Mechanical response and failure of bi-modulus and transversely-isotropic materials at different strain rates

This study investigates the response of bi-modulus and transversely-isotropic shale material with respect to elastic and mechanical properties. Primarily, these properties are characterized in to explore the effects of presence of bedding. In materials with well-defined bedding, the influence of bedding planes (Parallel and Perpendicular to loading direction) and stress state (Tensile or Compressive) on material properties are investigated. Several distinct shale materials from Oklahoma and Utah are subjected to quasi-static and dynamic loading resulting in strain rates of 3.28 × 10⁻⁵ s⁻¹ and 1500 s⁻¹ to characterize their elastic constants, mechanical strength, and strain rate dependence. A novel experimental--numerical technique based on full-field measurements of stresses, strains, and linear-least squares regression analysis is developed to quantify these properties. Full-field strains due to applied load are measured on the deforming specimen surface using Digital Image Correlation (DIC). Full-field stresses are numerically calculated using Finite Element Method (FEM) from experimental measured load. This technique is successfully utilized to characterize isotropic, transversely-isotropic, and bi-modulus materials. The unique advantage of field measurements allows to characterize bulk material response. Shale materials with distinct microstructure exhibited different responses; isotropic bi-modulus, transversely isotropic bi-modulus, and transversely-isotropic with no bi-modulus effect in quasi-static tests with varying failure strengths in tension and compression. Dynamic loading experiments are performed using split-Hopkinson pressure bar (SHPB) on same shale materials to evaluate rate dependence of material properties. The anisotropic damage distribution and bi-elastic properties exhibited by shale materials in different directions and stress states promises beneficial for optimizing hydraulic fracturing technology and enhancing efficiency.