Showing papers on "Crack closure published in 1994"

Numerical simulations of fast crack growth in brittle solids

Abstract: Dynamic crack growth is analysed numerically for a plane strain block with an initial central crack subject to tensile loading. The continuum is characterized by a material constitutive law that relates stress and strain, and by a relation between the tractions and displacement jumps across a specified set of cohesive surfaces. The material constitutive relation is that of an isotropic hyperelastic solid. The cohesive surface constitutive relation allows for the creation of new free surface and dimensional considerations introduce a characteristic length into the formulation. Full transient analyses are carried out. Crack branching emerges as a natural outcome of the initial-boundary value problem solution, without any ad hoc assumption regarding branching criteria. Coarse mesh calculations are used to explore various qualitative features such as the effect of impact velocity on crack branching, and the effect of an inhomogeneity in strength, as in crack growth along or up to an interface. The effect of cohesive surface orientation on crack path is also explored, and for a range of orientations zigzag crack growth precedes crack branching. Finer mesh calculations are carried out where crack growth is confined to the initial crack plane. The crack accelerates and then grows at a constant speed that, for high impact velocities, can exceed the Rayleigh wave speed. This is due to the finite strength of the cohesive surfaces. A fine mesh calculation is also carried out where the path of crack growth is not constrained. The crack speed reaches about 45% of the Rayleigh wave speed, then the crack speed begins to oscillate and crack branching at an angle of about 29° from the initial crack plane occurs. The numerical results are at least qualitatively in accord with a wide variety of experimental observations on fast crack growth in brittle solids.

High Temperature Component Life Assessment

Abstract: Introduction. Processes of deformation and fracture at high temperatures. Stress analysis of uncracked bodies. Stress analysis of cracked bodies. Models for creep crack initiation and growth. Creep-fatigue crack growth. Experimental determinations of high temperature crack growth. Practical applications. Index.

A review of crack closure, fatigue crack threshold and related phenomena

Abstract: Published fatigue crack closure mechanisms are reviewed in the context of the following five ideas that have been developed over the past twenty years to explain the closure effects on near-threshold crack growth behaviour: (1) oxide, (2) asperity, (3) plasticity, (4) phase transformation and (5) viscous fluids. The first three have been considered as more important than the last two. Our analysis indicates that (a) there can be no contribution from plasticity to crack closure, (b) the crack closure contribution can be significant only if it is closed fully, which is reflected as an infinite slope in the load-displacement curve, (c) formation of oxide asperities from a fretting action is a random process and not a deterministic one, and therefore cannot explain the deterministic behaviour of the effect of load ratio on the threshold, and (d) the closure contribution from the asperities resulting from oxides or corrosion products,or surface roughness, it is less than 20% of what has been deduced based on the change in the slope of the load-displacement curves. Thus the analyses show that crack closure can exist, but its magnitude is either small or negligible. The critical evaluation of the literature data on (1) the threshold stress intensity variation with load ratio on many materials, and (2) and examination of the experimentally observed load-displacement curves confirm the above conclusions. Hence to rationalize the observed variations in the fatigue crack threshold with load ratio, we have proposed a new model. It postulates a requirement for two critical stress intensity parameters, namely ΔK th ∗ and K max ∗ , that must be satisfied simultaneously as the crack tip driving forces for fatigue crack growth. This requirement is fundamental to fatigue, since an unambiguous description of cyclic loads requires two independent load parameters. Several experimental results from the literature are presented in support of this postulation. Using these two critical parameters, the entire functional relationship between Kmax, ΔKth and R is explained without invoking an entrinsic factor, namely crack closure. In addition, for a given material and its crack tip environment, a unique relationship between ΔKth and Kmax exists that is independent of test methods used in determining thresholds. Finally, because of this two parameter requirement, all fatigue crack growth data need to be represented in terms of three-dimensional plots involving da/dN, ΔK and Kmax. For a two-dimensional representation, the data need to be transformed correctly, defining the net driving force involving both ΔK and Kmax parameters. The concepts presented are independent of whether crack closure exists or not, or even whether cracks exist or not.

The activation energy for dislocation nucleation at a crack

Abstract: THE ACTIVATION energy for dislocation nucleation from a stressed crack tip is calculated within the Peierls framework, in which a periodic shear stress vs displacement relation is assumed to hold on a slip plane emanating from the crack tip. Previous results have revealed that the critical G (energy release rate corresponding to the “screened” crack tip stress field) for dislocation nucleation scales with y., (the unstable stacking energy), in an analysis which neglects any coupling between tension and shear along the slip plane. That analysis represents instantaneous nucleation and takes thermal effects into account only via the weak temperature dependence of the elastic constants. In this work, the energy required to thermally activate a stable, incipient dislocation into its unstable “saddle-point” configuration is directly calculated for loads less than that critical value. We do so only with the simplest case, for which the slip plane is a prolongation of the crack plane. A first calculation reported is 2D in nature, and hence reveals an activation energy per unit length. A more realistic scheme for thermal activation involves the emission of a dislocation loop, an inherently 3D phenomenon. Asymptotic calculations of the activation energy for loads close to the critical load are performed in 2D and in 3D. It is found that the 3D activation energy generally corresponds to the 2D activation energy per unit length multiplied by about 5 -10 Burgers vectors (but by as many as 17 very near to the critical loading). Implications for the emission of dislocations in copper. K-iron, and silicon at elevated temperature are discussed. The effects of thermal activation are very significant in lowering the load for emission. Also, the appropriate activation energy to correspond to molecular dynamics simulations of crack tips is discussed. Such simulations, as typically carried out with only a few atomic planes in a periodic repeat direction parallel to the crack tip. are shown to greatly exaggerate the (aheddy large) effects of temperature on dislocation nucleation. WE BUILD on recent advances in the modeling of dislocation nucleation at a crack tip

Electric-field-induced fatigue crack growth in piezoelectrics

Abstract: When subjected to large alternating electric fields, ferroelectric ceramics may experience cracking and mechanical degradation. This article describes an experimental procedure for characterizing crack extension from preexisting flaws in such materials subject to high-amplitude, alternating electric fields. A new mode of electric-field-induced fatigue crack growth is identified. Fracture mechanics concepts are applied to interpret the observed cracking.

Concepts for bridged cracks in fracture and fatigue

Abstract: Some recent work on length scales in bridged crack problems is reviewed and enlarged upon. Fundamental differences are highlighted between the solutions obtainable from bridged crack models (nonvanishing crack tip stress intensity factor, Ktip) and traditional cohesive zone models (vanishing Ktip); and from models for which the bridging tractions are nonincreasing (softening) and increasing (hardening) functions of the crack opening displacement. The roles of length scales in determining ductility, notch sensitivity, and whether failure is catastrophic or noncatastrophic are discussed. Length scales for monotonic loading are extended to cyclic loading.

Handbook of fatigue crack propagation in metallic structures

Abstract: Volume 1. Introductory Section. Failure criteria for anisotropic bodies (P.S. Theocaris). Introduction to fracture mechanics of fatigue (H. Kitagawa). Numerical methods for fracture parameters calculation (G.J. Tsamasphyros). Fatigue Behaviour of Metallic Materials. Fatigue of steels for concrete reinforcement and cables (M. Elices et al). Fatigue crack growth and crack shielding in a Fe-C-Cu sintered steel (Y-W. Mai et al). Fatigue and fracture properties of aerospace aluminium alloys (R.J.H. Wanhill). Fatigue crack propagation in titanium alloys (J.K. Gregory). Theoretical Models and Numerical Methods. Mechanical model for fatigue crack propagation in metals (X.-L. Zheng). Application of the three-dimensional boundary element method to quasistatic and fatigue crack propagation (M.H. Aliabadi, Y. Mi). Method of damage mechanics for prediction of structure member fatigue lives (X. Zhang et al.). Stochastic fatigue crack propagation (J.H. Yoon, Y.S. Yang). A fracture mechanics approach to the optimum design of cracked structures under cyclic loading (Z. Knesl). Fundamental Aspects of Fatigue Crack Propagation Phenomenon. Stable and unstable fatigue crack propagation in metals (V.T. Troshchenko). Fatigue crack growth from stress concentrations and fatigue life prediction in notched components (C.S. Shin). Propagation of surface cracks under cyclic loading (A. Carpinteri). Growth behaviour of small fatigue cracks and relating problems (H. Nisitani et al). Analytical and experimental study of crack closure behaviour (D.-h. Chen). Studies of fatigue crack closure (D. Francois). Fatigue threshold of metallic materials - a review (A. Hadrboletz et al). Mechanics of fatigue crack growth as a synthesis of micro-and macro-mechanics of fracture (V.V. Bolotin). Random material non-homogeneity effects on fatigue crack growth (K. Dolinski). Volume 2. Influence of Loading Conditions. Fatigue crack growth under variable amplitude loading (J. Dominguez). Mixed mode fatigue crack propagation (L.P. Pook). Numerical and. experimental study of mixed mode fatigue crack propagation (A.S. Kobayashi, M. Ramulu). Crack growth behaviour under repeated impact load conditions (T. Tanaka et al). Influence of Environmental Conditions. Influence of ambient atmosphere on fatigue crack growth behaviour of metals (J. Petit et al). Influence of hydrogen-containing environments on fatigue crack extension resistance of metals (V.V Panasyuk et al.). Fatigue crack propagation in aqueous environments (Y. Nakai). Application of fatigue crack growth data to low cycle fatigue at high temperature (L. Remy). Creep-fatigue interaction under high-temperature conditions (R. Ohtani, T. Kitamura). Fatigue crack propagation in metals at low temperatures (X.-L. Zheng, B.-T. Lu). (Part contents).

Toughness of an interface along a thin ductile layer joining elastic solids

Abstract: The contribution of plastic deformation to the effective work of fracture is computed for a crack lying along one of the interfaces of a thin ductile layer joining two elastic solids. A model is proposed for the joint whose major parameters are the layer thickness, the elastic-plastic properties of material in the layer, and the work of separation and peak separation stress of the local interface fracture process. A symmetric mode I loading of the joint is considered under conditions where the thickness of the layer and the extent of the plastic zone are small compared with the crack length. The crack growth resistance behaviour is computed, with special emphasis on the steady-state work of fracture. The role of the layer thickness in the development of the plasticity contribution to toughness is detailed. Plastic dissipation is fully realized for layers above a certain thickness, characteristic of a plastic zone dimension, and is negligible when the layer is thin relative to this dimension. Othe...

Micromechanics Modeling of Crack Initiation Under Contact Fatigue

Abstract: Using dislocation pileup theory, a model is givenfor the prediction of crack initiation life under contact fatigue. Near surface crack initiation is investigated by introducing the sliding contact boundary condition. Crack initiation originated at the surface and substrate are treated as extreme cases. The new model physically explains how a surface crack can be initiated and shows that the surface crack initiation life should be shorter than the subsurface crack initiation life under the same stress amplitude conditions. A discussion is given about the influence of residual stress, hardness, temperature, irreversibility of the plastic deformation, as well as other parameters that affect the crack initiation life

A reconciliation of dynamic crack velocity and Rayleigh wave speed in isotropic brittle solids

Abstract: Following earlier observations of multiple micro-crack formation accompanying crack propagation under dynamic conditions, the question regarding the discrepancy between the ‘theoretically anticipated’ maximal crack (Rayleigh wave) speed and those observed typically in amorphous, isotropic solids is examined experimentally It is shown that if the production of these multiple micro-cracks ahead of the main fracture is suppressed by fabricating a material possessing a thin uniform region of vanishing intrinsic (molecular/atomic) material strength, the crack speed is materially increased to the point of approaching the Rayleigh wave speed Moreover, it is also shown that the presence of small discreet flaws of sufficient spatial density similarly ‘weakens’ the material to produce fracture speeds comparable to the Rayleigh wave speed One deduces, therefore, that for a single crack front the linearized theory of elastodynamics correctly predicts the dynamic crack propagation behavior of a solid with sufficiently low material strength

Micromechanical modeling of reinforcement fracture in particle-reinforced metal-matrix composites

Abstract: Finite element analyses of the effect of particle fracture on the tensile response of particle-reinforced metal-matrix composites are carried out. The analyses are based on two-dimensional plane strain and axisymmetric unit cell models. The reinforcement is characterized as an isotropic elastic solid and the ductile matrix as an isotropically hardening viscoplastic solid. The reinforcement and matrix properties are taken to be those of an Al-3.5 wt pct Cu alloy reinforced with SiC particles. An initial crack, perpendicular to the tensile axis, is assumed to be present in the particles. Both stationary and quasi-statically growing cracks are analyzed. Resistance to crack growth in its initial plane and along the particle-matrix interface is modeled using a cohesive surface constitutive relation that allows for decohesion. Variations of crack size, shape, spatial distribution, and volume fraction of the particles and of the material and cohesive properties are explored. Conditions governing the onset of cracking within the particle, the evolution of field quantities as the crack advances within the particle to the particle-matrix interface, and the dependence of overall tensile stress-strain response during continued crack advance are analyzed.

Small-crack effects in high-strength aluminum alloys

Abstract: The National Aeronautics and Space Administration and the Chinese Aeronautical Establishment participated in a Fatigue and Fracture Mechanics Cooperative Program. The program objectives were to identify and characterize crack initiation and growth of small cracks (10 microns to 2 mm long) in commonly used US and PRC aluminum alloys, to improve fracture mechanics analyses of surface- and corner-crack configurations, and to develop improved life-prediction methods. Fatigue and small-crack tests were performed on single-edgenotch tension (SENT) specimens and large-crack tests were conducted on center-crack tension specimens for constant-amplitude (stress ratios of -1, 0, and 0.5) and Mini-TWIST spectrum loading. The plastic replica method was used to monitor the initiation and growth of small fatigue cracks at the semicircular notch. Crack growth results from each laboratory on 7075-T6 bare and LC9cs clad aluminum alloys agreed well and showed that fatigue life was mostly crack propagation from a material defect (inclusion particles or void) or from the cladding layer. Finite-element and weight-function methods were used to determine stress intensity factors for surface and corner cracks in the SENT specimens. Equations were then developed and used in a crack growth and crack-closure model to correlate small- and large-crack data and to make life predictions for various load histories. The cooperative program produced useful experimental data and efficient analysis methods for improving life predictions. The results should ultimately improve aircraft structural reliability and safety.

Experiments on 3-D crack growth in uniaxial compression

Abstract: A study of spatial crack propagation under uniaxial compression was undertaken in a brittle transparent plastic with artificially induced internal cracks. The specific issue addressed was whether there was a qualitative difference between 2-D and 3-D crack propagation. Fracture of heterogeneous materials (e.g., rocks) under uniaxial compression is produced by propagation of internal cracks towards the load, some of which propagate so extensively that they eventually split the material into columns. Experimental and theoretical studies of this phenomenon based on 2-D models (plates with through cracks.) showed that the extensive crack growth towards compression emerged from pre-existing (initial) defects such as through cracks in plates inclined to the compression axis, or cylindrical pores (e.g., [1-10]). The main point of these studies is that a single 2-D microdefect is capable of producing extensive crack growth sufficient to cause fracture. In reality however, pre-existing cracks are three-dimensional, which can result in a more complicated mechanism of their growth. A limited number of experiments: Adams and Sines [4] and Cannoo et al. [11], have been reported on studies of the growth of inclined disk-like cracks in uniaxially compressed transparent samples (from PMMA polimethilmethacrylate plastic). However, the dimensions of the PMMA samples used in these experiments were not sufficient to investigate the extensive crack growth. In order to study mechanisms of fracture in uniaxial compression, further experiments are required using sufficiently large samples which would provide enough room for extensive crack growth. In addition, the spatial interaction of several growing cracks has to be investigated. This paper reports the results of such experiments. 1. Sample preparation and experimental technique. In the experiments, parallelepiped samples made from transparent casting polyester resin "Polylite 61-209" with cross-section dimensions of 55 mm x 55 mm, and a height of 120 mm were used. When frozen to -17°C, this material is perfectly brittle, deforms without barrelling and has linear stress-strain behaviour up to its burst-like fracture. The mechanical properties evaluated during the tests are: Young's modulus _=_4GPa; uniaxial compressive strength _=_ 140MPa; fracture toughness _=_0.6MPa-m 1/2. Two methods of modelling the internal initial disk-like cracks were adopted: (1) embedding a thin disk-like inclusion into the resin block during casting. The inclusion consisted of two aluminium foil disks greased and put together and held within the sample by two cotton threads; (2) cutting two semi-circular slots of 0.3 mm thickness in two halves of the sample and then gluing the halves together (this method was used in [4]). In order to ensure the contact between the opposite faces of the crack, teflon or greased foil disks were inserted into the slots (According to the 2-D analysis [9], the initial cracks, i.e. voids with contacted lips are the strongest drivers of the extensive crack growth).

Ethanol‐induced crack healing in poly(methyl methacrylate)

Abstract: The crack healing induced by ethanol in poly(methyl methacrylate) (PMMA) has been studied at temperatures of 40–60°C. Crack healing occurs because the effective glass transition temperature of PMMA is reduced to below the test temperature by ethanol plasticization. It is found that crack closure rate is constant at a given temperature. The fracture strength of healed PMMA is lower than that of the original samples. By comparing the fracture stress with the morphology of the crack edge on the PMMA surface, we found that a high degree of swelling is responsible for the incomplete recovery of mechanical strength. The fractography of the completely healed sample shows a very different fracture morphology from that of virgin PMMA. The transport of ethanol in PMMA also is studied. At lower temperatures, transport is described by ideal Case II behavior. As the temperature increases, the kinetics shift from ideal Case II to anomalous behavior. The first stage of crack healing is controlled by Case I transport. © 1994 John Wiley & Sons, Inc.

Cleavage due to dislocation confinement in layered materials

Abstract: The effects of dislocation confinement on fracture behavior in laminates consisting of alternating submicron ductile and brittle layers are studied. When the ductile layer thickness is below the micron level, dislocations must be treated individually. Dislocations emitted from the crack tip have two effects: they blunt the crack and thereby reduce the tensile stress at the crack tip ; and pile up against an interface and send a back stress to the crack tip to hinder further dislocation emission. Consequently, an equilibrium number of dislocations exists at a given load level. We estimate this number by considering the stability conditions for dislocations threading in the ductile layer, and dislocation pile-up is treated as an equivalent superdislocation. Furthermore, the competition between further dislocation emission and cleavage at the blunted crack tip is considered. Our result shows that because of the confinement, as the applied load increases, the tensile stress at the blunted crack tip also increases. Cleavage occurs when the tensile stress at the crack tip reaches the theoretical strength. Given a sufficiently thin constraining layer, cleavage can even occur in ductile metals such as copper and aluminum. The implications of this model for several material systems are discussed.

The use of fatigue specimens precracked in compression for measuring threshold values and crack growth

Abstract: Precracks generated in purely compressive cycles permit a determination of threshold values ΔKth and crack growth curves with increasing load amplitude only. They also permit to measure the effective threshold ΔKeff th and the dependence of ΔKth on crack length. The production of such precracks is also discussed.

Fatigue crack growth in residual stress fields : experimental results and modelling

Abstract: — The paper discusses the results of fatigue crack growth rate tests conducted in the presence of residual stresses. Three different residual stress distributions, obtained by laser welds, were employed in order to characterize the crack propagation behaviour under different conditions, producing either an increase or a reduction of the stress intensity factor due to external loads. Test results are analysed by means of a non-linear numerical model (based on the weight function method) and a knowledge of the fatigue crack growth properties of the base material, free from residual stresses. The results of the analysis are discussed with reference to experimental trends, in order to clarify the predictive capabilities of the method and aspects needing further investigation.

Mesh-dependence and stress singularity in finite element analysis of creep crack growth by continuum damage mechanics

Abstract: A series of numerical analyses are first performed for a plate specimen of 316 stainless steel with a central crack to show the characteristics of the mesh dependence. It is shown that the effects of mesh size appear mainly in the initial stage of crack growth. Then, strain localization is discussed in relation to the mesh dependence phenomena. Finally, stress singularity due to boundary constraints such as observed at crack tip is shown to be a controlling factor in the present mesh dependence problem. Two simple approximate relations are proposed to describe the mesh dependent behavior for crack initiation time and crack growth rates, respectively

A review of modelling small-crack behavior and fatigue-life predictions for aluminum alloys

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

A Theory of Fatigue Crack Initiation

Abstract: A theory of fatigue crack initiation based on the Gibbs free energy consideration is proposed. The theory is extended to the cases when the free surface effect and interface effect are considered.