Breaking down chipping and fragmentation in sediment transport: the control of material strength
<p>As rocks are transported, they primarily undergo two breakdown mechanisms: fragmentation and chipping. Fragmentation is catastrophic breakup by fracture in the bulk – either by subcritical crack growth under repeated collisions, or from a single high-energy (supercritical) collision – and p...
| Main Authors: | , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2021-12-01
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| Series: | Earth Surface Dynamics |
| Online Access: | https://esurf.copernicus.org/articles/9/1531/2021/esurf-9-1531-2021.pdf |
| Summary: | <p>As rocks are transported, they primarily undergo two breakdown mechanisms: fragmentation and chipping. Fragmentation is catastrophic breakup by
fracture in the bulk – either by subcritical crack growth under repeated collisions, or from a single high-energy (supercritical) collision – and
produces angular shards. Chipping is a distinct low-energy mechanism of impact attrition that involves shallow cracking; this process rounds river
pebbles in a universal manner under bed-load transport. Despite its geophysical significance, the transition from chipping to fragmentation is not
well 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 experienced
monotonic rounding toward a spherical shape. As strength decreased, we observed the onset of a subcritical cracking regime associated with
fragmentation: mass of attrition products became larger and more varied; attrition rate was inversely proportional to material strength; and shape
evolution 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 material
strength, consistent with “Basquin's law” of fatigue failure. We suggest that bedrock erosion models must be clarified to incorporate distinct
attrition mechanisms, and that pebble or bedrock-channel shape may be utilized to deduce the operative mechanism in a given environment.</p> |
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| ISSN: | 2196-6311 2196-632X |