Non-contact ultrasonic detection of angled surface defects

Abstract

Non-destructive testing is an important technique, and improvements are constantly needed. Surface defects in metals are not necessarily confined to orientations normal to the sample surface; however, much of the previous work investigating the interaction of ultrasonic surface waves with surface breaking defects has assumed cracks inclined at 90• to the surface. This paper explores the interaction of Rayleigh waves with cracks which have a wide range of angles and depths relative to the surface, using a non-contact laser generation and detection system. Additional insight is acquired using a 3D model generated using finite element method software. A clear variation of the reflection and transmission coefficients with both crack angle and length is found, in both the out-of-plane and in-plane components. The 3D model is further used to understand the contributions of different wavemodes to B-Scans produced when scanning a sample, to enable understanding of the reflection and transmission behaviour, and help identify angled defects. Knowledge of these effects is essential to correctly gauge the severity of surface cracking.

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

“…The interaction of laser generated surface waves with surface-breaking defects has been investigated using finite element method (FEM) modelling using PZFlex, and with experiments [5,8]. The measurement configuration is shown in figure 1; the generation laser is focussed into a line source, while the detection laser measures at a point.…”

“…For the wedge sample and the simple angled defects, the generation was modelled as a dipole loading force of 10 ns duration with boundary conditions set to prevent unnecessary reflections. This force was chosen to match the experimental results [5]. For the branched defects, we implemented a thermoelastic generation model, which considered the flow of heat and resulting expansion in the aluminium sample.…”

“…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].…”

“…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]. 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].…”

“…RCF is modelled simply as a defect propagating at an angle to the surface [5,7], and we consider the far-field and near-field behaviour and the effect of the changing local thickness. SCC is modelled as a defect machined normal to the sample surface with a small branch; this is a simplification of the complicated branching nature of SCC, but is a first step towards understanding the behaviour of ultrasound in the region of such cracking [2].…”