The Enhancement of Nonlinear Ultrasonic Testing to Detect Micro-structural Defects in Metals

Second harmonic generation (SHG) is a nonlinear ultrasonic testing (NLUT) method, which is based on the distortion of a single frequency wave confronting the micro-structural features as it passes through the material. This distortion generates higher frequency components of the fundamental frequency. The amplitudes of the fundamental and second harmonics are used to calculate the acoustic non-linearity parameter (β). The acoustic non-linearity parameter is correlated with the extent of micro-structural features in materials; however, the major challenges of this method are high sensitivity of harmonic amplitudes to the signal processing method and the experimental variables, such as coupling condition and instrumental non-linearities, which introduce significant variability in the reported non-linearity parameters representing different defects. The objective of this research is to minimize the variability of non-linearity parameter in relation to damage by introducing wavelet-based signal decomposition approach, and blocking the non-linearity in medium and instruments by phononic crystals (PCs). The research has two major components: (i) understanding the major drawback of the most prevalent signal processing method and formulating a more robust harmonic decomposition method, and evaluating the proposed method in the NLUT experiments to detect plastic strain and creep damage; (ii) blocking the non-linearity generated in instrument and couplant (water for the case of immersed NLUT) by implementing one-dimensional PCs, known as superlattices (SLs). It is demonstrated that the wavelet-based signal decomposition reduces the variability of harmonic amplitudes as compared to Fourier transform-based signal decomposition. SLs are numerically modeled to block higher harmonics selected in this research, and evaluated experimentally. It is shown that non-linearity in water and electronics in the transmission side of data acquisition equipment can be blocked while the NLUT method can be used with immersion method with the reduced coupling effect. The NLUT method integrated with SLs is demonstrated on detecting plastic deformation in aluminum and creep damage in steel