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This study is part of a continuing exploration of the structural behavior of fiber-reinforced composite materials by NASA (Langley). The dual purposes of the present research are to (1) develop a whole-field, high sensitivity optical technique to measure load-induced changes of thickness of composite plates, and (2) measure changes of thickness of three composite plates, each with a central hole of different prescribed size, as a function of applied compressive loads. The specimens were graphite-epoxy plates.

Contour maps of thickness changes were obtained for three quasi-isotropIc graphite-epoxy plates with central holes, loaded in compression. Thickness changes were determined for six load increments from nearly zero to within a few percent of the failure load. The largest change of thickness occurred near the hole but not at the boundary of the hole. Below 90% of the failure load, the thickness changes were nearly proportional to load. irregularities of thickness changes occurred in zones of compressive stresses and they were attributed to localized fiber buckling. A new optical technique was developed to measure thickness changes with high sensitivity. it utilizes a omparatively simple means of holographic interferometry on both sides of the specimen, followed by additive moire to obtain thickness changes as the sum of the out-of-plane displacements. Sensitivity was 0.0000125 in. per fringe order. The fringe patterns represent thickness changes uniquely, even when specimen warpage and consequent out-of-plane disp%a tents are very large. (MM)