Time Reversal Analysis for Flaw Reconstruction using Elastic Property Determined by Ultrasonic Wave Field Data

Nakahata, Kazuyuki; Ehime University; Japan

Mizota, H.; Hitachi, Ltd.,; Japan
Nagashima, Y.; Hitachi, Ltd.,; Japan
Nakahata, K.; Ehime University; Japan

ID: ECNDT-0312-2018
Session: PAUT-Signal processing
Room: G2
Date: 2018-06-13
Time: 16:40 - 17:00

ost commercial ultrasonic phased array imaging systems implement linear and sector scan methods which use flight time and amplitude of flaw echoes. However, these methods are based on the fundamental ray–tracing theory in homogeneous media, and they are not directly applicable to heterogeneous media because of the beam skewing and interfacial diffraction, and so on. A time reversal (TR) technique is a promising method that can be used in nondestructive evaluation to improve the flaw detectability in heterogeneous media. In the TR technique, signals from a flaw are recorded at all the array elements, then the signal are reversed and re-transmitted into the medium. Since the signal is generated from the flaw, the time reversal waves return to the flaw position according to the reversibility of the elastic wave.
In this study, we propose a simulation–aided flaw reconstruction method using the time reversal approach. Here, scattered waves from a flaw are recorded and the time–reversed waves are re–transmitted in the image–based finite element (FE) simulation. The target flaw is an electric discharge machined slit in a unidirectional solidified SUS316L stainless steel. In the TR wave propagation in the FE simulation for SUS316L, the shape of the slit can be visually estimated from the focal point of the ultrasonic wave. The accuracy of the TR reconstruction method depends on the quality of the FE model. Before the flaw reconstruction, ultrasonic wave fields in SUS316L are experimentally visualized by a non-contact ultrasonic generation method using scanning laser source. Then the elastic constants are obtained from the wavefield data processed by the time-space Fourier transform. The elastic constants are fed into the FE model in the TR simulation.