Non-linear ultrasonic technique to assess fatigue damage in structural steel

A non-linear ultrasonic technique has been used to assess the fatigue damage in low carbon structural steel. The percentage of total harmonic distortion, which is the ratio of the amplitude of 2nd harmonic and the fundamental, has been introduced as the measurement parameter in this paper. It has also been observed that before crack initiation the 2nd harmonic amplitude is comparable to the fundamental. The results have been analysed in light of the dislocation theory for harmonic generation during material degradation.