Fig. 5. The effect of sub-Rayleigh–to–supershear transition on the equation of motion of supershear cracks.

Three different transition mechanisms are considered. Top: The first setup (brown curve) has a spatially nonuniform τ₀/τ_p profile with reduced local τ_p for l/l₀ < 50 (see main text for details). Two additional examples have spatially uniform τ₀/τ_p profiles (orange and red curves). Bottom: Colors represent the crack velocities C_f(l) corresponding to the stress profiles in the top panel. Brown curve indicates continuous crack acceleration to supershear speeds (direct transition) within a weakened nucleation (nucl.) patch (high τ₀/τ_p level). Orange curve indicates sub-Rayleigh rupture transitions at l/l₀ ≈ 65 to supershear speed through the Burridge-Andrews (BA) mechanism (4, 5). Red curve indicates an imposed supershear seed crack leads to a self-sustained supershear crack propagation. The black dashed line denotes theoretical prediction for a spatially uniform prestress level (τ₀/τ_p = 0.45).