Optical Measurement of Biaxial Strain in Thin Film Polymers

A non-contacting optical method for the measurement of strain simultaneously in two directions has been developed in the process of characterizing the behavior of thin film polymers. The thin film specimen geometry is a cylinder created by heat sealing two layers of the material together. The technique utilizes a random speckle pattern that is painted on the surface of the specimen. Images are captured by a CCD camera of a small portion of the painted region as the material creeps under load. A digital image correlation technique is used to analyze the movement of the speckle pattern over time. The resulting displacement field obtained from these images is then used to determine the strain on the surface of the specimen. This strain measurement technique has also been extended to the measurement of biaxial strains on the surface of thick sheets as well as thin films.