Diagnosis of fatigue crack in compression via nonlinear ultrasonic wave modulation

Abstract

This study presents nonlinear ultrasonic wave modulations that can be effectively used for crack detection in thin structural components. Typically, Fatigue cracks occur when a structure is exposed to repeated load, although the load causes the smaller stress than the yield stress. The crack widths of the fatigue cracks are extremely small in the early development stage. As the fatigue cracks grow by combining and coalescing, the overall size increases. The existence of cracks deteriorates the integrity of structures and reduces safety. Therefore, fatigue crack detection is very important to ensure the integrity of structures. To detect these cracks, nonlinear ultrasonic wave modulation technique, a promising fatigue crack-detection technique, has been proposed and used due to its high detecting sensitivity. The nonlinear modulated waves are generated by the truncation of the waves passing through cracks due to the opening and closing of the cracks, and the nonlinear ultrasonic modulation technique has been known to be effective in detecting finer cracks in comparison with other linear ultrasonic methods since the technique utilizes the breathing of the cracks rather than wave reflections or refractions. Although this phenomenon has been observed and used to detect microcracks, the underlying principles have not been thoroughly elucidated. Also, if the cracks are strongly compressed, the crack opening is hindered due to the excessive initial stress, and the nonlinearity does not show up. In this study, I present a technique for microcrack modeling in the finite element framework, and numerically investigate the occurrence of the nonlinear wave modulation. By analyzing photomicrographs of the cracks with crack width measuring algorithm, a realistic crack model is generated. Experiments with aluminum specimens before and after fatigue-crack generation show that cracks are detected correctly in both pristine and damaged specimens. Also, the improvement of the nonlinear modulation wave technique for the detection of microcracks under compression is devised. Nonlinear ultrasonic modulation is investigated with various frequency combinations, and an algorithm that can comprehensively analyze the results by comparison with ambient noise is proposed. When the developed crack-detection method is employed while applying an external force, specimen defects are correctly detected, although the magnitudes of modulated waves decreased. The time and effort required for monitoring the structure’s health can be reduced by using ambient noise as a reference.