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.