Scaling issues in resistance-welded thermoplastic composite joints

Resistance welding has been demonstrated as a viable technique for joining thermoplastic composites. Extensive research has been conducted in previous studies on the processing and postprocessing evaluation of coupon‐sized resistance welded specimens using this thermoplastic fusion bonding technique. This work focuses on the manufacturing and evaluation of large‐scale resistance welds used to join carbon fiber (CF)/polyetheretherketone (PEEK) thermoplastic composite parts. Applications using the present resistance welding technology are assessed, and potential applications of resistance welding are discussed. The sequential resistance welding process is introduced as an approach to large‐scale welding that increases joint quality and performance while maintaining modest pressure and weld power requirements. The single‐step and alternate multiple‐step (sequential) techniques are implemented for large‐scale resistance welds of CF/PEEK adherends comolded with polyetherimide (PEI) (i.e., the Thermabond® process). Nondestructive ultrasonic evaluation, mechanical testing, fractography, and microscopy show that: (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 shear strength; direct comparison of the weld quality and lap shear strength shows that the sequential resistance welding process yields a superior bond. With the principles developed in this investigation, resistance welding can be used to join large‐scale thermoplastic composite parts with consistent, high levels of performance and quality. © 1997 John Wiley & Sons, Inc. Adv in Polym Techn 16: 279–295, 1997