International Journal of Fracture最新文献

The hydrogen embrittlement (HE) characteristics of various stainless steels were investigated. In this study, as-received, heated (1100 °C, 15 h), and cold-rolled (30% strain) γ-austenite (AS), α-ferrite (FS), α′-martensite (MS), and γ–α duplex (DS) stainless steels were employed. For as-received stainless steels, severe HE occurred for DS and MS with static tensile loading, while no clear and weak HE was observed for AS and FS, respectively. This could be attributed to the different extent of hydrogen diffusivity in the stainless steel. A large amount of hydrogen penetrated to (i) lattice vacancy with low atomic density for body-centered cubic FS, DS, and MS, compared to that for face-centered cubic (AS); (ii) the phase boundary between γ-austenite and α-ferrite for DS; and (iii) the boundary between the Cr base precipitate and the martensite matrix for MS. HE also occurred strongly for heated-DS owing to the grain growth, i.e., a high hydrogen concentration in grain and phase boundaries. Although no clear HE was detected in as-received AS with static loading, HE occurred in cold-rolled AS, where hydrogen penetrated lattice vacancies and α′-martensite formed through strain-induced martensite. Owing to strain-induced martensite created during cyclic loading, HE was detected even for as-received AS, which is dissimilar to the result of the tensile test. Details of HE characteristics of the strainless steels were examined using the four stainless steels with different microstructures, diferent strain level and oxide layer. Moreover, those were investigated under different loading conditions, such as constant, static, and cyclic loading.

A computational model is formulated for studying dynamic crack propagation in quasi-brittle materials exposed to time-dependent loading conditions. Under such conditions, inertial effects of structural components play an important role in modelling crack propagation problems. The computational model is proposed within the theory of regularised cracks which uses a damage-like internal variable. Here, fracture considers phase-field damage which gives rise to a material degradation in a narrow material strip defining the regularised crack. Based on the energy formulation using the Lagrangian of the system, the proposed computational approach introduces a staggered scheme adopted to solve the coupled system and providing it in a variational form within the time stepping procedure. The numerical data are obtained by quadratic programming algorithms implemented together with a finite element code.

The importance of developing simple relationships for interpreting crack growth rate test results and a practical approach to predicting crack propagation under thermo-mechanical fatigue conditions based on readily available data is emphasised by many authors. In this study, a damage impact parameter was introduced to predict the crack growth rate and durability under isothermal and thermo-mechanical fatigue conditions. To validate the model, we used a single-edge notched specimen made of polycrystalline coarse-grained nickel-based alloy XH73M. The specimen was subjected to loading conditions that included in-phase and out-of-phase thermo-mechanical fatigue at a temperature range of 400–650 °C, as well as isothermal fatigue at 26 °C, 400 °C and 650 °C. A numerical analysis was used to simulate the material deformation behaviour at the crack tip according to a nonlinear kinematic hardening model. Numerical and experimental stress–strain state parameters based on strain energy density were used to formulate and estimate the damage impact parameter. Based on the correlation between the crack growth rate and the introduced damage impact parameter, a method for predicting crack propagation is proposed. The accuracy of the proposed method was experimentally validated.

We develop a computational framework to model damage and delamination in laminated polymer composite structures incorporating the effects of temperature and moisture content. The framework is founded on a recently developed comprehensive multi-layer thin-shell formulation based on Isogeometric Analysis, which includes continuum damage, plasticity and cohesive-interface models. To incorporate hygrothermal effects in the modeling, we propose a scaling law that is based on the Arrhenius equation and material glass transition temperature that establishes the dependence of the intra- and interlaminar material properties on the temperature and moisture content. We compute several classical test cases using a combination of environmental conditions and demonstrate that the resulting modeling approach shows a good agreement with the experimental data, both in terms of failure loads reached as well as failure modes predicted.

The present work aims to investigate the failure size effect on flattened disks containing an eccentric circular hole under mode I loading conditions. For this purpose, uniaxial compression tests are carried out on polymethyl methacrylate (PMMA) samples with holes. Depending on the hole radius and eccentricity, the energy release rate is either an increasing or decreasing function of the crack length, thus affecting the stability of crack propagation. Experimental results are interpreted and discussed through the coupled stress and energy criterion of Finite Fracture Mechanics. The approach lies on the assumption of a finite crack advance and it is implemented through the numerical estimation of the stress field and the Incremental Energy Release Rate functions. Finally, stability and crack speed propagation are discussed under the assumption of Linear Elastic Fracture Mechanics. Theoretical predictions reveal in agreement with experimental results thus demonstrating that the Coupled Criterion effectively captures the failure condition.

In the present paper, we extend results recently given by Ciavarella et al. (J Mech Phys Solids 169:105096, 2022) to show some actual calculations of the viscoelastic dissipation in a crack propagation at constant speed in a finite size specimen. It is usually believed that the cohesive models introduced by Knauss and Schapery and the dissipation-based theories introduced by de Gennes and Persson-Brener give very similar results for steady state crack propagation in viscoelastic materials, where usually only the asymptotic singular field is used for the stress. We show however that dissipation and the energy balance never reach a steady state, despite the constant propagation crack rate and stress intensity factor. Our loading protocol permits a rigorous solution, and implies a short phase with constant specimen elongation rate, but then possibly a very long phase of constant or decreasing elongation, which differs from typical experiments. For the external work we are therefore unable to use the de Gennes and Persson-Brener theories which suggested that the increase of effective fracture energy would go up to the ratio of instantaneous to relaxed modulus, at very fast rates. We show viscoelastic dissipation is in general a transient quantity, which can vary by orders of magnitude while the stress intensity factor is kept constant, and is largely affected by dissipation in the bulk rather than at the crack tip. The total work to break a specimen apart is found also to be possibly arbitrarily large for quite a large range of intermediate crack growth rates.

Williams asymptotic expansions are widely used to represent mechanical fields at the vicinity of crack-tips in plane elastic media. For practical applications, series solutions have to be truncated and it is believed that a better accuracy can be achieved by retaining more terms in the summations. The influence of the truncation on the accuracy can be quantified comparing truncated closed-form Williams series solutions available for some fracture configurations to their corresponding complex exact counterparts. The computation of 2D absolute error fields reveals astonishing patterns in which appear points with numerically zero error implying the existence of loci where truncated series can provide exact results. These loci of exactness gather on curves emanating from the crack-tips and pointing towards the outside of series convergence disks. An analytical investigation of this phenomenon allows to relate the number and tangency angle at the crack-tip of these curves to the number and values of the zeros of Williams series angular eigenfunctions. Beyond its analytical interest in the understanding of Williams series framework, this property of exactness for truncated series can also help to improve the accuracy of experimental and computational techniques based on Williams series.

The existing mechanical dicing process of single crystalline Silicon Carbide (SiC) is one of the main factors limiting the development of semiconductor process, which could be replaced by laser scribing potentially. To achieve efficient and low-damage SiC separation, the cracking behavior of SiC after laser grooving should be well understood and controllable. Since the laser grooving including thermal ablation and meltage solidification, the cracking behavior of the scribed SiC would be different to the original single crystal SiC. In this paper, cohesive zone model (CZM) is used to quantitively represent the cracking behavior of the nano-laser scribed SiC. The separation after scribing was conducted in a three-point bending (3 PB) fixture to characterize the cracking behavior. Therefore, by inverting the load–displacement curves of 3 PB with CZM embedded finite element model, the cohesive behavior is characterized by bilinear traction–separation law, which illustrated the whole cracking process numerically. The methodology established in current paper gives way to understand the SiC scribing and cracking process with quantitative cohesive parameters.

Construction of the Kitagawa–Takahashi (K–T) diagram requires inputs of two material properties, namely, endurance limit and threshold stress intensity range, ΔK_th. Both are sensitive to applied stress ratio. The effect of stress ratio on endurance limit is well known. Unfortunately, crack closure, associated with the nature of conventional testing practice obscures the effect of stress ratio on intrinsic, closure free ΔK_th that would apply to natural crack like defects and short cracks. This study was made possible by the development of a new test method to characterize closure free threshold conditions under controlled near-tip residual stress conditions that essentially determine near-tip stress ratio at threshold. A procedure is described to construct the K–T diagram, using ΔK_th values corrected for stress ratio and applicable to pre-existing defects and short cracks at notches that are unlikely to see closure. As a case study, a K–T diagram valid for different applied stress ratios is constructed for titanium alloy Ti-6Al-4V.