Microscopic Toughness of Viscous Solids via Scratching: From Amorphous Polymers to Gas Shale

Abstract

Rate effects are dominant in many fracture processes including the separation of mineralized collagen fibrils under tension, the failure of bulk metallic glasses under compression or the natural fracturing of layered sedimentary rocks. Here the rate-sensitivity of the scratch toughness is studied by integrating well-controlled microscopic experiments, scaling analysis, and theory. Starting from the Second Law of Thermodynamics, the different sources of energy dissipation, namely crack propagation and viscous mechanisms, are monitored so as to capture the scratch rate dependence in a unique curve. As illustrated for both a semicrystalline and amorphous polymers, this master-curve encapsulates the rate-induced ductile-to-brittle transition driving the scratch process. In turn, the analytical model is translated into a quantitative assay to measure the intrinsic microtoughness of a natural organic-inorganic composite: gas shale. The microtoughness of gas shale is found to be twice higher than that of shale materials, with several implications in geophysics and energy harvesting applications. The novel energy-based approach provides a rigorous framework to investigate the microscopic toughness of multiscale systems such as unconventional shale, biological materials, and bioinspired composites.