Breaking down chipping and fragmentation in sediment transport: the control of material strength

[摘要] As rocks are transported, they primarily undergo two breakdown mechanisms: fragmentation and chipping. Fragmentation is catastrophic breakup byfracture in the bulk – either by subcritical crack growth under repeated collisions, or from a single high-energy (supercritical) collision – andproduces angular shards. Chipping is a distinct low-energy mechanism of impact attrition that involves shallow cracking; this process rounds riverpebbles in a universal manner under bed-load transport. Despite its geophysical significance, the transition from chipping to fragmentation is notwell studied. Here, we examine this transition experimentally by measuring the shape and mass evolution of concrete particles of varying strength,subject to repeated collisions in a rotating drum. For sufficiently strong particles, chipping occurred and was characterized by the following:attrition products were orders of magnitude smaller than the parent; attrition rate was insensitive to material strength; and particles experiencedmonotonic rounding toward a spherical shape. As strength decreased, we observed the onset of a subcritical cracking regime associated withfragmentation: mass of attrition products became larger and more varied; attrition rate was inversely proportional to material strength; and shapeevolution fluctuated and became non-monotonic. Our results validate conceptual and numerical models for impact attrition: chipping follows“Sternberg's law” of exponential mass loss through time; for fragmentation, the lifetime of particles increases nonlinearly with materialstrength, consistent with “Basquin's law” of fatigue failure. We suggest that bedrock erosion models must be clarified to incorporate distinctattrition mechanisms, and that pebble or bedrock-channel shape may be utilized to deduce the operative mechanism in a given environment.

[学科分类] 土壤学