A. R. Clough

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.

Characterisation of hidden defects using the near-field ultrasonic enhancement of Lamb waves

Defects that propagate from the inside of a structure can be difficult to detect by traditional non-destructive inspection methods. A non-contact inspection method is presented here that uses the near-field interactions of ultrasonic Lamb waves to detect defects propagating into a 1.5 mm thick aluminium sheet from the opposite side to that which is inspected. Near-field interactions of the S0 Lamb waves with the defects are shown to give rise to a characteristic increase in the wave magnitude, which is used to position and characterise these hidden defects. It is shown that such defects are difficult to detect from a study of their influence on ultrasonic transmission alone. Single defects of different depths, and systems of multiple defects with varying separations and relative depths, are successfully detected in both experimental trials and FEM simulations. Reliable single defect detection is achieved for defects with a minimum depth of 30% of the plate thickness, and resolution of multiple defects is achieved for defect separations of 5mm.

Lamb wave near field enhancements for surface breaking defects in plates

Near field surface wave ultrasonic enhancements have previously been used to detect surface breaking defects in thick samples using Rayleigh waves. Here, we present analogous surface wave enhancements for Lamb waves propagating in plates. By tracking frequency intensities in selected regions of time-frequency representations, we observe frequency enhancement in the near field, due to constructive interference of the incident wave mode with those reflected and mode converted at the defect. This is explained using two test models; a square based notch and an opening crack, which are used to predict the contribution to the out-of-plane displacement from the reflected and mode converted waves. This method has the potential to provide a reliable method for the near field identification and characterisation of surface breaking defects in plates.

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.

Measuring elastic constants using non-contact ultrasonic techniques

The use of ultrasound for measuring elastic constants and phase transitions is well established. Standard measurements use piezoelectric transducers requiring couplant and contact with the sample. Recently, non-destructive testing (NDT) has seen an increase in the use of non-contact ultrasonic techniques, for example electromagnetic acoustic transducers (EMATs) and laser ultrasound, due to their many benefits. For measurements of single crystals over a range of temperatures non-contact techniques could also bring many benefits. These techniques do not require couplant, and hence do not suffer from breaking of the bond between transducer and sample during thermal cycling, and will potentially lead to a simpler and more adaptable measurement system with lower risk of sample damage. We present recent work adapting EMAT advances from NDT to measurements of single crystals at cryogenic temperatures and illustrate this with measurements of magnetic phase transitions in Gd64Sc36 using both contact and non-contac...

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.