Mechanical behaviour of +/-55 degrees filament-wound glass-fibre/epoxy-resin tubes .1. Microstructural analyses, mechanical behaviour and damage mechanisms of composite tubes under pure tensile loading, pure internal pressure, and combined loading

This series of papers on +/-55 degrees filament-wound glass-fibre/epoxy-resin tubes consists of three parts. In Part I, the microstructure of the composite tube was first investigated by image processing to qualify the microstructural defects produced during manufacturing. A series of mechanical tests was then carried out under various combinations of hoop and axial stresses to evaluate the tubes' mechanical behaviour under pure axial tensile load, pure internal pressure, and under combined loading. Observations on specimens loaded to 20-50% of the ultimate tensile strength showed that the main damage initiation mechanisms are microcracking (matrix and transverse cracks) and delamination. Depending on the loading conditions, one of the mechanisms dominates over the other. The effect of microstructural defects on the damage initiation was clearly demonstrated. In Part II, micromechanical modelling of the damage initiation will be conducted in order to determine (1) the mechanical conditions under which different microcracking mechanisms occur and (2) the critical sigma(zz)/sigma(theta theta) ratio which corresponds to the change from one mechanism to another. Emphasis is placed on assessing the influence of microstructural defects and the competition between the different mechanisms. In Part III, a macroscopical behaviour law for the tubing structure with and without damage will be presented, and the simulation results will be compared with experimental ones. (C) 1997 Elsevier Science Limited.