Microscopic toughness of viscous solids via scratching: From amorphous polymers to gas shale

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