Dual-energy wave subtraction imaging for damage detection in ultrasonic propagation imaging system

In various research fields, barely visible impact damages (BVID) caused by low velocity impacts have been studied as critical design factors of composite sandwich structures. Thus, dual-energy wave subtraction (DWS) algorithm using ultrasonic propagation imaging (UPI) system is proposed to evaluate BVID on sandwich panel. The UPI system, one of the laser ultrasonic techniques that generates ultrasound on structures with several tens kilohertz of scan speed and visualizes ultrasonic propagation. Amplitude of ultrasonic signals generated by the UPI system increases with increasing energy of excitation laser beam. Difference of amplitude level is able to be calibrated by multiplying a factor to the signal obtained with low laser excitation energy and the signals subtracted one from the other become almost noise level. However, the subtracted signals in the damaged area will not be zero because of change in ultrasonic frequency and material condition in laser ultrasonic generation mechanism, which will be shown in anomalous waves around the damage. The suggested algorithm has been developed based on this phenomenon. The DWS algorithm eliminates most of incident ultrasonic wave and relatively amplifies anomalous waves. In this paper, the sandwich panel with Nomex honeycomb core including ply delamination fabricated by mass drop has been inspected to validate the proposed algorithm, and the results of statistical analysis and damage visualization are presented.