OS02W0287 Ultrasonic characterization of interface of metals

Abstract

A laser ultrasonic method has been developed for inspection of hot-clad aluminum/steel interface. Because of high temperature, the conventional contact or immersion ultrasonic testing is not applicable for this purpose, therefore, non-contact nondestructive testing is required. Among non-contact ultrasonic methods, the laser ultrasonic method has relatively high S/N ratio and spatial resolution. Line-focused YAG pulse laser excites many ultrasonic waves on the plate surface. Beyond a certain distance from the irradiated area, only the Lamb waves of various modes prevail than other modes of waves. A broadband laser interferometer detects the out-of-plane displacement of many kinds of the Lamb waves. The received time-domain signals are superposition of the waveform of the individual Lamb wave, therefore, identification of interface delamination of aluminum/steel clad pate is difficult. We propose a new laser ultrasonic method for detecting interface delamination of hot-clad aluminum/steel plates with the group velocity dispersion of the Lamb waves. By apply the wavelet transformation on the received time-domain signals, we calculate the group velocity dispersion curves of several Lamb wave's modes. The dispersion curves agree well with theoretical dispersion curve of the defect-free clad plate. If some delamination appears within Lamb wave propagation distance, a noticeable distortion of the dispersion curve appears. By taking the differential of the dispersion maps of the intact and delaminated plate, we identify the delamination of the hot-clad plates.