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Register a new userRole of anisotropic strength and stiffness in governing the initiation and propagation of yielding in polycrystalline solids
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The ratio of directional strength-to-stiffness is important in governing the relative order in which individual crystals within a polycrystalline aggregate will yield as the aggregate is loaded. In this paper, a strength-to-stiffness parameter is formulated for multiaxial loading that extends the development of Wong and Dawson for uniaxial loading. Building on the principle of strength-to-stiffness, a methodology for predicting the macroscopic stresses at which elements in a finite element mesh yield is developed. This analysis uses elastic strain data from one increment of a purely elastic finite element simulation to make the prediction, given knowledge of the single-crystal yield surface. Simulations of austenitic strainless steel AL6XN are used to demonstrated the effectivness of the strength-to-stiffness parameter and yield prediction methodology.
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The elastoplastic behavior of a two-phase stainless steel alloy is explored at the crystal scale for five levels of stress biaxiality. The crystal lattice (elastic) strains were measured with neutron diffraction using tubular samples subjected to different combinations of axial load and internal pressure to achieve a range of biaxial stress ratios. Finite element simulations were conducted on virtual polycrystals using loading histories that mimicked the experimental protocols. For this, two-phase microstructures were instantiated based on microscopy images of the grain and phase topologies and on crystallographic orientation distributions from neutron diffraction. Detailed comparisons were made between the measured and computed lattice strains for several crystal reflections in both phases for scattering vectors in the axial, radial and hoop directions that confirm the models ability to accurate predict the evolving local stress states. A strength-to-stiffness parameter for multiaxial stress states was applied to explain the initiation of yielding across the polycrystalline samples across the five levels of stress biaxiality. Finally, building off the multiaxial strength-to-stiffness, the propagation of yielding over the volume of a polycrystal was estimated in terms of an equation that provides the local stress necessary to initiate within crystals in terms of the macroscopic stress.
Defects in solid commonly limit mechanical performance of the material. However, recent measurements reported that the extraordinarily high strength of graphene is almost retained with the presence of grain boundaries. We clarify in this work that lattice defects in the grain boundaries and distorted geometry thus induced define the mechanical properties characterized under specific loading conditions. Atomistic simulations and theoretical analysis show that tensile tests measure in-plane strength that is governed by defect-induced stress buildup, while nanoindentation probes local strength under the indenter tip and bears additional geometrical effects from warping. These findings elucidate the failure mechanisms of graphene under realistic loading conditions and assess the feasibility of abovementioned techniques in quantifying the strength of graphene, and suggest that mechanical properties of low-dimensional materials could be tuned by implanting defects and geometrical distortion they leads to.
Intermetallic phases in a recently developed Al-Li-Cu-Mg alloy have been investigated to understand their roles in the initiation and propagation processes of intergranular corrosion. Corrosion initiation involves trenching formation in the Al matrix adjacent to the large particles of Al7Cu2(Fe, Mn) phases. These phases containing Li are electrochemically active and susceptible to self-dissolution via a de-alloying mechanism during corrosion process. The subsurface particles of Al7Cu2(Fe, Mn) and Al20Cu2Mn3 phases act as the internal cathodes for continuous corrosion propagation along the particle-matrix interface and the associated grain boundaries. Corrosion propagation along the particle-matrix interface was facilitated by the anodic dissolution of the surrounding Al matrix due to the micro-galvanic interaction with the cathodic intermetallic phases. In addition, T1 (Al2CuLi) precipitates and the isolated particles of Al7Cu2(Fe, Mn) and Al20Cu2Mn3 phases were dissolved along the path of corrosion propagation. The dissolved metal ions were redeposited through the network of crevice.
Quasi-brittle behavior where macroscopic failure is preceded by stable damaging and intensive cracking activity is a desired feature of materials because it makes fracture predictable. Based on a fiber bundle model with global load sharing we show that blending strength and stiffness disorder of material elements leads to the stabilization of fracture, i.e. samples which are brittle when one source of disorder is present, become quasi-brittle as a consequence of blending. We derive a condition of quasi-brittle behavior in terms of the joint distribution of the two sources of disorder. Breaking bursts have a power law size distribution of exponent 5/2 without any crossover to a lower exponent when the amount of disorder is gradually decreased. The results have practical relevance for the design of materials to increase the safety of constructions.
The holographic principle has proven successful in linking seemingly unrelated problems in physics; a famous example is the gauge-gravity duality. Recently, intriguing correspondences between the physics of soft matter and gravity are emerging, including strong similarities between the rheology of amorphous solids, effective field theories for elasticity and the physics of black holes. However, direct comparisons between theoretical predictions and experimental/simulation observations remain limited. Here, we study the effects of non-linear elasticity on the mechanical and thermodynamic properties of amorphous materials responding to shear, using effective field and gravitational theories. The predicted correlations among the non-linear elastic exponent, the yielding strain/stress and the entropy change due to shear are supported qualitatively by simulations of granular matter models. Our approach opens a path towards understanding complex mechanical responses of amorphous solids, such as mixed effects of shear softening and shear hardening, and offers the possibility to study the rheology of solid states and black holes in a unified framework.
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