Effect of crack contact compression on the magnitude of nonlinear ultrasonic wave modulations

Abstract

We investigate the nonlinear wave caused by interaction of surfaces of a fatigue crack, and study the effect of the crack's contact compression on the magnitudes of nonlinear waves. Nonlinear wave modulation is generated when two ultrasonic waves having different frequencies passing through a crack, and the so-called nonlinear ultrasonic wave modulation technique is developed using this nonlinear waves. However, the magnitude of the nonlinear wave decreases as the crack contact compression increases because the large compression prevents the cracks from opening in motion. Even if the nonlinear wave modulation occurs in the damaged structures under compression, the magnitude might be different with the magnitude without compression. Consequently, finding the range of contact compression and the excitation directions with which the nonlinear wave modulation might occur is essential to use the technique for structural components under constraint compression. In order to examine the relations between the constraint compression and nonlinear wave modulation, we conduct numerical simulations under various compression by changing the excitation directions. The numerical model consists of a thin aluminum plate with a fatigue crack under constant compression, and the crack surfaces are modeled mimicking the shapes of real cracks. The roughness of the crack is determined using the crack widths obtained from optical measurement of fatigue cracks. Effective range of contact compression to generate nonlinear wave and the requirements to make the magnitudes of nonlinear waves non-trivial are described.