Interferometric and Holographic Imaging of Surface Wave Patterns for Characterization of Material Degradation

Abstract

Optical interferometry and holography systems provide an effective means for measuring surface motions and vibrations on nanometer scales, and have been used extensively in the past in a number of different applications [1–4] . For nondestructive evaluation (NDE) of material degradation, both techniques can be combined with classical ultrasonic testing methods to provide non-contact measurement capabilities for evaluating important parameters such as fatigue state [5], and the existence of microcracks [6] and corrosion [7]. Here we describe two advanced NDE systems capable of ‘visualizing’ ultrasonic fields on material surfaces with high sensitivity and spatial resolution. The systems use heterodyne interferometry and frequency-translated holography concepts to create detailed two-dimensional displacement-field images, which provide quantitative measurements of ultrasonic field parameters related to ultrasonic wave dispersion, phase velocity, attenuation, and localized scattering. Each of these parameters, in turn, can be used to assess the underlying material structure. Local ultrasonic scattering, for example, can be used to detect and characterize surface-breaking cracks [8–11]. As a limiting factor Hassan and Nagy have studied the effect of grain noise in detail [12].