Understanding the mechanical properties of fibre-matrix interfaces in flexible composite pipe materials

Abstract

There is an increasing demand for flexible thermoplastic composite pipes to be used in subsea oil and gas fields where production fluid temperatures and hydrostatic pressures are high. However, there is also concern that under those conditions, fibre-matrix interfaces within the pipes will gradually degrade, causing the pipe to become prone to collapse under hydrostatic pressure. Therefore, this thesis reports investigations of how the strength of interfaces in thermoplastic composite pipes changes due to prolonged exposure to water, oil, and elevated temperatures. The primary materials of this investigation were glass fibre-reinforced polypropylene (G/PP) and carbon fibre-reinforced polyamide 12 (C/PA12). Those materials were studied as they were received, as well as after they had been aged in water and oil (represented by a 50/50 pentane/toluene solution). In one approach, interfaces within the composites were incrementally damaged through single fibre push-out tests to simulate the effects of kink band propagation during the compressive failure of unidirectional composite laminates. In another approach, hereby named the hybrid-push test, fibres were pushed into a soft substrate to evaluate novel ways of analysing the strength of interfaces. A key finding was that the strength of a fibre-matrix interface may decline or improve depending upon the type of materials from which it is comprised and the environment in which the host composite is aged. Also, the rate at which axial load was applied to fibres in G/PP was positively correlated with the shear stress required to cause interface failure. Another finding was that the interfacial shear strength of G/PP reduced at elevated temperatures and recovered at room temperature. Moreover, interface shear strength values were lower in regions of high fibre density, interfaces degraded asymmetrically around the circumference of fibres, also distinct interface failure modes were observed and named as pulloff and pulldown failure modes.