Propagation of ultrasonic waves in complex media: Investigation of coda wave technique and traditional polar and C-scan imaging

Abstract

A discussion of nondestructive techniques is presented for the investigation of complex media, with a focus on composite samples. Traditionally, one applies ultrasonic C-scans, or polar scans, which are easy to implement and to interpret. However, in many realistic cases, it is important to use more sophisticated approaches as C-scans often do not reveal any useful information. Typically, the early part of received signals is used to extract information, whereas the later part is considered either as noise or as a useless coda wave as in musical acoustics. Nevertheless, it appears that the coda part carries useful information about the medium, and therefore, it is important to explore techniques to extract that information. In addition, it turns out that the coda is very sensitive to material properties and damage as those sound waves interact longer with the material than early arrival waves. First, earlier results will be shown which compare experimental polar scans with numerical simulations, then, for the same samples, coda wave results will be presented to show the effect of damage on the composite samples. The main damage indicator is the change in relative wave velocity which is caused by the damage.