Environmental Effects on Bonded Graphite/Bismaleimide Structural Joints

Abstract

There are many advantages to using adhesively bonded joints for aerospace applications, but their potential is not yet realized because not enough is known about the long-term durability of the polymeric adhesives. This paper focuses on the long-term durability of a toughened epoxy used to bond graphite/bismaleimide composites. Double cantilever beam (DCB), end-notched flexure (ENF), and cracked lap shear (CLS) tests were conducted to determine the fracture toughness of the adhesive system before and after environmental exposure. These specimens were exposed to either a hot/dry environment or thermal cycles between -54°C (-65°F) and 104°C (220°F). Unexposed and exposed specimens were tested at room temperature and it was found that the hot/dry environment lowered the fracture toughness of the joints. For the DCB and CLS geometry, unexposed specimens were tested at reduced and elevated temperatures. For some of the DCB and all of the CLS specimens, it was found that the cold temperature reduced the fracture toughness. In order to determine if the environment and temperature combined affect the toughness, the environmentally exposed DCB and CLS specimens were tested at the cold temperature. This combination of cold temperature and environmental exposure significantly lowered the fracture toughness of the material in all adhesive joints tested.