Experimental investigation of nonlinear acoustic effect at crack

The nonlinear ultrasonic technique using the feature of higher-harmonic generation that is one of nonlinear acoustic behavior at interfaces has been considered as a positive method for the detection of micro-cracks. However, most previous studies were limited to model development and its verification by testing two blocks contacted together. This paper investigates experimentally the nonlinear acoustic effect at a real crack. For this, we constructed a measurement system using contact PZT transducers, which was able to keep all equipments and their set-up conditions consistent and thus enabled a reliable measurement of nonlinear parameter. A CT specimen of Al6061 with a crack initiated by a fatigue test was tested, and the nonlinear parameter was measured by scanning along the crack depth direction. We could see that the nonlinear parameter had good correlation with the crack, which was identical to the theoretical prediction. From these results, we could show the applicability of the proposed methodology to micro-crack detection.