Showing papers on "Crack closure published in 2013"

A 50-year retrospective review of three-dimensional effects at cracks and sharp notches

Abstract: This review is a brief survey of three-dimensional effects at cracks and sharp notches. The overall aim is to review developments over the past 50 years leading up to the current state of the art. The review is restricted to linear elastic, homogeneous, isotropic materials, with any yielding confined to a small region at a crack or notch tip. It is also restricted to static loading and to constant amplitude fatigue loading. An enormous amount of theoretical and experimental information relevant to three-dimensional effects has been published in the past five decades, so the review is, of necessity, highly selective. Theoretical topics covered are linear elastic fracture mechanics, including Volterra distorsioni, stress intensity factors, corner point singularities, crack front line tension, displacement analysis of cracks and notches, and through thickness distributions of stresses and stress intensity factors. Crack path topics covered are fatigue crack path constraints, determination of fatigue crack paths, oscillating crack fronts in thin sheets and the transition to slant crack propagation in thin sheets. Plane strain fracture toughness testing, including standards, is covered. Overall, it can be concluded that the existence of three-dimensional effects at cracks and sharp notches has been known for many years, but understanding has been limited, and for some situations still is. Understanding improved when the existence of corner point singularities and their implications became known. Increasingly powerful computers made it possible to investigate three-dimensional effects numerically in detail. Despite increased understanding, three-dimensional effects are sometimes ignored in situations where they may be important.

The role of elastic anisotropy, length scale and crystallographic slip in fatigue crack nucleation

Abstract: Fatigue crack nucleation in polycrystal ferritic steel is investigated through experimental observation of multiple large-grained, notched, four-point bend tests combined with explicit microstructural representation of the same samples using crystal plasticity finite element techniques in order to assess fatigue indicator parameters, together with the roles of elastic anisotropy and length scale effects in slip development, and hence in crack nucleation. Elastic anisotropy has been demonstrated to play a pivotal role in the distribution and magnitude of polycrystal slip relative to observed crack nucleation sites in the context of constrained cyclic microplasticity. Length scale effects were found not to alter substantively the distributions or magnitudes of slip relative to the observed crack nucleation site, but in detailed analyses of an experimental sample, the location of highest magnitude of geometrically necessary dislocations was found to coincide precisely with the position of predicted peak plasticity and the experimentally observed crack nucleation site. The distributions of microplasticity within polycrystal samples were found to change quite significantly between the first yield and after multiple cycles. As a result, the effective plastic strain per cycle was found to be a better indicator of fatigue crack nucleation than peak effective plastic strain. In nine independently tested and analysed polycrystal samples, the cyclic effective plastic strain and crystallographic system peak accumulated slip were found to be good indicators of a fatigue crack nucleation site.

Effects of inclusions, grain boundaries and grain orientations on the fatigue crack initiation and propagation behavior of 2524-T3 Al alloy

Abstract: Microstructural aspects have fundamental influences on the fatigue crack characteristics of materials. In this paper, effects of inclusions, grain boundaries (GBs) and grain orientations on the fatigue crack initiation and propagation behavior in a 2524-T3 aluminum alloy have been investigated using in-situ scanning electron microscope (SEM) fatigue testing and electron back scattering diffraction (EBSD). The results show that, potential fatigue cracks tend to nucleate along coarse and closely spaced inclusion particles or high-angle GBs. Coarse inclusion particles drastically accelerate local crack growth rates. A model of series crack growing stages is given based on the observation of initiation and growth of cracks at the inclusion region. GBs serve to impede the crack tip from propagation and cause large angle crack deflections, which greatly affects local crack propagation behaviors. In addition, fatigue crack shows a strong tendency to propagate transgranularly grains with high Schmid factors (SFs) and avoid grains with low SFs.

Effect of Hydrogen on Fatigue Crack Growth of Metals

Abstract: Abstract The present paper shows several important phenomena obtained by investigations of the effect of hydrogen on fatigue crack growth behaviour, including the measurement of the hydrogen content in various materials such as low-carbon, Cr–Mo and stainless steels. Particularly important phenomena are the localization of fatigue slip bands, strain-induced martensite in Types 304, 316 and even 316L, and also strong frequency effects on fatigue crack growth rates. For example, with a decrease in frequency of fatigue loading down to the level of 0.2 Hz, the fatigue crack growth rate of a Cr–Mo steel is accelerated by 10–30 times. The same phenomenon also occurs even in austenitic stainless steels at the frequency of the level of 0.001 Hz. Striation morphology is also influenced by hydrogen. It has been revealed by re-analysing the results of the authors’ separately published reports that this basic hydrogen embrittlement mechanism is essentially the same throughout all the materials, i.e. low-carbon, Cr–Mo and stainless steels. Thus, the coupled effects of hydrogen content, hydrogen diffusion coefficient (for BCC or FCC), load frequency, localization of fatigue slip bands and strain-induced martensite must be always considered in fatigue test and analysis of hydrogen embrittlement.

A Thermodynamically Consistent Damage Model for Advanced Composites

Abstract: A continuum damage model for the prediction of damage onset and structural collapse of structures manufactured in fiber-reinforced plastic laminates is proposed. The principal damage mechanisms occurring in the longitudinal and transverse directions of a ply are represented by a damage tensor that is fixed in space. Crack closure under load reversal effects are taken into account using damage variables established as a function of the sign of the components of the stress tensor. Damage activation functions based on the LaRC04 failure criteria are used to predict the different damage mechanisms occurring at the ply level. The constitutive damage model is implemented in a finite element code. The objectivity of the numerical model is assured by regularizing the dissipated energy at a material point using Bazant's Crack Band Model. To verify the accuracy of the approach, analyses of coupon specimens were performed, and the numerical predictions were compared with experimental data.

Fatigue crack growth simulations of interfacial cracks in bi-layered FGMs using XFEM

Abstract: An investigation of fatigue crack growth of interfacial cracks in bi-layered materials using the extended finite element method is presented. The bi-material consists of two layers of dissimilar materials. The bottom layer is made of aluminium alloy while the upper one is made of functionally graded material (FGM). The FGM layer consists of 100 % aluminium alloy on the left side and 100 % ceramic (alumina) on the right side. The gradation in material property of the FGM layer is assumed to be exponential from the alloy side to the ceramic side. The domain based interaction integral approach is extended to obtain the stress intensity factors for an interfacial crack under thermo-mechanical load. The edge and centre cracks are taken at the interface of bi-layered material. The fatigue life of the interface crack plate is obtained using the Paris law of fatigue crack growth under cyclic mode-I, mixed-mode and thermal loads. This study reveals that the crack propagates into the FGM layer under all types of loads.

Oxidation and Crack Healing Behavior of a Fine‐Grained Cr2AlC Ceramic

Abstract: This work reports the oxidation and crack healing behavior of a fine-grained (~2 lm) Cr2AlC MAX phase ceramic. The oxidation behavior was investigated in the temperature range 900°C– 1200°C for times up to 100 h. The material showed a good oxidation resistance, owing to the formation of a dense and thin a-Al2O3 layer. The microstructure, composition and thickness of the oxide scale were characterized. Its oxidative crack healing behavior as a function of temperature, healing time, and initial crack size was studied systematically. The material showed excellent healing behavior. The main crack healing mechanism is the filling of the crack by oxides well adhering to the crack faces. The crack geometry before and after healing was characterized by X-ray tomography. Three-point bend tests showed the dependence of strength recovery at 1100° Ca s a function of initial crack length and healing time.

Investigation of sub-critical fatigue crack growth in FRP/concrete cohesive interface using digital image analysis

Abstract: The cohesive stress transfer during the sub-critical crack growth associated with the debonding of FRP from concrete under fatigue loading is experimentally investigated using the direct shear test set-up. The study focused on high-amplitude/low-cycle fatigue. The fatigue sub-critical crack growth occurs at a load that is smaller than the static bond capacity of the interface, obtained from monotonic quasi-static loading, and is also associated with a smaller value of the interfacial fracture energy. The strain distribution during debonding is obtained using digital image correlation. The results indicate that the strain distribution along the FRP during fatigue is similar to the strain distribution during debonding under monotonic quasi-static loading. The cohesive crack model and the shape of the strain distribution adopted for quasi-static monotonic loading is indirectly proven to be adequate to describe the stress transfer during fatigue loading. The length of the stress transfer zone during fatigue is observed to be smaller than the cohesive zone of the interfacial crack under quasi-static monotonic loading. The strain distribution across the width of the FRP sheet is not altered during and by fatigue loading. A new formulation to predict the debonding crack growth during fatigue is proposed.

Mixed mode crack propagation in low brittle rock-like materials

Abstract: Mixed mode fracture is quite common in rock structures Numerous investigators have used the Brazilian disk specimens with a central crack for investigating modes I, II, and mixed fracture toughness in brittle materials In this study, analytical, experimental, and numerical investigations were planned and performed on Central Straight Through Crack Brazilian Disk (CSCBD) specimens Ranking of geometrical parameters effective on the value of stress intensity factors (SIFs) of CSCBD specimens were obtained using stochastic analysis Furthermore, experimental tests were undertaken in order to evaluate the crack propagation in rock-like material of low brittleness Finally, numerical modeling was performed to assess the effect of crack length on the failure mode of CSCBD specimens Analytical analyses revealed that the inclination angle of the crack with respect to the diametrical load has the most important impact on the SIFs among the geometrical parameters of CSCBD specimen Performed experimental and numerical analyses also confirmed the effect of inclination angle and crack length and their impact on the mode of failure of the tested specimen

Plasticity and Fracture in Drying Colloidal Films

Abstract: Cracks in drying colloidal dispersions are typically modeled by elastic fracture mechanics, which assumes that all strains are linear, elastic, and reversible. We tested this assumption in films of a hard latex, by intermittently blocking evaporation over a drying film, thereby relieving the film stress. Here we show that although the deformation around a crack tip has some features of brittle fracture, only 20%-30% of the crack opening is relieved when it is unloaded. Atomic force micrographs of crack tips also show evidence of plastic deformation, such as microcracks and particle rearrangement. Finally, we present a simple scaling argument showing that the yield stress of a drying colloidal film is generally comparable to its maximum capillary pressure, and thus that the plastic strain around a crack will normally be significant. This also suggests that a film's fracture toughness may be increased by decreasing the interparticle adhesion.

Plastic strain localization and fatigue micro-crack formation in Hastelloy X

Abstract: In polycrystalline metals, local deformation heterogeneities induced by the microstructure influence fatigue crack initiation and micro-crack propagation. The localization in plastic strains associated with heterogeneous deformation has been described as a necessary condition and a precursor for the nucleation of fatigue cracks. However, a clear and quantitative assessment of the correlation between strain localization and fatigue micro-crack lengths requires further investigation. In this work, during interrupted loading experiments, high resolution deformation measurements using digital image correlation are made on polycrystalline Hastelloy X subjected to fatigue loading. The sub-grain level strain measurements are made prior to the formation of micro-cracks. The correlation between the localization of plastic strains very early on during the loading ( e.g ., less than 1000 cycles) and the micro-cracks which are detected later in the life of the sample ( e.g ., around 10,000) is discussed in this paper. Particular focus is given to the difference in grain boundary response, either blocking or transmitting slip, and the associated fatigue micro-crack lengths generated in the vicinity of these boundaries. The results show a clear correlation between both the locations and lengths of fatigue micro-cracks, which form later in loading, and the localization of plastic strains very early in the loading process. For the same number of cycles, the transmission of slip across grain boundaries resulted in longer transgranular cracks compared to cracks near grains surrounded by blocking grain boundaries which were shorter cracks and confined within single grains.