Practical considerations for the rapid inspection of composite materials using laser-based ultrasound

Abstract

An automated laser-based ultrasound system is described that has sufficient sensitivity to detect typical flaws of interest in composite materials without causing surface damage to the part under test. An important step in achieving a rapid scanning laser-in/layer-out system has been the integration of a galvanometer mirror scan head in conjunction with a novel data acquisition scheme. This has allowed both flat and curved composite materials to be inspected with scanning speeds in excess of 18.5 m 2h −1, based on a 12.5 mm × 12.5 mm pixel size. In addition, a reference signal technique has been developed which superimposes a known phase modulation onto the probe laser beam. Real-time processing of the reference signal allows changes in the ultrasonic signal amplitude due to ultrasonic attenuation to be distinguished from those due to the variations in the amount of light reflected from the surface of the test specimen as the specimen is scanned. This technique thus eliminates problems resulting from ‘scanning noise.’ These results represent a significant step toward developing a field-usable automated laser-based ultrasonic scanning system for flaw detection in advanced composite materials.