Nondestructive evaluation of cylindrical parts using laser ultrasonics

We applied the laser ultrasonic technique for detecting surface breaking slots in steel cylinders (25 mm in diameter). The observation of the detected signal over a long time (500 micros), shows that the interaction of the two contra-propagating incident Rayleigh waves reinforce the echoes coming from the defect. These echoes are slowly growing with time whereas the main signals decrease. This energy transfer occurring at each revolution of the waves around the cylinder allows the detection of cracks having a depth (h approximately 80 microm), very small compared to the Rayleigh wavelength (lambda approximately 2 mm). The evaluation of the material was performed by processing the detected signal in a sliding time window. A cross-correlation is made either between a reference signal and the signal from the tested sample or between two signals probed for two different positions of the sample. In both cases, the slope of the cross-correlation coefficient versus the number of turns is proportional to the depth of the slot.