Detection and characterisation of surface cracking using scanning laser techniques

Abstract

Abstract. The use of lasers for generating and detecting ultrasound is becoming more established in non-destructive testing. However, there is still scope in developing the techniques to fully realise the benefits of non-contact measurements. One application is the detection of surface defects in metals; for example, rolling contact fatigue in rails, and surface cracking on billets or plates. We present measurements using a pulsed Nd:YAG laser to generate surface ultrasonic waves and an interferometer to detect the surface displacement on the sample, and investigate the interaction of Rayleigh or Lamb waves with surface defects. Signal enhancement in the near-field is observed for Rayleigh waves when either the generator or detector is close to a defect. For a scanned detector measurement, enhancement is observed due to constructive interference of the incident and reflected waves. For a scanned generator measurement, 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 observed when the laser is above a defect. We discuss the implications of signal enhancements for detecting and characterising surface cracking.

Laser ultrasonic monitoring of reversible/irreversible modification of a real crack under photothermal loading

“…2 Among the various nondestructive fatigue crack detection techniques, such as piezoelectric transducer-based and laser-based ultrasonic testing (UT), [3][4][5][6][7][8] magnetic particle testing, 9 eddy current testing, 9 X-ray tomography, 10 and machinevision 11 and image processing, 12 the laser-based ultrasonic testing (LUT) technique is extensively studied because of its multiple advantages; furthermore, LUT finds its application in manufacturing, [13][14][15][16][17][18][19] construction, 20,21 aviation and aeronautics, 22,23 and several other industrial fields. 13,14,24 The sensitivity of these conventional techniques, 23,[25][26][27][28][29][30] which depend on linear property modifications of ultrasonic waves, has been reported to be insufficient to detect fatigue cracks until the cracks become visibly large; 29,31 this is because the changes caused in linear ultrasonic values by these cracks are not sufficiently large to be accurately measured using conventional linear ultrasonic techniques. 29,32 On the other hand, nonlinear acoustic 8, and vibro-thermography methods [56][57][58] have been proven to potentially overcome the limitatio...…”

“…The sensitivity of these conventional techniques, 23,25–30 which depend on linear property modifications of ultrasonic waves, has been reported to be insufficient to detect fatigue cracks until the cracks become visibly large; 29,31 this is because the changes caused in linear ultrasonic values by these cracks are not sufficiently large to be accurately measured using conventional linear ultrasonic techniques. 29,32…”

Using Laser Ultrasound to Detect Subsurface Defects in Metal Laser Powder Bed Fusion Components

“…Laser techniques are noncontacting; thus, they do not exhibit any coupling problems; they can be used for rapid scanning and are amenable to use in hostile environments. Although LU has been shown to be capable of detecting the types of defects generated during additive manufacture, 11,12 there have been a limited number of experiments applying laser ultrasonic inspection directly to additively manufactured materials.…”