Fracture process in cortical bone: X-FEM analysis of microstructured models

Bones tissues are heterogeneous materials that consist of various microstructural features at different length scales. The fracture process in cortical bone is affected significantly by the microstructural constituents and their heterogeneous distribution. Understanding mechanics of bone fracture is necessary for reduction and prevention of risks related to bone fracture. The aim of this study is to develop a finite-element approach to evaluate the fracture process in cortical bone at micro-scale. In this study, three microstructural models with various random distributions based on statistical realizations were constructed using the global model's framework together with a submodelling technique to investigate the effect of microstructural features on macroscopic fracture toughness and microscopic crack-propagation behaviour. Analysis of processes of crack initiation and propagation utilized the extended finite-element method using energy-based cohesive-segment scheme. The obtained results were compared with our experimental data and observations and demonstrated good agreement. Additionally, the microstructured cortical bone models adequately captured various damage and toughening mechanisms observed in experiments. The studies of crack length and fracture propagation elucidated the effect of microstructural constituents and their mechanical properties on the microscopic fracture propagation process. © 2013 Springer Science+Business Media Dordrecht

Small-scale assessment of corrosion-induced damage in hardmetals

In this work, the effect of corrosion-induced damage on the mechanical response of hardmetals was evaluated at small-scale level by means of nanoindentation and nanoscratch. Damage was introduced in a controlled way through immersion in acidic solution. It is found that surface degradation associated with corrosion leads to a strong reduction of hardness and elastic modulus, as compared to non-corroded samples. Similarly, significant differences are observed in nanoscratch response, regarding not only width and depth of tracks but also deformation mechanisms developed as contact load is progressively increased. Damage was already evidenced in corroded surfaces at scratching loads one order of magnitude lower than for virgin specimens. Cracking and fragmentation of individual WC grains, together with chipping of at the track edges were the main deformation and fracture micromechanisms identified. Changes in nanoindentation and nanoscratch response and damage scenario are discussed on the basis of the corrosion-induced changes within the intrinsic microstructural assemblage of hardmetals.Postprint (published version

An adaptive multi-level model for multi-scale ductile fracture analysis in heterogeneous aluminum alloys

This paper addresses the multi-scale modeling of ductile fracture in microstructures characterized by a dispersion of hard and brittle heterogeneities in a softer ductile matrix. An adaptive multi-level model is developed with different inter-scale transfer operators and interfaces. Micro-mechanical analysis in regions of dominant damage is performed to capture the important micro-mechanical damage modes that are responsible for deterring the overall failure properties of these alloys. Regions of macroscopic homogeneity are otherwise modeled with constitutive relations derived from homogenization of evolving variables in representative volume elements. These two length scales of analysis, in conjunction with an intermediate swing level, form a three-level coupled multi-scale model to capture ductile crack propagation. The capabilities of the proposed model are demonstrated for a cast aluminum alloy

The role of aggregates in the thermal stability of Mg-PSZ refractories for vacuum induction melting

Mg-PSZ refractories used as vacuum induction melting crucibles are particle-reinforced composites with aggregate and matrix phases comprising fused zirconia. Three commercial varieties were cycled eight times to service temperatures and their microstructural and thermomechanical evolution investigated, with focus placed on the aggregate populations. Two refractories, with large aggregates of similar size, were found to retain stiffness after cycling but in the refractory containing aggregates with high stabiliser levels, reaction between the stabiliser and Al and Si impurities produced secondary phases. Volume changes accompanying formation of these phases, and subsequent thermal expansion mismatches, led to aggregate break-up with consequent reductions in refractory toughness and strength. Secondary phases developed only rarely in the aggregates (with lower levels of stabiliser) of the second refractory. These aggregates remained intact and the refractory retained its toughness and strength. A third refractory contained small, unstabilised aggregates in a stabilised matrix and the strain mismatches that ensued during polymorphic transformation damaged microstructural interfaces. Refractory stiffness halved within eight cycles and toughness and strength were lost. All three refractories displayed R-curve behaviour and quasi-stable fracture curves were observed during bend tests. The study shows that when using fused zirconia aggregates to design refractories, engineers need to i) limit stabiliser concentrations - a difference of just ±1 wt% Mg (in the presence of impurity elements) may determine whether secondary phase formation occurs and ii) eliminate alumina and silica impurities when possible through substitution of zircon sand with baddeleyite as the source for fused zirconia.Open Acces

Microstructural topology effects on the onset of ductile failure in multi-phase materials - a systematic computational approach

Multi-phase materials are key for modern engineering applications. They are generally characterized by a high strength and ductility. Many of these materials fail by ductile fracture of the, generally softer, matrix phase. In this work we systematically study the influence of the arrangement of the phases by correlating the microstructure of a two-phase material to the onset of ductile failure. A single topological feature is identified in which critical levels of damage are consistently indicated. It consists of a small region of the matrix phase with particles of the hard phase on both sides in a direction that depends on the applied deformation. Due to this configuration, a large tensile hydrostatic stress and plastic strain is observed inside the matrix, indicating high damage. This topological feature has, to some extent, been recognized before for certain multi-phase materials. This study however provides insight in the mechanics involved, including the influence of the loading conditions and the arrangement of the phases in the material surrounding the feature. Furthermore, a parameter study is performed to explore the influence of volume fraction and hardness of the inclusion phase. For the same macroscopic hardening response, the ductility is predicted to increase if the volume fraction of the hard phase increases while at the same time its hardness decreases

Mechanics and mechanisms of fatigue in a WC-Ni hardmetal and a comparative study with respect to WC-Co hardmetals

There is a major interest in replacing cobalt binder in hardmetals (cemented carbides) aiming for materials with similar or even improved properties at a lower price. Nickel is one of the materials most commonly used as a binder alternative to cobalt in these metal-ceramic composites. However, knowledge on mechanical properties and particularly on fatigue behavior of Ni-base cemented carbides is relatively scarce. In this study, the fatigue mechanics and mechanisms of a fine grained WC-Ni grade is assessed. In doing so, fatigue crack growth (FCG) behavior and fatigue limit are determined, and the attained results are compared to corresponding fracture toughness and flexural strength. An analysis of the results within a fatigue mechanics framework permits to validate FCG threshold as the effective fracture toughness under cyclic loading. Experimentally determined data are then used to analyze the fatigue susceptibility of the studied material. It is found that the fatigue sensitivity of the WC-Ni hardmetal investigated is close to that previously reported for Co-base cemented carbides with alike binder mean free path. Additionally, fracture modes under stable and unstable crack growth conditions are inspected. It is evidenced that stable crack growth under cyclic loading within the nickel binder exhibit faceted, crystallographic features. This microscopic failure mode is rationalized on the basis of the comparable sizes of the cyclic plastic zone ahead of the crack tip and the characteristic microstructure length scale where fatigue degradation phenomena take place in hardmetals, i.e. the binder mean free path. (C) 2014 Elsevier Ltd. All rights reserved.Peer ReviewedPostprint (author’s final draft

Voronoi cell finite element modelling of the intergranular fracture mechanism in polycrystalline alumina

The mechanisms of fracture in polycrystalline alumina were investigated at the grain level using both the micromechanical tests and finite element (FE) model. First, the bending experiments were performed on the alumina microcantilever beams with a controlled displacement rate of 10 nm s–1 at the free end; it was observed that the intergranular fracture dominates the failure process. The full scale 3D Voronoi cell FE model of the microcantilever bending tests was then developed and experimentally validated to provide the insight into the cracking mechanisms in the intergranular fracture. It was found that the crystalline morphology and orientation of grains have a significant impact on the localised stress in polycrystalline alumina. The interaction of adjacent grains as well as their different orientations determines the localised tensile and shear stress state in grain boundaries. In the intergranular fracture process, the crack formation and propagation are predominantly governed by tensile opening (mode I) and shear sliding (mode II) along grain boundaries. Additionally, the parametric FE predictions reveal that the bulk failure load of the alumina microcantilever increases with the cohesive strength and total fracture energy of grain boundaries

Numerical simulation of grain-size effects on creep crack growth by means of grain elements

The effect of grain size on creep crack growth is investigated by means of a numerical technique in which the actual crack growth process is simulated in a discrete manner by grain elements and grain boundary elements. The grain elements account for the creep deformation of individual grains, while grain boundary cavitation and sliding are accounted for by grain boundary elements between the grains. This grain-element technique allows for an independent study of multiple grain size effects: a (direct) size effect related to the specimen size/grain size ratio or an (indirect) effect related to the effect of grain size on nucleation rate and creep resistance. Preliminary numerical results are presented concerning the direct effect of grain size, which predict that the crack growth rate and brittleness increase with grain size.

Fracture initiation in multi-phase materials: a systematic three-dimensional approach using a FFT-based solver

This paper studies a two-phase material with a microstructure composed of a hard brittle reinforcement phase embedded in a soft ductile matrix. It addresses the full three-dimensional nature of the microstructure and macroscopic deformation. A large ensemble of periodic microstructures is used, whereby the individual grains of the two phases are modeled using equi-sized cubes. A particular solution strategy relying on the Fast Fourier Transform is adopted, which has a high computational efficiency both in terms of speed and memory footprint, thus enabling a statistically meaningful analysis. This solution method naturally accompanies the regular microstructural model, as the Fast Fourier Transform relies on a regular grid. Using the many considered microstructures as an ensemble, the average arrangement of phases around fracture initiation sites is objectively identified by the correlation between microstructure and fracture initiation -- in three dimensions. The results show that fracture initiates where regions of the hard phase are interrupted by bands of the soft phase that are aligned with the direction of maximum shear. In such regions, the hard phase is arranged such that the area of the phase boundary perpendicular to the principal strain direction is maximum, leading to high hydrostatic tensile stresses, while not interrupting the shear bands that form in the soft phase. The local incompatibility that is present around the shear bands is responsible for a high plastic strain. By comparing the response to a two-dimensional microstructure it is observed that the response is qualitatively similar (both macroscopically and microscopically). One important difference is that the local strain partitioning between the two phases is over-predicted by the two-dimensional microstructure, leading to an overestimation of damage