Effects of processing conditions on bondline void formation in vacuum bag only adhesive bonding: Modelling, validation and guidelines

Analytical modelling and adhesive characterizations were used to predict void nucleation and growth in an adhesive film under ideal, and then realistic conditions of pre-bond moisture, consolidation pressure, and curing temperature. Void growth evolutions were measured in-situ by means of an instrumented transparent tool plate. Void development observations were found to be in close agreement with the analytical estimations obtained by a water diffusion-based model. The results identified the source of void formation: pre-bond moisture, while adhesive solvent was not found to cause void nucleation for the studied adhesive film. The findings also highlight the sensitivity of void formation to process variations. For example, the constructed processing maps identified that for very low content of pre-bond moisture (as received ~0.1 wt%), moisture-induced voids can nucleate and grow in the adhesive if the temperature exceeds the recommended cure temperature or if the applied pressure is reduced, for example, in repair shops at altitude. Processing widows are proposed to provide guidelines to adhesive bonding practitioners.