Damage Initiation and Ultimate Tensile Strength of Scaled [0deg\sub{n}, 90deg\sub{n}, 0deg\sub{n}]\subT Graphite-Epoxy Coupons

Previous research on scaling effects in composite materials has demonstrated that the stress levels at first ply failure and ultimate failure of composite laminates are dependent on the size of the laminate. In particular, the thickness dimension has been shown to be the most influential parameter in strength scaling of composite coupons loaded in tension. Geometrically and constitutively scaled laminates exhibit decreasing strength with increasing specimen size, and the magnitude of the strength-size effect is a function of both material properties and laminate stacking sequence. Some of the commonly used failure criteria for composite materials such as maximum stress, maximum strain, and tensor polynomial (e.g., Tsai-Wu) cannot account for the strength-size effect. In this paper, three concepts are developed and evaluated for incorporating size dependency into failure criteria for composite materials. An experimental program of limited scope was performed to determine the first ply failure stress in scaled cross-ply laminates loaded in tension. Test specimens were fabricated of AS-4/3502 graphite-epoxy composite material with laminate stacking sequences of [0deg\sub{n}/90deg\sub{n}/0deg\sub{n}]T where n=1-6. Two experimental techniques were used to determine first ply failure, defined as a transverse matrix crack in the 90deg ply: (1) step loading with dye penetrant x-ray of the specimen at each load interval, and (2) acoustic emission.