International Journal of Fracture最新文献

This study investigates the enhancement of fracture toughness in Ti-6Al-4V ELI, fabricated via laser powder bed fusion (LPBF), through a tailored cyclic heat treatment applied below the β-transus temperature to transform the martensitic microstructure into a bimodal configuration. Fracture toughness experiments were conducted using fatigue pre-cracked four-point bend specimens at room temperature, evaluating two orientations in additively manufactured (AM), heat-treated (HT) and wrought (WR) conditions. The findings reveal that stress-relieved AM samples demonstrated good ductility without compromising strength in uniaxial tension tests. However, they exhibited poor fracture toughness and pronounced anisotropy in crack initiation along directions parallel and perpendicular to the build orientation. This behavior is attributed to the (text{Widmanst}ddot{text{a}}text{tten}) microstructure and residual prior (upbeta ) grain boundaries. The cyclic heat treatment significantly enhanced fracture toughness in both orientations. This improvement is attributed to the larger colony size and higher initial strain hardening rate observed in the HT condition, achieving fracture toughness values comparable to wrought Ti-6Al-4V ELI. Fractographic analysis identified void-sheeting as the primary deformation mechanism governing crack propagation across all conditions. EBSD analysis further revealed that hard crystallographic orientations hindered crack initiation and propagation in HT samples. Additionally, ET1 twinning activity near the crack tip played a critical role in improving fracture toughness by blunting the crack tip and limiting its progression. This study offers valuable insights into the microstructural determinants of fracture toughness in additively manufactured Ti-6Al-4V ELI and underscores the potential of strategic heat treatments to achieve mechanical properties comparable to those of wrought materials.

Polymethyl methacrylate (PMMA) is a benchmark brittle material for dynamic crack propagation studies. Despite extensive research, significant inconsistencies persist in reported fracture parameter values, complicating the establishment of a consensus on their sensitivity to the cracking regime. This study aims to rigorously determine these properties while identifying the origins of these discrepancies. To minimize microbranching effects that can strongly influence fracture surface roughness, crack propagation was restricted to subcritical velocities using a strip-band-specimen (SBS) geometry and a dedicated experimental setup. This approach ensured a quasi-steady propagation regime with minimal inertial effects. Dynamic toughness was evaluated using resistance curves constructed from Williams series expansion and displacement fields obtained via digital image correlation (DIC). Fracture energy was assessed through two complementary methods: a global energy balance and an indirect analytical approach based on Irwin’s generalized relation. Two distinct propagation regimes were identified: a stable regime (90 – 180 (hbox {m}.hbox {s}^{-1})) with smooth fracture surfaces and an unstable regime (180 – 320 (hbox {m}.hbox {s}^{-1})) characterized by the emergence of conical microstructures, followed by a transition to fully disrupted propagation beyond 320 (hbox {m}.hbox {s}^{-1}), marking the onset of microbranches. A key outcome of this study is the validation of global fracture energy estimation through the local approach, and vice versa, allowing the derivation of one fracture property from the other – an unprecedented achievement for PMMA in dynamic crack propagation. This was made possible by the experimental setup and specimen geometry, which effectively minimized parasitic effects such as inertia and microbranching. Additionally, the findings confirm a strong correlation between surface roughness and the evolution of fracture energy from the earliest stages of dynamic propagation.

This study presents a novel phase-field modeling approach for brittle fracture that incorporates computational homogenization techniques to characterize the microstructural degradation of the material. Traditional phase-field models often implement degradation and dissipation functions in terms of the phase-field variable that, despite offering satisfactory results, their physical interpretation and their extension to anisotropic fracture behavior is not always clear. To address this challenge, we develop a framework inspired by the nucleation, growth, and coalescence of microstructural voids to model macroscopic fracture. The proposed approach employs homogenization techniques to calculate the effective material properties when introducing voids of varying sizes and shapes. By solving the homogenization problem for different void geometries, we obtain degradation functions that relate the size of microstructural voids to the homogenized constitutive tensor. These degradation functions provide a direct link between microscale damage mechanisms and macroscale fracture behavior. Comparative analyses with conventional AT1 and AT2 models reveal strong correlations between their response and those obtained via homogenization techniques. This relationship highlights the ability of homogenized models to not only replicate established results but also provide a new understanding of the phase-field variable.

Wedge splitting tests were conducted on a granite and a gneiss with similar mineralogy but different microstructure. The basic properties of the two rock types were characterized by petrographic analyses and mechanical tests. The granite specimens were split in one material direction, perpendicular to the rift plane, and the gneiss specimens were split in three different material directions, parallel and perpendicular to the foliation (and along and across a lineation). The effect of having a large blunt versus a sharp notch on the crack initiation was studied in the granite. The wedge splitting tests are unconventional for testing rocks and allowed to study the crack initiation and propagation under mode I loading condition in the quasi-brittle granite and brittle gneiss. The fracture energy and strain energy release rate were calculated. The strain energy release rate for gneiss, when splitting along and across the foliation, was around 45% and 60% of the values for the structurally isotropic granite. The fracture toughness was calculated from the strain energy release rate and was larger than corresponding values obtained from linear elastic fracture mechanics (LEFM). There was an effect on the early cracking stages by using a sharp notch compared with using a large blunt notch on the granite specimens, but the required largest force to split the specimens remained the same for the two notch types. The crack initiation started at a splitting force corresponding to 78% and 90% of the maximum splitting force on the specimens with a sharp notch and a large blunt notch, respectively. The results with a full force-displacement response during the crack propagation obtained for the brittle gneiss are unique. Most fracture mechanics results on rock materials are obtained from standard tests and LEFM and not via the measured strain energy release rate.

A novel failure criterion, named Strain-based Finite Fracture Mechanics, is proposed to predict the fatigue life of additively manufactured notched components under uniaxial loading conditions. The model relies on the simultaneous fulfillment of two conditions: a non-local strain requirement and the discrete energy balance. The inputs of the model are strain and the stress intensity factor at failure, which depend on the number of cycles according to power law equations. The inputs can be obtained based on strain-life and stress intensity factor-life data from plain and notched specimens. The present approach is comprehensively validated against experimental datasets on additively manufactured samples from the literature for different materials, raster angles, notch geometries and loading conditions. Predictions by other approaches, such as Finite Fracture Mechanics (in its original stress formulation) and the Theory of Critical Distances, are also considered, for the sake of completeness. Results show that, in general, the proposed strain-based model is more accurate and provides consistently precise predictions across different cases.

Modern phenomenological damage models use Lode parameter L and triaxiality (eta ) to describe the stress state of an isotropic material. Value pairs in the region between (L, eta = (0, 0)) and (L, eta = (0, frac{1}{sqrt{3}})) in plane stress condition can lead to ambiguous descriptions of the deformation. The case of simple shear is not defined separately. By using the difference in angles between the principal strain and principal stress axes, cases of coaxial stretch superposed with simple shear can be distinguished from cases of coaxial stretch without simple shear. In the case of anisotropic material or large elements, the distinction between these ambiguous cases can be utilized to optimize failure models. This study proposes a method to recover the deformation gradient and shear direction for proportional and non-proportional loading with an elastoplastic von Mises material. The deformation gradient is suitable for distinguishing stress states with simple shear from stress states without simple shear in plane stress condition.

Sharp kinks may be observed under shear loading or in materials containing weak directions, such as those produced by additive manufacturing. A better understanding of the fracture of these materials, both theoretically and experimentally, is required to deploy them in structural applications. This study focuses on the measurement of stress intensity factors (SIFs) around a sharp kink using digital image correlation (DIC). The performances of two DIC-based techniques, namely, integrated-DIC and post-processing of DIC-measured displacement fields, are assessed on a benchmark test using fused deposit modeling capabilities, and are compared to a reference finite element solution. It is shown that Williams’ expansion remains valid on a large enough region around the crack to extract reliable SIFs even very close to the crack kink. Both techniques are very trustworthy, provided the SIF identification zone is carefully defined to exclude the kink zone of influence.

This study proposes a fatigue life prediction method combining small-sample data expansion with the Weibull distribution function, incorporating the first order reliability factor (FOSM) to improve accuracy. Using Generalized Polynomial Chaos Expansion (GPC) and Latin Hypercube Sampling (LHS), small-sample fatigue data is expanded, followed by enhancing the two-parameter Weibull model with FOSM. Results show the generalized polynomial chaotic expansion method and Latin hypercube sampling are used to obtain the probability density curve when the stress level is 350 MPa, and the original data are all on this probability density curve, indicating that the expansion method is more credible. High prediction precision within a 1.5 × error range, with logarithmic safety life linearly related to stress level and decreasing with higher failure probability.

The rapid development of 3D printing of 316L stainless steel thin-walled structures obtained by direct energy deposition has generated an increased interest in the mechanical properties of such materials for use in applications; in particular, failure models are needed to ensure structural reliability. We consider the response of uniaxial tension specimens, with and without notches, to characterize the constitutive and failure behavior of the material. Specifically, we use numerical simulations of the notched tension experiment, achieved with a simple power-law strain hardening model and a failure criterion based on attaining a triaxiality-dependent critical strain-to-failure, to demonstrate that this model is capable of reproducing the material behavior accurately.

A particular failure mode of highly porous brittle materials consists in the propagation of cracks under uniaxial compressive loads. Such ’anticracks’ have been observed in a range of materials, from snow and porous sandstone to brittle foams. Here we present a computational model for the formation and propagation of anticrack-type failure in porous materials within the general computational framework of bond-based peridynamics. Random porosity is represented, on a scale well above the characteristic pore size, by random bond deletion (dilution disorder). We apply our framework to experimental data on anticrack propagation in silicate foams.