Detection and evaluation of fatigue cracks using a nonlinear ultrasonic sideband peak count technique with a pulse-echo experimental method

The nondestructive ultrasonic detection and evaluation of fatigue cracks is critical to ensure the safe operation of equipment. Although linear ultrasonic methods are easy to implement in practice, their parameters are insensitive to micro-scale cracks that are smaller than the wavelength. Conventional nonlinear techniques have better sensitivity but require a relatively complicated configuration and demanding experimental setups. In this work, a simple approach for the inspection of fatigue damage is presented to overcome the above shortcomings by combining nonlinear sideband peak count (SPC) techniques with the pulse-echo experimental method. Related nonlinear wave theories are briefly reviewed, and experiments are performed in stainless steel specimens with varying crack levels to verify the effectiveness of the proposed method. The linear and nonlinear parameters are extracted from the measurements for comparison. The results demonstrate that, under the same pulse-echo mode with low excitation amplitude, both the SPC-I (sideband peak count-index) and SPI (sideband peak intensity) variations are more sensitive to the quantitative detection of fatigue cracks than the conventional linear and nonlinear ultrasonic based parameters, especially in the initial stages of cracking. Experiments using pitch-catch configuration and an additional pulse-echo experiment using a different input frequency are carried out to verify the reliability and superiority of the proposed method. This work provides a convenient and efficient method for robust measurements of early damage of materials in practice.