The characteristic energy length scale χ {\displaystyle \chi } in fracture mechanics describes the size of the region from which energy flows to a rapidly moving crack. The length scale size decreases proportionally to the square of applied stress. If size decreases to the order of magnitude of a hyperelastic zone at the crack tip, local wave speeds can dominate crack dynamics. This can lead to supersonic fracture or to a reduction in fracture propagation speed, depending on whether the material undergoes hyperelastic stiffening or softening, respectively.
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