Modelling of the viscoelastic behaviour of steel reinforced thermoplastic pipes

This paper describes the analysis of the time dependent behaviour of a steel reinforced thermoplastic pipe. This new class of composite pipes is constructed of a HDPE (high-density polyethylene) liner pipe, which is over wrapped with two layers of thermoplastic tape. The thermoplastic tapes are reinforced by high strength steel strands. Although the steel strands, carrying the larger part of the loadings, behave in a linear elastic way, the steel reinforced thermoplastic pipe (s-RTP) shows considerable time dependent behaviour, which needs to be analysed and described. An existing model, in which a similar type of pipe is modelled as a multi-layered thick cylinder is modified and extended to take into account the time dependent behaviour of the HDPE. The actual viscoelastic deformations were found to be very sensitive to small variations of the angles of the reinforcement layers. The extended model shows very good agreement with the experimentally determined strains on a prototype s-RTP, subjected to an internal pressure. The model is expected to be suitable for the prediction of the time to failure of a s-RTP as a function of the pipe configuration and specific load-case.