Since the efficiency of a structure is established, in general, by its joints, the design of the joint in the assembly of separated parts has become an important research area. There are two kinds of joints such as a mechanical and an adhesively bonded joints. The adhesively bonded joint has been increased recently, because it can distribute the load over a larger area require no hole, add very little weight to structure and have superior fatigue resistance.
But the mechanical properties of the adhesively bonded joint are affected by service environments, because the properties of adhesive usually degrade as environmental temperature and humidity increase. When the adhesively bonded joint is exposed to hygrothermal environment, high humidity and temperature increase the volume of adhesive and decrease the glass transition temperature of adhesive. Since structures having joints are often exposed to hygrothermal environment in practical uses, the investigation of hygrothermal effects on the joint is indispensable for reliable design of structures.
In this thesis, adhesives and adhesively bonded joints were investigated experimentally and theoretically in order to predict the joint strength reduction under hygrothermal conditions. Using disk type and dog bone type adhesive specimens, diffusivity, maximum specific moisture concentration, moisture swelling coefficient, tensile modulus and tensile strength of the adhesives were tested with respect to the moisture content and temperature. Also chemical surface treatments of adherend for the improvement of strength retention of the joint under hygrothermal environments were investigated. From the investigation, it was found that chemical surface treatments improved the joint strength much. Using the adhesively bonded tubular single lap joint, the torque capacity and tensile load capacity were experimentally obtained with respect to the hygrothermal condition. From the experiment, it was found that the strength of the adhesively bonded joints were decreased in accordance with the absorbed moisture content and the steel adherends exposed to humid environments for a long time rusted.
In order to predict the joint strength under hygrothermal environment, the software and the failure model were developed with considering hygrothermal properties and the nonlinear behavior of the adhesive. When the absorbed moisture content of the joint was low, the calculated joint strength from the developed software agreed well with experimental result. But the calculated joint strength was a little lower than the experiment when the absorbed moisture content was increased, which might be caused by the friction between the failure surface and the swelled adhesive.