Fatigue Crack Detection in Metallic Members Using Spectral Anal Ysis of Ul Trasonic Rayleigh Waves

Ultrasonic testing for fatigue crack detection in metallic members are commonly performed using time domain analysis of Rayleigh waves. This approach will be successful only if the fatigue cracks produce strong interaction with the Rayleigh waves. The optimum frequency range of ultrasonic Rayleigh waves for scanning long structural members in the field is from 0.5 to 2.25 MHz because higher frequencies will not travel long distances. However, this frequency range may not always result in strong interaction with fatigue cracks. Hence, the data analysis procedure for characterizing such signals should provide enhanced sensitivity. Several authors have reported the advantages of using frequency domain analysis for detecting flaws [1-3]. This paper demonstrates the use of spectral analysis for detecting fatigue cracks in metallic members. The paper includes results from both direct-transmission and pulse-echo tests. Since Rayleigh waves are significantly affected by surface conditions, this study included crack detection in both painted and unpainted members.