Design of end plugs and specimen reinforcement for testing ±55° glass/epoxy composite tubes under biaxial compression

Finite element stress analysis predictions used in the development of a test specimen and grips for testing composite tubes under biaxial compression are reported. Specimen end reinforcement and end closures of different shapes (including cylindrical, conical and radiused plugs) were analyzed and a shape that gives minimal stress concentration was established. An axisymmetric model and linear elastic orthotropic material properties were employed in the finite element analysis and several different combinations of compressive loading were analysed. Experimental results are presented for two; ±55° filament wound glass/epoxy (GRP), strain-gauged cylinders, one with end reinforcement and the other without. Both specimens were tested to destruction under external pressure loading that produced biaxial compression with circumferential:axial stresses in the ratio SR = −2/−1. A good correlation was observed between the finite element analysis and experimental strain distributions in both cylinders and the optimised shape of end plug and end reinforcement gave low strain concentrations.