Effects of Static Loading and Environmental Conditions on the Aging Behavior of Aluminum Adhesive Joints

Abstract

During service, adhesive joints will be exposed to different aging environments, which tends to decrease the durability and the strength of the joint. Humidity and temperature are the most dominant environmental factors causing weakening of the joints. In the presence of static or dynamic loading, the weakening may be accelerated and cause an unexpected failure. Many studies have been carried out to determine the influence of the environmental factors (mainly temperature and humidity) or mechanical loading (cyclic or static) on the durability of the adhesive joints. Theoretical predictions concerning the combined effects of the environmental factors and mechanical loading on the aging behavior of the adhesive joints can be found in the literature, but there are no extensive experimental studies on this subject. Greater understanding of these combined effects is needed, and research work in this area gives better tools for predicting the durability of adhesive joints under different aging conditions.<br/><br/>This thesis studies the combined effects of static loading and environmental aging (temperature and humidity) on the strength and durability of aluminum adhesive joints. The strength behavior and durability of the studied joints aged in different aging environments with different static loading levels are determined. The influence of different pretreatment methods on the strength and durability of the studied joints is also determined. The fracture surfaces of the joints are studied to reveal the fracture type.<br/><br/>This study verifies the existence of a threshold stress for all specimens and all studied aging combinations. If the loading stress exceeds this threshold stress value, the joint strength drops quite rapidly to zero. The knowledge of the existence of such threshold stress is extremely important when designing the adhesively joined structures for conditions leading to aging. The threshold stress level is quite low, depending on the aging conditions and the joint pretreatment method. Typical threshold stress values measured in this study are 5% to 10% of the original unaged strength of the joint. Under milder aging conditions and with efficient pretreatment methods, the threshold stress value can be 10% to 15% of the original joint strength. For more severe aging conditions and less efficient pretreatment methods, the threshold stress level can be from 0% to 5% of the unaged joint strength.<br/><br/>Silane pretreatment of the adherend surfaces before joining enhances the durability of the joints. It also increases the threshold stress level when compared to specimens pretreated only by sand blasting and degreasing. The initiation of the rapid strength decrease and failure of the joint when loaded above the threshold stress level are moved towards longer times with the silane pretreatment of the specimens. Silane pretreatment enhances the joint durability especially in high humidity and high temperature environments.