Enhancing the Robustness of Nonlinear Ultrasonic Testing by Implementing 1D Phononic Crystals

Abstract

Nonlinear ultrasonic method is based on detecting higher order harmonic signals and relating their change with respect to the fundamental mode to microstructural features in a material. When ultrasonic wave propagates in a material, the waveform gets modified when it confronts microstructural features. Higher harmonics generated by the interaction of fundamental mode with microstructural features are measured to calculate the ultrasonic nonlinearity parameter known as β . However, typically, materials nonlinearity due to microstructural features is very weak and nonlinearity in the experimental instruments may produce significant error. This paper aims to enhance the robustness of the nonlinear ultrasonic method by blocking the nonlinearity induced by measurement process. Phononic crystals are periodic composite materials that generate a bandgap to restrict propagation of selected range of frequencies. In this study periodic phononic crystals and the nonlinear ultrasonic testing are integrated to block the higher harmonics generated in the experimental process. One dimensional phononic crystal, called superlattice, is numerically modeled using COMSOL Multiphysics software to block the frequency range of 4 MHz to 6 MHz. The model is tuned to block the higher harmonics while it allows the fundamental frequency of 2 MHz to pass. By placing the superlattice between the transmitter and the specimen, the higher harmonics generated in the experimental process are extensively decreased. This method has been experimentally implemented to detect the dislocation density changes in aluminum 1100 specimens. The results show that applying the superlattice into the nonlinear ultrasonic testing can improve the sensitivity of measuring microstructural features.