Pressure shock fronts formed by ultra-fast shear cracks in viscoelastic materials
Spontaneously propagating cracks in solids emit both pressure and shear waves. When a shear crack propagates faster than the shear wave speed of the material, the coalescence of the shear wavelets emitted by the near-crack-tip region forms a shock front that significantly concentrates particle motion. Such a shock front should not be possible for pressure waves, because cracks should not be able to exceed the pressure wave speed in isotropic linear-elastic solids. In this study, we present full-field experimental measurements of dynamic shear cracks in viscoelastic polymers that result in the formation of a pressure shock front, in addition to the shear one. The apparent violation of classic theories is explained by the strain-rate-dependent material behavior of polymers, where the crack speed remains below the highest pressure wave speed prevailing locally around the crack tip. These findings have important implications for the physics and dynamics of shear cracks such as earthquakes.
© The Author(s) 2018. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. Received: 31 May 2018 Accepted: 15 October 2018. Published online: 12 November 2018. This study was supported by the US National Science Foundation (NSF) (grant EAR 1321655 and EAR-1651235), the US Geological Survey (USGS) (grant G16AP00106), and the Southern California Earthquake Center (SCEC), contribution number 6276. SCEC is funded by NSF Cooperative Agreement EAR-1033462 and USGS Cooperative Agreement G12AC20038. We thank Drs Ravichandran and Knauss for helpful discussions. Author Contributions: M.G., V.R., A.J.R., and N.L. contributed to developing the main ideas, interpreting the results, and producing the manuscript. M.G. and V.R. performed the measurements on PMMA and Homalite-100, respectively. V.R. contributed in overseeing the experimental work. Data availability: Data supporting the findings of this study are available from the corresponding author upon request. The authors declare no competing interests.
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