PhotoStress coatings can be applied to almost any material; this includes applications to composite materials such as reinforced plastics, carbon fibers, concrete, wood, and metal-matrix composites. Due to their inhomogeneity, most composite materials have mechanical properties that vary from point to point. Very commonly, such materials are also anisotropic in their mechanical properties and the magnitudes of the properties (elastic modulus, Poisson’s ratio, ultimate strength, etc.) at each point vary with the direction at the point. As a result, the stress and strain measurements may be seriously misleading.
Because of its full field stress analysis capability, PhotoStress is ideally suited for preliminary stress analysis to test objects made from composite materials. It reveals the detailed strain distribution and the principal strain directions over the entire coated surface of the part. As exemplified by some of the illustrations shown here, the coating also tends to display the underlying structure of the inhomogeneity.
As shown below, a fiberglass plate and an aluminum plate of similar dimension were coated with PhotoStress plastic and tested in uniaxial tension. The resulting strain patterns that developed around the holes in both plates were similar in geometry, demonstrating a definite correspondence in the gross strain distribution in homogeneous and heterogeneous materials. However, the fringe patterns appeared as smooth unbroken lines for the homogeneous material (aluminum) as shown in Photo A, while for the heterogeneous material (fiberglass), they were discontinuous, with a scotch-plaid appearance as shown in Photo B.