ESPI with synchronized pressure stressing

Optical detection of disbonds in aluminum composites is demonstrated using electronic speckle pattern interferometry combined with synchronized pressure stressing. The surface on the test specimen is periodically pressure stressed in synchronization with the image acquisition rate of an image processor. This is achieved by using a two-port, low volume, transparent vacuum chamber mounted on the specimen. One of the ports of the vacuum chamber is connected to a constant vacuum source, and the other is connected to the ambient via a solenoid valve that is periodically opened and closed in synchronization with the image acquisition. Furthermore, illumination of the specimen is also synchronized with the stressing. Speckle images of the surface of the specimen undergoing high and low pressure stressing are combined with a reference speckle image and acquired at the image acquisition frequency of the detecting CCD camera. Every two consecutive images are then subtracted in the image processor and displayed in real-time. In this manner, excellent noise reduction is achieved, rejecting the effects of low frequency noise contributions such as slow object drift, air current, and thermal gradients in/around the specimen found in typical industrial environments.