Welding of bistable fibre-reinforced thermoplastic composite pipelines

This thesis addresses the issues associated with the transformation of a bistable composite slit tube, consisting of a thermoplastic matrix with glass fibres oriented in an antisymmetric lay-up, into a pipe by welding along the tube's longitudinal seam.

An in-depth review of 20 currently available thermoplastic composite joining techniques is presented. The six most promising are selected according to various criteria and a comparison is made between the tensile strength of welded single-lap samples for each technique and that of the parent material. It is found that laser welding and infrared (IR) welding are the most effective joining techniques with IR being the most applicable process for this application and will be investigated further. 

Two types of joint designs for the longitudinal seam of the bistable tube are described: single-lap and step-lap. A set of reference results for evaluation of the IR-welded samples is created from tensile tests of parent material single-ply and oven moulded single, five and six layer samples. It is concluded that the strength of the samples is significantly influenced by the 0± layer. 

The IR-welding technique and the welding process parameters are described in depth. An IR-welding machine is built and used for sample preparation. Tensile testing is carried out on IR-welded single-lap samples prepared using different parameter settings and compared with oven-moulded single-lap specimens. The results give a good indication of the performance of the IR-welding technique when each parameter is varied individually. The statistically and mathematically validated Design of Experiments approach is used to investigate the interaction between the IR-welding parameters when varied simultaneously. It is also applied to the optimisation of the IR-welding parameters. In addition, ¯ve and six layer step-lap joints produced using IR-welding are tensile tested. The most influential IR-welding parameters are heating time, heating power and dwell time.

The heat transfer taking place within the parent material during infrared welding is investigated with two cases of heat transfer being assessed and a simple one-dimensional analytical solution is proposed. The analytical results agree well with the experimental results and show that the rate of heat transfer of the parent material can be predicted reasonably accurately.