Dynamic Compressive Strength and Fragmentation in Felsic Crystalline Rocks

Abstract

Brittle deformation in rocks depends upon loading rate; with increasing rates, typically greater than~10 2 s −1 , rocks become significantly stronger and undergo increasingly severe fragmentation. Dynamic conditions required for rate-dependent brittle failure may be reached during impact events, seismogenic rupture, and landslides. Material characteristics and fragment characterization of specific geomaterials from dynamic loading are only approximately known. Here we determine the characteristic strain rate for dynamic behavior in felsic crystalline rocks, including anisotropy, and describe the resulting fragments. Regardless of the type of felsic crystalline rock or anisotropy, the characteristic strain rate is the same within uncertainties for all tested materials, with an average value of 229 ± 81 s −1. Despite the lack of variation of the critical strain rate with lithology, we find that the degree of fragmentation as a function of strain rate varies depending on material. Scaled or not, the fragmentation results are inconsistent with current theoretical models of fragmentation. Additionally, we demonstrate that conditions during impact cratering, where the impactor diameter is less than~100 m, are analogous to the experiments carried out here and therefore that dynamic strengthening and compressive fragmentation should be considered as important processes during impact cratering. Plain Language Summary When rocks deform quickly, they can behave with properties very different to the properties that would be measured when rocks are deformed slowly. In this study, we have measured the strength of rocks deformed at different rates to find how fast they must be deformed to cause substantial changes to their properties. We chose to look at granitic rocks and gneisses as they are broadly representative of the Earth's continental crust. We found that regardless of the exact rock type, the change from slow to fast deformation occurs at the same rate. When rocks break at fast rates, they break into many small fragments. We have measured the size of those fragments from our experiments to show how the average fragment size changes as a result of the deformation rate and the rock type. In addition, we show that the changes of properties that we see in our experiments is important for the formation of impact craters and potentially earthquake rupture and landslides. Rock strength can be investigated over a range of strain rate regimes (Zhang & Zhao, 2014): quasi-static (10 −5-10 −1 s −1), intermediate strain rate (10 −1-10 1 s −1), high strain rate (10 1-10 4 s −1), and very high strain rate (>10 4 s −1). Results across those regimes (

Dynamic Split Tensile Strength of Basalt, Granite, Marble and Sandstone: Strain Rate Dependency and Fragmentation

“…Since the results are available for rocks with the same lithologies in compression, comparing the characteristic strain rate among compression and tension experiments is interesting. The characteristic strain rate of Malsburg Granite, Seeberger Sandstone and Carrara Marble in compression was found to be 217 ± 95 /s, 322 ± 92 /s and 144 ± 33 /s, respectively (Rae et al 2020(Rae et al , 2022. The ratio of the characteristic strain rate in compression to the characteristic strain rate tension for the rock types are: MaGr = ~ 86; SeSa = ~ 123; CaMa = ~ 60; we observe that there is no definitive ratio among them, and they do not overlap as well.…”

“…In our study, we have used aluminium foam of 10 mm thickness and 90% porosity as a pulse shaper. The aluminium foam was pre-hit at a striker velocity of ~ 10 m/s resulting in a final thickness of ~ 7.5 mm (Rae et al 2022(Rae et al , 2020Zwiessler et al 2017). The pre-hit aluminium foam ensures a uniform contact between the bar and pulse shaper.…”

“…Measurement of strain rate during the deformation is an important aspect of dynamic testing. During a traditional compressional SHPB testing, the strain rate is normally calculated from the strain signals measured on the incident, and transmitted bar or an approximate value is deduced from the velocity of the striker bar and length of the specimen (Rae et al 2020;Shin and Kim 2019). Because of the nonuniform stress state in the Brazilian disk specimen, both methods will not yield a representative tensile strain rate.…”

“…The previous study from our research group (Rae et al 2020(Rae et al , 2022Zwiessler et al 2017) have used the same rock types, Malsburg Granite, Seeberger Sandstone and Carrara Marble for compressive SHPB experiments and have determined the characteristic strain rate values. Since the results are available for rocks with the same lithologies in compression, comparing the characteristic strain rate among compression and tension experiments is interesting.…”

Measuring crater volumes from bullet impacts: implications for field methods and cratering mechanics

“…The strengths of each target lithology were measured under quasi-static strain rates (< 10 s −1 ), but rock strength is strain rate dependent, increasing rapidly after a threshold strain rate of 49 . Rae et al 50,51 show that the dynamic compressive strength of rocks can be double the quasi-static strength at stain rates >10 2 s −1 . Cho et al 52 show that tensile strength increases at strain rates 100 -101 s −1 .…”