Experimental investigation of the resistance welding for thermoplastic-matrix composites. Part I: heating element and heat transfer

A comprehensive experimental investigation of the resistance welding of carbon-fibre and glass-fibre reinforced polyetherimide (PEI) laminates is presented. Lap-shear and double-cantilever-beam specimens were resistance welded by using fabric and unidirectional heating elements. The statistical distribution of the resistance of the heating elements was characterised, and the effects of the temperature on the heating element resistance were evaluated. The influence of the mechanical contact pressure on the contact resistance was also investigated. One of the main process parameters in resistance welding, i.e. the power density, was studied in detail. Heating uniformity in the heating elements was assessed through thermal imaging, allowing for a comparison between fabric and unidirectional heating elements. Temperature histories were measured and compared to those simulated by a three-dimensional transient heat transfer finite-element model. Factors limiting the size of the welded joint, i.e. temperature non-uniformity within the welding area and current leaking to the laminate, were investigated. Proper electrical insulation using a glass-fibre/PEI interlayer between the heating element and the laminate when joining carbon-fibre/PEI laminates effectively eliminated current leaking and enabled large-scale resistance welding.