Targeted delivery of acoustic energy for self-healing

Self-healing materials capable of autonomic crack healing are potentially important in terrestrial and space applications. Where damage-causing forces and healing forces compete, it is of interest to consider ways to increase the healing rate. This article investigates the use of acoustic energy for this purpose. As a means for targeted acoustic energy delivery to the crack site, we study time-reversal mirrors (iterative time reversal and playback) for stress wave focusing at a discontinuity. The article begins with a discussion of the key analytical results. Both pulse propagation and eigenfrequency vibrations are analyzed, and it is argued that if sufficient time is allowed between successive time-reversed playbacks, the focused pulse amplitude grows faster than the eigenfrequency vibrations. Experimental implementations of iterative time-reversed playback on solid circular steel and nylon rods with artificially introduced discontinuities show that even in the one-dimensional propagation studies here, time-reversal mirrors produce satisfactory focusing for a multitone pulse. Based on the significant amplification seen in the steel rods and the more dispersive and dissipative nylon rods, it appears that acoustic energy could be delivered in a targeted manner to a crack using iterative time-reversal mirrors.