Near-field ultrasonic scattering from surface-breaking cracks

Abstract

The near-field scattering of ultrasonic Rayleigh waves from surface-breaking cracks has been studied using scanning heterodyne interferometry. Distinct two-dimensional, localized displacement patterns were observed in the near field of the scattering sites, which provide an effective tool for detecting and characterizing the defects. The observed patterns showed a dramatic increase (2×–4×) in the ultrasonic displacement levels near the crack faces, allowing the cracks to be easily distinguished from background levels. A simple explanation for the increased near-field displacement amplitudes is presented that is based on wave propagation and free-boundary reflection arguments.

Enhancement of ultrasonic surface waves at wedge tips and angled defects

“…In the near-field, enhancement of the Rayleigh wave in the vicinity of a surface-breaking defect has drawn a lot of recent interest [12][13][14][15][16][17] . To quantify this enhancement the signal amplitude in a windowed region around the Rayleigh wave arrival time is measured, and the maximum amplitude compared to that of the incident Rayleigh wave.…”

Scanning laser source and scanning laser detection techniques for different surface crack geometries

“…Rayleigh wave enhancement at a defect has been considered by several groups [9,10,14,15]. For a generation laser passing over the defect (SLLS), we observe enhancement due to constructive interference of the direct and reflected Rayleigh waves.…”

“…It has been shown previously that, whilst reflection of Rayleigh waves has a dependence on crack angle, the transmission also has some angle dependence [3,[12][13][14]. This means that measurement of the depth of a defect using Rayleigh wave transmission must consider the crack angle when choosing a suitable depth calibration profile.…”

“…This means that measurement of the depth of a defect using Rayleigh wave transmission must consider the crack angle when choosing a suitable depth calibration profile. There are several ultrasonic surface wave methods with potential for measuring crack angle [4][5][6][7]13,14]; here, we consider the behaviour of the signal enhancement [4,6].…”

Enhanced damage characterization using wavefield imaging methods

“…By using a scanning laser vibrometry detection system, the transient displacement fields generated by an ultrasonic source can be visualized and studied in detail. As a tool for quantitative nondestructive evaluation, the visualization of elastic waves provides a unique opportunity for understanding the scattering of elastic waves from insipient damage, where the detection and characterization of damage features using ultrasound can be enhanced in many instances [1][2][3]. In the present effort, the detection and direct imaging of fatigue cracks in metals, and delaminations in composites, are described.…”