Nonlinear Ultrasonic Time Reversal Imaging for Closed Crack Location in Metallic Structures

Abstract

Initial crack in metallic materials inclines to be closed at rest. Such imperfection generally fails to be diagnosed and located with traditional linear ultrasonic techniques because ultrasonic waves penetrate the contact area of the closed crack. In this paper, an imaging algorithm based on nonlinear ultrasonic time reversal to detect closed crack using Lamb wave signals which are transmitted and received by surface-bonded piezoelectric transducers array is proposed. We develop a three-dimensional finite element model with a closed crack, the model makes use of local node splitting and nonlinear spring elements at the crack interface. Once excitation voltage is able to overcome the threshold to open the contact, the stiffness of the nonlinear spring will be changed and the nonlinear response which can be detected using the scaling subtraction method is produced. Applying a low amplitude excitation voltage as a comparison signal which, linearly rescaled in amplitudes, is not equal to the signal recorded at larger excitation amplitudes, it indicates the nonlinear response produced. The correlation coefficients between the original excitation signal and reconstructed time-reversed signal are calculated to define the damage index for individual sensing path, which are used to develop an imaging algorithm to locate the closed crack in the metallic structures. The results of the numerical simulation demonstrate that incident wave singles and their reconstructed singles can be used to accurately detect closed crack.