Monitoring frictional strength with acoustic wave transmission

We monitored acoustic transmissivity of a frictional interface during laboratory experiments where complex slip history was imposed. Frictional strength, which was also tracked continuously, showed complex changes due to various causes such as aging of the interface in quasi‐stationary contact and slip weakening that began with miniscule pre‐failure slip. Albeit such complexity, acoustic transmissivity was found to be a unique function of frictional strength throughout the experiment. This suggests that acoustic transmissivity reflects the resultant changes in load‐supporting contact population that governs the frictional strength of the moment, not the external conditions that have altered contact state. This simple non‐destructive acoustic measurement, which can be applied remotely and continuously, is therefore a robust technique to monitor frictional strength.