SCALING ISSUES IN RESISTANCE-WELDED THERMOPLASTIC COMPOSITE JOINTS

Resistance welding has been demonstrated as a viable technique for joi ning thermoplastic composites. Extensive research has been conducted i n previous studies on the processing and postprocessing evaluation of coupon-sized resistance welded specimens using this thermoplastic fusi on bonding technique. This work focuses on the manufacturing and evalu ation of large-scale resistance welds used to join carbon fiber (CF)/p olyetheretherketone (PEEK) thermoplastic composite parts. Applications using the present resistance welding technology are assessed, and pot ential applications of resistance welding are discussed. The sequentia l resistance welding process is introduced as an approach to large-sca le welding that increases joint quality and performance while maintain ing modest pressure and weld power requirements. The single-step and a lternate multiple-step (sequential) techniques are implemented for lar ge-scale resistance welds of CF/PEEK adherends comolded with polyether imide (PEI) (i.e., the Thermabond(R) process). Nondestructive ultrason ic evaluation, mechanical testing, fractography, and microscopy show t hat: (1) a higher weld uniformity was obtained with the multiple-step weld than the single-step weld; and (2) uneven heating and overheating were minimized by reducing the heating element length within the bond region through the use of the sequential resistance welding process. The different welding techniques produce a large variation in lap shea r strength; direct comparison of the weld quality and lap shear streng th shows that the sequential resistance welding process yields a super ior bond. With the principles developed in this investigation, resista nce welding call be used to join large-scale thermoplastic composite p arts with consistent, high levels of performance and quality. (C) 1997 John Wiley & Sons, Inc.