Interaction of Rayleigh Waves Induced by Interdigital Transducer with Fatigue Crack
Compared to bulk waves, Rayleigh waves can propagate much longer distances with lower signal attenuation levels, which are nominally pro portional to r / 1 for a given frequency (r=distance). In addition to the small attenuation, Ra yleigh waves are also non-dispersive and sensitive to surface-breaking defects. The surface penetra tion depth of Rayleigh waves is also approximately one wavelength, which can be beneficial for many NDT applications. However, one major drawback of using Rayleigh waves for nondestructive testing has been the difficulty of generating Rayleigh waves, where the most commonly used method involves the combination of a plastic wedge and a conventional longitudinal mode trans ducer. Unfortunately, a number of wedges with different angles are needed to generate Rayl eigh waves in different materials or at different operating frequencies to satisfy the critical Rayleigh launch angle requirement. In this study, instead of using wedge transducers, we have designed, fab ricated, and tested narrow-band Interdigital Transducers (IDTs), where precision elect rode patterns were etched on piezoelectric ceramic substrate materials using micro laser machining t echniques. These IDTs can be directly applied to the surface of test specimens to generate Rayle igh waves in MHz range. A laser interferometry system was used to characterize the I DT sensors, where a well-defined and directional Gaussian beam profile was observed. Inves tigations were also made for Rayleigh waves interacting with fatigue cracks in aircraft qualifi ed metallic alloys. It was observed that some of the Rayleigh wave energy can propagate through a tight crack at both normal and oblique incidence.
Interferometric and Holographic Imaging of Surface Wave Patterns for Characterization of Material Degradation
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].
Thermally Restorable Optical Degradation Effects in Heat‐Treated Cu2S–CdS Crystal Heterojunctions
We have shown that heat treatment alone does not produce appreciable degradation of the short‐circuit current in Cu2S–CdS single‐crystal photovoltaic heterojunctions. The degradation, which has been commonly observed, is the result of optically induced effects in the heat‐treated cell, subsequent to the heat treatment. These optically induced effects can be correlated with photochemical changes induced in single crystals of CdS: Cd: Cu in which the density of Cu acceptors exceeds the density of excess‐Cd donors. Major conclusions about the mechanism of the photovoltaic effect and heat‐treatment processes can be drawn.