Showing papers on "Fracture mechanics published in 2015"
TL;DR: A synergy of multiple deformation mechanisms is identified, rarely achieved in metallic alloys, which generates high strength, work hardening and ductility, including the easy motion of Shockley partials, their interactions to form stacking-fault parallelepipeds, and arrest at planar slip bands of undissociated dislocations.
Abstract: Damage tolerance can be an elusive characteristic of structural materials requiring both high strength and ductility, properties that are often mutually exclusive. High-entropy alloys are of interest in this regard. Specifically, the single-phase CrMnFeCoNi alloy displays tensile strength levels of ∼ 1 GPa, excellent ductility (∼ 60-70%) and exceptional fracture toughness (KJIc>200 MPa√m). Here through the use of in situ straining in an aberration-corrected transmission electron microscope, we report on the salient atomistic to micro-scale mechanisms underlying the origin of these properties. We identify a synergy of multiple deformation mechanisms, rarely achieved in metallic alloys, which generates high strength, work hardening and ductility, including the easy motion of Shockley partials, their interactions to form stacking-fault parallelepipeds, and arrest at planar slip bands of undissociated dislocations. We further show that crack propagation is impeded by twinned, nanoscale bridges that form between the near-tip crack faces and delay fracture by shielding the crack tip.
575 citations
TL;DR: In this article, a phase-field model for ductile fracture of elasto-plastic solids in the quasi-static kinematically linear regime is proposed, which captures the entire range of behavior of a ductile material exhibiting $$J_2$$J2-PLasticity, encompassing plasticization, crack initiation, propagation and failure.
Abstract: Phase-field modeling of brittle fracture in elastic solids is a well-established framework that overcomes the limitations of the classical Griffith theory in the prediction of crack nucleation and in the identification of complicated crack paths including branching and merging. We propose a novel phase-field model for ductile fracture of elasto-plastic solids in the quasi-static kinematically linear regime. The formulation is shown to capture the entire range of behavior of a ductile material exhibiting $$J_2$$J2-plasticity, encompassing plasticization, crack initiation, propagation and failure. Several examples demonstrate the ability of the model to reproduce some important phenomenological features of ductile fracture as reported in the experimental literature.
522 citations
TL;DR: In this paper, a generalization of recently developed continuum phase field models for brittle fracture towards fully coupled thermo-mechanical and multi-physics problems at large strains is presented.
416 citations
TL;DR: In this article, a generalization of recently developed continuum phase field models from brittle to ductile fracture coupled with thermo-plasticity at finite strains is presented, which uses a geometric approach to the diffusive crack modeling based on the introduction of a balance equation for a regularized crack surface.
407 citations
TL;DR: A comprehensive view of the state-of-the-art research on various aspects of the fracture of bulk metallic glasses, including fracture behavior and characteristics, fracture mode, fracture criterion, fracture toughness, and fracture morphology, is presented in this article.
391 citations
TL;DR: In this paper, an experimental study was conducted to investigate anisotropy effects on tensile properties of two short glass fiber reinforced thermoplastics, and mechanisms of failure were identified based on fracture surface microscopic analysis and crack propagation paths.
Abstract: An experimental study was conducted to investigate anisotropy effects on tensile properties of two short glass fiber reinforced thermoplastics. Tensile tests were performed in various mold flow directions and with two thicknesses. A shell–core morphology resulting from orientation distribution of fibers influenced the degree of anisotropy. Tensile strength and elastic modulus nonlinearly decreased with specimen angle and Tsai–Hill criterion was found to correlate variation of these properties with the fiber orientation. Variation of tensile toughness with fiber orientation and strain rate was evaluated and mechanisms of failure were identified based on fracture surface microscopic analysis and crack propagation paths. Fiber length, diameter, and orientation distribution mathematical models were also used along with analytical approaches to predict tensile strength and elastic modulus form tensile properties of constituent materials. Laminate analogy and modified Tsai–Hill criteria provided satisfactory predictions of elastic modulus and tensile strength, respectively.
340 citations
317 citations
TL;DR: In this article, a microcrack mechanism was proposed based on microscopy of the fracture surfaces, which indicated that the coalescence of microcracks may facilitate crack propagation, lowering the fracture toughness.
Abstract: The toughening effects of graphene and graphene-derived materials on thermosetting epoxies are investigated. Graphene materials with various structures and surface functional groups are incorporated into an epoxy resin by in situ polymerization. Graphene oxide (GO) and GO modified with amine-terminated poly(butadiene-acrylonitrile) (ATBN) are chosen to improve the dispersion of graphene nanosheets in epoxy and increase their interfacial adhesion. An impressive toughening effect is observed with less than 0.1 wt% graphene. A maximum in toughness at loadings as small as 0.02 wt% or 0.04 wt% is observed for all four types of graphene studied. An epoxy nanocomposite with ATBN-modified GO shows a 1.5-fold improvement in fracture toughness and a corresponding 2.4-fold improvement in fracture energy at 0.04 wt% of graphene loading. At such low loadings, these graphene-type materials become economically feasible components of nanocomposites. A microcrack mechanism is proposed based on microscopy of the fracture surfaces. Due to the stress concentration by graphene nanosheets, microcracks may be formed to absorb the fracture energy. However, above a certain graphene concentration, the coalescence of microcracks appears to facilitate crack propagation, lowering the fracture toughness. Crack defl ection and pinning likely contribute to the slow increase in fracture toughness at higher loadings.
300 citations
TL;DR: In this article, the fracture toughness and fatigue crack growth rate properties of selective laser melted (SLM) specimens produced from grade 5 Ti6Al4V powder metal have been investigated and three specimen orientations relative to the build direction as well as two different post-build heat treatments were considered.
Abstract: The fracture toughness (K 1c ) and fatigue crack growth rate (FCGR) properties of selective laser melted (SLM) specimens produced from grade 5 Ti6Al4V powder metal has been investigated. Three specimen orientations relative to the build direction as well as two different post-build heat treatments were considered. Specimens and test procedures were designed in accordance with ASTM E399 and ASTM E647 standard. The results show that there is a strong influence of post-build processing (heat treated versus ‘as built’) as well as specimen orientation on the dynamic behaviour of SLM produced Ti6Al4V. The greatest improvement in properties after heat treatment was demonstrated when the fracture plane is perpendicular to the SLM build direction. This behaviour is attributed to the higher anticipated influence of tensile residual stress for this orientation. The transformation of the initial rapidly solidified microstructure during heat treatment has a smaller beneficial effect on improving mechanical properties.
263 citations
TL;DR: In this paper, a meshless numerical method, called general particle dynamics (GPD), is proposed to simulate samples of rock-like brittle heterogeneous material containing four preexisting flaws under uniaxial compressive loads.
Abstract: A novel meshless numerical method, called general particle dynamics (GPD), is proposed to simulate samples of rock-like brittle heterogeneous material containing four preexisting flaws under uniaxial compressive loads. Numerical simulations are conducted to investigate the initiation, growth, and coalescence of cracks using a GPD code. An elasto-brittle damage model based on an extension of the Hoek–Brown strength criterion is applied to reflect crack initiation, growth, and coalescence and the macrofailure of the rock-like material. The preexisting flaws are simulated by empty particles. The particle is killed when its stresses satisfy the Hoek–Brown strength criterion, and the growth path of cracks is captured through the sequence of such damaged particles. A statistical approach is applied to model the heterogeneity of the rock-like material. It is found from the numerical results that samples containing four preexisting flaws may produce five types of cracks at or near the tips of preexisting flaws including wing, coplanar or quasi-coplanar secondary, oblique secondary, out-of-plane tensile, and out-of-plane shear cracks. Four coalescence modes are observed from the numerical results: tensile (T), compression (C), shear (S), and mixed tension/shear (TS). A higher load is required to induce crack coalescence in the shear mode (S) than the tensile (T) or mixed (TS) mode. It is concluded from the numerical results that crack coalescence occurs following the weakest coalescence path among all possible paths between any two flaws. The numerical results are in good agreement with reported experimental observations.
261 citations
TL;DR: In this article, the authors investigated fracture coalescence by loading rock-like specimens with two and three pre-existing flaws made by pulling out the embedded metal inserts in the pre-cured period.
TL;DR: In this article, the authors discuss the potential of alternative degradation functions in the context of crack nucleation and propagation, which is not desirable when modeling brittle materials, as the degradation function frequently found in the literature yields a pronounced softening behavior before the onset of fracture.
TL;DR: In this paper, a peridynamic model for dynamic fracture in brittle homogeneous and isotropic materials is presented, and three loading types are used to investigate the role of stress waves interactions on crack propagation and branching.
Abstract: In this paper we review the peridynamic model for brittle fracture and use it to investigate crack branching in brittle homogeneous and isotropic materials. The peridynamic simulations offer a possible explanation for the generation of dynamic instabilities in dynamic brittle crack growth and crack branching. We focus on two systems, glass and homalite, often used in crack branching experiments. After a brief review of theoretical and computational models on crack branching, we discuss the peridynamic model for dynamic fracture in linear elastic–brittle materials. Three loading types are used to investigate the role of stress waves interactions on crack propagation and branching. We analyze the influence of sample geometry on branching. Simulation results are compared with experimental ones in terms of crack patterns, propagation speed at branching and branching angles. The peridynamic results indicate that as stress intensity around the crack tip increases, stress waves pile-up against the material directly in front of the crack tip that moves against the advancing crack; this process “deflects” the strain energy away from the symmetry line and into the crack surfaces creating damage away from the crack line. This damage “migration”, seen as roughness on the crack surface in experiments, modifies, in turn, the strain energy landscape around the crack tip and leads to preferential crack growth directions that branch from the original crack line. We argue that nonlocality of damage growth is one key feature in modeling of the crack branching phenomenon in brittle fracture. The results show that, at least to first order, no ingredients beyond linear elasticity and a capable damage model are necessary to explain/predict crack branching in brittle homogeneous and isotropic materials.
TL;DR: In this article, the effect of temperature on the mechanical behavior of Strathbogie granite (fine-grained) was studied under unconfined stress conditions, and the results showed that the increasing temperature reduces the stress thresholds for crack initiation and crack damage and extends the duration of stable crack propagation.
TL;DR: A phase-field model for brittle fracture is implemented in the commercial finite element software Abaqus by means of UEL and UMAT subroutines and removes the need for numerical tracking of discontinuities in the displacement field that are characteristic of discrete crack methods.
TL;DR: In this article, a fully coupled non-planar hydraulic fracture propagation model based on the Extended Finite Element Method (XFEM) is presented, and the arbitrary solution-dependent fracture path can be determined by solving a set of discontinuity equations.
TL;DR: In this paper, a phase-field continuum theory for brittle fracture in elastic-plastic solids and its computational implementation are presented, based on a virtual-power formulation in which two additional and independent kinematical descriptors are introduced, namely the phase field and the accumulated plastic strain.
TL;DR: In this paper, an extended bond-based peridynamic (PD) approach is presented for quasi-static mechanical behavior and brittle failure analysis of materials and structures, where a local artificial damping is introduced into the peridynamics equations of motion, and a step-by-step loading method and a non-equilibrium criterion are employed to calculate the elastic response quantitatively, simulate the crack initiation and propagation and predict the extreme failure load of structures.
TL;DR: In this article, Semicircular bend tests on Marcellus Shale core samples containing calcite-filled natural fractures (veins) were performed to investigate the influence of weak planes on hydraulic fracture propagation.
Abstract: Field observations show that hydraulic fracture growth in naturally fractured formations like shale is complex. Preexisting discontinuities in shale, including natural fractures and bedding, act as planes of weakness that divert fracture propagation. To investigate the influence of weak planes on hydraulic fracture propagation, we performed Semicircular Bend tests on Marcellus Shale core samples containing calcite-filled natural fractures (veins). The approach angle of the induced fracture to the veins and the thickness of the veins have a strong influence on propagation. As the approach angle becomes more oblique to the induced fracture plane, and as the vein gets thicker, the induced fracture is more likely to divert into the vein. Microstructural analysis of tested samples shows that the induced fracture propagates in the middle of the vein but not at the interface between the vein and the rock matrix. Cleavage planes and fluid inclusion trails in the vein cements exert some control on the fracture path. Combining the experimental results with theoretical fracture mechanics arguments, the fracture toughness of the calcite veins was estimated to range from 0.24 MPa m1/2 to 0.83 MPa m1/2, depending on the value used for the Young's modulus of the calcite vein material. Measured fracture toughness of unfractured Marcellus Shale was 0.47 MPa m1/2.
TL;DR: An overview of past research on T-stress is presented in this article, where the authors discuss different phenomena associated with T-stress: crack path stability, isochromatic fringes pattern, plastic zone influence and constraint parameter.
TL;DR: This article validation of Francfort and Marigo’s variational approach to fracture based on some classical fracture experiments shows that this approach can be used to faithfully account for unknown crack paths even for complex loadings and geometry.
TL;DR: In this paper, a DGEBA epoxy resin cured using dicyandiamide hardener, was modified by using two types of CSR nano-particles over a range of volume fractions from 0.% to 38.%.
TL;DR: In this article, an extension of the extended isogeometric analysis (XIGA) for simulation of two-dimensional fracture mechanics problems in piezoelectric materials under dynamic and static coupled electromechanical loads is presented.
TL;DR: A phase field theory for modeling deformation and fracture of single crystals, polycrystals, and grain boundaries is developed in this paper, where an incremental energy minimization approach is used to predict equilibrium crack morphologies in finite element simulations.
TL;DR: In this paper, a variational phase-field model for strongly anisotropic fracture was proposed, which resorts to the extended Cahn-Hilliard framework proposed in the context of crystal growth.
Abstract: Crack propagation in brittle materials with anisotropic surface energy is important in applications involving single crystals, extruded polymers, or geological and organic materials. Furthermore, when this anisotropy is strong, the phenomenology of crack propagation becomes very rich, with forbidden crack propagation directions or complex sawtooth crack patterns. This problem interrogates fundamental issues in fracture mechanics, including the principles behind the selection of crack direction. Here, we propose a variational phase-field model for strongly anisotropic fracture, which resorts to the extended Cahn-Hilliard framework proposed in the context of crystal growth. Previous phase-field models for anisotropic fracture were formulated in a framework only allowing for weak anisotropy. We implement numerically our higher-order phase-field model with smooth local maximum entropy approximants in a direct Galerkin method. The numerical results exhibit all the features of strongly anisotropic fracture and reproduce strikingly well recent experimental observations.
TL;DR: It is unambiguously shown that fibers' interfacial areas along with parenchyma cells' boundaries were preferred routes for crack growth in both radial and longitudinal directions, and the tortuous crack propagation mode exhibited in the present study could be applicable to other cellular natural materials.
Abstract: Bamboo, as a natural hierarchical cellular material, exhibits remarkable mechanical properties including excellent flexibility and fracture toughness. As far as bamboo as a functionally graded bio-composite is concerned, the interactions of different constituents (bamboo fibers; parenchyma cells; and vessels.) alongside their corresponding interfacial areas with a developed crack should be of high significance. Here, by using multi-scale mechanical characterizations coupled with advanced environmental electron microscopy (ESEM), we unambiguously show that fibers' interfacial areas along with parenchyma cells' boundaries were preferred routes for crack growth in both radial and longitudinal directions. Irrespective of the honeycomb structure of fibers along with cellular configuration of parenchyma ground, the hollow vessels within bamboo culm affected the crack propagation too, by crack deflection or crack-tip energy dissipation. It is expected that the tortuous crack propagation mode exhibited in the present study could be applicable to other cellular natural materials as well.
TL;DR: In this article, the Edge Notched Disc Bend (ENDB) is proposed for experimental determination of KIIIc, which is a circular disc containing an edge crack created along the disc diameter which is subjected to three-point bend loading.
TL;DR: In this article, a review of atomistic aspects of fracture in crystalline materials where significant advances have been achieved over the last ten years and provides an outlook on future perspectives for atomistic modelling of fracture.
Abstract: Any fracture process ultimately involves the rupture of atomic bonds. Processes at the atomic scale therefore critically influence the toughness and overall fracture behavior of materials. Atomistic simulation methods including large-scale molecular dynamics simulations with classical potentials, density functional theory calculations and advanced concurrent multiscale methods have led to new insights e.g. on the role of bond trapping, dynamic effects, crack-microstructure interactions and chemical aspects on the fracture toughness and crack propagation patterns in metals and ceramics. This review focuses on atomistic aspects of fracture in crystalline materials where significant advances have been achieved over the last ten years and provides an outlook on future perspectives for atomistic modelling of fracture.
TL;DR: In this paper, an API X70 pipeline steel was electrochemically charged with hydrogen for different durations in order to find crack nucleation and propagation sites, and the results showed that fine grain colonies with dominant ND|| orientations were prone to intergranular HIC crack propagation.
TL;DR: In this article, a new model based on the cohesive zone method coupling stress-seepage-damage filed is developed to simulate the interaction between hydraulic fracture and natural fracture without introducing the crossing criterion.