Experimental investigation of the resistance welding of thermoplastic-matrix composites. Part II: optimum processing window and mechanical performance

An experimental investigation of the resistance welding of carbon-fibre and glass-fibre reinforced polyetherimide laminates is presented. The optimum resistance welding time based on a criterion of maximum lap shear strength was determined. The time required to achieve intimate contact predicted by a three-dimensional transient finite-element model featuring heat transfer and consolidation correlated well with the optimal welding time. The influence of the welding pressure on lap shear strength was investigated, and the consolidation quality obtained in the welded joint was related to the processing conditions. The extent of flow occurring during welding, or the reduction of thickness of the welded joints, was shown to be related to lap shear strength. Four failure mechanisms, leading to different values of lap shear strength, were identified including interfacial failure, cohesive failure of the heating element, tearing of the heating element and tearing of the laminate. Experimental and numerical processing windows were constructed and correlated well to each other. A comparison between fabric and unidirectional heating elements, in terms of lap shear strength and the interlaminar fracture toughness, GIc, was performed. It was demonstrated that large-scale lap-shear coupons and double cantilever beam specimens can be resistance welded providing that current leaking to the laminate is avoided.