Modeling Phase Transformation Induced Shear Failure in Geomaterial

One hypothesized mechanism that triggers deep-focus earthquakes in oceanic subducting slabs below ~300 km depth is transformational faulting due to the olivine-to-spinel phase transition. This study develops high-fidelity numerical models to investigate phase transformation-induced stress redistribution and material weakening in olivine. A thermodynamically consistent constitutive model is formulated and implemented in finite element modeling, incorporating viscoplastic behavior, thermo-mechanical coupling, and multiscale material features, including polycrystalline structures and mesoscale heterogeneity. Simulation results reveal that spinel formation under pressure initiates near inclusions and grain boundaries, aligning with experimental observations. At lower rates, thin spinel bands form diagonally to the compression direction, correlating with high pressure regions and suggesting fault initiation. At higher transformation rates and temperatures, more extensive spinel formation leads to ductile behavior. Overall, the numerical model captures mechanical features similar to laboratory experiments including spinel band formation and stress softening behavior. Further analyses on local stress-strain relationships and distributions also offer insights into the underlying mechanisms, demonstrating a reduction in material strength due to transformation and shedding light on fault initiation processes. To overcome conventional finite element method (FEM) limitations in capturing localized spinel bands, a Shearing Particle Method (SPM) in the framework of the Reproducing Kernel Particle Method (RKPM) is introduced. The SPM sharpens stress and strain localization by incorporating shear discontinuous enrichment functions. Comparisons with extended FEM (XFEM) model results show the SPM's enhanced ability to model localized spinel bands and provide insights into fault initiation mechanisms.