Online detection and evaluation of early fatigue cracks using sideband peak intensity technique with frequency-mismatched excitation pulse-echo method

This work presents a nonlinear ultrasonic (NLU) technique called sideband peak intensity (SPI) combining an improved pulse-echo (PE) experimental method for online detection and evaluation of fatigue cracks at their early stages. Advantages of the proposed technique are that it enjoys the high sensitivity and ease of application of NLU SPI technique and easy implementation of the PE experimental method. The PE experimental method is improved by adopting frequency-mismatched excitations to enhance the sensitivity and robustness of the SPI technique. In frequency-mismatched excitation mode, the frequency of the initial excitation differs from the nominal central frequency of the transducer, resulting in distinguishable sideband peaks compared to frequency-matched excitation. Experimental results in fatigue damaged specimens show that the SPI values obtained using the proposed frequency-mismatched excitation in PE method are more sensitive to early fatigue cracks than those obtained using the frequency-matched excitation method. Online ultrasonic experiments were also conducted to quantify wave signals from the specimen at various fatigue stages affixed to the fatigue testing apparatus, and it was found that online detection can achieve results consistent with offline detection. This work provides a more sensitive and robust NLU method for online measurements of fatigue cracks in engineering structures and can benefit the nondestructive testing and evaluation community.