Laser Generation and Detection of Surface Acoustic Waves Using Gas-Coupled Laser Acoustic Detection

Abstract

Laser generation and detection of ultrasound has the advantage of requiring no mechanical contact with the materials under investigation. We previously reported [1] laser-based measurements on Lamb waves in graphite/polymer composite laminates using a confocal Fabry-Perot interferometer for detection. Related work by other groups includes air-coupled detection of Lamb waves in similar composites using capacitive transducers [2,3] and interferometric detection of Lamb waves in paper [4]. Our earlier work has been extended using Gas-Coupled Laser Acoustic Detection (GCLAD), an economical alternative laser-based method which has the additional advantage that the detection laser beam is not reflected from the sample surface. GCLAD is thus particularly useful for materials with surfaces of poor optical quality. We demonstrate below that the combination of laser generation and GCLAD can be used to obtain well-resolved surface-acoustic waves (SAWs) in a variety of materials, including metals, paper, thin films, and composite pre-preg tape. We also show some preliminary SAW scans obtained with laser generation and GCLAD using metallic samples. Each pixel in the scans represents the strength of a SAW passing through a portion of the sample with an area of about 1 cm2. Scans of this type offer the possibility of economical testing of large sample areas, potentially on-line in a manufacturing environment.