Towards Optimum Carbon-Fibre-Reinforced Plastic End Closures

Abstract

Two series of external pressure tests on six-layer CFRP (carbon-fibre-reinforced plastic) torispherical domes are discussed in the paper. One series had a diameter-average thickness ratio of 80 and the other 120. The ratio R_s/D was the same for all the models and equalled 0.55 (R_s = spherical radius of a dome). The ratio r/D (r = toroidal or knuckle radius) varied from 0.2 to 0.5, the latter being a hemisphere. All the models were made by the hand lay-up/vacuum bag technique using male moulds.

The failure modes considered were bifurcation buckling and first-ply failure (FPF). The BOSOR 4 program was used to evaluate the theoretical stresses in the shells and various failure criteria (Owen, Tsai-Wu etc.) were employed to determine the FPF pressures. In the tests, all the models failed by material failure rather than shell buckling and the ratios of the experimental collapse pressures to the FPF predictions were in the range 0.85–1.30.

The tests also confirm a recent theoretical prediction that there is an optimum value of the r/D ratio insofar as dome strength is concerned. These CFRP torispherical domes are lighter and stronger than the comparable CFRP hemispheres.

The BOSOR 4 predictions of FPF pressures and failure locations were confirmed by the NISA finite element code. The calculations also showed that it is necessary to use finite deflection shell theory when analysing these composite domes.

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