Externally Pressurized Hemispherical Fibre-Reinforced Plastic Shells

Abstract

The results of external pressure tests on ten 0.8 m diameter torispherical shells made from FRP (fibre-reinforced plastic) were discussed in a recent paper by the authors. In the present paper, the results of similar tests on six hemispherical FRP shells are given. Four of the hemispheres were made from single sheets of woven pre-preg (three of carbon-fibre-reinforced plastic and one of glass-fibre-reinforced plastic): two of those made from carbon-fibre-reinforced plastic (CFRP) were petalled, that is they were made by butt-jointing together pieces of composite cloth. The diameter-average thickness (D/tav) ratios of the hemispherical domes varied from 66 to 108. The BOSOR 4 program was used to predict the stresses in the composite shells and their buckling pressures; the Tsai-Wu equation (in stress space) was utilized for the material failure criterion. For all the shells in this investigation, the controlling failure mode appeared to be first-ply failure (FPF). The latter is, of course, influenced by the compressive strength of the composite. Assuming a reasonable value for this (for example 570 N/mm2 for CFRP) gave ratios of Pexpt/PFPF which were in the range 1.05–1.19. In all cases, the experimental result was higher than the theoretical prediction, that is it was on the safe side. The authors measured the compressive strength of the CFRP composite experimentally. They obtained values in the range 510 ± 100 N/mm2. These were less than values obtained at Imperial College (664 ± 44 N/mm2). At the moment, it is not known why this wide variation in compressive strength was obtained. More work needs to be done on this aspect of the problem. Each of the butt-jointed CFRP hemispherical domes was weaker than its unjointed counterpart. In part, this is due to the increase in thickness of the unjointed hemispheres near the clamped edge, where the failures originated. With the CFRP torispherical domes, the failure location was at the spherical cap/knuckle junction (that is away from the clamped edge) and the butt-jointed torispherical dome turned out to be stronger than its unjointed counterpart. The 30-ply CFRP hemisphere (D/tav = 66) was 1.4 times as strong as the 36-ply GFRP hemisphere (D/tav = 73). In addition, the 30-ply CFRP hemisphere had the same collapse pressure as a geometrically similar welded steel hemisphere which had D/t ≈ 113 and a = σYP = 645 N/mm2. The CFRP dome would be much ligher than the steel one, in the ratio 1:2.8.