Quantitative non-destructive evaluation using laser generated ultrasonic pulses
The work presented here utilises features of laser generated ultrasound for the detection of defects in solids. Ultrasound is generated noncontactively by this method and likewise many of the detection devices used do not require direct coupling to the test pieces, thus acoustic pulses with high frequency components are able to be produced and monitored on a range of samples. Steel samples coated with between 3 and 7mm of plasma-transferred arc depositions are examined for bond quality via measurements of attenuation caused by porosity in the coatings, found to be related to weld current. Surface breaking cracks, (of depth < 3mm), in such claddings are quantitatively detected by a method which utilises Rayleigh pulses. A two sided automated scanning system is described which examines samples for subsurface defects. Results from the examinations of a dural test piece, plasma sprayed steel bars and carbon fibre composite samples are presented. Resolution of defects is shown to be within 0.5mm for a 1mm thick section of the composite material. A second scanning technique, requiring access to only one sample face, is presented which employs an interferometer for detection and is thus a truly remote system. Laminar flaws are modelled using flat-bottomed holes, the dimensions of which are measured using the resonance of the bodies. A theoretical investigation using various boundary conditions suggests possible applications for quantitative NDE of structures with well defined natural vibration frequencies. The propagation of acoustic transients in plates is also examined. Possible future work concerning laser/ultrasonic NDE is discussed.