Acoustic harmonic generation at unbonded interfaces and fatigue cracks

The generation of a second-harmonic signal at a flat unbonded interface between two solids has been observed. This signal is caused by passage of a longitudinal acoustic wave across the interface. The harmonic amplitude depends upon the pressure applied normal to the interface and is largest close to zero pressure, as expected theoretically. The effect has also been used to detect the presence of microcracks on the surface of Al 2024, developed during fatigue.