Detection of cracks in metal sheets using pulsed laser generated ultrasound and EMAT detection

Abstract

A pulsed Nd:YAG laser with an approximately Gaussian beam shape is directed onto the surface of an aluminium sheet at an energy density below which damage by laser ablation occurs, generating Lamb waves in the sheet. The laser beam is raster scanned across the surface of the sample. The Lamb waves travel radially outwards from the generation point and are detected some distance away by an electromagnetic acoustic transducer with sensitivity to in-plane displacements of the sheet. A number of static EMATs are located around the edges of the sheet, some distance from the generation point.The presence of a crack-like defect on the sheet can be detected by either a sudden change in the ultrasonic waveform or by an enhancement in the frequency content of the waveform when the laser beam illuminates directly onto the crack.

Multi-coil focused EMAT for characterisation of surface-breaking defects of arbitrary orientation

“…A 1 MHz driving frequency gives the strongest peak to peak signal for both cycle lengths. This is as expected, given the width of the coil, when approximating it as a linear coil [22]. The racetrack coil design has a more complicated dependence of frequency on width, but the linear approximation gives a reasonable prediction of frequency behaviour.…”

“…Typically, the narrower the coil, the higher the frequencies it is possible to generate [22]. However, narrowing the coil decreases the number of turns of wire, and thus weakens the overall signal.…”

“…The signal wave packet in figure 3 clearly starts distorting above about 2 MHz. This indicates that the coil spatial extent is too wide for the high frequency wave to be efficiently generated [22].…”

“…There is significant transmission around the 1 mm defect, but for the longer defects a near-constant frequency content is measured. The shape of the FFTs are due to the finite width of the EMAT coil [22]. Analysis looked at the peak magnitudes in three of the frequency 'lobes'; low frequency (0.35 -0.67 MHz), main lobe (0.67 -1.33 MHz), and high frequency (1.33 -1.65 MHz).…”

Detection and characterisation of surface cracking using scanning laser techniques

“…It has been shown that when the laser generation is over a defect it will generate higher order modes than are present away from the defect [14]. This, again, is a good indication that a surface defect is present.…”