High-rate interaction between strongly nonlinear solitary waves and an oscillating frictional boundary of a granular crystal

Understanding how frictional contacts evolve with sliding, plasticity, and wear remains a fundamental challenge in contact mechanics, partly because few experimental techniques can probe interfacial changes in situ without disrupting the contact. We address this gap by propagating strongly nonlinear Nesterenko solitary waves along an uncompressed one-dimensional granular chain that terminates at an oscillating frictional contact, probing the high-rate interaction between the fast, normally incident solitary wave and the slower oscillating frictional interface. We find that the solitary wave speed, which in the sonic-vacuum regime is governed by the nonlinear Hertzian contact stiffness along the path, including that of the frictional boundary contact, is highly sensitive to friction, plasticity, and wear effects. After 30 min of continuous 100 Hz sliding at the steel sphere-on-flat boundary contact, the wave speed increased by ∼ 5%; discrete element simulations match this change by varying only the effective Hertzian coefficient at the frictional boundary, implying a 60% increase in contact stiffness that we attribute to wear-driven evolution of the contact geometry. Further, the high peak forces at the interface due to the arrival of the solitary wave dominate any pre-existing contact preload, making the measurement sensitive to stiffness changes but mostly insensitive to the instantaneous stick–slip state, in contrast to low-amplitude ultrasonic probing. These findings reveal largely unexplored extreme mechanics at an evolving frictional boundary and suggest using unique properties of Nesterenko solitary waves propagating in granular chains in the state of sonic vacuum for in situ monitoring of frictional interfaces.