A nonlinear ultrasonic stimulated thermography method for detecting fatigue cracks
Abstract »It is well known that sonically exciting damage or defected region leads to heating due to friction, which can be evaluated using an infrared camera. If sufficient heating is not generated at damaged regions it becomes difficult to evaluate material damage, thus sonic excitation at local defect resonance is important. This work evaluated how closely further harmonic generation can be linked to the heating process in damaged regions. An aluminium fatigue crack was excited using an ultrasound transducer. The out-of-plane velocity was evaluated around the cracked region using a Laser Vibrometer in order to determine the fundamental and second order harmonic responses while the temperature was evaluated using an infrared camera. Using these velocity measurements it was possible to compare the local nonlinear parameter β for the fatigue crack and undamaged regions with the temperature gradients of the crack at various excitation frequencies. The results show a strong correlation between local nonlinear harmonics near the crack region and crack heating. Assuming frictional work is proportional to the out-of-plane velocity it was possible to individual evaluate the fundamental and second harmonics effect on total heating. This showed that the production of further harmonics can have a significant effect on the efficiency of these methods.