Abstract
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.
Highlights
Propagating cracks in solids emit both pressure and shear waves
The study of shear cracks propagating along frictional interfaces and the associated shock fronts is relevant to earthquake dynamics[19,20,21,22]
Propagating cracks are driven by elastodynamic waves, where the energy released by the crack motion is transferred through the medium to the crack tip region with the pressure wave speed and shear wave speed
Summary
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. The apparent violation of classic theories is explained by the strainrate-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. In fracture mechanics and geophysics, shear shock fronts have been observed to arise by the coalescence of shear waves emitted by tips of spontaneously propagating shear ruptures exceeding the shear wave speed of the surrounding material[6,7,8,9] These ruptures are commonly referred to as intersonic or supershear. The presence of the pressure shock fronts enables us to refer to our cracks as supersonic
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