We investigated the imaging capability of fixed-voltage fundamental wave amplitude difference (fixed-voltage FAD), which is one of the nonlinear ultrasonic phased array imaging techniques, for three-dimensional crack open/closed distribution. We fabricated two fatigue crack specimens with different thicknesses, which were selected to create different open/closed distributions in them. As a result of applying fixed-voltage FAD with mechanical scans to the thick and thin specimens, the difference in the crack open/closed distribution was successfully visualized. The results are in good agreement with fracture mechanics. Thus, it was demonstrated that fixed-voltage FAD is useful not only for nondestructive testing for aging infrastructures but also for the progress in fracture mechanics.
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