Interlaminar fracture of composites under tension and compression

Interlaminar fracture of the AS4/3502 graphite/epoxy material system is investigated using a double cracked-lap-shear (DCLS) specimen and a single cracked-lap-shear (SCLS) specimen. A fundamental feature of the designed specimens is their ability to be tested under net tensile and compressive loadings. The specimens exhibit mixed-mode or mode II behavior depending on the loading direction. The specimens are designed to precipitate crack growth at a designed-in site in a gage section. In the specimen design process, overall dimensions of the specimens are selected so that local disturbances in the stress field will not interact, there is adequate length to permit crack growth, and overall buckling will not occur under compressive loading. The experimental results confirm that the specimens and tests perform as designed. It is observed that: (1) There is an increasing resistance to crack growth under tensile loading; (2) interlaminar fracture under compression is a totally unstable process; and (3) tension and compression behaviors are considerably different. Fracture surfaces in the unstable regions from short beam shear and DCLS specimen tests exhibit similar characteristics.