Laser ultrasonics for detection of elastic nonlinearity using collinear mixing of surface acoustic waves

The inherent material nonlinearity of many materials leads to deviation from Hooke's law, as the elastic moduli are not constant under applied stress. Acoustic waves traveling in the material are sensitive to changes in the elastic moduli, and this sensitivity can be used as a metric of intrinsic material nonlinearity. As static stressing of a component is in most cases impractical, an alternative technique, explored here, is to use a large amplitude surface acoustic wave (SAW) "pump" to generate stresses which propagate along the surface of a sample. A laser generated high frequency tone burst SAW "probe" is co-propagated with the pump, and is subject to the same stress as it propagates; the stress will be determined by both the height of the pump and the relative position of the probe with respect to the pump, which we can adjust. This collinear mixing results in a small change in the velocity of the probe wave, which is detected using another laser. By relating the change of velocity to the applied stress, a measure of the material's intrinsic nonlinearity can be obtained. We present quantitative measurements relating to material characterization, and implications for its use in damage detection are discussed.