Relative acoustic nonlinearity parameter variation with local defect resonance frequencies and its different harmonics

Abstract

In recent times, nonlinear ultrasonic inspection has become a very reliable technique to characterize micro-scale cracks and small-scale delaminations. In this paper, an analytical framework is developed for the orthotropic composite plate to calculate the variation of relative acoustic nonlinearity parameter, a quantitative measurement of nonlinearity at the delamination region. The analytical investigation is justified with numerical and experimental research using a carbon fiber-reinforced polymer (CFRP) plate with three delaminations. Two incident waves are used for excitation, out of which the first wave frequency is varied as local defect resonance (LDR) frequency (f_d), subharmonic LDR frequency (2f_d), second-order superharmonic LDR frequency (f_d/2), and third-order superharmonic LDR frequency (f_d/3) while the second incident wave is taken as a single periodic frequency. Different delamination sizes, positions, and depths were used to understand the second-order relative acoustic nonlinearity parameter (β₂) with other LDR frequencies as well as its harmonics. It has been observed from analytical, numerical, and experimental results that the relative acoustic nonlinearity parameter is maximum at LDR frequencies. It increases in case of surface level delaminations.