Photoacoustic and photothermal evaluation of stratified materials

Abstract

Photoacoustic and photothermal nondestructive evaluation techniques are an emerging approach for inspection of several materials of industrial interest. This paper focuses on stratified materials such as aluminum-epoxy or graphite epoxy laminates as well as metallurgically coated materials which are widely used, particularly in the aerospace industry. Such materials are usually inspected for layer-to-layer delaminations after manufacture by ultrasonic or radiographic techniques. Optical techniques provide an attractive alternative to such methods because they require no contact with the inspected surface so that they can be deployed in hostile and remote-located environments, while being easy to scan over large surfaces. This paper describes some applications of photoacoustic and photothermal approaches to the inspection of layered materials. Photoacoustic techniques using both microphone detection in air and interferometric detection of the laser-generated ultrasonic wave in the material are reviewed. Photothermal techniques using either a thermoelastic displacement approach or an IR time-resolved temperature monitoring method are also described, and their applicability to different materials is discussed.