272-4 Dynamic Rock Strength and Fracture Roughness in Heterogenous Basalts and Gabbros

Session: Faults, Fractures, and Geomechanics for the Energy Transition (Posters)

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Abstract:

High strain rate (~10² s^-1) failure occurs in a number of geologic systems, representing a potentially significant energy sink in these dynamic environments. However, due to the short duration and small spatial scale over which fracture occurs under these conditions, the effects of rock microstructure on the micromechanics of dynamic fracture are poorly understood. Previous research shows that fragmentation of material at these high strain rates occurs by the activation of many flaws, like grain boundaries and pre-existing microcracks, in the fracture process zone. However, the effects of anisotropy and heterogeneous material structure may be poorly captured in previous models of failure under these conditions, which are typically built on observations in homogenous industrial materials. To evaluate how fragmentation behavior is influenced by heterogeneity in crystalline grain size, we utilize mid-continent rift basalt and gabbro rock collected from northern Minnesota. The basalt has a bulk composition of plagioclase feldspar and minor pyroxene with a fine grain size of < 1mm. The gabbro grains are larger (1-5 mm) and have a similar bulk composition of dominant plagioclase feldspar with some pyroxene crystals present. The dynamic fracture initiation toughness of these materials was determined using a notched semi-circular bend specimen and a split-Hopkinson pressure bar (SHPB) at loading rates of ~10² s^-1. During the loading event, the SHPB records the stress, strain, and strain rate history, which are used to calculate the fracture initiation toughness. After fracture, we measured the roughness of the fracture path and identified post-mortem microstructure damage to grains in thin section to determine the extent of the fracture process zone and the type of fractures produced as a response to high stress and strain rates. In coarse-grained gabbro, which have larger pre-existing flaws in the form of grain boundaries between larger mineral grains, we observed a rougher fracture profile compared to our basalt samples, representing a wider process zone and greater energy consumption during fragmentation when compared to finer-grained materials. A full understanding of dynamic rock failure under these conditions could potentially reduce discrepancies between experimental observations and model results and can have important applications where heterogeneous materials are subject to these types of high strain rate conditions.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9075

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