B. Dutton

Enhancement of ultrasonic surface waves at wedge tips and angled defects

The behaviour of sound waves interacting with wedges has attracted interest from researchers in geophysics and non-destructive testing. We consider here the near-field 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

Standard test samples typically contain simulated defects such as slots machined normal to the surface. However, real defects will not always propagate in this manner; for example, rolling contact fatigue on rails propagates at around 25o to the surface, and corrosion cracking can grow in a branched manner. Therefore, there is a need to understand how ultrasonic surface waves interact with different crack geometries. We present measurements of machined slots inclined at an angle to the surface normal, or with simple branched geometries, using laser ultrasound. Recently, Rayleigh wave enhancements observed when using the scanning laser source technique, where a generation laser is scanned along a sample, have been highlighted for their potential in detecting surface cracks. We show that the enhancement measured with laser detector scanning can give a more significant enhancement when different crack geometries are considered. We discuss the behaviour of an incident Rayleigh wave in the region of an angled defect, and consider mode-conversions which lead to a very large enhancement when the detector is close to the opening of a shallow defect. This process could be used in characterising defects, as well as being an excellent fingerprint of their presence.

Interaction of laser generated ultrasonic waves with wedge-shaped samples

Wedge-shaped samples can be used as a model of acoustic interactions with samples ranging from ocean wedges, to angled defects such as rolling contact fatigue, to thickness measurements of samples with non-parallel faces. We present work on laser generated ultrasonic waves on metal samples; one can measure the dominant Rayleigh-wave mode, but longitudinal and shear waves are also generated. We present calculations, models, and measurements giving the dependence of the arrival times and amplitudes of these modes on the wedge apex angle and the separation of generation and detection points, and hence give a measure of the wedge characteristics.

DEFECT FEATURE EXTRACTION USING SURFACE WAVE INTERACTIONS AND TIME‐FREQUENCY BEHAVIOR

In this paper we exploit the interaction of ultrasonic surface waves with surface cracks in order to extract defect characteristics. An experimentally validated computer model was developed to simulate laser generated surface waves interacting with defects of several depths and angles. The Rayleigh reflection coefficient vs. crack angle and depth was explored. Amplitude and frequency behaviour at the defect interface in aluminium samples was also considered. Additionally, the phase component of the FFT was used to extract similar information. Finally, we explore the time‐frequency behaviour using the Wigner transform.

IDENTIFYING SURFACE ANGLED CRACKS ON ALUMINIUM BAR USING EMATS AND AUTOMATED COMPUTER SYSTEM

Electromagnetic acoustic transducers (EMATs) have been used to generate and detect Rayleigh waves in order to identify surface cracking in aluminium bars and rails. B‐scans produced during scans of samples were used to determine the presence of surface defects. Additionally, the differences between signal enhancements due to wave interference at the crack produced by normal (90°) and angled cracks in the B‐scans were used to classify samples in order to decide an appropriate depth calibration curve for depth estimation. Classification was done using an image processing algorithm that selected the best features for classification, and used these to identify similar patterns in unclassified B‐scans.

Detection and characterisation of surface cracking using scanning laser techniques

The use of lasers for generating and detecting ultrasound is becoming more established in NDT. However, there is still scope in developing the techniques to fully realise the benefits of non-contact measurements for different applications. One particular application is the detection of surface defects in metals; for example, rolling contact fatigue in rails, and surface cracking on billets. For these applications scanning techniques can prove beneficial. We present measurements and models based on a system using a pulsed Nd:YAG laser to generate surface ultrasonic waves and an IOS two-wave mixer interferometer to detect the surface displacement on the sample, to investigate the interaction of Rayleigh or Lamb waves with surface defects. Changes in the transmission of surface waves in the vicinity of surface defects can be used for depth characterisation. This can then be combined with other techniques, such as signal enhancement, in order to pinpoint the defect position. This is observed for Rayleigh waves when either the generator (scanning laser source, SLS, technique) or detector is close to a defect. For a scanned detector measurement, enhancement is observed due to constructive interference of the incident and reflected Rayleigh waves with the mode converted surface skimming longitudinal wave. For SLS, the mode-converted wave is attenuated before reaching the detector, but the change in generation conditions when the laser is over the defect also lead to an enhancement. In measurements of plate samples we observe similar enhancement effects whereby higher order modes are generated when the laser is above a defect. The defect geometry significantly affects the enhancement observed when scanning the detection laser, such that a shallow angled crack will give an enhancement of over 10 times the incident signal amplitude, whereas the angle dependence of the SLS technique is relatively small. We discuss the reasons for this extra enhancement, and the implications for scanning laser techniques used for detecting and characterising surface cracking.

ULTRASONIC RAYLEIGH WAVE ENHANCEMENTS FROM ANGLED DEFECTS IN ALUMINIUM

Non-linear enhancements of ultrasonic surface wave amplitude and frequency have been observed when an incident wave interacts with a surface defect. Previous measurements of surface wave interactions with defects have considered only those that are inclined normal to the surface. Here, the enhancement effects have been studied in aluminium samples with machined slots of fixed length and of varying angle to the horizontal; the degree of enhancement was studied as a function of defect angle using both a scanning laser source, and a scanning laser detector. An automated scanning system has been developed for use with the detector, an IOS two-wave mixer interferometer, capable of measuring the out-of-plane surface displacement on rough surfaces. B-scans, consisting of many A-scans stacked together, were used to identify wave modes present in the near field, the arrival times of which are dependent on the angle of the defect. The observed enhancement is caused by superposition of the incident Rayleigh wave with reflected and mode converted waves, thereby making it angle dependent.

Exploring surface wave interaction with angled defects in the near and far field

This paper explores some effects that occur when using laser ultrasound to scan defective samples. Surface defects can often propagate at an angle to the surface; however, for calibration, slots machined normal to the surface of the sample are typically used. Several interesting angle-dependent effects are observed when Rayleigh waves interact with angled surface defects, and are explored here using measurements and models for a scanning laser detector (SLD) or scanning laser line source (SLLS) scanned across the defect. Reflection and transmission coefficients are calculated for different crack angles and lengths. Additionally, interesting angle-dependent effects are observed in the Rayleigh wave amplitude and frequency enhancements in the near field when using SLD or SLLS.

Effects of branched defect geometry on the propagation of Rayleigh waves

Rayleigh waves can be used for characterisation of surface-breaking defects, giving a measure of the depth and the angle of propagation of a defect with simple (i.e. single crack) geometry. However, surface breaking defects will often grow with a more complicated geometry. We present here results of experimental measurements using laser generated and detected Rayleigh waves on aluminium samples containing machined slots with varied branched geometries. The signal enhancement found in the near-field, and the reflection and transmission of different wavemodes can be used to position the defect and gain an idea of its geometry. This research can be applied to monitor components prone to developing stress corrosion cracking (branched-like defects). Results are shown of the near-field interactions of Rayleigh waves with this type of cracking in stainless steel pipe samples, in order to resolve the spatial extent and geometric alignment of those defects.

Laser generation and detection for surface wave interaction with different defect geometries

The enhancement of surface waves in the near-field of a defect has been reported by several authors. It has been demonstrated that the interaction between the incident Rayleigh wave with the reflected Rayleigh wave, plus the mode-converted surface skimming longitudinal wave, explains the significant increase in signal amplitude that is encountered as a detection point approaches a smooth machined defect inclined normal to the surface. However, this is not a typical defect geometry. For example, rolling contact fatigue in rails grows at an angle to the surface, and stress corrosion cracking grows as branched defects. We present results of experimental measurements on machined slots with varied geometries, including defects which are normal or inclined to the surface, and show the effect of branched defect geometries on the wave propagation and signal enhancement. We use laser generation and detection, and compare results with finite element method (FEM) models. We also investigate frequency enhancements for angled and branched defects, to highlight further potential measurement techniques when using scanning laser detection.