Ultrasonic Lamb wave mixing based fatigue crack detection using a deep learning model and higher-order spectral analysis.

• Conventional ultrasonic fatigue crack detection is unreliable in noisy environments. • Hybrid deep learning model can achieve reliable on-field fatigue crack detection. • Hybrid method comprises LSTM network and trispectrum-based spectral analysis. • Better noise reduction and fatigue crack detection achieved using hybrid approach. Recently, the technique of nonlinear Lamb wave mixing has been developed for the detection of fatigue crack in engineering structures. In this technique, two or three Lamb waves with distinct frequencies are applied to a structure. The cross-mixing between these waves results in nonlinear mixed components depending on the sum and difference of the incident frequencies. However, the amplitude of the mixed components generated by the fatigue crack becomes weak in a noisy environment. Thus, noise elimination is critical for reliable crack detection. To address this gap, a novel hybrid method that incorporates a deep learning (DL) model with higher-order spectral analysis is proposed in this study. First, a nonlinear Lamb wave mixing technique is developed to capture ultrasonic data from the aluminum plates during fatigue testing. Subsequently, the DL model based on long short-term memory (LSTM) accepts an original ultrasonic time signal as input and yields an output of a reconstructed ultrasonic signal after noise reduction. Finally, the random noise in the reconstructed signal is eliminated and the mixed components are extracted by trispectrum (TS)-based higher-order spectral analysis. Furthermore, the proposed and existing methods (e.g., power spectrum) are applied to the ultrasonic data collected from fatigue experiments. The results validated the improved performance of the proposed LSTM-TS method for reliable fatigue crack detection in noisy environments. [ABSTRACT FROM AUTHOR]