Fig. 1.

Prominent shear stress peaks due to accelerating or suddenly arrested frictional rupture fronts. (A) Nineteen rosette strain gauges (blue squares), mounted at $y\sim 3.5$ mm measure the three 2D strain tensor components simultaneously every 1 μs. These are synchronized with real contact area, $A\left(x,t\right)$, measurements. (B, Top) $A\left(x,t\right)$ evolution (normalized at nucleation time, $t=0$), along the quasi-1D interface due to a rupture front that nucleated at $x\sim 0$, rapidly accelerated to $\sim C_R$, and transitioned to supershear at $x\approx 155$ mm. (Bottom) Shear strain variations, $\mathrm{\Delta }{\epsilon }_{xy}={\epsilon }_{xy}-{\epsilon }_{xy}^r$, relative to the rupture tip arrival time, $t_{\text{tip}}$, at the two locations, $x_1$ (red) and $x_2$ (blue) denoted above, show prominent amplitude shear strain peaks preceding the rupture tip arrival. For simplicity, we refer to these peaks as ‟shear stress peaks.” ${\epsilon }_{xy}^r$ are residual values after the rupture front’s passage. Successive measurements (black points in upper panel) reveal that these peaks propagate at $C_S$, and trigger supershear rupture. (C) Measurements of ${\epsilon }_{xy}$ (Bottom) and the contact area (Top) reveal an inverted shear peak propagating at $C_S$ long after rupture arrest. Extrapolation (solid line) indicates that its origin coincides with rupture arrest. Arrest is due to decreasing values of ${\epsilon }_{xy}^0$ with x (22, 24). (Bottom) ${\epsilon }_{xy}$ measurements at three spatial points denoted above. The red (green) y axis is shifted by $0.06\cdot {10}^{-3}$ ($-0.06\cdot {10}^{-3}$) relative to initial values, ${\epsilon }_{xy}^0$, for clarity. Black points in upper panels are measured peak locations.