Design of adhesive joints at high environmental temperature

Abstract

When adhesive joints are exposed to high environmental temperatures, the tensile load capabilities of adhesive joints decrease because the elastic modulus and failure strength of rubber toughened structural epoxy adhesives decrease. Therefore, adhesive joints have been restricted to low environmental temperatures although they have many advantages over mechanical joints. In this thesis, the mechanical properties of rubber toughened structural epoxy adhesives were investigated with respect to filler content and environmental temperature. To predict the stiffness of filler containing epoxy adhesives with respect to filler content and environmental temperature, the modified Halpin equation and a thermal degradation function were proposed. Also several means to improve the performance of adhesive joints at higher temperatures such as addition of fillers in the adhesive, modification of the cure conditions of adhesive joints and the surface treatments of adherends were investigated. In order to estimate the filler effects, the coefficient of thermal expansion of adhesive layer was calculated and a crack propagation model in the filler containing adhesive layer were proposed to predict the tensile load capabilities of the adhesive joint. From the tensile tests and in-situ cure monitoring of the adhesive joint, it was revealed that the tensile load capability of the adhesive joint was much dependent on actual cure finish temperature rather than autoclave inside temperature. For the investigation of the adhesion characteristics of plasma surface treated adherends, the surface free energies of plasma surface treated adherends and the load capabilities of the plasma surface treated adhesive joints were measured with respect to gas flow rate, chamber pressure, power intensity, and surface treatment time. The chemical bonding mechanism between the adhesive and adherend was analyzed with XPS (X-ray Photoelectron Spectroscopy), from which the load capabilities of the adhesiv...