Failure of ±55 Degree Filament Wound Glass/Epoxy Composite Tubes under Biaxial Compression

New experimental results are presented which describe the crushing behaviour of ±55° filament wound E-glass/epoxy tubes subjected to biaxial compression at various ratios of circumferential to axial stress. The tubes were tested to fracture under combined external pressure and axial compression using a recently developed high pressure testing rig. In some cases it was necessary to use tubes with radius to thickness ratio less than 5, in order to avoid buckling under external pressure. These new results have been combined with previous results from tests involving tensile loads to form a comprehensive biaxial failure stress envelope. The experimental results showed that for ±55° tubes under biaxial loading the biaxial compressive strength is substantially greater than the uniaxial strength, reaching a maximum at a hoop to axial stress ratio of -2/-1. The maximum compressive strength under -2/-1 loading was up to three times greater than the uniaxial compressive strength. The experimental envelope is compared with theoretical predictions which utilise simple failure theories. Good correlation was observed between the experimental results and the prediction of simple 2-D laminate model which incorporated a progressive maximum stress ply failure criterion.