Surface and interior strain fileds measured by multiple-embedded-grid moiré and strain gages

Surface and interior straini fields in thick specimens of polycarbonate have been measured by an embedded-multiple-grid moiré method. Interior gratings are photographed through intervening gratings. Coherent optical processing of grating records creates moiré fringe pattern. A correction procedure was developed to eliminate moiré errors caused by strain-induced gradients of refractive index. This procedure utilizes the refraction distribution as obtained from observation of a grating from opposite sides of a specimen. No measurements other than the normal moiré observations are required in order to find actual strains. The correction technique should be useful in refining the results obtained when other optical methods are used in three-dimensional situations. The method is utilized to determine strains on the surfaces and at the mid-and quarter-planes of thick compact tension specimens. Results obtained, after correction, agree with measurements made with embedded strain gages. Maximum tensile strain occurs on the midplane. The findings are rationalized by consideration of thickness and proximity to the crack tip.