Crack initiation under generalized plane strain conditions
Shum, D.K.M.; Merkle, J.G.
1991-01-01
A method for estimating the decrease in crack-initiation toughness, from a reference plane strain value, due to positive straining along the crack front of a circumferential flaw in a reactor pressure vessel is presented in this study. This method relates crack initiation under generalized plane strain conditions with material failure at points within a distance of a few crack-tip-opening displacements ahead of a crack front, and involves the formulation of a micromechanical crack-initiation model. While this study is intended to address concerns regarding the effects of positive out-of- plane straining on ductile crack initiation, the approach adopted in this work can be extended in a straightforward fashion to examine conditions of macroscopic cleavage crack initiation. Provided single- parameter dominance of near-tip fields exists in the flawed structure, results from this study could be used to examine the appropriateness of applying plane strain fracture toughness to the evaluation of circumferential flaws, in particular to those in ring-forged vessels which have no longitudinal welds. In addition, results from this study could also be applied toward the analysis of the effects of thermal streaming on the fracture resistance of circumferentially oriented flaws in a pressure vessel. 37 refs., 8 figs., 1 tab.
Analysis of crack closure under plane strain conditions
NASA Technical Reports Server (NTRS)
Fleck, Norman A.; Newman, James C., Jr.
1988-01-01
The phenomenon of plasticity-induced crack closure is associated with the development of residual material on the flanks of an advancing fatigue crack. While it is easy to see that this residual material can come from the side faces of a specimen under plane stress conditions, it is difficult to discover the origin of this extra volume of material on the crack flanks when it is assumed that plane deformations occur and plastic flow is incompressible. The purpose of this paper is to determine whether plasticity-induced fatigue crack closure occurs in an elastic-perfectly plastic body under plane strain conditions.
Analysis of crack closure under plane strain conditions
NASA Technical Reports Server (NTRS)
Fleck, N. A.; Newman, J. C.
1986-01-01
The phenomenon of plasticity-induced crack closure is associated with the development of residual material on the flanks of an advancing fatigue crack. While it is easy to see that this residual material can come from the side faces of a specimen under plane stress conditions, it is difficult to discover the origin of this extra volume of material on the crack flanks when it is assumed that plane deformations occur and plastic flow is incompressible. The purpose of this paper is to determine whether plasticity-induced fatigue crack closure occurs in an elastic-perfectly plastic body under plane strain conditions.
Comparison of experiment and theory for elastic-plastic plane strain crack growth
Hermann, L; Rice, J R
1980-02-01
Recent theoretical results on elastic-plastic plane strain crack growth, and experimental results for crack growth in a 4140 steel in terms of the theoretical concepts are reviewed. The theory is based on a recent asymptotic analysis of crack surface opening and strain distributions at a quasi-statically advancing crack tip in an ideally-plastic solid. The analysis is incomplete in that some of the parameters which appear in it are known only approximately, especially at large scale yielding. Nevertheless, it suffices to derive a relation between the imposed loading and amount of crack growth, prior to general yielding, based on the assumption that a geometrically similar near-tip crack profile is maintained during growth. The resulting predictions for the variation of J with crack growth are found to fit well to the experimental results obtained on deeply cracked compact specimens.
Tests and analyses for fully plastic fracture mechanics of plane strain mode I crack growth
McClintock, F.A.; Parks, D.M.; Kim, Y.J.
1995-12-31
Under monotonic loading, structures should ideally be ductile enough to provide continued resistance during crack growth. For fully plastic crack growth in low strength alloys, existing asymptotic solutions for elastic-plastic growing cracks are not applicable because they reach the fracture strain only in regions small compared to the inhomogeneities of the actual fracture process. For the limiting case of non-hardening fully-plastic plane strain crack growth, in a number of geometries and loadings the near-tip fields are characterized in terms of three parameters: an effective angle 2{theta}{sub s} between a pair of slip planes, and the normal stress {sigma}{sub s} and the increment of displacement {delta}u{sub s} across the planes. This three-parameter characterization is in contrast to the one- or two-parameter (K or J and T or Q) characterization in linear or non-linear elastic fracture mechanics. These {theta}{sub s}, {sigma}{sub s}, and {delta}u{sub s} parameters are found form the far-field geometries and loadings through slip line fields or least upper bound analyses based on circular arcs. The resulting crack growth, in terms of the crack tip opening angle (CTOA), is a function of {theta}{sub s}, {sigma}{sub s}, and the material. The geometry of the crack growing between two moving slip planes emanating from its tip reduces this function to the critical fracture shear strain left behind the slip planes, {gamma}f, as a function of {sigma}{sub s}. {gamma}f({sigma}{sub s}) is found theoretically from a hole initiation and growth model. It is also found from preliminary fully plastic crack growth experiments on unequally grooved specimens with fixed-grip extension or 4-point bending of a 1018 CF steel.
NASA Astrophysics Data System (ADS)
Begley, Matthew R.; Creton, Costantino; McMeeking, Robert M.
2015-11-01
A general asymptotic plane strain crack tip stress field is constructed for linear versions of neo-Hookean materials, which spans a wide variety of special cases including incompressible Mooney elastomers, the compressible Blatz-Ko elastomer, several cases of the Ogden constitutive law and a new result for a compressible linear neo-Hookean material. The nominal stress field has dominant terms that have a square root singularity with respect to the distance of material points from the crack tip in the undeformed reference configuration. At second order, there is a uniform tension parallel to the crack. The associated displacement field in plane strain at leading order has dependence proportional to the square root of the same coordinate. The relationship between the amplitude of the crack tip singularity (a stress intensity factor) and the plane strain energy release rate is outlined for the general linear material, with simplified relationships presented for notable special cases.
Nobrega, B.N.
1984-01-01
The segmented expanding mandrel test (SEMT) method is generally regarded as a good laboratory simulator of pellet-cladding interactions (PCI) in LWR fuel rods. Yet it does not reproduce the low strain failures in Zircaloy cladding typical of PCI-failed fuel elements and commonly observed in other types of laboratory specimens. This investigation addressed this apparent inconsistency. Iodine-stress corrosion cracking (I-SCC) of cold worked, unirradiated Zircaloy-2 cladding was induced in three different types of tubing specimens (known as regular, thin-wall, and chamfered) in a modified SEMT apparatus designed to test mechanical conditions that could lead to slow strain failures. Only the chamfered sample, which has been shown to be subjected to more nearly plane strain conditions than either of the other two specimen types, failed consistently at low (0.8%) total diametral strains in good agreement with in-reactor failure data. Such conditions were numerically and experimentally quantified by means of finite element calculational models and local strain measurements. The numerical analyses and strain measurements provide valuable insight into the PCI simulating power of the segmented expanding mandrel test and its experimental limitations. Failure-strain results for chamfered barrier claddings were obtained and compared with available literature data. The improved I-SCC resistance of this type of cladding was confirmed but the failure strains were significantly lower than reported for regular barrier tubes.
Supersonic crack growth in a solid of upturn stress?strain relation under anti-plane shear
NASA Astrophysics Data System (ADS)
Guo, Gaofeng; Yang, Wei; Huang, Y.
2003-11-01
This paper examines, from the prospect of continuum analysis, the possibility for a supersonic crack growth in a solid with an upturn stress-strain relation. The stress has a linear-upturn power-law relation with the strain, resulting in an elastic modulus, and consequently a wave speed, that increase with the strain. Though appearing to be "supersonic", the local wave speed in the crack tip vicinity of the solid with a sufficient upturn stress-strain relation exceeds the crack extension speed. A pre-request for such a supersonic crack growth is the storage of sufficient deformation energy within the solid to nurse the energy flux drawn to the crack tip that extends at an "apparent supersonic" speed. The idea is demonstrated for the simplest case, the anti-plane shear. We examine the steady-state supersonic crack growth in a hyperelastic material. The governing equation is elliptical in the crack tip vicinity but hyperbolic elsewhere. The boundary between two regions is determined with a certain extent. An asymptotic solution is constructed within the super-hardening zone. The solution connects to the hyperbolic radiation strips by weak discontinuity boundaries and to the pre-stressed frontal field by a strong discontinuity boundary.
Bonded half planes containing an arbitrarily oriented crack
NASA Technical Reports Server (NTRS)
Erdogan, F.; Aksogan, O.
1973-01-01
The plane elastostatic problem for two bonded half planes containing an arbitrarily oriented crack in the neighborhood of the interface is considered. Using Mellin transforms, the problem is formulated as a system of singular integral equations. The equations are solved for various crack orientations, material combinations, and external loads. The numerical results given include the stress intensity factors, tHe strain energy release rates, and tHe probable cleavage angles giving the direction of crack propagation.
The crack problem for a nonhomogeneous plane
NASA Technical Reports Server (NTRS)
Delale, F.; Erdogan, F.
1983-01-01
The plane elasticity problem for a nonhomogeneous medium containing a crack is considered. It is assumed that the Poisson's ratio of the medium is constant and the Young's modulus E varies exponentially with the coordinate parallel to the crack. First the half plane problem is formulated and the solution is given for arbitrary tractions along the boundary. Then the integral equation for the crack problem is derived. It is shown that the integral equation having the derivative of the crack surface displacement as the density function has a simple Cauchy type kernel. Hence, its solution and the stresses around the crack tips have the conventional square root singularity. The solution is given for various loading conditions. The results show that the effect of the Poisson's ratio and consequently that of the thickness constraint on the stress intensity factors are rather negligible.
Crack problem for a nonhomogeneous plane
Delale, F.; Erdogan, F.
1983-09-01
This study considers the plane elasticity problem for a nonhomogeneous medium containing a crack. It is assumed that the Poisson's ratio of the medium is constant and the Young's modulus E varies exponentially with the coordinate parallel to the crack. First the half plane problem is formulated and the solution is given for arbitrary tractions along the boundary. Then, the integral equation for the crack problem is derived. It is shown that the integral equation having the derivative of the crack surface displacement as the density function has a simple Cauchy-type kernel. Hence, its solution and the stresses around the crack tips have the conventional square-root singularity. The solution is given for various loading conditions. The results show that the effect of the Poisson's ratio and consequently that of the thickness constraint on the stress intensity factors are rather negligible. 14 references.
The crack problem for a nonhomogeneous plane
NASA Technical Reports Server (NTRS)
Delale, F.; Erdogan, F.
1982-01-01
The plane elasticity problem for a nonhomogeneous medium containing a crack is considered. It is assumed that the Poisson's ratio of the medium is constant and the Young's modulus E varies exponentially with the coordinate parallel to the crack. First the half plane problem is formulated and the solution is given for arbitrary tractions along the boundary. Then the integral equation for the crack problem is derived. It is shown that the integral equation having the derivative of the crack surface displacement as the density function has a simple Cauchy type kernel. Hence, its solution and the stresses around the crack tips have the conventional square root singularity. The solution is given for various loading conditions. The results show that the effect of the Poisson's ratio and consequently that of the thickness constraint on the stress intensity factors are rather negligible.
A plane stress finite element model for elastic-plastic mode I/II crack growth
NASA Astrophysics Data System (ADS)
James, Mark Anthony
A finite element program has been developed to perform quasi-static, elastic-plastic crack growth simulations. The model provides a general framework for mixed-mode I/II elastic-plastic fracture analysis using small strain assumptions and plane stress, plane strain, and axisymmetric finite elements. Cracks are modeled explicitly in the mesh. As the cracks propagate, automatic remeshing algorithms delete the mesh local to the crack tip, extend the crack, and build a new mesh around the new tip. State variable mapping algorithms transfer stresses and displacements from the old mesh to the new mesh. The von Mises material model is implemented in the context of a non-linear Newton solution scheme. The fracture criterion is the critical crack tip opening displacement, and crack direction is predicted by the maximum tensile stress criterion at the crack tip. The implementation can accommodate multiple curving and interacting cracks. An additional fracture algorithm based on nodal release can be used to simulate fracture along a horizontal plane of symmetry. A core of plane strain elements can be used with the nodal release algorithm to simulate the triaxial state of stress near the crack tip. Verification and validation studies compare analysis results with experimental data and published three-dimensional analysis results. Fracture predictions using nodal release for compact tension, middle-crack tension, and multi-site damage test specimens produced accurate results for residual strength and link-up loads. Curving crack predictions using remeshing/mapping were compared with experimental data for an Arcan mixed-mode specimen. Loading angles from 0 degrees to 90 degrees were analyzed. The maximum tensile stress criterion was able to predict the crack direction and path for all loading angles in which the material failed in tension. Residual strength was also accurately predicted for these cases.
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.
Turbulent Plane Wakes Subjected to Successive Strains
NASA Technical Reports Server (NTRS)
Rogers, Michael M.
2003-01-01
Six direct numerical simulations of turbulent time-evolving strained plane wakes have been examined to investigate the response of a wake to successive irrotational plane strains of opposite sign. The orientation of the applied strain field has been selected so that the flow is the time-developing analogue of a spatially developing wake evolving in the presence of either a favourable or an adverse streamwise pressure gradient. The magnitude of the applied strain rate a is constant in time t until the total strain e(sup at) reaches about four. At this point, a new simulation is begun with the sign of the applied strain being reversed (the original simulation is continued as well). When the total strain is reduced back to its original value of one, yet another simulation is begun with the sign of the strain being reversed again back to its original sign. This process is done for both initially "favourable" and initially "adverse" strains, providing simulations for each of these strain types from three different initial conditions. The evolution of the wake mean velocity deficit and width is found to be very similar for all the adversely strained cases, with both measures rapidly achieving exponential growth at the rate associated with the cross-stream expansive strain e(sup at). In the "favourably" strained cases, the wake widths approach a constant and the velocity deficits ultimately decay rapidly as e(sup -2at). Although all three of these cases do exhibit the same asymptotic exponential behaviour, the time required to achieve this is longer for the cases that have been previously adversely strained (by at approx. equals 1). These simulations confirm the generality of the conclusions drawn in Rogers (2002) regarding the response of plane wakes to strain. The evolution of strained wakes is not consistent with the predictions of classical self-similar analysis; a more general equilibrium similarity solution is required to describe the results. At least for the cases
On the modified Westergaard equations for certain plane crack problems
NASA Technical Reports Server (NTRS)
Eftis, J.; Liebowitz, H.
1972-01-01
An error in Westergaard's equation for a certain class of plane crack problems, originally pointed out by Sih, is briefly discussed. The source of the difficulty is traced to an oversight in an earlier work by MacGregor, upon whose work Westergaard based his equations. Several examples of interest illustrating the consequences of the necessary correction to these equations are given.
Biaxial load effects on the crack border elastic strain energy and strain energy rate
NASA Technical Reports Server (NTRS)
Eftis, J.; Subramonian, N.; Liebowitz, H.
1977-01-01
The validity of the singular solution (first term of a series representation) is investigated for the crack tip stress and displacement field in an infinite sheet with a flat line crack with biaxial loads applied to the outer boundaries. It is shown that if one retains the second contribution to the series approximations for stress and displacement in the calculation of the local elastic strain energy density and elastic strain energy rate in the crack border region, both these quantities have significant biaxial load dependency. The value of the J-integral does not depend on the presence of the second term of the series expansion for stress and displacement. Thus J(I) is insensitive to the presence of loads applied parallel to the plane of the crack.
NASA Technical Reports Server (NTRS)
Shbeeb, N.; Binienda, W. K.; Kreider, K.
1999-01-01
The driving forces for a generally oriented crack embedded in a Functionally Graded strip sandwiched between two half planes are analyzed using singular integral equations with Cauchy kernels, and integrated using Lobatto-Chebyshev collocation. Mixed-mode Stress Intensity Factors (SIF) and Strain Energy Release Rates (SERR) are calculated. The Stress Intensity Factors are compared for accuracy with previously published results. Parametric studies are conducted for various nonhomogeneity ratios, crack lengths. crack orientation and thickness of the strip. It is shown that the SERR is more complete and should be used for crack propagation analysis.
Opening and closing of cracks at high cyclic strains
NASA Technical Reports Server (NTRS)
Iyyer, N. S.; Dowling, N. E.
1986-01-01
The closure behavior of cracks of different length and at different cyclic strain levels (ranging from predominantly elastic to grossly plastic strains) was studied to observe the effect of residual crack-tip plasticity on crack closure. Cracks were initiated either naturally or artificially (from electric discharge machining pits) in uniaxial test specimens of strengthened alloy steel AISI 4340 with a grain size of 0.016 mm. It was found that, at high strains, cracks closed only when the lowest stress level in the cycle was approached. The stress or the strain opening level depended upon the exact point along the crack length where the observations were made. As the plastic deformation increased, the relative crack opening level was found to decrease and approach the value of stress ratio R. The experimental results were compared with those of three analytical models of crack closure and opening, demonstrating the limitations of the currently available elastic-plastic crack growth analysis.
Axial strain redistribution resulting from off-axis ply cracking in polymer composites
NASA Astrophysics Data System (ADS)
Katerelos, Dionisis G.; Galiotis, Costas
2004-10-01
The initial mode of damage in multidirectional composites is the accumulation of matrix cracks in the off-axis plies. Remote laser Raman spectroscopy, using aramid (Kevlar 49®) fibers as Raman strain sensors, has been employed to monitor the local strains in cracked crossply composites. The strain magnification in the 0° ply caused by 90° matrix cracking are measured at different levels of loading. A relationship between the values of strain magnification and the distance between the position of the fibers sensors vis-à-vis the crack front has been established. The effect of damage progression within the 0° plies on the remaining/surviving glass fibers is modeled satisfactorily and verified against predictions on the basis of a generalized plane strain model by assuming that the 90° ply "expands" in relative size against the 0° ply thickness.
NASA Technical Reports Server (NTRS)
Bassani, J. L.; Erdogan, F.
1979-01-01
The antiplane shear problem for two bonded dissimilar half planes containing a semi-infinite crack or two arbitrarily located collinear cracks is considered. For the semi-infinite crack the problem is solved for a concentrated wedge load and the stress intensity factor and the angular distribution of stresses are calculated. For finite cracks the problem is reduced to a pair of integral equations. Numerical results are obtained for cracks fully imbedded in a homogeneous medium, one crack tip touching the interface, and a crack crossing the interface for various crack angles.
NASA Technical Reports Server (NTRS)
Bassani, J. L.; Erdogan, F.
1978-01-01
The antiplane shear problem for two bonded dissimilar half planes containing a semi-infinite crack or two arbitrarily located collinear cracks was considered. For the semi-infinite crack the problem was solved for a concentrated wedge load and the stress intensity factor and the angular distribution of stresses were calculated. For finite cracks the problem was reduced to a pair of integral equations. Numerical results were obtained for cracks fully imbedded in a homogeneous medium, one crack tip touching the interface, and a crack crossing the interface for various crack angles.
Dynamic crack propagation in elastic-perfectly plastic solids under plane stress conditions
NASA Astrophysics Data System (ADS)
Deng, Xiaomin; Rosakis, Ares J.
THE phenomenon of steady-state dynamic crack propagation in elastic-perfectly plastic solids under mode I plane stress, small-scale yielding conditions is investigated numerically. An Eulerian finite element scheme is employed. The materials are assumed to obey the von Mises yield criterion and the associated flow rule. The ratio of the crack tip plastic zone size to that of the element nearest to the crack tip is of the order of 1.6 × 10 4. Two subjects of general interest are discussed. These are the asymptotic structure of the crack tip stress and deformation fields, and the appropriateness of a crack growth fracture criterion based on the far-field dynamic stress intensity factor. The crack-line solution by ACHENBACH and LI (Report NU-SML-TR-No. 84-1, Dept. of Civil Engineering, Northwestern University, Evanston, IL 60201, 1984a; in Fundamentals of Deformation and Fracture (edited by B.A. Brilby et al.). Cambridge University Press, 1984b) is discussed and compared to the numerical solution. The results of this study strongly indicate that the crack tip strain and velocity fields possess logarithmic singularities, which is consistent with the assumptions in the asymptotic analysis by Gao ( Int. J. Fracture34, 111, 1987). However, it is revealed that the crack tip field variations in Gao's solution present features often contrary to the numerical findings. To this end, a preliminary asymptotic analysis is performed in an effort to resolve certain issues. Finally, the critical plastic strain criterion ( MCCLINTOCK and IRWIN, in Fracture Toughness Testing and Its Applications, ASTM STP 381, p. 84, 1964) is adopted to obtain theoretical relations between the critical dynamic stress intensity factor and the crack propagation speed. These relations are found to agree well with experimental measurements by Rosakis et al. ( J. Mech. Phys. Solids32, 443, 1984) and by ZEHNDER and ROSAKIS ( Int. J. Fracture, to appear 1990), performed on thin 4340 steel plates whose
Strain oxidation cracking of austenitic stainless steels at 610 C
Calvar, M. Le; Scott, P.M.; Magnin, T.; Rieux, P.
1998-02-01
Strain oxidation cracking of both forged and welded austenitic stainless steels (SS) was studied. Creep and slow strain rate tests (SSRT) were performed in vacuum, air, and a gas furnace environment (air + carbon dioxide [CO{sub 2}] + water [H{sub 2}O]). Results showed cracking was environmentally dependent. Almost no cracking was observed in vacuum, whereas intergranular cracking was observed with increasing severity in passing from an air to a gas furnace environment. The most severe cracking was associated with formation of a less protective film formed in the gas furnace environment (air: haematite-like M{sub 2}O{sub 3} oxide; gas furnace environment: spinel M{sub 3}O{sub 4} oxide). Cracking depended strongly on the carbon content and the sensitization susceptibility of the material: the higher the carbon content, the more susceptible the alloy. This cracking was believed to be similar to other oxidation-induced cracking phenomena.
Finite strain crack tip fields in soft incompressible elastic solids.
Krishnan, Venkat R; Hui, Chung Yuen; Long, Rong
2008-12-16
A finite element model (FEM) is used to study the behavior of the large deformation field near the tip of a crack in a soft incompressible plane stress fracture specimen loaded in mode I. Results are obtained for the case of a neo-Hookean solid (ideal rubber) and a hyperelastic solid with exponentially hardening behavior. In contrast to the predictions of linear elastic fracture mechanics (LEFM), the near tip stress fields are dominated by the opening stress which shows a 1/R singularity for the neo-Hookean material and a -1/(R ln R) singularity for the exponential hardening solid. We found very similar qualitative behavior in the near tip stress fields despite the very large difference in strain hardening behavior of the two material models. Our result shows that the near tip opening stress is controlled by the far field energy release rate for large applied loads. PMID:19053624
Intergranular Strain Evolution near Fatigue Crack Tips in Polycrystalline Metals
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.
The crack problem for a half plane stiffened by elastic cover plates
NASA Technical Reports Server (NTRS)
Delale, F.; Erdogan, F.
1981-01-01
An elastic half plane containing a crack and stiffened by a cover plate is discussed. The asymptotic nature of the stress state in the half plane around an end point of the stiffener to determine the likely orientation of a possible fracture initiation and growth was studied. The problem is formulated for an arbitrary oriented radial crack in a system of singular integral equations. For an internal crack and for an edge crack, the problem is solved and the stress intensity factors at the crack tips and the interface stress are calculated. A cracked half plane with two symmetrically located cover plates is also considered. It is concluded that the case of two stiffeners appears to be more severe than that of a single stiffener.
A half plane and a strip with an arbitrarily located crack
NASA Technical Reports Server (NTRS)
Erdogan, F.; Arin, K.
1973-01-01
A technique is presented for dealing with the problem of an elastic domain containing an arbitrarily oriented internal crack. The problem is formulated as a system of integral equations for a fictitious layer of body forces imbedded in the plane along a closed smooth curve encircling the original domain. The problems of a half plane with a crack in the neighborhood of its free boundary and of an infinite strip containing a symmetrically located internal crack with an arbitrary orientation are considered as examples. In each case the stress intensity factors are computed and are given as functions of the crack angle.
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).
Panontin, T.L.; Sheppard, S.D.
1997-12-01
Large-strain, 3-D finite element analyses with incremental plasticity were performed for a variety of crack geometries to study local crack-tip stress-strain fields and associated global fracture parameters under conditions of large-scale yielding. The geometries analyzed include thin, single-edge crack tension, single-edge crack bending, and center-crack tension fracture specimens with varying crack depth (a/W) ratios. Two materials were investigated: a high-hardening, low-strength steel and a moderate-hardening, high-strength steel. Mesh refinement studies were performed to ensure convergence of the finite element predictions. The studies examine the effects of in-plane crack-tip element size, initial crack-tip radius size, and number of through-thickness layers on predicted distributions of crack-tip stress and plastic strain and predicted values of the J-integral and CTOD. In addition, the finite element predictions of specimen behavior were verified experimentally by direct measurements, namely load displacement, load longitudinal strain, and load CTOS, made during and following testing of the fracture specimens. Representative results of the finite element analyses are presented and compared to previously published data where pertinent. Results from the mesh refinement studies and the verification testing are shown. Predicted trends among the specimens and materials in local distributions of crack-tip plastic strain, triaxiality, and opening stress as well as in global parameters, J-integral and m-factor, are discussed.
Strain-age cracking in Rene 41 alloy
NASA Technical Reports Server (NTRS)
Prager, M.; Thompson, E. G.
1969-01-01
Weldability test determines the effects of material and process variables on the occurrence of strain-age cracking, and demonstrates effective and practical means for its reduction. Studies consist of tensile, impact, and stress-rupture tests.
Characterization of oriented cracks with differential strain analysis
NASA Technical Reports Server (NTRS)
Siegfried, R.; Simmons, G.
1978-01-01
Linear strain of a rock sample as a function of hydrostatic pressure can be measured with a precision of 2 millionths. Such high-precision data for three orthogonal directions allow calculation of the distribution function for the porosity due to cracks' closing completely at a given pressure. Such data for at least six independent directions yield the zero-pressure strain tensor due to cracks' closing completely at a given pressure. The principal values and axes of this tensor distribution function provide information about the orientation of cracks as a function of closure pressure. The mathematical basis for the technique is developed, and the technique is illustrated with differential strain data for two samples, the Westerly (Rhode Island) granite and the Twin Sisters (Washington) dunite. Strain-tensor calculations reveal that each of these samples has a different type of anisotropic crack distribution.
Out-of-plane deviation of a mode I+III crack encountering a tougher obstacle
NASA Astrophysics Data System (ADS)
Leblond, Jean-Baptiste; Ponson, Laurent
2016-07-01
One possible explanation of out-of-plane deviations of cracks loaded in mode I+III was suggested by Gao and Rice in 1986. These authors noted that small in-plane undulations of the crack front, arising from fluctuations of the fracture toughness, should generate a small local mode-II component, causing the crack to depart from planarity. Their analysis is completed here by explicitly calculating the evolution in time of the out-of-plane deviation of a mode-I+III crack encountering a tougher obstacle. The calculation is based on (i) first-order formulae for the stress intensity factors of a crack slightly perturbed within and out of its plane; and (ii) a "double" propagation criterion combining a Griffith condition on the local energy-release rate and a Goldstein-Salganik condition on the local stress intensity factor of mode II. It is predicted that the crack must evolve toward a stationary state, wherein the orthogonal distance from the average fracture plane to the perturbed crack front is constant outside the obstacle and varies linearly across it. We hope that this theoretical prediction will encourage comparison with experiments, and propose a fracture test involving propagation of a mode-I+III crack through a 3D-printed specimen containing some designed obstacle. xml:lang="fr"
Mechanics of fatigue crack closure
NASA Technical Reports Server (NTRS)
Newman, J. C., Jr. (Editor); Elber, Wolf (Editor)
1988-01-01
Papers are presented on plasticity induced crack closure, crack closure in fatigue crack growth, the dependence of crack closure on fatigue loading variables, and a procedure for standardizing crack closure levels. Also considered are a statistical approach to crack closure determination, the crack closure behavior of surface cracks under pure bending, closure measurements on short fatigue cracks, and crack closure under plane strain conditions. Other topics include fatigue crack closure behavior at high stress ratios, the use of acoustic waves for the characterization of closed fatigue cracks, and the influence of fatigue crack wake length and state of stress on crack closure.
The Stress-Strain Condition Estimation of Detail in Crack Tip by Integral Strain Gauges
NASA Astrophysics Data System (ADS)
Syzrantsev, V.; Syzrantseva, K.
2016-04-01
The paper considers the task of stress-strain condition calculation of experimental sample in fatigue crack tip on weld boundary at its cyclic deforming. For this task decision authors use the information obtained by original means of cyclic strains measurement: Integral Strain Gauges. The results of carried experimental researches are compared with data of stress-strain condition estimation of detail in crack tip calculated by Finish Element Method.
Numerical simulation of out-of-plane distortion fatigue crack growth in bridge girders
NASA Astrophysics Data System (ADS)
MIller, Paula A.
Aging of the United States infrastructure systems has resulted in the degradation of many operational bridge structures throughout the country. Structural deficiencies can result from material fatigue caused by cyclical loadings leading to localized structural damage. While fatigue crack growth is viewed as a serviceability problem, unstable crack growth can compromise the integrity of the structure. Multi-girder bridges designed with transverse cross bracing systems can be prone to distortion fatigue at unstiffened web gaps. Cracking is exhibited within this fatigue prone region from the application of cyclical multi-mode loadings. Focus of fatigue analysis has largely been directed at pure Mode I loading through the development of AASHTO fatigue classifications for crack initiation and the Paris Law for crack propagation. Numerical modeling approaches through the ABAQUS Extended Finite Element Method offers a unique avenue in which this detail can be assessed. Finite element simulations were developed to first evaluate the applicability of the Paris Law crack propagation under multi-mode loading against experimental data. Following the validation, fatigue crack growth in plate girders with various web gap sizes was assessed due to mixed-mode loadings. Modeling results showed enlargement of horizontal initial crack lengths within stiffer web gap regions arrested crack development. Crack directionality was also seen to change as initial crack lengths were increased. From this research it is hypothesized that deterioration of the transverse stiffener connection can be minimized by increasing the horizontal length of initial fatigue cracks. Enlargement of the crack plane away from regions of localized stress concentrations within the web gap may result in arrestment of the out-of-plane distortion induced cracking.
Near tip stress and strain fields for short elastic cracks
NASA Technical Reports Server (NTRS)
Soediono, A. H.; Kardomateas, G. A.; Carlson, R. L.
1994-01-01
Recent experimental fatigue crack growth studies have concluded an apparent anomalous behavior of short cracks. To investigate the reasons for this unexpected behavior, the present paper focuses on identifying the crack length circumstances under which the requirements for a single parameter (K(sub I) or delta K(sub I) if cyclic loading is considered) characterization are violated. Furthermore, an additional quantity, the T stress, as introduced by Rice, and the related biaxiality ratio, B, are calculated for several crack lengths and two configurations, the single-edge-cracked and the centrally-cracked specimen. It is postulated that a two-parameter characterization by K and T (or B) is needed for the adequate description of the stress and strain field around a short crack. To further verify the validity of this postulate, the influence of the third term of the Williams series on the stress, strain and displacement fields around the crack tip and in particular on the B parameter is also examined. It is found that the biaxiality ratio would be more negative if the third term effects are included in both geometries. The study is conducted using the finite element method with linearly elastic material and isoparametric elements and axial (mode I) loading. Moreover, it is clearly shown that it is not proper to postulate the crack size limits for 'short crack' behavior as a normalized ratio with the specimen width, a/w; it should instead be stated as an absolute, or normalized with respect to a small characteristic dimension such as the grain size. Finally, implications regarding the prediction of cyclic (fatigue) growth of short cracks are discussed.
Growth behavior of surface cracks in the circumferential plane of solid and hollow cylinders
NASA Technical Reports Server (NTRS)
Forman, R. G.; Shivakumar, V.
1986-01-01
Experiments were conducted to study the growth behavior of surface fatigue cracks in the circumferential plane of solid and hollow cylinders. In the solid cylinders, the fatigue cracks were found to have a circular arc crack front with specific upper and lower limits to the arc radius. In the hollow cylinders, the fatigue cracks were found to agree accurately with the shape of a transformed semiellipse. A modification to the usual nondimensionalization expression used for surface flaws in flat plates was found to give correct trends for the hollow cylinder problem.
Inclined edge crack in two bonded elastic quarter planes under out-of-plane loading
NASA Astrophysics Data System (ADS)
Hwang, E. H.; Choi, S. R.; Earmme, Y. Y.
1992-08-01
The problem of the interfacial edge crack in which the crack-inclination angle = zero is solved analytically by means of the Wiener-Hopf technique with the Mellin transform. The results are found to confirm the result by Bassani and Erdogan (1979) showing that there is no stress singularity for the interface perpendicular to the free boundary at the junction with a straight inclined interface with no crack.
Lee, S.Y.; Huang, E.-W.; Wu, W.; Liaw, P.K.; Paradowska, A.M.
2013-05-15
In-situ neutron diffraction was employed to directly measure the crystallographic-orientation-dependent (i.e. hkl) internal strains as a function of distance from the crack tip on the pre-cracked Hastelloy C-2000 compact-tension specimen. Both in-plane (IP) and through-thickness (TT) strain evolutions for various grain orientations were examined during tensile overloading and compressive underloading cycles. After overloading, underloading and their combination loadings were applied and unloaded, the significantly different (hkl) residual strain profiles were obtained in the vicinity of the crack tip. The load responses of the (200) grain orientation in both the IP and TT directions were more significant than those of any other orientations. It is suggested that the different orientation-dependent strain distributions around the crack tip are caused by the combined effects of elastic and plastic anisotropy of each (hkl) reflection upon loading and the subsequent development of residual stresses generated near the crack tip during unloading as a result of the plastic deformation. - Highlights: ► (hkl) strains are examined in situ using neutron diffraction. ► Distinct strain responses are developed around the crack tip under loading. ► The strain response of the (200) grain orientation is more significant. ► Possible mechanisms for the orientation-dependent strain responses are provided.
Dynamic crack propagation in a 2D elastic body: The out-of-plane case
NASA Astrophysics Data System (ADS)
Nicaise, Serge; Sandig, Anna-Margarete
2007-05-01
Already in 1920 Griffith has formulated an energy balance criterion for quasistatic crack propagation in brittle elastic materials. Nowadays, a generalized energy balance law is used in mechanics [F. Erdogan, Crack propagation theories, in: H. Liebowitz (Ed.), Fracture, vol. 2, Academic Press, New York, 1968, pp. 498-586; L.B. Freund, Dynamic Fracture Mechanics, Cambridge Univ. Press, Cambridge, 1990; D. Gross, Bruchmechanik, Springer-Verlag, Berlin, 1996] in order to predict how a running crack will grow. We discuss this situation in a rigorous mathematical way for the out-of-plane state. This model is described by two coupled equations in the reference configuration: a two-dimensional scalar wave equation for the displacement fields in a cracked bounded domain and an ordinary differential equation for the crack position derived from the energy balance law. We handle both equations separately, assuming at first that the crack position is known. Then the weak and strong solvability of the wave equation will be studied and the crack tip singularities will be derived under the assumption that the crack is straight and moves tangentially. Using the energy balance law and the crack tip behavior of the displacement fields we finally arrive at an ordinary differential equation for the motion of the crack tip.
The Evolution of Plane Wakes Subjected to Irrotational Strains
NASA Technical Reports Server (NTRS)
Rogers, M. R.; Rai, Man Mohan (Technical Monitor)
1995-01-01
Three direct numerical simulations of time-evolving turbulent plane wakes developing in the presence of various irrotational plane strains have been generated. A pseudospectral numerical method with up to 26 million modes is used to solve the equations in a reference frame moving with the irrotational strain. The initial condition for each simulation is taken from a previous turbulent self-similar plane wake direct numerical simulation at a velocity deficit Reynolds number, Re, of about 2000. Three different plane strains (of the same magnitude) are imposed. In the first two simulations the strain is in a plane normal to the streamwise wake direction (the two cases having strain of opposite sign); in the third the wake is compressed in the streamwise direction and stretched in the inhomogeneous cross-stream direction. The two flows that are stretched in the cross-stream direction experience an exponential increase of Re; flow visualization indicates many small-scale vortices with little or no organized large-scale structure. In the flow that is compressed in the cross-stream direction Re decays exponentially and the layer appears to be relaminarizing. The evolution of several turbulence statistics in each of these flows is examined.
Barker, D.B.; Chona, R.; Fourney, W.L.; Irwin, G.R.
1988-01-01
This report summarizes the results from a Round Robin Program that was conducted during 1983-1985 to evaluate a proposed ASTM Test Method for Crack Arrest Fracture Toughness of Ferritic Materials. The Round Robin attracted a total of twenty-seven participants from the United States, Canada, Europe and Japan, each of whom agreed to test three specimens of A514 bridge steel, three specimens of A588 bridge steel, and six specimens of A533 Grade B Class 1 reactor pressure vessel steel. Twenty-one participants completed their testing and forwarded test results for inclusion and discussion in this report. Modifications to the proposed test method that are called for in the light of the experience gained from the Round Robin are also discussed. The test method has been revised and is currently in the process of being considered by ASTM Committee E-24 on Fracture Testing as a Proposed Standard Test Method. 20 refs., 44 figs., 6 tabs.
Fracture Mechanics of Thin, Cracked Plates Under Tension, Bending and Out-of-Plane Shear Loading
NASA Technical Reports Server (NTRS)
Zehnder, Alan T.; Hui, C. Y.; Potdar, Yogesh; Zucchini, Alberto
1999-01-01
Cracks in the skin of aircraft fuselages or other shell structures can be subjected to very complex stress states, resulting in mixed-mode fracture conditions. For example, a crack running along a stringer in a pressurized fuselage will be subject to the usual in-plane tension stresses (Mode-I) along with out-of-plane tearing stresses (Mode-III like). Crack growth and initiation in this case is correlated not only with the tensile or Mode-I stress intensity factor, K(sub I), but depends on a combination of parameters and on the history of crack growth. The stresses at the tip of a crack in a plate or shell are typically described in terms of either the small deflection Kirchhoff plate theory. However, real applications involve large deflections. We show, using the von-Karman theory, that the crack tip stress field derived on the basis of the small deflection theory is still valid for large deflections. We then give examples demonstrating the exact calculation of energy release rates and stress intensity factors for cracked plates loaded to large deflections. The crack tip fields calculated using the plate theories are an approximation to the actual three dimensional fields. Using three dimensional finite element analyses we have explored the relationship between the three dimensional elasticity theory and two dimensional plate theory results. The results show that for out-of-plane shear loading the three dimensional and Kirchhoff theory results coincide at distance greater than h/2 from the crack tip, where h/2 is the plate thickness. Inside this region, the distribution of stresses through the thickness can be very different from the plate theory predictions. We have also explored how the energy release rate varies as a function of crack length to plate thickness using the different theories. This is important in the implementation of fracture prediction methods using finite element analysis. Our experiments show that under certain conditions, during fatigue crack
Real-time measurement system for in-plane displacement and strain based on vision
NASA Astrophysics Data System (ADS)
Luo, Tao; Jin, Yi; Zhu, Ye; Zhai, Chao
2013-08-01
In this paper, combining optical measurement with conventional material testing machine, a real-time in-plane displacement and strain measurement system is built, which is applied to the material testing machine. This system can realize displacement and strain measurement of a large deformation sample moreover it can observe the sample crack on line. The change of displacement field is obtained through the change of center coordinate of each point of a grid lattice in the surface of the testing sample, according to two-dimensional sort coding for the grid in the traditional automated grid method, in this paper, an improved one-dimensional code method is adopted which make calculating speed much faster and the algorithm more adaptable. The measurement of the stability and precision of this system are made using the calibration board whose position precision is about 1.5 micron. The results show that the short-time stability of this system is about 0.5micron. At last, this system is used for strain measurement in a sample tension test, and the result shows that the system can acquire in-plane displacement and strain measurement results accurately and real-time, the velocity of image processing can reach 10 frame per second; or it can observe sample crack on line and storage the test process, the max velocity of observation and storage is 100 frame per second.
Ultrafast vascular strain compounding using plane wave transmission.
Hansen, H H G; Saris, A E C M; Vaka, N R; Nillesen, M M; de Korte, C L
2014-03-01
Deformations of the atherosclerotic vascular wall induced by the pulsating blood can be estimated using ultrasound strain imaging. Because these deformations indirectly provide information on mechanical plaque composition, strain imaging is a promising technique for differentiating between stable and vulnerable atherosclerotic plaques. This paper first explains 1-D radial strain estimation as applied intravascularly in coronary arteries. Next, recent methods for noninvasive vascular strain estimation in a transverse imaging plane are discussed. Finally, a compounding technique that our group recently developed is explained. This technique combines motion estimates of subsequently acquired focused ultrasound images obtained at various insonification angles. However, because the artery moves and deforms during the multi-angle acquisition, errors are introduced when compounding. Recent advances in computational power have enabled plane wave ultrasound acquisition, which allows 100 times faster image acquisition and thus might resolve the motion artifacts. In this paper the performance of strain imaging using plane wave compounding is investigated using simulations of an artery with a vulnerable plaque and experimental data of a two-layered vessel phantom. The results show that plane wave compounding outperforms 0° focused strain imaging. For the simulations, the root mean squared error reduced by 66% and 50% for radial and circumferential strain, respectively. For the experiments, the elastographic signal-to-noise and contrast-to-noise ratio (SNR(e) and CNR(e)) increased with 2.1 dB and 3.7 dB radially, and 5.6 dB and 16.2dB circumferentially. Because of the high frame rate, the plane wave compounding technique can even be further optimized and extended to 3D in future. PMID:24484646
The plane strain tests in the PROMETRA program
NASA Astrophysics Data System (ADS)
Cazalis, B.; Desquines, J.; Carassou, S.; Le Jolu, T.; Bernaudat, C.
2016-04-01
A fuel cladding mechanical test, performed under conditions of plane strain deformation in the transverse direction of tube axis, was originally developed at Pennsylvania State University. It was decided to implement this original test within the PROMETRA program using the same experimental procedure and its optimization for a ring mechanical testing on plane strain conditions (PST tests) in hot cells laboratory. This paper presents a detailed description and an interpretation of the Plane Strain Tensile (PST) tests performed in the framework of the PROMETRA program on fresh and irradiated claddings. At first, the context of the PST tests is situated and the specificities of these tests implemented at CEA are justified. Indeed, a significant adjustment of the original experimental procedure is carried out in order to test the irradiated fuel cladding in the best possible conditions. Then, the tests results on fresh Zircaloy-4 and on irradiated Zircaloy-4, M5™ and ZIRLO® specimens are gathered. The main analyses in support of these tests, such as metallographies, fractographic examinations and finite element simulations are detailed. Finally, a synthesis of the interpretation of the tests is proposed. The PST test seems only representative of plane strain fracture conditions when the test material is very ductile (fresh or high temperature or low hydride material like M5TM). However, it provides a relevant representation of the RIA rupture initiation which is observed in irradiated cladding resulting from hydride rim damage due to the strong irradiation of a fuel rod.
Crack-free AlGaN for solar-blind focal plane arrays through reduced area epitaxy
NASA Astrophysics Data System (ADS)
Cicek, E.; McClintock, R.; Vashaei, Z.; Zhang, Y.; Gautier, S.; Cho, C. Y.; Razeghi, M.
2013-02-01
We report on crack reduction for solar-blind ultraviolet detectors via the use of a reduced area epitaxy (RAE) method to regrow on patterned AlN templates. With the RAE method, a pre-deposited AlN template is patterned into isolated mesas in order to reduce the formation of cracks in the subsequently grown high Al-content AlxGa1-xN structure. By restricting the lateral dimensions of the epitaxial growth area, the biaxial strain is relaxed by the edges of the patterned squares, which resulted in ˜97% of the pixels being crack-free. After successful implementation of RAE method, we studied the optical characteristics, the external quantum efficiency, and responsivity of average pixel-sized detectors of the patterned sample increased from 38% and 86.2 mA/W to 57% and 129.4 mA/W, respectively, as the reverse bias is increased from 0 V to 5 V. Finally, we discussed the possibility of extending this approach for focal plane array, where crack-free large area material is necessary for high quality imaging.
NASA Astrophysics Data System (ADS)
Nibur, Kevin A.; Somerday, Brian P.; Marchi, Chris San; Foulk, James W.; Dadfarnia, Mohsen; Sofronis, Petros
2013-01-01
Threshold stress intensity factors were measured in high-pressure hydrogen gas for a variety of low alloy ferritic steels using both constant crack opening displacement and rising crack opening displacement procedures. Thresholds for crack extension under rising displacement, K THi, for crack extension under constant displacement, K_{{THi}}^{*} , and for crack arrest under constant displacement K THa, were identified. These values were not found to be equivalent, i.e. K THi < K THa < K_{{THi}}^{*} . The hydrogen assisted fracture mechanism was determined to be strain controlled for all of the alloys in this study, and the micromechanics of strain controlled fracture are used to explain the observed disparities between the different threshold measurements. K THa and K THi differ because the strain singularity of a stationary crack is stronger than that of a propagating crack; K THa must be larger than K THi to achieve equivalent crack tip strain at the same distance from the crack tip. Hydrogen interacts with deformation mechanisms, enhancing strain localization and consequently altering both the nucleation and growth stages of strain controlled fracture mechanisms. The timing of load application and hydrogen exposure, i.e., sequential for constant displacement tests and concurrent for rising displacement tests, leads to differences in the strain history relative to the environmental exposure history and promotes the disparity between K_{{THi}}^{*} and K THi. K THi is the only conservative measurement of fracture threshold among the methods presented here.
The inspection of crack initiation in mid-plane by ultrasonic method
NASA Astrophysics Data System (ADS)
Liu, Quanliang; Jiang, Yuejin; Ge, Sen; Guo, Wanlin
2010-03-01
Ultra-sonic is one of the major methods to inspect and locate the damage or flaws of materials in the components or structures in engineering1. The technique to inspect the crack initiation in mid-plane by ultrasonic in real time is presented and developed upon the requirement of detecting the initiation of cracks for thick compact tension specimen (CTS) to set up the I-II mixed mode fracture criterion. The paper is concerned with the application of an ultra-sonic scheme to detect initiation or growth of through-thickness crack at mid-plane from the surface during the loading. The technique in relation to the inspection of crack initiation in mid-plane by ultra-sonic is described in detail. The principles of the technique, and the equipment required to facilitate it are reviewed. The verification of the technique which is supported by a specific example of a CTS inspection performed on an I-II mixed mode fracture test shows that the method is valid and the technique can be capable of detecting delaminations, disbonds and lack-of-adhesion in a wide diversity of coated and uncoated materials. It is also capable of assessing the quality of composites.
The inspection of crack initiation in mid-plane by ultrasonic method
NASA Astrophysics Data System (ADS)
Liu, Quanliang; Jiang, Yuejin; Ge, Sen; Guo, Wanlin
2009-12-01
Ultra-sonic is one of the major methods to inspect and locate the damage or flaws of materials in the components or structures in engineering1. The technique to inspect the crack initiation in mid-plane by ultrasonic in real time is presented and developed upon the requirement of detecting the initiation of cracks for thick compact tension specimen (CTS) to set up the I-II mixed mode fracture criterion. The paper is concerned with the application of an ultra-sonic scheme to detect initiation or growth of through-thickness crack at mid-plane from the surface during the loading. The technique in relation to the inspection of crack initiation in mid-plane by ultra-sonic is described in detail. The principles of the technique, and the equipment required to facilitate it are reviewed. The verification of the technique which is supported by a specific example of a CTS inspection performed on an I-II mixed mode fracture test shows that the method is valid and the technique can be capable of detecting delaminations, disbonds and lack-of-adhesion in a wide diversity of coated and uncoated materials. It is also capable of assessing the quality of composites.
NASA Astrophysics Data System (ADS)
Jacques, Patrick; Lefebvre, Florence; Lemaignan, Clément
1999-01-01
For a better understanding of the initiation step of iodine induced stress corrosion cracking (SCC) in Zr alloys, responsible for pellet-cladding interaction (PCI) fuel rod failures, an analytical study has been undertaken, the aim of which being focused on the respective roles of local chemistry and stress/strain state on the crack nucleation. This second part is mostly related to the local stress induced by strain incompatibilities between grains. Using EBSP (electron back-scattering pattern) to analyze the crystallographic orientation of all the grains of the samples tested in SCC, it was possible to conclude that the major parameter controlling the nucleation of the intergranular cracks is not related to grain to grain strain incompatibilities, but to the orientation of the grain boundary planes with respect to the tensile stress.
Luo, P.F.; Wang, J.S.; Chao, Y.J.; Sutton, M.A.
1996-12-31
The stereo vision is used to study the fracture behavior in the compact tension (CT) specimen made from 304L stainless steel. During crack tip blunting, initiation, and growth in the CT specimen, both in-plane and out-of-plane displacement fields near the crack tip are measured by the stereo vision. Based on the plane stress assumption and the deformation theory of plasticity, the J integral is evaluated along several rectangular paths surrounding the crack tip by using the measured in-plane displacement field. Prior to crack growth, the J integral is path independent. For crack extension up to {Delta}a {approx} 3 mm, the near field J integral values are 6% to 10% lower than far field J integral values. For the crack extension of {Delta}a {approx} 4 mm, the J integral lost path independence. The far field J integral values are in good agreement with results obtained from Merkle-Corten`s formula. Both J-{Delta}a and CTOA-{Delta}a are obtained by computing the J integral value and crack tip opening angle (CTOA) at each {Delta}a. Results indicate that CTOA reached a nearly constant value at a crack extension of {Delta}a = 3 mm with a leveled resistance curve thereafter. Also, the J integral value is determined by the maximum transverse diameter of the shadow spots, which are generated by using the out-of-plane displacement field. Results indicate that for crack extension up to 0.25 mm, the J integral values evaluated by using the out-of- plane displacement are close to those obtained by using in-plane displacements and Merkle-Corten`s formula.
The influence of strain rate and hydrogen on the plane-strain ductility of Zircaloy cladding
Link, T.M.; Motta, A.T.; Koss, D.A.
1998-03-01
The authors studied the ductility of unirradiated Zircaloy-4 cladding under loading conditions prototypical of those found in reactivity-initiated accidents (RIA), i.e.: near plane-strain deformation in the hoop direction (transverse to the cladding axis) at room temperature and 300 C and high strain rates. To conduct these studies, they developed a specimen configuration in which near plane-strain deformation is achieved in the gage section, and a testing methodology that allows one to determine both the limit strain at the onset of localized necking and the fracture strain. The experiments indicate that there is little effect of strain rate (10{sup {minus}3} to 10{sup 2} s{sup {minus}1}) on the ductility of unhydrided Zircaloy tubing deformed under near plane-strain conditions at either room temperature or 300 C. Preliminary experiments on cladding containing 190 ppm hydrogen show only a small loss of fracture strain but no clear effect on limit strain. The experiments also indicate that there is a significant loss of Zircaloy ductility when surface flaws are present in the form of thickness imperfections.
Transient response of a single-edge crack in an elastic half-plane
Chen, E.P.
1980-01-01
The elasto-dynamic response of a single-edge crack in a semi-infinite elastic medium is considered in this study. The crack is assumed to appear suddenly in an elastic half-plane that is being stretched by a remote stress field. Near the crack, the interactions between the wave emanating from the tip and those reflected from the free edge take place. This investigation focuses its attention on the determination of the effect of this interaction on the stress intensity factor. Laplace and Fourier transforms are used to reduce the solution to a pair of dual integral equations. A numerical Laplace inversion procedure is used to obtain the time dependence of the solution. Numerical results on the dynamic stress intensity factor are obtained and are presented in a graphical form.
Crack Growth Mechanisms under Anti-Plane Shear in Composite Laminates
NASA Astrophysics Data System (ADS)
Horner, Allison Lynne
The research conducted for this dissertation focuses on determining the mechanisms associated with crack growth in polymer matrix composite laminates subjected to anti-plane shear (mode III) loading. For mode III split-beam test methods were proposed, and initial evaluations were conducted. A single test method was selected for further evaluation. Using this test method, it was determined that the apparent mode III delamination toughness, GIIIc , depended on geometry, which indicated a true material property was not being measured. Transverse sectioning and optical microscopy revealed an array of transverse matrix cracks, or echelon cracks, oriented at approximately 45° and intersecting the plane of the delamination. Subsequent investigations found the echelon array formed prior to the onset of planar delamination advance and that growth of the planar delamination is always coupled to echelon array formation in these specimens. The evolution of the fracture surfaces formed by the echelon array and planar delamination were studied, and it was found that the development was similar to crack growth in homogenous materials subjected to mode III or mixed mode I-III loading, although the composite laminate architecture constrained the fracture surface development differently than homogenous materials. It was also found that, for split-beam specimens such as those used herein, applying an anti-plane shear load results in twisting of the specimen's uncracked region which gives rise to a mixed-mode I-III load condition. This twisting has been related to the apparent mode III toughness as well as the orientation of the transverse matrix cracks. A finite element model was then developed to study the mechanisms of initial echelon array formation. From this, it is shown that an echelon array will develop, but will become self-limiting prior to the onset of planar delamination growth.
Shear Band Formation in Plane Strain Experiments of Sand
NASA Technical Reports Server (NTRS)
Alshibli, Khalid A.; Sture, Stein
2000-01-01
A series of biaxial (plane strain) experiments were conducted on three sands under low (15 kPa) and high (100 kPa) confining pressure conditions to investigate the effects of specimen density, confining pressure, and sand grain size and shape on the constitutive and stability behavior of granular materials. The three sands used in the experiments were fine-, medium-, and coarse-grained uniform silica sands with rounded, subangular, and angular grains, respectively. Specimen deformation was readily monitored and analyzed with the help of a grid pattern imprinted on the latex membrane. The overall stress-strain behavior is strongly dependent on the specimen density, confining pressure, sand grain texture, and the resulting failure mode(s). That became evident in different degrees of softening responses at various axial strains. The relationship between the constitutive behavior and the specimens' modes of instability is presented. The failure in all specimens was characterized by two distinct and opposite shear bands. It was found that the measured dilatancy angles increase as the sand grains' angularities and sizes increase. The measured shear band inclination angles are also presented and compared with classical Coulomb and Roscoe solutions.
Investigation of flaw geometry and loading effects on plane strain fracture in metallic structures
NASA Technical Reports Server (NTRS)
Hall, L. R.; Finger, R. W.
1971-01-01
The effects on fracture and flaw growth of weld-induced residual stresses, combined bending and tension stresses, and stress fields adjacent to circular holes in 2219-T87 aluminum and 5AI-2.5Sn(ELI) titanium alloys were evaluated. Static fracture tests were conducted in liquid nitrogen; fatigue tests were performed in room air, liquid nitrogen, and liquid hydrogen. Evaluation of results was based on linear elastic fracture mechanics concepts and was directed to improving existing methods of estimating minimum fracture strength and fatigue lives for pressurized structure in spacecraft and booster systems. Effects of specimen design in plane-strain fracture toughness testing were investigated. Four different specimen types were tested in room air, liquid nitrogen and liquid hydrogen environments using the aluminum and titanium alloys. Interferometry and holograph were used to measure crack-opening displacements in surface-flawed plexiglass test specimens. Comparisons were made between stress intensities calculated using displacement measurements, and approximate analytical solutions.
Strained layer superlattice focal plane array having a planar structure
Kim, Jin K; Carroll, Malcolm S; Gin, Aaron; Marsh, Phillip F; Young, Erik W; Cich, Michael J
2012-10-23
An infrared focal plane array (FPA) is disclosed which utilizes a strained-layer superlattice (SLS) formed of alternating layers of InAs and In.sub.xGa.sub.1-xSb with 0.ltoreq.x.ltoreq.0.5 epitaxially grown on a GaSb substrate. The FPA avoids the use of a mesa structure to isolate each photodetector element and instead uses impurity-doped regions formed in or about each photodetector for electrical isolation. This results in a substantially-planar structure in which the SLS is unbroken across the entire width of a 2-D array of the photodetector elements which are capped with an epitaxially-grown passivation layer to reduce or eliminate surface recombination. The FPA has applications for use in the wavelength range of 3-25 .mu.m.
To Crack or Not to Crack: Strain in High TemperatureSuperconductors
Godeke, Arno
2007-08-22
Round wire Bi 2212 is emerging as a viable successor ofNb3Sn in High Energy Physics and Nuclear Magnetic Resonance, to generatemagnetic fields that surpass the intrinsic limitations of Nb3Sn. Ratherbold claims are made on achievable magnetic fields in applications usingBi 2212, due to the materials' estimated critical magnetic field of 100 Tor higher. High transport currents in high magnetic fields, however, leadto large stress on, and resulting large strain in the superconductor. Theeffect of strain on the critical properties of Bi-2212 is far fromunderstood, and strain is, as with Nb3Sn, often treated as a secondaryparameter in the design of superconducting magnets. Reversibility of thestrain induced change of the critical surface of Nb3Sn, points to anelectronic origin of the observed strain dependence. Record breaking highfield magnets are enabled by virtue of such reversible behavior. Straineffects on the critical surface of Bi-2212, in contrast, are mainlyirreversible and suggest a non-electronic origin of the observed straindependence, which appears to be dominated by the formation of cracks inthe superconductor volumes. A review is presented of available results onthe effects of strain on the critical surface of Bi-2212, Bi-2223 andYBCO. It is shown how a generic behavior emerges for the (axial) straindependence of the critical current density, and how the irreversiblereduction of the critical current density is dominated by strain inducedcrack formation in the superconductor. From this generic model it becomesclear that magnets using high temperature superconductors will be strainlimited far before the intrinsic magnetic field limitations will beapproached, or possibly even before the magnetic field limitation ofNb3Sn can be surpassed. On a positive note, in a very promising recentresult from NIST on the axial strain dependence of the critical currentdensity in extremely well aligned YBCO, reversible behavior was observed.This result emphasizes the need
Stress-Strain Behaviour of a Micacious Sand in Plane Strain Condition
NASA Astrophysics Data System (ADS)
Yasin, S. J. M.; Tatsuoka, F.
Unusual failures of river banks and river training structures have been reported during construction and shortly after commissioning of several structures along Jamuna river in Bangladesh that raised widespread questions regarding the design principles and parameters used. The natural sand deposit along the Jamuna river contain relatively larger amount of mica than most other natural soils. Jamuna sand needs to be studied under wide range of loading conditions (such as triaxial, plane strain, simple shear etc.), drainage and density conditions (i.e. drained / undrained, dry / saturated, dense/loose state etc.) to reveal the extent of variation of its strength and deformation characteristics in order to facilitate understanding of the mechanism of past failures of structures and suggest rational design parameters. A series of plane strain compression tests were performed on Jamuna sand. It is observed that Jamuna sand is highly contractive under shear and more anisotropic than other non-mica sands.
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.
Crack growth measured on flat and curved surfaces at cryogenic temperatures
NASA Technical Reports Server (NTRS)
Orange, T. W.; Sullivan, T. L.
1967-01-01
Multiple element continuity gage measures plane stress crack growth plus surface crack growth under plane strain conditions. The gage measures flat and curved surfaces and operates at cryogenic temperatures.
NASA Astrophysics Data System (ADS)
Hu, Keqiang; Chen, Zengtao
2012-12-01
An opening crack in a magnetoelectroelastic strip under in-plane mechanical, electric, and magnetic impact loadings is considered for magneto-electrically impermeable and permeable crack surface boundary conditions. Laplace and Fourier transforms are applied to reduce the mixed boundary value problem of the crack to dual integral equations, which are expressed in terms of Fredholm integral equations of the second kind. The asymptotic fields near the crack tip are obtained in explicit form and the corresponding field intensity factors are defined. The crack curving phenomena are investigated by applying the criterion of maximum hoop stress intensity factors. Numerical results show that the hoop stress intensity factors are influenced by the electric and magnetic loadings and the geometric size ratios.
Seven-year-long crack detection monitoring by Brillouin-based fiber optic strain sensor
NASA Astrophysics Data System (ADS)
Imai, Michio
2015-03-01
As an optical fiber is able to act as a sensing medium, a Brillouin-based sensor provides continuous strain information along an optical fiber. The sensor has been used in a wide range of civil engineering applications because no other tool can satisfactorily detect discontinuity such as a crack. Cracking generates a local strain change on the embedded optical fiber, thus Brillouin optical correlation domain analysis (BOCDA), which offers a high spatial resolution by stimulated Brillouin scattering, is expected to detect a fine crack on concrete structures. The author installed the surface-mounted optical fiber on a concrete deck and periodically monitored strain distribution for seven years. This paper demonstrates how a BOCDA-based strain sensor can be employed to monitor cracks in a concrete surface. Additionally, focusing on another advantage of the sensor, the natural frequency of the deck is successfully measured by dynamic strain history.
NASA Astrophysics Data System (ADS)
Zhao, Yanlin; Zhang, Lianyang; Wang, Weijun; Pu, Chengzhi; Wan, Wen; Tang, Jingzhou
2016-07-01
This paper investigates the cracking and stress-strain behavior, especially the local strain concentration near the flaw tips, of rock-like material containing two flaws. A series of uniaxial compression tests were carried out on rock-like specimens containing two flaws, with strain gauges mounted near the flaw tips to measure the local strain concentration under the uniaxial compressive loading. Four different types of cracks (wing cracks, anti-wing cracks, coplanar shear cracks and oblique shear cracks) and seven patterns of crack coalescences (T1 and T2; S1 and S2; and TS1, TS2 and TS3) are observed in the experiments. The type of crack coalescence is related to the geometry of the flaws. In general, the crack coalescence varies from the S-mode to the TS-mode and then to the T-mode with the increase of the rock bridge ligament angle. The stress-strain curves of the specimens containing two flaws are closely related to the crack development and coalescence process. The strain measurements indicate that the local tensile strain concentration below or above the pre-existing flaw tip causes wing or anti-wing cracks, while the local compressive strain concentration near the flaw tip is related to the shear crack. The measured local tensile strain shows a jump at the initiation of wing- and anti-wing cracks, reflecting the instant opening of the wing- and anti-wing crack propagating through the strain gauge. During the propagation of wing- and anti-wing cracks, the measured local tensile strain gradually increases with few jumps, implying that the opening deformation of wing- and anti-wing cracks occurs in a stable manner. The shear cracks initiate followed by a large and abrupt compressive strain jump and then quickly propagate in an unstable manner resulting in the failure of specimens.
Ulaganathan, Jaganathan Newman, Roger C.
2014-06-01
The dynamic strain rate ahead of a crack tip formed during stress corrosion cracking (SCC) under a static load is assumed to arise from the crack propagation. The strain surrounding the crack tip would be redistributed as the crack grows, thereby having the effect of dynamic strain. Recently, several studies have shown cold work to cause accelerated crack growth rates during SCC, and the slip-dissolution mechanism has been widely applied to account for this via a supposedly increased crack-tip strain rate in cold worked material. While these interpretations consider cold work as a homogeneous effect, dislocations are generated inhomogeneously within the microstructure during cold work. The presence of grain boundaries results in dislocation pile-ups that cause local strain concentrations. The local strains generated from cold working α-brass by tensile elongation were characterized using electron backscatter diffraction (EBSD). The role of these local strains in SCC was studied by measuring the strain distributions from the same regions of the sample before cold work, after cold work, and after SCC. Though, the cracks did not always initiate or propagate along boundaries with pre-existing local strains from the applied cold work, the local strains surrounding the cracked boundaries had contributions from both the crack propagation and the prior cold work. - Highlights: • Plastic strain localization has a complex relationship with SCC susceptibility. • Surface relief created by cold work creates its own granular strain localization. • Cold work promotes crack growth but several other factors are involved.
NASA Astrophysics Data System (ADS)
Bhadauria, S. S.; Pathak, K. K.; Hora, M. S.
2012-09-01
It is widely accepted that failure due to plastic deformation in metals greatly depends on the stress triaxiality factor (TF). This article investigates the variation of stress triaxiality along the yield locus of ductile materials. Von Mises yield criteria and triaxiality factor have been used to determine the critical limits of stress triaxiality for the materials under plane strain condition. A generalized mathematical model for triaxiality factor has been formulated and a constrained optimization has been carried out using genetic algorithm. Finite element analysis of a two dimensional square plate has been carried out to verify the results obtained by the mathematical model. It is found that the set of values of the first and the second principal stresses on the yield locus, which results in maximum stress triaxiality, can be used to determine the location at which crack initiation may occur. Thus, the results indicate that while designing a certain component, such combination of stresses which leads the stress triaxiality to its critical value, should be avoided.
Back-Face Strain for Monitoring Stable Crack Extension in Precracked Flexure Specimens
NASA Technical Reports Server (NTRS)
Salem, Jonathan A.; Ghosn, Louis J.
2010-01-01
Calibrations relating back-face strain to crack length in precracked flexure specimens were developed for different strain gage sizes. The functions were verified via experimental compliance measurements of notched and precracked ceramic beams. Good agreement between the functions and experiments occurred, and fracture toughness was calculated via several operational methods: maximum test load and optically measured precrack length; load at 2 percent crack extension and optical precrack length; maximum load and back-face strain crack length. All the methods gave vary comparable results. The initiation toughness, K(sub Ii) , was also estimated from the initial compliance and load.The results demonstrate that stability of precracked ceramics specimens tested in four-point flexure is a common occurrence, and that methods such as remotely-monitored load-point displacement are only adequate for detecting stable extension of relatively deep cracks.
NASA Technical Reports Server (NTRS)
Yuan, F. G.
1998-01-01
Determination of all the coefficients in the crack tip field expansion for monoclinic materials under two-dimensional deformation is presented in this report. For monoclinic materials with a plane of material symmetry at x(sub 3) = 0, the in-plane deformation is decoupled from the anti-plane deformation. In the case of in-plane deformation, utilizing conservation laws of elasticity and Betti's reciprocal theorem, together with selected auxiliary fields, T-stress and third-order stress coefficients near the crack tip are evaluated first from path-independent line integrals. To determine the T-stress terms using the J-integral and Betti's reciprocal work theorem, auxiliary fields under a concentrated force and moment acting at the crack tip are used respectively. Through the use of Stroh formalism in anisotropic elasticity, analytical expressions for all the coefficients including the stress intensity factors are derived in a compact form that has surprisingly simple structure in terms of the Barnett-Lothe tensors, L. The solution forms for degenerated materials, orthotropic, and isotropic materials are presented.
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.
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
In situ SEM observation of microscale strain fields around a crack tip in polycrystalline molybdenum
NASA Astrophysics Data System (ADS)
Li, J. J.; Li, W. C.; Jin, Y. J.; Wang, L. F.; Zhao, C. W.; Xing, Y. M.; Lang, F. C.; Yan, L.; Yang, S. T.
2016-06-01
In situ scanning electron microscopy was employed to investigate the crack initiation and propagation in polycrystalline molybdenum under uniaxial tensile load at room temperature. The microscale grid pattern was fabricated using the sputtering deposition technology on the specimen surface covered with a fine square mesh copper grid. The microscale strain fields around the crack tip were measured by geometric phase analysis technique and compared with the theoretical solutions based on the linear elastic fracture mechanics theory. The results showed that as the displacement increases, the crack propagated mainly perpendicular to the tensile direction during the fracture process of molybdenum. The normal strain ɛ xx and shear strain ɛ xy are relatively small, and the normal strain ɛ yy holds a dominant position in the deformation fields and plays a key role in the whole fracture process of molybdenum. With the increase in displacement, the ɛ yy increases rapidly and the two lobes grow significantly but maintain the same shape and orientation. The experimental ɛ yy is in agreement with the theoretical solution. Along the x-axis in front of the crack tip, there is minor discrepancy between the experimental ɛ yy and theoretical ɛ yy within 25 μm from the crack tip, but the agreement between them is very good far from the crack tip (>25 μm).
NASA Astrophysics Data System (ADS)
Thongnum, Anusit; Pinsook, Udomsilp
2015-03-01
Anisotropic transport properties of a two-dimensional electron gas in nonpolar m-plane AlN/GaN heterostructures with the interface roughness coupled anisotropic in-plane strain scattering were investigated theoretically using a path-integral framework. The scattering potential was composed of the interface roughness and the effective field from the electron charge and the net piezoelectric polarization. We showed that the anisotropic biaxial strains generate only the net piezoelectric polarization along the [0 0 0 1]-direction and cause anisotropy in electron mobility with a magnitude lower than the ≤ft[11\\bar{2}0\\right] -direction. We also showed that the anisotropy in electron mobility reduced with increasing electron density. Moreover, the anisotropic electron mobility disappeared when the anisotropic in-plane strain scattering was removed, and the relation for pure interface roughness scattering was reestablished. This formulation with existing roughness parameters gave a good description for the experimental results of polar c-plane AlN/GaN heterostructures.
Detection of Steel Fatigue Cracks with Strain Sensing Sheets Based on Large Area Electronics
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
Lee, S. Y.; Sun, Yinan; An, Ke; Choo, Hahn; Hubbard, Camden R; Liaw, Peter K
2010-01-01
Neutron diffraction was employed to investigate the crack-growth retardation phenomenon after a single tensile overload by mapping both one-dimensional and two-dimensional residual-strain distributions around the crack tip in a series of compact-tension specimens representing various crack-growth stages through an overload-induced retardation period. The results clearly show a large compressive residual-strain field near the crack tip immediately after the overload. As the fatigue crack propagates through the overload-induced plastic zone, the compressive residual strains are gradually relaxed, and a new compressive residual-strain field is developed around the propagating crack tip, illustrating that the subsequent fatigue-induced plastic zone grows out of the large plastic zone caused by the overloading. The relationship between the overload-induced plastic zone and subsequent fatigue-induced plastic zone, and its influence on the residual-strain distributions in the perturbed plastic zone are discussed.
T{sub {epsilon}}{sup *} integral under plane stress crack growth
Omori, Yoshika; Ma, Leong; Kobayashi, A.S.; Okada, Hiroshi; Atluri, S.N.
1997-12-01
The T{sub {epsilon}}{sup *} integral values associated with stable crack growth in A606 HSLA steel single-edge-notched (SEN) specimens were determined experimentally and numerically. The displacement fields obtained through Moire interferometry and elastic-plastic finite element analysis were used to evaluate T{sub {epsilon}}{sup *} associated with the moving crack tip. T{sub {epsilon}}{sup *} decreased with the reduction in the size of the integration contour, {Gamma}{sub {epsilon}}, and continually increased with stable crack growth in this specimen. The measured and computed crack-tip opening angle (CTOA), on the other hand, was a constant 15{degree} during stable crack growth after dropping from the measured and computed initial high values of 35 and 57{degree}, respectively.
Influence of pre-strain on fracture toughness and stable crack growth in low carbon steels
Miyata, Takashi; Tagawa, Tetsuya; Aihara, Syuji
1997-12-31
Experimental investigations demonstrate a significant effect of pre-strain on fracture toughness and stable crack growth resistance of a low carbon structural steel. Fracture toughness, Ji for the onset of stable ductile crack growth is decreased to one half with a 9% pre-strain due to cold rolling. The characteristic distance model for ductile crack initiation was applied to analyze parameters affecting the degradation of fracture resistance. The model predicts that value of Ji is given as a linear function of yield strength and ductility of the material. In order to confirm the theoretical prediction, notched round bar tensile tests were performed and ductility under a high triaxial stress state was measured. Critical plastic strain for micro-void coalescence is significantly decreased with increasing pre-strain. Degradation in Ji due to pre-strain can be well explained by the characteristic distance model. To clarify micro-mechanisms of degradation in ductility due to pre-strain, fracture process in notched round bar specimens was investigated emphasizing the role of micro-void nucleation and growth. Experimental observation indicates that the significant decrease of the critical strain due to pre-strain is attributed to the increase of void nucleation sites under a high triaxial stress state.
A semi-implicit finite strain shell algorithm using in-plane strains based on least-squares
NASA Astrophysics Data System (ADS)
Areias, P.; Rabczuk, T.; de Sá, J. César; Natal Jorge, R.
2015-04-01
The use of a semi-implicit algorithm at the constitutive level allows a robust and concise implementation of low-order effective shell elements. We perform a semi-implicit integration in the stress update algorithm for finite strain plasticity: rotation terms (highly nonlinear trigonometric functions) are integrated explicitly and correspond to a change in the (in this case evolving) reference configuration and relative Green-Lagrange strains (quadratic) are used to account for change in the equilibrium configuration implicitly. We parametrize both reference and equilibrium configurations, in contrast with the so-called objective stress integration algorithms which use a common configuration. A finite strain quadrilateral element with least-squares assumed in-plane shear strains (in curvilinear coordinates) and classical transverse shear assumed strains is introduced. It is an alternative to enhanced-assumed-strain (EAS) formulations and, contrary to this, produces an element satisfying ab-initio the Patch test. No additional degrees-of-freedom are present, contrasting with EAS. Least-squares fit allows the derivation of invariant finite strain elements which are both in-plane and out-of-plane shear-locking free and amenable to standardization in commercial codes. Two thickness parameters per node are adopted to reproduce the Poisson effect in bending. Metric components are fully deduced and exact linearization of the shell element is performed. Both isotropic and anisotropic behavior is presented in elasto-plastic and hyperelastic examples.
Texture development and hardening characteristics of steel sheets under plane-strain compression
Friedman, P.A.; Liao, K.C.; Pan, J.; Barlat, F.
1999-04-01
Crystallographic texture development and hardening characteristics of a hot-rolled, low-carbon steel sheet due to cold rolling were investigated by idealizing the cold rolling process as plane-strain compression. The starting anisotropy of the test material was characterized by examination of the grain structure by optical microscopy and the preferred crystal orientation distribution by x-ray diffraction. Various heat treatments were used in an effort to remove the initial deformation texture resulting from hot rolling. The plastic anisotropy of the starting material was investigated with tensile tests on samples with the tensile axis parallel, 45{degree}, and perpendicular to the rolling direction. The grain structure after plane-strain compression was studied by optical microscopy, and the new deformation texture was characterized by x-ray diffraction pole figures. These figures are compared with the theoretical pole figures produced from a Taylor-like polycrystal model based on a pencil-glide slip system. The uniaxial tensile stress-strain curve and the plane-strain, compressive stress-strain curve of the sheet were used to calibrate the material parameters in the model. The experimental pole figures were consistent with the findings in the theoretical study. The experimental and theoretical results suggest that the initial texture due to hot rolling was insignificant as compared with the texture induced by large strains under plane-strain compression.
Crack-tip strain field mapping and the toughness of metallic glasses.
Hufnagel, Todd C; Vempati, Uday K; Almer, Jonathan D
2013-01-01
We have used high-energy x-ray scattering to map the strain fields around crack tips in fracture specimens of a bulk metallic glass under load at room temperature and below. From the measured strain fields we can calculate the components of the stress tensor as a function of position and determine the size and shape of the plastic process zone around the crack tip. Specimens tested at room temperature develop substantial plastic zones and achieve high stress intensities ((K(If) = 76 MPa m(1/2)) prior to fracture. Specimens tested at cryogenic temperatures fail at reduced but still substantial stress intensities (K(If) = 39 MPa m(1/2)) and show only limited evidence of crack-tip plasticity. We propose that the difference in behavior is associated with changes in the flow stress and elastic constants, which influence the number density of shear bands in the plastic zone and thus the strain required to initiate fracture on an individual band. A secondary effect is a change in the triaxial state of stress around the crack tip due to the temperature dependence of Poisson's ratio. It is likely that this ability to map elastic strains on the microscale will be useful in other contexts, although interpreting shifts in the position of the scattering peaks in amorphous materials in terms of elastic strains must be done with caution. PMID:24386172
Crack-Tip Strain Field Mapping and the Toughness of Metallic Glasses
Hufnagel, Todd C.; Vempati, Uday K.; Almer, Jonathan D.
2013-01-01
We have used high-energy x-ray scattering to map the strain fields around crack tips in fracture specimens of a bulk metallic glass under load at room temperature and below. From the measured strain fields we can calculate the components of the stress tensor as a function of position and determine the size and shape of the plastic process zone around the crack tip. Specimens tested at room temperature develop substantial plastic zones and achieve high stress intensities () prior to fracture. Specimens tested at cryogenic temperatures fail at reduced but still substantial stress intensities () and show only limited evidence of crack-tip plasticity. We propose that the difference in behavior is associated with changes in the flow stress and elastic constants, which influence the number density of shear bands in the plastic zone and thus the strain required to initiate fracture on an individual band. A secondary effect is a change in the triaxial state of stress around the crack tip due to the temperature dependence of Poisson's ratio. It is likely that this ability to map elastic strains on the microscale will be useful in other contexts, although interpreting shifts in the position of the scattering peaks in amorphous materials in terms of elastic strains must be done with caution. PMID:24386172
Strain-mediated control of orbital ordering planes in heteroepitaxial lanthanum manganite thin films
NASA Astrophysics Data System (ADS)
Kim, Yong-Jin; Lee, Jin Hong; Koo, Tae Yeong; Yang, Chan-Ho
Strain engineering which controls the misfit strain of heteroepitaxial thin films leads to distinctive physical properties in contrast to the intrinsic properties of unstrained bulk materials Perovskite LaMnO3 (LMO) has attracted considerable attention due to strong coupling among the lattice, charge, spin and orbital degrees of freedom. Bulk LMO is known to be an A-type antiferromagnetic (TN~140 K) Mott insulator, and its orbital ordering plane is established due to cooperative Jahn-Teller distortion below ~750 K. Previous studies have focused on the orbital ordering planes of the bulk LMO but not researched on correlation between orbital planes and misfit stain. To figure out the strain dependence of orbital ordering planes, we have grown LMO thin films on four different substrates, i . e . , DyScO3(110), GaScO3(110), SrTiO3(001), and LSAT(001), using the pulsed laser deposition technique. The films have been characterized by atomic force microscopy and x-ray diffraction. We have performed resonant x-ray scattering to identify orbital ordering plane on each film. We have found that orbital ordering planes can be modulated depending on the misfit strain.
NASA Astrophysics Data System (ADS)
Adumitroaie, Adi
applications in the range of cryogenic temperatures. The process of matrix cracking under
Paul, S.C.; Pirskawetz, S.; Zijl, G.P.A.G. van; Schmidt, W.
2015-03-15
This paper presents the analysis of crack propagation in strain-hardening cement-based composite (SHCC) under tensile and flexural load by using acoustic emission (AE). AE is a non-destructive technique to monitor the development of structural damage due to external forces. The main objective of this research was to characterise the cracking behaviour in SHCC in direct tensile and flexural tests by using AE. A better understanding of the development of microcracks in SHCC will lead to a better understanding of pseudo strain-hardening behaviour of SHCC and its general performance. ARAMIS optical deformation analysis was also used in direct tensile tests to observe crack propagation in SHCC materials. For the direct tensile tests, SHCC specimens were prepared with polyvinyl alcohol (PVA) fibre with three different volume percentages (1%, 1.85% and 2.5%). For the flexural test beam specimens, only a fibre dosage of 1.85% was applied. It was found that the application of AE in SHCC can be a good option to analyse the crack growth in the specimens under increasing load, the location of the cracks and most importantly the identification of matrix cracking and fibre rupture or slippage.
NASA Astrophysics Data System (ADS)
Zhang, Siyuan; Cui, Ying; Griffiths, James T.; Fu, Wai Y.; Freysoldt, Christoph; Neugebauer, Jörg; Humphreys, Colin J.; Oliver, Rachel A.
2015-12-01
I nxG a1 -xN structures epitaxially grown on a -plane or m -plane GaN exhibit in-plane optical polarization. Linear elasticity theory treats the two planes equivalently and is hence unable to explain the experimentally observed higher degree of linear polarization for m -plane than a -plane I nxG a1 -xN . Using density functional theory, we study the response of I nxG a1 -xN random alloys to finite biaxial strains on both nonpolar planes. The calculated m -plane I nxG a1 -xN valence band splitting is larger than that of the a plane, due to a greater degree of structural relaxation in a -plane I nxG a1 -xN . We provide a parametrization of the valence band splitting of I nxG a1 -xN strained to a -plane and m -plane GaN for In compositions between 0 and 0.5, which agrees with experimental measurements and qualitatively explains the experimentally observed difference between a -plane and m -plane polarization.
Stress Corrosion Cracking Behavior of Interstitial Free Steel Via Slow Strain Rate Technique
NASA Astrophysics Data System (ADS)
Murkute, Pratik; Ramkumar, J.; Mondal, K.
2016-07-01
An interstitial free steel is subjected to slow strain rate tests to investigate the stress corrosion cracking (SCC) behavior at strain rates ranging from 10-4 to 10-6s-1 in air and 3.5 wt.% NaCl solution. The ratios of time to failure, failure strain, and ultimate tensile stress at different strain rates in air to that in corrosive were considered as SCC susceptibility. Serrated stress-strain curve observed at lowest strain rate is explained by the Portevin-Le Chatelier effect. Maximum susceptibility to SCC at lowest strain rate is attributed to the soluble γ-FeOOH in the rust analyzed by Fourier Transformed Infrared spectroscopy. Mechanism for SCC relates to the anodic dissolution forming the groove, where hydrogen embrittlement can set in and finally fracture happens due to triaxiality.
Tunable biaxial in-plane compressive strain in a Si nanomembrane transferred on a polyimide film
Kim, Munho; Mi, Hongyi; Cho, Minkyu; Seo, Jung-Hun; Ma, Zhenqiang; Zhou, Weidong; Gong, Shaoqin
2015-05-25
A method of creating tunable and programmable biaxial compressive strain in silicon nanomembranes (Si NMs) transferred onto a Kapton{sup ®} HN polyimide film has been demonstrated. The programmable biaxial compressive strain (up to 0.54%) was generated utilizing a unique thermal property exhibited by the Kapton HN film, namely, it shrinks from its original size when exposed to elevated temperatures. The correlation between the strain and the annealing temperature was carefully investigated using Raman spectroscopy and high resolution X-ray diffraction. It was found that various amounts of compressive strains can be obtained by controlling the thermal annealing temperatures. In addition, a numerical model was used to evaluate the strain distribution in the Si NM. This technique provides a viable approach to forming in-plane compressive strain in NMs and offers a practical platform for further studies in strain engineering.
Tunable biaxial in-plane compressive strain in a Si nanomembrane transferred on a polyimide film
NASA Astrophysics Data System (ADS)
Kim, Munho; Mi, Hongyi; Cho, Minkyu; Seo, Jung-Hun; Zhou, Weidong; Gong, Shaoqin; Ma, Zhenqiang
2015-05-01
A method of creating tunable and programmable biaxial compressive strain in silicon nanomembranes (Si NMs) transferred onto a Kapton® HN polyimide film has been demonstrated. The programmable biaxial compressive strain (up to 0.54%) was generated utilizing a unique thermal property exhibited by the Kapton HN film, namely, it shrinks from its original size when exposed to elevated temperatures. The correlation between the strain and the annealing temperature was carefully investigated using Raman spectroscopy and high resolution X-ray diffraction. It was found that various amounts of compressive strains can be obtained by controlling the thermal annealing temperatures. In addition, a numerical model was used to evaluate the strain distribution in the Si NM. This technique provides a viable approach to forming in-plane compressive strain in NMs and offers a practical platform for further studies in strain engineering.
NASA Astrophysics Data System (ADS)
Avitzur, Boaz
1993-04-01
There is a long-standing interest in developing a capability to predict the distribution of retained stresses in thick-walled pressure vessels after the removal of an internal pressure--post autofrettage. The key to such a prediction is in the capacity to compute the stress distribution in a vessel while under externally imposed stress sufficient enough to cause at least partial plastic deformation. A good approximation of the stress distribution was developed by Mises in his 1913 plane-stress solution. The fact that such vessels are not representative of the plane-stress condition not withstanding, Mises recognized that his solution was mathematically restricted to a limited range of vessels' wall ratios. More recently, Avitzur offered a solution similar to that of Mises, but for a plane-strain condition. Depending on the material's Poisson's factor, Avitzur's solution is also mathematically applicable for a limited range of vessels' wall ratios only. The wall ratio, beyond which Avitzur's solution in plane-strain is not applicable, is a few times larger than that which limits Mises' solution in plane-stress. This work introduces a modification to Avitzur's solution in plane-strain, which makes its applicability unlimited.
Cracking monitoring by FBG strain sensor in the small scale dam model
NASA Astrophysics Data System (ADS)
Ren, Liang; Li, Hongnan; Li, Dongsheng
2009-03-01
Accurate measurement of strain variation and effective prediction of failure within models has been a major objective for strain sensors in dam model tests. In this paper, a FBG strain sensor with enhanced strain sensitivity and packaged by two gripper tubes is presented and applied in the seismic tests of a small scale dam model. This paper discusses the principle of enhanced sensitivity of the FBG strain sensor. Calibration experiments were conducted to evaluate the sensor's strain transferring characteristics on plates of different material. This paper also investigates the applicability of the FBG strain sensors in seismic tests of a dam model by conducting a comparison between the test measurements of FBG sensors and analytical predictions, monitoring the failure progress and predicting the cracking inside the dam model. Results of the dam model tests prove that this FBG strain sensor has the advantages of small size, high precision and embedability, demonstrate promising potential in cracking and failure monitoring and in identification of the dam model.
... sound the drug makes as it heats up. Short-Term Effects Crack is a stimulant that is absorbed through ... quickly, after about 5 or 10 minutes. Other short-term effects include: higher heart rate, breathing rate, blood pressure , ...
Strain and hysteresis by stochastic matrix cracking in ceramic matrix composites
NASA Astrophysics Data System (ADS)
Ahn, B. K.; Curtin, W. A.
1997-02-01
A theory is presented to predict the stress/strain relations and unload/reload hysteresis behavior during the evolution of multiple matrix cracking in unidirectional fiber reinforced ceramic matrix composites (CMCs). The theory is based on the similarity between multiple matrix cracking and fiber fragmentation in a single fiber composite, and determines the crack and strain evolution as a function of the statistical distribution of initial flaws in the material, the interfacial sliding resistance τ, and the thermal residual stresses in the composite. The model properly includes matrix fragments of all lengths, from lengths smaller than the current slip length δ(σ) to larger than 2δ(σ), at applied stress σ, and accounts for their respective and differing contributions to the overall strain and hysteresis behavior of the composite. The procedure by which experimental stress/strain and hysteresis data can be interpreted to derive values for the interfacial shear stress, thermal stresses, and intrinsic matrix flaw distribution is discussed. The actual physical crack spacing needs only to be determined at one load level, such as post-fracture, which greatly simplifies the data acquisition and analysis. Several detailed examples are presented, and the results compared with a widely-used approach in which the crack spacing is assumed constant and equal to the average spacing obtained directly from experiment. The discrepancy between the previous and present theories is manifest in an incorrect estimate for the interfacial sliding, but only by approximately 10%. The effect of changing temperature, and hence residual stresses, without changing either matrix flaws or interfacial sliding resistance, is studied.
NASA Astrophysics Data System (ADS)
Ontiveros, Victor Luis
A critical challenge to the continued use of engineering structures as they are asked to perform longer than their design life is the prediction of an initiating crack and the prevention of damage, estimation of remaining useful life, schedule maintenance and to reduce costly downtimes and inspections. The research presented in this dissertation explores the cumulative plastic strain energy density and thermodynamic entropy generation up to crack initiation. Plastic strain energy density and thermodynamic entropy generation are evaluated to investigate whether they would be capable of providing a physical basis for fatigue life and structural risk and reliability assessments. Navy aircraft, specifically, the Orion P-3C, which represent an engineered structure currently being asked to perform past is design life, which are difficult and time consuming to inspect from carrier based operations and are currently evaluated using an empirically based damage index the, fatigue life expended, is used as an example in this investigation. A set of experimental results for aluminum alloy 7075-T651, used in airframe structures, are presented to determine the correlation between plastic strain energy dissipation and the thermodynamic entropy generation versus fatigue crack initiation over a wide range of fatigue loadings. Cumulative plastic strain energy and thermodynamic entropy generation measured from hysteresis energy and temperature rise proved to be valid physical indices for estimation of the probability of crack initiation. Crack initiation is considered as a major evidence of fatigue damage and structural integrity risk. A Bayesian estimation and validation approach is used to determine systematic errors in the developed models as well as other model uncertainties. Comparisons of the energy-based and entropy-based models are presented and benefits of using one over the other are discussed.
A numerical method for determining the strain rate intensity factor under plane strain conditions
NASA Astrophysics Data System (ADS)
Alexandrov, S.; Kuo, C.-Y.; Jeng, Y.-R.
2016-07-01
Using the classical model of rigid perfectly plastic solids, the strain rate intensity factor has been previously introduced as the coefficient of the leading singular term in a series expansion of the equivalent strain rate in the vicinity of maximum friction surfaces. Since then, many strain rate intensity factors have been determined by means of analytical and semi-analytical solutions. However, no attempt has been made to develop a numerical method for calculating the strain rate intensity factor. This paper presents such a method for planar flow. The method is based on the theory of characteristics. First, the strain rate intensity factor is derived in characteristic coordinates. Then, a standard numerical slip-line technique is supplemented with a procedure to calculate the strain rate intensity factor. The distribution of the strain rate intensity factor along the friction surface in compression of a layer between two parallel plates is determined. A high accuracy of this numerical solution for the strain rate intensity factor is confirmed by comparison with an analytic solution. It is shown that the distribution of the strain rate intensity factor is in general discontinuous.
Matsui, Hiroaki Tabata, Hitoshi; Hasuike, Noriyuki; Harima, Hiroshi
2014-09-21
In-plane anisotropic strains in A-plane layers on the electronic band structure of ZnO were investigated from the viewpoint of optical polarization anisotropy. Investigations utilizing k·p perturbation theory revealed that energy transitions and associated oscillation strengths were dependent on in-plane strains. The theoretical correlation between optical polarizations and in-plane strains was experimentally demonstrated using A-plane ZnO layers with different in-plane strains. Finally, optical polarization anisotropy and its implications for in-plane optical properties are discussed in relation to the energy shift between two orthogonal directions. Higher polarization rotations were obtained in an A-plane ZnO layer with in-plane biaxially compressive strains as compared to strain-free ZnO. This study provides detailed information concerning the role played by in-plane strains in optically polarized applications based on nonpolar ZnO in the ultra-violet region.
Hall, M.M. Jr.; Symons, D.M.
1996-05-01
A strain energy density-distance criterion was previously developed and used to correlate rising-load K{sub c} initiation data for notched and fatigue precracked specimens of hydrogen precharged Alloy X-750. This criterion, which was developed for hydrogen embrittlement (HE) cracking, is used here to correlate static-load stress corrosion cracking (SCC) initiation times obtained for smooth geometry, notched and fatigue precracked specimens. The onset of SCC crack growth is hypothesized to occur when a critical strain, which is due to environment-enhanced creep, is attained within the specimen interior. For notched and precracked specimens, initiation is shown by analysis to occur at a variable distance from notch and crack tips. The initiation site varies from very near the crack tip, for highly loaded sharp cracks, to a site that is one grain diameter from the notch, for lower loaded, blunt notches. The existence of hydrogen gradients, which are due to strain-induced hydrogen trapping in the strain fields of notch and crack tips, is argued to be controlling the site for initiation of cracking. By considering the sources of the hydrogen, these observations are shown to be consistent with those from the previous HE study, in which the characteristic distance for crack initiation was found to be one grain diameter from the notch tip, independent of notch radius, applied stress intensity factor and hydrogen level.
Jin, Y.J.; Lu, H. Yu, C.; Xu, J.J.
2013-10-15
Intergranular reheat cracking in the coarse-grained heat-affected zone of T23 steel was produced by strain to fracture tests on a Gleeble 3500 thermal–mechanical simulator. Then the grain boundary character, as well as the strain distribution after reheat crack propagation, was studied by electron backscatter diffraction technique. The results showed that incoherent Σ3 boundaries were seldom found on the prior austenite grain boundaries. Therefore, only the type of random high-angle boundaries played a crucial role in the intergranular cracking. Microstructurally cavities and small cracks were preferentially initiated from high-angle grain boundaries. Low-angle grain boundaries and high-angle ones with misorientation angles less than 15° were more resistant to the cracking. More importantly, the fraction of high-angle grain boundaries increased with the plastic strain induced by both temperature gradient and stress in the coarse-grained heat-affected zone, which contributed to the crack initiation and propagation. Furthermore, the strain distributions in the vicinity of cavities and cracks revealed the accommodation processes of plastic deformation during stress relaxation. It also reflected the strength differences between grain interior and grain boundary at different heat-treated temperatures, which had a large influence on the cracking mechanism. - Highlights: • The coincidence site lattice boundaries play little role in the reheat cracking. • Cavity and crack occur at high-angle grain boundaries rather than low-angle ones. • The strain leads low-angle grain boundaries to transform to high-angle ones. • Strain distribution differs for cavity and crack zones at different temperatures.
NASA Astrophysics Data System (ADS)
Wong, Meng Fei; Zeng, Kaiyang
2010-06-01
This paper presents recent studies on surface and cross-sectional domain structures of Pb(Zn1/3Nb2/3)O3-(6-7)%PbTiO3 (PZN-PT) single crystals using piezoresponse force microscopy and three-point bending technique. The surface domain structures for the rhombohedral-based single crystals in (001) orientation are found to be influenced by polishing process, whereas the surface domains on the (011)-oriented single crystals are aligned along [011¯] direction, unaffected by the polishing process. On the other hand, the domain structures on the cross-sectional fracture surface show preferential alignment which agrees reasonably with the rhombohedral dipoles on the {100} and {110} planes. The differences between the surface and cross-sectional domain structures could be attributed to stress compensation between the surface strain effect and the minimization of elastic energy. In addition, both surface and cross-sectional surface demonstrate nanoscale domains, about 100-200 nm in size. Further fractography observation suggests that the preferred cracking planes for the PZN-PT single crystals are {110} and {100} planes. The {110} planes may be the slip planes along which material pile up is observed upon indentation loading. The pile up results in tensile hoop stress, producing radial cracks along the {100} cleavage planes. To accommodate the localized stress change, new ferroelastic domains by mechanical stress are then formed without interrupting the out-of-plane piezoelectric response. Since the material pile up is thought to cause enhanced toughness along {110} planes, the PZN-PT single crystal in [011]-poled orientation exhibits more superior piezoelectric properties compared to that of the [001]-poled counterpart.
Strain Profiling of Fatigue Crack Overload Effects Using Energy Dispersive X-Ray Diffraction
Croft,M.; Zhong, Z.; Jisrawi, N.; Zakharchenko, I.; Holtz, R.; Gulak, Y.; Skaritka, J.; Fast, T.; Sadananda, K.; Tsakalakos, T.
2005-01-01
In this paper, an assessment of commonly used assumptions associated with {Delta}K{sub eff} and their implications on FCG predictions in light of existing experimental and numerical data is presented. In particular, the following assumptions are examined: (1). {Delta}K{sub eff} fully describes cyclic stresses and strains at the crack-tip vicinity. (2). K{sub op} can be determined experimentally or numerically with certain accuracy. (3). Overload alters K{sub op} but not K{sub max} and associated s{sub max} at the crack-tip 'process zone'. (4). Contact of crack faces curtails the crack driving force in terms of {Delta}K{sub eff}. The analysis indicates that there is insufficient support to justify the above assumptions. In contrary, the analysis demonstrates that a two-parameter fatigue crack driving force in terms of {Delta}K and K{sub max}, which accounts for both applied and the internal stresses should be used in FCG analyses and predictions.
NASA Astrophysics Data System (ADS)
Connolly, Matthew; Slifka, Andrew; Drexler, Elizabeth; Hydrogen Pipeline Safety Team
Hydrogen (H2) is desirable for energy storage as it is cleaner burning and can store a larger amount of energy than an equal mass of gasoline. One problem in the development of a hydrogen economy is to find or develop materials that ensure the safe, reliable, and cost-effective flow of energy from the source to the user. It is expected steels will be needed to serve this function. However, the existing network of natural gas pipeline, for example, is constructed of ferrous materials which are susceptible to embrittlement and subsequent increased fatigue crack growth rates after exposure to hydrogen. In order to improve current modeling efforts, experimental determination of hydrogen concentration, hydrogen diffusion rates, and strain fields are required to inform and validate the model. Here we report neutron imaging measurements of the hydrogen concentration near a fatigue crack and the corresponding strain field, measured via neutron transmission Bragg edge spectroscopy. Nist Materials Measurement Laboratory, Applied Chemicals and Materials Division.
The plane strain shear fracture of the advanced high strength steels
Sun, Li
2013-12-16
The “shear fracture” which occurs at the high-curvature die radii in the sheet metal forming has been reported to remarkably limit the application of the advanced high strength steels (AHSS) in the automobile industry. However, this unusual fracture behavior generally cannot be predicted by the traditional forming limit diagram (FLD). In this research, a new experimental system was developed in order to simulate the shear fracture, especially at the plane strain state which is the most common state in the auto-industry and difficult to achieve in the lab due to sample size. Furthermore, the system has the capability to operate in a strain rate range from quasi-static state to the industrial forming state. One kinds of AHSS, Quenching-Partitioning (QP) steels have been performed in this test and the results show that the limiting fracture strain is related to the bending ratio and strain rate. The experimental data support that deformation-induced heating is an important cause of “shear fracture” phenomena for AHSS: a deformation-induced quasi-heating caused by smaller bending ratio and high strain rate produce a smaller limiting plane strain and lead a “shear fracture” in the component.
NASA Astrophysics Data System (ADS)
Shatskii, I. P.; Makoviichuk, N. V.
2011-05-01
The problem of closure of collinear cracks during bending of a shallow shell is considered within the framework of the Kirchhoff theory. Crack closure is described using the model of contact along a line on one of the shell faces. Strain and moment intensity factors and fracture load are studied as functions of shell curvature and defect location, and the distribution of contact forces along the cracks is investigated.
Imaging in-plane and normal stresses near an interface crack using traction force microscopy
Xu, Ye; Engl, Wilfried C.; Jerison, Elizabeth R.; Wallenstein, Kevin J.; Hyland, Callen; Wilen, Larry A.; Dufresne, Eric R.
2010-01-01
Colloidal coatings, such as paint, are all around us. However, we know little about the mechanics of the film-forming process because the composition and properties of drying coatings vary dramatically in space and time. To surmount this challenge, we extend traction force microscopy to quantify the spatial distribution of all three components of the stress at the interface of two materials. We apply this approach to image stress near the tip of a propagating interface crack in a drying colloidal coating and extract the stress intensity factor. PMID:20696929
Mathematical model of the strain-softening curve for crack extension
NASA Astrophysics Data System (ADS)
Fujii, K.; Nakagawa, K.; Duan, S. J.
1994-07-01
We show in this paper a method of simulating the load-displacement diagrams of notched four-point bending beams with crack extension and the corresponding relation between stress distribution and crack opening displacement (also called the tensile strain-softening curve). As is well-known, the whole fracture behavior of concrete structures as expressed by a complete load-displacement diagram is generally influenced by fracture energy G(sub f) and the shape of the strain-softening curve, which has to be simulated by the FEM or other numerical methods. The present simulation is based on the superposition of the complex stress functions with the finite stress concentration at the crack tips derived from the authors' proposed weight integral method. The character of this model is such that it is possible to approach the material's strain-softening behavior and the fracture process by simply changing the fracture process zone length and the ligament length, although only the elastic solutions are used.
Super switching and control of in-plane ferroelectric nanodomains in strained thin films
NASA Astrophysics Data System (ADS)
Matzen, S.; Nesterov, O.; Rispens, G.; Heuver, J. A.; Biegalski, M.; Christen, H. M.; Noheda, B.
2014-07-01
With shrinking device sizes, controlling domain formation in nanoferroelectrics becomes crucial. Periodic nanodomains that self-organize into so-called ‘superdomains’ have been recently observed, mainly at crystal edges or in laterally confined nanoobjects. Here we show that in extended, strain-engineered thin films, superdomains with purely in-plane polarization form to mimic the single-domain ground state, a new insight that allows a priori design of these hierarchical domain architectures. Importantly, superdomains behave like strain-neutral entities whose resultant polarization can be reversibly switched by 90°, offering promising perspectives for novel device geometries.
NASA Astrophysics Data System (ADS)
Miao, Yu; Chen, L.; Sammynaiken, R.; Lin, Y.; Zhang, W. J.
2011-12-01
The use of carbon nanotubes (CNT) for the application in in-plane strain detection is promising. In recent years, in-plane strain sensors constructed from CNT networks have been developed; however, few studied optimization of these sensors. In this paper, a study of the optimization of pure CNT networks in terms of piezoresistive response is reported. The so-called pure CNT networks are CNT networks free of surfactants. The performances of piezoresistive response are gauge factor (GF) and linearity. The variables are the number of layers of networks, concentration of CNT solution, and length of sonication time. As a result, the study concluded an optimal pure CNT networks sensor (GF: 2.59, linearity 0.98) with ten layers of networks, 0.8 mg/ml concentration, and 2 h of sonication time.
The unique effect of in-plane anisotropic strain in the magnetization control by electric field
NASA Astrophysics Data System (ADS)
Zhao, Y. Y.; Wang, J.; Hu, F. X.; Liu, Y.; Kuang, H.; Wu, R. R.; Sun, J. R.; Shen, B. G.
2016-05-01
The electric field control of magnetization in both (100)- and (011)-Pr0.7Sr0.3MnO3/Pb(Mg1/3Nb2/3)0.7Ti0.3O3(PSMO/PMN-PT) heterostructures were investigated. It was found that the in-plane isotropic strain induced by electric field only slightly reduces the magnetization at low temperature in (100)-PSMO/PMN-PT film. On the other hand, for (011)-PSMO/PMN-PT film, the in-plane anisotropic strain results in in-plane anisotropic, nonvolatile change of magnetization at low-temperature. The magnetization, remanence and coercivity along in-plane [100] direction are suppressed by the electric field while the ones along [01-1] direction are enhanced, which is ascribed to the extra effective magnetic anisotropy induced by the electric field via anisotropic piezostrains. More interestingly, such anisotropic modulation behaviors are nonvolatile, demonstrating a memory effect.
Optical Strain and Crack-Detection Measurements on a Rotating Disk
NASA Technical Reports Server (NTRS)
Woike, Mark; Abdul-Aziz, Ali; Clem, Michelle M.; Fralick, Gustave
2013-01-01
The development of techniques for the in-situ measurement and structural health monitoring of the rotating components in gas turbine engines is of major interest to NASA. As part of this on-going effort, several experiments have been undertaken to develop methods for detecting cracks and measuring strain on rotating turbine engine like disks. Previous methods investigated have included the use of blade tip clearance sensors to detect the presence of cracks by monitoring the change in measured blade tip clearance and analyzing the combined disk-rotor system's vibration response. More recently, an experiment utilizing a novel optical Moiré based concept has been conducted on a subscale turbine engine disk to demonstrate a potential strain measurement and crack detection technique. Moiré patterns result from the overlap of two repetitive patterns with slightly different spacing. When this technique is applied to a rotating disk, it has the potential to allow for the detection of very small changes in spacing and radial growth in a rotating disk due to a flaw such as a crack. This investigation was a continuation of previous efforts undertaken in 2011 to 2012 to validate this optical concept. The initial demonstration attempted on a subscale turbine engine disk was inconclusive due to the minimal radial growth experienced by the disk during operation. For the present experiment a new subscale Aluminum disk was fabricated and improvements were made to the experimental setup to better demonstrate the technique. A circular reference pattern was laser etched onto a subscale engine disk and the disk was operated at speeds up to 12 000 rpm as a means of optically monitoring the Moiré created by the shift in patterns created by the radial growth due the presence of the simulated crack. Testing was first accomplished on a clean defect free disk as a means of acquiring baseline reference data. A notch was then machined in to the disk to simulate a crack and testing was
Optical Strain and Crack-Detection Measurements on a Rotating Disk
NASA Technical Reports Server (NTRS)
Woike, Mark; Abdul-Aziz, Ali; Clem, Michelle; Fralick, Gustave
2013-01-01
The development of techniques for the in-situ measurement and structural health monitoring of the rotating components in gas turbine engines is of major interest to NASA. As part of this on-going effort, several experiments have been undertaken to develop methods for detecting cracks and measuring strain on rotating turbine engine like disks. Previous methods investigated have included the use of blade tip clearance sensors to detect the presence of cracks by monitoring the change in measured blade tip clearance and analyzing the combined disk-rotor system's vibration response. More recently, an experiment utilizing a novel optical Moiré based concept has been conducted on a subscale turbine engine disk to demonstrate a potential strain measurement and crack detection technique. Moiré patterns result from the overlap of two repetitive patterns with slightly different spacing. When this technique is applied to a rotating disk, it has the potential to allow for the detection of very small changes in spacing and radial growth in a rotating disk due to a flaw such as a crack. This investigation was a continuation of previous efforts undertaken in 2011-2012 to validate this optical concept. The initial demonstration attempted on a subscale turbine engine disk was inconclusive due to the minimal radial growth experienced by the disk during operation. For the present experiment a new subscale Aluminum disk was fabricated and improvements were made to the experimental setup to better demonstrate the technique. A circular reference pattern was laser etched onto a subscale engine disk and the disk was operated at speeds up to 12 000 rpm as a means of optically monitoring the Moiré created by the shift in patterns created by the radial growth due the presence of the simulated crack. Testing was first accomplished on a clean defect free disk as a means of acquiring baseline reference data. A notch was then machined in to the disk to simulate a crack and testing was repeated
NASA Astrophysics Data System (ADS)
Cherukuri, Harish P.; Ulysse, Patrick; Smelser, Ronald E.; Subramanian, Kannan; Kotaru, Deepti
2010-06-01
Rapid quenching of aluminum extrusions often results in residual stresses and distortion. The out-of-plane normal component of the residual stress is typically very large and results in undesirable bending (bowing) of the extruded shape. Three-dimensional models to predict the residual stresses and bending of extruded thin-walled shapes are difficult to implement since the wall-thicknesses are often very small compared with the axial dimensions. In this paper, a generalized plane-strain model is presented to predict the residual stresses and distortion. For illusrative purposes of the model, a Z-shaped extrusion is chosen. The model predicts the bowing of the extruded shape along with the in-plane and out-of-plane stress components. An internal state-variable model is used for the constitutive description. The residual stresses and distortion are studied for cold and warm water quenching and three different cases of spray quenching. The numerical results indicate that cold water quenching and the two spray quenching cases with the higher discharge rates lead to significantly larger residual stresses compared to the remaining two cases. For each case, the out-of-plane bows of the extruded shapes are also shown to be significant.
Bio-inspired sensitive and reversible mechanochromisms via strain-dependent cracks and folds.
Zeng, Songshan; Zhang, Dianyun; Huang, Wenhan; Wang, Zhaofeng; Freire, Stephan G; Yu, Xiaoyuan; Smith, Andrew T; Huang, Emily Y; Nguon, Helen; Sun, Luyi
2016-01-01
A number of marine organisms use muscle-controlled surface structures to achieve rapid changes in colour and transparency with outstanding reversibility. Inspired by these display tactics, we develop analogous deformation-controlled surface-engineering approaches via strain-dependent cracks and folds to realize the following four mechanochromic devices: (1) transparency change mechanochromism (TCM), (2) luminescent mechanochromism (LM), (3) colour alteration mechanochromism (CAM) and (4) encryption mechanochromism (EM). These devices are based on a simple bilayer system that exhibits a broad range of mechanochromic behaviours with high sensitivity and reversibility. The TCM device can reversibly switch between transparent and opaque states. The LM can emit intensive fluorescence as stretched with very high strain sensitivity. The CAM can turn fluorescence from green to yellow to orange as stretched within 20% strain. The EM device can reversibly reveal and conceal any desirable patterns. PMID:27389480
Bio-inspired sensitive and reversible mechanochromisms via strain-dependent cracks and folds
NASA Astrophysics Data System (ADS)
Zeng, Songshan; Zhang, Dianyun; Huang, Wenhan; Wang, Zhaofeng; Freire, Stephan G.; Yu, Xiaoyuan; Smith, Andrew T.; Huang, Emily Y.; Nguon, Helen; Sun, Luyi
2016-07-01
A number of marine organisms use muscle-controlled surface structures to achieve rapid changes in colour and transparency with outstanding reversibility. Inspired by these display tactics, we develop analogous deformation-controlled surface-engineering approaches via strain-dependent cracks and folds to realize the following four mechanochromic devices: (1) transparency change mechanochromism (TCM), (2) luminescent mechanochromism (LM), (3) colour alteration mechanochromism (CAM) and (4) encryption mechanochromism (EM). These devices are based on a simple bilayer system that exhibits a broad range of mechanochromic behaviours with high sensitivity and reversibility. The TCM device can reversibly switch between transparent and opaque states. The LM can emit intensive fluorescence as stretched with very high strain sensitivity. The CAM can turn fluorescence from green to yellow to orange as stretched within 20% strain. The EM device can reversibly reveal and conceal any desirable patterns.
Bio-inspired sensitive and reversible mechanochromisms via strain-dependent cracks and folds
Zeng, Songshan; Zhang, Dianyun; Huang, Wenhan; Wang, Zhaofeng; Freire, Stephan G.; Yu, Xiaoyuan; Smith, Andrew T.; Huang, Emily Y.; Nguon, Helen; Sun, Luyi
2016-01-01
A number of marine organisms use muscle-controlled surface structures to achieve rapid changes in colour and transparency with outstanding reversibility. Inspired by these display tactics, we develop analogous deformation-controlled surface-engineering approaches via strain-dependent cracks and folds to realize the following four mechanochromic devices: (1) transparency change mechanochromism (TCM), (2) luminescent mechanochromism (LM), (3) colour alteration mechanochromism (CAM) and (4) encryption mechanochromism (EM). These devices are based on a simple bilayer system that exhibits a broad range of mechanochromic behaviours with high sensitivity and reversibility. The TCM device can reversibly switch between transparent and opaque states. The LM can emit intensive fluorescence as stretched with very high strain sensitivity. The CAM can turn fluorescence from green to yellow to orange as stretched within 20% strain. The EM device can reversibly reveal and conceal any desirable patterns. PMID:27389480
Dislocation microstructures and strain-gradient plasticity with one active slip plane
NASA Astrophysics Data System (ADS)
Conti, Sergio; Garroni, Adriana; Müller, Stefan
2016-08-01
We study dislocation networks in the plane using the vectorial phase-field model introduced by Ortiz and coworkers, in the limit of small lattice spacing. We show that, in a scaling regime where the total length of the dislocations is large, the phase field model reduces to a simpler model of the strain-gradient type. The limiting model contains a term describing the three-dimensional elastic energy and a strain-gradient term describing the energy of the geometrically necessary dislocations, characterized by the tangential gradient of the slip. The energy density appearing in the strain-gradient term is determined by the solution of a cell problem, which depends on the line tension energy of dislocations. In the case of cubic crystals with isotropic elasticity our model shows that complex microstructures may form in which dislocations with different Burgers vector and orientation react with each other to reduce the total self-energy.
NASA Astrophysics Data System (ADS)
Sakaguchi, Junichi; Nakajima, Akinori; Suzuki, Yasuo
The sophisticated numerical analysis method is required to simulate the strain behavior of the reinforcement and the crack width of the RC slab, although many researches are conducted on the cracking behavior of the RC slab in the composite girder under the negative bending. In this paper, the numerical analysis method is constructed to evaluate the strain behavior of the reinforcement and the crack width of the RC slab in the steel-concrete composite girder subjected to the negative bending. The analysis method using the rigid body spring model takes into account the imperfect composite action between the steel girder and the RC slab, the bond between the reinforcement and the concrete, and the shrinkage of the concrete. As a result, it is confirmed that the analysis method can simulate the crack and the strain behavior of RC slab quantitatively in the steel-concrete composite girder under the negative bending.
NASA Astrophysics Data System (ADS)
Tung, S.-T.; Yao, Y.; Glisic, B.
2014-07-01
Increasing concerns regarding the conditions of civil structures and infrastructure give rise to the need for efficient strategies to identify and repair structural anomalies. ‘Sensing sheets’ based on large-area electronics consist of a dense array of unit strain sensors. These are an effective and affordable structural health monitoring tool that can identify and continuously monitor the growth of cracks in structures. This paper presents a study on the quantitative relationship between crack width and strain, the latter measured by an individual sensor that would be part of a sensing sheet. We investigate the sensitivity of thin-film full-bridge strain sensors to concrete cracks by conducting laboratory experiments in temperature-controlled settings. The results show a distribution of near-linear relationships with an average sensitivity of 31 µɛ µm-1. Experiments were also conducted to investigate the effect of crack position and orientation with respect to the sensor, and it appears that both variables affect the sensitivity of strain sensors to cracks. Overall, this study confirms that full-bridge resistive strain sensors can successfully detect and quantify cracks in structural materials and are therefore appropriate as part of a dense array of sensors on a sensing sheet.
Analysis of plane-plastic stress problems with axial symmetry in strain-hardening range
NASA Technical Reports Server (NTRS)
Wu, M H Lee
1951-01-01
A simple method is developed for solving plane-plastic-stress problems with axial symmetry in the strain-hardening range which is based on the deformation theory of plasticity employing the finite-strain concept. The equations defining the problems are first reduced to two simultaneous nonlinear differential equations involving two dependent variables: (a) the octahedral shear strain, and (b) a parameter indicating the ratio of principal stresses. By multiplying the load and dividing the radius by an arbitrary constant, it is possible to solve these problems without iteration for any value of the modified load. The constant is determined by the boundary condition. This method is applied to a circular membrane under pressure, a rotating disk without and with a central hole, and an infinite plate with a circular hole. Two materials, inconel x and 16-25-6, the octahedral shear stress-strain relations of which do not follow the power law, are used. Distributions of octahedral shear strain, as well as of principal stresses and strains, are obtained. These results are compared with the results of the same problems in the elastic range.
Xiao, Xia; Zhang, Yaoyuan; Jiang, Guiyuan; Liu, Jia; Han, Shanlei; Zhao, Zhen; Wang, Ruipu; Li, Cong; Xu, Chunming; Duan, Aijun; Wang, Yajun; Liu, Jian; Wei, Yuechang
2016-08-01
Nanosheet ZSM-5 zeolite with highly exposed (010) crystal planes demonstrates high reactivity and good anti-coking stability for the catalytic cracking of n-heptane, which is attributed to the synergy of high external surface area and acid sites, fully accessible channel intersection acid sites, and hierarchical porosity caused by the unique morphology. PMID:27458616
In-plane displacement and strain measurements using a camera phone and digital image correlation
NASA Astrophysics Data System (ADS)
Yu, Liping; Pan, Bing
2014-05-01
In-plane displacement and strain measurements of planar objects by processing the digital images captured by a camera phone using digital image correlation (DIC) are performed in this paper. As a convenient communication tool for everyday use, the principal advantages of a camera phone are its low cost, easy accessibility, and compactness. However, when used as a two-dimensional DIC system for mechanical metrology, the assumed imaging model of a camera phone may be slightly altered during the measurement process due to camera misalignment, imperfect loading, sample deformation, and temperature variations of the camera phone, which can produce appreciable errors in the measured displacements. In order to obtain accurate DIC measurements using a camera phone, the virtual displacements caused by these issues are first identified using an unstrained compensating specimen and then corrected by means of a parametric model. The proposed technique is first verified using in-plane translation and out-of-plane translation tests. Then, it is validated through a determination of the tensile strains and elastic properties of an aluminum specimen. Results of the present study show that accurate DIC measurements can be conducted using a common camera phone provided that an adequate correction is employed.
Dependence of electronic properties of germanium on the in-plane biaxial tensile strains
NASA Astrophysics Data System (ADS)
Yang, C. H.; Yu, Z. Y.; Liu, Y. M.; Lu, P. F.; Gao, T.; Li, M.; Manzoor, S.
2013-10-01
The hybrid HSE06 functional with the spin-orbit coupling effects is used to calculate the habituation of the electronic properties of Ge on the (0 0 1), (1 1 1), (1 0 1) in-plane biaxial tensile strains (IPBTSs). Our motivation is to explore the nature of electronic properties of tensile-strained Ge on different substrate orientations. The calculated results demonstrate that one of the most effective and practical approaches for transforming Ge into a direct transition semiconductor is to introduce (0 0 1) IPBTS to Ge. At 2.3% (0 0 1) IPBTS, Ge becomes a direct bandgap semiconductor with 0.53 eV band gap, in good agreement with the previous theoretical and experimental results. We find that the (1 1 1) and (1 0 1) IPBTSs are not efficient since the shear strain and inner displacement of atoms introduced by them quickly decrease the indirect gap of Ge. By investigating the dependence of valence band spin-orbit splitting on strain, we prove that the dependency relationship and the coupled ways between the valence-band states of tensile-strained Ge are closely related to the symmetry of strain tensor, i.e., the symmetry of the substrate orientation. The first- and second-order coefficients describing the dependence of indirect gap, direct gap, the valence band spin-orbit coupling splitting, and heavy-hole-light-hole splitting of Ge on IPBTSs have been obtained by the least squares polynomial fitting. These coefficients are significant to quantitatively modulate the electronic properties of Ge by tensile strain and design tensile-strained Ge devices by semiconductor epitaxial technique.
NASA Technical Reports Server (NTRS)
Gabb, Timothy P.; Telesman, Jack; Banik, Anthony; McDevitt, Erin
2014-01-01
Intergranular fatigue crack initiation and growth due to environmental degradation, especially at notched features, can often limit the fatigue life of disk superalloys at high temperatures. For clear comparisons, the effects of alloy composition on cracking in air needs to be understood and compared separately from variables associated with notches and cracks such as effective stress concentration, plastic flow, stress relaxation, and stress redistribution. The objective of this study was to attempt using simple tensile tests of specimens with uniform gage sections to compare the effects of varied alloy composition on environment-assisted cracking of several powder metal and cast and wrought superalloys including ME3, LSHR, Udimet 720, ATI 718Plus alloy, Haynes 282, and Inconel 740. Slow and fast strain-rate tensile tests were found to be a useful tool to compare propensities for intergranular surface crack initiation and growth. The effects of composition and heat treatment on tensile fracture strain and associated failure modes were compared. Environment interactions were determined to often limit ductility, by promoting intergranular surface cracking. The response of various superalloys and heat treatments to slow strain rate tensile testing varied substantially, showing that composition and microstructure can significantly influence environmental resistance to cracking.
Analysis of crack propagation as an energy absorption mechanism in metal matrix composites
NASA Technical Reports Server (NTRS)
Adams, D. F.; Murphy, D. P.
1981-01-01
The crack initiation and crack propagation capability was extended to the previously developed generalized plane strain, finite element micromechanics analysis. Also, an axisymmetric analysis was developed, which contains all of the general features of the plane analysis, including elastoplastic material behavior, temperature-dependent material properties, and crack propagation. These analyses were used to generate various example problems demonstrating the inelastic response of, and crack initiation and propagation in, a boron/aluminum composite.
An Accurate Upper Bound Solution for Plane Strain Extrusion through a Wedge-Shaped Die
Mustafa, Yusof; Lyamina, Elena
2014-01-01
An upper bound method for the process of plane strain extrusion through a wedge-shaped die is derived. A technique for constructing a kinematically admissible velocity field satisfying the exact asymptotic singular behavior of real velocity fields in the vicinity of maximum friction surfaces (the friction stress at sliding is equal to the shear yield stress on such surfaces) is described. Two specific upper bound solutions are found using the method derived. The solutions are compared to an accurate slip-line solution and it is shown that the accuracy of the new method is very high. PMID:25101311
Biologically inspired crack delocalization in a high strain-rate environment
Knipprath, Christian; Bond, Ian P.; Trask, Richard S.
2012-01-01
Biological materials possess unique and desirable energy-absorbing mechanisms and structural characteristics worthy of consideration by engineers. For example, high levels of energy dissipation at low strain rates via triggering of crack delocalization combined with interfacial hardening by platelet interlocking are observed in brittle materials such as nacre, the iridescent material in seashells. Such behaviours find no analogy in current engineering materials. The potential to mimic such toughening mechanisms on different length scales now exists, but the question concerning their suitability under dynamic loading conditions and whether these mechanisms retain their energy-absorbing potential is unclear. This paper investigates the kinematic behaviour of an ‘engineered’ nacre-like structure within a high strain-rate environment. A finite-element (FE) model was developed which incorporates the pertinent biological design features. A parametric study was carried out focusing on (i) the use of an overlapping discontinuous tile arrangement for crack delocalization and (ii) application of tile waviness (interfacial hardening) for improved post-damage behaviour. With respect to the material properties, the model allows the permutation and combination of a variety of different material datasets. The advantage of such a discontinuous material shows notable improvements in sustaining high strain-rate deformation relative to an equivalent continuous morphology. In the case of the continuous material, the shockwaves propagating through the material lead to localized failure while complex shockwave patterns are observed in the discontinuous flat tile arrangement, arising from platelet interlocking. The influence of the matrix properties on impact performance is investigated by varying the dominant material parameters. The results indicate a deceleration of the impactor velocity, thus delaying back face nodal displacement. A final series of FE models considered the
Biologically inspired crack delocalization in a high strain-rate environment.
Knipprath, Christian; Bond, Ian P; Trask, Richard S
2012-04-01
Biological materials possess unique and desirable energy-absorbing mechanisms and structural characteristics worthy of consideration by engineers. For example, high levels of energy dissipation at low strain rates via triggering of crack delocalization combined with interfacial hardening by platelet interlocking are observed in brittle materials such as nacre, the iridescent material in seashells. Such behaviours find no analogy in current engineering materials. The potential to mimic such toughening mechanisms on different length scales now exists, but the question concerning their suitability under dynamic loading conditions and whether these mechanisms retain their energy-absorbing potential is unclear. This paper investigates the kinematic behaviour of an 'engineered' nacre-like structure within a high strain-rate environment. A finite-element (FE) model was developed which incorporates the pertinent biological design features. A parametric study was carried out focusing on (i) the use of an overlapping discontinuous tile arrangement for crack delocalization and (ii) application of tile waviness (interfacial hardening) for improved post-damage behaviour. With respect to the material properties, the model allows the permutation and combination of a variety of different material datasets. The advantage of such a discontinuous material shows notable improvements in sustaining high strain-rate deformation relative to an equivalent continuous morphology. In the case of the continuous material, the shockwaves propagating through the material lead to localized failure while complex shockwave patterns are observed in the discontinuous flat tile arrangement, arising from platelet interlocking. The influence of the matrix properties on impact performance is investigated by varying the dominant material parameters. The results indicate a deceleration of the impactor velocity, thus delaying back face nodal displacement. A final series of FE models considered the
NASA Astrophysics Data System (ADS)
Zaitsev, V. Yu.; Matveev, L. A.
2012-05-01
Presently, experimental evidence for extremely high strain-sensitivity of dissipation in rocks and similar microstructured materials is obtained both in laboratory and field conditions, in particular observations of pronounced amplitude modulation of the radiation of high-stability seismo-acoustic sources by tidal deformations of rocks with typical strains ~ 10-8. Such data indicate the presence of some thresholdless in amplitude and very efficient mechanism of strain-dependent dissipation. Conventionally, its origin is discussed in the context of frictional or adhesion-hysteretic loss at cracks in rocks. However, such dissipation mechanisms are not relevant to weak perturbations with displacements smaller than atomic size. Here, we revise thresholdless thermoelastic loss in dry cracks and viscous loss in saturated cracks taking into account wavy asperities typical of real cracks, which can create elongated (strip-like) contacts or almost closed "waists" in cracks. Thermoelastic loss at these contacts can be very efficient. Besides, the state of such contacts can already be strongly perturbed by the average strain which yet practically does not change the mean opening of the entire crack. Thus the dissipation localized at such contacts can be significantly affected by quite small average strain (e.g., 10-8), which is usually believed to be unable to produce any appreciable effect on the dissipation. Next, for liquid-saturated cracks, the presence of inner elongated asperities also drastically changes the character of squirt-type viscous dissipation. Velocity gradients and consequently the dissipation are localized in the vicinity of the nearly-closed waists which almost harness the liquid flow in the crack. This dissipation can be comparable in magnitude with viscous dissipation at the entire crack with smooth interface, but the decrement maximum is strongly shifted downwards on the frequency axis. Since near the waist the gap is much smaller than the average crack
NASA Astrophysics Data System (ADS)
Sun, Y. W.; Holec, D.; Dunstan, D. J.
2015-09-01
Stacking graphene sheets forms graphite. Two in-plane vibrational modes of graphite, E1 u and E2g (2 ), are derived from the graphene E2 g mode, the shifts of which under compression are considered as results of the in-plane bond shortening. Values of the Grüneisen parameter have been reported to quantify such a relation. However, the reason why the shift rates of these three modes with pressure differ is unclear. In this work, we introduce new parameters γE2g'=-0.0131 and γE1u'=0.0585 to quantify the contribution of out-of-plane strain to the shift of the in-plane vibrational frequencies, suggesting that the compression of the π - electrons plays a non-negligible part in both graphite and graphene under high pressure.
NASA Astrophysics Data System (ADS)
Kim, Young Suk; Kim, Sung Soo
2016-07-01
We show that enhanced stress corrosion cracking (SCC) initiation in cold-rolled Alloy 690 with decreasing strain rate is related to the rate of short-range ordering (SRO) but not to the time-dependent corrosion process. Evidence for SRO is provided by aging tests on cold-rolled Alloy 690 at 623 K and 693 K (350 °C and 420 °C), respectively, which demonstrate its enhanced lattice contraction and hardness increase with aging temperature and time, respectively. Secondary intergranular cracks formed only in thermally treated and cold-rolled Alloy 690 during SCC tests, which are not SCC cracks, are caused by its lattice contraction by SRO before SCC tests but not by the orientation effect.
NASA Astrophysics Data System (ADS)
Kim, Young Suk; Kim, Sung Soo
2016-09-01
We show that enhanced stress corrosion cracking (SCC) initiation in cold-rolled Alloy 690 with decreasing strain rate is related to the rate of short-range ordering (SRO) but not to the time-dependent corrosion process. Evidence for SRO is provided by aging tests on cold-rolled Alloy 690 at 623 K and 693 K (350 °C and 420 °C), respectively, which demonstrate its enhanced lattice contraction and hardness increase with aging temperature and time, respectively. Secondary intergranular cracks formed only in thermally treated and cold-rolled Alloy 690 during SCC tests, which are not SCC cracks, are caused by its lattice contraction by SRO before SCC tests but not by the orientation effect.
The impact of strain, bedding plane friction and overburden pressure on joint spacing
NASA Astrophysics Data System (ADS)
Arslan, Arzu; Schöpfer, Martin P. J.; Walsh, John J.; Childs, Conrad
2010-05-01
In layered sequences, rock joints usually best develop within the more brittle layers and commonly display a regular spacing that scales with layer thickness. A variety of conceptual and mechanical models have been advanced to explain this relationship. A limitation of previous approaches, however, is that fracture initiation and associated bedding-parallel slip are not explicitly simulated; instead, fractures were predefined and interfaces were welded. To surmount this problem, we have modelled the formation and growth of joints in layered sequences by using the two-dimensional Distinct Element Method (DEM) as implemented in the Particle Flow Code (PFC-2D). In PFC-2D, rock is represented by an assemblage of circular particles that are bonded at particle-particle contacts, with failure occurring when either the tensile or shear strength of a bond is exceeded. Model materials with different rheological properties can be generated by calibrating the results of synthetic mechanical test procedures with those of real rocks. Our simple models of jointing comprise a central brittle layer with high Young's modulus, which is embedded in a low Young's modulus matrix. The interfaces between the layers (i.e. bedding planes) are defined by ‘smooth joint' contacts, a modelling feature that eliminates interparticle bumpiness and associated interlocking friction. Consequently, this feature allows the user to assign macroscopic properties such as friction along layer interfaces in a controlled manner. Layer parallel extension is applied by assigning a velocity to particles at the lateral boundaries of the model while maintaining a constant vertical confining pressure. Models were extended until joint saturation was reached in the central layer. We thereby explored the impact of strain, bedding plane friction and overburden pressure on joint spacing. The modelling revealed that joint spacing decreases as strain, bedding plane friction and overburden pressure are increased
Effect of material damage on the stress-strain state near a crack tip in creep
NASA Astrophysics Data System (ADS)
Astaf'ev, V. I.; Grigorova, T. V.; Pastukhov, V. A.
1992-02-01
The asymptotic stress and damage fields near the tip of a growing crack are determined for a creep-damaged material described by Rabotnov-Hayhurst-Leckie constitutive equations. It is found that the singular stress field, characteristic of the crack theory, is absent near the crack tip, which is consistent with the results of finite element solutions for tearing cracks. A crack growth law is obtained which provides a qualitative description of the crack growth process in stainless steels under constant loading.
NASA Astrophysics Data System (ADS)
Kim, K.; Okayasu, K.; Fukutomi, H.
2015-04-01
The formation behavior of basal texture during high temperature deformation of AZ80 magnesium alloys in single phase was investigated by plane strain compression deformation. Three kinds of specimens with different initial textures were machined out from an extruded bar having a <101¯0> texture. Plane strain compression tests were conducted at temperatures of 623K and 723K and a strain rate of 5.0×10-2s-1, with a strain range of between - 0.4 and -1.0. After deformation, the specimens were immediately quenched in oil. Texture measurement was carried out on the compression planes by the Schulz reflection method using nickel filtered Cu Kα radiation. Electron backscatter diffraction (EBSD) measurements were also conducted in order to examine the spatial distribution of orientations. Three kinds of specimens named A, B and C were prepared from the same extruded bar. In the specimens A, B and C, {0001} was distributed preferentially parallel to ND, TD, and RD, respectively. After deformation, texture evaluation was conducted on the mid-plane section. At the plane strain compression deformation, peaks appeared in the true stress-true strain curves irrespective of the kinds of specimen used. It was found that the main components and the pole densities of the textures vary depending on deformation condition and initial texture. Six kinds of texture components were observed after deformation. The (0001)<101¯0> has formed regardless of the initial texture. There are two types of texture components; one exists before the deformation, and the other does not. Either types are considered to have stable orientations for plane strain compression. Also, the basal texture is composed of two crystal orientation components - (0001)<101¯0> and (0001)<112¯0>. When (0001) existed before deformation, an extremely sharp (0001) (compression plane) texture is formed.
Assessment of Constitutive and Stability Behavior of Sands Under Plane Strain Condition
NASA Technical Reports Server (NTRS)
Alshibli, Khalid A.; Sture, Stein
2000-01-01
A series of biaxial (plane strain) experiments were conducted on three sands under low (15 kPa) and high (100 kPa) confining pressure conditions to investigate the effects of specimen density, confining pressure, and sand grains size and shape on the constitutive and stability behavior of granular materials. The three sands used in the experiments were fine, medium, and coarse-grained uniform silica sands with rounded, sub-angular, and angular grains, respectively. Specimen deformation was readily monitored and analyzed with the help of a grid pattern imprinted on the latex membrane. The overall stress-strain behavior is strongly dependent on the specimen density, confining pressure, sand grain texture, and the resulting failure mode(s). That became evident in different degrees of softening responses at various axial strains. The relationship between the constitutive behavior and the specimens' modes of instability is presented. The failure in all specimens was characterized by two distinct and opposite shear bands. It was found that the measured dilatancy angles increase as the sand grains' angularity and size increase. The measured shear band inclination angles are also presented and compared with classical Coulomb and Roscoe solutions.
Development of a new model for plane strain bending and springback analysis
Zhang, Z.T.; Lee, D.
1995-06-01
A new mathematical model is presented for plane strain bending and springback analysis in sheet metal forming. This model combines effects associated with bending and stretching, considers stress and strain distributions and different thickness variations in the thickness direction, and takes force equilibrium into account. An elastic-plastic material model and Hill`s nonquadratic yield function are incorporated in the model. The model is used to obtain force, bending moment, and springback curvature. A typical two-dimensional draw bending part is divided into five regions along the strip, and the forces and moments acting on each region and the deformation history of each region are examined. Three different methods are applied to the two-dimensional draw bending problems: the first using the new model, the second using the new model but also including a kinematic directional hardening material model to consider the bending and unbending deformation in the wall, and the third using membrane theory plus bending strain. Results from these methods, including those from the recent benchmark program, are compared.
Uniqueness of the interior plane strain time-harmonic viscoelastic inverse problem
NASA Astrophysics Data System (ADS)
Zhang, Yixiao; Barbone, Paul E.; Harari, Isaac; Oberai, Assad A.
2016-07-01
Elasticity imaging has emerged as a promising medical imaging technique with applications in the detection, diagnosis and treatment monitoring of several types of disease. In elasticity imaging measured displacement fields are used to generate images of elastic parameters of tissue by solving an inverse problem. When the tissue excitation, and the resulting tissue motion is time-harmonic, elasticity imaging can be extended to image the viscoelastic properties of the tissue. This leads to an inverse problem for the complex-valued shear modulus at a given frequency. In this manuscript we have considered the uniqueness of this inverse problem for an incompressible, isotropic linear viscoelastic solid in a state of plane strain. For a single measured displacement field we conclude that the solution is infinite dimensional, and the data required to render it unique is determined by the measured strain field. In contrast, for two independent displacement fields such that the principal directions of the resulting strain fields are different, the space of possible solutions is eight dimensional, and given additional data, like the value of the shear modulus at four locations, or over a calibration region, we may determine the shear modulus everywhere. We have also considered simple analytical examples that verify these results and offer additional insights. The results derived in this paper may be used as guidelines by the practitioners of elasticity imaging in designing more robust and accurate imaging protocols.
NASA Astrophysics Data System (ADS)
Ridzuan, M. J. M.; Hafis, S. M.; Saifullah, K. N.; Syahrullail, S.
2012-06-01
Large quantities of lubricant are being widely used in the metal forming industry and this high consumption is negatively affecting the environment. Finding an alternative to this current situation is getting more serious and urgent in response to environmental and operational cost pressures. This paper deals with an experimental investigation to obtain the minimum quantity of lubricant (MQL) of RBD palm stearin, which is used as lubricant between the contact sliding surfaces of the taper die and billet via plane-strain-extrusion apparatus. The symmetrical workpieces are designed as combined billets made from pure aluminium A1100. The dies of the apparatus are made of SKD 11 steel. The extrusion ratio of the processes is 3 and the workpieces are extruded by hydraulic press machine. Four conditions of the quantity selected are 0.1 mg, 1 mg, 5 mg, and 20 mg. The analysis of the result shows that the conditions of the quantity are in the load reducing order from 0.1 mg, 1mg and 5 mg. The highest distribution of surface roughness is at 0.1 mg, whereby for others, the conditions are quite similar. However, the distribution of velocity and effective strain are lowest at 5 mg. The minimum quantity of lubricant (MQL) of the RBD palm stearin as lubricant on the contact sliding surfaces in planestrain-extrusion is determined based on the results of load, surface roughness, velocity and effective strain.
Plane strain finite element analysis of sheet forming operations including bending effects
NASA Astrophysics Data System (ADS)
Cho, Uk Youn
1993-01-01
An improved finite element method suitable for the plane-strain analysis of sheet metal forming operations is presented. The method incorporates a computationally efficient shell model and a consistent frictional contact algorithm through an implicit updated Lagrangian formulation. The workpiece material model is rigid-viscoplastic with a choice of power law hardening and plastic normal anisotropy and is capable of modeling a wide variety of sheet metals. A simplified nonlinear incremental shell theory is employed along with an optional reduced integration through the thickness for computational efficiency, while retaining the advantages of the kinematic model containing the bending effects. Complex tool geometry can be handled by discrete data points, by primitives (lines and arcs), or by analytical functions. The capabilities of the method are demonstrated through verification problems and comparisons with experimental data, benchmark results, and published data for several practical problems of the sheet metal forming industry. The problems include stretching and/or deep drawing operations, simulation of automobile panel section, and brake bending operation. As an independent investigation from the first portion of the dissertation, measured data from a set of simple bending experiments of two types of aluminum are presented and analyzed. Generated data from the experiments include strain histories (loading and unloading), spring back information (spring back angle and strains), and friction coefficients. As a by-product, a simple way of estimating the friction coefficient (Coulomb's law) during a bending operation is proposed and demonstrated.
NASA Astrophysics Data System (ADS)
Kvasha, Oleg V.; Boström, Anders; Glushkova, Natalia V.; Glushkov, Evgeny V.
2011-08-01
The propagation of in-plane (P-SV) waves in a symmetrically three-layered thick plate with a periodic array of interface cracks is investigated. The exact dispersion relation is derived based on an integral equation approach and Floquet's theorem. The interface cracks can be a model for interface damage, but a much simpler model is a recently developed spring boundary condition. This boundary condition is used for the thick plate and also in the derivation of plate equations with the help of power series expansions in the thickness coordinate. For low frequencies (cracks small compared to the wavelength) the three approaches give more or less coinciding dispersion curves, and this is a confirmation that the spring boundary condition is a reasonable approximation at low frequencies.
Influence of load interactions on crack growth as related to state of stress and crack closure
NASA Technical Reports Server (NTRS)
Telesman, J.
1985-01-01
Fatigue crack propagation (FCP) after an application of a low-high loading sequence was investigated as a function of specimen thickness and crack closure. No load interaction effects were detected for specimens in a predominant plane strain state. However, for the plane stress specimens, initially high FCP rates after transition to a higher stress intensity range were observed. The difference in observed behavior was explained by examining the effect of the resulting closure stress intensity values on the effective stress intensity range.
NASA Astrophysics Data System (ADS)
EL-Bealy, Mostafa Omar
2012-06-01
In a study of the early stages of dendritic solidification in the direct-chill cast sheet ingots, the coupled effect of interdendritic strain and macrosegregation on the interdendritic cracks formation in dendritic equiaxed structure has been investigated by the metallographic study of ingot samples and by performing a set of mathematical analyses for AA-6061 and AA-1050 aluminum alloys. The metallographic investigation contains microstructure examinations and macrosegregation measurements of collected samples from plant trials. The mathematical analysis consists of a two-dimensional (2-D) fluid flow, heat flow, interdendritic strain, and macrosegregation-coupled model. Also, a simple approach to measure interdendritic crack has been developed based on the accumulative interdendritic strain criterion, local dendritic phases, and the crystal distortion correlation factor resulting from steep positive local segregation. The model predications have clarified the effect of high positive macrosegregation on the surface and subsurface interdendritic crack formation. It has been revealed that interdendritic strain starts to generate just below the liquidus temperature, resulting from shrinkage of liquid→solid phase transformation and contraction of dendritic solid in the incoherent mushy region. In this region, the coupled effect of the shrinkage/contraction mechanism increases the interdendritic distances between equiaxed crystals and the interdendritic crack begins to nucleate. Subsequently, in the coherent mushy region, the different interdendritic strain sources start to affect significantly the distances between equiaxed crystals in a diverse way, and therefore, the final morphology of interdendritic crack begins to form. The mechanism of interdendritic crack formation during dendritic equiaxed structure solidification and the possible solutions to this problem are discussed.
NASA Technical Reports Server (NTRS)
Obrien, T. K.
1991-01-01
An analysis utilizing laminated plate theory is developed to calculate the strain energy release rate associated with local delaminations originating at off-axis, single ply, matrix cracks in laminates subjected to uniaxial loads. The analysis includes the contribution of residual thermal and moisture stresses to the strain energy released. Examples are calculated for the strain energy release rate associated with local delaminations originating at 90 degrees and angle-ply (non-90 degrees) matrix ply cracks in glass epoxy and graphite epoxy laminates. The solution developed may be used to assess the relative contribution of mechanical, residual thermal, and moisture stresses on the strain energy release rate for local delamination for a variety of layups and materials.
Strained-layer superlattice focal plane array having a planar structure
Kim, Jin K.; Carroll, Malcolm S.; Gin, Aaron; Marsh, Phillip F.; Young, Erik W.; Cich, Michael J.
2010-07-13
An infrared focal plane array (FPA) is disclosed which utilizes a strained-layer superlattice (SLS) formed of alternating layers of InAs and In.sub.xGa.sub.1-xSb with 0.ltoreq.x.ltoreq.0.5 epitaxially grown on a GaSb substrate. The FPA avoids the use of a mesa structure to isolate each photodetector element and instead uses impurity-doped regions formed in or about each photodetector for electrical isolation. This results in a substantially-planar structure in which the SLS is unbroken across the entire width of a 2-D array of the photodetector elements which are capped with an epitaxially-grown passivation layer to reduce or eliminate surface recombination. The FPA has applications for use in the wavelength range of 3-25 .mu.m.
Infrared focal plane arrays based on dots in a well and strained layer superlattices
NASA Astrophysics Data System (ADS)
Krishna, Sanjay
2009-01-01
In this paper, we will review some of the recent progress that we have made on developing single pixel detectors and focal plane arrays based on dots-in-a-well (DWELL) heterostructure and Type II strained layer superlattice (SLS). The DWELL detector consists of an active region composed of InAs quantum dots embedded in InGaAs/GaAs quantum wells. By varying the thickness of the InGaAs well, the DWELL heterostructure allows for the manipulation of the operating wavelength and the nature of the transitions (bound-to-bound, bound-to-quasibound and bound-to-continuum) of the detector. Based on these principles, DWELL samples were grown using molecular beam epitaxy and fabricated into 320 x 256 focal plane arrays (FPAs) with Indium bumps using standard lithography at the University of New Mexico. The FPA evaluated was hybridized to an Indigo 9705 readout integrated circuit (ROIC). From this evaluation, we have reported the first two-color, co-located quantum dot based imaging system that can be used to take multicolor images using a single FPA. We have also been investigating the use of miniband transitions in Type II SLS to develop infrared detectors using PIN and nBn based designs.
NASA Astrophysics Data System (ADS)
Salvo, Maxime; Sercombe, Jérôme; Helfer, Thomas; Sornay, Philippe; Désoyer, Thierry
2015-05-01
In this paper, the behavior of a dense UO2 (porosity less than 2%) was studied experimentally on a range of temperatures (1100-1700 °C) and strain rates (10-4-10-1 /s) representative of RIA loading conditions. The yield stress was found to increase with strain rate and to decrease with temperature. Macroscopic cracking of the samples was apparent after the tests at 1100 °C. Scanning Electron Microscopy (SEM) image analyses revealed a pronounced grain boundary cracking in the core of the samples tested at 10-1 /s and at 1550-1700 °C. A hyperbolic sine model for the viscoplastic strain rate with a clear dependency on porosity was first developed. It was completed by a Drucker-Prager yield criterion with associated plastic flow to account for the porosity increase induced by grain boundary cracking. Finite Elements simulations of the compression tests on the dense UO2 were then successfully compared to the stress-strain curves, post-test diameter profiles and porosities at the pellets' center, periphery and top extremity. The response of the grain boundary cracking model was then studied in biaxial compression, this condition being closer to that of the pellet during a RIA power transient.
NASA Astrophysics Data System (ADS)
Bao, Yi; Chen, Genda
2016-01-01
This study aims at evaluating the feasibility of strain measurement and crack detection in thin unbonded concrete pavement overlays with pulse prepump Brillouin optical time domain analysis. Single-mode optical fibers with two-layer and three-layer coatings, respectively, were applied as fully distributed sensors, their performances were compared with analytical predictions. They were successfully protected from damage during concrete casting of three full-scale concrete panels when 5 to 10-cm-thick protective mortar covers had been set for 2 h. Experimental results from three-point loading tests of the panels indicated that the strain distributions measured from the two types of sensors were in good agreement, and cracks can be detected at sharp peaks of the measured strain distributions. The two-layer and three-layer coated fibers can be used to measure strains up to 2.33% and 2.42% with a corresponding sensitivity of 5.43×10-5 and 4.66×10-5 GHz/μɛ, respectively. Two cracks as close as 7 to 9 cm can be clearly detected. The measured strains in optical fiber were lower than the analytical prediction by 10% to 25%. Their difference likely resulted from strain transfer through various coatings, idealized point loading, varying optical fiber embedment, and concrete heterogeneity.
Andrews, D.J.
1985-01-01
A numerical boundary integral method, relating slip and traction on a plane in an elastic medium by convolution with a discretized Green function, can be linked to a slip-dependent friction law on the fault plane. Such a method is developed here in two-dimensional plane-strain geometry. Spontaneous plane-strain shear ruptures can make a transition from sub-Rayleigh to near-P propagation velocity. Results from the boundary integral method agree with earlier results from a finite difference method on the location of this transition in parameter space. The methods differ in their prediction of rupture velocity following the transition. The trailing edge of the cohesive zone propagates at the P-wave velocity after the transition in the boundary integral calculations. Refs.
A New Look at Self-Similarity in Strained Plane Wakes. 1.3
NASA Technical Reports Server (NTRS)
Rogers, Michael M.; Mansour, Nagi N. (Technical Monitor)
2001-01-01
In early experiments, A. J. Reynolds and J. F. Keffer sought to determine whether plane wakes of circular cylinders, when strained by a wind tunnel of varying cross-section, evolved in accordance with an analytically derived self-similar solution. As pointed out by Reynolds, for the strain geometry considered this self-similar solution indicated exponential growth of the viscous term in the mean momentum equation, a result which he interpreted as suggesting that such wakes would eventually relaminarize. The experimental results were found not to agree with the similarity theory and recent direct numerical simulations confirm this. However, a more general self-similar analysis of the kind suggested by W. K. George is found to lead to families of possible similarity solutions, some of which do indeed describe the observed flaw behavior. These equilibrium similarity solutions result from creating a balance in the governing equations by grouping certain terms. For these solutions the viscous terms can be retained in the analysis.
Wireless Open-Circuit In-Plane Strain and Displacement Sensor Requiring No Electrical Connections
NASA Technical Reports Server (NTRS)
Woodard, Stanley E. (Inventor)
2014-01-01
A wireless in-plane strain and displacement sensor includes an electrical conductor fixedly coupled to a substrate subject to strain conditions. The electrical conductor is shaped between its ends for storage of an electric field and a magnetic field, and remains electrically unconnected to define an unconnected open-circuit having inductance and capacitance. In the presence of a time-varying magnetic field, the electrical conductor so-shaped resonates to generate harmonic electric and magnetic field responses. The sensor also includes at least one electrically unconnected electrode having an end and a free portion extending from the end thereof. The end of each electrode is fixedly coupled to the substrate and the free portion thereof remains unencumbered and spaced apart from a portion of the electrical conductor so-shaped. More specifically, at least some of the free portion is disposed at a location lying within the magnetic field response generated by the electrical conductor. A motion guidance structure is slidingly engaged with each electrode's free portion in order to maintain each free portion parallel to the electrical conductor so-shaped.
Strain-induced crack formations in PDMS/DXA drug collars.
Warner, J A; Polkinghorne, J C; Gonerka, J; Meyer, S; Luo, B; Frethem, C; Haugstad, G
2013-07-01
Drug-eluting systems are currently used in cardiac leads in order to reduce inflammation and fibrosis at the lead-tissue interface. Drug release from these drug delivery systems can be modulated by the manufacturing processes used to create the drug systems and assemble them onto the cardiac lead. In this study, scanning electron microscopy, atomic force microscopy and Raman microscopy are employed to explore the material characteristics of a polydimethylsiloxane-dexamethasone acetate drug collar used on cardiac leads when varying the strain during collar assembly on the lead. A novel test fixture was created in order to investigate these drug collars under simulated stresses. Measurements of the collar while fitted to a rod revealed microcracks that are hypothesized to affect the drug release performance, resulting in increased drug elution. It was found that the strain that occurs during assembly of the collar onto the lead is a key factor in the formation of these microcracks. Results also suggest that cracks tend to form in areas of high drug particle density, and propagate between drug particles. PMID:23541599
Wang, Cheng-Cai; Mao, Yun-Wei; Shan, Zhi-Wei; Dao, Ming; Li, Ju; Sun, Jun; Ma, Evan; Suresh, Subra
2013-12-01
Metallic glasses (MGs) exhibit greater elastic limit and stronger resistance to plastic deformation than their crystalline metal counterparts. Their capacity to withstand plastic straining is further enhanced at submicrometer length scales. For a range of microelectromechanical applications, the resistance of MGs to damage and cracking from thermal and mechanical stress or strain cycling under partial or complete constraint is of considerable scientific and technological interest. However, to our knowledge, no real-time, high-resolution transmission electron microscopy observations are available of crystallization, damage, and failure from the controlled imposition of cyclic strains or displacements in any metallic glass. Here we present the results of a unique in situ study, inside a high-resolution transmission electron microscope, of glass-to-crystal formation and fatigue of an Al-based MG. We demonstrate that cyclic straining progressively leads to nanoscale surface roughening in the highly deformed region of the starter notch, causing crack nucleation and formation of nanocrystals. The growth of these nanograins during cyclic straining impedes subsequent crack growth by bridging the crack. In distinct contrast to this fatigue behavior, only distributed nucleation of smaller nanocrystals is observed with no surface roughening under monotonic deformation. We further show through molecular dynamics simulation that these findings can be rationalized by the accumulation of strain-induced nonaffine atomic rearrangements that effectively enhances diffusion through random walk during repeated strain cycling. The present results thus provide unique insights into fundamental mechanisms of fatigue of MGs that would help shape strategies for material design and engineering applications. PMID:24255113
Wang, Cheng-Cai; Mao, Yun-Wei; Shan, Zhi-Wei; Dao, Ming; Li, Ju; Sun, Jun; Ma, Evan; Suresh, Subra
2013-01-01
Metallic glasses (MGs) exhibit greater elastic limit and stronger resistance to plastic deformation than their crystalline metal counterparts. Their capacity to withstand plastic straining is further enhanced at submicrometer length scales. For a range of microelectromechanical applications, the resistance of MGs to damage and cracking from thermal and mechanical stress or strain cycling under partial or complete constraint is of considerable scientific and technological interest. However, to our knowledge, no real-time, high-resolution transmission electron microscopy observations are available of crystallization, damage, and failure from the controlled imposition of cyclic strains or displacements in any metallic glass. Here we present the results of a unique in situ study, inside a high-resolution transmission electron microscope, of glass-to-crystal formation and fatigue of an Al-based MG. We demonstrate that cyclic straining progressively leads to nanoscale surface roughening in the highly deformed region of the starter notch, causing crack nucleation and formation of nanocrystals. The growth of these nanograins during cyclic straining impedes subsequent crack growth by bridging the crack. In distinct contrast to this fatigue behavior, only distributed nucleation of smaller nanocrystals is observed with no surface roughening under monotonic deformation. We further show through molecular dynamics simulation that these findings can be rationalized by the accumulation of strain-induced nonaffine atomic rearrangements that effectively enhances diffusion through random walk during repeated strain cycling. The present results thus provide unique insights into fundamental mechanisms of fatigue of MGs that would help shape strategies for material design and engineering applications. PMID:24255113
Wentzell, Scott; Nesbitt, Robert Sterling; Macione, James; Kotha, Shiva
2016-07-01
The maintenance of healthy bone tissue depends upon the ability of osteocytes to respond to mechanical cues on the cellular level. The combination of digital volume correlation and second harmonic generation microscopy offers the opportunity to investigate the mechanical microenvironment of intact bone on the scale of individual osteocytes. Adult human femurs were imaged under tensile loads of 5 and 15MPa and volumes of approximately 492×429×31μm(3) were analyzed, along with an image of a bone microcrack under the same loading conditions. Principal strains were significantly higher in three-dimensional digital volume correlation when compared to two-dimensional digital image correlation. The average maximum principal strain magnitude was 5.06-fold greater than the applied global strain, with peak strains of up to 23.14-fold over global strains measured at the borders of osteocyte lacunae. Finally, a microcrack that initiated at an osteocyte lacunae had its greatest tensile strain magnitudes at the crack expansion front in the direction of a second lacunae, but strain at the crack border was reduced to background strain magnitudes upon breaching the second lacunae. This serveed to demonstrate the role of lacunae in initiating, mediating and terminating microcrack growth. PMID:26807766
NASA Astrophysics Data System (ADS)
Benito, L.; Ballesteros, C.; Ward, R. C. C.
2014-04-01
We report on the magnetic and structural characterization of high lattice-mismatched [Dy2nm/SctSc] superlattices, with variable Sc thickness tSc= 2-6 nm. We find that the characteristic in-plane effective hexagonal magnetic anisotropy K66,ef reverses sign and undergoes a dramatic reduction, attaining values of ≈13-24 kJm-3, when compared to K66=-0.76 MJm-3 in bulk Dy. As a result, the basal plane magnetic anisotropy is dominated by a uniaxial magnetic anisotropy (UMA) unfound in bulk Dy, which amounts to ≈175-142 kJm-3. We attribute the large downsizing in K66,ef to the compression epitaxial strain, which generates a competing sixfold magnetoelastic (MEL) contribution to the magnetocrystalline (strain-free) magnetic anisotropy. Our study proves that the in-plane UMA is caused by the coupling between a giant symmetry-breaking MEL constant Mγ ,22≈1 GPa and a morphic orthorhombiclike strain ɛγ ,1≈10-4, whose origin resides on the arising of an in-plane anisotropic strain relaxation process of the pseudoepitaxial registry between the nonmagnetic bottom layers in the superstructure. This investigation shows a broader perspective on the crucial role played by epitaxial strains at engineering the magnetic anisotropy in multilayers.
NASA Technical Reports Server (NTRS)
Buczek, M. B.; Gregory, M. A.; Herakovich, C. T.
1983-01-01
CLFE2D is a two dimensional generalized plane strain finite element code, using a linear, four node, general quadrilateral, isoparametric element. The program is developed to calculate the displacements, strains, stresses, and strain energy densities in a finite width composite laminate. CLFE2D offers any combination of the following load types: nodal displacements, nodal forces, uniform normal strain, or hygrothermal. The program allows the user to input one set of three dimensional orthotropic material properties. The user can then specify the angle of material principal orientation for each element in the mesh. Output includes displacements, stresses, strains and strain densities at points selected by the user. An option is also available to plot the underformed and deformed finite element meshes.
Line Spring Model and Its Applications to Part-Through Crack Problems in Plates and Shells
NASA Technical Reports Server (NTRS)
Erdogan, F.; Aksel, B.
1986-01-01
The line spring model is described and extended to cover the problem of interaction of multiple internal and surface cracks in plates and shells. The shape functions for various related crack geometries obtained from the plane strain solution and the results of some multiple crack problems are presented. The problems considered include coplanar surface cracks on the same or opposite sides of a plate, nonsymmetrically located coplanar internal elliptic cracks, and in a very limited way the surface and corner cracks in a plate of finite width and a surface crack in a cylindrical shell with fixed end.
Line spring model and its applications to part-through crack problems in plates and shells
NASA Technical Reports Server (NTRS)
Erdogan, Fazil; Aksel, Bulent
1988-01-01
The line spring model is described and extended to cover the problem of interaction of multiple internal and surface cracks in plates and shells. The shape functions for various related crack geometries obtained from the plane strain solution and the results of some multiple crack problems are presented. The problems considered include coplanar surface cracks on the same or opposite sides of a plate, nonsymmetrically located coplanar internal elliptic cracks, and in a very limited way the surface and corner cracks in a plate of finite width and a surface crack in a cylindrical shell with fixed end.
NASA Astrophysics Data System (ADS)
Terasaki, Hidenori; Shintome, Yutaro; Komizo, Yu-ichi; Ohata, Mitsuru; Moriguchi, Koji; Tomio, Yusaku
2015-05-01
Global effect of Bain-zone boundaries and the local effect of close-packed plane (CP) boundaries on the path of secondary cleavage cracks (observed on a fractured V-notch Charpy specimen) were visualized and discussed in simulated coarse-grained HAZ of bainitic steel. Microstructural unit map (Bain-zone map and CP map) was obtained by electron backscatter diffraction method for a prior austenite grain of a few hundred micrometers. Furthermore, a correlation between CP boundaries and sites that favored formation of the blocky martensite-austenite (M-A) constituent was confirmed. It was clarified that two crack deviation/local changing factors (CP boundaries and blocky M-A) are paired.
Zaitsev, V Yu; Matveev, L A
2012-01-01
Mechanisms of acoustic energy dissipation in heterogeneous solids attract much attention in view of their importance for material characterization, nondestructive testing, and geophysics. Due to the progress in measurement techniques in recent years, it has been revealed that rocks can demonstrate extremely high strain sensitivity of seismoacoustic loss. In particular, it has been found that strains of order 10(-8) produced by lunar and solar tides are capable of causing variations in the seismoacoustic decrement on the order of several percent. Some laboratory data (although obtained for higher frequencies) also indicate the presence of very high dissipative nonlinearity. Conventionally discussed dissipation mechanisms (thermoelastic loss in dry solids, Biot and squirt-type loss in fluid-saturated ones) do not suffice to interpret such data. Here the dissipation at individual cracks is revised taking into account the influence of wavy asperities of their surfaces quite typical of real cracks, which can drastically change the values of the relaxation frequencies and can result in giant strain sensitivity of the dissipation without the necessity of assuming the presence of unrealistically thin (and, therefore, unrealistically soft) cracks. In particular, these mechanisms suggest interpretation for observations of pronounced amplitude modulation of seismo-acoustic waves by tidal strains. PMID:22280566
NASA Astrophysics Data System (ADS)
Komaragiri, Uday; Agnew, Sean R.; Gangloff, Richard P.; Begley, Matthew R.
This paper quantifies the effect of strain gradient plasticity (SGP) on crack tip stress elevation for a broad range of applied loading conditions and constitutive model parameters, including both macroscopic hardening parameters and individual material length-scales controlling gradient effects. Finite element simulations incorporating the Fleck-Hutchinson SGP theory are presented for an asymptotically sharp stationary crack. Results identify fundamental scaling relationships describing (i) the physical length-scales over which strain gradients are prominent, and (ii) the degree of stress elevation over conventional Hutchinson-Rice-Rosengren (HRR) fields. Results illustrate that the three length-scale theory predicts much larger SGP effects than the single length-scale theory. Critically, the first length-scale parameter dominates SGP stress elevation: this suggests that SGP effects in fracture can be predicted using the length-scales extracted from nanoindentation, which exhibits similar behavior. Transitional loading/material parameters are identified that establish regimes of SGP relevance: this provides the foundation for the rational application of SGP when developing new micromechanical models of crack tip damage mechanisms and associated subcritical crack propagation behavior in structural alloys.
NASA Technical Reports Server (NTRS)
Oldrieve, R. E.
1978-01-01
Analysis of high temperature low cycle fatigue of AISI 304LC and 316 stainless steels by the method of strainrange partitioning results in four separate strainrange versus life relationships, depending upon the way in which creep-strain and plastic strain are combined within a cycle. Fractography is used in this investigation of the creep-fatigue interaction associated with these cycles. The PP and PC-cycle fractures were transgranular. The PC-cycle resulted in fewer cycles of initiation and shorter total cyclic life for the same applied inelastic strainrange. The CC-cycle had mixed transgranular and intergranular fracture, fewer cycles of initiation and shorter cycle life than PP or PC. The CP-cycle had fully integranular cracking, and failed in fewer cycles than were required for cracks to initate for PP,PC, and CC.
Cracked shells under skew-symmetric loading
NASA Technical Reports Server (NTRS)
Lelale, F.
1982-01-01
A shell containing a through crack in one of the principal planes of curvature and under general skew-symmetric loading is considered. By employing a Reissner type shell theory which takes into account the effect of transverse shear strains, all boundary conditions on the crack surfaces are satisfied separately. Consequently, unlike those obtained from the classical shell theory, the angular distributions of the stress components around the crack tips are shown to be identical to the distributions obtained from the plane and antiplane elasticity solutions. Extensive results are given for axially and circumferentially cracked cylindrical shells, spherical shells, and toroidal shells under uniform inplane shearing, out of plane shearing, and torsion. The effect of orthotropy on the results is also studied.
Stress corrosion cracking of Alloy 600 using the constant strain rate test
Bulischeck, T.S.; Van Rooyen, D.
1981-10-01
Nuclear grade production tubing of Alloy 600 was evaluated for stress corrosion cracking (SCC) susceptibility in high purity water at 365, 345, 325, and 290 C. Reverse tube U-bend specimens provided crack initiation data and constant extension rate tests were employed to determine the crack velocities experienced in th crack propagation stage. Initial results indicate that a linear extrapolation of data received from high temperature tests can be used to predict the service life of steam generator tubing that has been plastically deformed or is continually deforming by ''denting.''
NASA Astrophysics Data System (ADS)
Li, Hong; Tsai, Charlie; Koh, Ai Leen; Cai, Lili; Contryman, Alex W.; Fragapane, Alex H.; Zhao, Jiheng; Han, Hyun Soon; Manoharan, Hari C.; Abild-Pedersen, Frank; Nørskov, Jens K.; Zheng, Xiaolin
2016-01-01
As a promising non-precious catalyst for the hydrogen evolution reaction (HER; refs ,,,,), molybdenum disulphide (MoS2) is known to contain active edge sites and an inert basal plane. Activating the MoS2 basal plane could further enhance its HER activity but is not often a strategy for doing so. Herein, we report the first activation and optimization of the basal plane of monolayer 2H-MoS2 for HER by introducing sulphur (S) vacancies and strain. Our theoretical and experimental results show that the S-vacancies are new catalytic sites in the basal plane, where gap states around the Fermi level allow hydrogen to bind directly to exposed Mo atoms. The hydrogen adsorption free energy (ΔGH) can be further manipulated by straining the surface with S-vacancies, which fine-tunes the catalytic activity. Proper combinations of S-vacancy and strain yield the optimal ΔGH = 0 eV, which allows us to achieve the highest intrinsic HER activity among molybdenum-sulphide-based catalysts.
Cracked shells under skew-symmetric loading. [Reissner theory
NASA Technical Reports Server (NTRS)
Delale, F.
1981-01-01
The general problem of a shell containing a through crack in one of the principal planes of curvature and under general skew-symmetric loading is considered. By employing a Reissner type shell theory which takes into account the effect of transverse shear strains, all boundary conditions on the crack surfaces are satisfied separately. Consequently, unlike those obtained from the classical shell theory, the angular distributions of the stress components around the crack tips are shown to be identical to the distributions obtained from the plane and anti-plane elasticity solutions. Results are given for axially and circumferentially cracked cylindrical shells, spherical shells, and toroidal shells under uniform in-plane shearing, out of plane shearing, and torsion. The problem is formulated for specially orthostropic materials, therefore, the effect of orthotropy on the results is also studied.
Fracture Analysis of Semi-Elliptical Surface Cracks in Ductile Materials
NASA Technical Reports Server (NTRS)
Daniewicz, S. R.; Newman, J. C., Jr.; Leach, A. M.
2004-01-01
Accurate life assessment of structural components may require advanced life prediction criteria and methodologies. Structural components often exhibit several different types of defects, among the most prevalent being surface cracks. A semi-elliptical surface crack subjected to monotonic loading will exhibit stable crack growth until the crack has reached a critical size, at which the crack loses stability and fracture ensues (Newman, 2000). The shape and geometry of the flaw are among the most influential factors. When considering simpler crack configurations, such as a through-the-thickness crack, a three-dimensional (3D) geometry may be modeled under the approximation of two-dimensional (2D) plane stress or plane strain. The more complex surface crack is typically modeled numerically with the Finite Element Method (FEM). A semi-elliptical surface crack is illustrated in Figure 1-1.
NASA Astrophysics Data System (ADS)
Wu, Huaping; Ma, Xuefu; Zhang, Zheng; Zhu, Jun; Wang, Jie; Chai, Guozhong
2016-04-01
A nonlinear thermodynamic model based on the vertically aligned nanocomposite (VAN) thin films of ferroelectric-metal oxide system has been developed to investigate the physical properties of the epitaxial Ba0.6Sr0.4TiO3 (BST) films containing vertical Sm2O3 (SmO) nanopillar arrays on the SrTiO3 substrate. The phase diagrams of out-of-plane lattice mismatch vs. volume fraction of SmO are calculated by minimizing the total free energy. It is found that the phase transformation and dielectric response of BST-SmO VAN systems are extremely dependent on the in-plane misfit strain, the out-of-plane lattice mismatch, the volume fraction of SmO phase, and the external electric field applied to the nanocomposite films at room temperature. In particular, the BST-SmO VAN systems exhibit higher dielectric properties than pure BST films. Giant dielectric response and maximum tunability are obtained near the lattice mismatch where the phase transition occurs. Under the in-plane misfit strain of umf=0.3 % and the out-of-plane lattice mismatch of u3=0.002 , the dielectric tunability can be dramatically enhanced to 90% with the increase of SmO volume fraction, which is well consistent with previous experimental results. This work represents an approach to further understand the dependence of physical properties on the lattice mismatch (in-plane and out-of-plane) and volume fraction, and to manipulate or optimize functionalities in the nanocomposite oxide thin films.
Comparison of fatigue crack propagation in Modes I and III
Ritchie, R.O.
1985-06-01
The propagation behavior of fatigue cracks in Mode III (anti-plane shear), measured under cyclic torsion, is described and compared with more commonly encountered behavior under Mode I (tensile opening) loads. It is shown that a unique, global characterization of Mode III growth rates, akin to the Paris ''law'' in Mode I, is only possible if characterizating parameters appropriate to large-scale yielding are employed and allowance is made for crack tip shielding from sliding crack surface interference (i.e., friction and abrasion) between mating fracture surfaces. Based on the crack tip stress and deformation fields for Mode III stationary cracks, the cyclic crack tip displacement, (..delta..CTD/sub III/, and plastic strain intensity range ..delta..GAMMA/sub III/, have been proposed and are found to provide an adequate description of behavior in a range of steels, provided crack surface interference is minimized. The magnitude of this interference, which is somewhat analogous to crack closure in Mode I, is further examined in the light of the complex fractography of torsional fatigue failures and the question of a ''fatigue threshold'' for Mode III crack growth. Finally, micro-mechanical models for cyclic crack extension in anti-plane shear are briefly described, and the contrasting behavior between Mode III and Mode I cracks subjected to simple variable amplitude spectra is examined in terms of the differing role of crack tip blunting and closure in influencing shear, as opposed to tensile opening, modes of crack growth.
NASA Astrophysics Data System (ADS)
Garcia, V.; Sidis, Y.; Marangolo, M.; Vidal, F.; Eddrief, M.; Bourges, P.; Maccherozzi, F.; Ott, F.; Panaccione, G.; Etgens, V. H.
2007-09-01
The α-β magnetostructural phase transition in MnAs/GaAs(111) epilayers is investigated by elastic neutron scattering. The in-plane parameter of MnAs remains almost constant with temperature from 100 to 420 K, following the thermal evolution of the GaAs substrate. This induces a temperature dependent biaxial strain that is responsible for an α-β phase coexistence and, more importantly, for the stabilization of the ferromagnetic α phase at a higher temperature than in the bulk. We explain the premature appearance of the β phase at 275 K and the persistence of the ferromagnetic α phase up to 350 K with thermodynamical arguments based on the MnAs phase diagram. It results that the biaxial strain in the hexagonal plane is the key parameter to extend the ferromagnetic phase well over room temperature.
Garcia, V; Sidis, Y; Marangolo, M; Vidal, F; Eddrief, M; Bourges, P; Maccherozzi, F; Ott, F; Panaccione, G; Etgens, V H
2007-09-14
The alpha-beta magnetostructural phase transition in MnAs/GaAs(111) epilayers is investigated by elastic neutron scattering. The in-plane parameter of MnAs remains almost constant with temperature from 100 to 420 K, following the thermal evolution of the GaAs substrate. This induces a temperature dependent biaxial strain that is responsible for an alpha-beta phase coexistence and, more importantly, for the stabilization of the ferromagnetic alpha phase at a higher temperature than in the bulk. We explain the premature appearance of the beta phase at 275 K and the persistence of the ferromagnetic alpha phase up to 350 K with thermodynamical arguments based on the MnAs phase diagram. It results that the biaxial strain in the hexagonal plane is the key parameter to extend the ferromagnetic phase well over room temperature. PMID:17930469
NASA Astrophysics Data System (ADS)
Peeters, Michael; Panajotov, Krassimir P.; Verschaffelt, Guy; Nagler, Bob; Albert, Jan; Thienpont, Hugo; Veretennicoff, Irina P.; Danckaert, Jan
2002-06-01
It is well known that vertical-cavity surface-emitting lasers (VCSELs) can abruptly switch between two orthogonal linear polarization states if the current is changed. The impact of externally induced in-plane anisotropic strain on this switching was experimentally demonstrated in proton-implanted devices. In this contribution we present a further and thorough experimental investigation of the polarization behavior of different types of VCSELs (proton-implanted, air-post and oxide-confined), under varying strain conditions. We first measure the influence of the strain on the orientation of the axes of the linear polarization states. These axes can be rotated from the crystallographic direction [110] over [100] to [110]. At the same time, we monitor the exact birefringence. From the combination of these two measurements the amount of residual strain in these devices is deduced. Applying strain not only changes the frequency splitting between the two modes (due to birefringence) and their orientation, but also lifts the degeneracy in the gain of the polarization modes. We therefore also measure the gain difference (dichroism) as a function of the applied strain, via the mode suppression ratio and the optical spectrum. Due to the effect on both the birefringence and the dichroism, strain also changes the position of the polarization switching point as a function of current and can lead to the observation of double (consecutive) polarization switching. All this experimental evidence will help to build up a better understanding of the physics of polarization switching in VCSELs.
Shimada, T; Okuno, J; Ishii, Y; Kitamura, T
2012-03-01
We investigated a nanometer-sharp magnetic domain wall (DW) structure in a free-standing Fe(110) monolayer and studied the crucial role of in-plane strain using fully unconstrained noncollinear ab initio spin-density-functional theory calculations within the generalized gradient approximation. The DW width is calculated to be 0.86 nm. A precise vector-field description of the magnetization density revealed that a noncollinear character in the DW was spatially confined between atoms, whereas a collinear and high magnetization density was localized around each atom. In the rapid rotation of magnetic moments in the DW, we found an electron rearrangement from the d(zx) and d(x(2)-y(2)) states to the d(xy), d(yz) and d(z(2)) states due to a shift of band structures. Applied tensile and compressive in-plane strains both bring about narrower DWs in the monolayer except when the strain is small. The strain dependence of the DW width is discussed in terms of both exchange interaction and magnetocrystalline anisotropy. PMID:22322862
Kirkwood, Jonah; Ghetler, Andrew; Sedman, Jacqueline; Leclair, Daniel; Pagotto, Franco; Austin, John W; Ismail, Ashraf A
2006-10-01
A method was developed for whole-organism fingerprinting of Clostridium botulinum isolates by focal plane array Fourier transform infrared (FPA-FTIR) spectroscopy. A database of 150,000 infrared spectra of 44 strains of C. botulinum was acquired using a FPA-FTIR imaging spectrometer equipped with a 16 x 16 array detector to evaluate the ability of FTIR spectroscopy to differentiate the 44 strains. The database contained strains from C. botulinum groups I and II producing botulinum neurotoxin of serotypes A, B, E, and F. All strains were grown on each of three agar media (brain heart infusion, McClung Toabe agar base, and universal) prior to spectral acquisition. Given the dependence of the infrared spectra of microorganisms on the composition of the growth medium, the spectra were initially separated into three subsets corresponding to the three growth media employed. However, the replicate spectra of all strains, regardless of growth medium, were properly clustered by hierarchical cluster analysis based on differences in their infrared spectral profiles in three narrow spectral regions (1,428 to 1,412, 1,296 to 1,284, and 1,112 to 1,100 cm(-1)). The dendrogram generated from the FTIR data revealed complete separation between group I and group II strains. The spectral differences between group I and group II strains allowed accurate classification of C. botulinum strains at the group level in two blind validation studies (n = 40). These results demonstrate that FPA-FTIR spectroscopy has the potential for rapid discrimination of group I and group II C. botulinum strains in less than 3 min per sample. PMID:17066916
NASA Technical Reports Server (NTRS)
Morscher, Gregory N.; Yun, Hee Mann; DiCarlo, James A.
2007-01-01
The tensile mechanical properties of ceramic matrix composites (CMC) in directions off the primary axes of the reinforcing fibers are important for architectural design of CMC components that are subjected to multi-axial stress states. In this study, 2D-woven melt-infiltrated (MI) SiC/SiC composite panels with balanced fiber content in the 0 degree and 90 degree directions were tensile loaded in-plane in the 0 degree direction and at 45 degree to this direction. In addition, a 2D triaxially-braided MI composite panel with balanced fiber content in the plus or minus 67 degree bias directions and reduced fiber content in the axial direction was tensile loaded perpendicular to the axial direction tows (i.e., 23 degrees from the bias fibers). Stress-strain behavior, acoustic emission, and optical microscopy were used to quantify stress-dependent matrix cracking and ultimate strength in the panels. It was observed that both off-axis loaded panels displayed higher composite onset stresses for through-thickness matrix cracking than the 2D-woven 0/90 panels loaded in the primary 0 degree direction. These improvements for off-axis cracking strength can in part be attributed to higher effective fiber fractions in the loading direction, which in turn reduces internal stresses on critical matrix flaws for a given composite stress. Also for the 0/90 panel loaded in the 45 degree direction, an improved distribution of matrix flaws existed due to the absence of fiber tows perpendicular to the loading direction. In addition, for the +67/0/-67 braided panel, the axial tows perpendicular to the loading direction were not only low in volume fraction, but were also were well separated from one another. Both off-axis oriented panels also showed relatively good ultimate tensile strength when compared to other off-axis oriented composites in the literature, both on an absolute strength basis as well as when normalized by the average fiber strength within the composites. Initial
C-shaped specimen plane strain fracture toughness tests. [metallic materials
NASA Technical Reports Server (NTRS)
Buzzard, R. T.; Fisher, D. M.
1977-01-01
Test equipment, procedures, and data obtained in the evaluation of C-shaped specimens are presented. Observations reported on include: specimen preparation and dimensional measurement; modifications to the standard ASTM E 399 displacement gage, which permit punch mark gage point engagement; and a measurement device for determining the interior and exterior radii of ring segments. Load displacement ratios were determined experimentally which agreed with analytically determined coefficients for three different gage lengths on the inner surfaces of radially-cracked ring segments.
Full in-plane strain tensor analysis using the microscale ring-core FIB milling and DIC approach
NASA Astrophysics Data System (ADS)
Lunt, Alexander J. G.; Salvati, Enrico; Ma, Lifeng; Dolbyna, Igor P.; Neo, Tee K.; Korsunsky, Alexander M.
2016-09-01
Microscale Full In-plane Strain Tensor (FIST) analysis is crucial for improving understanding of residual stress and mechanical failure in many applications. This study outlines the first Focused Ion Beam (FIB) milling and Digital Image Correlation (DIC) based technique capable of performing precise, reliable and rapid quantification of this behaviour. The nature of semi-destructive FIB milling overcomes the main limitations of X-Ray Diffraction (XRD) strain tensor quantification: unstrained lattice parameter estimates are not required, analysis is performed in within a precisely defined 3D microscale volume, both amorphous and crystalline materials can be studied and access to X-ray/neutron facilities is not required. The FIST FIB milling and DIC experimental technique is based on extending the ring-core milling geometry to quantify the strain variation with angle and therefore benefits from the excellent precision and simple analytical approach associated with this method. In this study in-plane strain analysis was performed on sample of commercial interest: a porcelain veneered Yttria Partially Stabilised Zirconia (YPSZ) dental prosthesis, and was compared with the results of XRD. The two methods sample different gauge volumes and mechanical states: approximately plane stress for ring-core milling, and a through-thickness average for XRD. We demonstrate using complex analysis methods and Finite Element (FE) modelling that valid comparisons can be drawn between these two stress states. Excellent agreement was obtained between principal stress orientation and magnitudes, leading to realistic residual stress estimates that agree well with the literature (σAv ≈ 460 MPa) . As a measure of validity of the matching approach we report the upper and lower bounds on the (101) interplanar spacing of YPSZ that are found to correspond to the range 2.9586 - 2.9596 Å , closely matching published values.
NASA Technical Reports Server (NTRS)
Gabb, Timothy P.; Telesman, Jack; Banik, Anthony; McDevitt, Erin
2014-01-01
Intergranular fatigue crack initiation and growth due to environmental degradation, especially at notched features, can often limit the fatigue life of disk superalloys at high temperatures. For clear comparisons, the effects of alloy composition on cracking in air needs to be understood and compared separately from variables associated with notches and cracks such as effective stress concentration, plastic flow, stress relaxation, and stress redistribution. The objective of this study was to attempt using simple tensile tests of specimens with uniform gage sections to compare the effects of varied alloy composition on environment-assisted cracking of several powder metal and cast and wrought superalloys including ME3, LSHR, Udimet 720(TradeMark) ATI 718Plus(Registered TradeMark) alloy, Haynes 282(Trademark), and Inconel 740(TradeMark) Slow and fast strain-rate tensile tests were found to be a useful tool to compare propensities for intergranular surface crack initiation and growth. The effects of composition and heat treatment on tensile fracture strain and associated failure modes were compared. Environment interactions were determined to often limit ductility, by promoting intergranular surface cracking. The response of various superalloys and heat treatments to slow strain rate tensile testing varied substantially, showing that composition and microstructure can significantly influence environmental resistance to cracking.
NASA Astrophysics Data System (ADS)
Shin, J. K.; Ochiai, S.; Okuda, H.; Sugano, M.; Oh, S. S.
2008-11-01
The critical current at 77 K of multifilamentary Bi2223 composite tape was studied under applied tensile strain experimentally and analytically. Beyond the irreversible strain, the critical currents (IC) decreased significantly with increasing applied tensile strain (ɛc), due to the enhanced cracking of the Bi2223 filaments. The voltage generation in the voltage-current relation was calculated by the current share model in which the transport current is shared by the Bi2223 filament and Ag near the cracked portion. Then the critical current was estimated with a 1 µV cm-1 criterion. By the application of the current share model to the experimental result, the effective crack length responsible for the reduction in critical current was estimated, with which the change of critical current could be described satisfactorily as a function of applied strain.
Cracking phenomena in In{sub 0.25}Ga{sub 0.75}As films on InP substrates
Wu, X.; Weatherly, G.C.
1999-09-29
The strain relaxation of a series of In{sup 0.25}Ga{sub 0.75}As films grown on (100) InP substrates (lattice mismatch = 2%) has been studied by electron microscopy. The mechanisms of strain relief (in the first stages of growth) occurred by cracking on (0{bar 1}1), and by twinning on (111) and ({bar 1}11) planes. Cracking was a transitory process with the density of cracks being highest in a 20 nm thick film, while a 500 nm thick film was crack-free. These results are discussed in the context of different cracking and crack-healing models.
Stress corrosion cracking of zirconium in nitric acid
Beavers, J A; Griess, J C; Boyd, W K
1980-01-01
The susceptibility of zirconium and its common alloys to stress corrosion cracking (SCC) in nitric acid was investigated by slow strain-rate and constant deflection techniques. Cracking occurred at 25/sup 0/C over a wide range of acid concentrations and electrochemical potentials. The crack velocity increased slightly with increasing temperature. The failure mode was transgranular and the morphology was similar to SCC failures of zirconium alloys in other environments. The fracture was very orientation-dependent suggesting that it occurs on a single crystallographic plane in the material. The results of the study are not consistent with a hydrogen mechanism for cracking.
NASA Astrophysics Data System (ADS)
Zhang, P.; Wittmann, F. H.; Zhao, T. J.; Lehmann, E. H.; Tian, L.; Vontobel, P.
2010-08-01
Durability of reinforced concrete structures has become a crucial issue with respect to economy, ecology and sustainability. One major reason for durability problems of concrete structures is the limited strain capacity of cement-based materials under imposed tensile stress. By adding PVA fibers, a new material named Strain Hardening Cement-based Composites (SHCC) with high strain capacity can be produced. Due to the formation of multiple micro-cracks, wide cracks can be avoided in SHCC under an imposed strain. The high strain capacity, however, is beneficial with respect to durability only if the multi-crack formation in SHCC does not lead to significantly increased water penetration. If water and aggressive chemical compounds such as chlorides and sulfates dissolved in water penetrate into the cement-based matrix and reach the steel reinforcement service-life of reinforced concrete structures will be reduced significantly. In this project, neutron radiography was applied to observe and quantify the process of water penetration into uncracked SHCC and after the multi-crack formation. In addition, water penetration into integral water repellent cracked and uncracked SHCC, which has been produced by adding a silane-based water repellent agent to the fresh SHCC mortar has been investigated. Results will be discussed with respect to durability.
Albocher, U.; Barbone, P.E.; Richards, M.S.; Oberai, A.A.; Harari, I.
2014-01-01
We apply the adjoint weighted equation method (AWE) to the direct solution of inverse problems of incompressible plane strain elasticity. We show that based on untreated noisy displacements, the reconstruction of the shear modulus can be very poor. We link this poor performance to loss of coercivity of the weak form when treating problems with discontinuous coefficients. We demonstrate that by smoothing the displacements and appending a regularization term to the AWE formulation, a dramatic improvement in the reconstruction can be achieved. With these improvements, the advantages of the AWE method as a direct solution approach can be extended to a wider range of problems. PMID:25383085
NASA Technical Reports Server (NTRS)
Fisher, D. M.; Buzzard, R. J.
1979-01-01
Standard round specimen fracture test results compared satisfactorily with results from standard rectangular compact specimens machined from the same material. The location of the loading pin holes was found to provide adequate strength in the load bearing region for plane strain fracture toughness testing. Excellent agreement was found between the stress intensity coefficient values obtained from compliance measurements and the analytic solution proposed for inclusion in the standard test method. Load displacement measurements were made using long armed displacement gages and hollow loading cylinders. Gage points registered on the loading hole surfaces through small holes in the walls of the loading cylinders.
An orthotropic laminate composite containing a layer with a crack
NASA Technical Reports Server (NTRS)
Arin, K.
1974-01-01
A laminate composite containing an orthotropic layer with a crack situated normal to the interfaces, and bonded to two orthotropic half-planes of dissimilar materials was considered. The solutions for two different classes of orthotropic materials are presented. In each case, the problem was first reduced to a system of dual integral equations, then to a singular integral equation which was subsequently solved numerically for the stress intensity factors at the tip of the crack. The effect of the material properties on the stress intensity factor was investigated. The generalized plane stress and the plane strain were treated simultaneously.
NASA Astrophysics Data System (ADS)
Fournier, L.; Serres, A.; Auzoux, Q.; Leboulch, D.; Was, G. S.
2009-01-01
The radiation-induced microstructure, strain localization, and iodine-induced stress corrosion cracking (I-SCC) behaviour of recrystallized Zircaloy-4 proton-irradiated to 2 dpa at 305 °C was examined. type dislocation loops having 1/3<1 1 2¯ 0> Burgers vector and a mean diameter and density of, respectively, 10 nm and 17 × 10 21 m -3 were observed while no Zr(Fe,Cr) 2 precipitates amorphization or Fe redistribution were detected after irradiation. After transverse tensile testing to 0.5% macroscopic plastic strain at room temperature, almost exclusively basal channels were imaged. Statistical Schmid factor analysis shows that irradiation leads to a change in slip system activation from prismatic to basal due to a higher increase of critical resolved shear stresses for prismatic slip systems than for basal slip system. Finite element calculations suggest that dislocation channeling occurs in the irradiated proton layer at an equivalent stress close to 70% of the yield stress of the irradiated material, i.e. while the irradiated layer is still in the elastic regime for a 0.5% applied macroscopic plastic strain. Comparative constant elongation rate tensile tests performed at a strain rate of 10 -5 s -1 in iodized methanol solutions at room temperature on specimens both unirradiated and proton-irradiated to 2 dpa demonstrated a detrimental effect of irradiation on I-SCC.
Development of plane strain fracture toughness test for ceramics using Chevron notched specimens
NASA Technical Reports Server (NTRS)
Bubsey, R. T.; Shannon, J. L., Jr.; Munz, D.
1983-01-01
Chevron-notched four-point-bend and short-bar specimens have been used to determine the fracture toughness of sintered aluminum oxide and hot-pressed silicon nitride ceramics. The fracture toughness for Si3N4 is found to be essentially independent of the specimen size and chevron notch configuration, with values ranging from 4.6 to 4.9 MNm exp -3/2. In contrast, significant specimen size and notch geometry effects have been observed for Al2O3, with the fracture toughness ranging from 3.1 to 4.7 MNm exp -3/2. These effects are attributed to a rising crack growth resistance curve for the Al2O3 tested.
NASA Astrophysics Data System (ADS)
Dai, Ming; Schiavone, Peter; Gao, Cun-Fa
2016-06-01
We re-examine the conclusion established earlier in the literature that in the presence of a homogeneously imperfect interface, the circular inhomogeneity is the only shape of inhomogeneity which can achieve a uniform internal strain field in an isotropic or anisotropic material subjected to anti-plane shear. We show that under certain conditions, it is indeed possible to design such non-circular inhomogeneities despite the limitation of a homogeneously imperfect interface. Our method proceeds by prescribing a uniform strain field inside a non-circular inhomogeneity via perturbations of the uniform strain field inside the analogous circular inhomogeneity and then subsequently identifying the corresponding (non-circular) shape via the use of a conformal mapping whose unknown coefficients are determined from a system of nonlinear equations. We illustrate our results with several examples. We note also that, for a given size of inhomogeneity, the minimum value of the interface parameter required to guarantee the desired uniform internal strain increases as the elastic constants of the inclusion approach those of the matrix. Finally, we discuss in detail the relationship between the curvature of the interface and the displacement jump across the interface in the design of such inhomogeneities.
Crack propagation and arrest in CFRP materials with strain softening regions
NASA Astrophysics Data System (ADS)
Dilligan, Matthew Anthony
Understanding the growth and arrest of cracks in composite materials is critical for their effective utilization in fatigue-sensitive and damage susceptible applications such as primary aircraft structures. Local tailoring of the laminate stack to provide crack arrest capacity intermediate to major structural components has been investigated and demonstrated since some of the earliest efforts in composite aerostructural design, but to date no rigorous model of the crack arrest mechanism has been developed to allow effective sizing of these features. To address this shortcoming, the previous work in the field is reviewed, with particular attention to the analysis methodologies proposed for similar arrest features. The damage and arrest processes active in such features are investigated, and various models of these processes are discussed and evaluated. Governing equations are derived based on a proposed mechanistic model of the crack arrest process. The derived governing equations are implemented in a numerical model, and a series of simulations are performed to ascertain the general characteristics of the proposed model and allow qualitative comparison to existing experimental results. The sensitivity of the model and the arrest process to various parameters is investigated, and preliminary conclusions regarding the optimal feature configuration are developed. To address deficiencies in the available material and experimental data, a series of coupon tests are developed and conducted covering a range of arrest zone configurations. Test results are discussed and analyzed, with a particular focus on identification of the proposed failure and arrest mechanisms. Utilizing the experimentally derived material properties, the tests are reproduced with both the developed numerical tool as well as a FEA-based implementation of the arrest model. Correlation between the simulated and experimental results is analyzed, and future avenues of investigation are identified
Effects of surface cracks and strain rate on the tensile behavior of Balmoral Red granite
NASA Astrophysics Data System (ADS)
Mardoukhi, Ahmad; Hokka, Mikko; Kuokkala, Veli-Tapani
2015-09-01
This paper presents an experimental procedure for studying the effects of surface cracks on the mechanical behavior of Balmoral Red granite under dynamic and quasi-static loading. Three different thermal shocks were applied on the surface of the Brazilian Disc test samples by keeping a flame torch at a fixed distance from the sample surface for 10, 30, and 60 seconds. Microscopy clearly shows that the number of the surface cracks increases with the duration of the thermal shock. After the thermal shock, the Brazilian Disc tests were performed using a servohydraulic materials testing machine and a compression Split Hopkinson Pressure Bar (SHPB) device. The results show that the tensile strength of the rock decreases and the rate sensitivity of the rock increases as more cracks are introduced to the structure. The DIC analysis of the Brazilian disc tests shows that the fracture of the sample initiates at the center of the samples or slightly closer to the incident bar contact point. This is followed by crushing of the samples at both contact points with the stress bars.
NASA Technical Reports Server (NTRS)
James, Mark Anthony
1999-01-01
A finite element program has been developed to perform quasi-static, elastic-plastic crack growth simulations. The model provides a general framework for mixed-mode I/II elastic-plastic fracture analysis using small strain assumptions and plane stress, plane strain, and axisymmetric finite elements. Cracks are modeled explicitly in the mesh. As the cracks propagate, automatic remeshing algorithms delete the mesh local to the crack tip, extend the crack, and build a new mesh around the new tip. State variable mapping algorithms transfer stresses and displacements from the old mesh to the new mesh. The von Mises material model is implemented in the context of a non-linear Newton solution scheme. The fracture criterion is the critical crack tip opening displacement, and crack direction is predicted by the maximum tensile stress criterion at the crack tip. The implementation can accommodate multiple curving and interacting cracks. An additional fracture algorithm based on nodal release can be used to simulate fracture along a horizontal plane of symmetry. A core of plane strain elements can be used with the nodal release algorithm to simulate the triaxial state of stress near the crack tip. Verification and validation studies compare analysis results with experimental data and published three-dimensional analysis results. Fracture predictions using nodal release for compact tension, middle-crack tension, and multi-site damage test specimens produced accurate results for residual strength and link-up loads. Curving crack predictions using remeshing/mapping were compared with experimental data for an Arcan mixed-mode specimen. Loading angles from 0 degrees to 90 degrees were analyzed. The maximum tensile stress criterion was able to predict the crack direction and path for all loading angles in which the material failed in tension. Residual strength was also accurately predicted for these cases.
NASA Astrophysics Data System (ADS)
Teomete, Egemen
2016-07-01
Earthquakes, material degradations and other environmental factors necessitate structural health monitoring (SHM). Metal foil strain gages used for SHM have low durability and low sensitivity. These factors motivated researchers to work on cement based strain sensors. In this study, the effects of temperature and moisture on electrical resistance, compressive and tensile strain gage factors (strain sensitivity) and crack sensitivity were determined for steel fiber reinforced cement based composite. A rapid increase of electrical resistance at 200 °C was observed due to damage occurring between cement paste, aggregates and steel fibers. The moisture—electrical resistance relationship was investigated. The specimens taken out of the cure were saturated with water and had a moisture content of 9.49%. The minimum electrical resistance was obtained at 9% moisture at which fiber–fiber and fiber–matrix contact was maximum and the water in micro voids was acting as an electrolyte, conducting electrons. The variation of compressive and tensile strain gage factors (strain sensitivities) and crack sensitivity were investigated by conducting compression, split tensile and notched bending tests with different moisture contents. The highest gage factor for the compression test was obtained at optimal moisture content, at which electrical resistance was minimum. The tensile strain gage factor for split tensile test and crack sensitivity increased by decreasing moisture content. The mechanisms between moisture content, electrical resistance, gage factors and crack sensitivity were elucidated. The relations of moisture content with electrical resistance, gage factors and crack sensitivities have been presented for the first time in this study for steel fiber reinforced cement based composites. The results are important for the development of self sensing cement based smart materials.
Insight into the band structure engineering of single-layer SnS2 with in-plane biaxial strain.
Zhou, Wei; Umezawa, Naoto
2016-03-21
The effects of in-plane biaxial strain on the electronic structure of a photofunctional material, single-layer SnS2, were systematically investigated using hybrid density functional calculations. The bonding diagram for the band gap was firstly proposed based on the crystal orbital overlap population analysis. The conduction band-edge of single-layer SnS2 is determined by the anti-bonding interaction between Sn-5s and S-3p orbitals, while the valence band-edge comes from the anti-bonding between the neighboring S atoms. It is found that the compressive strain not only decreases the indirect band gap of single-layer SnS2, but also effectively promotes the band-edges of the conduction band to realize the overall water splitting. Besides, the dispersion of the valence band of single-layer SnS2 becomes weaker with increasing tensile strain which is beneficial for the photo-excitation through direct transitions. PMID:26912413
Analysis of fatigue crack propagation
NASA Technical Reports Server (NTRS)
Liu, H. W.
1972-01-01
The correlation between fatigue crack propagation and stress intensity factor is analyzed. When determining fatigue crack propagation rate, a crack increment, delta a, and its corresponding increment in load cycles, delta N, are measured. Fatigue crack propagation must be caused by a shear and/or a normal separation mode. Both of these two processes are discrete if one looks at the atomic level. If the average deformation and fracture properties over the crack increments, delta a, can be considered as homogeneous, if the characteristic discrete lengths of sigma a, if the plastic zone size is small, and if a plate is thick enough to insure a plane strain case, da/dN is proportional to delta K squared. Any deviation of empirical data from this relation must be caused by the fact that one or more of these conditions are not satisfied. The effects of plate thickness and material inhomogeneity are discussed in detail. A shear separation mode of fatigue crack propagation is described and is used to illustrate the effects of material inhomogeneity.
NASA Astrophysics Data System (ADS)
Harikumar, M.; Sankar, N.; Chandrakaran, S.
2015-09-01
Since 1969, when the concept of earth reinforcing was brought about by Henry Vidal, a large variety of materials such as steel bars, tire shreds, polypropylene, polyester, glass fibres, coir and jute fibres etc. have been widely added to soil mass randomly or in a regular, oriented manner. The conventional reinforcements in use were two dimensional or planar, in the form of strips with negligible widths or in the form of sheets. In this investigation, a novel concept of multi oriented plastic reinforcement (hexa-pods) is discussed. Direct shear tests were conducted on unreinforced and reinforced dry fine, medium and coarse sands. Detailed parametric studies with respect to the effective grain size of soil (d10), normal stress (σ) and the volume ratio of hexa-pods (Vr) were performed. It was noticed that addition of hexa-pods resulted in increase in the shear strength parameters viz. peak deviatoric stresses and increased angle of internal friction. The hexa-pods also changed the brittle behaviour of unreinforced sand samples to ductile ones. Although the peak shear stress did not show a considerable improvement, the angle of internal friction improved noticeably. Addition of a single layer of reinforcement along the shear plane also reduced the post peak loss of strength and changed the soil behavior from brittle to a ductile one.
Development of a Plane Strain Tensile Geometry to Assess Shear Fracture in Dual Phase Steels
NASA Astrophysics Data System (ADS)
Taylor, M. D.; Matlock, D. K.; De Moor, E.; Speer, J. G.
2014-10-01
A geometrically modified sample capable of generating a triaxial stress state when tested on a standard uniaxial tensile frame was developed to replicate shear fractures observed during stretch bend tests and industrial sheet stamping operations. Seven commercially produced dual phase (DP) steels were tested using the geometrically modified sample, and the modified sample successfully produced shear fractures on a unique shear plane for all steels. For each steel, void densities were determined, based on metallographic analyses, as a function of imposed displacement. Microstructural properties of ferrite and martensite grain size, martensite volume fraction (MVF), retained austenite content, Vickers hardness, average nanoindentation hardness, average ferrite and martensite constituent hardness, and tensile properties were obtained in order to evaluate potential correlations with void data. A linear correlation was observed between Vickers hardness and the average nanoindentation hardness, verifying the ability of nanoindentation to produce data consistent with more traditional hardness measurement techniques. A linear relationship was observed between the number of voids present at 90% failure displacement and the martensite/ferrite hardness ratio, indicating that a decrease in relative hardness difference in a microstructure can suppress void formation, and potentially extend formability limits. The void population appeared independent of MVF, grain size, and tensile properties suggesting that constituent hardness may be a dominant parameter when considering suppression of void nucleation in DP steels.
Comparison of acoustic and strain gauge techniques for crack closure measurements
NASA Technical Reports Server (NTRS)
Buck, O.; Inman, R. V.; Frandsen, J. D.
1976-01-01
A quantitative study on the systems performances of the COD gauge and the acoustic transmission techniques to elastic deformation of part-through crack and compact tension specimens has been conducted. It is shown that the two instruments measure two completely different quantities: The COD gauge yields information on the length change of the specimen whereas the acoustic technique is sensitive directly to the amount of contract area between two surfaces, interfering with the acoustic signal. In another series of experiments, compression tests on parts with specifically prepared surfaces were performed so that the surface contact area could be correlated with the transmitted acoustic signal, as well as the acoustic with the COD gauge signal. A linear relation between contact area and COD gauge signal was obtained until full contact had been established.
NASA Astrophysics Data System (ADS)
Dustin, Joshua S.
A state-of-the-art multi-scale analysis was performed to predict failure initiation at the free-edge of an angle-ply laminate using the Strain Invariant Failure Theory (SIFT), and multiple improvements to this analysis methodology were proposed and implemented. Application of this analysis and theory led to the conclusion that point-wise failure criteria which ignore the singular stress and strain fields from a homogenized analysis and the presence of free-edge damage in the form of micro-cracking, may do so at the expense of failure prediction capability. The main contributions of this work then are made in the study of the laminate free-edge singularity and in the effects of micro-cracking at the composite laminate free-edge. Study of both classical elasticity and finite element solutions of the laminate free-edge stress field based upon the assumption of homogenized lamina properties reveal that the order of the free-edge singularity is sufficiently small such that the domain of dominance of this term away from the laminate free-edge is much smaller than the relevant dimensions of the microstructure. In comparison to a crack-tip field, these free-edge singularities generate stress and strain fields which are half as intense as those at the crack-tip, leading to the conclusion that existing flaws at the free-edge in the form of micro-cracks would be more prone to the initiation of free-edge failure than the existence of a singularity in the free-edge elasticity solutions. A methodical experiment was performed on a family of [±25°/90°] s laminates made of IM7/8552 carbon/epoxy composite, to both characterize micro-cracks present at the laminate free-edge and to study their behavior under the application of a uniform extensional load. The majority of these micro-cracks were of length on the order of a few fiber diameters, though larger micro-cracks as long as 100 fiber diameters were observed in thicker laminates. A strong correlation between the application of
NASA Astrophysics Data System (ADS)
Qiu, J. H.; Jiang, Q.
2007-10-01
A phenomenological Landau Devonshire thermodynamic theory is used to describe the effects of anisotropic in-plane misfit strains on equilibrium polarization states and dielectric properties of single domain epitaxial Pb(Zr1-xTix)O3 thin films grown on dissimilar orthorhombic substrates. Compared with the “isotropic in-plane misfit strains-temperature” phase diagrams, the characteristic features of “misfit strain-misfit strain” and “misfit strain-temperature” phase diagrams under the circumstance of strain anisotropy are the presence of four different phases (a, a, ac, and ac) and the direct 90° polarization switching between c phase and a phase (or a phase), between a phase and a phase. The misfit strain dependence of polarization components, the small-signal dielectric responses and the tunabilities at room temperature are also calculated. We find that the phase diagrams and dielectric properties largely depend on anisotropic in-plane misfit strains as well. Moreover, the strain anisotropy will lead to the polarization and dielectric anisotropy.
NASA Technical Reports Server (NTRS)
Finger, R. W.
1978-01-01
Static fracture tests were performed on surface flawed specimens of aluminum and titanium alloys. A simulated proof overload cycle was applied prior to all of the cyclic tests. Variables included in each test series were flaw shapes and thickness. Additionally, test temperature was a variable for the aluminum test series. The crack opening displacement and stress-strain data obtained are presented.
Crack curving in a ductile pressurized fuselage
NASA Astrophysics Data System (ADS)
Lam, Paul W.
magnitude of CTOA and CTOD depends on the structural geometry, and including plasticity increases the crack tip displacements. An estimate of the strain in the skin flaps at the crack tip may indicate the tendency for flapping. Out-of-plane effects become significant as the crack propagates and curves.
Deformation fields near a steady fatigue crack with anisotropic plasticity
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.
Deformation fields near a steady fatigue crack with anisotropic plasticity
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
NASA Technical Reports Server (NTRS)
Lu, M. C.; Erdogan, F.
1980-01-01
The basic crack problem which is essential for the study of subcritical crack propagation and fracture of layered structural materials is considered. Because of the apparent analytical difficulties, the problem is idealized as one of plane strain or plane stress. An additional simplifying assumption is made by restricting the formulation of the problem to crack geometries and loading conditions which have a plane of symmetry perpendicular to the interface. The general problem is formulated in terms of a coupled system of four integral equations. For each relevant crack configuration of practical interest, the singular behavior of the solution near and at the ends and points of intersection of the cracks is investigated and the related characteristic equations are obtained. The edge crack terminating at and crossing the interface, the T-shaped crack consisting of a broken layer and a delamination crack, the cross-shaped crack which consists of a delamination crack intersecting a crack which is perpendicular to the interface, and a delamination crack initiating from a stress-free boundary of the bonded layers are some of the practical crack geometries considered.
Constraint effects observed in crack initiation stretch
Lambert, D.M.; Ernst, H.A.
1995-12-31
The current paper characterizes constraint in fracture: J-modified resistance (Jr) curves were developed for two tough structural materials, 6061-T651 (aluminum) and IN718-STA1 (nickel-base superalloy). A wide variety of configurations was tested to consider load configurations from bending to tension including three specimen types (compact tension, center-crack tension, and single-edge notched tension), and a range of ligament lengths and thicknesses, as well as side-grooved and smooth-sided ligaments. The Jr curves exhibited an inflection point after some crack extension, and the data were excluded beyond the inflection. Qualified Jr curves for the two materials showed similar behavior, but R-curves were identical for equal ligament length-to-thickness ratio (RL), for the aluminum alloy, with increasing slope for increasing RL, while for the nickel, the resistance curves aligned for equal ligament thickness, B, and the slope increased for decreasing B. Displacements at the original crack tip (CToD) were recorded throughout the test for several specimens. CToD-versus-crack extension curves were developed, and data were excluded beyond the inflection point (as with the Jr curves). The data collapsed into two distinct curves, thought to represent the surface, plane stress effect and the central, plane strain effect. This was observed for both materials. A technique called profiling is presented for the aluminum alloy only, where the crack face displacements are recorded at the final point of the test as a function of the position throughout the crack cavity, along with an effort to extract the observations in a usable form. Displacements were consistent throughout the cross-section at and behind the original crack tip. In the region where the crack grew, this displacement was developed by a combination of stretch and crack growth. The stretch required to initiate crack extension was a function of the depth beneath the surface into the cross-section.
The application of a logic framework for fatigue crack growth analyses to microstructural effects
Xu, J.G.; Liu, H.W.
1995-12-31
{Delta}K has been widely used to correlate da/dN data. The relation between da/dN and {Delta}K is usually found empirically. However, fatigue crack growth relations can also be derived theoretically. Three fatigue crack growth theories are derived for the state of small scale yielding and plane strain. These three theories constitute a logic framework useful for fatigue crack growth analyses. The application of the logic framework to the analyses of microstructural effects on fatigue crack growth is illustrated. The fatigue crack growth curve of 7075-T651 aluminum alloy has five distinct regions. A fatigue crack grows by crack-tip shear decohesion forming striations and by brittle fractures of particles followed by localized shear decohesion at these microcracks forming dimples. The logic framework helps to relate the fatigue crack growth behaviors in these five regions to the fractures of inclusions and to the resistance of grain boundaries and dispersoids to shear decohesion.
NASA Astrophysics Data System (ADS)
Oliveira, M. C.; Baptista, A. J.; Alves, J. L.; Menezes, L. F.; Green, D. E.; Ghaei, A.
2007-05-01
The main purpose of the "Numisheet'05 Benchmark♯3: Channel Draw/Cylindrical Cup" was to evaluate the forming characteristics of materials in multi-stage processes. The concept was to verify the strain fields achieved during the two stage forming process and also to test the ability of numerical models to predict both strain and stress fields. The first stage consisted of forming channel sections in an industrial-scale channel draw die. The material that flows through the drawbead and over the die radii into the channel sidewalls is prestrained by cyclic bending and unbending. The prestrained channel sidewalls are subsequently cut and subjected to near plane-strain Marciniak-style cup test. This study emphasizes the analysis of the first stage process, the Channel Draw, since accurate numerical results for the first stage forming and springback are essential to guarantee proper initial state variables for the subsequent stage simulation. Four different sheet materials were selected: mild steel AKDQ-HDG, high strength steel HSLA-HDG, dual phase steel DP600-HDG and an aluminium alloy AA6022-T43. The four sheet materials were formed in the same channel draw die, but with drawbead penetrations of 25%, 50% and 100%. This paper describes the testing and measurement procedures for the numerical simulation of these conditions with DD3IMP FE code. A comparison between experimental and numerical simulation results for the first stage is presented. The experimental results indicate that an increase in drawbead penetration is accompanied by a general decrease in springback, with both sidewall radius of curvature and the sidewall angle increasing with increasing drawbead penetration. An exception to this trend occurs at the shallowest bead penetration: the radius of curvature in the sidewall is larger than expected. The sequence of cyclic tension and compression is numerically studied for each drawbead penetration in order to investigate this phenomenon.
Oliveira, M. C.; Baptista, A. J.; Menezes, L. F.; Alves, J. L.; Green, D. E.; Ghaei, A.
2007-05-17
The main purpose of the 'Numisheet'05 Benchmark no. 3: Channel Draw/Cylindrical Cup' was to evaluate the forming characteristics of materials in multi-stage processes. The concept was to verify the strain fields achieved during the two stage forming process and also to test the ability of numerical models to predict both strain and stress fields. The first stage consisted of forming channel sections in an industrial-scale channel draw die. The material that flows through the drawbead and over the die radii into the channel sidewalls is prestrained by cyclic bending and unbending. The prestrained channel sidewalls are subsequently cut and subjected to near plane-strain Marciniak-style cup test. This study emphasizes the analysis of the first stage process, the Channel Draw, since accurate numerical results for the first stage forming and springback are essential to guarantee proper initial state variables for the subsequent stage simulation. Four different sheet materials were selected: mild steel AKDQ-HDG, high strength steel HSLA-HDG, dual phase steel DP600-HDG and an aluminium alloy AA6022-T43. The four sheet materials were formed in the same channel draw die, but with drawbead penetrations of 25%, 50% and 100%. This paper describes the testing and measurement procedures for the numerical simulation of these conditions with DD3IMP FE code. A comparison between experimental and numerical simulation results for the first stage is presented. The experimental results indicate that an increase in drawbead penetration is accompanied by a general decrease in springback, with both sidewall radius of curvature and the sidewall angle increasing with increasing drawbead penetration. An exception to this trend occurs at the shallowest bead penetration: the radius of curvature in the sidewall is larger than expected. The sequence of cyclic tension and compression is numerically studied for each drawbead penetration in order to investigate this phenomenon.
Crack propagation and fracture in silicon wafers under thermal stress
Danilewsky, Andreas; Wittge, Jochen; Kiefl, Konstantin; Allen, David; McNally, Patrick; Garagorri, Jorge; Elizalde, M. Reyes; Baumbach, Tilo; Tanner, Brian K.
2013-01-01
The behaviour of microcracks in silicon during thermal annealing has been studied using in situ X-ray diffraction imaging. Initial cracks are produced with an indenter at the edge of a conventional Si wafer, which was heated under temperature gradients to produce thermal stress. At temperatures where Si is still in the brittle regime, the strain may accumulate if a microcrack is pinned. If a critical value is exceeded either a new or a longer crack will be formed, which results with high probability in wafer breakage. The strain reduces most efficiently by forming (hhl) or (hkl) crack planes of high energy instead of the expected low-energy cleavage planes like {111}. Dangerous cracks, which become active during heat treatment and may shatter the whole wafer, can be identified from diffraction images simply by measuring the geometrical dimensions of the strain-related contrast around the crack tip. Once the plastic regime at higher temperature is reached, strain is reduced by generating dislocation loops and slip bands and no wafer breakage occurs. There is only a small temperature window within which crack propagation is possible during rapid annealing. PMID:24046487
Lai, Chih-Ming; Huang, Yu-En; Feng, Shih-Wei; Kou, Kuang-Yang; Chen, Chien-Hsun; Tu, Li-Wei
2015-07-13
Anisotropic strain relaxation and the resulting degree of polarization of photoluminescence (PL) in nonpolar a-plane textured ZnO are experimentally and theoretically studied. A thicker nonpolar a-plane textured ZnO film enhances the anisotropic in-plane strain relaxation, resulting in a larger degree of polarization of PL and better sample quality. Anisotropic in-plane strains, sample quality, and degree of polarization of PL in nonpolar a-plane ZnO are consequences of the degree of anisotropic in-plane strain relaxation. By the k·p perturbation approach, simulation results of the variation of the degree of polarization for the electronic transition upon anisotropic in-plane strain relaxation agree with experimental results.
Mode I and mixed I/III crack initiation and propagation behavior of V-4Cr-4Ti alloy at 25{degrees}C
Li, H.X.; Kurtz, R.J.; Jones, R.H.
1997-04-01
The mode I and mixed-mode I/III fracture behavior of the production-scale heat (No. 832665) of V-4Cr-4Ti has been investigated at 25{degrees}C using compact tension (CT) specimens for a mode I crack and modified CT specimens for a mixed-mode I/III crack. The mode III to mode I load ratio was 0.47. Test specimens were vacuum annealed at 1000{degrees}C for 1 h after final machining. Both mode I and mixed-mode I/III specimens were fatigue cracked prior to J-integral testing. It was noticed that the mixed-mode I/III crack angle decreased from an initial 25 degrees to approximately 23 degrees due to crack plane rotation during fatigue cracking. No crack plane rotation occurred in the mode I specimen. The crack initiation and propagation behavior was evaluated by generating J-R curves. Due to the high ductility of this alloy and the limited specimen thickness (6.35 mm), plane strain requirements were not met so valid critical J-integral values were not obtained. However, it was found that the crack initiation and propagation behavior was significantly different between the mode I and the mixed-mode I/III specimens. In the mode I specimen crack initiation did not occur, only extensive crack tip blunting due to plastic deformation. During J-integral testing the mixed-mode crack rotated to an increased crack angle (in contrast to fatigue precracking) by crack blunting. When the crack initiated, the crack angle was about 30 degrees. After crack initiation the crack plane remained at 30 degrees until the test was completed. Mixed-mode crack initiation was difficult, but propagation was easy. The fracture surface of the mixed-mode specimen was characterized by microvoid coalescence.
Wu, Huaping; Chai, Guozhong; Zhou, Ting; Zhang, Zheng; Kitamura, Takayuki; Zhou, Haomiao
2014-03-21
The strain-mediated magnetoelectric (ME) property of self-assembled vertical multiferroic nanocomposite films epitaxially grown on cubic substrates was calculated by a nonlinear thermodynamic theory combined with the elastic theory. The dependent relations of phase state of ferroelectric films with the in-plane misfit strain, out-of-plane misfit strain, temperature, and volume fraction of ferromagnetic phase were confirmed. The effects of in-plane misfit strain and ferromagnetic volume fraction on the polarization and dielectric constant of ferroelectric films at room temperature were elaborately analyzed for the vertical BaTiO{sub 3}-CoFe{sub 2}O{sub 4} and PbTiO{sub 3}-CoFe{sub 2}O{sub 4} nanocomposite films. Our calculated results confirmed the relationship among ME effect and in-plane misfit strain and ferromagnetic volume fraction in the nanocomposite films. The ME voltage coefficients of vertical BaTiO{sub 3}-CoFe{sub 2}O{sub 4} and PbTiO{sub 3}-CoFe{sub 2}O{sub 4} nanocomposite films displayed various maximums and abrupt points at special phases and phase transition boundaries. The ME voltage coefficients of lead-free BaTiO{sub 3}-CoFe{sub 2}O{sub 4} nanocomposite films epitaxially grown on different substrates could reach a comparative value of ∼2 V·cm{sup −1}·Oe{sup −1} under the controllable in-plane misfit strain induced by substrate clamping. Our results provided an available method for the optimal design of vertical multiferroic nanocomposites with adjustable ME effect by optimizing the ferromagnetic volume fraction and substrate type.
Fracture of Hydrided Zircaloy-4 Sheet under Through-Thickness Crack Growth Conditions
Raynaud, P.A.; Koss, D.A.; Motta, A.T.; Chan, K.S.
2007-07-01
The failure of thin-wall components such as fuel cladding may be caused by crack initiation on the component surface and subsequent crack growth through its thickness. This study has determined the fracture toughness of hydrided cold-worked stress relieved Zircaloy-4 sheet subject to through-thickness crack growth at 25 deg. C. The experimental approach utilizes a novel procedure in which a narrow linear strip of brittle hydride blister across the specimen width creates a well-defined pre-crack upon initial loading. The subsequent crack growth resistance is then characterized by four-point bending of the specimen and an elastic-plastic fracture mechanics analysis. At room temperature, the through-thickness fracture toughness (K{sub Q}) is sensitive to the orientation of the hydride platelets, and K{sub Q} {approx_equal} 25 MPavm for crack growth through a mixed in-plane/out-of-plane hydride field. In contrast, K{sub Q} is much higher ({approx_equal} 75 MPavm) when the hydride platelets are oriented predominantly in the plane of the sheet (and therefore normal to both the crack plane and the crack growth direction). The implication of these fracture toughness values to the fracture strain behavior of hydrided Zircaloy-4 under through-thickness crack growth conditions is illustrated. (authors)
Mapping and load response of overload strain fields: Synchrotron X-ray measurements
Shukla, V; Jisrawi, N M; Sadangi, R K; Pao, P S; Horvath, K; Sadananda, K; Ignatov, A; Skaritka, J; Tsakalakos, T
2009-02-05
High energy synchrotron X-ray diffraction measurements have been performed to provide quantitative microscopic guidance for modeling of fatigue crack growth. Specifically we report local strain mapping, along with in situ loading strain response, results on 4140 steel fatigue specimens exhibiting the crack growth retardation 'overload effect'. Detailed, 2D, {epsilon}{gamma}{gamma}-strain field mapping shows that a single overload (OL) cycle creates a compressive strain field extending millimeters above and below the crack plane. The OL strain field structures are shown to persist after the crack tip has grown well beyond the OL position. The specimen exhibiting the maximal crack growth rate retardation following overload exhibits a tensile residual strain region at the crack tip. Strain field results, on in situ tensile loaded specimens, show a striking critical threshold load, F{sub c}, phenomenon in their strain response. At loads below F{sub c} the strain response is dominated by a rapid suppression of the compressive OL feature with modest response at the crack tip. At loads above F{sub c} the strain response at the OL position terminates and the response at the crack tip becomes large. This threshold load response behavior is shown to exhibit lower F{sub c} values, and dramatically enhanced rates of strain change with load as the crack tip propagates farther beyond the OL position. The OL strain feature behind the crack tip also is shown to be suppressed by removing the opposing crack faces via an electron discharge cut passing through the crack tip. Finally unique 2D strain field mapping (imaging) results, through the depth of the specimen, of the fatigue crack front and the OL feature in the wake are also presented.
Støylen, Asbjørn; Ingul, Charlotte B; Torp, Hans
2003-01-01
Background We describe a method for 3-/4D reconstruction of tissue Doppler data from three standard apical planes, post processing to derived data of strain rate / strain and parametric colour imaging of the data. The data can be displayed as M-mode arrays from all six walls, Bull's eye projection and a 3D surface figure that can be scrolled and rotated. Numerical data and waveforms can be re-extracted. Methods Feasibility was tested by Strain Rate Imaging in 6 normal subjects and 6 patients with acute myocardial infarction. Reverberation artefacts and dyssynergy was identified by colour images. End systolic strain, peak systolic and mid systolic strain rate were measured. Results Infarcts were visualised in all patients by colour imaging of mid systolic strain rate, end systolic strain and post systolic shortening by strain rate. Reverberation artefacts were visible in 3 of 6 normals, and 2 of 6 patients, and were identified both on bull's eye and M-mode display, but influenced quantitative measurement. Peak systolic strain rate was in controls minimum -1.11, maximum -0.89 and in patients minimum -1.66, maximum 0.02 (p = 0.04). Mid systolic strain rate and end systolic strain did not separate the groups significantly. Conclusion 3-/4D reconstruction and colour display is feasible, allowing quick visual identification of infarcts and artefacts, as well as extension of area of post systolic shortening. Strain rate is better suited to colour parametric display than strain. PMID:12956886
NASA Astrophysics Data System (ADS)
Edsinger, K.; Odette, G. R.; Lucas, G. E.; Sheckherd, J. W.
1996-10-01
Fracture toughness K( T) curves for F82H were determined as a function of specimen size, crack size and strain rate. It was found that F82H shows a relatively abrupt transition from lower-shelf, quasi-cleavage fracture to upper-shelf ductile fracture. However, decreasing specimen size, crack size and strain rate resulted in a shift of the K( T) curve to lower temperatures. The differences in the lower shelf/knee regime were reconciled by combining a critical stressed area criteria for quasi-cleavage fracture with stress fields determined by finite element methods for the different specimen geometries. The results demonstrated that the large effective toughness for small specimens or specimens with shallow cracks are a consequence of having to deform the specimens to much higher Js before the stressed-area criteria are met ahead of the crack. Such large toughnesses and low transition temperatures support the contention that thin-walled ferritic structures should remain a viable option for advanced fusion reactors.
NASA Astrophysics Data System (ADS)
Hu, Lun-Hui; Xu, Dong-Hui; Zhang, Fu-Chun; Zhou, Yi
2016-08-01
Motivated by the recent discovery of quantized spin Hall effect in InAs/GaSb quantum wells [Du, Knez, Sullivan, and Du, Phys. Rev. Lett. 114, 096802 (2015), 10.1103/PhysRevLett.114.096802], we theoretically study the effects of in-plane magnetic field and strain effect to the quantization of charge conductance by using Landauer-B ütikker formalism. Our theory predicts a robustness of the conductance quantization against the in-plane magnetic field up to a very high field of 20 T. We use a disordered hopping term to model the strain and show that the strain may help the quantization of the conductance. Relevance to the experiments will be discussed.
NASA Astrophysics Data System (ADS)
Ning, Xiaoguang; Murayama, Hideaki; Kageyama, Kazuro; Wada, Daichi; Kanai, Makoto; Ohsawa, Isamu; Igawa, Hirotaka
2014-10-01
In this study, the dynamic strain distribution measurement of an adhesive-bonded single-lap joint was carried out in a cyclic load test using a fiber Bragg grating (FBG) sensor embedded into the adhesive/adherend interface along the overlap length direction. Unidirectional carbon fiber reinforced plastic (CFRP) substrates were bonded by epoxy resin to form the joint, and the FBG sensor was embedded into the surface of one substrate during its curing. The measurement was carried out with a sampling rate of 5 Hz by the sensing system, based on the optical frequency domain reflectometry (OFDR) throughout the test. A finite element analysis (FEA) was performed for the measurement evaluation using a three-dimensional model, which included the embedded FBG sensor. The crack detection method, based on the longitudinal strain distribution measurement, was introduced and performed to estimate the cracks that occurred at the adhesive/adherend interface in the test.
Thermal-Mechanical Response of Cracked Satin Weave CFRP Composites at Cryogenic Temperatures
NASA Astrophysics Data System (ADS)
Watanabe, S.; Shindo, Y.; Narita, F.; Takeda, T.
2008-03-01
This paper examines the thermal-mechanical response of satin weave carbon fiber reinforced polymer (CFRP) laminates with internal and/or edge cracks subjected to uniaxial tension load at cryogenic temperatures. Cracks are considered to occur in the transverse fiber bundles and extend through the entire thickness of the fiber bundles. Two-dimentional generalized plane strain finite element models are developed to study the effects of residual thermal stresses and cracks on the mechanical behavior of CFRP woven laminates. A detailed examination of the Young's modulus and stress distributions near the crack tip is carried out which provides insight into material behavior at cryogenic temperatures.
THERMAL-MECHANICAL RESPONSE OF CRACKED SATIN WEAVE CFRP COMPOSITES AT CRYOGENIC TEMPERATURES
Watanabe, S.; Shindo, Y.; Narita, F.; Takeda, T.
2008-03-03
This paper examines the thermal-mechanical response of satin weave carbon fiber reinforced polymer (CFRP) laminates with internal and/or edge cracks subjected to uniaxial tension load at cryogenic temperatures. Cracks are considered to occur in the transverse fiber bundles and extend through the entire thickness of the fiber bundles. Two-dimentional generalized plane strain finite element models are developed to study the effects of residual thermal stresses and cracks on the mechanical behavior of CFRP woven laminates. A detailed examination of the Young's modulus and stress distributions near the crack tip is carried out which provides insight into material behavior at cryogenic temperatures.
NASA Astrophysics Data System (ADS)
Kelestemur, Mehmet Halidun
1998-12-01
acceleration right after the overload application was observed. However, the acceleration was not detected on the interior of the material along the crack tip. Only the surface in which the plane stress was effective showed crack propagation in acceleration zone. This result were explained by strain hardening and plastic deformation of grains at the overload zone. Discontinuous crack closure occurred following the overload. In other words, at the overload position, fracture surfaces on which stretching occurred came into contact at high stress intensity factor values and far away from the overload position. As a result of discontinuous crack closure, crack opening or closing data were very high although the fatigue crack growth rate reached its original value.
Diffraction-based study of fatigue crack initiation and propagation in aerospace aluminum alloys
NASA Astrophysics Data System (ADS)
Gupta, Vipul K.
crystallographic {111} slip-plane cracking typical of the Stage I crack growth mode observed in single crystals and high purity polycrystals of face centered cubic metals, and which has presently been assumed for the present materials within fatigue crack initiation models. Rather, the facets tend to have near-Mode I spatial orientation, which is another indicator of the importance of environmentally affected fatigue damage. The results provide a physical basis to develop microstructurally-based next generation multi-stage fatigue (MSF) models that should include a new crack decohesion criteria based upon environmental fatigue cracking mechanisms. EBSD study of small-cracks in alloy 7050-T7451, stressed in warm-humid environment, showed that crack-path orientation changes and crack-branching occurred at both low/high-angle grain and subgrain boundaries. Single surface trace analysis suggests that the crack-path differs substantially from crystallographic slip-planes. EBSD-based observations of small-crack propagation through subgrain structure, either formed by cyclic plastic strain accumulation or pre-existing (typical of unrecrystallized grain structure in the present materials), suggest that subgrain structure plays a crucial role in small fatigue crack propagation. As mentioned earlier, local fluctuations in small-crack growth rates appear to be caused by frequent interaction with subgrain boundaries, and multiple occurrences of crack-branching and crack-path orientation changes at low/high-angle grain and subgrain boundaries. The aforementioned deviation from low-index {001}/{101}-planes and the occurrence of high-index cracking planes observed by EBSD/Stereology, in this study and others, are interpreted as trans-subgranular decohesion or inter-subgranular cracking, due to trapped hydrogen. In summary, the results provide a firmer experimental foundation for, and clearer understanding of, the mechanisms of environmental fatigue cracking of aluminum alloys, especially the
NASA Technical Reports Server (NTRS)
Lu, M.-C.; Erdogan, F.
1983-01-01
The basic crack problem which is essential for the study of subcritical crack propagation and fracture of layered structural materials is considered. Because of the apparent analytical difficulties, the problem is idealized as one of plane strain or plane stress. An additional simplifying assumption is made by restricting the formulation of the problem to crack geometries and loading conditions which have a plane of symmetry perpendicular to the interface. The general problem is formulated in terms of a coupled systems of four integral equations. For each relevant crack configuration of practical interest, the singular behavior of the solution near and at the ends and points of intersection of the cracks is investigated and the related characteristic equations are obtained. The edge crack terminating at and crossing the interface, the T-shaped crack consisting of a broken layer and a delamination crack, the cross-shaped crack which consists of a delamination crack intersecting a crack which is perpendicular to the interface, and a delamination crack initiating from a stress-free boundary of the bonded layers are some of the practical crack geometries considered. Previously announced in STAR as N80-18428 and N80-18429
Subcritical crack growth in two titanium alloys.
NASA Technical Reports Server (NTRS)
Williams, D. N.
1973-01-01
Measurement of subcritical crack growth during static loading of precracked titanium alloys in salt water using samples too thin for plane strain loading to predominate was examined as a method for determining the critical stress intensity for crack propagation in salt water. Significant internal crack growth followed by arrest was found at quite low stress intensities, but crack growth rates were relatively low. Assuming these techniques provided a reliable measurement of the critical stress intensity, the value for annealed Ti-4Al-1.5Mo-0.5V alloy was apparently about 35 ksi-in. to the 1/2 power, while that for annealed Ti-4Al-3Mo-1V was below 45 ksi-in. to the 1/2 power. Crack growth was also observed in tests conducted in both alloys in an air environment. At 65 ksi-in. to the 1/2 power, the extent of crack growth was greater in air than in salt water. Ti-4Al-3Mo-1V showed arrested crack growth in air at a stress intensity of 45 ksi-in. to the 1/2 power.
Frefer, Abdulbaset Ali; Raddad, Bashir S.
2013-12-16
In the present study, stress corrosion cracking (SCC) behavior of naturally aged advanced silver-bearing Al-Li alloy in NaCl solution was investigated using slow strain rate test (SSRT) method. The SSRT’s were conducted at different strain rates and applied potentials at room temperature. The results were discussed based on percent reductions in tensile elongation in a SCC-causing environment over those in air tended to express the SCC susceptbility of the alloy under study at T3. The SCC behavior of the alloy was also discussed based on the microstructural and fractographic examinations.
Finite element solutions for crack-tip behavior in small-scale yielding
NASA Technical Reports Server (NTRS)
Tracey, D. M.
1976-01-01
The subject considered is the stress and deformation fields in a cracked elastic-plastic power law hardening material under plane strain tensile loading. An incremental plasticity finite element formulation is developed for accurate analysis of the complete field problem including the extensively deformed near tip region, the elastic-plastic region, and the remote elastic region. The formulation has general applicability and was used to solve the small scale yielding problem for a set of material hardening exponents. Distributions of stress, strain, and crack opening displacement at the crack tip and through the elastic-plastic zone are presented as a function of the elastic stress intensity factor and material properties.
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.
Stable crack growth during actuation in shape memory alloys
NASA Astrophysics Data System (ADS)
Jape, Sameer; Baxevanis, Theocharis; Lagoudas, Dimitris C.
2014-03-01
A finite element analysis of crack growth is carried out in an in nite center-cracked shape memory alloy plate subjected to thermal variations under plane strain mode I constant applied loading. Crack is assumed to propagate when the energy release rate reaches a material specific critical value. The virtual crack growth technique is employed to calculate the energy release rate, which was shown to increase an order of magnitude at constant applied loading as a result of phase transformation induced by thermal variations.1 A fracture toughening is observed associated with the energy dissipated by the transformed material in the wake of the growing crack and its sensitivity over key thermomechanical parameters is presented.
Residual strength of thin panels with cracks
NASA Technical Reports Server (NTRS)
Madenci, Erdogan
1994-01-01
The previous design philosophies involving safe life, fail-safe and damage tolerance concepts become inadequate for assuring the safety of aging aircraft structures. For example, the failure mechanism for the Aloha Airline accident involved the coalescence of undetected small cracks at the rivet holes causing a section of the fuselage to peel open during flight. Therefore, the fuselage structure should be designed to have sufficient residual strength under worst case crack configurations and in-flight load conditions. Residual strength is interpreted as the maximum load carrying capacity prior to unstable crack growth. Internal pressure and bending moment constitute the two major components of the external loads on the fuselage section during flight. Although the stiffeners in the form of stringers, frames and tear straps sustain part of the external loads, the significant portion of the load is taken up by the skin. In the presence of a large crack in the skin, the crack lips bulge out with considerable yielding; thus, the geometric and material nonlinearities must be included in the analysis for predicting residual strength. Also, these nonlinearities do not permit the decoupling of in-plane and out-of-plane bending deformations. The failure criterion combining the concepts of absorbed specific energy and strain energy density addresses the aforementioned concerns. The critical absorbed specific energy (local toughness) for the material is determined from the global specimen response and deformation geometry based on the uniaxial tensile test data and detailed finite element modeling of the specimen response. The use of the local toughness and stress-strain response at the continuum level eliminates the size effect. With this critical parameter and stress-strain response, the finite element analysis of the component by using STAGS along with the application of this failure criterion provides the stable crack growth calculations for residual strength predictions.
The structure of the near-tip field during transient elastodynamic crack growth
NASA Astrophysics Data System (ADS)
Freund, L. B.; Rosakis, A. J.
T HE PROCESS of dynamic crack growth in a nominally elastic malerial under conditions of plane strain or plane stress is considered. Of particular concern is the influence of the transient nature of the process on the stress field in the immediate vicinity of the crack tip during nonsteady growth. Asymptotically, the crack tip stress field is square root singular at the crack tip, with the angular variation of the singular field depending weakly on the instantaneous crack tip speed and with the instantaneous stress intensity factor being a scalar multiplier of the singular field. However, for a material particle at a small distance from the moving crack, the local stress field depends not only on instantaneous values of crack speed and stress intensity factor, but also on the past history of these lime-dependent quantities. A representation of the crack tip field is obtained in the form of an expansion about the crack up in powers of radial coordinate, with the coefficients depending on the time rates of change of crack tip speed and stress intensity factor. This representation is used to interpret some experimental observations, with the conclusion that the higher-order expansion provides an accurate description of crack tip fields under fairly severe transient conditions. In addition, some estimates are made of the practical limits of using a stress intensity factor field alone to characterize the local fields.
The application of Newman crack-closure model to predicting fatigue crack growth
NASA Technical Reports Server (NTRS)
Si, Erjian
1994-01-01
Newman crack-closure model and the relevant crack growth program were applied to the analysis of crack growth under constant amplitude and aircraft spectrum loading on a number of aluminum alloy materials. The analysis was performed for available test data of 2219-T851, 2024-T3, 2024-T351, 7075-T651, 2324-T39, and 7150-T651 aluminum materials. The results showed that the constraint factor is a significant factor in the method. The determination of the constraint factor is discussed. For constant amplitude loading, satisfactory crack growth lives could be predicted. For the above aluminum specimens, the ratio of predicted to experimental lives, Np/Nt, ranged from 0.74 to 1.36. The mean value of Np/Nt was 0.97. For a specified complex spectrum loading, predicted crack growth lives are not in very good agreement with the test data. Further effort is needed to correctly simulate the transition between plane strain and plane stress conditions, existing near the crack tip.
The application of Newman crack-closure model to predicting fatigue crack growth
NASA Astrophysics Data System (ADS)
Si, Erjian
1994-09-01
Newman crack-closure model and the relevant crack growth program were applied to the analysis of crack growth under constant amplitude and aircraft spectrum loading on a number of aluminum alloy materials. The analysis was performed for available test data of 2219-T851, 2024-T3, 2024-T351, 7075-T651, 2324-T39, and 7150-T651 aluminum materials. The results showed that the constraint factor is a significant factor in the method. The determination of the constraint factor is discussed. For constant amplitude loading, satisfactory crack growth lives could be predicted. For the above aluminum specimens, the ratio of predicted to experimental lives, Np/Nt, ranged from 0.74 to 1.36. The mean value of Np/Nt was 0.97. For a specified complex spectrum loading, predicted crack growth lives are not in very good agreement with the test data. Further effort is needed to correctly simulate the transition between plane strain and plane stress conditions, existing near the crack tip.
NASA Astrophysics Data System (ADS)
Larsen, James M.; Allison, John E.
This book contains chapters on fracture mechanics parameters for small fatigue cracks, monitoring small-crack growth by the replication method, measurement of small cracks by photomicroscopy (experiments and analysis), and experimental mechanics of microcracks. Other topics discussed are the real-time measurement of small-crack-opening behavior using an interferometric strain/displacement gage; direct current electrical potential measurement of the growth of small cracks; an ultrasonic method for the measurement of the size and opening behavior of small fatigue cracks; and the simulation of short crack and other low closure loading conditions, utilizing constant K(max) Delta-K-decreasing fatigue crack growth procedures.
A Continuum-Atomistic Analysis of Transgranular Crack Propagation in Aluminum
NASA Technical Reports Server (NTRS)
Yamakov, V.; Saether, E.; Glaessgen, E.
2009-01-01
A concurrent multiscale modeling methodology that embeds a molecular dynamics (MD) region within a finite element (FEM) domain is used to study plastic processes at a crack tip in a single crystal of aluminum. The case of mode I loading is studied. A transition from deformation twinning to full dislocation emission from the crack tip is found when the crack plane is rotated around the [111] crystallographic axis. When the crack plane normal coincides with the [112] twinning direction, the crack propagates through a twinning mechanism. When the crack plane normal coincides with the [011] slip direction, the crack propagates through the emission of full dislocations. In intermediate orientations, a transition from full dislocation emission to twinning is found to occur with an increase in the stress intensity at the crack tip. This finding confirms the suggestion that the very high strain rates, inherently present in MD simulations, which produce higher stress intensities at the crack tip, over-predict the tendency for deformation twinning compared to experiments. The present study, therefore, aims to develop a more realistic and accurate predictive modeling of fracture processes.
NASA Astrophysics Data System (ADS)
Kim, Ji Hoon; Park, Jung Ho; Hwang, Sung-Min; Baik, Kwang Hyeon
2012-05-01
We studied the growth and the characteristics of nonpolar Si-doped a-plane GaN grown on r-plane sapphire substrates with different off-cut angles which were changed in the range of -0.2° ˜ +0.4°. Samples grown by using -0.2° and +0.2° off-cut angles showed triangular pit-free and smooth surfaces, which resulted from enhanced lateral growth owing to the epitaxial films having a Ga face. On the other hand, the sample grown by using +0.4° off-cut angles revealed a high density of pits and low crystalline quality due to a high density of dislocations. The strain determined by using calculations with the lattice parameters also showed a dependence on the off-cut angles. We expect r-plane sapphire with off-cut angles in the range of -0.2° ˜ +0.2° to be very effective for improving the crystalline quality and the surface morphology of a-plane GaN.
A Study of Failure in Small Pressurized Cylindrical Shells Containing a Crack
NASA Technical Reports Server (NTRS)
Barwell, Craig A.; Eber, Lorenz; Fyfe, Ian M.
1998-01-01
The deformation in the vicinity of axial cracks in thin pressurized cylinders is examined using small experimental The deformation in the vicinity of axial cracks in thin pressurized cylinders is examined using small experimental models. The loading applied was either symmetric or unsymmetric about the crack plane, the latter being caused by structural constraints such as stringers. The objective was two fold - one, to provide the experimental results which will allow computer modeling techniques to be evaluated for deformations that are significantly different from that experienced by flat plates, and the other to examine the deformations and conditions associated with the onset of crack kinking which often precedes crack curving. The stresses which control crack growth in a cylindrical geometry depend on conditions introduced by the axial bulging, which is an integral part of this type of failure. For the symmetric geometry, both the hoop and radial strain just ahead off the crack, r = a, were measured and these results compared with those obtained from a variety of structural analysis codes, in particular STAGS [1], ABAQUS and ANSYS. In addition to these measurements, the pressures at the onset of stable and unstable crack growth were obtained and the corresponding crack deformations measured as the pressures were increased to failure. For the unsymmetric cases, measurements were taken of the crack kinking angle, and the displacements in the vicinity of the crack. In general, the strains ahead of the crack showed good agreement between the three computer codes and between the codes and the experiments. In the case of crack behavior, it was determined that modeling stable tearing with a crack-tip opening displacement fracture criterion could be successfully combined with the finite-element analysis techniques as used in structural analysis codes. The analytic results obtained in this study were very compatible with the experimental observations of crack growth
NASA Astrophysics Data System (ADS)
Liu, Z. Y.; Li, X. G.; Cheng, Y. F.
2011-10-01
The effect of strain rate on cathodic reactions of X70 pipeline steel during stress corrosion cracking in a near-neutral pH solution was investigated by electrochemical impedance spectroscope and potentiodynamic polarization curve measurements as well as slow strain rate tests. A local additional potential model was used to understand mechanistically the role of strain rate in electrochemical cathodic reaction. It was found that an application of elastic stress would not affect the electrochemical stable state of the steel specimen at a macroscopic scale. Under a weak cathodic polarization, the interfacial charge-transfer process occurring on steel contains both cathodic and anodic reactions. Since the anodic reaction process is still significant, localized dissolution could occur even at such a cathodic potential, resulting in generation of corrosion pits. These pits could be the start sites to initiate stress corrosion cracks. Strain rate affects the corrosion reaction, which is associated with the generation of dislocation emergence points and slip steps on the specimen surface, resulting in a negative local additional potential to enhance the cathodic reaction locally.
The surface and through crack problems in layered orthotropic plates
NASA Technical Reports Server (NTRS)
Erdogan, Fazil; Wu, Binghua
1991-01-01
An analytical method is developed for a relatively accurate calculation of Stress Intensity Factors in a laminated orthotropic plate containing a through or part-through crack. The laminated plate is assumed to be under bending or membrane loading and the mode 1 problem is considered. First three transverse shear deformation plate theories (Mindlin's displacement based first-order theory, Reissner's stress-based first-order theory, and a simple-higher order theory due to Reddy) are reviewed and examined for homogeneous, laminated and heterogeneous orthotropic plates. Based on a general linear laminated plate theory, a method by which the stress intensity factors can be obtained in orthotropic laminated and heterogeneous plates with a through crack is developed. Examples are given for both symmetrically and unsymmetrically laminated plates and the effects of various material properties on the stress intensity factors are studied. In order to implement the line-spring model which is used later to study the surface crack problem, the corresponding plane elasticity problem of a two-bonded orthotropic plated containing a crack perpendicular to the interface is also considered. Three different crack profiles: an internal crack, an edge crack, and a crack terminating at the interface are considered. The effect of the different material combinations, geometries, and material orthotropy on the stress intensity factors and on the power of stress singularity for a crack terminating at the interface is fully examined. The Line Spring model of Rice and Levy is used for the part-through crack problem. The surface crack is assumed to lie in one of the two-layered laminated orthotropic plates due to the limitation of the available plane strain results. All problems considered are of the mixed boundary value type and are reduced to Cauchy type of singular integral equations which are then solved numerically.
NASA Technical Reports Server (NTRS)
Smith, Robert W.; Smith, Gordon T.
1960-01-01
Thermal-fatigue crack-growth characteristics of notched- and unnotched-disk specimens of A-286, Discaloy, hot-cold worked 16-25-6, and overaged 16-25-6 were experimentally studied. Separately controlled variables were total strain range (0.0043 to 0.0079 in./in.), maximum cycle temperature (1300 and 1100 F), and hold time at maximum temperature (O and 5 min). A limited number of mechanical, push-pull, constant-strain cycle tests at room temperature were made using notched and un-notched bars of the same materials. In these tests the number of cycles to failure as well as the variation of load change with accumulated cycles was measured, and the effects of mean stress were observed. Constant-strain-range mechanical-fatigue tests at room temperature revealed notched-bar fatigue life to be strongly influenced by mean stress. For a specific strain range, the longest fatigue life was always found to be associated with the least-tensile (or most compressive) mean stress. By defining thermal-fatigue life as the number of cycles required to produce a crack area of 6000 square mils, the relative thermal-fatigue resistances of the test materials were established. Notched-disk specimens of A-286 and Discaloy steels exhibited longer fatigue lives than either hot-cold worked or overaged 16-25-6. On the other hand, unnotched-disk specimens of Discaloy and hot-cold worked 16-25-6 had longer lives than A-286 and overaged 16-25-6. Separation of the crack-growth data into microstage and macrostage periods revealed that the macrostage period accounted for the greatest part of the difference among materials when tested in the notched configuration, while the microstage was largely responsible for the differences encountered in unnotched disks.
Characteristics of thermally-induced transverse cracks in graphite epoxy composite laminates
NASA Technical Reports Server (NTRS)
Adams, D. S.; Bowles, D. E.; Herakovich, C. T.
1983-01-01
The characteristics of thermally induced transverse cracks in T300/5208 graphite-epoxy cross-ply and quasi-isotropic laminates were investigated both experimentally and analytically. The formation of transverse cracks and the subsequent crack spacing present during cool down to -250 F (116K) and thermal cycling between 250 and -250 F (116 and 394K) was investigated. The state of stress in the vicinity of a transverse crack and the influence of transverse cracking on the laminate coefficient of thermal expansion (CTE) was predicted using a generalized plane strain finite element analysis and a modified shear lag analysis. A majority of the cross-ply laminates experienced transverse cracking during the initial cool down to -250 F whereas the quasi-isotropic laminates remained uncracked. The in situ transverse strength of the 90 degree layers was more than 1.9 times greater than the transverse strength of the unidirectional 90 degree material for all laminates investigated.
Matrix cracking in laminated composites under monotonic and cyclic loadings
NASA Technical Reports Server (NTRS)
Allen, David H.; Lee, Jong-Won
1991-01-01
An analytical model based on the internal state variable (ISV) concept and the strain energy method is proposed for characterizing the monotonic and cyclic response of laminated composites containing matrix cracks. A modified constitution is formulated for angle-ply laminates under general in-plane mechanical loading and constant temperature change. A monotonic matrix cracking criterion is developed for predicting the crack density in cross-ply laminates as a function of the applied laminate axial stress. An initial formulation for a cyclic matrix cracking criterion for cross-ply laminates is also discussed. For the monotonic loading case, a number of experimental data and well-known models are compared with the present study for validating the practical applicability of the ISV approach.
NASA Astrophysics Data System (ADS)
Feng, Shih-Wei; Chen, Yu-Yu; Lai, Chih-Ming; Tu, Li-Wei; Han, Jung
2013-12-01
Anisotropic strain relaxation and the resulting degree of polarization of the electronic transition in nonpolar a-plane GaN using one- and two-step growth are studied. By using two-step growth, a slower coalescence and a longer roughening-recovery process lead to larger anisotropic strain relaxation, a less striated surface, and lower densities of basal stacking fault (BSF) and prismatic stacking fault (PSF). It is suggested that anisotropic in-plane strains, surface striation, and BSF and PSF densities in nonpolar a-GaN are consequences of the rate of coalescence, the period of roughening-recovery process, and the degree of anisotropic strain relaxation. In addition, the two-step growth mode can enhance the degree of polarization of the electronic transition. The simulation results of the kṡp perturbation approach show that the oscillator strength and degree of polarization of the electronic transition strongly depend on the in-plane strains upon anisotropic in-plane strain relaxation. The research results provide important information for optimized growth of nonpolar III-nitrides. By using two-step growth and by fabricating the devices on the high-quality nonpolar free-standing GaN substrates, high-efficiency nonpolar a-plane InGaN LEDs can be realized. Nonpolar a-plane InGaN/GaN LEDs can exhibit a strongly polarized light to improve the contrast, glare, eye discomfort and eye strain, and efficiency in display application.
Feng, Shih-Wei Chen, Yu-Yu; Lai, Chih-Ming; Tu, Li-Wei; Han, Jung
2013-12-21
Anisotropic strain relaxation and the resulting degree of polarization of the electronic transition in nonpolar a-plane GaN using one- and two-step growth are studied. By using two-step growth, a slower coalescence and a longer roughening-recovery process lead to larger anisotropic strain relaxation, a less striated surface, and lower densities of basal stacking fault (BSF) and prismatic stacking fault (PSF). It is suggested that anisotropic in-plane strains, surface striation, and BSF and PSF densities in nonpolar a-GaN are consequences of the rate of coalescence, the period of roughening-recovery process, and the degree of anisotropic strain relaxation. In addition, the two-step growth mode can enhance the degree of polarization of the electronic transition. The simulation results of the k⋅p perturbation approach show that the oscillator strength and degree of polarization of the electronic transition strongly depend on the in-plane strains upon anisotropic in-plane strain relaxation. The research results provide important information for optimized growth of nonpolar III-nitrides. By using two-step growth and by fabricating the devices on the high-quality nonpolar free-standing GaN substrates, high-efficiency nonpolar a-plane InGaN LEDs can be realized. Nonpolar a-plane InGaN/GaN LEDs can exhibit a strongly polarized light to improve the contrast, glare, eye discomfort and eye strain, and efficiency in display application.
Stable Crack Growth During Thermal Actuation of Shape Memory Alloys
NASA Astrophysics Data System (ADS)
Jape, S.; Baxevanis, T.; Lagoudas, D. C.
2016-03-01
A finite element analysis of crack growth is carried out in shape memory alloys subjected to thermal variations under plane strain, mode I, constant applied loading. The crack is assumed to propagate at a critical level of the crack-tip energy release rate which is modeled using the virtual crack closure technique. The load level, applied at a high temperature at which the austenite phase is stable, is assumed sufficiently low so that the resulting crack-tip energy release rate is smaller than the critical value but sufficiently high so that the critical value is reached during cooling, initiating crack growth (Baxevanis and Lagoudas in Int J Fract 191:191-213, 2015). Stable crack growth is observed, mainly associated with the shielding effect of the transformed material left in the wake of the advancing crack. Results pertaining to the near-tip mechanical fields and fracture toughness are presented and their sensitivity to phase transformation metrics and bias load levels is investigated.
Elasto-plastic bending of cracked plates, including the effects of crack closure. Ph.D. Thesis
NASA Technical Reports Server (NTRS)
Jones, D. P.
1972-01-01
A capability for solving elasto-plastic plate bending problems is developed using assumptions consistent with Kirchhoff plate theory. Both bending and extensional modes of deformation are admitted with the two modes becoming coupled as yielding proceeds. Equilibrium solutions are obtained numerically by determination of the stationary point of a functional which is analogous to the potential strain energy. The stationary value of the functional for each load increment is efficiently obtained through use of the conjugate gradient. This technique is applied to the problem of a large centrally through cracked plate subject to remote circular bending. Comparison is drawn between two cases of the bending problem. The first neglects the possibility of crack face interference with bending, and the second includes a kinematic prohibition against the crack face from passing through the symmetry plane. Results are reported which isolate the effects of elastoplastic flow and crack closure.
Energy absorption mechanisms during crack propagation in metal matrix composites
NASA Technical Reports Server (NTRS)
Murphy, D. P.; Adams, D. F.
1979-01-01
The stress distributions around individual fibers in a unidirectional boron/aluminum composite material subjected to axial and transverse loadings are being studied utilizing a generalized plane strain finite element analysis. This micromechanics analysis was modified to permit the analysis of longitudinal sections, and also to incorporate crack initiation and propagation. The analysis fully models the elastoplastic response of the aluminum matrix, as well as temperature dependent material properties and thermal stress effects. The micromechanics analysis modifications are described, and numerical results are given for both longitudinal and transverse models loaded into the inelastic range, to first failure. Included are initially cracked fiber models.
Structure and switching of in-plane ferroelectric nano-domains in strained PbxSr1-xTiO3 thin films
Matzen, Sylivia; Nesterov, Okeksiy; Rispens, Gregory; Heuver, J. A.; Bark, C; Biegalski, Michael D; Christen, Hans M; Noheda, Beatriz
2014-01-01
Nanoscale ferroelectrics, the active elements of a variety of nanoelectronic devices, develop denser and richer domain structures than the bulk counterparts. With shrinking device sizes understanding and controlling domain formation in nanoferroelectrics is being intensely studied. Here we show that a precise control of the epitaxy and the strain allows stabilizing a hierarchical domain architecture in PbxSr1-xTiO3 thin films, showing periodic, purely in-plane polarized, ferroelectric nano-domains that can be switched by a scanning probe.
Line-spring model for surface cracks in a Reissner plate
NASA Technical Reports Server (NTRS)
Delale, F.; Erdogan, F.
1981-01-01
In this paper the line-spring model developed by Rice and Levy for a surface crack in elastic plates is reconsidered. The problem is formulated by using Reissner's plate bending theory. For the plane strain problem of a strip containing an edge crack and subjected to tension and bending new expressions for stress intensity factors are used which are valid up to a depth-to-thickness ratio of 0.8. The stress intensity factors for a semi-elliptic and a rectangular crack are calculated. Considering the simplicity of the technique and the severity of the underlying assumptions, the results compare rather well with the existing finite element solutions.
Automatic crack propagation tracking
NASA Technical Reports Server (NTRS)
Shephard, M. S.; Weidner, T. J.; Yehia, N. A. B.; Burd, G. S.
1985-01-01
A finite element based approach to fully automatic crack propagation tracking is presented. The procedure presented combines fully automatic mesh generation with linear fracture mechanics techniques in a geometrically based finite element code capable of automatically tracking cracks in two-dimensional domains. The automatic mesh generator employs the modified-quadtree technique. Crack propagation increment and direction are predicted using a modified maximum dilatational strain energy density criterion employing the numerical results obtained by meshes of quadratic displacement and singular crack tip finite elements. Example problems are included to demonstrate the procedure.
NASA Astrophysics Data System (ADS)
Ren, Zhenhua; Zeng, Xiantao; Liu, Hanlong; Zhou, Fengjun
2013-03-01
The application of fiber reinforced plastic (FRP), including carbon FRP and glass FRP, for structural repair and strengthening has grown due to their numerous advantages over conventional materials such as externally bonded reinforcement (EBR) and near-surface mounted (NSM) strengthening techniques. This paper summarizes the results from 21 reinforced concrete beams strengthened with different methods, including externally-bonded and near-surface mounted FRP, to study the strain coordination of the FRP and steel rebar of the RC beam. Since there is relative slipping between the RC beam and the FRP, the strain of the FRP and steel rebar of the RC beam satisfy the quasi-plane-hypothesis; that is, the strain of the longitudinal fiber that parallels the neutral axis of the plated beam within the scope of the effective height ( h 0) of the cross section is in direct proportion to the distance from the fiber to the neutral axis. The strain of the FRP and steel rebar satisfies the equation: ɛ FRP= βɛ steel, and the value of β is equal to 1.1-1.3 according to the test results.
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.