Nonlinear ultrasonic testing for fatigue damage in stainless steel using frequency-mismatched excitation

Fatigue damage of materials or components affects their reliability. Traditional linear ultrasonic techniques struggle to detect micro-cracks smaller than the wavelength, while nonlinear ultrasonics are more sensitive to micro-damage. However, current nonlinear ultrasonic methods face problems such as complex equipment and strict operation requirements, which limit their application scenarios and efficiency. This paper proposes a simple approach for fatigue damage inspection using the pulse-echo experimental system. The frequency-mismatched excitation mode is introduced to perform the nonlinear sideband peak count technique. Six stainless steel test blocks with different degrees of fatigue are fabricated, and the fatigue cracks are detected and evaluated using the designed experiments to verify the effectiveness and accuracy of the proposed method. The results show that the sideband peak count-index variations have a strong correlation with the fatigue crack length, and the varying degree of damage can be effectively and easily detected by the proposed method. Compared with normal excitation modes, it is less affected by system nonlinearity and artificial interference, showing better robustness and reliability. The proposed detection method combines the advantages of simple operation of linear ultrasonic and high sensitivity of nonlinear ultrasonic techniques.