Analysis of Wood Bonding Failures that Initiated Before Adhesive Solidification: Air Fingers and Cavitation

Abstract

Bond line openings while an adhesive is still viscous (liquid or paste-like) leave characteristic marks. Air fingers and cavitation develop on the surface of the adhesive when joint parts are fully or partially separated before solidifying of the adhesive. Therefore, the observation of air fingers or cavitation provides important clues on the root cause of the failure of bond lines. There is still limited knowledge about the factors that lead to the formation of either air fingers or cavitation. Additionally, a resoftening of thermoplastic adhesives by high temperatures may be confused with air fingers or cavitation that developed before the initial curing. To improve the understanding of the adhesive structures, we assemble some experiments on this phenomenon. Air fingers dominate when air easily penetrates the adhesive, the adhesive thickness is high, and the adhesive has a low viscosity (early in the process). Cavitation dominates when the adhesive layer is thin, when the viscosity is high (late in the process) or when the air ingress is restricted.