An analysis of mechanisms governing fusion bonding of thermoplastic composites

A number of mechanisms have been proposed in the literature as contributors to the strength development at the polymer-polymer interface during fusion bonding of thermoplastic composites. Of these, healing and intimate contact emerge as fundamental mechanisms governing bonding. Intimate contact refers to the development of the amount of surface area that is physically contacted at the interface at any time, and healing describes the migration of polymer chains across the interface in intimate contact. This work provides a new theoretical development of a coupled bonding model that accounts for variability in initiation time for healing due to growth in the area in intimate contact. The generalized coupled bonding model is valid for any set of processing conditions and reduces to the proper controlling mechanism as dictated by the process. Analysis revealed a key dimensionless group, Omega, that captures the coupled nature of the mechanisms governing fusion bonding. By evaluating Omega which is a function of material and process parameters, one can determine the relative contributions of each mechanism. Experimental validation of the coupled model using two different processes, tow placement and resistance welding, is also presented. An evaluation of Omega for the tow-placement process indicates that both mechanisms are controlling. For this case, the coupled model demonstrates better strength predictions than the conventional healing model alone, in contrast, the resistance welding process is shown to be intimate-contact controlled, in which case the coupled model reduces to a more simplified model. The ability to rigorously determine the controlling mechanisms is of critical importance to accurately model the strength development during fusion bonding processes.