack depth on crack electrochemistry and fatigue crack growth, and nondamaging notches in fatigue. Consideration is also given to models of small fatigue cracks, short crack theory, assessment of the growth of small flaws from residual strength data, the relevance of short crack behavior to the integrity of major rotating aero engine components, and the relevance of short fatigue crack growth data to the durability and damage tolerance analyses of aircraft.

  9. 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.

  10. 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

  11. The detection of fatigue cracks by nondestructive testing methods

    NASA Technical Reports Server (NTRS)

    Rummel, W. D.; Todd, P. H., Jr.; Frecska, S. A.; Rathke, R. A.

    1974-01-01

    X-radiographic penetrant, ultrasonic, eddy current, holographic, and acoustic emission techniques were optimized and applied to the evaluation of 2219-T87 aluminum alloy test specimens. One hundred eighteen specimens containing a total of 328 fatigue cracks were evaluated. The cracks ranged in length from 0.500 inch (1.27 cm) to 0.007 inch (0.018 cm) and in depth from 0.178 inch (0.451 cm) and 0.001 inch (0.003 cm). Specimen thicknesses were nominally 0.060 inch (0.152 cm) and 0.210 inch (0.532 cm) and surface finishes were nominally 32 and 125 rms and 64 and 200 rms respectively. Specimens were evaluated in the as-milled surface condition, in the chemically milled surface condition and, after proof loading, in a randomized inspection sequence. Results of the nondestructive test (NDT) evaluations were compared with actual crack size obtained by measurement of the fractured specimens. Inspection data was then analyzed to provide a statistical basis for determinating the threshold crack detection sensitivity (the largest crack size that would be missed) for each of the inspection techniques at a 95% probability and 95% confidence level.

  12. Accelerated Threshold Fatigue Crack Growth Effect-Powder Metallurgy Aluminum Alloy

    NASA Technical Reports Server (NTRS)

    Piascik, R. S.; Newman, J. A.

    2002-01-01

    Fatigue crack growth (FCG) research conducted in the near threshold regime has identified a room temperature creep crack growth damage mechanism for a fine grain powder metallurgy (PM) aluminum alloy (8009). At very low (Delta) K, an abrupt acceleration in room temperature FCG rate occurs at high stress ratio (R = K(sub min)/K(sub max)). The near threshold accelerated FCG rates are exacerbated by increased levels of K(sub max) (K(sub max) = 0.4 K(sub IC)). Detailed fractographic analysis correlates accelerated FCG with the formation of crack-tip process zone micro-void damage. Experimental results show that the near threshold and K(sub max) influenced accelerated crack growth is time and temperature dependent.

  13. 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.

  14. Corrosion-fatigue crack propagation of aluminum alloys for high-speed trains

    NASA Astrophysics Data System (ADS)

    Shen, Lin; Chen, Hui; Che, Xiaoli; Xu, Lidong

    2017-07-01

    A modified single-edge notch tension (SENT) specimen exposed to saline environment was utilized to investigate the corrosion-fatigue crack growth behaviors of 5083, 6005 and 7N01 aluminum alloys. The fatigue crack propagation life, corrosion-fatigue crack rate (da/dN) were tested. The microstructures and fracture surfaces of specimens were examined by optical microscopy and scanning electron microscopy (SEM). The results showed that fatigue crack propagation rate of 7N01 in 3.5% NaCl was significantly higher than 6005 and 5083 alloys. The mechanisms of anodic dissolution and hydrogen embrittlement are used to explain the results.

  15. Low-cycle fatigue-cracking mechanisms in fcc crystalline materials

    NASA Astrophysics Data System (ADS)

    Zhang, P.; Qu, S.; Duan, Q. Q.; Wu, S. D.; Li, S. X.; Wang, Z. G.; Zhang, Z. F.

    2011-01-01

    The low-cycle fatigue (LCF) cracking behavior in various face-centered-cubic (fcc) crystalline materials, including Cu single crystals, bicrystals and polycrystals, Cu-Al and Cu-Zn alloys, ultrafine-grained (UFG) Al-Cu and Cu-Zn alloys, was systematically investigated and reviewed. In Cu single crystals, fatigue cracking always nucleates along slip bands and deformation bands. The large-angle grain boundary (GB) becomes the preferential site in bicrystals and polycrystals. In addition, fatigue cracking can also nucleate along slip bands and twin boundaries (TBs) in polycrystalline materials. However, shear bands and coarse deformation bands are observed to the preferential sites for fatigue cracking in UFG materials with a large number of GBs. Based on numerous observations on fatigue-cracking behavior, the fatigue-cracking mechanisms along slip bands, GBs, TBs, shear bands and deformation bands were systematically compared and classified into two types, i.e. shear crack and impingement crack. Finally, these fatigue-cracking behaviors are discussed in depth for a better understanding of their physical nature and the transition from intergranular to transgranular cracking in various fcc crystalline materials. These comprehensive results for fatigue damage mechanisms should significantly aid in obtaining the optimum design to further strengthen and toughen metallic materials in practice.

  16. Use of Marker Bands for Determination of Fatigue Crack Growth Rates and Crack Front Shapes in Pre-Corroded Coupons

    NASA Technical Reports Server (NTRS)

    Willard, S. A.

    1997-01-01

    Groups of striations called marker bands generated on a fatigue fracture surface can be used to mark the position of an advancing fatigue crack at known intervals. A technique has been developed that uses the distance between multiple sets of marker bands to obtain a vs. N, crack front shape, and fatigue crack growth rate data for small cracks. This technique is particularly usefull for specimens that require crack length measurements during testing that cannot be obtained because corrosion obscures the surface of the specimen. It is also useful for specimens with unusual or non-symmetric shapes where it is difficult to obtain accurate crack lengths using traditional methods such as compliance or electric potential difference in the early stages of testing.

  17. Fatigue-crack propagation in gamma-based titanium aluminide alloys at large and small crack sizes

    SciTech Connect

    Kruzic, J.J.; Campbell, J.P.; Ritchie, R.O.

    1999-07-01

    Most evaluations of the fracture and fatigue-crack propagation properties of {gamma}+{alpha}{sub 2} titanium aluminide alloys to date have been performed using standard large-crack samples, e.g., compact-tension specimens containing crack sizes which are on the order of tens of millimeters, i.e., large compared to microstructural dimensions. However, these alloys have been targeted for applications, such as blades in gas-turbine engines, where relevant crack sizes are much smaller ({lt}500 {micro}m) and where the small-crack fatigue threshold may be the most relevant design parameter. In this study, the authors compare and contrast the cyclic crack-growth behavior of both large (a {gt} 5 mm) and (c {approximately} 25--300 {micro}m) cracks in a {gamma}-TiAl based alloy, of composition Ti-47Al-2Nb-2Cr-0.2B (at.%), specifically for duplex (average grain size {approximately}17 {micro}m) and refined lamellar (average colony size {approximately}150 {micro}m) microstructures. It is found that, whereas the lamellar microstructure displays far superior fracture toughness and fatigue-crack growth resistance in the presence of large cracks, in small-crack testing the duplex microstructure exhibits a better combination of properties. The reasons for such contrasting behavior are examined in terms of the intrinsic and extrinsic (i.e., crack bridging) contributions to cyclic crack advance.

  18. Fatigue Crack Propagation in Freshwater Ice

    DTIC Science & Technology

    1993-07-01

    Dempsey, J.P., and Wei, Y. (1989). Fracture toughness K,and fractography of SI type freshwater ice. Advances in Fracture Research, (K. Salama , K. Ravi...188-200. Perez , J., Mai, C., Tatibouet, J. and Vassoille, R. (1980). Dynamic Behaviour of Dislocations in HF-Doped Ice Ih. Journal of Glaciology, vol...Denmark, pp. 351-362. Tatibouet, J., Perez , J. and Vassoille, R. (1987). Study of Grain Boundaries in Ice by Internal Friction. J. Phys. Colloq. C1

  19. The influence of crack closure on fatigue crack growth thresholds in 2024-T3 aluminum alloy

    NASA Technical Reports Server (NTRS)

    Phillips, Edward P.

    1988-01-01

    Crack opening loads were determined in load-shedding fatigue crack growth threshold tests on 2024-T3 aluminum alloy at stress ratios R of -2, -1, 0, 0.33, 0.5, and 0.7. The effects of load-shedding procedure and rate were investigated. Values of threshold Delta-K were found to vary significantly with R, whereas values of threshold effective Delta-K did not. That is, the variation of threshold Delta-K with R was almost completely explained by accounting for the measured variation in crack opening load behavior with R. The load-shedding guidelines of ASTM Test Method for Measurement of Fatigue Crack Growth (E 647) produced a threshold Delta-K value for R = 0.7 that was in agreement with the value determined using a procedure that should minimize closure effects. At both R = 0 and R = 0.7, high load-shedding rates produced high values of threshold Delta-K caused by large closure effects.

  20. Fatigue crack localization with near-field acoustic emission signals

    NASA Astrophysics Data System (ADS)

    Zhou, Changjiang; Zhang, Yunfeng

    2013-04-01

    This paper presents an AE source localization technique using near-field acoustic emission (AE) signals induced by crack growth and propagation. The proposed AE source localization technique is based on the phase difference in the AE signals measured by two identical AE sensing elements spaced apart at a pre-specified distance. This phase difference results in canceling-out of certain frequency contents of signals, which can be related to AE source direction. Experimental data from simulated AE source such as pencil breaks was used along with analytical results from moment tensor analysis. It is observed that the theoretical predictions, numerical simulations and the experimental test results are in good agreement. Real data from field monitoring of an existing fatigue crack on a bridge was also used to test this system. Results show that the proposed method is fairly effective in determining the AE source direction in thick plates commonly encountered in civil engineering structures.

  1. Fatigue Crack Growth of the Gun Tube Steel under Spectrum Loading.

    DTIC Science & Technology

    1986-05-23

    Vessel and Piping Technology, June 19-24, 1983, Portland, Oregon. 8. Palmgren , A., "Durability of Ball Bearings ," ZDVDI, Vol. 68, No. 14, p. 339 (in...has become another important area of current research. Early attempts at accounting for variable load effects ignored sequence effects. Palmgren (8...the exception of the thickness, which was decreased to conform to the requirements of ASTM Standard E647(27) on fatigue crack growth. B. Test Equipment

  2. Hydride-phase formation and its influence on fatigue crack propagationbehavior in a Zircaloy-4 alloy

    SciTech Connect

    Garlea, Elena; Choo, H.; Wang, G Y; Liaw, Peter K; Clausen, B; Brown, D. W.; Park, Jae-Sung; Rack, P. D.; Kenik, Edward A

    2010-01-01

    The hydride-phase formation and its influence on the fatigue behavior of a Zircaloy-4 alloy charged with hydrogen gas are investigated. First, the microstructure and fatigue crack propagation rate of the alloy in the as-received condition are studied. Second, the formation and homogeneous distribution of delta zirconium hydride ( -ZrH2) in the bulk, and its effect on the fatigue crack propagation rate are presented. The results show that in the presence of hydrides the zirconium alloy exhibits reduced toughness and enhanced crack growth rates. Finally, the influence of a pre-existing fatigue crack in the specimen and the subsequent hydride formation were investigated. The residual lattice strain profile around the fatigue crack tip was measured using neutron diffraction. The combined effects of residual strains and hydride precipitation on the fatigue behavior are discussed.

  3. Fatigue crack growth behaviors in Al-Si-Mg sand cast alloys

    NASA Astrophysics Data System (ADS)

    Han, Sang-Won; Kim, Sug-Won

    2004-02-01

    The fatigue crack growth behavior of Al-Si-Mg sand cast alloys has been investigated with reference to the effects of solidification structure and aging condition. Fatigue crack growth tests have been carried out under constant load amplitude and a stress ratio of R=0.1 using CT specimens. The amount of pores in the matrix was limited by performing HIP treatment. The pores tended to promote deflection of fatigue cracks, which decreased the fatigue crack growth rate at low ΔK regions and increased the number of cycles until final fatigue fracture. Refining and spheroidizing of eutectic Si particles increased the fatigue crack growth rates over a wide range of ΔK up to larger ΔK values. The difference of aging conditions significantly affected the da/dN-ΔKeff relationship.

  4. Plastic deformation - Its role in fatigue crack propagation

    NASA Technical Reports Server (NTRS)

    Mazumdar, P. K.; Jeelani, S.

    1986-01-01

    Recognizing the fact that the effective driving force Delta-K(eff) determines the fatigue crack propagation (FCP) rate and that the shear strain, which is considered to develop due to an occurrence of crack closure, primarily contributes to the plastic deformation, an effort is made here to elucidate the role of plastic deformation in FCP by developing a correlation between the Delta-K(eff) and the applied driving force (Delta-K) with shear strain as variable. The disparity between Delta(K)eff) and Delta-K, which apparently increases with shear strain level, persists at lower values of Delta-K. This suggests a strong influence of the degree of localized deformation on the FCP rates in the near threshold level. Hence, an improvement of FCP rates in the near threshold level should follow an effort that promotes the plastic deformation near the crack tip to a greater degree. This approach could explain the effect of the grain size, microstructure, environment, R-ratio and crack size on the near-threshold FCP rates.

  5. Probabilistic Prognosis of Non-Planar Fatigue Crack Growth

    NASA Technical Reports Server (NTRS)

    Leser, Patrick E.; Newman, John A.; Warner, James E.; Leser, William P.; Hochhalter, Jacob D.; Yuan, Fuh-Gwo

    2016-01-01

    Quantifying the uncertainty in model parameters for the purpose of damage prognosis can be accomplished utilizing Bayesian inference and damage diagnosis data from sources such as non-destructive evaluation or structural health monitoring. The number of samples required to solve the Bayesian inverse problem through common sampling techniques (e.g., Markov chain Monte Carlo) renders high-fidelity finite element-based damage growth models unusable due to prohibitive computation times. However, these types of models are often the only option when attempting to model complex damage growth in real-world structures. Here, a recently developed high-fidelity crack growth model is used which, when compared to finite element-based modeling, has demonstrated reductions in computation times of three orders of magnitude through the use of surrogate models and machine learning. The model is flexible in that only the expensive computation of the crack driving forces is replaced by the surrogate models, leaving the remaining parameters accessible for uncertainty quantification. A probabilistic prognosis framework incorporating this model is developed and demonstrated for non-planar crack growth in a modified, edge-notched, aluminum tensile specimen. Predictions of remaining useful life are made over time for five updates of the damage diagnosis data, and prognostic metrics are utilized to evaluate the performance of the prognostic framework. Challenges specific to the probabilistic prognosis of non-planar fatigue crack growth are highlighted and discussed in the context of the experimental results.

  6. Inclusion size effect on the fatigue crack propagation mechanism and fracture mechanics of a superalloy

    NASA Astrophysics Data System (ADS)

    Denda, Takeshi; Bretz, Perter L.; Tien, John K.

    1992-02-01

    Low cycle fatigue life of nickel-base superalloys is enhanced as a consequence of inclusion reduction in the melt process; however, the functional dependencies between fatigue characteristics and inclusions have not been well investigated. In this study, the propagation mechanism of the fatigue crack initiated from inclusions is examined in fine-grained IN718, which is a representative turbine disc material for jet engines. There is a faceted-striated crack transition on the fracture surfaces. This faceted-striated transition also appears in the da/dN vs crack length curves. It is observed that the faceted crack propagation time can be more than 50 pct of total lifetime in the low cycle fatigue test. The significance of inclusion size effect is explained on the premise that the faceted fatigue crack propagation time scales with the inclusion size, which is taken as the initial crack length. A predictive protocol for determining inclusion size effect is given.

  7. Non-contact visualization of nonlinear ultrasonic modulation for reference-free fatigue crack detection

    NASA Astrophysics Data System (ADS)

    Lim, Hyung Jin; Song, Byeongju; Park, Byeongjin; Liu, Peipei; Sohn, Hoon

    2014-03-01

    This paper presents a fatigue crack detection technique based on visualization of nonlinear ultrasonic wave modulation produced by a fatigue crack. When distinctive low frequency (LF) and high frequency (HF) inputs are generated and 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. In this study, the two input signals are created by two air-coupled transducers (ACT), and the corresponding ultrasonic responses are scanned over a target specimen using a 3D laser Doppler vibrometer (LDV). The crack-induced spectral sidebands are isolated using a combination of linear response subtraction (LRS), and continuous wavelet transform (CWT) filtering. Then, the extracted spectral sideband components are visualized near the fatigue crack. The effectiveness of the proposed non-contact scanning technique is tested using an aluminum plate with a real fatigue crack.

  8. Fractographic analysis of initiation and growth of fatigue cracks at rivet holes

    NASA Astrophysics Data System (ADS)

    Pelloux, R.; Warren, A.; O'Grady, J.

    A series of fatigue tests were performed on riveted panels of clad 2024-T3 without epoxy bonds. Fatigue crack initiation occurred at the apex of the rivet hole chamfers. Transgranular fatigue crack growth by ductile striation formation occurred through the sheet. The fracture features at low, medium and high growth rates were examined with the SEM. Microscopic crack propagation rates as measured by fatigue striation spacings correlate with macroscopic crack growth rates observed. The fatigue crack growth rate is fairly constant over a length of 6 mm (0.25 in.) from the edge of the rivet hole, due to the fact that the stress intensity range is approximately constant in this region. Transition to fast fracture and unstable crack propagation is readily identified due to marked yielding of the cladding material.

  9. Fatigue crack growth in 2024-T3 aluminum under tensile and transverse shear stresses

    NASA Technical Reports Server (NTRS)

    Viz, Mark J.; Zehnder, Alan T.

    1994-01-01

    The influence of transverse shear stresses on the fatigue crack growth rate in thin 2024-T3 aluminum alloy sheets is investigated experimentally. The tests are performed on double-edge cracked sheets in cyclic tensile and torsional loading. This loading generates crack tip stress intensity factors in the same ratio as the values computed for a crack lying along a lap joint in a pressurized aircraft fuselage. The relevant fracture mechanics of cracks in thin plates along with the details of the geometrically nonlinear finite element analyses used for the test specimen calibration are developed and discussed. Preliminary fatigue crack growth data correlated using the fully coupled stress intensity factor calibration are presented and compared with fatigue crack growth data from pure delta K(sub I)fatigue tests.

  10. Influence of Residual Stress Field on the Fatigue Crack Propagation in Prestressing Steel Wires

    PubMed Central

    Toribio, Jesús; Matos, Juan-Carlos; González, Beatriz; Escuadra, José

    2015-01-01

    This paper deals with the effect of several residual stress profiles on the fatigue crack propagation in prestressing steel wires subjected to tension loading or bending moment. To this end, a computer program was developed to evaluate the crack front evolution on the basis of the Walker law. Results demonstrate that the absence of residual stresses makes the crack propagate towards a preferential crack path. When surface residual stresses are tensile and, correspondingly, core residual stresses are compressive, the fatigue crack fronts rapidly converge towards a quasi-straight shape. When surface residual stresses are compressive, with their corresponding tensile stresses in the core area, a preferential crack path also appears. PMID:28793661

  11. 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.

  12. Contributions of Aging to the Fatigue Crack Growth Resistance of Human Dentin

    PubMed Central

    Ivancik, Juliana; Majd, Hessam; Bajaj, Devendra; Romberg, Elaine; Arola, Dwayne

    2012-01-01

    An evaluation of the fatigue crack resistance of human dentin was conducted to identify the degree of degradation that arises with aging and the dependency on tubule orientation. Fatigue crack growth was achieved in specimens of coronal dentin through application of Mode I cyclic loading and over clinically relevant lengths (0 ≤ a ≤ 2 mm). The study considered two directions of cyclic crack growth in which the crack was either in-plane (0°) or perpendicular (90°) to the dentin tubules. Results showed that regardless of tubule orientation, aging of dentin is accompanied by a significant reduction in the resistance to the initiation of fatigue crack growth, as well as a significant increase in the rate of incremental extension. Perpendicular to the tubules, the fatigue crack exponent increased significantly (from m=14.2±1.5 to 24.1±5.0), suggesting an increase in brittleness of the tissue with age. For cracks extending in plane with the tubules, the fatigue crack growth exponent does not change significantly with patient age (from m=25.4±3.03 to 22.9±5.3), but there is a significant increase in the incremental crack growth rate. Regardless of age, coronal dentin exhibits the lowest resistance to fatigue crack growth perpendicular to the tubules. While there are changes in the cyclic crack growth rate and mechanisms of cyclic extension with aging, this tissue maintains its anisotropy. PMID:22484693

  13. Cyclic deformation, fatigue and fatigue crack propagation in Ni-base alloys

    NASA Technical Reports Server (NTRS)

    Antolovich, Stephen D.; Lerch, Brad

    1989-01-01

    Ni-base superalloys' cumulative glide behavior, damage accumulation, low-cycle fatigue, and crack propagation characteristics are directly dependent on deformation behavior which is in turn a strong function of microstructural characteristics. Microstructural instabilities and environmental interactions become additional factors at elevated temperatures. An account is presently given of microstructural, chemical, and processing techniques that may be used to obtain the properties that appear most critical or desirable in specific applications.

  14. Effects of Dynamic and Static Loading on Eddy Current Nde of Fatigue Cracks

    NASA Astrophysics Data System (ADS)

    Lo, C. C. H.; Nakagawa, N.

    2009-03-01

    This paper reports on a study of the effects of dynamic and static loading on eddy current (EC) crack signals in aerospace materials. In situ EC measurements were performed on a series of fatigue crack samples grown in Al 6061 bars under dynamic loading. The EC signals detected at a fixed location on the fatigue cracks were found to vary periodically with cyclic loading. Under dynamic tension, the amplitude of the vertical component of the crack signal, which is perpendicular to the lift off direction, increases with the load amplitude and the effect is stronger for longer cracks. Such changes in EC signals are attributable to crack morphology changes (i.e. crack opening) under tension. In contrast, the amplitude of crack signal remains relatively unchanged under compression, possibly due to the insulation effect of the oxidation layer on the crack face. The loading effects on crack signals in the Al samples were confirmed by obtaining c-scan images of the fatigue cracks under static loads. C-scans conducted on a series of Ti 64 fatigue crack samples under static loads revealed significantly stronger stress-induced changes of crack signal for Ti 64 than for the Al samples. The results are interpreted in terms of load-induced crack morphology changes, taking into account of likely differences between the Al and Ti alloy samples.

  15. Fatigue and environmentally assisted cracking in light water reactors

    SciTech Connect

    Kassner, T.F.; Ruther, W.E.; Chung, H.M.; Hicks, P.D.; Hins, A.G.; Park, J.Y.; Shack, W.J.

    1992-03-01

    Fatigue and stress corrosion cracking (SCC) for low-alloy steel used in piping and in steam generator and reactor pressure vessels have been investigated. Fatigue data were obtained on medium-sulfur-content A533-Gr B and A106-Gr B steels in high-purity (HP) deoxygenated water, in simulated pressurized water reactor water, and in air. Analytical studies focused on the behavior of carbon steels in boiling water reactor (BWR) environments. Crack-growth rates of composite fracture-mechanics specimens of A533-Gr B/Inconel-182/Inconel-600 (plated with nickel) and homogeneous specimens of A533-Gr B steel were determined under small-amplitude cyclic loading in HP water with {approx}300 pbb dissolved oxygen. Radiation-induced segregation and irradiation-assisted SCC of Type 304 SS after accumulation of relatively high fluence also have been investigated. Microchemical and microstructural changes in HP and commercial-purity Type 304 SS specimens from control-blade absorber tubes used in two operating BWRs were studied by Auger electron spectroscopy and scanning electron microscopy, and slow-strain-rate tensile tests were conducted on tubular specimens in air and in simulated BWR water at 289{degrees}C.

  16. An unsupervised learning algorithm for fatigue crack detection in waveguides

    NASA Astrophysics Data System (ADS)

    Rizzo, Piervincenzo; Cammarata, Marcello; Dutta, Debaditya; Sohn, Hoon; Harries, Kent

    2009-02-01

    Ultrasonic guided waves (UGWs) are a useful tool in structural health monitoring (SHM) applications that can benefit from built-in transduction, moderately large inspection ranges, and high sensitivity to small flaws. This paper describes an SHM method based on UGWs and outlier analysis devoted to the detection and quantification of fatigue cracks in structural waveguides. The method combines the advantages of UGWs with the outcomes of the discrete wavelet transform (DWT) to extract defect-sensitive features aimed at performing a multivariate diagnosis of damage. In particular, the DWT is exploited to generate a set of relevant wavelet coefficients to construct a uni-dimensional or multi-dimensional damage index vector. The vector is fed to an outlier analysis to detect anomalous structural states. The general framework presented in this paper is applied to the detection of fatigue cracks in a steel beam. The probing hardware consists of a National Instruments PXI platform that controls the generation and detection of the ultrasonic signals by means of piezoelectric transducers made of lead zirconate titanate. The effectiveness of the proposed approach to diagnose the presence of defects as small as a few per cent of the waveguide cross-sectional area is demonstrated.

  17. Extending non-fatigue Mode I subcritical crack growth data to subcritical fatigue crack growth: Demonstration of the equivalence of the Charles' law and Paris law exponents

    NASA Astrophysics Data System (ADS)

    Keanini, Russell; Eppes, Martha-Cary

    2016-04-01

    Paris's law connects fatigue-induced subcritical crack growth and fatigue loading. Environmentally-driven subcritical crack growth, while a random process, can be decomposed into a spectrum of cyclic processes, where each spectral component is governed by Paris's law. Unfortunately, almost no data exists concerning the Paris law exponent, m; rather, the great majority of existing sub-critical crack growth measurements on rock have been carried out via Mode I tensile tests, where corresponding data are generally correlated using Charles' law, and where the latter, similar to Paris's law, exposes a power law relationship between crack growth rate and stress intensity. In this study, a statistical argument is used to derive a simple, rigorous relationship between the all-important Paris law and Charles law exponents, m and n. This result has a significant practical implication: subcritical fatigue crack growth in rock, driven by various random environmental weathering processes can now be predicted using available Mode I stress corrosion indices, n.

  18. Comparative Study on Prediction Effects of Short Fatigue Crack Propagation Rate by Two Different Calculation Methods

    NASA Astrophysics Data System (ADS)

    Yang, Bing; Liao, Zhen; Qin, Yahang; Wu, Yayun; Liang, Sai; Xiao, Shoune; Yang, Guangwu; Zhu, Tao

    2017-05-01

    To describe the complicated nonlinear process of the fatigue short crack evolution behavior, especially the change of the crack propagation rate, two different calculation methods are applied. The dominant effective short fatigue crack propagation rates are calculated based on the replica fatigue short crack test with nine smooth funnel-shaped specimens and the observation of the replica films according to the effective short fatigue cracks principle. Due to the fast decay and the nonlinear approximation ability of wavelet analysis, the self-learning ability of neural network, and the macroscopic searching and global optimization of genetic algorithm, the genetic wavelet neural network can reflect the implicit complex nonlinear relationship when considering multi-influencing factors synthetically. The effective short fatigue cracks and the dominant effective short fatigue crack are simulated and compared by the Genetic Wavelet Neural Network. The simulation results show that Genetic Wavelet Neural Network is a rational and available method for studying the evolution behavior of fatigue short crack propagation rate. Meanwhile, a traditional data fitting method for a short crack growth model is also utilized for fitting the test data. It is reasonable and applicable for predicting the growth rate. Finally, the reason for the difference between the prediction effects by these two methods is interpreted.

  19. Crack paths, microstructure, and fatigue crack growth in annealed and cold-rolled AISI 304 stainless steels

    NASA Astrophysics Data System (ADS)

    Gao, Ming; Chen, Shuchun; Wei, Robert P.

    1992-01-01

    To assist in the understanding of micromechanisms for corrosion fatigue crack growth in metastable austenitic steels, the relationships between the crack paths and the underlying microstructure were investigated for annealed and cold-rolled (CR) 304 stainless steels that had been tested in a deaerated 3.5 pct NaCl solution, air, and vacuum. Corrosion fatigue in the deleterious environments (3.5 pct NaCl and air) was brittle and occurred primarily by {001}γ and other unidentified, quasi-cleavage (QC), accompanied by preferential cracking along {111}γ twin and grain boundaries. In contrast, fatigue cracking in vacuum was ductile, fully transgranular, and noncrystallographic. Transformation to alpha prime (α'-) martensite by fatigue was found to be essentially complete in the CR steel, which contained ɛ-martensite, and in the annealed steel tested in vacuum, but was substantially less in the annealed steel tested in air and 3.5 pct NaCl solution. These results, taken in conjunction with the crack growth and electrochemical reaction data, support hydrogen embrittlement (HE) as the mechanism for corrosion fatigue crack growth in 304 stainless steels in 3.5 pct NaCl solution. Martensitic transformation appears not to be the only responsible factor for embrittlement. Other microstructural components, such as twin and grain boundaries, slip bands, and cold work-induced lattice defects, may play more important roles in enhancing crack growth rates.

  20. Fatigue Crack Measurement in Composite Materials by Ultrasonic Methods

    NASA Technical Reports Server (NTRS)

    Walker, James L.; Russell, Samuel S.; Suits, Michael W.; Workman, Gary L.; Watson, Jason M.; Thom, Robert (Technical Monitor)

    2002-01-01

    The nondestructive detection of intra-ply microcracking in unlined pressure vessels fabricated from composite materials is critical to ensuring mission success. Microcracking in composite structures due to combined fatigue and cryogenic thermal loading can be very troublesome to detect in-service and when it begins to link through the thickness can cause leakage and failure of the structure. These leaks may lead to loss of pressure/propellant, increased risk of explosion and possible cryo-pumping. The work presented herein develops a method and an instrument to locate and measure intraply fatigue cracking through the thickness of laminated composite material by means of correlation with ultrasonic resonance. Resonant ultrasound spectroscopy provides measurements which are, sensitive to both the microscopic and macroscopic properties of an object. Elastic moduli, acoustic attenuation, and geometry can all be probed. The approach is based on the premise of half-wavelength resonance. The method injects a broadband ultrasonic wave into the test structure using a swept frequency technique. This method provides dramatically increased energy input into the test article, as compared to conventional spike pulsed ultrasonics. This relative energy increase improves the ability to measure finer details in the materials character, such as micro-cracking and porosity. As the micro-crack density increases, more interactions occur with the higher frequency (small wavelength) components of the signal train causing the spectrum to shift toward lower frequencies. Preliminary experiments have verified a measurable effect on the resonance spectrum of the ultrasonic data to detect microcracking. Methods involving self organizing neural networks and other clustering algorithms show that the resonance ultrasound signatures from composites vary with the degree of microcracking and can be separated and identified.

  1. Fatigue Crack Measurement in Composite Materials by Ultrasonic Methods

    NASA Technical Reports Server (NTRS)

    Walker, James L.; Russell, Samuel S.; Suits, Michael W.; Workman, Gary L.; Watson, Jason M.; Thom, Robert (Technical Monitor)

    2002-01-01

    The nondestructive detection of intra-ply microcracking in unlined pressure vessels fabricated from composite materials is critical to ensuring mission success. Microcracking in composite structures due to combined fatigue and cryogenic thermal loading can be very troublesome to detect in-service and when it begins to link through the thickness can cause leakage and failure of the structure. These leaks may lead to loss of pressure/propellant, increased risk of explosion and possible cryo-pumping. The work presented herein develops a method and an instrument to locate and measure intraply fatigue cracking through the thickness of laminated composite material by means of correlation with ultrasonic resonance. Resonant ultrasound spectroscopy provides measurements which are, sensitive to both the microscopic and macroscopic properties of an object. Elastic moduli, acoustic attenuation, and geometry can all be probed. The approach is based on the premise of half-wavelength resonance. The method injects a broadband ultrasonic wave into the test structure using a swept frequency technique. This method provides dramatically increased energy input into the test article, as compared to conventional spike pulsed ultrasonics. This relative energy increase improves the ability to measure finer details in the materials character, such as micro-cracking and porosity. As the micro-crack density increases, more interactions occur with the higher frequency (small wavelength) components of the signal train causing the spectrum to shift toward lower frequencies. Preliminary experiments have verified a measurable effect on the resonance spectrum of the ultrasonic data to detect microcracking. Methods involving self organizing neural networks and other clustering algorithms show that the resonance ultrasound signatures from composites vary with the degree of microcracking and can be separated and identified.

  2. Characterization of a soft elastomeric capacitive strain sensor for fatigue crack monitoring

    NASA Astrophysics Data System (ADS)

    Kong, Xiangxiong; Li, Jian; Laflamme, Simon; Bennett, Caroline; Matamoros, Adolfo

    2015-04-01

    Fatigue cracks have been one of the major factors for the deterioration of steel bridges. In order to maintain structural integrity, monitoring fatigue crack activities such as crack initiation and propagation is critical to prevent catastrophic failure of steel bridges due to the accumulation of fatigue damage. Measuring the strain change under cracking is an effective way of monitoring fatigue cracks. However, traditional strain sensors such as metal foil gauges are not able to capture crack development due to their small size, limited measurement range, and high failure rate under harsh environmental conditions. Recently, a newly developed soft elastomeric capacitive sensor has great promise to overcome these limitations. In this paper, crack detection capability of the capacitive sensor is demonstrated through Finite Element (FE) analysis. A nonlinear FE model of a standard ASTM compact tension specimen is created which is calibrated to experimental data to simulate its response under fatigue loading, with the goal to 1) depict the strain distribution of the specimen under the large area covered by the capacitive sensor due to cracking; 2) characterize the relationship between capacitance change and crack width; 3) quantify the minimum required resolution of data acquisition system for detecting the fatigue cracks. The minimum resolution serves as a basis for the development of a dedicated wireless data acquisition system for the capacitive strain sensor.

  3. Evidence that abnormal grain growth precedes fatigue crack initiation in nanocrystalline Ni-Fe

    DOE PAGES

    Furnish, Timothy A.; Bufford, Daniel C.; Ren, Fang; ...

    2018-09-06

    Prior studies on the high-cycle fatigue behavior of nanocrystalline metals have shown that fatigue fracture is associated with abnormal grain growth (AGG). However, those previous studies have been unable to determine if AGG precedes fatigue crack initiation, or vice-versa. The present study shows that AGG indeed occurs prior to crack formation in nanocrystalline Ni-Fe by using a recently developed synchrotron X-ray diffraction modality that has been adapted for in-situ analysis. The technique allows fatigue tests to be interrupted at the initial signs of the AGG process, and subsequent microscopy reveals the precursor damage state preceding crack initiation.

  4. A Method for Sizing Small Fatigue Cracks in Stainless Steel Using Microwaves

    NASA Astrophysics Data System (ADS)

    Saka, Masumi; Ju, Yang; Luo, Daying; Abé, Hiroyuki

    To evaluate the depth of small fatigue cracks under conditions of no contact and without using any coupling medium, a novel microwave technique was demonstrated. An open-ended coaxial line sensor was used to increase the spatial resolution and the ratio of signal to noise. Closed fatigue cracks were detected successfully and a W-shaped characteristic signal was obtained. A dual frequency technique was proposed to evaluate the depth of small fatigue cracks for which crack closure characteristics were unknown. The evaluated results are shown to agree well with the actual values.

  5. Nonlinear ultrasonic imaging of thermal fatigue cracks of several tens nm gap in glass plates

    NASA Astrophysics Data System (ADS)

    Hertl, M.; Kawashima, K.; Sekino, K.; Yasui, H.; Aida, T.

    2015-10-01

    Thermal fatigue crack of which gap distance is several tens nm in glass plate is imaged by using an immersion higher harmonic imaging technique. Some parts of the thermal fatigue crack are clearly imaged by the third harmonic amplitude of the 3.5 MHz burst wave by angular incidence. For through-transmission mode across the crack face, the seventh harmonic of a through-thickness resonant frequency also visualizes the thermal fatigue crack. If spatial resolution will reach to a few micron meters, the technique could be applied for detection of disbonds in bonded wafers.

  6. Uncommon deformation mechanisms during fatigue-crack propagation in nanocrystalline alloys.

    PubMed

    Cheng, Sheng; Lee, Soo Yeol; Li, Li; Lei, Changhui; Almer, Jon; Wang, Xun-Li; Ungar, Tamas; Wang, Yinmin; Liaw, Peter K

    2013-03-29

    The irreversible damage at cracks during the fatigue of crystalline solids is well known. Here we report on in situ high-energy x-ray evidence of reversible fatigue behavior in a nanocrystalline NiFe alloy both in the plastic zone and around the crack tip. In the plastic zone, the deformation is fully recoverable as the crack propagates, and the plastic deformation invokes reversible interactions of dislocation and twinning in the nanograins. But around the crack tip lies a regime with reversible grain lattice reorientation promoted by a change of local stress state. These observations suggest unprecedented fatigue deformation mechanisms in nanostructured systems that are not addressed theoretically.

  7. An experimental approach to determining fatigue crack size in polyethylene tibial inserts.

    PubMed

    Lockard, Carly A; Sanders, Anthony P; Raeymaekers, Bart

    2016-02-01

    A major limiting factor to the longevity of prosthetic knee joints is fatigue crack damage of the polyethylene tibial insert. Existing methods to quantify fatigue crack damage have several shortcomings, including limited resolution, destructive testing approach, and high cost. We propose an alternative fatigue crack damage visualization and measurement method that addresses the shortcomings of existing methods. This new method is based on trans-illumination and differs from previously described methods in its ability to non-destructively measure subsurface fatigue crack damage while using a simple and cost-effective bench-top set-up. We have evaluated this method to measure fatigue crack damage in two tibial inserts. This new method improves on existing image-based techniques due to its usability for subsurface damage measurement and its decreased reliance on subjective damage identification and measurement.

  8. Factors influencing fatigue crack propagation behavior of austenitic steels

    NASA Astrophysics Data System (ADS)

    Kim, Sangshik; Kwon, Jaeki; Kim, Youngju; Jang, Wookil; Lee, Soongi; Choi, Jongkyo

    2013-07-01

    In the present study, the fatigue crack propagation (FCP) behaviors of austenitic single phase steels, including STS304, Fe18Mn and Fe22Mn with different grain sizes ranging from 12 μm to 98 μm were investigated. The FCP tests were conducted in air at an R ratio of 0.1 using compact tension specimens and the crack paths and fracture surfaces were documented by using an SEM. The highest ΔKth value of 9.9MPa·m1/2 was observed for the Fe18Mn specimen, followed by 5.2MPa·m1/2 for the Fe22Mn specimen and 4.6MPa·m1/2 for the STS304 specimen, showing a substantial difference in the near-threshold FCP resistance for each microstructure. The crack path and fractographic analyses suggested that the near-threshold FCP behavior of these austenitic steels was largely influenced by the degree of slip planarity, as determined by stacking fault energy and grain size, rather than the tensile properties. In the Paris' regime, the slip planarity still played an important role while the tensile properties began to affect the FCP. The FCP behavior of austenitic steels with different microstructural features are discussed based on detailed fractographic and micrographic observations.

  9. Evaluation of Fatigue Crack Growth and Fracture Properties of Cryogenic Model Materials

    NASA Technical Reports Server (NTRS)

    Newman, John A.; Forth, Scott C.; Everett, Richard A., Jr.; Newman, James C., Jr.; Kimmel, William M.

    2002-01-01

    The criteria used to prevent failure of wind-tunnel models and support hardware were revised as part of a project to enhance the capabilities of cryogenic wind tunnel testing at NASA Langley Research Center. Specifically, damage-tolerance fatigue life prediction methods are now required for critical components, and material selection criteria are more general and based on laboratory test data. The suitability of two candidate model alloys (AerMet 100 and C-250 steel) was investigated by obtaining the fatigue crack growth and fracture data required for a damage-tolerance fatigue life analysis. Finally, an example is presented to illustrate the newly implemented damage tolerance analyses required of wind-tunnel model system components.

  10. Grain boundary oxidation and oxidation accelerated fatigue crack nucleation and propagation

    NASA Technical Reports Server (NTRS)

    Liu, H. W.; Oshida, Y.

    1986-01-01

    Fatigue life at elevated temperatures is often shortened by oxidation. Grain boundary oxidation penetrates deeper than the surface oxidation. Therefore, grain boundary oxide penetration could be the primary cause of accelerated fatigue crack nucleation and propagation, and the shortened fatigue life at elevated temperatures. Grain boundary oxidation kinetics was studied and its statistical scatter was analyzed by the Weibull's distribution function. The effects of grain boundary oxidation on shortened fatigue life was analyzed and discussed. A model of intermittent microruptures of the grain boundary oxide was proposed for the fatigue crack growth in the low frequency region. The proposed model is consistent with the observations that fatigue crack growth rate in the low frequency region with hold time at K sub max is inversely proportional to cyclic frequency and that crack growth is intergranular.

  11. Life extension of self-healing polymers with rapidly growing fatigue cracks.

    PubMed

    Jones, A S; Rule, J D; Moore, J S; Sottos, N R; White, S R

    2007-04-22

    Self-healing polymers, based on microencapsulated dicyclopentadiene and Grubbs' catalyst embedded in the polymer matrix, are capable of responding to propagating fatigue cracks by autonomic processes that lead to higher endurance limits and life extension, or even the complete arrest of the crack growth. The amount of fatigue-life extension depends on the relative magnitude of the mechanical kinetics of crack propagation and the chemical kinetics of healing. As the healing kinetics are accelerated, greater fatigue life extension is achieved. The use of wax-protected, recrystallized Grubbs' catalyst leads to a fourfold increase in the rate of polymerization of bulk dicyclopentadiene and extends the fatigue life of a polymer specimen over 30 times longer than a comparable non-healing specimen. The fatigue life of polymers under extremely fast fatigue crack growth can be extended through the incorporation of periodic rest periods, effectively training the self-healing polymeric material to achieve higher endurance limits.

  12. 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

  13. Proof test and fatigue crack growth modeling on 2024-T3 aluminum alloy

    NASA Technical Reports Server (NTRS)

    Newman, J. C., Jr.; Poe, C. C., Jr.; Dawicke, D. S.

    1990-01-01

    Pressure proof testing of aircraft fuselage structures has been suggested as a means of screening critical crack sizes and of extending their useful life. The objective of this paper is to study the proof-test concept and to model the crack-growth process on a ductile material. Simulated proof and operational fatigue life tests have been conducted on cracked panels made of 2024-T3 aluminum alloy sheet material. A fatigue crack-closure model was modified to simulate the proof test and operational fatigue cycling. Using crack-growth rate and resistance-curve data, the model was able to predict crack growth during and after the proof load. These tests and analyses indicate that the proof test increases fatigue life; but the beneficial life, after a 1.33 or 1.5 proof, was less than a few hundred cycles.

  14. Crack Growth Behavior in the Threshold Region for High Cycle Fatigue Loading

    NASA Technical Reports Server (NTRS)

    Forman, Royce G.; Zanganehgheshlaghi, Mohannad

    2014-01-01

    The research results described in this paper presents a new understanding of the behavior of fatigue crack growth in the threshold region. It is believed by some crack growth experts that the ASTM load shedding test method does not produce true or valid threshold properties. The concern involves the observed fanning of threshold region da/dN data plots for some materials in which the low R-ratio data fans out or away from the high R-ratio data. This data fanning or elevation of threshold values is obviously caused by an increase in crack closure in the low R-ratio tested specimens. This increase in crack closure is assumed by some investigators to be caused by a plastic wake on the crack surfaces that was created during the load shedding test phase. This study shows that the increase in crack closure is the result of an extensive occurrence of crack bifurcation behavior in some materials, particularly in aluminum alloys, when the crack tip cyclic yield zone size becomes less than the grain size of the alloy. This behavior is related to the high stacking fault energy (SFE) property of aluminum alloys which results in easier slip characteristics. Therefore, the particular fanning behavior in aluminum alloys is a function of intrinsic dislocation property of the materials and that the fanned data represents valid material properties. However, for corrosion sensitive steel alloys used in this study the fanning was caused by a build-up of iron oxide at the crack tip from fretting corrosion.

  15. Critical plane analysis of multiaxial fatigue experiments leading to White Etching Crack formation

    NASA Astrophysics Data System (ADS)

    Averbeck, S.; Kerscher, E.

    2017-05-01

    Various researchers have shown that rolling contact fatigue can be reproduced with cyclic compression-torsion experiments, with the load components either in-phase or out of phase. As reported previously, the authors used such experiments to reproduce the rolling contact fatigue phenomenon “White Etching Cracks” which can cause premature failures of rolling element bearings. It is characterized by subsurface crack initiation and propagation coupled with microstructural changes alongside the crack flanks. Surprisingly, only in-phase load superposition caused these microstructural changes to occur. This suggests that White Etching Crack formation is somehow linked to the multiaxial stress state in the specimens, as this was the only variable that changed between in-phase and out-of-phase testing. In this study, the multiaxial stress state in the two experiments is analysed and compared using different critical plane criteria. In contrast to common ways of characterizing the stress state, e.g. equivalent stress approaches, this class of criteria is explicitly designed for multiaxial stress states. Special attention is given to the Dang Van criterion, which has been used in a number of rolling contact fatigue studies.

  16. Corrosion Fatigue Crack Growth Behavior at Notched Hole in 7075 T6 Under Different Biaxial Stress Ratios

    DTIC Science & Technology

    2016-08-18

    constant for Paris law (unit less) CF corrosion fatigue da/dN rate of crack growth per cycle (mm/cycle) DOD Department of Defense G...31]. When these different factors failure modes are combined, the failure occurs sooner. Corrosion fatigue ( CF ) occurs when the crack generated by...Intrusion and Extrusion of Fatigue Crack Initiation [30]. 2.2 Corrosion Fatigue Corrosion fatigue ( CF ) is one of the most complicated topics of

  17. A surface acoustic wave technique for monitoring the growth behavior of small surface fatigue cracks

    SciTech Connect

    Resch, M.T.; Nelson, D.V.; Ramvsat, G.F.; Yuce, H.H.

    1985-03-01

    The theory of Kino and Auld which relates the reflection coefficient of acoustic waves from a crack to its size is summarized. A scattering model is evaluated from this theory concerning the reflection of surface acoustic waves (SAW) from a small surface fatigue crack at a frequency such that the crack depth is much smaller than the acoustic wavelength. Acoustic predictions of crack depth are compared to postfracture measurements of depth for small surface cracks in Pyrex glass, 7075-T651 aluminum, and 4340 steel. Additionally, the minimum detectable crack depth as limited by the acoustic noise level is determined for several typical aluminum and steel alloys. The utility of SAW reflection coefficient measurements for inferring crack depth, crack growth, and crack opening behavior in situ during fatigue cycling is discussed.

  18. Fatigue-life behavior and matrix fatigue crack spacing in unnotched SCS-6/Timetal 21S metal matrix composites

    NASA Technical Reports Server (NTRS)

    Ward, G. T.; Herrmann, D. J.; Hillberry, B. M.

    1993-01-01

    Fatigue tests of the SCS-6/Timetal 21S composite system were performed to characterize the fatigue behavior for unnotched conditions. The stress-life behavior of the unnotched (9/90)2s laminates was investigated for stress ratios of R = 0.1 and R = 0.3. The occurrence of matrix cracking was also examined in these specimens. This revealed multiple matrix crack initiation sites throughout the composite, as well as evenly spaced surface cracks along the length of the specimens. No difference in fatigue lives were observed for stress ratios of R = 0.1 and R = 0.3 when compared on a stress range basis. The unnotched SCS-6/Timetal 21S composites had shorter fatigue lives than the SCS-6/Ti-15-3 composites, however the neat Timetal 21S matrix material had a longer fatigue life than the neat Ti-15-3.

  19. Characterization of Mode I fatigue crack growth in GFRP woven laminates at low temperatures

    SciTech Connect

    Shindo, Yasuhide . E-mail: shindo@material.tohoku.ac.jp; Inamoto, Akihiro; Narita, Fumio

    2005-03-01

    This paper describes an experimental and analytical study on the cryogenic fatigue behavior of glass fiber reinforced polymer woven laminates under Mode I loading. Fatigue crack growth rate tests were performed using compact tension specimens at room temperature, liquid nitrogen temperature (77 K), and liquid helium temperature (4 K). The fracture surfaces were also examined by scanning electron microscopy to correlate with the fatigue properties. A finite element method coupled with fatigue damage was adopted for the extensional analysis. The effects of temperature and loading condition on the fatigue crack growth rates are examined.

  20. Noncontact monitoring of fatigue crack growth using high frequency guided waves

    NASA Astrophysics Data System (ADS)

    Masserey, B.; Fromme, P.

    2014-03-01

    The development of fatigue cracks at fastener holes due to stress concentration is a common problem in aircraft maintenance. This contribution investigates the use of high frequency guided waves for the non-contact monitoring of fatigue crack growth in tensile, aluminium specimens. High frequency guided ultrasonic waves have a good sensitivity for defect detection and can propagate along the structure, thus having the potential for the inspection of difficult to access parts by means of non-contact measurements. Experimentally the required guided wave modes are excited using standard wedge transducers and measured using a laser interferometer. The growth of fatigue cracks during cyclic loading was monitored optically and the resulting changes in the signal caused by crack growth are quantified. Full three-dimensional simulation of the scattering of the high frequency guided ultrasonic waves at the fastener hole and crack has been implemented using the Finite Difference (FD) method. The comparison of the results shows a good agreement of the measured and predicted scattered field of the guided wave at quarter-elliptical and through-thickness fatigue cracks. The measurements show a good sensitivity for the early detection of fatigue damage and for the monitoring of fatigue crack growth at a fastener hole. The sensitivity and repeatability are ascertained, and the robustness of the methodology for practical in-situ ultrasonic monitoring of fatigue crack growth is discussed.

  1. Effects of microstructure banding on hydrogen assisted fatigue crack growth in X65 pipeline steels

    DOE PAGES

    Ronevich, Joseph A.; Somerday, Brian P.; San Marchi, Chris W.

    2015-09-10

    Banded ferrite-pearlite X65 pipeline steel was tested in high pressure hydrogen gas to evaluate the effects of oriented pearlite on hydrogen assisted fatigue crack growth. Test specimens were oriented in the steel pipe such that cracks propagated either parallel or perpendicular to the banded pearlite. The ferrite-pearlite microstructure exhibited orientation dependent behavior in which fatigue crack growth rates were significantly lower for cracks oriented perpendicular to the banded pearlite compared to cracks oriented parallel to the bands. Thus the reduction of hydrogen assisted fatigue crack growth across the banded pearlite is attributed to a combination of crack-tip branching and impededmore » hydrogen diffusion across the banded pearlite.« less

  2. Acoustic Emission Detection and Prediction of Fatigue Crack Propagation in Composite Patch Repairs Using Neural Networks

    SciTech Connect

    Okafor, A. Chukwujekwu; Singh, Navdeep; Singh, Navrag

    2007-03-21

    An aircraft is subjected to severe structural and aerodynamic loads during its service life. These loads can cause damage or weakening of the structure especially for aging military and civilian aircraft, thereby affecting its load carrying capabilities. Hence composite patch repairs are increasingly used to repair damaged aircraft metallic structures to restore its structural efficiency. This paper presents the results of Acoustic Emission (AE) monitoring of crack propagation in 2024-T3 Clad aluminum panels repaired with adhesively bonded octagonal, single sided boron/epoxy composite patch under tension-tension fatigue loading. Crack propagation gages were used to monitor crack initiation. The identified AE sensor features were used to train neural networks for predicting crack length. The results show that AE events are correlated with crack propagation. AE system was able to detect crack propagation even at high noise condition of 10 Hz loading; that crack propagation signals can be differentiated from matrix cracking signals that take place due to fiber breakage in the composite patch. Three back-propagation cascade feed forward networks were trained to predict crack length based on the number of fatigue cycles, AE event number, and both the Fatigue Cycles and AE events, as inputs respectively. Network using both fatigue cycles and AE event number as inputs to predict crack length gave the best results, followed by Network with fatigue cycles as input, while network with just AE events as input had a greater error.

  3. Acoustic Emission Detection and Prediction of Fatigue Crack Propagation in Composite Patch Repairs Using Neural Networks

    NASA Astrophysics Data System (ADS)

    Okafor, A. Chukwujekwu; Singh, Navdeep; Singh, Navrag

    2007-03-01

    An aircraft is subjected to severe structural and aerodynamic loads during its service life. These loads can cause damage or weakening of the structure especially for aging military and civilian aircraft, thereby affecting its load carrying capabilities. Hence composite patch repairs are increasingly used to repair damaged aircraft metallic structures to restore its structural efficiency. This paper presents the results of Acoustic Emission (AE) monitoring of crack propagation in 2024-T3 Clad aluminum panels repaired with adhesively bonded octagonal, single sided boron/epoxy composite patch under tension-tension fatigue loading. Crack propagation gages were used to monitor crack initiation. The identified AE sensor features were used to train neural networks for predicting crack length. The results show that AE events are correlated with crack propagation. AE system was able to detect crack propagation even at high noise condition of 10 Hz loading; that crack propagation signals can be differentiated from matrix cracking signals that take place due to fiber breakage in the composite patch. Three back-propagation cascade feed forward networks were trained to predict crack length based on the number of fatigue cycles, AE event number, and both the Fatigue Cycles and AE events, as inputs respectively. Network using both fatigue cycles and AE event number as inputs to predict crack length gave the best results, followed by Network with fatigue cycles as input, while network with just AE events as input had a greater error.

  4. Near-threshold fatigue behaviors of small shear cracks in bearing steel

    NASA Astrophysics Data System (ADS)

    Koyanagi, D.; Shomura, N.; Endo, M.; Matsunaga, H.; Moriyama, S.

    2009-12-01

    Failures of engineering components caused by rolling contact fatigue, such as flaking in bearings, are closely related to the initiation and growth of share-mode, i.e. Modes II and III, fatigue cracks. In order to evaluate quantitatively the fatigue strength of those components, it is necessary to elucidate the propagation and threshold behaviors of share-mode cracks, particularly for small cracks, on the basis of fracture mechanics. In this study, fatigue tests of fully-reversed cyclic torsion superposed upon static compression were carried out using SAE52100 bearing steel shafts into which semi-elliptical cracks smaller than 1 mm in size were initially introduced in the axial direction. Propagation and nonpropagation of shear-mode fatigue cracks were controlled by changing the torsional stress amplitude. The threshold stress intensity factor (SIF) range for a share-mode crack was defined at the minimum stress required for crack propagation. Crack face interference was responsible for the reduction in crack driving force. An intrinsic value of threshold SIF range that does not include the effect of crack face interference was obtained to be approximately 13 MPa+m0.5.

  5. Near-threshold fatigue behaviors of small shear cracks in bearing steel

    NASA Astrophysics Data System (ADS)

    Koyanagi, D.; Shomura, N.; Endo, M.; Matsunaga, H.; Moriyama, S.

    2010-03-01

    Failures of engineering components caused by rolling contact fatigue, such as flaking in bearings, are closely related to the initiation and growth of share-mode, i.e. Modes II and III, fatigue cracks. In order to evaluate quantitatively the fatigue strength of those components, it is necessary to elucidate the propagation and threshold behaviors of share-mode cracks, particularly for small cracks, on the basis of fracture mechanics. In this study, fatigue tests of fully-reversed cyclic torsion superposed upon static compression were carried out using SAE52100 bearing steel shafts into which semi-elliptical cracks smaller than 1 mm in size were initially introduced in the axial direction. Propagation and nonpropagation of shear-mode fatigue cracks were controlled by changing the torsional stress amplitude. The threshold stress intensity factor (SIF) range for a share-mode crack was defined at the minimum stress required for crack propagation. Crack face interference was responsible for the reduction in crack driving force. An intrinsic value of threshold SIF range that does not include the effect of crack face interference was obtained to be approximately 13 MPa+m0.5.

  6. NDE detectability of fatigue type cracks in high strength alloys

    NASA Technical Reports Server (NTRS)

    Christner, B. K.; Rummel, W. D.

    1983-01-01

    Specimens suitable for investigating the reliability of production nondestructive evaluation (NDE) to detect tightly closed fatigue cracks in high strength alloys representative of those materials used in spacecraft engine/booster construction were produced. Inconel 718 was selected as representative of nickel base alloys and Haynes 188 was selected as representative of cobalt base alloys used in this application. Cleaning procedures were developed to insure the reusability of the test specimens and a flaw detection reliability assessment of the fluorescent penetrant inspection method was performed using the test specimens produced to characterize their use for future reliability assessments and to provide additional NDE flaw detection reliability data for high strength alloys. The statistical analysis of the fluorescent penetrant inspection data was performed to determine the detection reliabilities for each inspection at a 90% probability/95% confidence level.

  7. Fracture mechanics applied to nonisothermal fatigue crack growth

    NASA Technical Reports Server (NTRS)

    Jordan, E. H.; Meyers, G. J.

    1986-01-01

    Twelve nonisothermal fatigue crack growth tests were performed on Hastelloy-X tubular specimens in which strain and temperature varied simultaneously. Conditions were selected to include nominally elastic and nominally plastic conditions and temperatures up to 982 C. A number of parameters, including the stress intensity factor, strain intensity factor, and J-integral, were examined for their ability to correlate the data. There was no decisive difference between the success of the three parameters. Each parameter correlated data from different strain ranges to within no worse than a factor of 2.1 on da/dn. The effect of strain temperature cycle shape was investigated and found to be moderate, while a strain hold of 1 min had very little effect. An attempt was made to predict nonisothermal test results from isothermal data. These predictions were better than those made by using peak test temperature isothermal data but still not within scatter.

  8. Fatigue-crack propagation in advanced aerospace materials: Aluminum-lithium alloys

    SciTech Connect

    Venkateswara Rao, K.T.; Ritchie, R.O.

    1988-10-01

    Characteristics of fatigue-crack propagation behavior are reviewed for recently developed commercial aluminum-lithium alloys, with emphasis on the underlying micromechanisms associated with crack advance and their implications to damage-tolerant design. Specifically, crack-growth kinetics in Alcoa 2090-T8E41, Alcan 8090 and 8091, and Pechiney 2091 alloys, and in certain powder-metallurgy alloys, are examined as a function of microstructure, plate orientation, temperature, crack size, load ratio and loading sequence. In general, it is found that growth rates for long (> 10 mm) cracks are nearly 2--3 orders of magnitude slower than in traditional 2000 and 7000 series alloys at comparable stress-intensity levels. In additions, Al-Li alloys shown enhanced crack-growth retardations following the application of tensile overloads and retain superior fatigue properties even after prolonged exposure at overaging temperatures; however, they are less impressive in the presence of compression overloads and further show accelerated crack-growth behavior for microstructurally-small (2--1000 {mu}m) cracks (some three orders of magnitude faster than long cracks). These contrasting observations are attributed to a very prominent role of crack-tip shielding during fatigue-crack growth in Al-Li alloys, promoted largely by the tortuous and zig-zag nature of the crack-path morphologies. Such crack paths result in locally reduced crack-tip stress intensities, due to crack deflection and consequent crack wedging from fracture-surface asperities (roughness-induced crack closure); however, such mechanisms are far less potent in the presence of compressive loads, which act to crush the asperities, and for small cracks, where the limited crack wake severely restricts the shielding effect. 50 refs., 21 figs.

  9. A Review Of Modelling Small-Crack Behavior And Fatigue-Life Predictions For Aluminum Alloys

    NASA Technical Reports Server (NTRS)

    Newman, J. C., Jr.

    1994-01-01

    The small-crack effect, where small fatigue cracks grow faster and at lower stress-intensity factors than large cracks, has been found to be significant for many materials and loading conditions. In this paper, plasticity effects and crack-closure modelling of small fatigue cracks are reviewed. A crack-closure model with a cyclic-plastic zone-corrected effective stress-intensity factor range (related to the cyclic J-integral) and microstructural data on crack-initiation sites were used to calculate small-crack growth rates and fatigue lives for unnotched and notched specimens made of two aluminum alloys. The crack-closure transient from the plastic wake was shown to be the dominant cause of the small-crack effect and plasticity effects on the cyclic-plastic zone-corrected stress-intensity factor range were negligible except at extremely high stress levels. Small-crack growth rates and fatigue lives under both constant-amplitude and spectrum loading from tests and analyses agreed well.

  10. Review of modelling small-crack behavior and fatigue-life predictions for aluminum alloys

    NASA Technical Reports Server (NTRS)

    Newman, J. C., Jr.

    1994-01-01

    The small-crack effect, where small fatigue cracks grow faster and at lower stress-intensity factors than large cracks, has been found to be significant for many materials and loading conditions. In this paper, plasticity effects and crack-closure modelling of small fatigue cracks are reviewed. A crack-closure model with a cyclic-plastic-zone-corrected effective stress-intensity factor range (related to the cyclic J-integral) and microstructural data on crack-initiation sites were used to calculate small-crack growth rates and fatigue lives for unnotched and notched specimens made of two aluminum alloys. The crack-closure transient from the plastic wake was shown to be the dominant cause of the small-crack effect and plasticity effects on the cyclic-plastic-zone-corrected stress-intensity factor range were negligible except at extremely high stress levels. Small-crack growth rates and fatigue lives under both constant-amplitude and spectrum loading from tests and analyses agreed well.

  11. Recent advances in the modelling of crack growth under fatigue loading conditions

    NASA Technical Reports Server (NTRS)

    Dekoning, A. U.; Tenhoeve, H. J.; Henriksen, T. K.

    1994-01-01

    Fatigue crack growth associated with cyclic (secondary) plastic flow near a crack front is modelled using an incremental formulation. A new description of threshold behaviour under small load cycles is included. Quasi-static crack extension under high load excursions is described using an incremental formulation of the R-(crack growth resistance)- curve concept. The integration of the equations is discussed. For constant amplitude load cycles the results will be compared with existing crack growth laws. It will be shown that the model also properly describes interaction effects of fatigue crack growth and quasi-static crack extension. To evaluate the more general applicability the model is included in the NASGRO computer code for damage tolerance analysis. For this purpose the NASGRO program was provided with the CORPUS and the STRIP-YIELD models for computation of the crack opening load levels. The implementation is discussed and recent results of the verification are presented.

  12. Fatigue crack growth study of SCS6/Ti-15-3 composite

    NASA Technical Reports Server (NTRS)

    Kantzos, P.; Telesman, J.

    1990-01-01

    A study was performed to determine the fatigue crack growth (FCG) behavior and the associated fatigue damage processes in a (0)8- and (90)8-oriented SCS6/Ti-15-3 composite. Companion testing was also done on identically processed Ti-15-3 unreinforced material. The active fatigue crack growth failure processes were very similar for both composite orientations tested. For both orientations, fatigue crack growth was along the fiber direction. It was found that the composite constituent most susceptible to fatigue damage was the interface region and, in particular, the carbon coating surrounding the fiber. The failure of the interface region led to crack initiation and also strongly influenced the FCG behavior in this composite. The failure of the interface region was apparently driven by normal stresses perpendicular to the fiber direction. The FCG rates were considerably higher for the (90)8-oriented CT specimens in comparison to the unreinforced material.

  13. High frequency guided waves for hidden fatigue crack growth monitoring in multi-layer aerospace structures

    NASA Astrophysics Data System (ADS)

    Chan, Henry; Fromme, Paul

    2015-03-01

    Varying loading conditions of aircraft structures result in stress concentration at fastener holes, where multi-layered components are connected, possibly leading to the development of fatigue cracks. High frequency guided waves propagating along the structure allow for the non-destructive testing of such components, e.g., aircraft wings. However, the sensitivity for the detection of small, potentially hidden, fatigue cracks has to be ascertained. The type of multi-layered model structure investigated consists of two adhesively bonded aluminium plate-strips. Fatigue experiments were carried out. The sensitivity of the high frequency guided wave modes to monitor fatigue crack growth at a fastener hole during cyclic loading was investigated, using both standard pulse-echo equipment and laser interferometry. The sensitivity and repeatability of the measurements were ascertained, having the potential for fatigue crack growth monitoring at critical and difficult to access fastener locations from a stand-off distance.

  14. Initiation of fatigue cracks in AZ91 Mg alloy processed by ECAP

    NASA Astrophysics Data System (ADS)

    Fintová, S.; Kunz, L.

    2014-08-01

    Mechanism of fatigue crack initiation was investigated in ultrafine-grained (UFG) magnesium alloy AZ91 processed by equal channel angular pressing (ECAP). Fatigue behaviour of UFG material was compared to the behaviour of material in an initial as-cast state. Focused ion beam technique (FIB) was applied to reveal the surface relief and early fatigue cracks. Two substantially different mechanisms of crack initiation were observed in UFG structure, which can be characterized as bimodal even after 6 ECAP passes by route Bc. The bimodality consists in a coexistence of very fine grained areas with higher content of Mg17Al12 particles and areas exhibiting somewhat larger grains and lower density of particles. The fatigue cracks which initiate in areas of larger grains are related to the cyclic slip bands; this initiation mechanism is similar to that observed in cast alloy. The second initiation mechanism is related to the grain boundary cracking which takes place predominantly in the fine grained areas.

  15. Selective Reinforcement to Improve Fracture Toughness and Fatigue Crack Growth Resistance in Metallic Structures

    NASA Technical Reports Server (NTRS)

    Farley, Gary L.; Newman, John A.; James, Mark A.

    2004-01-01

    Experimental and analytical investigations of the fatigue crack growth and fracture response of aluminum selectively reinforced compact tension specimens were performed. It was shown that selective reinforcement significantly improved these responses primarily through load sharing by the reinforcement. With the appropriate combination of reinforcement architecture and mechanical properties, as well as reinforcement to base aluminum interface properties, fatigue cracks can be arrested using selective reinforcement. Maximum load associated with fracture increased up to 20 percent for the cases investigated and crack growth at maximum load increased as much as 150 percent. For both fatigue crack growth and fracture, the three most influential properties identified within the bounds of this investigation that influence this response are reinforcement width, reinforcement stiffness and interface stiffness. Considerable coupling occurs between the different fiber architecture and material properties and how they influence fatigue crack growth and fracture responses.

  16. Fatigue crack growth and fracture toughness properties of 304 stainless steel pipe for LNG transmission

    NASA Astrophysics Data System (ADS)

    Baek, Jong-Hyun; Kim, Cheol-Man; Kim, Woo-Sik; Kho, Young-Tai

    2001-11-01

    The fatigue crack growth rate and fracture toughness tests of type 304 stainless steel were studied over a temperature range of -162°C to room temperature. Girth weld metal specimens were fabricated using a combination of gas-tungsten-arc-welding and shielded-metal-arc-welding. The seam weld metal was made with submerged arc welding. Fatigue crack growth rate tests were conducted using compact tension specimens in accordance with ASTM E647. Fracture toughness was evaluated through CTOD tests with three point bend specimens. The CTOD values were affected by crack orientation with respect to the rolling direction, but orientation had no influence on fatigue crack growth rates. The fatigue crack growth rates and the CTOD values decreased with decreasing test temperature.

  17. The effect of pre-existing corrosion on the fatigue cracking behavior of aluminum alloys

    SciTech Connect

    Hagerdorn, E.L.; Koch, G.H.

    1996-10-01

    In order to assess the effect of preexisting corrosion on the fatigue crack behavior of aluminum alloys 2024-T3 and 7074-T6 crack initiation and growth data were obtained using fracture mechanics specimens. These specimens incorporated mechanically thinned areas and areas that had been preexposed to environments which produced various degrees of pitting or exfoliation corrosion. The data obtained from these laboratory experiments indicate that specific corrosive was most pronounced in the fatigue cracking behavior of aluminum alloys. The effect of preexisting corrosion was most pronounced in the fatigue crack initiation stage. Based on the results of this study, it was concluded that the effect of preexisting corrosion on the fatigue cracking behavior of both aluminum alloys 2024-T3 and 7075-T6 is a combination of stress concentrations as a result of material loss, and altered material properties, possible as a result of hydrogen entry into the lattice.

  18. Development of Standard Methods of Testing and Analyzing Fatigue Crack Growth Rate Data

    DTIC Science & Technology

    1978-05-01

    S,,,c0AEVEII# AFML-TR-78-40 0 m DEVELOPMENT OF STANDARD METHODS OF TES(ING AND ANALYZING FATIGUE CRACK GROWTH RATE DATA S.J. HUDAK, Jr. A. SAXENA 0...s i n a O i t e l a d 1 - 8 5 l u m n m a l y Approed matheuatica repreasen itariobuftideong faimtiged cakgothrt. N. ~ ~ ~ ~ ~ ~ ~ ~ SC~T...Del Research N. E. Dowling - Westinghouse R&D Center A. W. Gunderson - Air Force Materials Laboratory B. M . Kapadia - U.S. Steel Research Laboratory C

  19. NASGRO(registered trademark): Fracture Mechanics and Fatigue Crack Growth Analysis Software

    NASA Technical Reports Server (NTRS)

    Forman, Royce; Shivakumar, V.; Mettu, Sambi; Beek, Joachim; Williams, Leonard; Yeh, Feng; McClung, Craig; Cardinal, Joe

    2004-01-01

    This viewgraph presentation describes NASGRO, which is a fracture mechanics and fatigue crack growth analysis software package that is used to reduce risk of fracture in Space Shuttles. The contents include: 1) Consequences of Fracture; 2) NASA Fracture Control Requirements; 3) NASGRO Reduces Risk; 4) NASGRO Use Inside NASA; 5) NASGRO Components: Crack Growth Module; 6) NASGRO Components:Material Property Module; 7) Typical NASGRO analysis: Crack growth or component life calculation; and 8) NASGRO Sample Application: Orbiter feedline flowliner crack analysis.

  20. Chemical and metallurgical aspects of environmentally assisted fatigue crack growth in 7075-T651 aluminum alloy

    NASA Astrophysics Data System (ADS)

    Gao, Ming; Wei, R. P.; Pao, P. S.

    1988-07-01

    A comprehensive study has been carried out on a 7075-T651 alloy to examine the influence of water vapor on fatigue crack growth. The kinetics of fatigue crack growth were determined as a function of water vapor pressure at room temperature and at 353 K. Detailed fractographic analyses and surface chemistry studies were carried out to identify the micromechanisms and to quantify the chemical interactions for corrosion fatigue crack growth in this alloy. Experiments were also carried out in ultra-high vacuum and in oxygen to provide for comparisons. Two regions of fatigue crack growth response were identified. In the low pressure region (below 67 Pa at 5 Hz), crack growth is controlled by the rate of transport of water vapor to the crack tip, and the response can be described by a model for transport controlled crack growth. At pressures above 67 Pa, additional increases in crack growth rate occurred, which are attributed to the further reactions of water vapor with segregated magnesium in this alloy. Different micromechanisms for crack growth have been identified for vacuum, oxygen, and water vapor. These micromechanisms are considered in relation to the environmental parameters through a modified superposition model for corrosion fatigue.

  1. 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.

  2. Effects of Laser Shock Processing on Fatigue Crack Growth in Ti-17 Titanium Alloy

    NASA Astrophysics Data System (ADS)

    Huang, Shuai; Zhu, Ying; Guo, Wei; Peng, Peng; Qiao, Hongchao; Diao, Xungang; Chu, Paul K.

    2017-02-01

    The effects of laser shock processing (LSP) on the fatigue crack properties of Ti-17 titanium alloy are investigated. Surfaces on either side of a fatigue slot are subjected to LSP. The residual stress of the irradiated surface is measured by x-ray diffraction measurement and fatigue crack growth testing of the treated and untreated specimens. The fatigue fracture morphology and microstructure are examined by scanning electron microscopy and transmission electron microscopy. Proliferation and tangles of dislocations occur in the Ti-17, and the density of dislocation increases after the LSP treatment. The fine spacing of the fatigue striations indicates that LSP produces residual compressive stress on the irradiated surfaces which can delay micro-crack formation and expansion. Consequently, the fatigue propagation life of the specimen increases considerably after LSP.

  3. Investigation of Fatigue Crack Propagation in Spot-Welded Joints Based on Fracture Mechanics Approach

    NASA Astrophysics Data System (ADS)

    Hassanifard, S.; Bonab, M. A. Mohtadi; Jabbari, Gh.

    2013-01-01

    In this paper, fatigue crack propagation life of resistance spot welds in tensile-shear specimens is investigated based on the calculation of stress intensity factors and J-integral using three-dimensional finite element method. For comparison, experimental works on 5083-O aluminum alloy spot-welded joints have been carried out to verify the numerical predictions of fatigue crack propagation of welded joints. A lot of analyses have been performed to obtain stress intensity factors and J-integral in tensile-shear specimens of spot-welded joints by using commercial software ANSYS. These gathered data have been formulated by using statistical software SPSS. The results of fatigue propagation life and predicted fatigue crack path revealed very good agreement with the experimental fatigue test data and photograph of cross-section of the fatigued spot-weld specimens.

  4. Frequency-dependent environmental fatigue crack propagation in the 7XXX alloy/aqueous chloride system

    NASA Astrophysics Data System (ADS)

    Gasem, Zuhair Mattoug

    The need to predict the fatigue performance of aging aerospace structures has focused interest on environmentally assisted cracking in thick-section damage-tolerant aluminum alloys (AA). The objective of this research is to characterize and understand the time-dependent processes that govern environmental fatigue crack propagation (EFCP) in 7XXX series aluminum alloys exposed to an aggressive environment. Results are utilized to identify the rate-controlling step in growth enhancement in order to develop a mechanistic model describing the time dependency of EFCP. Aluminum alloy 7075, tested in the sensitive (SL) orientation and exposed to aqueous chloride solution, is studied. Da/dNcrit for different D K levels depends on 1/√fcrit, as predicted by process zone hydrogen-diffusion-limited crack growth modeling. A model based on hydrogen diffusion controlled growth is modified to include a stress-dependent critical hydrogen concentration normalized with the crack tip hydrogen concentration (Ccrit/CS). It is proposed that da/dNcrit for a given D K and R corresponds to the distance ahead of the crack tip where the local tensile stress associated with Kmax is maximum. The reversed plasticity estimate of this location equals da/dNcrit for two aging conditions of 7075 (SL)/NaCl at R = 0.1. The EFCP dependencies on alloy microstructure (T6 vs. T7), crack orientation (SL vs. LT), and stress ratio are measured and interpreted based on their effect on da/dN crit and fcrit as well as environmental closure. Chromate addition to the chloride solution eliminates the environmental acceleration of crack growth and reduces corrosion-product induced closure. In chromate-inhibited solution, the frequency dependence of EFCP in 7075 (SL) is unique. Da/dN is reduced at moderate and low frequencies to a value similar to crack growth rate in moist air, probably due to formation of a passive film which inhibits hydrogen uptake. Inhibition is mitigated by increasing frequency or increasing

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

    SciTech Connect

    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 the 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

  6. Fatigue Crack Length Sizing Using a Novel Flexible Eddy Current Sensor Array

    PubMed Central

    Xie, Ruifang; Chen, Dixiang; Pan, Mengchun; Tian, Wugang; Wu, Xuezhong; Zhou, Weihong; Tang, Ying

    2015-01-01

    The eddy current probe, which is flexible, array typed, highly sensitive and capable of quantitative inspection is one practical requirement in nondestructive testing and also a research hotspot. A novel flexible planar eddy current sensor array for the inspection of microcrack presentation in critical parts of airplanes is developed in this paper. Both exciting and sensing coils are etched on polyimide films using a flexible printed circuit board technique, thus conforming the sensor to complex geometric structures. In order to serve the needs of condition-based maintenance (CBM), the proposed sensor array is comprised of 64 elements. Its spatial resolution is only 0.8 mm, and it is not only sensitive to shallow microcracks, but also capable of sizing the length of fatigue cracks. The details and advantages of our sensor design are introduced. The working principal and the crack responses are analyzed by finite element simulation, with which a crack length sizing algorithm is proposed. Experiments based on standard specimens are implemented to verify the validity of our simulation and the efficiency of the crack length sizing algorithm. Experimental results show that the sensor array is sensitive to microcracks, and is capable of crack length sizing with an accuracy within ±0.2 mm. PMID:26703608

  7. Fatigue Crack Length Sizing Using a Novel Flexible Eddy Current Sensor Array.

    PubMed

    Xie, Ruifang; Chen, Dixiang; Pan, Mengchun; Tian, Wugang; Wu, Xuezhong; Zhou, Weihong; Tang, Ying

    2015-12-21

    The eddy current probe, which is flexible, array typed, highly sensitive and capable of quantitative inspection is one practical requirement in nondestructive testing and also a research hotspot. A novel flexible planar eddy current sensor array for the inspection of microcrack presentation in critical parts of airplanes is developed in this paper. Both exciting and sensing coils are etched on polyimide films using a flexible printed circuit board technique, thus conforming the sensor to complex geometric structures. In order to serve the needs of condition-based maintenance (CBM), the proposed sensor array is comprised of 64 elements. Its spatial resolution is only 0.8 mm, and it is not only sensitive to shallow microcracks, but also capable of sizing the length of fatigue cracks. The details and advantages of our sensor design are introduced. The working principal and the crack responses are analyzed by finite element simulation, with which a crack length sizing algorithm is proposed. Experiments based on standard specimens are implemented to verify the validity of our simulation and the efficiency of the crack length sizing algorithm. Experimental results show that the sensor array is sensitive to microcracks, and is capable of crack length sizing with an accuracy within ±0.2 mm.

  8. The Variability of Fatigue Crack Growth Life of Aluminum Casting Alloy A357-T6

    DTIC Science & Technology

    1986-07-01

    34,FWAL-TR-86-4115 . A THE VARIABILITY OF FATIGUE CRACK GROWTH LIFE OF ALUMINUM CASTING ALLOY A357 -T6 .D. TIRPAK, CAPT, USAF Materials Engineering...Fatigue Crack Growth Life of Aluminum Casting Alloy A357 -T6 17 COSATI CODES 18. SUBJECT 1%iRMS (Continue on reverse if necessary and identify by...fContinue on reverse if necessary and identify by block number) "This investigation considers the variability of fatigue crack growth (FCG) life of A357 -T6

  9. Stochastic model for fatigue crack size and cost effective design decisions. [for aerospace structures

    NASA Technical Reports Server (NTRS)

    Hanagud, S.; Uppaluri, B.

    1975-01-01

    This paper describes a methodology for making cost effective fatigue design decisions. The methodology is based on a probabilistic model for the stochastic process of fatigue crack growth with time. The development of a particular model for the stochastic process is also discussed in the paper. The model is based on the assumption of continuous time and discrete space of crack lengths. Statistical decision theory and the developed probabilistic model are used to develop the procedure for making fatigue design decisions on the basis of minimum expected cost or risk function and reliability bounds. Selections of initial flaw size distribution, NDT, repair threshold crack lengths, and inspection intervals are discussed.

  10. Detection and characterization of fatigue cracks in thin metal plates by low frequency resonant model analysis

    NASA Technical Reports Server (NTRS)

    Wincheski, B.; Namkung, M.; Birt, E. A.

    1992-01-01

    Low-frequency resonant model analysis, a technique for the detection and characterization of fatigue cracks in thin metal plates, which could be adapted to rapid scan or large area testing, is considered. Experimental data displaying a direct correlation between fatigue crack geometry and resonance frequency for the second vibrational plate mode are presented. FEM is used to calculate the mechanical behavior of the plates, and provides a comparison basis for the experimentally determined resonance frequency values. The waveform of the acoustic emission generated at the resonant frequency is examined; it provides the basis for a model of the interaction of fatigue crack faces during plate vibration.

  11. Detection of rolling contact sub-surface fatigue cracks using acoustic emission technique

    SciTech Connect

    Yoshioka, T. )

    1993-04-01

    A method of locating the position of acoustic emission sources has been developed to analyze the mechanism of rolling contact fatigue. Using this method, sub-surface fatigue cracks were found at positions corresponding to the actual source positions of acoustic emissions. When fatigue tests were run under maximum stresses of 5.75 GPa and lubricant film parameters of 0.19, the cracks propagated parallel to the surface, had a maximum length of approximately 200 microns in the rolling direction of balls, and were distributed between 50 microns and 200 microns below the surface. Although the lubricant film parameter was small, no cracks from the surface were found. 12 refs.

  12. Effect of interstitial content on high- temperature fatigue crack propagation and low- cycle fatigue of alloy 720

    NASA Astrophysics Data System (ADS)

    Bashir, S.; Thomas, M. C.

    1993-08-01

    Alloy 720 is a high-strength cast and wrought turbine disc alloy currently in use for temperatures up to about 650 °C in Allison’s T800, T406, GMA 2100, and GMA 3007 engines. In the original composition in-tended for use as turbine blades, large carbide and boride stringers formed and acted as preferred crack initiators. Stringering was attributed to relatively higher boron and carbon levels. These interstitials are known to affect creep and ductility of superalloys, but the effects on low-cycle fatigue and fatigue crack propagation have not been studied. Recent emphasis on the total life approach in the design of turbine discs necessitates better understanding of the interactive fatigue crack propagation and low-cycle fatigue behavior at high temperatures. The objective of this study was to improve the damage tolerance of Alloy 720 by systematically modifying boron and carbon levels in the master melt, without altering the low-cy-cle fatigue and strength characteristics of the original composition. Improvement in strain-controlled low-cycle fatigue life was achieved by fragmenting the continuous stringers via composition modifica-tion. The fatigue crack propagation rate was reduced by a concurrent reduction of both carbon and bo-ron levels to optimally low levels at which the frequency of brittle second phases was minimal. The changes in composition have been incorporated for production disc forgings.

  13. An inverse method for the calculation of through-thickness fatigue crack closure behavior

    NASA Technical Reports Server (NTRS)

    Dawicke, D. S.; Shivakumar, K. N.; Newman, J. C., Jr.; Grandt, A. F., Jr.

    1992-01-01

    An inverse technique was used to calculate through-thickness fatigue crack closure behavior. The through-thickness variation in crack opening stress-intensity factor was calculated by considering the variation in the three-dimensional stress-intensity factor, the variation in crack growth rate along the crack front, and a relationship between the crack growth rate and effective stress-intensity factor range (da/dN-Delta-K(eff)). The three-dimensional stress-intensity factor variation was obtained from an elastic finite element analysis of specific crack-front profiles observed experimentally. The variation in crack growth rate along the crack front was obtained experimentally from comparison of observed crack front changes. The da/dN-Delta-K(eff) relationship was estimated from high stress ratio, constant load amplitude, and fatigue crack growth tests. The through-thickness crack opening stress-intensity factor results agreed with crack opening measurements obtained from fatigue striations, near-tip strain gages, and remote strain and displacement gages.

  14. A study on the influence of microstructure on small fatigue cracks

    NASA Astrophysics Data System (ADS)

    Castelluccio, Gustavo M.

    In spite of its significance in industrial applications, the prediction of the influence of microstructure on the early stages of crack formation and growth in engineering alloys remains underdeveloped. The formation and early growth of fatigue cracks in the high cycle fatigue regime lasts for much of the fatigue life, and it is strongly influenced by microstructural features such as grain size, twins and morphological and crystallographic texture. However, most fatigue models do not predict the in uence of the microstructure on early stages of crack formation, or they employ parameters that should be calibrated with experimental data from specimens with microstructures of interest. These post facto strategies are adequate to characterize materials, but they are not fully appropriate to aid in the design of fatigue-resistant engineering alloys. This thesis considers finite element computational models that explicitly render the microstructure of selected FCC metallic systems and introduces a fatigue methodology that estimates transgranular and intergranular fatigue growth for microstructurally small cracks. The driving forces for both failure modes are assessed by means of fatigue indicators, which are used along with life correlations to estimate the fatigue life. Furthermore, cracks with meandering paths are modeled by considering crack growth on a grain-by-grain basis with a damage model embedded analytically to account for stress and strain redistribution as the cracks extend. The methodology is implemented using a crystal plasticity constitutive model calibrated for studying the effect of microstructure on early fatigue life of a powder processed Ni-base RR1000 superalloy at elevated temperature under high cycle fatigue conditions. This alloy is employed for aircraft turbine engine disks, which undergo a thermomechanical production process to produce a controlled bimodal grain size distribution. The prediction of the fatigue life for this complex

  15. 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.

  16. Fracture mechanics of propagating 3-D fatigue cracks with parametric dislocations

    NASA Astrophysics Data System (ADS)

    Takahashi, Akiyuki; Ghoniem, Nasr M.

    2013-07-01

    Propagation of 3-D fatigue cracks is analyzed using a discrete dislocation representation of the crack opening displacement. Three dimensional cracks are represented with Volterra dislocation loops in equilibrium with the applied external load. The stress intensity factor (SIF) is calculated using the Peach-Koehler (PK) force acting on the crack tip dislocation loop. Loading mode decomposition of the SIF is achieved by selection of Burgers vector components to correspond to each fracture mode in the PK force calculations. The interaction between 3-D cracks and free surfaces is taken into account through application of the superposition principle. A boundary integral solution of an elasticity problem in a finite domain is superposed onto the elastic field solution of the discrete dislocation method in an infinite medium. The numerical accuracy of the SIF is ascertained by comparison with known analytical solution of a 3-D crack problem in pure mode I, and for mixed-mode loading. Finally, fatigue crack growth simulations are performed with the Paris law, showing that 3-D cracks do not propagate in a self-similar shape, but they re-configure as a result of their interaction with external boundaries. A specific numerical example of fatigue crack growth is presented to demonstrate the utility of the developed method for studies of 3-D crack growth during fatigue.

  17. Fatigue crack propagation in dual-phase steels: Effects of ferritic-martensitic microstructures on crack path morphology

    NASA Astrophysics Data System (ADS)

    Dutta, V. B.; Suresh, S.; Ritchie, R. O.

    1984-06-01

    microstructures with maximum resistance to fatigue crack extension while maintaining high strength levels. A wide range of crack growth rates has been examined, from ~10-8 to 10-3 mm per cycle, in a series of duplex microstructures of comparable yield strength and prior austenite grain size where intercritical heat treatments were used to vary the proportion, morphology, and distribution of the ferrite and martensite phases. Results of fatigue crack propagation tests, conducted on “long cracks” in room temperature moist air environments, revealed a very large influence of microstructure over the entire spectrum of growth rates at low load ratios. Similar trends were observed at high load ratio, although the extent of the microstructural effects on crack growth behavior was significantly less marked. Specifically, microstructures containing fine globular or coarse martensite in a coarse-grained ferritic matrix demonstrated exceptionally high resistance to crack growth without loss in strength properties. To our knowledge, these microstructures yielded the highest ambient temperature fatigue threshold stress intensity range ΔK0 values reported to date, and certainly the highest combination of strength and ΔK0 for steels ( i.e., ΔK0 values above 19 MPa√m with yield strengths in excess of 600 MPa). Such unusually high crack growth resistance is attributed primarily to a tortuous morphology of crack path which results in a reduction in the crack driving force from crack deflection and roughness-induced crack closure mechanisms. Quantitative metallography and experimental crack closure measurements, applied to currently available analytical models for the deflection and closure processes, are presented to substantiate such interpretations.

  18. Development of short fatigue cracks in aluminum alloy 2524-T3 specimens

    NASA Astrophysics Data System (ADS)

    Botvina, L. R.; Nesterenko, G. I.; Soldatenkov, A. P.; Demina, Yu. A.; Sviridov, A. A.

    2017-04-01

    The development of short fatigue cracks in a 2524-T3 alloy is studied under cyclic tension conditions. Flat specimens with a stress concentrator in the form of a central hole are analyzed. The replica technique is used to determine the microcrack parameters and to estimate the cyclic damage characteristics of the alloy in the stress concentrator zone. The experimental results are compared to the fatigue lives estimated by a calculation-experimental method using the NASGRO software package. The experimental fatigue life at the stage of short crack initiation is found to be significantly shorter than the calculated fatigue life.

  19. Constant amplitude and post-overload fatigue crack growth behavior in PM aluminum alloy AA 8009

    NASA Technical Reports Server (NTRS)

    Reynolds, A. P.

    1991-01-01

    A recently developed, rapidly solidified, powder metallurgy, dispersion strengthened aluminum alloy, AA 8009, was fatigue tested at room temperature in lab air. Constant amplitude/constant delta kappa and single spike overload conditions were examined. High fatigue crack growth rates and low crack closure levels compared to typical ingot metallurgy aluminum alloys were observed. It was proposed that minimal crack roughness, crack path deflection, and limited slip reversibility, resulting from ultra-fine microstructure, were responsible for the relatively poor da/dN-delta kappa performance of AA 8009 as compared to that of typical IM aluminum alloys.

  20. Development of detection technique for fatigue crack due to thermal stratification phenomena in RCS piping

    NASA Astrophysics Data System (ADS)

    Lee, Sam Lai; Kim, Byoung Chul; Lim, Hyung Taik; Park, Chi Sung; Lee, Jong Po; Chang, Kee Ok

    1993-01-01

    Area highly vulnerable to crack generation has been chosen by calculating the stress and deformation using computer program ANSYS considering thermal stratification phenomena, and for weld area, stress distribution has been investigated in order to perform the intensive ISI. The manufacturing of specimens with inside natural cracks has been done and basic procedure to manufacture mechanical fatigue crack by implanting fatigue cracked specimen into parent metal has been set up. This is a distribution that meets the requirement of ASME code Sec.XI, App.VII and VIII (1989 edition).

  1. Effects of R-ratio on fatigue crack growth in a Ti-24Al-11Nb alloy

    NASA Technical Reports Server (NTRS)

    Bae, K.; Nelson, H. G.

    1993-01-01

    The microscopic fatigue crack behavior in a Ti-24Al-11Nb alloy was investigated. Particular attention was given to the path of the fatigue crack through the microstructure, the fracture mode, and the effects of R-ratio and crack closure on the fatigue crack growth behavior. The FCGR of the alloy at R = 0.5 was an order of magnitude higher than that at R = 0.1.

  2. Degradation in the fatigue crack growth resistance of human dentin by lactic acid.

    PubMed

    Orrego, Santiago; Xu, Huakun; Arola, Dwayne

    2017-04-01

    The oral cavity frequently undergoes localized changes in chemistry and level of acidity, which threatens the integrity of the restorative material and supporting hard tissue. The focus of this study was to evaluate the changes in fatigue crack growth resistance of dentin and toughening mechanisms caused by lactic acid exposure. Compact tension specimens of human dentin were prepared from unrestored molars and subjected to Mode I opening mode cyclic loads. Fatigue crack growth was achieved in samples from mid- and outer-coronal dentin immersed in either a lactic acid solution or neutral conditions. An additional evaluation of the influence of sealing the lumens by dental adhesive was also conducted. A hybrid analysis combining experimental results and finite element modeling quantified the contribution of the toughening mechanisms for both environments. The fatigue crack growth responses showed that exposure to lactic acid caused a significant reduction (p≤0.05) of the stress intensity threshold for cyclic crack extension, and a significant increase (p≤0.05) in the incremental fatigue crack growth rate for both regions of coronal dentin. Sealing the lumens had negligible influence on the fatigue resistance. The hybrid analysis showed that the acidic solution was most detrimental to the extrinsic toughening mechanisms, and the magnitude of crack closure stresses operating in the crack wake. Exposing dentin to acidic environments contributes to the development of caries, but it also increases the chance of tooth fractures via fatigue-related failure and at lower mastication forces.

  3. Cryogenic S-N Fatigue and Fatigue Crack Propagation Behaviors of High Manganese Austenitic Steels

    NASA Astrophysics Data System (ADS)

    Jeong, Dae-Ho; Lee, Soon-Gi; Jang, Woo-Kil; Choi, Jong-Kyo; Kim, Young-Ju; Kim, Sangshik

    2013-10-01

    In the current study, the S-N fatigue and the fatigue crack propagation (FCP) behaviors of high manganese austenitic steels, including Fe24Mn and Fe22Mn, were studied, and the results were compared with STS304 (Fe-1Si-2Mn-20Cr-10Ni). The S-N fatigue tests were conducted at 298 K and 110 K (25 °C and -163 °C), respectively, and at an R ratio of 0.1 under a uniaxial loading condition. The FCP tests were conducted at 298 K and 110 K (25 °C and -163°C), respectively, and at R ratios of 0.1 and 0.5, respectively, using compact tension specimens. The resistance to S-N fatigue of each specimen increased greatly with decreasing temperature from 298 K to 110 K (25 °C to -163 °C) and showed a strong dependency on the flow stress. The FCP behaviors of the austenitic steels currently studied substantially varied depending on testing temperature, applied Δ K (stress intensity factor range), and R ratio. The enhanced FCP resistance was observed for the Fe24Mn and the Fe22Mn specimens particularly in the near-threshold Δ K regime, while the enhancement was significant over the entire Δ K regimes for the STS304 specimen, with decreasing temperature from 298 K to 110 K (25 °C to -163 °C). The S-N fatigue and the FCP behaviors of high manganese austenitic steels are compared with STS304 and discussed based on the fractographic and the micrographic observations.

  4. An investigation of environmental effects on fatigue crack growth in Q1N (HY80) steel

    NASA Astrophysics Data System (ADS)

    Soboyejo, W. O.; Knott, J. F.

    1990-11-01

    Fatigue threshold tests have been conducted on through-thickness and semielliptic cracks in laboratory air, vacuum, and salt water at stress ratios (R = Kmin/Kmax @#@) of 0.2 and 0.7. The effects of stress ratio are rationalized by crack closure concepts. Environmental effects are explained by considerations of the irreversibility of slip at the crack tip and the role of debris on the fracture surfaces. Differences in the fatigue crack growth rates in the three environments are attributed largely to the extent of the irreversibility of slip due to the chemisorption of water/ water vapor at the crack tip. Debris in saltwater solutions is also shown to significantly affect the near-threshold growth through its influence on crack closure and the transportation of environment to the crack tip.

  5. Interlaminar crack growth in fiber reinforced composites during fatigue, part 3

    NASA Technical Reports Server (NTRS)

    Wang, S. S.; Wang, H. T.

    1981-01-01

    Interlaminar crack growth behavior in fiber-reinforced composites subjected to fatigue loading was investigated experimentally and theoretically. In the experimental phase, inter-laminar crack propagation rates and mechanisms were determined for the cases of various geometries, laminate parameters and cyclic stress levels. A singular hybrid-stress finite element method was used in conjuction with the experimental results to examine the local crack-tip behavior and to characterize the crack propagation during fatigue. Results elucidate the basic nature of the cyclic delamination damage, and relate the interlaminar crack growth rate to the range of mixed-mode crack-tip stress intensity factors. The results show that crack growth rates are directly related to the range of the mixed-mode cyclic stress intensity factors by a power law relationship.

  6. Localization and characterization of fatigue cracks around fastener holes using spherically focused ultrasonic probes

    NASA Astrophysics Data System (ADS)

    Hopkins, Deborah; Datuin, Marvin; Aldrin, John; Warchol, Mark; Warchol, Lyudmila; Forsyth, David

    2017-02-01

    Results are presented from laboratory experiments and simulations that demonstrate the ability to localize fatigue cracks around fastener holes using spherically focused ultrasonic probes for shear-wave inspections. For the experiments, fatigue cracks were created in aluminum plates in a testing frame under cyclic loading. With the exceptions of one specimen with a mid-bore crack and another with a "through" crack, the remaining specimens contain surface-breaking cracks. All of the specimens were inspected for the cracks intersecting the back wall, and some were flipped over and re-inspected with the crack intersecting the front surface. Parameter and variable sensitivity studies were performed using CIVA Simulation Software. In contrast to C-scans where detection and localization of small cracks can be very difficult, modeling and initial experimental results demonstrate that cracks can be accurately located in "True" B-scans (B-scans projected in the part along the beam path). Initial results show that small-amplitude diffracted/scattered signals from the crack tips and edges are essential in obtaining clear crack traces in the True B-scans. It is important therefore that experimental data be acquired with sufficient gain to capture the diffracted/scattered signals. In all of the cases studied here, saturating the high-amplitude specular reflections from the fastener hole and crack enhanced the crack trace in the True B-scans.

  7. Fracture resistance and fatigue crack growth characteristics of two Al-Cu-Mg-Zr alloys

    NASA Technical Reports Server (NTRS)

    Sarkar, Bhaskar; Lisagor, W. B.

    1992-01-01

    The dependence of strength, fracture resistance, and fatigue crack growth rate on the aging conditions of two alloy compositions based on Al-3.7Cu-1.85Mg-0.2Mn is investigated. Mechanical properties were evaluated in two heat treatment conditions and in two orientations (longitudinal and transverse). Compact tension specimens were used to determine fatigue crack growth characteristics and fracture resistance. The aging response was monitored on coupons using hardness measurements determined with a standard Rockwell hardness tester. Fracture resistance is found to increase with increasing yield strength during artificial aging of age-hardenable 2124-Zr alloys processed by powder metallurgy techniques. Fatigue crack growth rate increases with increasing strength. It is argued that these changes are related to deformation modes of the alloys; a homogeneous deformation mode tends to increase fracture resistance and to decrease the resistance to the fatigue crack propagation rate.

  8. Influence of environment on the fatigue crack growth behaviour of 12% Cr steel.

    PubMed

    Schönbauer, Bernd M; Stanzl-Tschegg, Stefanie E

    2013-12-01

    In the present work, the influence of different environments on the fatigue crack growth behaviour of 12% Cr steam turbine blade steel is investigated. Fatigue crack growth rates (FCGRs) in the near threshold regime are measured with ultrasonic fatigue testing technique. Fatigue tests are performed in vacuum, air and different aqueous environments with defined chloride and oxygen content. Furthermore, the influence of different stress ratios is investigated. It is found that crack propagation is not necessarily enhanced with increasing corrosiveness. In the aqueous environments, the FCGRs below 10⁻⁸ m/cycle are lower than in air. The threshold stress intensity factor ranges are higher or equal. Observation of the fracture surfaces shows oxide formation and partly intergranular fracture for specimens tested in aqueous environments. Crack closure effects seem to be responsible for this unexpected behaviour.

  9. Fracture resistance and fatigue crack growth characteristics of two Al-Cu-Mg-Zr alloys

    SciTech Connect

    Sarkar, B.; Lisagor, W.B. NASA, Langley Research Center, Hampton, VI )

    1992-01-01

    The dependence of strength, fracture resistance, and fatigue crack growth rate on the aging conditions of two alloy compositions based on Al-3.7Cu-1.85Mg-0.2Mn is investigated. Mechanical properties were evaluated in two heat treatment conditions and in two orientations (longitudinal and transverse). Compact tension specimens were used to determine fatigue crack growth characteristics and fracture resistance. The aging response was monitored on coupons using hardness measurements determined with a standard Rockwell hardness tester. Fracture resistance is found to increase with increasing yield strength during artificial aging of age-hardenable 2124-Zr alloys processed by powder metallurgy techniques. Fatigue crack growth rate increases with increasing strength. It is argued that these changes are related to deformation modes of the alloys; a homogeneous deformation mode tends to increase fracture resistance and to decrease the resistance to the fatigue crack propagation rate. 12 refs.

  10. Fracture resistance and fatigue crack growth characteristics of two Al-Cu-Mg-Zr alloys

    NASA Technical Reports Server (NTRS)

    Sarkar, Bhaskar; Lisagor, W. B.

    1992-01-01

    The dependence of strength, fracture resistance, and fatigue crack growth rate on the aging conditions of two alloy compositions based on Al-3.7Cu-1.85Mg-0.2Mn is investigated. Mechanical properties were evaluated in two heat treatment conditions and in two orientations (longitudinal and transverse). Compact tension specimens were used to determine fatigue crack growth characteristics and fracture resistance. The aging response was monitored on coupons using hardness measurements determined with a standard Rockwell hardness tester. Fracture resistance is found to increase with increasing yield strength during artificial aging of age-hardenable 2124-Zr alloys processed by powder metallurgy techniques. Fatigue crack growth rate increases with increasing strength. It is argued that these changes are related to deformation modes of the alloys; a homogeneous deformation mode tends to increase fracture resistance and to decrease the resistance to the fatigue crack propagation rate.

  11. A test procedure for determining the influence of stress ratio on fatigue crack growth

    NASA Technical Reports Server (NTRS)

    Fitzgerald, J. H.; Wei, R. P.

    1974-01-01

    A test procedure is outlined by which the rate of fatigue crack growth over a range of stress ratios and stress intensities can be determined expeditiously using a small number of specimens. This procedure was developed to avoid or circumvent the effects of load interactions on fatigue crack growth, and was used to develop data on a mill annealed Ti-6Al-4V alloy plate. Experimental data suggest that the rates of fatigue crack growth among the various stress ratios may be correlated in terms of an effective stress intensity range at given values of K max. This procedure is not to be used, however, for determining the corrosion fatigue crack growth characteristics of alloys when nonsteady-state effects are significant.

  12. Intrinsically higher fatigue cracking resistance of the penetrable and movable incoherent twin boundary

    PubMed Central

    Li, L. L.; Zhang, P.; Zhang, Z. J.; Zhang, Z. F.

    2014-01-01

    Incoherent twin boundaries (ITBs) are widespread and play a crucial role in unidirectional deformation behavior of materials, however, the intrinsic role of individual ITB under cyclic loading remains elusive. Here we show the fatigue cracking behavior of Cu bicrystal with an ITB as its sole interface for the first time. The slip bands (SBs) could transfer through the ITB; meanwhile, the ITB could migrate with the motion of partial dislocations. Both the penetrability and mobility contribute to the higher fatigue cracking resistance of the ITB and hence the fatigue crack nucleates along the SBs preferentially. These new findings not only shed light on the fatigue cracking mechanisms of a penetrable boundary with direct evidence but also could provide important implications for future interfacial optimization of metallic materials. PMID:24434787

  13. A test procedure for determining the influence of stress ratio on fatigue crack growth

    NASA Technical Reports Server (NTRS)

    Fitzgerald, J. H.; Wei, R. P.

    1974-01-01

    A test procedure is outlined by which the rate of fatigue crack growth over a range of stress ratios and stress intensities can be determined expeditiously using a small number of specimens. This procedure was developed to avoid or circumvent the effects of load interactions on fatigue crack growth, and was used to develop data on a mill annealed Ti-6Al-4V alloy plate. Experimental data suggest that the rates of fatigue crack growth among the various stress ratios may be correlated in terms of an effective stress intensity range at given values of K max. This procedure is not to be used, however, for determining the corrosion fatigue crack growth characteristics of alloys when nonsteady-state effects are significant.

  14. Fatigue crack initiation in riveted lap joints and in pressurized fuselages

    NASA Astrophysics Data System (ADS)

    Mueller, Richard P. G.

    1993-06-01

    Riveted joints in pressurized fuselages are exposed to severe fatigue loading. The study was carried out to increase fundamental understanding of the behavior of riveted fuselage joints. Areas of interest include rivet flexibility, load transfer, residual stress distribution, fatigue crack location, secondary bending and inter-sheet friction. These aspects depend on the squeezing force used to drive the rivet. Flat uniaxially loaded riveted lap joint specimens show longer fatigue lives than curved riveted panels loaded by internal pressure in a barrel test setup. Strain gauge measurements on a barrel test setup show more severe loading of the non-countersunk inner sheet compared to the countersunk outer sheet. Finite element calculations gave insight to the improved fatigue crack initiation performance for increased sqeezing force and to the crack initiation location. The early crack initiation at the edges of flat riveted lap joint panels is explained.

  15. Analysis of fatigue, fatique-crack propagation, and fracture data. [design of metallic aerospace structural components

    NASA Technical Reports Server (NTRS)

    Jaske, C. E.; Feddersen, C. E.; Davies, K. B.; Rice, R. C.

    1973-01-01

    Analytical methods have been developed for consolidation of fatigue, fatigue-crack propagation, and fracture data for use in design of metallic aerospace structural components. To evaluate these methods, a comprehensive file of data on 2024 and 7075 aluminums, Ti-6A1-4V, and 300M and D6Ac steels was established. Data were obtained from both published literature and unpublished reports furnished by aerospace companies. Fatigue and fatigue-crack-propagation analyses were restricted to information obtained from constant-amplitude load or strain cycling of specimens in air at room temperature. Fracture toughness data were from tests of center-cracked tension panels, part-through crack specimens, and compact-tension specimens.

  16. Fatigue Crack Growth Behavior of Metastable Austenitic Stainless Steel in Cryogenic High Magnetic Field Environments

    NASA Astrophysics Data System (ADS)

    Shindo, Yasuhide; Takeda, Tomo; Suzuki, Masato; Narita, Fumio

    2009-08-01

    This article studies the fatigue crack growth in a metastable austenitic stainless steel in cryogenic high magnetic field environments. Fatigue crack growth tests were performed with the compact tension (CT) specimens at liquid helium temperature (4 K) in magnetic fields of 0 and 6 T, and the crack growth rate data were expressed in terms of the J-integral range during fatigue loading. The J-integral range values were evaluated using an elastic-plastic finite element analysis. The measurement of martensite phase in the test specimens and the fractographic examination were also carried out. The high magnetic field effect on the fatigue crack growth rate properties at 4 K is discussed in detail.

  17. The effect of material heterogeneity and random loading on the mechanics of fatigue crack growth

    NASA Technical Reports Server (NTRS)

    Srivatsan, T. S.; Sambandham, M.; Bharucha-Reid, A. T.

    1985-01-01

    This paper reviews experimental work on the influence of variable amplitude or random loads on the mechanics and micromechanisms of fatigue crack growth. Implications are discussed in terms of the crack driving force, local plasticity, crack closure, crack blunting, and microstructure. Due to heterogeneity in the material's microstructure, the crack growth rate varies with crack tip position. Using the weakest link theory, an expression for crack growth rate is obtained as the expectation of a random variable. This expression is used to predict the crack growth rates for aluminum alloys, a titanium alloy, and a nickel steel in the mid-range region. It is observed, using the present theory, that the crack growth rate obeys the power law for small stress intensity factor range, and that the power is a function of a material constant.

  18. Fatigue crack growth spectrum simplification: Facilitation of on-board damage prognosis systems

    NASA Astrophysics Data System (ADS)

    Adler, Matthew Adam

    2009-12-01

    Better lifetime predictions of systems subjected to fatigue loading are needed in support of the optimization of the costs of life-cycle engineering. In particular, the climate is especially encouraging for the development of safer aircraft. One issue is that aircraft experience complex fatigue loading and current methods for the prediction of fatigue damage accumulation rely on intensive computational tools that are not currently carried onboard during flight. These tools rely on complex models that are made more difficult by the complicated load spectra themselves. This presents an overhead burden as offline analysis must be performed at an offsite facility. This architecture is thus unable to provide online, timely information for on-board use. The direct objective of this research was to facilitate the real-time fatigue damage assessments of on-board systems with a particular emphasis on aging aircraft. To achieve the objective, the goal of this research was to simplify flight spectra. Variable-amplitude spectra, in which the load changes on a cycle-by-cycle basis, cannot readily be supported by an onboard system because the models required to predict fatigue crack growth during variable-amplitude loading are too complicated. They are too complicated because variable-amplitude fatigue crack growth analysis must be performed on a cycle-by-cycle basis as no closed-form solution exists. This makes these calculations too time-consuming and requires impractical, heavy onboard systems or offsite facilities. The hypothesis is to replace a variable-amplitude spectrum with an equivalent constant-amplitude spectrum. The advantage is a dramatic reduction in the complexity of the problem so that damage predictions can be made onboard by simple, fast calculations in real-time without the need to add additional weight to the aircraft. The intent is to reduce the computational burden and facilitate on-board projection of damage evolution and prediction for the accurate

  19. Ultrasonic Characterization of Fatigue Cracks in Composite Materials

    NASA Technical Reports Server (NTRS)

    Workman, Gary L.; Watson, Jason; Johnson, Devin; Walker, James; Russell, Sam; Thom, Robert (Technical Monitor)

    2002-01-01

    Microcracking in composite structures due to combined fatigue and cryogenic loading can cause leakage and failure of the structure and can be difficult to detect in-service. In aerospace systems, these leaks may lead to loss of pressure/propellant, increased risk of explosion and possible cryo-pumping. The success of nondestructive evaluation to detect intra-ply microcracking in unlined pressure vessels fabricated from composite materials is critical to the use of composite structures in future space systems. The work presented herein characterizes measurements of intraply fatigue cracking through the thickness of laminated composite material by means of correlation with ultrasonic resonance. Resonant ultrasound spectroscopy provides measurements which are sensitive to both the microscopic and macroscopic properties of the test article. Elastic moduli, acoustic attenuation, and geometry can all be probed. The approach is based on the premise of half-wavelength resonance. The method injects a broadband ultrasonic wave into the test structure using a swept frequency technique. This method provides dramatically increased energy input into the test article, as compared to conventional pulsed ultrasonics. This relative energy increase improves the ability to measure finer details in the materials characterization, such as microcracking and porosity. As the microcrack density increases, more interactions occur with the higher frequency (small wavelength) components of the signal train causing the spectrum to shift toward lower frequencies. Several methods are under investigation to correlate the degree of microcracking from resonance ultrasound measurements on composite test articles including self organizing neural networks, chemometric techniques used in optical spectroscopy and other clustering algorithms.

  20. Fatigue Crack Growth of Age-Hardened Al Alloy Under Ultrasonic Loading

    NASA Astrophysics Data System (ADS)

    Chen, Q.; Kawagoishi, N.; Kariya, K.; Nu, Y.; Goto, M.

    An age-hardened and extruded Al alloy 7075-T6 was fatigued under both ultrasonic loading (20kHz) and rotating bending (50Hz) in the environments of controlled humidity, distilled water and oxygen gas respectively, to investigate the availability of ultrasonic fatigue test as a time-saving tool for the reliability evaluation of materials subjected to conventional frequency loading. Although fatigue strength decreased slightly at relative humidity below 60-70%, it degraded significantly when the humidity was increased beyond that level, irrespective of the loading frequency. However, the mechanisms of strength degradation involved in high humidity are quite different. Under rotating bending, fatigue strength decreased because crack growth was accelerated due to brittle fracture, whileas the decrease in fatigue strength under ultrasonic loading was caused by crack propagation transition from tensile mode to shear mode cracking.

  1. Acoustic emission during fatigue crack propagation in SiC particle reinforced Al matrix composites

    SciTech Connect

    Niklas, A.; Froyen, L.; Wevers, M.; Delaey, L.

    1995-12-01

    The acoustic emission (AE) behavior during fatigue propagation in aluminum 6061 and aluminum 6061 matrix composites containing 5, 10, and 20 wt pct SiC particle reinforcement was investigated under tension-tension fatigue loading. The purpose of this investigation was to monitor fatigue crack propagation by the AE technique and to identify the source(s) of AE. Most of the AEs detected were observed at the top of the load cycles. The cumulative number of AE events was found to correspond closely to the fatigue crack growth and to increase with increasing SiC content. Fractographic studies revealed an increasing number of fractured particles and to a lesser extent decohered particles on the fatigue fracture surface as the crack propagation rate (e.g., {Delta}K) or the SiC content was increased.

  2. High-cycle fatigue crack initiation and propagation in laser melting deposited TC18 titanium alloy

    NASA Astrophysics Data System (ADS)

    Wang, Yang; Zhang, Shu-quan; Tian, Xiang-jun; Wang, Hua-ming

    2013-07-01

    This article examines fatigue crack nucleation and propagation in laser deposited TC18 titanium alloy. The Widmanstätten structure was obtained by double-annealing treatment. High-cycle fatigue (HCF) tests were conducted at room temperature with the stress ratio of 0.1 and the notch concentration factor K t = 1. Fatigue cracks initiated preferentially at micropores, which had great effect on the HCF properties. The effect decreased with the decrease of pore size and the increase of distance from the pore location to the specimen surface. The crack initiation region was characterized by the cleavage facets of α lamella and the tearing of β matrix. The soft α precipitated-free zone formed along grain boundaries accelerated the crack propagation. Subsurface observation indicated that the crack preferred to propagate along the grain boundary α or border of α lamella or vertical to α lamella.

  3. On the growth of small fatigue cracks in aluminum-lithium alloy 2090

    SciTech Connect

    Venkatesward-Rao, K.T.; Yu, W.; Ritchie, R.O.

    1986-01-01

    It is the objective of this article to examine the behavior of small (2 to 1000 ..mu..m) fatigue cracks in a commercial Al-Li-Cu-Zr alloy, and to compare results with those determined on conventional long (greater than or equal to 20 mm) crack samples. The development of ultra-lightweight aluminum-lithium alloys has aroused much interest in the aerospace industry with the prospect of the design of aircraft with alloys of lower density and increased modulus. Moreover, although Li additions can cause low ductility and toughness properties, Al-Li-X alloys generally show far superior fatigue crack growth resistance to traditional aluminum alloys, such as 2124 and 7050. However, the latter observations are based exclusively on long (greater than or equal to 10 mm) crack studies; to date little information is available on the behavior of small fatigue cracks in these alloys.

  4. A statistical approach of fatigue crack detection for a structural hotspot

    NASA Astrophysics Data System (ADS)

    Jin, Pei; Zhou, Li

    2012-04-01

    This work focuses on an unsupervised, data driven statistical approach to detect and monitor fatigue crack growth in lug joint samples using surface mounted piezoelectric sensors. Early and faithful detection of fatigue cracks in a lug joint can guide in taking preventive measures, thus avoiding any possible fatal structural failure. The on-line damage state at any given fatigue cycle is estimated using a damage index approach as the dynamical properties of a structure change with the initiation of a new crack or the growth of an existing crack. Using the measurements performed on an intact lug joint as baseline, damage indices are evaluated from the frequency response of the lug joint with an unknown damage state. As the damage indices are evaluated, a Bayesian analysis is committed and a statistical metric is evaluated to identify damage state(say crack length).

  5. Modal analysis for characterization of fatigue cracks in thin metal plates

    NASA Technical Reports Server (NTRS)

    Wincheski, B.; Namkung, M.

    1991-01-01

    Detailed experimental investigation and numerical analysis were performed on the characterization of fatigue cracks in thin aluminum alloys by a low frequency resonant modal analysis technique. The resonance frequency of plate modes is altered by the presence of fatigue cracks. This frequency shift was analyzed using a finite element approach and the results were compared to experimental data. Acoustic sensors were used to detect the fatigue cracks which were stimulated by standing waves in 0.1 cm aluminum alloy plates. The response of several different sensors to the crack noise was studied, as was the response at higher order modes. It was found that critical crack information can be obtained from the amplitude and phase of the emitted signal with respect to that of the external driving force.

  6. Fatigue flaw growth and NDI evaluation for preventing through cracks in spacecraft tankage structures

    NASA Technical Reports Server (NTRS)

    Pettit, D. E.; Hoeppner, D. W.

    1972-01-01

    A program was conducted to determine the fatigue-crack propagation behavior of parent and welded 2219-T87 aluminum alloy sheet under controlled cyclic stress conditions in room temperature air and 300 F air. Specimens possessing an initial surface defect of controlled dimensions were cycled under constant load amplitude until the propagating fatigue crack penetrated the back surface of the specimen. A series of precracked specimens were prepared to determine optimum penetrant, X-ray, ultrasonic, and eddy current nondestructive inspection procedures.

  7. Fatigue Crack Growth Rate of Inconel 718 Sheet at Cryogenic Temperatures

    NASA Technical Reports Server (NTRS)

    Wells, Douglas; Wright, Jonathan; Hastings, Keith

    2005-01-01

    Inconel 718 sheet material was tested to determine fatigue crack growth rate (FCGR) at cryogenic conditions representative of a liquid hydrogen (LH2) environment at -423 degree F. Tests utilized M(T) and ESE(T) specimen geometries and environments were either cold gaseous helium or submersion in LH2. The test results support a significant improvement in the fatigue crack growth threshold at -423 degree F compared to -320 degree F or 70 degree F.

  8. Neutron Irradiation Effects on Fatigue Crack Propagation in Type 316 Stainless Steels at 649 C.

    DTIC Science & Technology

    1980-08-01

    maintaining the maximum tensile load constant for selected time periods duriig each cycle. Induction heating was employed to achieve a test temperature of...7 AD-A OB 052 NAVAL RESEAR ICH LAS WASHINGTON DC F/6 11/BNEUTRON IRRADIATION EFFECTS ON FATIGUE CRACK PROPAGATION IN TYP-EC(UlU UNL AUG 80 0 .J...and Identify by block number) Radiation Microstruture B Irradiation Fatigue Stainless steels Crack propagation Radiation effects High temperature V

  9. The application of acoustic emission technique to fatigue crack measurement. [in aluminum alloys

    NASA Technical Reports Server (NTRS)

    Singh, J. J.; Davis, W. T.; Crews, J. H., Jr.

    1974-01-01

    The applicability of acoustic emission technique to measure fatigue cracks in aluminum alloy specimens was investigated. There are several variables, such as the metallurgical and the physical treatment of the specimen, that can affect the level of acoustic activity of a fatigue specimen. It is therefore recommended that the acoustic emission technique be supplemented by other nondestructive evaluation methods to obtain quantitative data on crack growth.

  10. Effects of precrack environment on subsequent corrosion fatigue crack growth behavior of a squeeze-cast aluminum alloy

    SciTech Connect

    Shiozawa, Kazuaki; Sun, S.

    1995-11-01

    Corrosion fatigue occurs in all materials exposed to a corrosive environment and subjected to fatigue-type stresses. As in corrosion fatigue cracking, there are several aspects of the problem arising from mechanical, environmental, and metallurgical properties, which affect corrosion fatigue susceptibility. Corrosion fatigue crack propagation behavior will be obviously affected by different precrack conditions. However, studies regarding prefatigue crack environmental effects on subsequent corrosion fatigue crack growth and associated damage mechanisms are lacking to date. The present article gives a corrosion fatigue growth behavior of a through crack artificially obtained for long cracks relating to a different experimental precrack program in the air and the aqueous aggressive environments. A squeeze-cast Al-Si-Mg-Cu aluminum alloy (AC8A-T6) was used in this study. Chemical composition of the alloy is (in wt pct) 12Si-1.1Mg-1.1Cu-1.3Ni and balance Al.

  11. Experimental measurements and influence of overload-induced residual stress field on constant amplitude fatigue crack growth in aluminum alloy

    NASA Astrophysics Data System (ADS)

    Wahab, M. A.; Rohrsheim, G. R.; Brown, I. H.

    1997-03-01

    The aim of this research is to investigate current methods in Linear Elastic Fracture Mechanics for their suitability to predict crack growth in Aluminum alloy 7050 - T7451, when a compressive residual stress field is introduced by an overload. A comparative study has been made on the effect of various levels of tensile overload on the crack growth rate in Aluminum alloy. Experiments were performed on center-cracked tension specimens at various values of range of stress- intensity-factor ((Delta) K). Crack growth measurements are performed using crack propagation gauges and a travelling microscope. The average crack growth rate is used to determine an effective (Delta) K value for each interval using the fatigue crack propagation curve. After the application of overloads, the propagation gauges revealed a period of significant retardation before the crack growth rates returned to their baseline levels. The results from the numerical predictions are compared with the experimental results. The prediction model produces conservative results for both constant amplitude crack growth and overload induced retarded growth.

  12. Damage detection of fatigue cracks under nonlinear boundary condition using subharmonic resonance.

    PubMed

    Zhang, Mengyang; Xiao, Li; Qu, Wenzhong; Lu, Ye

    2017-05-01

    In recent years, the nonlinear ultrasonic technique has been widely utilized for detecting fatigue crack, one of the most common forms of damage. However, one of limitations associated with this technique is that nonlinearities can be produced not only by damage but also by various intrinsic effects such as boundary conditions. The objective of this paper is to demonstrate the application of a nonlinear ultrasonic subharmonic method for detecting fatigue cracks with nonlinear boundary conditions. The fatigue crack was qualitatively modeled as two elastic, frictionless half spaces that enter into contact during vibration and where the contact obeys the basic Hertz contact law. The nonlinear ordinary differential equation drawn from the developed model was solved with the method of multiple scales. The threshold of subharmonic generation was studied. Different threshold behaviors between the nonlinear boundary condition and the fatigue crack were found that can be used to distinguish the source of nonlinear subharmonic features. To evaluate the proposed method, experiments using an aluminum plate with a fatigue crack were conducted to quantitatively verify the subharmonic resonance range. Two surface-bonded piezoelectric transducers were used to generate and receive ultrasonic wave signals. The experimental results demonstrated that the subharmonic component of the sensing signal could be used to detect the fatigue crack and further to distinguish it from inherent nonlinear boundary conditions.

  13. Observations of fatigue crack initiation and damage growth in notched titanium matrix composites

    NASA Technical Reports Server (NTRS)

    Naik, R. A.; Johnson, W. S.

    1990-01-01

    The purpose was to characterize damage initiation and growth in notched titanium matrix composites at room temperature. Double edge notched or center open hole SCS-6/Ti-15-3 specimens containing 0 deg plies or containing both 0 and 90 deg plies were fatigued. The specimens were tested in the as-fabricated (ASF) and in heat-treated conditions. A local strain criterion using unnotched specimen fatigue data was successful in predicting fatigue damage initiation. The initiation stress level was accurately predicted for both a double edge notched unidirectional specimen and a cross-plied center hole specimen. The fatigue produced long multiple cracks growing from the notches. These fatigue cracks were only in the matrix material and did not break the fibers in their path. The combination of matrix cracking and fiber/matrix debonding appears to greatly reduce the stress concentration around the notches. The laminates that were heat treated showed a different crack growth pattern. In the ASF specimens, matrix cracks had a more tortuous path and showed considerable more crack branching. For the same specimen geometry and cyclic stress, the (0/90/0) laminate with a hole had far superior fatigue resistance than the matrix only specimen with a hole.

  14. The transition of corrosion pitting to surface fatigue cracks in 2024-T3 aluminum alloy

    NASA Astrophysics Data System (ADS)

    Clark, Paul Nolan

    Pitting corrosion experiments were executed to characterize the dynamics. of accelerated corrosion for the combination of 2024-T3 aluminum and a 3.5% NaCl solution at room temperature. Platinum was used as a counter electrode and a silver-silver chloride electrode was used as a reference. A corrosion protocol was developed to accelerate pitting corrosion on 2024-T3 aluminum. The developed protocol allowed for the relative control of pitting depth on fatigue dog-bone specimens. A pitting damage threshold was encountered for depths beyond an average value of approximately 135mum. Corrosion fatigue experiments were performed on specimens that had been prior corroded using the developed corrosion protocol. Pitting depths from approximately 40mum-200mum were examined for fatigue characterization. Corrosion pitting to surface fatigue crack transition was captured and fatigue crack growth behavior was documented. As well as the pit to crack transition, the short crack behavior to long crack behavior transition was captured and analyzed. Fractographic analysis of corrosion fatigue specimens revealed cracks nucleating, propagating and causing subsequent failure from accelerated prior pitting corrosion damage.

  15. Characterization of fatigue crack initiation and propagation in Ti-6Al-4V with electrical potential drop technique

    NASA Technical Reports Server (NTRS)

    Kalluri, Sreeramesh; Telesman, Jack

    1988-01-01

    Electrical potential methods have been used in the past primarily to monitor crack length in long crack specimens subjected to fatigue loading. An attempt was made to develop test procedures for monitoring the fatigue crack initiation and the growth of short fatigue cracks in a turbine disk alloy with the electrical potential drop technique (EPDT). In addition, the EPDT was also applied to monitor the fatigue crack growth in long crack specimens of the same alloy. The resolution of the EPDT for different specimen geometries was determined. Factors influencing the EPDT are identified and the applicability of EPDT in implementing damage tolerant design concepts for turbine disk materials is discussed. The experimental procedure adopted and the results obtained is discussed. No substantial differences were observed between the fatigue crack growth data of short and long crack specimens.

  16. Thermal fatigue crack behavior of SiCp/A356 composites prepared by stirring casting

    NASA Astrophysics Data System (ADS)

    Pan, Like; Han, Jianmin; Yang, Zhiyong; Li, Xiang; Wang, Jialin; Li, Zhiqiang; Li, Weijing

    The thermal fatigue crack initiation and propagation behavior of SiCp/A356 composites which is produced by stirring casting were studied. Specimens with a V-shaped notch were used in the thermal fatigue experiment. Optical microscope (OM) and scanning electron microscope (SEM) were used to observe the crack growth. Crack initiated at the notch tip after about 150 cycles of heating and cooling from room temperature to 250 °C. The crack propagation stage was dominated during the whole crack growth. Cracks mainly propagated along the interface of particle and through the matrix. The propagation stage experienced the cycle of slow propagation and fast propagation, and a Step-Like shape was observed during the crack growth process. The micro cracks appeared in the interface of particle and the matrix after repeated thermal cycles, and induced fast propagation by coalescing with the main cracks. Distributions of the particle play an important role in hindering thermal fatigue crack propagation. Hardness of the composites decreased with the increase of the number of cycles, and decreased by 46.1% after thermal 290 cycles.

  17. Fatigue crack growth study of SCS6/Ti-15-3 composite

    NASA Technical Reports Server (NTRS)

    Kantzos, Peter; Telesman, Jack

    1989-01-01

    A study was performed to determine the fatigue crack growth (FCG) behavior and the associated fatigue damage processes in a (0)(8) and (90)(8) oriented SCS(6)/Ti-15-3 composite. Companion testing (CT) was also done on identically processed Ti-15-3 unreinforced material. The active fatigue crack growth failure processes were very similar for both composite orientations tested. For both orientations, fatigue crack growth was along the fiber direction. It was found that the composite constituent most susceptible to fatigue damage was the interface region and in particular the carbon coating surrounding the fiber. The failure of the interface region lead to crack initiation and also strongly influenced the FCG behavior in this composite. The failure of the interface region was apparently driven by normal stresses perpendicular to the fiber direction. The FCG rates were considerably higher for the (90)(8) oriented CT specimens in comparison to the unreinforced material. This is consistent with the scenario in which the interface has lower fatigue resistance than the matrix, causing lower composite fatigue resistance. The FCG rates of the (0)(8) composite could not be directly compared to the (90)(8) composite but were shown to increase with an increase in the crack length.

  18. Fatigue Life Prediction Based on Crack Closure and Equivalent Initial Flaw Size.

    PubMed

    Wang, Qiang; Zhang, Wei; Jiang, Shan

    2015-10-21

    Failure analysis and fatigue life prediction are necessary and critical for engineering structural materials. In this paper, a general methodology is proposed to predict fatigue life of smooth and circular-hole specimens, in which the crack closure model and equivalent initial flaw size (EIFS) concept are employed. Different effects of crack closure on small crack growth region and long crack growth region are considered in the proposed method. The EIFS is determined by the fatigue limit and fatigue threshold stress intensity factor △Kth. Fatigue limit is directly obtained from experimental data, and △Kth is calculated by using a back-extrapolation method. Experimental data for smooth and circular-hole specimens in three different alloys (Al2024-T3, Al7075-T6 and Ti-6Al-4V) under multiple stress ratios are used to validate the method. In the validation section, Semi-circular surface crack and quarter-circular corner crack are assumed to be the initial crack shapes for the smooth and circular-hole specimens, respectively. A good agreement is observed between model predictions and experimental data. The detailed analysis and discussion are performed on the proposed model. Some conclusions and future work are given.

  19. Fatigue Life Prediction Based on Crack Closure and Equivalent Initial Flaw Size

    PubMed Central

    Wang, Qiang; Zhang, Wei; Jiang, Shan

    2015-01-01

    Failure analysis and fatigue life prediction are necessary and critical for engineering structural materials. In this paper, a general methodology is proposed to predict fatigue life of smooth and circular-hole specimens, in which the crack closure model and equivalent initial flaw size (EIFS) concept are employed. Different effects of crack closure on small crack growth region and long crack growth region are considered in the proposed method. The EIFS is determined by the fatigue limit and fatigue threshold stress intensity factor △Kth. Fatigue limit is directly obtained from experimental data, and △Kth is calculated by using a back-extrapolation method. Experimental data for smooth and circular-hole specimens in three different alloys (Al2024-T3, Al7075-T6 and Ti-6Al-4V) under multiple stress ratios are used to validate the method. In the validation section, Semi-circular surface crack and quarter-circular corner crack are assumed to be the initial crack shapes for the smooth and circular-hole specimens, respectively. A good agreement is observed between model predictions and experimental data. The detailed analysis and discussion are performed on the proposed model. Some conclusions and future work are given. PMID:28793625

  20. Mechanisms of Slow Fatigue Crack Growth in High Strength Aluminum Alloys: Role of Microstructure and Environment

    NASA Astrophysics Data System (ADS)

    Suresh, S.; Vasudévan, A. K.; Bretz, P. E.

    1984-02-01

    The role of microstructure and environment in influencing ultra-low fatigue crack propagation rates has been investigated in 7075 aluminum alloy heat-treated to underaged, peak-aged, and overaged conditions and tested over a range of load ratios. Threshold stress intensity range, ΔK0, values were found to decrease monotonically with increasing load ratio for all three heat treatments fatigue tested in 95 pct relative humidity air, with Δ K 0 decreasing at all load ratios with increased extent of aging. Comparison of the near-threshold fatigue behavior obtained in humid air with the data for vacuo, however, showed that the presence of moisture leads to a larger reduction in ΔK0 for the underaged microstructure than the overaged condition, at all load ratios. An examination of the nature of crack morphology and scanning Auger/SIMS analyses of near-threshold fracture surfaces revealed that although the crack path in the underaged structure was highly serrated and nonlinear, crack face oxidation products were much thicker in the overaged condition. The apparent differences in slow fatigue crack growth resistance of the three aging conditions are ascribed to a complex interaction among three mechanisms: the embrittling effect of moisture resulting in conventional corrosion fatigue processes, the role of microstructure and slip mode in inducing crack deflection, and crack closure arising from a combination of environmental and microstructural contributions.

  1. The Significance of Small Cracks in Fatigue Design Concepts as Related to Rotorcraft Metallic Dynamic Components

    NASA Technical Reports Server (NTRS)

    Everett, R. A., Jr.; Elber, W.

    2000-01-01

    In this paper the significance of the "small" crack effect as defined in fracture mechanics will be discussed as it relates to life managing rotorcraft dynamic components using the conventional safe-life, the flaw tolerant safe-life, and the damage tolerance design philosophies. These topics will be introduced starting with an explanation of the small-crack theory, then showing how small-crack theory has been used to predict the total fatigue life of fatigue laboratory test coupons with and without flaws, and concluding with how small cracks can affect the crack-growth damage tolerance design philosophy. As stated in this paper the "small" crack effect is defined in fracture mechanics where it has been observed that cracks on the order of 300 microns or less in length will propagate at higher growth rates than long cracks and also will grow at AK values below the long crack AK threshold. The small-crack effect is illustrated herein as resulting from a lack of crack closure and is explained based on continuum mechanics principles using crack-closure concepts in fracture mechanics.

  2. Resolved shear stress intensity coefficient and fatigue crack growth in large crystals

    NASA Technical Reports Server (NTRS)

    Chen, QI; Liu, Hao-Wen

    1988-01-01

    Fatigue crack growth in large grain Al alloy was studied. Fatigue crack growth is caused primarily by shear decohesion due to dislocation motion in the crack tip region. The crack paths in the large crystals are very irregular and zigzag. The crack planes are often inclined to the loading axis both in the inplane direction and the thickness direction. The stress intensity factors of such inclined cracks are approximated from the two dimensional finite element calculations. The plastic deformation in a large crystal is highly anisotropic, and dislocation motion in such crystals are driven by the resolved shear stress. The resolved shear stress intensity coefficient in a crack solid, RSSIC, is defined, and the coefficients for the slip systems at a crack tip are evaluated from the calculated stress intensity factors. The orientations of the crack planes are closely related to the slip planes with the high RSSIC values. If a single slip system has a much higher RSSIC than all the others, the crack will follow the slip plane, and the slip plane becomes the crack plane. If two or more slip systems have a high RSSIC, the crack plane is the result of the decohesion processes on these active slip planes.

  3. The Significance of Small Cracks in Fatigue Design Concepts as Related to Rotorcraft Metallic Dynamic Components

    NASA Technical Reports Server (NTRS)

    Everett, R. A., Jr.; Elber, W.

    2000-01-01

    In this paper the significance of the "small" crack effect as defined in fracture mechanics will be discussed as it relates to life managing rotorcraft dynamic components using the conventional safe-life, the flaw tolerant safe-life, and the damage tolerance design philosophies. These topics will be introduced starting with an explanation of the small-crack theory, then showing how small-crack theory has been used to predict the total fatigue life of fatigue laboratory test coupons with and without flaws, and concluding with how small cracks can affect the crack-growth damage tolerance design philosophy. As stated in this paper the "small" crack effect is defined in fracture mechanics where it has been observed that cracks on the order of 300 microns or less in length will propagate at higher growth rates than long cracks and also will grow at AK values below the long crack AK threshold. The small-crack effect is illustrated herein as resulting from a lack of crack closure and is explained based on continuum mechanics principles using crack-closure concepts in fracture mechanics.

  4. Fatigue of polycrystalline silicon for MEMS applications: Crack growth and stability under resonant loading conditions

    SciTech Connect

    Muhlstein, C.L.; Howe, R.T.; Ritchie, R.O.

    2001-12-05

    Although bulk silicon is not known to exhibit susceptibility to cyclic fatigue, micron-scale structures made from silicon films are known to be vulnerable to degradation by fatigue in ambient air environments, a phenomenon that has been recently modeled in terms of a mechanism of sequential oxidation and stress-corrosion cracking of the native oxide layer.

  5. 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.

  6. Fatigue-life prediction methodology using a crack-closure model

    SciTech Connect

    Newman, J.C. Jr.

    1995-10-01

    This paper reviews the capabilities of a plasticity-induced crack-closure model and life-prediction code, FASTRAN, to predict fatigue lives of metallic materials using small-crack theory. 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{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 to predict total fatigue lives, for notched specimens made of several aluminum alloys and a titanium alloy 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. An equivalent-initial-flaw-size concept was used to bound the fatigue lives for the titanium alloy. Results from the tests and analyses agreed well.

  7. Detection of Fatigue Cracks at Rivets with Self-Nulling Probe

    NASA Technical Reports Server (NTRS)

    Wincheski, Buzz; Fulton, Jim; Nath, Shridhar; Namkung, Min

    1994-01-01

    A new eddy current probe developed at NASA Langley Research Center has been used to detect small cracks at rivets in aircraft lap splices [1]. The device has earlier been used to detect isolated fatigue cracks with a minimum detectable flaw size of roughly 1/2 to 1/3 the diameter of the probe [2]. The present work shows that the detectable flaw size for cracks originating at rivets can be greatly improved upon from that of isolated flaws. The use of a rotating probe method combined with spatial filtering has been used to detect 0.18 cm EDM notches, as measured from the rivet shank, with a 1.27 cm diameter probe and to detect flaws buried under the rivet head, down to a length of 0.076 cm, using a 0.32 cm diameter probe. The Self-Nulling Electromagnetic Flaw Detector induces a high density eddy current ring in the sample under test. A ferromagnetic flux focusing lens is incorporated such that in the absence of any inhomogeneities in the material under test only a minimal magnetic field will reach the interior of the probe. A magnetometer (pickup coil) located in the center of the probe therefore registers a null voltage in the absence of material defects. When a fatigue crack or other discontinuity is present in the test article the path of the eddy currents in the material is changed. The magnetic field associated with these eddy currents then enter into the interior of the probe, producing a large output voltage across the pickup coil leads. Further

  8. Sensing sheets based on large area electronics for fatigue crack detection

    NASA Astrophysics Data System (ADS)

    Yao, Yao; Glisic, Branko

    2015-03-01

    Reliable early-stage damage detection requires continuous structural health monitoring (SHM) over large areas of structure, and with high spatial resolution of sensors. This paper presents the development stage of prototype strain sensing sheets based on Large Area Electronics (LAE), in which thin-film strain gauges and control circuits are integrated on the flexible electronics and deposited on a polyimide sheet that can cover large areas. These sensing sheets were applied for fatigue crack detection on small-scale steel plates. Two types of sensing-sheet interconnects were designed and manufactured, and dense arrays of strain gauge sensors were assembled onto the interconnects. In total, four (two for each design type) strain sensing sheets were created and tested, which were sensitive to strain at virtually every point over the whole sensing sheet area. The sensing sheets were bonded to small-scale steel plates, which had a notch on the boundary so that fatigue cracks could be generated under cyclic loading. The fatigue tests were carried out at the Carleton Laboratory of Columbia University, and the steel plates were attached through a fixture to the loading machine that applied cyclic fatigue load. Fatigue cracks then occurred and propagated across the steel plates, leading to the failure of these test samples. The strain sensor that was close to the notch successfully detected the initialization of fatigue crack and localized the damage on the plate. The strain sensor that was away from the crack successfully detected the propagation of fatigue crack based on the time history of measured strain. Overall, the results of the fatigue tests validated general principles of the strain sensing sheets for crack detection.

  9. Cyclic-fatigue crack initiation and propagation in smooth alumina specimens

    SciTech Connect

    Kishimoto, Hidehiro; Ueno, Akira; Matsunaga, Atsushi; Kondo, Takuya

    1998-01-01

    Crack-initiation sites and crack-propagation rates of small cracks in smooth specimens of alumina with two grain sizes have been studied. The principal results that have been obtained are as follows: (1) in most cases, the crack-initiation life comprises a large portion of the fatigue life, (2) small cracks that are initiated in smooth specimens propagate with stress intensity factors that are much lower than the apparent threshold stress intensity factors (K{sub Imax}) of artificial cracks that are 200 {micro}m in length, and (3) the critical crack lengths beyond which the crack-propagation rate is described uniquely by K{sub Imax} are 120 {micro}m for fine-grained alumina and 250 {micro}m for coarse-grained alumina.

  10. A Study of the Fatigue Behavior of Small Cracks in Nickel-Base Superalloys

    DTIC Science & Technology

    1988-02-24

    later successfully used in a study of stress corrosion crack initiation in a nickel-base superalloy tested under monotonic loading in a PWR environment...growth ratcs were measured over crack lengths ranging from AI lOom to lmni. Mos’ of the testing was performed in load control. with stress ranges...elevated temperatures in the elastic-plastic regime where the maximum cyclic stress reaches the macroscopki yield stress , the fatigue crack growth rates

  11. Fatigue crack growth rate of Ti-6Al-4V considering the effects of fracture toughness and crack closure

    NASA Astrophysics Data System (ADS)

    Zhang, Junhong; Yang, Shuo; Lin, Jiewei

    2015-03-01

    Fatigue fracture is one of the main failure modes of Ti-6Al-4V alloy, fracture toughness and crack closure have strong effects on the fatigue crack growth(FCG) rate of Ti-6Al-4V alloy. The FCG rate of Ti-6Al-4V is investigated by using experimental and analytical methods. The effects of stress ratio, crack closure and fracture toughness on the FCG rate are studied and discussed. A modified prediction model of the FCG rate is proposed, and the relationship between the fracture toughness and the stress intensity factor(SIF) range is redefined by introducing a correcting coefficient. Notched plate fatigue tests (including the fracture toughness test and the FCG rate test) are conducted to investigate the influence of affecting factors on the FCG rate. Comparisons between the predicted results of the proposed model, the Paris model, the Walker model, the Sadananda model, and the experimental data show that the proposed model gives the best agreement with the test data particularly in the near-threshold region and the Paris region, and the corresponding calculated fatigue life is also accurate in the same regions. By considering the effects of fracture toughness and crack closure, the novel FCG rate prediction model not only improves the estimating accuracy, but also extends the adaptability of the FCG rate prediction model in engineering.

  12. Experimental study of crack initiation and propagation in high- and gigacycle fatigue in titanium alloys

    SciTech Connect

    Bannikov, Mikhail E-mail: oborin@icmm.ru Oborin, Vladimir E-mail: oborin@icmm.ru Naimark, Oleg E-mail: oborin@icmm.ru

    2014-11-14

    Fatigue (high- and gigacycle) crack initiation and its propagation in titanium alloys with coarse and fine grain structure are studied by fractography analysis of fracture surface. Fractured specimens were analyzed by interferometer microscope and SEM to improve methods of monitoring of damage accumulation during fatigue test and to verify the models for fatigue crack kinetics. Fatigue strength was estimated for high cycle fatigue regime using the Luong method [1] by “in-situ” infrared scanning of the sample surface for the step-wise loading history for different grain size metals. Fine grain alloys demonstrated higher fatigue resistance for both high cycle fatigue and gigacycle fatigue regimes. Fracture surface analysis for plane and cylindrical samples was carried out using optical and electronic microscopy method. High resolution profilometry (interferometer-profiler New View 5010) data of fracture surface roughness allowed us to estimate scale invariance (the Hurst exponent) and to establish the existence of two characteristic areas of damage localization (different values of the Hurst exponent). Area 1 with diameter ∼300 μm has the pronounced roughness and is associated with damage localization hotspot. Area 2 shows less amplitude roughness, occupies the rest fracture surface and considered as the trace of the fatigue crack path corresponding to the Paris kinetics.

  13. Environmental effects on crack healing and static fatigue behavior of glass and polycrystalline ceramics

    NASA Astrophysics Data System (ADS)

    Wilson, Brett Allen

    An Environmental Scanning Electron Microscope (ESEM) was used for in-situ studies of static fatigue growth of Vickers indent cracks in polycrystalline alumina. Clarks advanced via static fatigue along a tortuous path with grain bridging and crack deflections on the order of a single grain diameter. In-situ ESEM studies of healing of Vickers indent cracks in soda-lime silicate glass found healing to occur at 8% r.h. and at 400sp°C. Increased levels of initial humidity decreased the temperature at which healing initiated. The crack morphology included slow crack regression from the crack tip towards the indent impression and multiple crack pinch-off (above 550sp°C). Debris in a crack hindered complete healing. Conventional heating was used in healing of soda-lime silica glass where Vickers crack lengths were measured via optical microscopy. Microscopy showed crack tip blunting, pinch-off, and sub-surface spheroidization. The more complete healing observed for cracks aged in 45% r.h. than in 0% r.h. was attributed to water vapor entering the glass structure and decreasing the local glass transition temperature and viscosity along the crack faces. Crack healing experiments on polycrystalline alumina were performed using both conventional and microwave heating, No effect of aging environment was found for conventional healing of alumina. The relative crack healing ((2csbinitial-2csbheal) /2csbinitial) behavior for 49 and 98 N indent cracks was nearly identical. Crack healing was modeled by a diffusive transport model originally developed by Dutton and Stevens showing significantly lower activation energies for conventional compared to microwave heating. Microwave heating at 10sp°C/minute had much greater healing than at 75sp°C/minute.

  14. ECCI observations of dislocation structures around fatigue cracks in ferritic stainless steel single crystals

    NASA Astrophysics Data System (ADS)

    Taniguchi, T.; Kaneko, Y.; Hashimoto, S.

    2009-07-01

    Dislocation structures around the crack tips of ferritic stainless steel single crystals were observed with electron channelling contrast imaging (ECCI) method. The ECCI method enables us to observe dislocations lying near surface using a scanning electron microscope. Fatigue crack growth tests were conducted on compact tension (CT) specimens having loading axes of [221] and [110] directions. In the specimen having the [110] loading axis at which the fatigue crack having Mode I and II component propagated, a thin band-like structure consisting of dislocation wall array was observed ahead of the crack tip. On the other hand, the dislocation structures around the crack having Mode I and III components could be divided into three regions in the specimen with the [221] loading axis: the cell structure, the dislocation wall structure and the vein structure were observed in order of ascending distance from crack tip. Difference between the dislocation structures near the fatigue cracks could be understood from the crack mode by which edge and screw dislocation emissions from the crack tips are strongly affected.

  15. Mixed-mode static and fatigue crack growth in central notched and compact tension shear specimens

    SciTech Connect

    Shlyannikov, V.N.

    1999-07-01

    Elastic-plastic crack growth under mixed Mode I and 2 in six types of aluminum alloys and three types of steel were investigated. The experimental study of fatigue crack growth in six types of the aluminum alloys and one type of the steel is performed on biaxially loaded eight-petal specimens (EPS). All specimens for biaxial loading contained inclined through thickness central cracks. Mixed Mode I/2 static and fatigue crack growth experiments on the three types of steels and one type of the aluminum alloy used compact tension shear (CTS) specimens. Two approaches are developed for geometrical modeling of crack growth trajectories for the central notched and compact tension shear specimens respectively. The principal feature of such modeling is the determination of crack growth direction and the definition of crack length increment in this direction. On the basis of the analysis of the experimental data for the aluminum alloys and the steels an empirical crack reorientation criterion is suggested for both brittle and ductile materials. The damage process zone size concept is used for calculations and mixed-mode crack path. The influence of specimen geometry, biaxial loading and properties of the aluminum alloys and the steels on both crack growth direction and crack path at the macroscopic scale is discussed.

  16. Effects of inert gases on fatigue crack growth and their transportation into subsurface regions in titanium

    SciTech Connect

    Shimojo, M.; Higo, Y.; Oya-Seimiya, Y.

    2000-05-01

    To clarify the effects of inert gases on the fatigue behavior of titanium, fatigue crack growth tests were carried out in pure inert gases and in vacuum. Fatigue crack growth rates increased, and the fracture surface appearance was changed in inert gases, as compared to those in vacuum. The transportation of inert gases into subsurface regions of fracture surfaces was confirmed using Auger electron spectroscopy. This transportation is considered to be due to the reverse slip of slip planes on which inert gas atoms have adsorbed.

  17. 7075-T6 and 2024-T351 Aluminum Alloy Fatigue Crack Growth Rate Data

    NASA Technical Reports Server (NTRS)

    Forth, Scott C.; Wright, Christopher W.; Johnston, William M., Jr.

    2005-01-01

    Experimental test procedures for the development of fatigue crack growth rate data has been standardized by the American Society for Testing and Materials. Over the past 30 years several gradual changes have been made to the standard without rigorous assessment of the affect these changes have on the precision or variability of the data generated. Therefore, the ASTM committee on fatigue crack growth has initiated an international round robin test program to assess the precision and variability of test results generated using the standard E647-00. Crack growth rate data presented in this report, in support of the ASTM roundrobin, shows excellent precision and repeatability.

  18. Some observations on loss of static strength due to fatigue cracks

    NASA Technical Reports Server (NTRS)

    Illg, Walter; Hardrath, Herbert F

    1955-01-01

    Static tensile tests were performed on simple notched specimens containing fatigue cracks. Four types of aluminum alloys were investigated: 2024-T3(formerly 24S-T3) and 7075-T6(formerly 75S-T6) in sheet form, and 2024-T4(formerly 24S-T4) and 7075-T6(formerly 75S-T6) in extruded form. The cracked specimens were tested statically under four conditions: unmodified and with reduced eccentricity of loading by three methods. Results of static tests on C-46 wings containing fatigue cracks are also reported.

  19. The influence of temperature on fatigue-crack growth in a mill annealed Ti-6Al-4V alloy

    NASA Technical Reports Server (NTRS)

    Wei, R. P.; Ritter, D. L.

    1972-01-01

    To understand the influence temperature on the rate of fatigue crack growth in high strength metal alloys, constant load amplitude, fatigue crack growth experiments were carried out using a 1/4-inch-thick (6.35 mm) mill annealed Ti-6Al-4V alloy plate as a model material. The rates of fatigue crack growth were determined as a function of temperature, ranging from room temperature to about 290 C (or, about 550 F/563K) and as a function of the crack tip stress intensity factor in a dehumidified high purity argon environment. Limited correlative experiments indicate that dehumidified oxygen and hydrogen have no effect on the rate of fatigue crack growth in this alloy, while distilled water increased the rate of crack growth slightly in the range tested. Companion fractographic examinations suggest that the mechanisms for fatigue crack growth in the various environments are essentially the same.

  20. A comparison of fatigue crack growth in human enamel and hydroxyapatite.

    PubMed

    Bajaj, Devendra; Nazari, Ahmad; Eidelman, Naomi; Arola, Dwayne D

    2008-12-01

    Cracks and craze lines are often observed in the enamel of human teeth, but they rarely cause tooth fracture. The present study evaluates fatigue crack growth in human enamel, and compares that to the fatigue response of sintered hydroxyapatite (HAp) with similar crystallinity, chemistry and density. Miniature inset compact tension (CT) specimens were prepared that embodied a small piece of enamel (N=8) or HAp (N=6). The specimens were subjected to mode I cyclic loads and the steady state crack growth responses were modeled using the Paris Law. Results showed that the fatigue crack growth exponent (m) for enamel (m=7.7+/-1.0) was similar to that for HAp (m=7.9+/-1.4), whereas the crack growth coefficient (C) for enamel (C=8.7 E-04 (mm/cycle)x(MPa m(0.5))(-m)) was significantly lower (p<0.0001) than that for HAp (C=2.0 E+00 (mm/cycle)x(MPa m(0.5))(-m)). Micrographs of the fracture surfaces showed that crack growth in the enamel occurred primarily along the prism boundaries. In regions of decussation, the microstructure promoted microcracking, crack bridging, crack deflection and crack bifurcation. Working in concert, these mechanisms increased the crack growth resistance and resulted in a sensitivity to crack growth (m) similar to bone and lower than that of human dentin. These mechanisms of toughening were not observed in the crack growth response of the sintered HAp. While enamel is the most highly mineralized tissue of the human body, the microstructural arrangement of the prisms promotes exceptional resistance to crack growth.

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

    SciTech Connect

    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 conventional 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.

  2. A Comparison of Fatigue Crack Growth in Human Enamel and Hydroxyapatite

    PubMed Central

    Bajaj, Devendra; Nazari, Ahmad; Eidelman, Naomi; Arola, Dwayne

    2008-01-01

    Cracks and craze lines are often observed in the enamel of human teeth, but they rarely cause tooth fracture. The present study evaluates fatigue crack growth in human enamel, and compares that to the fatigue response of sintered hydroxyapatite (HAp) with similar crystallinity, chemistry and density. Miniature inset compact tension (CT) specimens were prepared that embodied a small piece of enamel (N=8) or HAp (N=6). The specimens were subjected to mode I cyclic loads and the steady state crack growth responses were modeled using the Paris Law. Results showed that the fatigue crack growth exponent (m) for enamel (m = 7.7±1.0) was similar to that for HAp (m = 7.9±1.4), whereas the crack growth coefficient (C) for enamel (C=8.7E-04 (mm/cycle)·(MPa·m0.5)-m) was significantly lower (p<0.0001) than that for HAp (C = 2.0E+00 (mm/cycle)·(MPa·m0.5)-m). Micrographs of the fracture surfaces showed that crack growth in the enamel occurred primarily along the prism boundaries. In regions of decussation, the microstructure promoted microcracking, crack bridging, crack deflection and crack bifurcation. Working in concert, these mechanisms increased the crack growth resistance and resulted in a sensitivity to crack growth (m) similar to bone and lower than that of human dentin. These mechanisms of toughening were not observed in the crack growth response of the sintered HAp. While enamel is the most highly mineralized tissue of the human body, the microstructural arrangement of the prisms promotes exceptional resistance to crack growth. PMID:18804277

  3. Quality Factor and Microslipping of Fatigue Cracks in Thin Plates at Resonant Vibration

    NASA Technical Reports Server (NTRS)

    Wincheski, B.; Namkung, M.; Fulton, J. P.

    1993-01-01

    Resonant vibrations have been stimulated in thin metal plates using a non-contacting electromagnetic driver. A sinusoidal force was applied in a swept frequency fashion and the resulting surface displacements were monitored through the use of an acoustic microphone. It has been found that the presence of a fatigue crack in the sample causes a broadening of the second resonance peak. The Q factors of the resonance curves were determined and are directly correlated with the presence of fatigue cracks in the samples. The broadening of the curves is explained in terms of a microslipping at the crack face walls which reduces the amplitude of the resonant vibration by increasing the damping of the system. A comparison is made between the resonance characteristics of fatigue damaged and notched samples, where the stiffness of the two systems is nearly constant while the interaction between crack face walls is eliminated in the latter.

  4. Effect of band-overload on fatigue crack growth rate of HSLA steel

    NASA Astrophysics Data System (ADS)

    Abhinay, S. V.; Tenduwe, Om Prakash; Kumar, Ajit; Dutta, K.; Verma, B. B.; Ray, P. K.

    2015-02-01

    Fatigue crack growth behavior is important parameter of structural materials. This parameters can be used to predict their life, service reliability and operational safety in different conditions. The material used in this investigation is an HSLA steel. In this investigation effect of single overload and band-overload on fatigue crack growth of same steel are studied using compact tension (CT) specimens under mode-I condition and R=0.3. It is observed that overload and band-overload applications resulted retardation on the fatigue crack growth rate in most of the cases. It is also noticed that maximum retardation took place on application of seven successive overload cycles. Application of ten and more overload cycles caused no crack growth retardation.

  5. Role of hydrogen embrittlement in crack propagation in corrosion fatigue of steels

    SciTech Connect

    Marichev, V.A.; Shipilov, S.A.

    1987-09-01

    In earlier reports of the corrosion fatigue of steels higher crack growth rates were observed with long cathodic polarization and the hydrogen mechanism of corrosion fatigue was concluded. Not taken into consideration, however, was the possibility of a change in the electrochemical conditions in the crack to which long polarization leads. In this paper the influence of cathodic polarization on crack growth in corrosion fatigue of SP-33sh steel was investigated in an 0.5 NaCl solution. It was found that cathodic polarization increases the rate of electrochemical liberation of hydrogen and that conditions of cyclic loading provide the formation of a fresh surface of metal at the tip of the crack with the resulting increase in the ease of hydrogen penetration into the metal.

  6. Oxidation effects on the fatigue crack growth behavior of alloy 718 at high temperature

    SciTech Connect

    Molins, R.; Hochstetter, G.; Chassaigne, J.C.; Andrieu, E.

    1997-02-01

    The purpose of this study was to investigate oxidation assisted crack growth phenomena encountered in nickel-based alloys at high temperatures. Fatigue crack growth tests conducted at 650 C and under a range of oxygen partial pressures revealed the existence of a transition pressure. This pressure is in no way correlated to the loading conditions, but rather it varies with the chromium content in the alloy, and is furthermore directly linked to the oxidation mechanisms which were identified by using analytical TEM. By means of specific mechanical tests, superimposing a square wave oxygen pressure cycle to a fatigue or creep-fatigue mechanical cycle, various fundamental aspects of the local interaction between oxidation and deformation at the crack tip were investigated. Embrittlement is due partly to the nickel oxide nucleation and partly to the stress relaxation ability of the material. Chemical and microstructural modifications are recommended in order to improve the cracking resistance.

  7. Inhibition of environmental fatigue crack propagation in age-hardenable aluminum alloys

    NASA Astrophysics Data System (ADS)

    Warner, Jenifer S.

    Age-hardenable aluminum alloys, such as C47A-T86 (Al-Cu-Li) and 7075-T651 (Al-Zn-Mg-Cu), used in aerospace structures are susceptible to environment assisted fatigue crack propagation (EFCP) by hydrogen environment embrittlement. This research demonstrates effective inhibition of EFCP in C47A-T86 and 7075-T651 under both full immersion in aqueous chloride solution and atmospheric exposure which more accurately describes aircraft service conditions. Inhibition is attributed to the presence of a crack tip passive film reducing H production and uptake, as explained by the film rupture-hydrogen embrittlement mechanism, and can be accomplished through both addition of a passivating ion (ion-assisted inhibition) and localized-alloy corrosion creating passivating conditions (self inhibition). Addition of molybdate to both bulk chloride solution and surface chloride droplets eliminates the effect of environment on fatigue crack propagation in C47A-T86 and 7075-1651 at sufficiently low loading frequencies and high stress ratio by yielding crack growth rates equivalent to those for fatigue in ultra high vacuum. The preeminent corrosion inhibitor, chromate, has not been reported to produce such complete inhibition. Inhibition is promoted by reduced loading frequency, increased crack tip molybdate concentration, and potential at or anodic to free corrosion; each of which favors passivity. The inhibiting effect of molybdate parallels chromate, establishing molybdate as a viable chromate replacement inhibitor. The ability of molybdate to inhibit EFCP is enhanced by atmospheric exposures producing surface electrolyte droplets; crack growth rates are reduced by an order of magnitude under loading frequencies as high as 30 Hz, a frequency at which inhibition was not possible under full immersion. Al-Cu-Mg/Li alloys, including 2024-T351, are capable of self inhibition of EFCP. This behavior is attributed to localized corrosion through dealloying of anodic Al2CuMg or Al2Cu

  8. The effects of pitting on fatigue crack nucleation in 7075-T6 aluminum alloy

    NASA Technical Reports Server (NTRS)

    Ma, LI; Hoeppner, David W.

    1994-01-01

    A high-strength aluminum alloy, 7075-T6, was studied to quantitatively evaluate chemical pitting effects of its corrosion fatigue life. The study focused on pit nucleation, pit growth, and fatigue crack nucleation. Pitting corrosion fatigue experiments were conducted in 3.5 percent NaCl aqueous solution under constant amplitude sinusoidal loading at two frequencies, 5 and 20 Hz. Smooth and unnotched specimens were used in this investigation. A video recording system was developed to allow in situ observation of the surface changes of the specimens during testing. The results indicated that pitting corrosion considerably reduces the fatigue strength by accelerating fatigue crack nucleation. A metallographic examination was conducted on the specimens to evaluate the nature of corrosion pits. First, the actual shapes of the corrosion pits were evaluated by cross-sectioning the pits. Secondly, the relation between corrosion pits and microstructure was also investigated. Finally, the possibility of another corrosion mechanism that might be involved in pitting was explored in this investigation. The fractography of the tested specimens showed that corner corrosion pits were responsible for fatigue crack nucleation in the material due to the associated stress concentration. The pits exhibited variance of morphology. Fatigue life for the experimental conditions appeared to be strongly dependent on pitting kinetics and the crack nucleation stage.

  9. 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.

  10. Intrinsic fatigue crack growth rates for Al-Li-Cu-Mg alloys in vacuum

    NASA Technical Reports Server (NTRS)

    Slavik, D. C.; Blankenship, C. P., Jr.; Starke, E. A., Jr.; Gangloff, R. P.

    1993-01-01

    The influences of microstructure and deformation mode on inert environment intrinsic fatigue crack propagation were investigated for Al-Li-Cu-Mg alloys AA2090, AA8090, and X2095 compared to AA2024. The amount of coherent shearable delta-prime (Al3Li) precipitates and extent of localized planar slip deformation were reduced by composition (increased Cu/Li in X2095) and heat treatment (double aging of AA8090). Intrinsic growth rates, obtained at high constant K(max) to minimize crack closure and in vacuum to eliminate any environmental effect, were alloy dependent; da/dN varied up to tenfold based on applied Delta-K or Delta-K/E. When compared based on a crack tip cyclic strain or opening displacement parameter, growth rates were equivalent for all alloys except X2095-T8, which exhibited unique fatigue crack growth resistance. Tortuous fatigue crack profiles and large fracture surface facets were observed for each Al-Li alloy independent of the precipitates present, particularly delta-prime, and the localized slip deformation structure. Reduced fatigue crack propagation rates for X2095 in vacuum are not explained by either residual crack closure or slip reversibility arguments; the origin of apparent slip band facets in a homogeneous slip alloy is unclear.

  11. 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.

  12. Intrinsic fatigue crack growth rates for Al-Li-Cu-Mg alloys in vacuum

    NASA Technical Reports Server (NTRS)

    Slavik, D. C.; Blankenship, C. P., Jr.; Starke, E. A., Jr.; Gangloff, R. P.

    1993-01-01

    The influences of microstructure and deformation mode on inert environment intrinsic fatigue crack propagation were investigated for Al-Li-Cu-Mg alloys AA2090, AA8090, and X2095 compared to AA2024. The amount of coherent shearable delta-prime (Al3Li) precipitates and extent of localized planar slip deformation were reduced by composition (increased Cu/Li in X2095) and heat treatment (double aging of AA8090). Intrinsic growth rates, obtained at high constant K(max) to minimize crack closure and in vacuum to eliminate any environmental effect, were alloy dependent; da/dN varied up to tenfold based on applied Delta-K or Delta-K/E. When compared based on a crack tip cyclic strain or opening displacement parameter, growth rates were equivalent for all alloys except X2095-T8, which exhibited unique fatigue crack growth resistance. Tortuous fatigue crack profiles and large fracture surface facets were observed for each Al-Li alloy independent of the precipitates present, particularly delta-prime, and the localized slip deformation structure. Reduced fatigue crack propagation rates for X2095 in vacuum are not explained by either residual crack closure or slip reversibility arguments; the origin of apparent slip band facets in a homogeneous slip alloy is unclear.

  13. Crack Nucleation in β Titanium Alloys under High Cycle Fatigue Conditions - A Review

    NASA Astrophysics Data System (ADS)

    Benjamin, Rohit; Nageswara Rao, M.

    2017-05-01

    Beta titanium (β-Ti) alloys have emerged over the last 3 to 4 decades as an important class of titanium alloys. Many of the applications that they found, particularly in aerospace sector, are such that their high cycle fatigue (HCF) behavior becomes critical. In HCF regime, crack nucleation accounts for major part of the life. Consequently it becomes important to understand the mechanisms underlying the nucleation of cracks under HCF type loading conditions. The purpose of this review is to document the best understanding we have on date on crack nucleation in β-Ti alloys under HCF conditions. Role of various microstructural features encountered in β-Ti alloys in influencing the crack nucleation under HCF conditions has been reviewed. It has been brought out that changes in processing can result in changes in microstructure which in turn influence the time for crack nucleation/fatigue life and fatigue limit. While majority of fatigue failures originate at the surface, subsurface cracking is not uncommon with β-Ti alloys and the factors leading to subsurface cracking have been discussed in this review.

  14. Microstructurally-sensitive fatigue crack nucleation in Ni-based single and oligo crystals

    NASA Astrophysics Data System (ADS)

    Chen, Bo; Jiang, Jun; Dunne, Fionn P. E.

    2017-09-01

    An integrated experimental, characterisation and computational crystal plasticity study of cyclic plastic beam loading has been carried out for nickel single crystal (CMSX4) and oligocrystal (MAR002) alloys in order to assess quantitatively the mechanistic drivers for fatigue crack nucleation. The experimentally validated modelling provides knowledge of key microstructural quantities (accumulated slip, stress and GND density) at experimentally observed fatigue crack nucleation sites and it is shown that while each of these quantities is potentially important in crack nucleation, none of them in its own right is sufficient to be predictive. However, the local (elastic) stored energy density, measured over a length scale determined by the density of SSDs and GNDs, has been shown to predict crack nucleation sites in the single and oligocrystals tests. In addition, once primary nucleated cracks develop and are represented in the crystal model using XFEM, the stored energy correctly identifies where secondary fatigue cracks are observed to nucleate in experiments. This (Griffith-Stroh type) quantity also correctly differentiates and explains intergranular and transgranular fatigue crack nucleation.

  15. 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.

  16. 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.

  17. Fatigue behavior of Long and Short Cracks in Aluminum Alloys.

    DTIC Science & Technology

    1987-05-01

    deposits ,37 40 irregular fracture morphologies coupled with crack tip shear displacements, 4 1 4 3 and fluid-induced pressure44 in addition to...Associated Auger measurements of the extent of crack surface corrosion deposits are shown in Fig. 4.6. In marked contrast to behavior in lower...6 8 10 12 CRACK LENGTH MEASURED FROM NOTCH (mm) Fig. 4.6: Scanning Auger spectroscopic measurements of excess crack surface oxide deposits as a

  18. Fatigue crack growth properties of a cryogenic structural steel at liquid helium temperature

    SciTech Connect

    Konosu, Shinji; Kishiro, Tomohiro; Ivano, O.; Nunoya, Yoshihiko; Nakajima, Hideo; Tsuji, Hiroshi

    1996-01-01

    The structural materials of the coils of superconducting magnets utilized in thermonuclear fusion reactors are used at liquid helium (4.2 K) temperatures and are subjected to repeated thermal stresses and electromagnetic forces. A high strength, high toughness austenitic stainless steel (12Cr-12Ni-10Mn-5Mo-0.2N) has recently been developed for large, thick-walled components used in such environments. This material is non-magnetic even when subjected to processing and, because it is a forging material, it is advantageous as a structural material for large components. In the current research, a large forging of 12Cr-12Ni-10Mn-5Mo-0.2N austenitic stainless steel, was fabricated to a thickness of 250 mm, which is typical of section thicknesses encountered in actual equipment. The tensile fatigue crack growth properties of the forging were examined at liquid helium temperature as a function of specimen location across the thickness of the forging. There was virtually no evidence of variation in tensile strength or fatigue crack growth properties attributable to different sampling locations in the thickness direction and no effect of thickness due to the forging or solution treatment associated with large forgings was observed.

  19. Factors Influencing Dwell Fatigue Cracking in Notches of Powder Metallurgy Superalloys

    NASA Technical Reports Server (NTRS)

    Gabb, T. P.; Telesman, J.; Ghosn, L.; Garg, A.; Gayda, J.

    2011-01-01

    The influences of heat treatment and cyclic dwells on the notch fatigue resistance of powder metallurgy disk superalloys were investigated for low solvus high refractory (LSHR) and ME3 disk alloys. Disks were processed to produce material conditions with varied microstructures and associated mechanical properties. Notched specimens were first subjected to baseline dwell fatigue cycles having a dwell at maximum load, as well as tensile, stress relaxation, creep rupture, and dwell fatigue crack growth tests at 704 C. Several material heat treatments displayed a bimodal distribution of fatigue life with the lives varying by two orders-of-magnitude, while others had more consistent fatigue lives. This response was compared to other mechanical properties, in search of correlations. The wide scatter in baseline dwell fatigue life was observed only for material conditions resistant to stress relaxation. For selected materials and conditions, additional tests were then performed with the dwells shifted in part or in total to minimum tensile load. The tests performed with dwells at minimum load exhibited lower fatigue lives than max dwell tests, and also exhibited early crack initiation and a substantial increase in the number of initiation sites. These results could be explained in part by modeling evolution of peak stresses in the notch with continued dwell fatigue cycling. Fatigue-environment interactions were determined to limit life for the fatigue cycles with dwells.

  20. High-temperature cyclic fatigue-crack growth behavior in an in situ toughened silicon carbide

    SciTech Connect

    Chen, D.; Gilbert, C.J.; Zhang, X.F.; Ritchie, R.O.

    2000-02-09

    The growth of fatigue cracks at elevated temperatures (25--1,300 C) is examined under cyclic loading in an in situ toughened, monolithic silicon carbide with Al-B-C additions (termed ABC-SiC), with specific emphasis on the roles of temperature, load ratio, cyclic frequency, and loading mode (static vs cyclic). Extensive crack-growth data are presented, based on measurements form an electrical potential-drop crack-monitoring technique, adapted for use on ceramics at high temperatures. It was found that at equivalent stress-intensity levels, crack velocities under cyclic loads were significantly faster than those under static loads. Fatigue thresholds were found to decrease with increasing temperature up to 1,200 C; behavior at 1,300 C, however, was similar to that at 1,200 C. Moreover, no effect of frequency was detected (between 3 and 1,000 Hz), no evidence of creep cavitation or crack bridging by viscous ligaments of grain-boundary glassy phases in the crack wake. Indeed, fractography and crack-path sectioning revealed a fracture mode at 1,200--1,300 C that was essentially identical to that at room temperature, i.e., predominantly intergranular cracking with evidence of grain bridging in the crack wake. Such excellent crack-growth resistance is attributed to a process of grain-boundary microstructural evolution at elevated temperatures, specifically involving crystallization of the amorphous grain-boundary films/phases.

  1. Modeling of the variability of fatigue crack growth using cohesive zone elements

    PubMed Central

    Beaurepaire, P.; Schuëller, G.I.

    2011-01-01

    By its nature, metal fatigue has random characteristics, leading to extensive scatter in the results. Both initiation and propagation of a fatigue crack can be seen as random processes. This manuscript develops a numerical analysis using cohesive zone elements allowing the use of one single model in the finite element simulation of the complete fatigue life. The present formulation includes a damage evolution mechanism that reflects gradual degradation of the cohesive strength under cyclic loading. The uncertainties inherent to the fatigue process are assumed to be caused by the variability of the material properties, which are modeled using random fields. An extrapolation scheme is proposed to reduce the computational time. First, the accuracy of the proposed formulation is assessed considering a deterministic crack growth problem. Second, the effect of randomness in the material properties on the total fatigue life of a structure is then analyzed. PMID:22049246

  2. Fatigue failure in polysilicon not due to simple stress corrosion cracking.

    PubMed

    Kahn, H; Ballarini, R; Bellante, J J; Heuer, A H

    2002-11-08

    In the absence of a corrosive environment, brittle materials such as silicon should be immune to cyclic fatigue. However, fatigue effects are well known in micrometer-sized polycrystalline silicon (polysilicon) samples tested in air. To investigate the origins of this phenomenon in polysilicon, we developed a fixed-grip fracture mechanics microspecimen but could find no evidence of static stress corrosion cracking. The environmental sensitivity of the fatigue resistance was also investigated under cyclic loading. For low-cycle fatigue, the behavior is independent of the ambient conditions, whether air or vacuum, but is strongly influenced by the ratio of compressive to tensile stresses experienced during each cycle. The fatigue damage most likely originates from contact stresses at processing-related surface asperities; subcritical crack growth then ensues during further cyclic loading. The lower far-field stresses involved in high-cycle fatigue induce reduced levels of fatigue damage. Under these conditions, a corrosive ambient such as laboratory air exacerbates the fatigue process. Without cyclic loading, polysilicon does not undergo stress corrosion cracking.

  3. Evaluation of fatigue cracks using nonlinearities of acousto-ultrasonic waves acquired by an active sensor network

    NASA Astrophysics Data System (ADS)

    Zhou, Chao; Hong, Ming; Su, Zhongqing; Wang, Qiang; Cheng, Li

    2013-01-01

    There has been increasing interest in using the nonlinear features of acousto-ultrasonic (AU) waves to detect damage onset (e.g., micro-fatigue cracks) due to their high sensitivity to damage with small dimensions. However, most existing approaches are able to infer the existence of fatigue damage qualitatively, but fail to further ascertain its location and severity. A damage characterization approach, in conjunction with the use of an active piezoelectric sensor network, was established, capable of evaluating fatigue cracks in a quantitative manner (including the co-presence of multiple fatigue cracks, and their individual locations and severities). Fundamental investigations, using both experiment and enhanced finite element analysis dedicated to the simulation of nonlinear AU waves, were carried out to link the accumulation of nonlinearities extracted from high-order AU waves to the characteristic parameters of a fatigue crack. A probability-based diagnostic imaging algorithm was developed, facilitating an intuitive presentation of identification results in images. The approach was verified experimentally by evaluating multi-fatigue cracks near rivet holes of a fatigued aluminum plate, showing satisfactory precision in characterizing real, barely visible fatigue cracks. Compared with existing methods, this approach innovatively (i) uses permanently integrated active sensor networks, conducive to automatic and online health monitoring; (ii) characterizes fatigue cracks at a quantitative level; (iii) allows detection of multiple fatigue cracks; and (iv) visualizes identification results in intuitive images.

  4. Introducing the K{sub max} sensitivity concept for correlating fatigue crack growth data

    SciTech Connect

    Donald, J.K.; Bray, G.H.; Bush, R.W.

    1997-12-31

    Previous attempts to correlate stress ratio effects in fatigue crack growth rate data have often emphasized one important contribution, such as crack closure, without considering other mitigating factors such as K{sub max}. In the past, correlating stress ratio effects using closure have been hampered by non-repeatable measurements of closure and contradictory results. A new analysis technique for determining {Delta}KJ{sub eff} is based on an interpretation of crack closure as a stress redistribution (or load transfer) on a relatively compliant crack wake and is able to account for the contribution of cyclic crack-tip strain below the opening load. This results in the observation that the fatigue crack growth rate is not determined solely by {Delta}K{sub eff} but also depends on K{sub max}. It was observed that this K{sub max} dependence takes the form of a power-law with the magnitude of the exponent being a measure of K{sub max} sensitivity. A procedure for determining the intrinsic crack growth rate curve without the measurement or modeling of crack closure will be described. The implications for fatigue-life prediction are profound.

  5. 3D characterization of trans- and inter-lamellar fatigue crack in (α + β) Ti alloy

    SciTech Connect

    Babout, Laurent; Jopek, Łukasz; Preuss, Michael

    2014-12-15

    This paper presents a three dimensional image processing strategy that has been developed to quantitatively analyze and correlate the path of a fatigue crack with the lamellar microstructure found in Ti-6246. The analysis is carried out on X-ray microtomography images acquired in situ during uniaxial fatigue testing. The crack, the primary β-grain boundaries and the α lamellae have been segmented separately and merged for the first time to allow a better characterization and understanding of their mutual interaction. This has particularly emphasized the role of translamellar crack growth at a very high propagation angle with regard to the lamellar orientation, supporting the central role of colonies favorably oriented for basal 〈a〉 slip to guide the crack in the fully lamellar microstructure of Ti alloy. - Highlights: • 3D tomography images reveal strong short fatigue crack interaction with α lamellae. • Proposed 3D image processing methodology makes their segmentation possible. • Crack-lamellae orientation maps show prevalence of translamellar cracking. • Angle study comforts the influence of basal/prismatic slip on crack path.

  6. Assessment of Crack Path Prediction in Non-Proportional Mixed-Mode Fatigue

    NASA Technical Reports Server (NTRS)

    Highsmith, Shelby, Jr.; Johnson, Steve; Swanson, Gregory; Sayyah, Tarek; Pettit, Richard

    2008-01-01

    Non-proportional mixed-mode loading is present in many systems and a growing crack can experience any manner of mixed-mode loading. Prediction of the resulting crack path is important when assessing potential failure modes or when performing a failure investigation. Current crack path selection criteria are presented along with data for Inconel 718 under non-proportional mixed-mode loading. Mixed-mode crack growth can transition between path deflection mechanisms with very different orientations. Non-proportional fatigue loadings lack a single parameter for input to current crack path criteria. Crack growth transitions were observed in proportional and non-proportional FCG tests. Different paths displayed distinct fracture surface morphologies. New crack path drivers & transition criteria must be developed.

  7. Gear fatigue crack prognosis using embedded model, gear dynamic model and fracture mechanics

    NASA Astrophysics Data System (ADS)

    Li, C. James; Lee, Hyungdae

    2005-07-01

    This paper presents a model-based method that predicts remaining useful life of a gear with a fatigue crack. The method consists of an embedded model to identify gear meshing stiffness from measured gear torsional vibration, an inverse method to estimate crack size from the estimated meshing stiffness; a gear dynamic model to simulate gear meshing dynamics and determine the dynamic load on the cracked tooth; and a fast crack propagation model to forecast the remaining useful life based on the estimated crack size and dynamic load. The fast crack propagation model was established to avoid repeated calculations of FEM and facilitate field deployment of the proposed method. Experimental studies were conducted to validate and demonstrate the feasibility of the proposed method for prognosis of a cracked gear.

  8. Small fatigue crack behavior in 7075-T651 aluminum as monitored with Rayleigh wave reflection

    SciTech Connect

    Hirao, M.; Tojo, K.; Fukuoka, H. )

    1993-08-01

    Small fatigue crack growth in 7075-T651 aluminum is periodically monitored with an ultrasonic Rayleigh wave technique. The wideband reflection signals are digitized and stored in the computer memory to permit the signal processing in the frequency domain. With the help of the constructed three-dimensional (3-D) reflection coefficient, the reflection amplitude spectra yield the measurements of the crack depth, if larger than 0.062 mm, and the crack closure stress. Acoustically obtained da/dN-[Delta]K[sub eff] relations describe the anomalous growth behavior of small surface crack, extending from the late small crack regime to the large crack regime passing through the minimum growth rate. The split spectrum processing proves to be useful in removing the grain noises to illuminate the target echoes, which is necessary to detect the nucleation and characterize the cracks smaller than 0.1 mm in depth. Discussions on the computer-controlled, automated, in situ monitoring system are provided.

  9. Developing the Capability to Monitor Small Fatigue Crack Growth Under Elevated Temperature, Ultra-High Vacuum Conditions (Preprint)

    DTIC Science & Technology

    2011-07-01

    design new grips to allow testing of low cycle fatigue buttonhead specimens inside the chamber. The grips were designed to be identical to...cracks did not grow from the notches, both specimens demonstrated longer fatigue lifetimes then those specimens that have been tested under identical ...Superalloys 2008. pp. 565 - 572. 2. The Effect of Vacuum on Fatigue Crack Growth. Grinberg , N. M. s.l. : International Journal of Fatigue, 1982. 3

  10. Application of cyclic J-integral to low cycle fatigue crack growth of Japanese carbon steel pipe

    SciTech Connect

    Miura, N.; Fujioka, T.; Kashima, K.

    1997-04-01

    Piping for LWR power plants is required to satisfy the LBB concept for postulated (not actual) defects. With this in mind, research has so far been conducted on the fatigue crack growth under cyclic loading, and on the ductile crack growth under excessive loading. It is important, however, for the evaluation of the piping structural integrity under seismic loading condition, to understand the fracture behavior under dynamic and cyclic loading conditions, that accompanies large-scale yielding. CRIEPI together with Hitachi have started a collaborative research program on dynamic and/or cyclic fracture of Japanese carbon steel (STS410) pipes in 1991. Fundamental tensile property tests were conducted to examine the effect of strain rate on tensile properties. Cracked pipe fracture tests under some loading conditions were also performed to investigate the effect of dynamic and/or cyclic loading on fracture behavior. Based on the analytical considerations for the above tests, the method to evaluate the failure life for a cracked pipe under cyclic loading was developed and verified. Cyclic J-integral was introduced to predict cyclic crack growth up to failure. This report presents the results of tensile property tests, cracked pipe fracture tests, and failure life analysis. The proposed method was applied to the cracked pipe fracture tests. The effect of dynamic and/or cyclic loading on pipe fracture was also investigated.

  11. Analysis of the Influence of Laser Welding on Fatigue Crack Growth Behavior in a Newly Developed Nickel-Base Superalloy

    NASA Astrophysics Data System (ADS)

    Buckson, R. A.; Ojo, O. A.

    2015-01-01

    The influence of laser welding on fatigue crack growth (FCG) behavior of a newly developed nickel-base superalloy, Haynes 282 was studied. Laser welding resulted in cracking in the heat affected zone (HAZ) of the alloy during welding and FCG test results show that this produces deleterious effect on the fatigue crack growth behavior of Haynes 282. However, two post weld heat treatments, including a new thermal treatment schedule developed in this work, are used to significantly improve the resistance of the Haynes 282 fatigue crack growth after laser welding. The effects of laser welding and thermal treatments are discussed in terms of HAZ cracking and heterogeneity of slip, respectively.

  12. The effect of pre-stress cycles on fatigue crack growth - An analysis of crack growth mechanism. [in Al alloy plates

    NASA Technical Reports Server (NTRS)

    Kang, T. S.; Liu, H. W.

    1974-01-01

    Cyclic prestress increases subsequent fatigue crack growth rate in 2024-T351 aluminum alloy. This increase in growth rate, caused by the prestress, and the increased rate, caused by temper embrittlement as observed by Ritchie and Knott (1973), cannot be explained by the crack tip blunting model alone. Each fatigue crack increment consists of two components, a brittle and a ductile component. They are controlled by the ductility of the material and its cyclic yield strength, respectively.

  13. Crack growth behavior under creep-fatigue conditions using compact and double edge notch tension-compression specimens

    NASA Astrophysics Data System (ADS)

    Narasimha Chary, Santosh Balaji

    inspection of fatigue surfaces, it has been found that that better alignment control procedures are needed to ensure symmetric crack fronts for the DEN(T-C) specimen. Creep-fatigue crack growth tests were conducted on 9Cr-1Mo (P91) steels at 625°C with various hold times. These tests were conducted using C(T) specimens under constant load amplitude conditions (tension-tension) and DEN(T-C) specimens under displacement like conditions (tension-compression). Crack growth data generated under creep-fatigue conditions using standard C(T) specimens correlated well with crack growth data generated using DEN(T-C) specimens. The crack growth rates per cycle increased significantly with increase in hold time when crack growth data were plotted with the cyclic stress intensity parameter, Delta-K. A transient behavior in the initial portion of da/dN versus Delta-K plots were observed for the hold time tests, as reported previously by several other researchers. It is shown for the C(T) specimens that the creep-fatigue interactions during crack growth for various hold times are represented better by the (Ct)avg parameter implying that the P91 steel behaves in a creep-ductile manner. Significant differences (factors of 2 to 5) were observed between the calculated values of (Ct)avg and those based on measured values of force-line deflection. It is also shown that there is a high risk of obtaining invalid data in longer hold time tests under force-control conditions. The usefulness of DEN(T-C) specimens for crack growth studies under displacement controlled conditions to combat ratcheting problems in tests conducted under load conditions is established. The tests conditions for the round-robin program on creep-fatigue crack growth testing in support of ASTM E-2760 are finalized. Further developments needed in creep-fatigue crack growth testing are also presented.

  14. Near-Threshold Fatigue Crack Growth Behavior of Fine-Grain Nickel-Based Alloys

    NASA Technical Reports Server (NTRS)

    Newman, John A.; Piascik, Robert S.

    2003-01-01

    Constant-Kmax fatigue crack growth tests were performed on two finegrain nickel-base alloys Inconel 718 (DA) and Ren 95 to determine if these alloys exhibit near-threshold time-dependent crack growth behavior observed for fine-grain aluminum alloys in room-temperature laboratory air. Test results showed that increases in K(sub max) values resulted in increased crack growth rates, but no evidence of time-dependent crack growth was observed for either nickel-base alloy at room temperature.

  15. Hydrogen accelerated fatigue crack growth of friction stir welded X52 steel pipe

    DOE PAGES

    Ronevich, Joseph Allen; Somerday, Brian P.; Feng, Zhili

    2016-11-17

    Friction stir welded steel pipelines were tested in high pressure hydrogen gas to examine the effects of hydrogen accelerated fatigue crack growth. Fatigue crack growth rate (da/dN) vs. stress-intensity factor range (ΔK) relationships were measured for an X52 friction stir welded pipe tested in 21 MPa hydrogen gas at a frequency of 1 Hz and R = 0.5. Tests were performed on three regions: base metal (BM), center of friction stir weld (FSW), and 15 mm off-center of the weld. For all three material regions, tests in hydrogen exhibited accelerated fatigue crack growth rates that exceeded an order of magnitudemore » compared to companion tests in air. Among tests in hydrogen, fatigue crack growth rates were modestly higher in the FSW than the BM and 15 mm off-center tests. Select regions of the fracture surfaces associated with specified ΔK levels were examined which revealed intergranular fracture in the BM and 15 mm off-center specimens but an absence of intergranular features in the FSW specimens. In conclusion, the X52 friction stir weld and base metal tested in hydrogen exhibited fatigue crack growth rate relationships that are comparable to those for conventional arc welded steel pipeline of similar strength found in the literature.« less

  16. Hydrogen accelerated fatigue crack growth of friction stir welded X52 steel pipe

    SciTech Connect

    Ronevich, Joseph Allen; Somerday, Brian P.; Feng, Zhili

    2016-11-17

    Friction stir welded steel pipelines were tested in high pressure hydrogen gas to examine the effects of hydrogen accelerated fatigue crack growth. Fatigue crack growth rate (da/dN) vs. stress-intensity factor range (ΔK) relationships were measured for an X52 friction stir welded pipe tested in 21 MPa hydrogen gas at a frequency of 1 Hz and R = 0.5. Tests were performed on three regions: base metal (BM), center of friction stir weld (FSW), and 15 mm off-center of the weld. For all three material regions, tests in hydrogen exhibited accelerated fatigue crack growth rates that exceeded an order of magnitude compared to companion tests in air. Among tests in hydrogen, fatigue crack growth rates were modestly higher in the FSW than the BM and 15 mm off-center tests. Select regions of the fracture surfaces associated with specified ΔK levels were examined which revealed intergranular fracture in the BM and 15 mm off-center specimens but an absence of intergranular features in the FSW specimens. In conclusion, the X52 friction stir weld and base metal tested in hydrogen exhibited fatigue crack growth rate relationships that are comparable to those for conventional arc welded steel pipeline of similar strength found in the literature.

  17. The effect of moisture absorption on the fatigue crack propagation resistance of acrylic bone cement.

    PubMed

    Schmitt, S; Krzypow, D J; Rimnac, C M

    2004-03-01

    In vivo, bone cement is subject to cyclic loading in a fluid environment. However, little is known about the effect of moisture absorption on the fatigue crack propagation resistance of bone cement. The effect of moisture absorption at 37 degrees C on the fatigue crack propagation resistance of a common bone cement (Endurance, DePuy, Orthopaedics, Inc.) was examined. Preliminary fracture toughness tests were conducted on disk-shaped, vacuum-mixed cement specimens (compact tension type) that were cyclically pre-cracked. Plain-strain fracture toughness K(IC) (MPa square root(m)) was determined. To study the effect of moisture absorption four treatment groups, with different soaking periods in Ringer's at 37 degrees C, of Endurance cement were tested. The specimens weights prior to and following soaking showed a significant increase in mean weight for specimens soaked for 8 and 12 weeks. Linear regression analysis of log(da/dN) vs. log (deltaK) was conducted on the combined data in each fatigue test group. Soaking bone cement in Ringer's at 37 degrees C for 8 and 12 weeks lead to an improvement in fatigue crack propagation resistance, that may be related to water sorption that increases polymer chain mobility, with enhanced crack tip blunting. It may be more physiologically relevant to conduct in vitro studies of fatigue and fracture toughness of bone cements following storage in a fluid environment.

  18. EFFECT OF CARBODIIMIDE ON THE FATIGUE CRACK GROWTH RESISTANCE OF RESIN-DENTIN BONDS

    PubMed Central

    Zhang, Zihou; Beitzel, Dylan; Majd, Hessam; Mutluay, Mustafa; Tezvergil-Mutluay, Arzu; Tay, Franklin R.; Pashley, David H.; Arola, Dwayne

    2015-01-01

    Recent studies have shown that ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) inactivates endogenous dentin proteases, thereby preventing collagen degradation and improving the durability of adhesive bonds to dentin. Bond durability is routinely assessed by monotonic microtensile testing, which does not consider the cyclic nature of mastication. Objective to characterize the effect of an EDC pretreatment on the fatigue crack growth behavior of resin-dentin bonds. Methods Bonded interface Compact Tension (CT) specimens were prepared using a three-step etch-and-rinse adhesive and hybrid resin-composite. Adhesive bonding of the treated groups included a 1 min application of an experimental EDC conditioner to the acid-etched dentin. The control groups did not receive EDC treatment. The fatigue crack growth resistance was examined after storage in artificial saliva for 0, 3 and 6 months. Results There was no significant difference in the immediate fatigue crack growth resistance of the EDC-treated and control groups at 0 months. However, after the 3 and 6 months storage periods the EDC-treated groups exhibited significantly greater (p≤0.05) fatigue crack growth resistance than the control specimens. Significance Although the EDC treatment maintained the fatigue crack growth resistance of the dentin bonds through 6 months of storage, additional studies are needed to assess its effectiveness over longer periods and in relation to other cross-linking agents. PMID:26739775

  19. A Comparison Study of Machine Learning Based Algorithms for Fatigue Crack Growth Calculation.

    PubMed

    Wang, Hongxun; Zhang, Weifang; Sun, Fuqiang; Zhang, Wei

    2017-05-18

    The relationships between the fatigue crack growth rate ( d a / d N ) and stress intensity factor range ( Δ K ) are not always linear even in the Paris region. The stress ratio effects on fatigue crack growth rate are diverse in different materials. However, most existing fatigue crack growth models cannot handle these nonlinearities appropriately. The machine learning method provides a flexible approach to the modeling of fatigue crack growth because of its excellent nonlinear approximation and multivariable learning ability. In this paper, a fatigue crack growth calculation method is proposed based on three different machine learning algorithms (MLAs): extreme learning machine (ELM), radial basis function network (RBFN) and genetic algorithms optimized back propagation network (GABP). The MLA based method is validated using testing data of different materials. The three MLAs are compared with each other as well as the classical two-parameter model ( K * approach). The results show that the predictions of MLAs are superior to those of K * approach in accuracy and effectiveness, and the ELM based algorithms show overall the best agreement with the experimental data out of the three MLAs, for its global optimization and extrapolation ability.

  20. A Comparison Study of Machine Learning Based Algorithms for Fatigue Crack Growth Calculation

    PubMed Central

    Wang, Hongxun; Zhang, Weifang; Sun, Fuqiang; Zhang, Wei

    2017-01-01

    The relationships between the fatigue crack growth rate (da/dN) and stress intensity factor range (ΔK) are not always linear even in the Paris region. The stress ratio effects on fatigue crack growth rate are diverse in different materials. However, most existing fatigue crack growth models cannot handle these nonlinearities appropriately. The machine learning method provides a flexible approach to the modeling of fatigue crack growth because of its excellent nonlinear approximation and multivariable learning ability. In this paper, a fatigue crack growth calculation method is proposed based on three different machine learning algorithms (MLAs): extreme learning machine (ELM), radial basis function network (RBFN) and genetic algorithms optimized back propagation network (GABP). The MLA based method is validated using testing data of different materials. The three MLAs are compared with each other as well as the classical two-parameter model (K* approach). The results show that the predictions of MLAs are superior to those of K* approach in accuracy and effectiveness, and the ELM based algorithms show overall the best agreement with the experimental data out of the three MLAs, for its global optimization and extrapolation ability. PMID:28772906

  1. Visualization of non-propagating Lamb wave modes for fatigue crack evaluation

    NASA Astrophysics Data System (ADS)

    An, Yun-Kyu; Sohn, Hoon

    2015-03-01

    This article develops a non-propagating Lamb wave mode (NPL) imaging technique for fatigue crack visualization. NPL has a great potential for crack evaluation in that it significantly contributes local mode amplitudes in the vicinity of a crack without spatial propagation. Such unique physical phenomenon is theoretically proven and experimentally measured through laser scanning. Although its measurement is a quite challenging work due to the fact that it is quite localized and coexists with complex propagating Lamb wave modes, a NPL filter proposed in this article overcomes the technical challenge by eliminating all propagating Lamb modes from laser scanned full Lamb wavefields. Through the NPL filtering process, only fatigue crack-induced NPLs can be measured and retained. To verify such physical observation and the corresponding NPL filter, a real micro fatigue crack is created by applying repeated tensile loading, and its detectability is tested using a surface-mounted piezoelectric transducer for generating Lamb waves and a laser Doppler vibrometer for measuring the corresponding responses. The experimental results confirm that even an invisible fatigue crack can be instantaneously visualized and effectively evaluated through the proposed NPL measurement and filtering processes.

  2. Fatigue Crack Nucleation Studies on Sulfuric Acid Anodized 7075-T73 Aluminum

    NASA Astrophysics Data System (ADS)

    Savas, Terence P.; Earthman, James C.

    2014-06-01

    The influence of a sulfuric acid anodic coating process on the fatigue crack nucleation behavior of 7075-T73 aluminum alloy was investigated. Silicone surface replication in combination with carbon sputter coating and scanning electron microscopy (SEM) allowed for in situ monitoring of the number of cycles for crack nucleation. A single edge circular notch (SECN) coupon was designed for the present study to localize fatigue damage thus enhancing fatigue crack detection and capture the effects of multiaxial stress conditions indicative of a majority engineering applications. Linear elastic finite element modeling of the SECN coupon was performed to quantify the von Mises equivalent stress distribution and the stress concentration factor of the notched region. The experimental results indicate that the presence of localized pitting corrosion initiated during the anodic coating pretreatment process had an adverse effect on fatigue performance. Specifically, multiple crack nucleation sites were evident as opposed to a single crack origin for the untreated specimens. Post-cycling SEM surface examinations displayed networks of micro-cracks in the anodic coating emanating from the pits although these were not found to be fatigue crack origin sites during post SEM fractographic exams. Thus, the stress concentration effect of the corrosion pits was found to be predominant. The total cycles to failure on average was reduced by approximately 60% for the anodic coated versus untreated specimens. A strategy is also discussed on how to mitigate accelerated crack nucleation by controlled surface pretreatment and use of a chromated chemical conversion coating in lieu of an anodic coating for selective applications.

  3. Comparison of the Fatigue Crack Propagation Behavior of Two Different Forms of PMMA Using Two-Stage Zone Model

    NASA Astrophysics Data System (ADS)

    Hao, Wenfeng; Ma, Liting; Chen, Xinwen; Yuan, Yanan; Ma, Yinji

    2016-02-01

    The fatigue crack propagation behavior of two different forms of PMMA was studied using two-stage zone model. First, the fatigue crack length and fatigue crack propagation velocities of different specimens were obtained experimentally. Then the effect of material forms and specimen types on the fatigue crack propagation velocities was analyzed. Finally, the data scatter of da/ dN-Δ K curves in different forms and different types of specimens was analyzed. The results show that the expressions of fatigue crack propagation velocities of middle crack tension (MT) specimens and compact tension (CT) specimens in the same form PMMA are similar. And the scatter of MT specimens is larger than CT specimens in two forms of PMMA.

  4. Mechanism and estimation of fatigue crack initiation in austenitic stainless steels in LWR environments.

    SciTech Connect

    Chopra, O. K.; Energy Technology

    2002-08-01

    The ASME Boiler and Pressure Vessel Code provides rules for the construction of nuclear power plant components. Figures I-9.1 through I-9.6 of Appendix I to Section III of the Code specify fatigue design curves for structural materials. However, the effects of light water reactor (LWR) coolant environments are not explicitly addressed by the Code design curves. Existing fatigue strain-vs.-life ({var_epsilon}-N) data illustrate potentially significant effects of LWR coolant environments on the fatigue resistance of pressure vessel and piping steels. This report provides an overview of fatigue crack initiation in austenitic stainless steels in LWR coolant environments. The existing fatigue {var_epsilon}-N data have been evaluated to establish the effects of key material, loading, and environmental parameters (such as steel type, strain range, strain rate, temperature, dissolved-oxygen level in water, and flow rate) on the fatigue lives of these steels. Statistical models are presented for estimating the fatigue {var_epsilon}-N curves for austenitic stainless steels as a function of the material, loading, and environmental parameters. Two methods for incorporating environmental effects into the ASME Code fatigue evaluations are presented. The influence of reactor environments on the mechanism of fatigue crack initiation in these steels is also discussed.

  5. Evaluation of sub-critical fatigue crack propagation in a restorative composite.

    PubMed

    Loughran, Galyna M; Versluis, Antheunis; Douglas, William H

    2005-03-01

    Fracture is a major reason for clinical failure of dental restorations. Many clinical fractures are likely to be preceded by a slow sub-critical crack propagation. The purpose of this study was to determine slow crack propagation in a restorative composite. Notched composite (Z100, 3M ESPE) specimens were fatigued in a four-point bending test. The notch (1 mm) was created by embedding a sharpened razor blade in the specimen mold. The specimens were load-cycled at 5 Hz between -5 and -30 N until failure. Displacement and load were recorded during the fatigue tests and used to derive crack propagation based on beam-compliance. The number of cycles until failure ranged between 34 and 82,481. In the last 1500 cycles prior to final fracture, the beam compliance increased consistently, indicating sub-critical crack propagation. It was calculated from the compliance change that the crack length increased 8% (77 +/- 14 microm) before final failure. The crack growth rate during sub-critical crack propagation was determined as a function of the stress intensity for the last 1500 cycles before fracture. A method is presented to determine slow crack propagation using a four-point bending test. Although fatigue lifetime varied widely, stable crack growth prior to fracture was consistent. This consistency allowed formulation of stress-based crack propagation relationships that can be used in concert with numerical simulations to predict composite restoration performance. The large variation found for specimen lifetime was attributed to the initiation process that precedes sub-critical crack propagation.

  6. Three-dimensional EBSD characterization of thermo-mechanical fatigue crack morphology in compacted graphite iron

    SciTech Connect

    Pirgazi, Hadi; Ghodrat, Sepideh; Kestens, Leo A.I.

    2014-04-01

    In cylinder heads made of compacted graphitic iron (CGI), heating and cooling cycles can lead to localized cracking due to thermo-mechanical fatigue (TMF). To meticulously characterize the complex crack path morphology of CGI under TMF condition, in relation to microstructural features and to find out how and by which mechanisms the cracks predominantly develop, three-dimensional electron back scattering diffraction (EBSD) was employed. Based on the precise quantitative microstructural analysis, it is found that graphite particles not only play a crucial role in the crack initiation, but also are of primary significance for crack propagation, i.e. crack growth is enhanced by the presence of graphite particles. Furthermore, the density of graphite particles on the fracture plane is more than double as high as in any other arbitrary plane of the structure. The obtained results did not indicate a particular crystallographic preference of fracture plane, i.e. the crystal plane parallel to the fracture plane was nearly of random orientation. - Highlights: • Crystallographic features of a thermo-mechanical fatigue (TMF) crack were studied. • Wide-field 3D EBSD is used to characterize the TMF crack morphology. • Data processing was applied on a large length scale of the order of millimeters. • Graphite density in the fracture plane is much higher than any other random plane. • It is revealed that crack growth is enhanced by the presence of graphite particles.

  7. Three-Orthogonal-Direction Stress Mapping around a Fatigue-Crack Tip Using Neutron Diffraction

    NASA Astrophysics Data System (ADS)

    Huang, E.-Wen; Lee, Soo Yeol; Woo, Wanchuck; Lee, Kuan-Wei

    2012-08-01

    Quantitative determination of the stress fields around the crack tip is a challenging and important subject to understand the fatigue crack-growth mechanism. In the current study, we measured the distribution of residual stresses and the evolution of the stress fields around a fatigue crack tip subjected to the constant-amplitude cyclic loading in a 304L stainless steel compact-tension (CT) specimen. The three orthogonal stress components ( i.e., crack growth, crack opening, and through thickness) of the CT specimen were determined as a function of distance from the crack tip with 1-mm spatial resolution along the crack-propagation direction. In-situ neutron-diffraction results show that the enlarged tensile stresses were developed during loading along the through-thickness direction at a localized volume close to the crack tip, resulting in the lattice expansion in all three orthogonal directions during P max. The current study suggests that the atypical plane strainlike behavior observed at the midthickness position might be the reason for the mechanism of the faster crack-growth rate inside the interior than that near the surface.

  8. Fatigue Crack Growth Rate and Stress-Intensity Factor Corrections for Out-of-Plane Crack Growth

    NASA Technical Reports Server (NTRS)

    Forth, Scott C.; Herman, Dave J.; James, Mark A.

    2003-01-01

    Fatigue crack growth rate testing is performed by automated data collection systems that assume straight crack growth in the plane of symmetry and use standard polynomial solutions to compute crack length and stress-intensity factors from compliance or potential drop measurements. Visual measurements used to correct the collected data typically include only the horizontal crack length, which for cracks that propagate out-of-plane, under-estimates the crack growth rates and over-estimates the stress-intensity factors. The authors have devised an approach for correcting both the crack growth rates and stress-intensity factors based on two-dimensional mixed mode-I/II finite element analysis (FEA). The approach is used to correct out-of-plane data for 7050-T7451 and 2025-T6 aluminum alloys. Results indicate the correction process works well for high DeltaK levels but fails to capture the mixed-mode effects at DeltaK levels approaching threshold (da/dN approximately 10(exp -10) meter/cycle).

  9. Residual strength of five boron/aluminum laminates with crack-like notches after fatigue loading

    NASA Technical Reports Server (NTRS)

    Simonds, R. A.

    1986-01-01

    Boron/aluminum specimens were made with crack-like slits in the center and with various proportions of 0 and + or - 45 deg plies. They were fatigue loaded and then fractured to determine their residual strengths. The fatigue loads were generally in the range of 60 to 80 percent of the static tensile strength of the specimen as determined from a previous study, and the stress ratio was .05. For virtually all of the specimens the fatigue loading was continued for 100,000 cycles. The specimens were radiographed after the fatigue loading to determine the nature of the fatigue damage. A few specimens were sectioned and examined in a scanning electron microscope after being radiographed in order to verify the interpretation of the radiographs and also to get a better insight into the nature of the fatigue damage. The results indicate that the fatiguing does not significantly affect the strength of the specimens tested. The results of the radiography and of the scanning electron microscopy indicate that the 45 deg plies suffer extensive damage in the form of split and broken fibers and matrix cracking in the vicinity of the ends of the split. By contrast, the only significant damage to the 0 deg plies was a single 0 deg matric crack growing from the ends of the slit and between the 0 deg fibers.

  10. Residual strength of five boron/aluminum laminates with crack-like notches after fatigue loading

    NASA Technical Reports Server (NTRS)

    Simonds, R. A.

    1984-01-01

    Boron/aluminum specimens were made with crack-like slits in the center and with various proportions of 0 and + or - 45 deg plies. They were fatigue loaded and then fractured to determine their residual strengths. The fatigue loads were generally in the range of 60 to 80 percent of the static tensile strength of the specimen as determined from a previous study, and the stress ratio was .05. For virtually all of the specimens the fatigue loading was continued for 100,000 cycles. The specimens were radiographed after the fatigue loading to determine the nature of the fatigue damage. A few specimens were sectioned and examined in a scanning electron microscope after being radiographed in order to verify the interpretation of the radiographs and also to get a better insight into the nature of the fatigue damage. The results indicate that the fatiguing does not significantly affect the strength of the specimens tested. The results of the radiography and of the scanning electron microscopy indicate that the 45 deg plies suffer extensive damage in the form of split and broken fibers and matrix cracking in the vicinity of the ends of the split. By contrast, the only significant damage to the 0 deg plies was a single 0 deg matric crack growing from the ends of the slit and between the 0 deg fibers.

  11. Fatigue Life and Crack Growth Behavior in Annealed and Normalized 0.83% Carbon Steel

    NASA Astrophysics Data System (ADS)

    Makabe, Chobin; Yamazaki, Shinya; Miyazaki, Tatsujiro; Fujikawa, Masaki

    2015-09-01

    The variations of fatigue limit and fatigue life of a plain specimen of annealed and normalized 0.83% carbon steel were investigated. This material is used for cutting tools and the original microstructure includes a spherical microstructure. After heat treatment under some conditions, the microstructure changed to a lamellar microstructure. However, the fatigue lives of the plain specimens of this material showed almost the same tendency even after heat treatment under some conditions. In those cases, the initial crack length in the fatigue process is related to the size of the crystal structure and related to the distribution of ferrite. In the present study, the relationship between the distribution of hardness and the fatigue limit was investigated. Also, it was discussed that the fatigue limit of heat-treated 0.83% carbon steel could be evaluated by a relationship in which the parameters are the hardness and initial crack length. Finally, the tendencies of fatigue life of heat-treated 0.83% carbon steel were discussed based on the observations of crack growth behavior.

  12. Modeling of nonlinear interactions between guided waves and fatigue cracks using local interaction simulation approach.

    PubMed

    Shen, Yanfeng; Cesnik, Carlos E S

    2017-02-01

    This article presents a parallel algorithm to model the nonlinear dynamic interactions between ultrasonic guided waves and fatigue cracks. The Local Interaction Simulation Approach (LISA) is further developed to capture the contact-impact clapping phenomena during the wave crack interactions based on the penalty method. Initial opening and closure distributions are considered to approximate the 3-D rough crack microscopic features. A Coulomb friction model is integrated to capture the stick-slip contact motions between the crack surfaces. The LISA procedure is parallelized via the Compute Unified Device Architecture (CUDA), which enables parallel computing on powerful graphic cards. The explicit contact formulation, the parallel algorithm, as well as the GPU-based implementation facilitate LISA's high computational efficiency over the conventional finite element method (FEM). This article starts with the theoretical formulation and numerical implementation of the proposed algorithm, followed by the solution behavior study and numerical verification against a commercial finite element code. Numerical case studies are conducted on Lamb wave interactions with fatigue cracks. Several nonlinear ultrasonic phenomena are addressed. The classical nonlinear higher harmonic and DC response are successfully captured. The nonlinear mode conversion at a through-thickness and a half-thickness fatigue crack is investigated. Threshold behaviors, induced by initial openings and closures of rough crack surfaces, are depicted by the proposed contact LISA model.

  13. The Effect of the Laboratory Specimen on Fatigue Crack Growth Rate

    NASA Technical Reports Server (NTRS)

    Forth, S. C.; Johnston, W. M.; Seshadri, B. R.

    2006-01-01

    Over the past thirty years, laboratory experiments have been devised to develop fatigue crack growth rate data that is representative of the material response. The crack growth rate data generated in the laboratory is then used to predict the safe operating envelope of a structure. The ability to interrelate laboratory data and structural response is called similitude. In essence, a nondimensional term, called the stress intensity factor, was developed that includes the applied stresses, crack size and geometric configuration. The stress intensity factor is then directly related to the rate at which cracks propagate in a material, resulting in the material property of fatigue crack growth response. Standardized specimen configurations and experimental procedures have been developed for laboratory testing to generate crack growth rate data that supports similitude of the stress intensity factor solution. In this paper, the authors present laboratory fatigue crack growth rate test data and finite element analyses that show similitude between standard specimen configurations tested using the constant stress ratio test method is unobtainable.

  14. Fatigue of Self-Healing Nanofiber-based Composites: Static Test and Subcritical Crack Propagation.

    PubMed

    Lee, Min Wook; Sett, Soumyadip; Yoon, Sam S; Yarin, Alexander L

    2016-07-20

    Here, we studied the self-healing of composite materials filled with epoxy-containing nanofibers. An initial incision in the middle of a composite sample stretched in a static fatigue test can result in either crack propagation or healing. In this study, crack evolution was observed in real time. A binary epoxy, which acted as a self-healing agent, was encapsulated in two separate types of interwoven nano/microfibers formed by dual-solution blowing, with the core containing either epoxy or hardener and the shell being formed from poly(vinylidene fluoride)/ poly(ethylene oxide) mixture. The core-shell fibers were encased in a poly(dimethylsiloxane) matrix. When the fibers were damaged by a growing crack in this fiber-reinforced composite material because of static stretching in the fatigue test, they broke and released the healing agent into the crack area. The epoxy used in this study was cured and solidified for approximately an hour at room temperature, which then conglutinated and healed the damaged location. The observations were made for at least several hours and in some cases up to several days. It was revealed that the presence of the healing agent (the epoxy) in the fibers successfully prevented the propagation of cracks in stretched samples subjected to the fatigue test. A theoretical analysis of subcritical cracks was performed, and it revealed a jumplike growth of subcritical cracks, which was in qualitative agreement with the experimental results.

  15. A study on ultrasonic detection and characterization of partially closed fatigue cracks

    SciTech Connect

    Van Wyk, L.M.

    1990-02-01

    This study consists of an ultrasonic characterization of two different fatigue cracks in 2024-T351 Aluminum. The first crack was grown with a 5 cycle overload, and the second was grown with a decreasing stress intensity. The first part of the study used through transmitted longitudinal ultrasonic waves to find a distinct change in the transmission coefficient as fatigue cycles were added following the overload and as the crack was statically loaded during the ultrasonic scanning. The second part of the study compared the differences in the through transmitted signals and the shear diffracted signals of the decreasing {Delta}K crack to a crack in the constant {Delta}K condition. In both parts of the study, estimates of the size of the closure region and the magnitude of the residual compressive stress intensity at the crack tip were made through the use of the distributed spring model. This model mathematically characterizes the reaction of an ultrasonic wave to a fatigue crack in a solid. 15 refs., 18 figs., 2 tabs.

  16. Fatigue Crack Growth Behavior of Nickel-base Superalloy Haynes 282 at 550-750 °C

    NASA Astrophysics Data System (ADS)

    Rozman, K. A.; Kruzic, J. J.; Hawk, J. A.

    2015-08-01

    The fatigue crack growth rates for nickel-based superalloy Haynes 282 were measured at temperatures of 550, 650, and 750 °C using compact tension specimens with a load ratio of 0.1 and cyclic loading frequencies of 25 Hz and 0.25 Hz. Increasing the temperature from 550 to 750 °C caused the fatigue crack growth rates to increase from ~20 to 60% depending upon the applied stress intensity level. The effect of reducing the applied loading frequency increased the fatigue crack growth rates from ~20 to 70%, also depending upon the applied stress intensity range. The crack path was observed to be transgranular for the temperatures and frequencies used during fatigue crack growth rate testing. At 750 °C, there were some indications of limited intergranular cracking excursions at both loading frequencies; however, the extent of intergranular crack growth was limited and the cause is not understood at this time.

  17. Response of a structural health monitoring fastener to fatigue crack growth and loads in metallic joints

    NASA Astrophysics Data System (ADS)

    Rakow, Alexi S.; Chang, Fu-Kuo

    2009-03-01

    Fatigue cracks initiating at fastener hole locations in metallic structure are among the most common form of airframe damage. Current methods for inspecting airframes for these cracks are manual, whereby inspectors rely on nondestructive inspection equipment or hand-held probes to scan over areas to be monitored. Use of this equipment often demands disassembly of the airframe to search appropriate hole locations for cracks, which elevates the complexity and cost of maintenance inspections. In this study an Additive, Interleaved, Multi-layer Electromagnetic (AIME) sensor was developed and integrated with the shank of a fastener to form a Structural Health Monitoring Fastener, a new technology targeted at insitu detection of fastener hole cracks. The major advantages of the Structural Health Monitoring (SHM) Fastener over other SHM technologies are its installation, which does not require joint layer disassembly, its capability to detect inner layer cracks in a multi-layer joint, and its capability to operate in a continuous monitoring mode. The AIME sensor design, SHM Fastener, and complete SHM system are presented along with experimental results from a series of single-layer and bolted double lap-joint aluminum specimens to validate the capability of these sensors to monitor metallic joints for fastener hole cracks and loads. Fatigue cracks were successfully tracked to over 0.7 inches from the fastener hole in these tests. Sensor output obtained from single-layer fatigue specimens was compared with analytical predictions for fatigue crack growth versus cycle number showing a good correlation in trend between sensor output and predicted crack size.

  18. Detection of Fatigue Crack in Basalt FRP Laminate Composite Pipe using Electrical Potential Change Method

    NASA Astrophysics Data System (ADS)

    Altabey, Wael A.; Noori, Mohammed

    2017-05-01

    Novel modulation electrical potential change (EPC) method for fatigue crack detection in a basalt fibre reinforced polymer (FRP) laminate composite pipe is carried out in this paper. The technique is applied to a laminate pipe with an embedded crack in three layers [0º/90º/0º]s. EPC is applied for evaluating the dielectric properties of basalt FRP pipe by using an electrical capacitance sensor (ECS) to discern damages in the pipe. Twelve electrodes are mounted on the outer surface of the pipe and the changes in the modulation dielectric properties of the piping system are analyzed to detect damages in the pipe. An embedded crack is created by a fatigue internal pressure test. The capacitance values, capacitance change and node potential distribution of ECS electrodes are calculated before and after crack initiates using a finite element method (FEM) by ANSYS and MATLAB, which are combined to simulate sensor characteristics and fatigue behaviour. The crack lengths of the basalt FRP are investigated for various number of cycles to failure for determining crack growth rate. Response surfaces are adopted as a tool for solving inverse problems to estimate crack lengths from the measured electric potential differences of all segments between electrodes to validate the FEM results. The results show that, the good convergence between the FEM and estimated results. Also the results of this study show that the electrical potential difference of the basalt FRP laminate increases during cyclic loading, caused by matrix cracking. The results indicate that the proposed method successfully provides fatigue crack detection for basalt FRP laminate composite pipes.

  19. Crack Growth Behavior in the Threshold Region for High Cycle Fatigue Loading

    NASA Technical Reports Server (NTRS)

    Forman, R. G.; Zanganeh, M.

    2014-01-01

    This paper describes the results of a research program conducted to improve the understanding of fatigue crack growth rate behavior in the threshold growth rate region and to answer a question on the validity of threshold region test data. The validity question relates to the view held by some experimentalists that using the ASTM load shedding test method does not produce valid threshold test results and material properties. The question involves the fanning behavior observed in threshold region of da/dN plots for some materials in which the low R-ratio data fans out from the high R-ratio data. This fanning behavior or elevation of threshold values in the low R-ratio tests is generally assumed to be caused by an increase in crack closure in the low R-ratio tests. Also, the increase in crack closure is assumed by some experimentalists to result from using the ASTM load shedding test procedure. The belief is that this procedure induces load history effects which cause remote closure from plasticity and/or roughness changes in the surface morphology. However, experimental studies performed by the authors have shown that the increase in crack closure is a result of extensive crack tip bifurcations that can occur in some materials, particularly in aluminum alloys, when the crack tip cyclic yield zone size becomes less than the grain size of the alloy. This behavior is related to the high stacking fault energy (SFE) property of aluminum alloys which results in easier slip characteristics. Therefore, the fanning behavior which occurs in aluminum alloys is a function of intrinsic dislocation property of the alloy, and therefore, the fanned data does represent the true threshold properties of the material. However, for the corrosion sensitive steel alloys tested in laboratory air, the occurrence of fanning results from fretting corrosion at the crack tips, and these results should not be considered to be representative of valid threshold properties because the fanning is

  20. Updated Fatigue-Crack-Growth And Fracture-Mechanics Software

    NASA Technical Reports Server (NTRS)

    Forman, Royce G.; Shivakumar, Venkataraman; Newman, James C., Jr.

    1995-01-01

    NASA/FLAGRO 2.0 developed as analytical aid in predicting growth and stability of preexisting flaws and cracks in structural components of aerospace systems. Used for fracture-control analysis of space hardware. Organized into three modules to maximize efficiency in operation. Useful in: (1) crack-instability/crack-growth analysis, (2) processing raw crack-growth data from laboratory tests, and (3) boundary-element analysis to determine stresses and stress-intensity factors. Written in FORTRAN 77 and ANSI C.

  1. Effect of Stress Ratio on Fatigue Crack Growth Rate at Notched Hole in 7075-T6 Aluminum Alloy Under Biaxial Fatigue

    DTIC Science & Technology

    2016-08-18

    and corrosion. This approach has been used to delay crack in structure in aerospace field [26]. In addition, laterally many studies have described...loading conditions. And it is a link between previous studies and future studies in the field . II. Background 2.1 Fatigue Since fracture mechanics...steady, but other forms of stresses such as rotation, torsion , pending can lead to fatigue failure [5,34]. Fatigue crack start as invisible microcrack

  2. Temperature dependence of the intrinsic small fatigue crack growth behavior in ni-base superalloys based on measurement of crack closure

    NASA Astrophysics Data System (ADS)

    Okazaki, M.; Yamada, H.; Nohmi, S.

    1996-04-01

    The effect of temperature on the small fatigue crack growth behavior of a single crystal and directionally solidified Ni-base superalloys was investigated at temperatures between 873 to 1123 K by measuring the crack closure. The results were also compared with those of the physically long crack. It was found that the propagation resistance and the fatigue threshold of the long cracks increased with temperature in all the materials. The long crack growth rates at three temperatures were approximately represented by an unique curve, after taking account of crack closure level and elastic modulus. In contrast, the small crack growth resistance decreased with temperature even when the crack closure phenomenon was taken into consideration. Furthermore, the small fatigue cracks exhibited considerably higher growth rates than the long cracks at a given effective stress intensity factor range and also grew under effective stress intensity factor ranges below the long crack threshold. The factors responsible for the lack of similitude in propagation rates between small and long cracks were also discussed, based on these observations and the chemical analysis near the crack tip using the electron probe microanalyzer.

  3. Location, location &size: defects close to surfaces dominate fatigue crack initiation.

    PubMed

    Serrano-Munoz, Itziar; Buffiere, Jean-Yves; Mokso, Rajmund; Verdu, Catherine; Nadot, Yves

    2017-03-27

    Metallic cast components inevitably contain defects such as shrinkage cavities which are inherent to the solidification process. Those defects are known to significantly alter the fatigue life of components. Yet very little is known, quantitatively, on the dangerosity of internal casting defects compared to surface ones. In this study, fatigue specimens containing controlled internal defects (shrinkage pores) are used to foster internal cracking. In situ fatigue tests monitored by X ray synchrotron tomography revealed that the internal nucleation and propagation of cracks was systematically overran by surface cracking initiated at castings defects up to ten times smaller than the internal ones. These findings indicate that the presence of internal defects in cast components can be tolerated to a larger extent than is allowed by nowadays standards.

  4. Location, location & size: defects close to surfaces dominate fatigue crack initiation

    PubMed Central

    Serrano-Munoz, Itziar; Buffiere, Jean-Yves; Mokso, Rajmund; Verdu, Catherine; Nadot, Yves

    2017-01-01

    Metallic cast components inevitably contain defects such as shrinkage cavities which are inherent to the solidification process. Those defects are known to significantly alter the fatigue life of components. Yet very little is known, quantitatively, on the dangerosity of internal casting defects compared to surface ones. In this study, fatigue specimens containing controlled internal defects (shrinkage pores) are used to foster internal cracking. In situ fatigue tests monitored by X ray synchrotron tomography revealed that the internal nucleation and propagation of cracks was systematically overran by surface cracking initiated at castings defects up to ten times smaller than the internal ones. These findings indicate that the presence of internal defects in cast components can be tolerated to a larger extent than is allowed by nowadays standards PMID:28345599

  5. Study of Near-Threshold Fatigue Crack Propagation in Pipeline Steels in High Pressure Environments

    NASA Technical Reports Server (NTRS)

    Mitchell, M.

    1981-01-01

    Near threshold fatigue crack propagation in pipeline steels in high pressure environments was studied. The objective was to determine the level of threshold stress intensity for fatigue crack growth rate behavior in a high strength low alloy X60 pipeline-type steel. Complete results have been generated for gaseous hydrogen at ambient pressure, laboratory air at ambient pressure and approximately 60% relative humidity as well as vacuum of 0.000067 Pa ( 0.0000005 torr) at R-ratios = K(min)/K(max) of 0.1, 0.5, and 0.8. Fatigue crack growth rate behavior in gaseous hydrogen, methane, and methane plus 10 percent hydrogen at 6.89 MPa (100 psi) was determined.

  6. 77 K Fatigue Crack Growth Rate of Modified CF8M Stainless Steel Castings

    SciTech Connect

    Walsh, R. P.; Toplosky, V. J.; Han, K.; Heitzenroeder, P. J.; Nelson, B. E.

    2006-03-31

    The National Compact Stellerator Experiment (NCSX) is the first of a new class of stellarators. The modular superconducting coils in the NCSX have complex geometry that are manufactured on cast stainless steel (modified CF8M) winding forms. Although CF8M castings have been used before at cryogenic temperature there is limited data available for their mechanical properties at low temperatures. The fatigue life behavior of the cast material is vital thus a test program to generate data on representative material has been conducted. Fatigue test specimens have been obtained from key locations within prototype winding forms to determine the 77 K fatigue crack growth rate. The testing has successfully developed a representative database that ensures confident design. The measured crack growth rates are analyzed in terms of the Paris law parameters and the crack growth properties are related to the materials microstructure.

  7. Fatigue crack initiation and microcrack propagation in X7091 type aluminum P/M alloys

    NASA Astrophysics Data System (ADS)

    Hirose, S.; Fine, M. E.

    1983-06-01

    Fatigu crack initiation in extruded X7091 RSP-P/M aluminum type alloys o°Curs at grain boundaries at both low and high stresses. By a process of elimination this grain boundary embrittlement was attributed to Al2O3 particles formed mainly during atomization and segregated to some grain boundaries. It is not due to the small grain size, to Co2Al9, to η precipitates at grain boundaries, nor to a precipitate free zone. Thermomechanical processing after extrusion of X7091 with 0.8 pct Co was done by Alcoa to produce large recrystallized grains. This resulted in initiation of fatigue cracks at slip bands, and the resistance to initiation of fatigue cracks at low stresses was much greater. Microcrack growth is, however, much faster in the thermomechanically treated samples, as well as in ingot alloys, than in extruded and aged X7091.

  8. Influence of Initial Inclined Surface Crack on Estimated Residual Fatigue Lifetime of Railway Axle

    NASA Astrophysics Data System (ADS)

    Náhlík, Luboš; Pokorný, Pavel; Ševčík, Martin; Hutař, Pavel

    2016-11-01

    Railway axles are subjected to cyclic loading which can lead to fatigue failure. For safe operation of railway axles a damage tolerance approach taking into account a possible defect on railway axle surface is often required. The contribution deals with an estimation of residual fatigue lifetime of railway axle with initial inclined surface crack. 3D numerical model of inclined semi-elliptical surface crack in railway axle was developed and its curved propagation through the axle was simulated by finite element method. Presence of press-fitted wheel in the vicinity of initial crack was taken into account. A typical loading spectrum of railway axle was considered and residual fatigue lifetime was estimated by NASGRO approach. Material properties of typical axle steel EA4T were considered in numerical calculations and lifetime estimation.

  9. Transition probabilities matrix of Markov Chain in the fatigue crack growth model

    NASA Astrophysics Data System (ADS)

    Nopiah, Zulkifli Mohd; Januri, Siti Sarah; Ariffin, Ahmad Kamal; Masseran, Nurulkamal; Abdullah, Shahrum

    2016-10-01

    Markov model is one of the reliable method to describe the growth of the crack from the initial until fracture phase. One of the important subjects in the crack growth models is to obtain the transition probability matrix of the fatigue. Determining probability transition matrix is important in Markov Chain model for describing probability behaviour of fatigue life in the structure. In this paper, we obtain transition probabilities of a Markov chain based on the Paris law equation to describe the physical meaning of fatigue crack growth problem. The results show that the transition probabilities are capable to calculate the probability of damage in the future with the possibilities of comparing each stage between time.

  10. The early stage wheel fatigue crack detection using eddy current pulsed thermography

    NASA Astrophysics Data System (ADS)

    Peng, Jianping; Zhang, Kang; Yang, Kai; He, Zhu; Zhang, Yu; Peng, Chaoyong; Gao, Xiaorong

    2017-02-01

    The in-service wheel-set quality is one of critical challenges for railway safety, especially for the high-speed train. The defect in wheel tread, initiated by rolling contact fatigue (RCF) damage, is one of the most significant phenomena and has serious influence on rail industry. Eddy current pulsed thermography is studied to compensate the UT method for detection these early stage of fatigue cracks in wheel tread surface. This paper proposes approximately uniform magnetic field, excited by Helmholtz coils, based pulsed eddy current thermography to achieve open-view image and meet the irregular surface in wheel tread through numerical way. Some features are extracted and studied also to quantify the fatigue crack in term of eddy current pulsed thermography. The proposed method enhances the capability for cracks detection and quantitative evaluation compared with previous NDT method in railway.

  11. An experimental investigation of fatigue-crack growth in drillstring tubulars

    SciTech Connect

    Dale, B.A.

    1988-12-01

    Drillstring failures continue to plague the oil industry, often costing millions of dollars each year. This problem is frequently intensified with the drilling of deep, deviated wellbores or ''hard rock'' drilling conditions. The drilling industry attempts to guard against these costly failures by performing periodic nondestructive inspections to remove damaged tubulars from service. This paper describes the results of full-scale fatigue-crack-growth tests of drill collars under rotating and bending loads. In addition, corrosion fatigue-crack-growth data are also presented for API drillpipe steels in air and in three representative water-based drilling-fluid environments. Based on this experimental investigation, the test data support the practical application of fatigue-crack-growth mechanics principles for the development of nondestructive inspection intervals to reduce drillstring failures.

  12. Methodology Developed for Modeling the Fatigue Crack Growth Behavior of Single-Crystal, Nickel-Base Superalloys

    NASA Technical Reports Server (NTRS)

    1996-01-01

    Because of their superior high-temperature properties, gas generator turbine airfoils made of single-crystal, nickel-base superalloys are fast becoming the standard equipment on today's advanced, high-performance aerospace engines. The increased temperature capabilities of these airfoils has allowed for a significant increase in the operating temperatures in turbine sections, resulting in superior propulsion performance and greater efficiencies. However, the previously developed methodologies for life-prediction models are based on experience with polycrystalline alloys and may not be applicable to single-crystal alloys under certain operating conditions. One of the main areas where behavior differences between single-crystal and polycrystalline alloys are readily apparent is subcritical fatigue crack growth (FCG). The NASA Lewis Research Center's work in this area enables accurate prediction of the subcritical fatigue crack growth behavior in single-crystal, nickel-based superalloys at elevated temperatures.

  13. Influence of dissolved hydrogen on the fatigue crack growth behaviour of AISI 4140 steel

    NASA Astrophysics Data System (ADS)

    Ramasagara Nagarajan, Varun

    Many metallic structural components come into contact with hydrogen during manufacturing processes or forming operations such as hot stamping of auto body frames and while in service. This interaction of metallic parts with hydrogen can occur due to various reasons such as water molecule dissociation during plating operations, interaction with atmospheric hydrogen due to the moisture present in air during stamping operations or due to prevailing conditions in service (e.g.: acidic or marine environments). Hydrogen, being much smaller in size compared to other metallic elements such as Iron in steels, can enter the material and become dissolved in the matrix. It can lodge itself in interstitials locations of the metal atoms, at vacancies or dislocations in the metallic matrix or at grain boundaries or inclusions (impurities) in the alloy. This dissolved hydrogen can affect the functional life of these structural components leading to catastrophic failures in mission critical applications resulting in loss of lives and structural component. Therefore, it is very important to understand the influence of the dissolved hydrogen on the failure of these structural materials due to cyclic loading (fatigue). For the next generation of hydrogen based fuel cell vehicles and energy systems, it is very crucial to develop structural materials for hydrogen storage and containment which are highly resistant to hydrogen embrittlement. These materials should also be able to provide good long term life in cyclic loading, without undergoing degradation, even when exposed to hydrogen rich environments for extended periods of time. The primary focus of this investigation was to examine the influence of dissolved hydrogen on the fatigue crack growth behaviour of a commercially available high strength medium carbon low alloy (AISI 4140) steel. The secondary objective was to examine the influence of microstructure on the fatigue crack growth behaviour of this material and to determine the

  14. On the micromechanisms of fatigue-crack propagation in aluminum- lithium alloys: Sheet vs. plate material

    SciTech Connect

    Rao Venkateswara, K.T.; Ritchie, R.O. California Univ., Berkeley, CA . Dept. of Materials Science and Mineral Engineering); Bucci, R.J. . Alcoa Labs.)

    1989-12-01

    Micromechanisms influencing the propagation of long (>10 mm) fatigue cracks in aluminum-lithium alloys are examined by specifically comparing crack-growth kinetics in a peak-aged Al-Li-Cu-Zr alloy 2090, processed as 1.6-mm thin (T83) sheet and 12.7-mm thick (T81) plate. It is found that in general crack-growth rates are significantly faster in the sheet material at equivalent stress-intensity levels, due to differences in the role of crack-tip shielding, resulting from crack deflection and consequent crack closure from wedging of fracture-surface asperities. Microstructurally, such differences are related to variations in the degree of recrystallization, grain structure and deformation texture in the two wrought-product forms. 14 refs., 4 figs.

  15. Acoustic emission analysis of fatigue crack growth in 2024-T4 aluminum

    SciTech Connect

    Wu, J.Y.; Ono, K.

    1995-12-31

    Fatigue crack growth experiments have been performed on single-edge cracked Al 2024-T4 specimens. Acoustic emission (AE) signals are collected using a Fracture Wave Detector at various crack growth rate, ranging from 0.25 to 50 {mu}m/cycle. Relationships between signal amplitude, RMS voltage, stress intensity factor range and crack growth rate are examined. Characteristics of AE signals generated are investigated by ICEPAK-based pattern recognition analysis with a trained K-nearest neighbor classifier. AE signals from crack propagation are studied to discriminate features of various signal types and to correlate the waveforms with respect to crack growth rate with different types of AE source. Classification results will be given.

  16. The characterization of small fatigue crack growth in PH13-8 molybdenum stainless steel

    NASA Astrophysics Data System (ADS)

    Jin, Ohchang

    The rotor hubs of Navy CH-46 helicopters have been made of 4340 steel and had extensive corrosion fatigue problems. Since these helicopters have to be used until the year 2020, the Navy decided to replace 4340 steel with PH 13-8 Mo stainless steel. Because the rotors are exposed to high frequency high cycle fatigue, small fatigue cracks are important in estimating remaining lifetime of the components. The objective of this study was to characterize the small crack growth behavior in the PH 13-8 Mo stainless steel under various loading conditions. Constant amplitude loading was conducted at the stress ratios, R, 0.1 and 0.4. The crack growth rate was affected by the microstructures in early stage of the growth, mainly by the size of the martensite packets and oscillated up to the crack length of 200 mum. It was found that the crack growth rate was little influenced by the stress amplitudes and stress ratios. In addition, the small crack growth rate was found to be similar to the long crack growth rate at R = 0.1 and 0.4. Overload tests and simple block loading were performed to understand load interaction effects on the small crack growth rate. The overload tests indicated that the crack growth rate was little affected by the overload. This might result from the fact that the overload ratio used in this study was low (<1.3). However, the results of the simple block loading showed overall crack growth retardation. The compressive residual stress present at the notch root of the specimen tested at R = 0.1 may lower the effective stress ratio, Reff, from 0.1 to negative R, and may result in the crack growth retardation. The small crack growth behavior was also examined under the saltwater. There was no difference in the crack growth rate between under air and under saltwater. In addition, the crack growth rate of the specimens tested under the saltwater was not affected by the test frequencies of 10, 1 and 0.1 Hz. It was shown that under the saltwater the PH 13-8 Mo

  17. Near-threshold fatigue crack growth behavior of 2195 aluminum-lithium-alloy—prediction of crack propagation direction and influence of stress ratio

    NASA Astrophysics Data System (ADS)

    Chen, D. L.; Chaturvedi, M. C.

    2000-06-01

    Tensile properties and fatigue crack propagation behavior of a 2195-T8 Al-Li alloy were investigated at different stress ratios, with particular emphasis on their dependence on specimen orientation. Specimens with orientations of 0, 15, 30, 45, and 90 deg to the rolling direction were tested. The alloy contained a strong brass-type texture and a profuse distribution of platelike precipitates of T 1 (Al2CuLi) phase on {111} matrix planes. Both tensile strength and fatigue thresholds were found to be strongly dependent on the specimen orientation, with the lowest values observed along the direction at 45 deg to the rolling direction. The effect of stress ratio on fatigue threshold could generally be explained by a modified crack closure concept. The growth of fatigue crack in this alloy was found to exhibit a significant crystallographic cracking and especially macroscopic crack deflection. The specimens oriented in the L-T + 45 deg had the smallest deflection angle, while the specimens in the L-T and T-L orientations exhibited a large deflection angle. The dependence of the fatigue threshold on the specimen orientation could be rationalized by considering an equivalent fatigue threshold calculated from both mode I and mode II values due to the crack deflection. A four-step approach on the basis of Schmid’s law combined with specific crystallographic textures is proposed to predict the fatigue crack deflection angle. Good agreement between the theoretical prediction and experimental results was observed.

  18. Crossing grain boundaries in metals by slip bands, cleavage and fatigue cracks.

    PubMed

    Pineau, André

    2015-03-28

    The size and the character (low and large angle, special boundaries, tilt and twist boundaries, twins) of the grain boundaries (GBs) in polycrystalline materials influence their strength and their fracture toughness. Recent studies devoted to nanocrystalline (NC) materials have shown a deviation from the Hall-Petch law. Special GBs formed by Σ3 twins in face-centred cubic metals are also known to have a strong effect on the mechanical behaviour of these metals, in particular their work-hardening rate. Grain orientation influences also crack path, the fracture toughness of body-centred cubic (BCC) metals and the fatigue crack growth rate of microstructurally short cracks. This paper deals both with slip transfer at GBs and with the interactions between propagating cracks with GBs. In the analysis of slip transfer, the emphasis is placed on twin boundaries (TBs) for which the dislocation reactions during slip transfer are analysed theoretically, experimentally and using the results of atomic molecular simulations published in the literature. It is shown that in a number of situations this transfer leads to a normal motion of the TB owing to the displacement of partial dislocations along the TB. This motion can generate a de-twinning effect observed in particular in NC metals. Crack propagation across GBs is also considered. It is shown that cleavage crack path behaviour in BCC metals is largely dependent on the twist component of the GBs. A mechanism for the propagation of these twisted cracks involving a segmentation of the crack front and the existence of intergranular parts is discussed and verified for a pressure vessel steel. A similar segmentation seems to occur for short fatigue cracks although, quite surprisingly, this crossing mechanism for fatigue cracks does not seem to have been examined in very much detail in the literature. Metallurgical methods used to improve the strength of the materials, via grain boundaries, are briefly discussed.

  19. Effects of a Hydrogen Gas Environment on Fatigue Crack Growth of a Stable Austenitic Stainless Steel

    NASA Astrophysics Data System (ADS)

    Kawamoto, Kyohei; Oda, Yasuji; Noguchi, Hiroshi; Higashida, Kenji

    In order to clarify the effects of a hydrogen gas environment on the fatigue crack growth characteristics of stable austenitic stainless steels, bending fatigue tests were carried out in a hydrogen gas, in a nitrogen gas at 1.0 MPa and in air on a SUS316L using the Japanese Industrial Standards (type 316L). Also, in order to discuss the difference in the hydrogen sensitivity between austenitic stainless steels, the fatigue tests were also carried out on a SUS304 using the Japanese Industrial Standards (type 304) metastable austenitic stainless steel as a material for comparison. The main results obtained are as follows. Hydrogen gas accelerates the fatigue crack growth rate of type 316L. The degree of the fatigue crack growth acceleration is low compared to that in type 304. The fracture surfaces of both the materials practically consist of two parts; the faceted area seemed to be brittle and the remaining area occupying a greater part of the fracture surface and seemed to be ductile. The faceted area does not significantly contribute to the fatigue crack growth rate in both austenitic stainless steels. The slip-off mechanism seems to be valid not only in air and in nitrogen, but also in hydrogen. Also, the main cause of the fatigue crack growth acceleration of both materials occurs by variation of the slip behaviour. The difference in the degree of the acceleration, which in type 316L is lower than in type 304, seems to be caused by the difference in the stability of the γ phase.

  20. Classification of fatigue cracking data in a simulated aircraft fuselage using a self-organizing map

    SciTech Connect

    Marsden, M.L.; Hill, E.V.K.

    1994-12-31

    Many aircraft are being flown beyond their design lifespans and have therefore fallen victim to fatigue cracking. In some cases, such as the 1988 Aloha Airlines 737-200 incident, catastrophic fatigue growth has caused the loss of life. Acoustic emission (AE) nondestructive testing has been used to detect and classical the growth of fatigue cracks in complex structures, such as aircraft fuselages and wings since as early as 1979. In order to simulate an aircraft fuselage undergoing pressurization cycle fatigue, a test was developed in which a thin-walled aluminum pressure vessel was instrumented with AE sensors and cyclically fatigued to promote crack growth at a stress concentration built into the vessel. The AE data acquisition system. extracted the six AE parameters - amplitude, counts, duration, energy, risetime, and count-to-peak from each of the sensor signals. One-third of these parameter data sets were used to tram a Kohonen self-organizing map (SOM) neural network. The remaining data sets were used to test the SOM. The SOM output is a two-dimensional map with similar input data sets located at similar coordinates on the map. Because the continuous AE parameter data are grouped into discrete bands or intervals, e.g., all the events having amplitudes between 51.00 dB and 51.99 dB are classified as 51 dB events, the initial SOM output showed no distinct clustering. However, when the output was transformed into three-dimensions, with the third dimension being the frequency of occurrence of each two-dimensional coordinate, several distinct peaks were evident. These peaks correspond to the three AE source in the vessel: metal rubbing, rivet fretting, and fatigue cracking. Thus, the three-dimensional SOM was able to unambiguously classify fatigue crack growth events in a simulated aircraft fuselage structure.

  1. A Critical Review of the Short Crack Problem in Fatigue

    DTIC Science & Technology

    1983-01-01

    including foreign nations. This technical report has been reviewed and is approved for publication. J N P. HENDERSON, JAMES M. LARSEN Metals Behavior ...Branch Project Engineer Metals and Ceramics Division Metals Behavior Branch FOR THE COMMANDER LWRENCE N. HJALM, As set ief Metals and Ceramics...Finite Growth Rate Behavior of Short Cracks in Unnotched Samples ................................................ 33 Near Threshold Growth of Short Cracks

  2. Strength Behaviour of Fatigue Cracked Lugs (Festigkeitsverhalten von Rissbehafteten Augenstaeben),

    DTIC Science & Technology

    1981-01-01

    either surface cracks or corner cracks at holes. NASA TN 1)-8244 64 A.F. Grandt Stress intensity factors for some through fracked fastener holes...with Hydropuise L~ngszylinder longitudinal cylinder Druckblversorgung =pressure oil Supply Hydraulikaggregat = hydraulic control unit Fig 7.5 Plan of

  3. Influence of microstructure on fatigue crack growth behavior of Sn-Ag solder interfaces

    NASA Astrophysics Data System (ADS)

    Liu, Pi Lin; Shang, Jian Ku

    2000-05-01

    The relationship between microstructure and fatigue crack growth behavior was examined at Sn-Ag solder interfaces on copper and electroless-nickel metallizations. On copper metallization, the solder interface was lined with a coarse Ag3Sn intermetallic phase in addition to the Cu6Sn5 intermetallic and the adjacent solder alloy contained nodular Ag3Sn phase. This interfacial microstructure was shown to result in inferior fatigue resistance, with the fatigue crack path following the interfacial Ag3Sn intermetallic phase. In contrast, the solder interface on the electroless-nickel metallization was covered with a thin layer of Ni3Sn4 intermetallic phase, and the solder microstructure was composed of fine needles of Ag3Sn phase dispersed in the Sn-rich matrix. This solder interface was found to be significantly more resistant to fatigue than the copper/Sn-Ag solder interface.

  4. 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.

  5. Effect of Environment on Fatigue and Creep Crack Growth in Inconel X-750 at Elevated Temperature

    NASA Astrophysics Data System (ADS)

    Gabrielli, F.; Pelloux, R. M.

    1982-06-01

    The fatigue crack growth rates (FCGR) of Inconel X-750 were measured in air and in vacuum at 25 °C and 650 °C as a function of test frequency. The wave shape was triangular and the frequency varied from 10 Hz to 0.01 Hz. The creep crack growth rates (CCGR) were also measured on single edge notch specimens at 650 °C in air and in purified argon. For a given AK, the FCGR increases when temperature increases and frequency decreases. At low frequency the FCGR approach the creep crack growth rates. The mode of fracture changes from transgranular at 10 Hz to intergranular at 0.01 Hz. The effect of air environment is to accelerate the transition from transgranular to intergranular fracture modes with decreasing frequency. The role of oxidation in accelerating crack growth rate in fatigue and in creep is discussed in detail.

  6. 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.

  7. Algorithm of crack tracking during fatigue test through calculating the optical flow

    NASA Astrophysics Data System (ADS)

    Panin, S. V.; Chemezov, V. O.; Lyubutin, P. S.; Titkov, V. V.

    2016-11-01

    An algorithm of crack detection during fatigue testing of materials, designed to automate the process of cyclic loading and tracking the crack tip, was proposed and tested. The ultimate goal of the study is aimed at controlling the displacements of the optical system with regard to specimen under fatigue loading to ensure observation of the area of interest. It is shown that the image region that contains the crack may be detected and positioned with an average error of 1.17% for noisy images and 11.5% when blurred images are analyzed. In terms of determining the crack tip position. the algorithm provides the accuracy of its localization with the average error value of 9.23 pixels when processing noisy images, while it makes 42.7 pixels for blurry ones.

  8. Extreme stress gradient effects on microstructural fatigue crack propagation rates in Ni microbeams

    SciTech Connect

    Sadeghi-Tohidi, F.; Pierron, O. N.

    2015-05-18

    The fatigue crack propagation behavior of microstructurally small cracks growing under extreme stress gradients was investigated in Ni microbeams under fully reversed cyclic loading. A technique to calculate the crack growth rates in microbeams with two different normalized stress gradients (17% and 50% μm{sup −1}) is developed and validated. Decreasing crack propagation rates are observed over the first 2 μm, and the rates are more than 1 order of magnitude slower for the devices with 50% μm{sup −1} stress gradients. This fundamental knowledge is critical to predict the fatigue reliability of advanced metallic microcomponents under bending such as in microelectromechanical systems or flexible/stretchable electronics.

  9. Effect of material heat treatment on fatigue crack initiation in austenitic stainless steels in LWR environments.

    SciTech Connect

    Chopra, O. K.; Alexandreanu, B.; Shack, W. J.; Energy Technology

    2005-07-31

    The ASME Boiler and Pressure Vessel Code provides rules for the design of Class 1 components of nuclear power plants. Figures I-9.1 through I-9.6 of Appendix I to Section III of the Code specify design curves for applicable structural materials. However, the effects of light water reactor (LWR) coolant environments are not explicitly addressed by the Code design curves. The existing fatigue strain-vs.-life ({var_epsilon}-N) data illustrate potentially significant effects of LWR coolant environments on the fatigue resistance of pressure vessel and piping steels. Under certain environmental and loading conditions, fatigue lives of austenitic stainless steels (SSs) can be a factor of 20 lower in water than in air. This report presents experimental data on the effect of heat treatment on fatigue crack initiation in austenitic Type 304 SS in LWR coolant environments. A detailed metallographic examination of fatigue test specimens was performed to characterize the crack morphology and fracture morphology. The key material, loading, and environmental parameters and their effect on the fatigue life of these steels are also described. Statistical models are presented for estimating the fatigue {var_epsilon}-N curves for austenitic SSs as a function of material, loading, and environmental parameters. Two methods for incorporating the effects of LWR coolant environments into the ASME Code fatigue evaluations are presented.

  10. Multi-Mode Excitation and Data Reduction for Fatigue Crack Characterization in Conducting Plates

    NASA Technical Reports Server (NTRS)

    Wincheski, B.; Namkung, M.; Fulton, J. P.; Clendenin, C. G.

    1992-01-01

    Advances in the technique of fatigue crack characterization by resonant modal analysis have been achieved through a new excitation mechanism and data reduction of multiple resonance modes. A non-contacting electromagnetic device is used to apply a time varying Lorentz force to thin conducting sheets. The frequency and direction of the Lorentz force are such that resonance modes are generated in the test sample. By comparing the change in frequency between distinct resonant modes of a sample, detecting and sizing of fatigue cracks are achieved and frequency shifts caused by boundary condition changes can be discriminated against. Finite element modeling has been performed to verify experimental results.

  11. Natural Fatigue Crack Initiation and Detection in High Quality Spur Gears

    DTIC Science & Technology

    2012-06-01

    Natural Fatigue Crack Initiation and Detection in High Quality Spur Gears by David “Blake” Stringer, Ph.D., Kelsen E. LaBerge, Ph.D., Cory...0383 June 2012 Natural Fatigue Crack Initiation and Detection in High Quality Spur Gears David “Blake” Stringer and Ph.D., Kelsen E. LaBerge...Quality Spur Gears 5a. CONTRACT NUMBER 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) David “Blake” Stringer, Ph.D., Kelsen E

  12. Study on optimization method of test conditions for fatigue crack detection using lock-in vibrothermography

    NASA Astrophysics Data System (ADS)

    Min, Qing-xu; Zhu, Jun-zhen; Feng, Fu-zhou; Xu, Chao; Sun, Ji-wei

    2017-06-01

    In this paper, the lock-in vibrothermography (LVT) is utilized for defect detection. Specifically, for a metal plate with an artificial fatigue crack, the temperature rise of the defective area is used for analyzing the influence of different test conditions, i.e. engagement force, excitation intensity, and modulated frequency. The multivariate nonlinear and logistic regression models are employed to estimate the POD (probability of detection) and POA (probability of alarm) of fatigue crack, respectively. The resulting optimal selection of test conditions is presented. The study aims to provide an optimized selection method of the test conditions in the vibrothermography system with the enhanced detection ability.

  13. Application of fiber bridging models to fatigue crack growth in unidirectional titanium matrix composites

    NASA Technical Reports Server (NTRS)

    Bakuckas, J. G., Jr.; Johnson, W. S.

    1992-01-01

    Several fiber bridging models were reviewed and applied to study the matrix fatigue crack growth behavior in center notched (0)(sub 8) SCS-6/Ti-15-3 and (0)(sub 4) SCS-6/Ti-6Al-4V laminates. Observations revealed that fatigue damage consisted primarily of matrix cracks and fiber matrix interfacial failure in the (0)(sub 8) SCS-6/Ti-15-3 laminates. Fiber-matrix interface failure included fracture of the brittle reaction zone and cracking between the two carbon rich fiber coatings. Intact fibers in the wake of the matrix cracks reduce the stress intensity factor range. Thus, an applied stress intensity factor range is inappropriate to characterize matrix crack growth behavior. Fiber bridging models were used to determine the matrix stress intensity factor range in titanium metal matrix composites. In these models, the fibers in the wake of the crack are idealized as a closure pressure. An unknown constant frictional shear stress is assumed to act along the debond or slip length of the bridging fibers. The frictional shear stress was used as a curve fitting parameter to available data (crack growth data, crack opening displacement data, and debond length data). Large variations in the frictional shear stress required to fit the experimental data indicate that the fiber bridging models in their present form lack predictive capabilities. However, these models provide an efficient and relatively simple engineering method for conducting parametric studies of the matrix growth behavior based on constituent properties.

  14. Detection of surface breaking fatigue crack on a complex aircraft structure with Rayleigh surface waves

    NASA Astrophysics Data System (ADS)

    Na, Jeong K.; Blackshire, James L.; Kuhr, Samuel J.

    2009-03-01

    As part of an on-going, multi-year effort focused on developing a practical structural health monitoring (SHM) sensor for critical structural components in aircraft, a miniature Rayleigh surface wave sensor has been developed and tested. The sensor was specifically designed to detect localized, deterministic cracking in targeted locations in critical locations where fatigue cracking is prevalent. A representative aircraft component was used in the present investigation. Miniature interdigital transducers (IDTs) operating in the low megahertz frequency range were designed, fabricated, and tested on compact tension (CT) fatigue specimens in the laboratory before they were strategically placed on the structure, where surface wave signals were monitored in both pitch-catch and pulse-echo detection modes simultaneously. Under a high-cycle fatigue loading to the structure, the IDT sensors performed well with three of the sensors successfully detecting the existence of a critical fatigue crack. Visual and eddy current inspection methods subsequently verified the presence of the crack and its location. In this paper, the entire effort from the design and characterization of the IDT sensors to the final fatigue test on an actual aircraft part is discussed.

  15. Effect of heat treatment upon the fatigue-crack growth behavior of Alloy 718 weldments

    SciTech Connect

    Mills, W.J.; James, L.A.

    1981-05-01

    The microstructural features that influenced the room and elevated temperature fatigue-crack growth behavior of as-welded, conventional heat-treated, and modified heat-treated Alloy 718 GTA weldments were studied. Electron fractographic examination of fatigue fracture surfaces revealed that operative fatigue mechanisms were dependent on microstructure, temperatures and stress intensity factor. All specimens exhibited three basic fracture surface appearances at temperatures up to 538{degrees}C: crystallographic faceting at low stress intensity range ({Delta}K) levels, striation, formation at intermediate values, and dimples coupled with striations in the highest ({Delta}K) regime. At 649{degrees}C, the heat-treated welds exhibited extensive intergranular cracking. Laves and {delta} particles in the conventional heat-treated material nucleated microvoids ahead of the advancing crack front and caused on overall acceleration in crack growth rates at intermediate and high {Delta}K levels. The modified heat treatment removed many of these particles from the weld zone, thereby improving its fatigue resistance. The dramatically improved fatigue properties exhibited by the as-welded material was attributed to compressive residual stresses introduced by the welding process. 19 refs., 16 figs.

  16. Influence of Experimental Parameters on Fatigue Crack Growth and Heat Build-Up in Rubber

    PubMed Central

    Stadlbauer, Franziska; Koch, Thomas; Archodoulaki, Vasiliki-Maria; Planitzer, Florian; Fidi, Wolfgang; Holzner, Armin

    2013-01-01

    Loading parameters (frequency, amplitude ratio and waveform) are varied to determine their influence on fatigue crack growth in rubber. Up to three different rubber blends are investigated: one actual engineering material and two model materials. Fatigue crack growth curves and strain distributions of pure shear and faint waist pure shear samples are compared for a model material. Fatigue behavior is studied for three different frequencies (1 Hz, 3 Hz and 5 Hz). Amplitude ratio appears to be another important influence factor concerning fatigue crack growth in rubber. The beneficial effect of positive amplitude ratios (tensional loading conditions) is shown for different materials. However, fatigue crack growth is considerably increased for negative amplitude ratios (tensional-compressional loading conditions). Furthermore, the influence of the waveform is determined for three different waveform shapes. One is sinusoidal, and two have a square shape, including dwell periods and sinusoidal slopes. Special focus lies on heat build-up, which is substantial, especially for large loads, high frequencies and/or highly filled rubber blends. Plateau temperatures are determined for various loading conditions and rubber blends. A very simple linear relationship with dissipated energy per time and unit area is obtained. Results gathered with dynamic mechanical analyses show, likewise, a linear trend, but the heat build-up is very small, due to different sample geometries. PMID:28788405

  17. Influence of Experimental Parameters on Fatigue Crack Growth and Heat Build-Up in Rubber.

    PubMed

    Stadlbauer, Franziska; Koch, Thomas; Archodoulaki, Vasiliki-Maria; Planitzer, Florian; Fidi, Wolfgang; Holzner, Armin

    2013-11-27

    Loading parameters (frequency, amplitude ratio and waveform) are varied to determine their influence on fatigue crack growth in rubber. Up to three different rubber blends are investigated: one actual engineering material and two model materials. Fatigue crack growth curves and strain distributions of pure shear and faint waist pure shear samples are compared for a model material. Fatigue behavior is studied for three different frequencies (1 Hz, 3 Hz and 5 Hz). Amplitude ratio appears to be another important influence factor concerning fatigue crack growth in rubber. The beneficial effect of positive amplitude ratios (tensional loading conditions) is shown for different materials. However, fatigue crack growth is considerably increased for negative amplitude ratios (tensional-compressional loading conditions). Furthermore, the influence of the waveform is determined for three different waveform shapes. One is sinusoidal, and two have a square shape, including dwell periods and sinusoidal slopes. Special focus lies on heat build-up, which is substantial, especially for large loads, high frequencies and/or highly filled rubber blends. Plateau temperatures are determined for various loading conditions and rubber blends. A very simple linear relationship with dissipated energy per time and unit area is obtained. Results gathered with dynamic mechanical analyses show, likewise, a linear trend, but the heat build-up is very small, due to different sample geometries.

  18. An analytical model which combines roughness- and plasticity- induced fatigue crack closure

    NASA Astrophysics Data System (ADS)

    Chen, Nong

    In this study an analytical PICC-RICC Model was developed to describe better the near-threshold fatigue behavior. The PICC-RICC Model was built upon a strip-yield type PICC model originally proposed by Newman and later modified by Hou and Lawrence. A zigzag crack growth path was introduced to simulate surface roughness. The two opposing crack surfaces were considered to be translated and thus mismatched by the mixed-mode displacements occurring near the deflected crack tip. The model is powerful and unique in that it combines the effects of RICC and PICC. Thus, the gradual transition from RICC to PICC dominated crack closure is handled naturally by this model. The influences of the geometrical features of the surface roughness, R-ratio and the cyclic load range on RICC were examined using the PICC-RICC Model. Near-threshold fatigue behavior of various materials was predicted. The effect of microstructure on the RICC level was studied. The predicted results compared favorably with experimental data. The fatigue notch size effect was investigated using the PICC-RICC model. The initial crack length (asb{i}) for propagation was estimated. The predicted notch fatigue strength compared favorably with the Initiation-Propagation (I-P) Model prediction and test data. The existence of a "worst case notch" previously postulated using the I-P Model was confirmed.

  19. Can acoustic emission detect the initiation of fatigue cracks: Application to high-strength light alloys used in aeronautics

    NASA Technical Reports Server (NTRS)

    Bathias, C.; Brinet, B.; Sertour, G.

    1978-01-01

    Acoustic emission was used for the detection of fatigue cracking in a number of high-strength light alloys used in aeronautical structures. Among the features studied were: the influence of emission frequency, the effect of surface oxidation, and the influence of grains. It was concluded that acoustic emission is an effective nondestructive technique for evaluating the initiation of fatigue cracking in such materials.

  20. The Influence on Microstructure and Microtexture on Fatigue Crack Initiation and Growth in Alpha + Beta Titanium

    DTIC Science & Technology

    2011-10-01

    discovered by fatigue testing, however, it would be useful to rank the resistance of a particular microstructure to its fatigue crack growth... resistance . In the present work, we present a methodology for this purpose that correlates, microstructure including microtexture as affected by...thermomechanical processing sequences were all typical of α + β worked material consisting of globular α particles in a matrix of transformed β. The

  1. Experimental investigation of fatigue of cracked aluminium specimens repaired with fibre composite patches

    SciTech Connect

    Nahas, M.N. )

    1992-08-01

    Standard notched fatigue test specimens are subjected to cyclic loading conditions to a number of cycles enough to initiate cracks, and then the specimens are repaired with fiber-reinforced composite patches to investigate the influence of the repair on their fatigue life. It is found that the repair provides longer life for the test specimens, and substantial reduction in the stress intensity is achieved. 10 refs.

  2. A New Modelling of Crack Propagation with Fatigue-Creep-Oxidation Interaction under Non Isothermal Loading

    DTIC Science & Technology

    2003-02-01

    propagation of metallic materials at high temperature such as Ni-base superalloy for turbine discs . The strong requirements in design procedures have led to the...This paper deals with the extension of a crack growth model to high temperature complex loading and application to turbine disc . The proposed model is...which comprises fatigue with or without hold times and special sequence tests representative to the disc in service. The crack growth model is built up

  3. Acoustic-emission inspection of the kinetics of fatigue cracks in turbomachinery disks

    SciTech Connect

    Banov, M.D.; Shanyavskii, A.A.; Urbakh, A.I.; Troenkin, D.A.; Konyaev, E.A.; Pykhtin, Yu.A.; Minatsevich, S.F.; Kashin, V.N.

    1988-07-01

    The kinetics of crack growth in turbomachinery disks under low-cycle fatigue conditions was investigated. The relationship between the change in acoustic emission parameters and the processes of loss of continuity of the disk material in cyclic loading was established on the basis of fratographic investigations. A comparative evaluation of the effectiveness of traditional methods of nondestructive testing applicable to the problem of early detection of cracks in disks is given.

  4. [A microstructural approach to fatigue crack processes in poly crystalline BCC materials]. Progress report

    SciTech Connect

    Gerberich, W.W.

    1992-12-31

    Objective was to study fatigue where a combination of low temperature and cyclic loading produced cyclic cleavage in bcc Fe-base systems. Both dislocation dynamics and quasi-statics of crack growth were probed. This document reviews progress over the past 6 years: hydrogen embrittlement and cleavage, computations (stress near crack tip), dislocation emission from grain boundaries, fracture process zones, and understanding brittle fracture at the atomistic/dislocation scales and at the microscopic/macroscopic scale.

  5. [A microstructural approach to fatigue crack processes in poly crystalline BCC materials

    SciTech Connect

    Gerberich, W.W.

    1992-01-01

    Objective was to study fatigue where a combination of low temperature and cyclic loading produced cyclic cleavage in bcc Fe-base systems. Both dislocation dynamics and quasi-statics of crack growth were probed. This document reviews progress over the past 6 years: hydrogen embrittlement and cleavage, computations (stress near crack tip), dislocation emission from grain boundaries, fracture process zones, and understanding brittle fracture at the atomistic/dislocation scales and at the microscopic/macroscopic scale.

  6. Detection of Fatigue Damage Prior to Crack Initiation withScanning SQUID Microscopy

    SciTech Connect

    Lee, Tae-Kyu; Morris Jr., J.W.; Lee, Seungkyun; Clarke, John

    2005-11-07

    The remanence fields of fatigued ferritic steel specimens were measured using a scanning microscope based on a high transition temperature Superconducting Quantum Interference Device (SQUID). The results show an overall increase of remanence until dislocation density saturates and an additional local remanence increase after saturation during cyclic loading. Because of the combined magnetic and spatial resolution of the SQUID microscope, these local changes of dislocation structures can be detected before a crack actually initiates, and identify the sites where crack nucleation will occur.

  7. A Problem of Fatigue Fracture Mechanics of a Two-Layer Material with Edge Cracks

    NASA Astrophysics Data System (ADS)

    Seyfullayev, A. I.; Rustamova, M. A.; Kerimova, Sh. A.

    2017-07-01

    A problem for an elastic half-plane consisting of two tightly coupled homogeneous isotropic elastic media and containing edge cracks is considered. Its lateral surface is subjected to a given external load. The growth of a fatigue crack in the two-layer half-plane is investigated. For some materials, the number of cycles to failure is calculated. The stress intensity factor is determined, and a numerical analysis is performed.

  8. The Significance of Small Cracks in Fatigue Design Concepts as Related to Rotorcraft Metallic Dynamic Components

    DTIC Science & Technology

    2000-02-01

    REFERENCES Mechs., Vol. 7, No. 2, pp 235-247, 1975. 1. Palmgren , A., Ball and Roller Bearing 11. Zocher, H., (ed.), "Behavior of Short Cracks in 1...flaw (not a sharp fatigue times. The replacement times are determined using crack) pre-exists in the structure. The flaw tolerant the Palmgren /Miner...nominal stress rule [Ref. 1,2] method still uses the Palmgren /Miner rule to establish where the stress versus life curves, S/N curves, are retirement

  9. On Microstructural Control of Near-Threshold Fatigue Crack Growth in 7000-Series Aluminum Alloys.

    DTIC Science & Technology

    1982-04-02

    crack growth rate behavior for different microstruc - tural conditions in aluminum alloys is also in quantitative agreement with the predictions of the...34 .. . -~ Introduction ! A number of recent studies have been conducted to ascertain the influence of microstructure on fatigue crack growth behavior in aluminum...161. The da/dN data, obtained over a very broad spectrum of ,K, characterize the near-threshold growth-rate behavior unusually well. Predictions of

  10. Fatigue-crack initiation and growth in notched 2024-T3 specimens monitored by a video tape system

    NASA Technical Reports Server (NTRS)

    Sova, J. A.; Crews, J. H., Jr.; Exton, R. J.

    1976-01-01

    Fatigue-crack initiation and early stages of growth in notched 2024-T3 aluminum alloy sheet specimens were monitored during constant-amplitude fatigue tests conducted for a range of stress ratios. A video tape system was developed to detect fatigue cracks 0.1 mm in length on the sheet surface and to monitor their growth. Fatigue cracks initiated either at the notch corner or on the notch surface. Most of the first fatigue cracks started as notch-corner cracks, and the crack-initiation period was defined as the number of cycles for which the crack was 0.1 mm in length on the sheet surface. For each of the stress ratios used, the results showed that at the low stress levels, the crack-initiation period constituted at least 90 percent of the fatigue lives. However, for the negative stress ratios at the high stress levels, cracks started as early as at about 40 percent of the fatigue lives.

  11. 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

  12. Fatigue crack growth of Fe{sub 3}Al,Cr alloys

    SciTech Connect

    Alven, D.A.; Stoloff, N.S.

    1996-06-15

    Iron aluminides are currently being considered for use in applications where excellent corrosion resistance, moderate strength at temperatures up to 500 C, and low cost are desired. Binary iron aluminides do not possess high ductility at room temperature in air; however, when tested in vacuum or gaseous oxygen environments iron aluminides have demonstrated ductility of greater than 10%. This indicates that Fe{sub 3}Al is an inherently ductile material and the low ductility in air is due to an environment interaction. The purpose of this research was to examine the changes in mechanical properties of two Fe{sub 2}Al,Cr intermetallic alloys in various environments. In particular, tensile behavior and fatigue crack growth resistance were examined and the resulting data used to gain insight into the role of hydrogen in embrittlement of iron aluminides.

  13. AE analysis during corrosion, stress corrosion cracking and corrosion fatigue processes

    SciTech Connect

    Yuyama, S.; Kishi, T.

    1983-01-01

    Current theoretical and experimental research on the use of acoustic emission (AE) techniques for studying corrosion problems is reviewed. In particular, attention is given to the AE behavior of Type 304 stainless steel in aqueous environment, and a new method for analyzing corrosion, stress corrosion cracking, and corrosion fatigue in Type 304 steel is described. Results are also presented for other steels, aluminum and magnesium alloys, copper and its alloys, uranium alloys, and titanium and zirconium alloys. It is concluded that the AE method is a prommising approach to the detection and monitoring of localized corrosion in both laboratory specimens and engineering structures. Care must be taken, however, to discriminate valid AE signals from the background noise and to interpret the results correctly. 95 references.

  14. Detection of Steel Fatigue Cracks with Strain Sensing Sheets Based on Large Area Electronics

    PubMed Central

    Yao, Yao; Glisic, Branko

    2015-01-01

    Reliable early-stage damage detection requires continuous monitoring over large areas of structure, and with sensors of high spatial resolution. Technologies based on Large Area Electronics (LAE) can enable direct sensing and can be scaled to the level required for Structural Health Monitoring (SHM) of civil structures and infrastructure. Sensing sheets based on LAE contain dense arrangements of thin-film strain sensors, associated electronics and various control circuits deposited and integrated on a flexible polyimide substrate that can cover large areas of structures. This paper presents the development stage of a prototype strain sensing sheet based on LAE for crack detection and localization. Two types of sensing-sheet arrangements with size 6 × 6 inch (152 × 152 mm) were designed and manufactured, one with a very dense arrangement of sensors and the other with a less dense arrangement of sensors. The sensing sheets were bonded to steel plates, which had a notch on the boundary, so the fatigue cracks could be generated under cyclic loading. The sensors within the sensing sheet that were close to the notch tip successfully detected the initialization of fatigue crack and localized the damage on the plate. The sensors that were away from the crack successfully detected the propagation of fatigue cracks based on the time history of the measured strain. The results of the tests have validated the general principles of the proposed sensing sheets for crack detection and identified advantages and challenges of the two tested designs. PMID:25853407

  15. Small fatigue crack propagation in Y2O3 strengthened steels

    NASA Astrophysics Data System (ADS)

    Hutař, P.; Kuběna, I.; Ševčík, M.; Šmíd, M.; Kruml, T.; Náhlík, L.

    2014-09-01

    This paper is focused on two type of Y2O3 strengthened steels (Fe-14Cr ODS and ODS-EUROFER). Small fatigue crack propagation was experimentally measured using special small cylindrical specimens (diameter 2 and 2.6 mm) with shallow notch grinded in the gauge length. In the middle of this notch, a pre-crack of length of 50 μm was fabricated using a focused ion beam technique. Fatigue crack growth rate was measured for different applied total strain amplitudes and described using plastic part of the J-integral. Obtained results were compared with published data of EUROFER 97. The effect of the oxide dispersion on small fatigue crack propagation was found rather insignificant. Ferritic Fe-14Cr ODS steel shows more brittle behaviour, i.e. for the same cyclic plasticity, characterised by the plastic part of the J-integral, the small cracks grow faster. A new methodology for residual lifetime prediction of structures containing physically small cracks, based on plastic part of the J-integral, is presented.

  16. Overload and Underload Effects on the Fatigue Crack Growth Behavior of the 2024-T3 Aluminum Alloy

    NASA Technical Reports Server (NTRS)

    Dawicke, David S.

    1997-01-01

    Fatigue crack growth tests were conducted on 0.09 inch thick, 3.0 inch wide middle-crack tension specimens cut from sheets of 2024-T3 aluminum alloy. The tests were conducted using a load sequence that consisted of a single block of 2,500 cycles of constant amplitude loading followed by an overload/underload combination. The largest fatigue crack growth life occurred for the tests with the overload stress equal to 2 times the constant amplitude stress and the underload stress equal to the constant amplitude minimum stress. For the tests with compressive underloads, the fatigue crack growth life decreased with increasing compressive underload stress.

  17. Subcritical crack-growth behavior of borosilicate glass under cyclic loads: Evidence of a mechanical fatigue effect

    SciTech Connect

    Dill, S.J.; Dauskardt, R.H.; Bennison, S.J.

    1997-03-01

    Amorphous glasses are generally considered immune to mechanical fatigue effects associated with cyclic loading. In this study surprising new evidence is presented for a mechanical fatigue effect in borosilicate glass, in both moist air and dry nitrogen environments. The fatigue effect occurs at near threshold subcritical crack-growth rates (da/dt < 3 {times} 10{sup {minus}8} m/s) as the crack extension per cycle approaches the dimensions of the borosilicate glass network. While subcritical crack growth under cyclic loads at higher load levels is entirely consistent with environmentally assisted crack growth, lower growth rates actually exceed those measured under monotonic loads. This suggests a mechanical fatigue effect which accelerates subcritical crack-growth rates. Likely mechanisms for the mechanical fatigue effect are presented.

  18. Advanced Finite Element Modeling of Low Cycle Fatigue Crack Growth

    NASA Technical Reports Server (NTRS)

    Gregg, Wayne; McGill, Preston; Swanson, Greg; Wells, Doug; Throckmorton, D. A. (Technical Monitor)

    2001-01-01

    This document (a viewgraph presentation) assumes a crack-like defect of a size which may be missed in inspection will exist in most critical location of any critical structure or component. Flaw existence assumption is usually, but not always, conservative based on past experiences in NASA and knowledge of manufacturing processes. Cyclic, environmental, and sustained loads used to generate stresses on models. Fracture Mechanics analysis used to predict crack growth and residual strength. Must show that defective structure will still provide four times required mission lifetime. Special exemptions cover redundant structures, low risk parts, etc. Assessments require specialized software tools, experienced analysts, and reliable material crack growth rate test database.

  19. Fatigue crack initiation potential from defects in terms of local stress analysis

    NASA Astrophysics Data System (ADS)

    Zhu, Mingliang; Xuan, Fuzhen

    2014-05-01

    The competition of surface and subsurface crack initiation induced failure is critical to understand very high cycle fatigue(VHCF) behavior, which necessitates the elucidation of the underlying mechanisms for the transition of crack initiation from surface to interior defects. Crack initiation potential in materials containing defects is investigated numerically by focusing on defect types, size, shape, location, and residual stress influences. Results show that the crack initiation potency is higher in case of serious property mismatching between matrix and defects, and higher strength materials are more sensitive to soft inclusions(elastic modulus lower than the matrix). The stress localization around inclusions are correlated to interior crack initiation mechanisms in the VHCF regime such as inclusion-matrix debonding at soft inclusions and inclusion-cracking for hard inclusions(elastic modulus higher than the matrix). It is easier to emanate cracks from the subsurface pores with the depth 0.7 times as large as their diameter. There exists an inclusion size independent region for crack incubation, outside which crack initiation will transfer from the subsurface soft inclusion to the interior larger one. As for elliptical inclusions, reducing the short-axis length can decrease the crack nucleation potential and promote the interior crack formation, whereas the long-axis length controls the site of peak stress concentration. The compressive residual stress at surface is helpful to shift crack initiation from surface to interior inclusions. Some relaxation of residual stress can not change the inherent crack initiation from interior inclusions in the VHCF regime. The work reveals the crack initiation potential and the transition among various defects under the influences of both intrinsic and extrinsic factors in the VHCF regime, and is helpful to understand the failure mechanism of materials containing defects under long-term cyclic loadings.

  20. Fatigue crack growth monitoring of idealized gearbox spline component using acoustic emission

    NASA Astrophysics Data System (ADS)

    Zhang, Lu; Ozevin, Didem; Hardman, William; Kessler, Seth; Timmons, Alan

    2016-04-01

    The spline component of gearbox structure is a non-redundant element that requires early detection of flaws for preventing catastrophic failures. The acoustic emission (AE) method is a direct way of detecting active flaws; however, the method suffers from the influence of background noise and location/sensor based pattern recognition method. It is important to identify the source mechanism and adapt it to different test conditions and sensors. In this paper, the fatigue crack growth of a notched and flattened gearbox spline component is monitored using the AE method in a laboratory environment. The test sample has the major details of the spline component on a flattened geometry. The AE data is continuously collected together with strain gauges strategically positions on the structure. The fatigue test characteristics are 4 Hz frequency and 0.1 as the ratio of minimum to maximum loading in tensile regime. It is observed that there are significant amount of continuous emissions released from the notch tip due to the formation of plastic deformation and slow crack growth. The frequency spectra of continuous emissions and burst emissions are compared to understand the difference of sudden crack growth and gradual crack growth. The predicted crack growth rate is compared with the AE data using the cumulative AE events at the notch tip. The source mechanism of sudden crack growth is obtained solving the inverse mathematical problem from output signal to input signal. The spline component of gearbox structure is a non-redundant element that requires early detection of flaws for preventing catastrophic failures. In this paper, the fatigue crack growth of a notched and flattened gearbox spline component is monitored using the AE method The AE data is continuously collected together with strain gauges. There are significant amount of continuous emissions released from the notch tip due to the formation of plastic deformation and slow crack growth. The source mechanism of

  1. Separating plasticity-induced closure and residual stress contributions to fatigue crack retardation following an overload

    NASA Astrophysics Data System (ADS)

    Salvati, Enrico; Zhang, Hongjia; Fong, Kai Soon; Song, Xu; Korsunsky, Alexander M.

    2017-01-01

    The introduction of an overload or underload within a constant amplitude loading fatigue test leads to a retardation or acceleration of the Fatigue Crack Growth Rate (FCGR). The understanding of the causes of these effects is essential in the context of variable amplitude fatigue loading, since in principle any loading history can be represented as a sequence of overloads and underloads. In the case of overload, along with some other minor causes, the residual stress changes at the crack tip and crack closure behind the tip can be thought of as the main factors that affect the fatigue crack growth rate. Whilst this has been recognised and accepted for many decades, controversy persists regarding the relative significance and presence of these two effects, and consensus is yet to emerge. The effect of crack closure, when the baseline loading ratio is high enough, can be inhibited so that the main cause of retardation becomes doubtless the residual stress present ahead the crack tip. In the present paper we report our attempt to deconvolve the contributions of crack closure and residual stress on crack retardation following an overload. To accomplish this task we analyse the results of fatigue tests run at two baseline load ratios, namely R=0.1 and R=0.7. At the load ratio of R=0.7 the crack closure effect is not operative, as confirmed by Digital Image Correlation analysis of the crack flanks close to the tip, and post mortem fractographic analysis of crack surfaces. Therefore, for R=0.7 the compressive residual stress region created by the overload ahead of the crack tip is the sole mechanism causing crack retardation. Therefore, for R=0.7 the focus must be placed entirely on the strain field around the crack tip. To this end, line profiles along the crack bisector of elastic strain in the crack opening direction were collected at several stages of crack propagation past the overload using in situ Synchrotron X-ray Powder Diffraction (SXRPD) technique. By

  2. Fracture toughness and fatigue crack propagation rate of short fiber reinforced epoxy composites for analogue cortical bone.

    PubMed

    Chong, Alexander C M; Miller, Forrest; Buxton, McKee; Friis, Elizabeth A

    2007-08-01

    Third-generation mechanical analogue bone models and synthetic analogue cortical bone materials manufactured by Pacific Research Laboratories, Inc. (PRL) are popular tools for use in mechanical testing of various orthopedic implants and biomaterials. A major issue with these models is that the current third-generation epoxy-short fiberglass based composite used as the cortical bone substitute is prone to crack formation and failure in fatigue or repeated quasistatic loading of the model. The purpose of the present study was to compare the tensile and fracture mechanics properties of the current baseline (established PRL "third-generation" E-glass-fiber-epoxy) composite analogue for cortical bone to a new composite material formulation proposed for use as an enhanced fourth-generation cortical bone analogue material. Standard tensile, plane strain fracture toughness, and fatigue crack propagation rate tests were performed on both the third- and fourth-generation composite material formulations using standard ASTM test techniques. Injection molding techniques were used to create random fiber orientation in all test specimens. Standard dog-bone style tensile specimens were tested to obtain ultimate tensile strength and stiffness. Compact tension fracture toughness specimens were utilized to determine plane strain fracture toughness values. Reduced thickness compact tension specimens were also used to determine fatigue crack propagation rate behavior for the two material groups. Literature values for the same parameters for human cortical bone were compared to results from the third- and fourth-generation cortical analogue bone materials. Tensile properties of the fourth-generation material were closer to that of average human cortical bone than the third-generation material. Fracture toughness was significantly increased by 48% in the fourth-generation composite as compared to the third-generation analogue bone. The threshold stress intensity to propagate the crack

  3. Effect of Frequency on Fatigue Crack Growth Rate of Inconel 718 at High Temperature

    DTIC Science & Technology

    1987-06-01

    Potential 6 and Displacement Measurements. 2 Fractured Specimens of Inconel 718 Showing 13 Different Cracking*Regions Corresponding to Test Under Different...Conditions. 3 Typical a vs N Experimental Data with the 16 Fitted Linear Regression Line. 4 Fatigue Crack Growth Rate (da/dN) for 17 Inconel 718 as a...Temperature Air Data are Given. 5 Time Rate of Crack Growth, (da/dt) for 18 Inconel 718 as a Function of Frequency at Kmax = 40 MPa-ml/ 2 , R = 0.1

  4. Calorimetric Measurement of the Plastic Vtork of Fatigue Crack Propagation in 4140 Steel

    NASA Astrophysics Data System (ADS)

    Gross, Todd S.; Weertman, Johannes

    1982-12-01

    A calorimetric technique has been developed for measurement of the effective surface energy of fatigue crack propagation, U, and the cyclic plastic work in the plastic zone, Q. The technique has several distinct advantages over existing methods. Measurements on 4140 steel (650 °C temper) show that U and Q are direct functions of the stress intensity factor, hK, and indirect functions of crack growth rate, daldN. Measurement of the change of U and Q after the application of a tensile overload supports this conclusion and provides strong evidence supporting crack closure theories.

  5. NASA GRC Fatigue Crack Initiation Life Prediction Models

    NASA Technical Reports Server (NTRS)

    Arya, Vinod K.; Halford, Gary R.

    2002-01-01

    Metal fatigue has plagued structural components for centuries, and it remains a critical durability issue in today's aerospace hardware. This is true despite vastly improved and advanced materials, increased mechanistic understanding, and development of accurate structural analysis and advanced fatigue life prediction tools. Each advance is quickly taken advantage of to produce safer, more reliable, more cost effective, and better performing products. In other words, as the envelope is expanded, components are then designed to operate just as close to the newly expanded envelope as they were to the initial one. The problem is perennial. The economic importance of addressing structural durability issues early in the design process is emphasized. Tradeoffs with performance, cost, and legislated restrictions are pointed out. Several aspects of structural durability of advanced systems, advanced materials and advanced fatigue life prediction methods are presented. Specific items include the basic elements of durability analysis, conventional designs, barriers to be overcome for advanced systems, high-temperature life prediction for both creep-fatigue and thermomechanical fatigue, mean stress effects, multiaxial stress-strain states, and cumulative fatigue damage accumulation assessment.

  6. NASA GRC Fatigue Crack Initiation Life Prediction Models

    NASA Astrophysics Data System (ADS)

    Arya, Vinod K.; Halford, Gary R.

    2002-10-01

    Metal fatigue has plagued structural components for centuries, and it remains a critical durability issue in today's aerospace hardware. This is true despite vastly improved and advanced materials, increased mechanistic understanding, and development of accurate structural analysis and advanced fatigue life prediction tools. Each advance is quickly taken advantage of to produce safer, more reliable, more cost effective, and better performing products. In other words, as the envelope is expanded, components are then designed to operate just as close to the newly expanded envelope as they were to the initial one. The problem is perennial. The economic importance of addressing structural durability issues early in the design process is emphasized. Tradeoffs with performance, cost, and legislated restrictions are pointed out. Several aspects of structural durability of advanced systems, advanced materials and advanced fatigue life prediction methods are presented. Specific items include the basic elements of durability analysis, conventional designs, barriers to be overcome for advanced systems, high-temperature life prediction for both creep-fatigue and thermomechanical fatigue, mean stress effects, multiaxial stress-strain states, and cumulative fatigue damage accumulation assessment.

  7. High Temperature Fatigue Crack Growth Behavior of Alloy 10

    NASA Technical Reports Server (NTRS)

    Gayda, John

    2001-01-01

    Methods to improve the high temperature, dwell crack growth resistance of Alloy 10, a high strength, nickel-base disk alloy, were studied. Two approaches, heat treat variations and composition modifications, were investigated. Under the heat treat approach, solution temperature, cooling rates, and stabilization, were studied. It was found that higher solution temperatures, which promote coarser grain sizes, coupled with a 1550 F stabilization treatment were found to significantly reduce dwell crack growth rates at 1300 F Changes in the niobium and tantalum content were found to have a much smaller impact on crack growth behavior. Lowering the niobium:tantalum ratio did improve crack growth resistance and this effect was most pronounced for coarse grain microstructures. Based on these findings, a coarse grain microstructure for Alloy 10 appears to be the best option for improving dwell crack growth resistance, especially in the rim of a disk where temperatures can reach or exceed 1300 T. Further, the use of advanced processing technologies, which can produce a coarse grain rim and fine grain bore, would be the preferred option for Alloy 10 to obtain the optimal balance between tensile, creep, and crack growth requirements for small gas turbine engines.

  8. Small Fatigue Crack Growth and Failure Mode Transitions in a Ni-Base Superalloy at Elevated Temperature (Preprint)

    DTIC Science & Technology

    2010-02-01

    AFRL-RX-WP-TP-2010-4070 SMALL FATIGUE CRACK GROWTH AND FAILURE MODE TRANSITIONS IN A Ni-BASE SUPERALLOY AT ELEVATED TEMPERATURE (Preprint...CRACK GROWTH AND FAILURE MODE TRANSITIONS IN A Ni-BASE SUPERALLOY AT ELEVATED TEMPERATURE (Preprint) 5a. CONTRACT NUMBER IN HOUSE 5b. GRANT...by ANSI Std. Z39-18 Page 1 of 28 Small Fatigue Crack Growth and Failure Mode Transitions in a Ni-Base Superalloy at Elevated Temperature M. J

  9. Retardation of fatigue crack growth in ceramics by glassy ligaments: A rationalization

    SciTech Connect

    Ramamurty, U.

    1996-04-01

    In high-temperature fatigue crack growth (FCG) experiments on ceramic materials containing amorphous grain boundary phases, the crack growth rates under cyclic loads were observed to be lower than those predicted solely on the basis of crack growth velocities measured under static loads. In this paper, a rationalization was offered for such a behavior by means of a phenomenological glass-bridging model which takes the relaxation behavior of glass into account. In ceramics which exhibit subcritical crack growth through cavitation ahead of the crack tip, the maximum stress intensity factor of the fatigue cycle required to initiate FCG was observed to be always greater than or equal to the threshold stress intensity factor for crack growth under sustained far-field loads. This trend was also explained with the aid of the glass-bridging model and invoking the equivalence between bridging and damage zones. The elevated temperature FCG behavior of nitride-based ceramics which exhibit grain bridging in the wake during crack propagation was discussed and contrasted with oxide-based ceramics which show glass bridging.

  10. High Speed Research Program Sonic Fatigue

    NASA Technical Reports Server (NTRS)

    Rizzi, Stephen A. (Technical Monitor); Beier, Theodor H.; Heaton, Paul

    2005-01-01

    The objective of this sonic fatigue summary is to provide major findings and technical results of studies, initiated in 1994, to assess sonic fatigue behavior of structure that is being considered for the High Speed Civil Transport (HSCT). High Speed Research (HSR) program objectives in the area of sonic fatigue were to predict inlet, exhaust and boundary layer acoustic loads; measure high cycle fatigue data for materials developed during the HSR program; develop advanced sonic fatigue calculation methods to reduce required conservatism in airframe designs; develop damping techniques for sonic fatigue reduction where weight effective; develop wing and fuselage sonic fatigue design requirements; and perform sonic fatigue analyses on HSCT structural concepts to provide guidance to design teams. All goals were partially achieved, but none were completed due to the premature conclusion of the HSR program. A summary of major program findings and recommendations for continued effort are included in the report.

  11. A structural health monitoring fastener for tracking fatigue crack growth in bolted metallic joints

    NASA Astrophysics Data System (ADS)

    Rakow, Alexi Schroder

    Fatigue cracks initiating at fastener hole locations in metallic components are among the most common form of airframe damage. The fastener hole site has been surveyed as the second leading initiation site for fatigue related accidents of fixed wing aircraft. Current methods for inspecting airframes for these cracks are manual, whereby inspectors rely on non-destructive inspection equipment or hand-held probes to scan over areas of a structure. Use of this equipment often demands disassembly of the vehicle to search appropriate hole locations for cracks, which elevates the complexity and cost of these maintenance inspections. Improved reliability, safety, and reduced cost of such maintenance can be realized by the permanent integration of sensors with a structure to detect this damage. Such an integrated system of sensors would form a structural health monitoring (SHM) system. In this study, an Additive, Interleaved, Multi-layer Electromagnetic (AIME) sensor was developed and integrated with the shank of a fastener to form a SHM Fastener, a new SHM technology targeted at detection of fastener hole cracks. The major advantages of the SHM Fastener are its installation, which does not require joint layer disassembly, its capability to detect inner layer cracks, and its capability to operate in a continuous autonomous mode. Two methods for fabricating the proposed SHM Fastener were studied. The first option consisted of a thin flexible printed circuit film that was bonded around a thin metallic sleeve placed around the fastener shank. The second option consisted of coating sensor materials directly to the shank of a part in an effort to increase the durability of the sensor under severe loading conditions. Both analytical and numerical models were developed to characterize the capability of the sensors and provide a design tool for the sensor layout. A diagnostic technique for crack growth monitoring was developed to complete the SHM system, which consists of the

  12. Biaxial Fatigue Crack Growth Behavior in Aluminum Alloy 5083-H116 Under Ambient Laboratory and Saltwater Environments

    NASA Astrophysics Data System (ADS)

    Perel, V. Y.; Misak, H. E.; Mall, S.; Jain, V. K.

    2015-04-01

    Crack growth of aluminum alloy 5083 was investigated when subjected to the in-plane biaxial tension-tension fatigue with stress ratio of 0.5 under ambient laboratory and saltwater environments. Cruciform specimens with a center hole, containing a notch and precrack at 45° to the specimen's arms, were tested in a biaxial fatigue test machine. Two biaxiality ratios, λ = 1 and λ = 1.5, were studied. For λ = 1, crack propagated along a straight line collinearly with the precrack, while for λ = 1.5 case, the crack path was curved and non-collinear with the precrack. Uniaxial fatigue tests were also conducted. Crack growth rates were faster under the biaxiality fatigue in comparison to uniaxial fatigue at a given crack driving force (Δ K I or Δ G) in both environments. Further, an increase in biaxiality ratio increased the crack growth rate, i.e., faster for λ = 1.5 case than λ = 1 case. Both biaxial fatigue and saltwater environment showed detrimental effects on the fatigue crack growth resistance of 5083, and its combination is highly detrimental when compared to uniaxial fatigue.

  13. Monitoring of hidden fatigue crack growth in multi-layer aircraft structures using high frequency guided waves

    NASA Astrophysics Data System (ADS)

    Chan, H.; Masserey, B.; Fromme, P.

    2015-03-01

    Varying loading conditions of aircraft structures result in stress concentration at fastener holes, where multi-layered components are connected, potentially leading to the development of hidden fatigue cracks in inaccessible layers. High frequency guided waves propagating along the structure allow for the structural health monitoring (SHM) of such components, e.g., aircraft wings. Experimentally the required guided wave modes can be easily excited using standard ultrasonic wedge transducers. However, the sensitivity for the detection of small, potentially hidden, fatigue cracks has to be ascertained. The type of multi-layered model structure investigated consists of two adhesively bonded aluminum plate-strips with a sealant layer. Fatigue experiments were carried out and the growth of fatigue cracks at the fastener hole in one of the metallic layers was monitored optically during cyclic loading. The influence of the fatigue cracks of increasing size on the scattered guided wave field was evaluated. The sensitivity and repeatability of the high frequency guided wave modes to detect and monitor the fatigue crack growth was investigated, using both standard pulse-echo equipment and a laser interferometer. The potential for hidden fatigue crack growth monitoring at critical and difficult to access fastener locations from a stand-off distance was ascertained. The robustness of the methodology for practical in situ ultrasonic monitoring of fatigue crack growth is discussed.

  14. The Growth of Naturally-Generated Small Fatigue Cracks in a Nickel-Base Single-Crystal Superalloy

    NASA Astrophysics Data System (ADS)

    Yandt, Scott A.

    An experimental and analytical study on the formation and growth small fatigue cracks embedded in a notch in single-crystal superalloy has been investigated. The experimental program consisted of 12 constant amplitude fatigue tests performed on single-edge notch (SEN) fatigue specimens oriented with the loading axis along [010] and with a notch factor of 2.7. The fatigue