Behaviour of composite pipes under multi-axial stress

Abstract

This thesis describes an experimental investigation of the behaviour of filament wound glass fibre reinforced epoxy (GRE) composite pipe under hydrostatic and biaxial load conditions at temperatures up to 95⁰C. The project was intended to lead to improvements in reliability and quality, and ultimately a reduction in the cost of qualifying GRE oil and gas pipelines. The experiments were designed to be compatible with the procedure currently used by Future Pipe Industries (FPI), employing the concept of ultimate elastic wall stress (UEWS) in the qualification and production control of GRE pipe. The UEWS test appears to provide an attractive means of rating GRE pipes, where weepage resulting from the accumulation of matrix cracks is a common failure mechanism. A novel test rig capable of performing UEWS tests under various loading conditions from hydrostatic to multi-axial loadings was designed and developed. UEWS tests were conducted under six different stress ratios ranging from pure axial to pure hoop loading at room temperature (RT), 65°C and 95°C. The tests involved the application of groups of ten 1-minute hydrostatic pressure cycles at increasing pressure levels. The intention is to identify, by examining the stress-strain response, a stress level below which damage growth is either negligible or at least sufficiently low to prevent long term failure within the design life. In addition, acoustic emission measurements were also conducted to investigate the nature of the damage mechanisms involved as well as its compatibility to the UEWS results. Three distinct failure modes were observed: tensile axial failure under pure axial loading, weepage under axial dominated loading from 0.5:1 to 2:1 and localized leakage failure under hoop dominated loading of 4:1 and 1:0. Full tensile-tensile UEWS and leakage based failure envelopes were developed at a range of temperatures from 20°C (RT) to 95°C. Both envelopes showed a strong dependence on stress ratio and test temperature. It was also shown that the UEWS based failure envelope at elevated temperatures generally degraded, except for the 2:1 loading where UEWS strength increased. The Miner‟s law model developed, gives a good account of the effects of cyclic and static loading in UEWS tests. Using a crack growth model similar to Paris Law, damage development can be directly linked to the progressive nucleation of matrix micro cracks. It is also shown that cyclic rather than static loading dominates the UEWS test response. The general lifetime damage model developed in the study shows good agreement with the experimental data from the multiaxial UEWS tests. This approach may therefore be an appropriate procedure for describing the long term performance of GRE pipes under any required combination of static, cyclic fatigue, hydrostatic and non-hydrostatic loading.