CONSIDERING ENVIRONMENTAL CONDITIONS IN THE DESIGN OF BONDED STRUCTURES: A FRACTURE MECHANICS APPROACH

The continued airworthiness of ageing aircraft and long-term durability of new airframes depends, in part, on the integrity of adhesive bonds used for repairs and joining structural components. Additionally, the advent of composite materials and advanced repair techniques incorporating composites has increased the number of adhesively bonded joints specified for use in aerospace structures. Traditionally, adhesive bonds have been analysed and designed using a dependable and rigorous stress-based approach. However, the need to address the effect of bondline flaws and to understand the fatigue characteristics of bonded joints has led to the adoption of a discipline already common in the design of metallic components-fracture mechanics. To understand the durability of bonded structures, however, it is further necessary to examine the effect of environmental exposure on the performance of the adhesive bondline. Familiarity with the stress-based and fracture mechanics analytical approaches, as well as an understanding of environmentally induced trends in bond performance is paramount to quality design. This paper will briefly discuss the attributes of the two main forms of bonded joint analysis, and will broadly outline a design approach that uses fracture mechanics and accounts for environmental effects. Experiments discussed in this paper were performed specifically to use fracture mechanics in assessing the environmental effects on a toughened epoxy adhesive. Results indicate that the Mode I fracture toughness and fatigue crack growth threshold of this adhesive are significantly reduced upon exposure to a high temperature, high humidity aircraft service environment. These results will be used to illustrate the philosophical arguments supporting the design approach.