Enhancement of ultrasonic surface waves at wedge tips and angled defects

Abstract

The behaviour of sound waves interacting with wedges has attracted interest from researchers in geophysics and non-destructive testing. We consider here the nearfield behaviour of Rayleigh waves incident on wedges and surface-breaking defects which propagate at an angle to the surface, such as rolling contact fatigue on rails. It has been shown that, for a detection point on the edge of the crack tip, a very large signal enhancement is observed for shallow angles. We explain this behaviour through considering the effect of the defect geometry, with changes in the frequency·thickness product leading to mode-conversion of the incident Rayleigh wave.

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

“…The boundary conditions will change slightly, as will the reflection coefficient, but this leads only to a small angle dependence of the signal enhancement. This is shown in figure 2 by circles (open for experiment, closed for model, showing excellent agreement), and can be explained by the change in reflection coefficient and concentration of the laser energy into a smaller thickness of sample [4].…”

“…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].…”

“…When the generation laser is scanned over a defect and the detection point held fixed, this configuration is known as scanning laser line source, or SLLS [9,10]. When the detection point is scanned (and the generation laser held fixed) this is scanning laser detection (SLD) [4,6]. Several defect geometries in aluminium samples are considered, including simple angled defects (shown in figure 1), a wedge-shaped sample, and a set of thick samples containing branched defects (shown in figure 8).…”

“…For example, when considering a slot propagating at an angle to the surface, the reflection and transmission of Rayleigh waves show some angle dependence [5]. Furthermore, the signal enhancement as the incident and reflected wavemodes constructively interfere close to the defect shows a large dependence on the defect angle [4,6].…”

“…Typically, one considers a calibration based on a set of slots machined at 90° to the sample surface, and compares this to results from a real defect [3]. We have shown recently that, when considering the interaction of a Rayleigh wave with a surface-breaking defect, this is not always accurate [4][5][6]. For example, when considering a slot propagating at an angle to the surface, the reflection and transmission of Rayleigh waves show some angle dependence [5].…”