Damage Detection of Closed Crack in a Metallic Plate Using Nonlinear Ultrasonic Time Reversal Method

Initial cracks in metallic structures incline to be closed at rest. Such incipient damage generally fails to be detected 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 method is proposed to detect closed cracks in aluminum plates. Two surface-bonded piezoelectric transducer arrays are used to generate, receive, and reemit ultrasonic wave signals. The closed crack is simulated by tightening a bolt on the aluminum plate. By applying large amplitude excitation voltage on the PZT transducers, the closed crack could be opened and closed. The transmitted waves recorded by PZT array contain nonlinear components, the signals are time reversed and emitted back, and the tone burst reconstructions are achieved. The linear reciprocity and the time reversibility break down due to the presence of the nonlinear components. The correlation coefficient between the original excitation signal and the reconstructed signal is calculated to define the damage index for individual sensing path and is used to develop an imaging algorithm to locate the closed crack on the plate. The experimental results demonstrate that incident wave signals and their reconstructed signals can be used to accurately detect and locate closed cracks. 1. Introduction In mechanical, aerospace, and civil infrastructures, metallic components made of aluminum are ubiquitous. Failures in metallic structures are often caused by closed cracks developed due to fatigue. In practice, closed cracks usually cannot be avoided. Thus, there is a need in nondestructive inspection for detection of such incipient damage. The closed crack stays in contact and presents the same as linear damage at rest, and only when the excitation overcomes a threshold, the closed crack can be opened and produces nonlinear response, so it is hard to detect closed crack with traditional linear ultrasonic technology. Since the nonlinearity signatures are much more sensitive to small damage features than the measurement of the variations of linear elastic properties [1], it is a very promising field of research to detect nonlinear scatterers in metallic structures using nonlinear ultrasound. However, the nonlinear response is usually small and hard to be detected, and hence imaging the closed crack is a challenging problem. Recently, more and more attentions had been drawn to the damage detection with application of time reversal wave signals. This technique uses the