We introduce a novel approach for probing sliding contact interfaces using solitary waves generated in a granular crystal. Unlike conventional ultrasonic methods for non-destructive evaluation of contacts, solitary waves offer superior spatial localization and minimal dispersion, enabling in situ detection of interfacial changes with high precision. To demonstrate this concept, we designed an experimental setup where a solitary wave propagates through a 1-D granular crystal and impacts a sphere-on-flat interface in the normal direction, while simultaneous tangential sliding is imposed at 100 Hz. The terminal bead in the chain makes direct contact with the vibrating substrate, allowing the pulse to interact with a dynamically evolving frictional interface. This setup couples tribology, nonlinear dynamics, and acoustics across distinct time and length scales, enabling the separation of acoustic and dynamics effects. We find that the solitary wave speed is highly sensitive to interfacial stiffness variations caused by friction and wear. For example, after several hysteresis cycles, we observe a 6% increase in wave speed that correlates with a 30% increase in interfacial stiffness, attributed to wear evolution of the contact geometry. These findings show that solitary waves, traditionally used to assess elastic properties, can serve as real-time, non-invasive probes of frictional state and wear evolution, opening new possibilities for tribological diagnostics at the interface of contact mechanics and nonlinear acoustics.
Fantetti et al. (Wed,) studied this question.