Shear properties of polyimide adhesives at various temperatures

In composite material structures, one of the primary limitations in minimising structural weight is the joining of various structural components. In composite structures joining is largely limited to employing adhesive bonding¹ or mechanical fasteners,² such as bolts or rivets, or possibly a combination of the two. Since mechanical fastening requires intermittent load paths, resulting in large stress concentrations, as well as the cutting of fibres to make the necessary holes, adhesive bonding is considered far more desirable for joining composite structural components to either other composite or metallic structural components.

Concerning various adhesive systems, in order to design, analyse and optimise bonded structures^3–13 it is necessary to know the mechanical properties of the adhesives in both shear and tension. Furthermore, it is necessary to know these properties for each temperature and moisture level to which the structure will be subjected, because the adhesives today are polymeric, and therefore subjected to ‘hygrothermal’ effects, i.e. the deleterious effects of combined high moisture and high temperature.^8,10,11

Unfortunately, not many of the moderate temperature adhesives, which are mostly epoxies, have been characterised. However, a few dozen have been characterised to some extent.^{11–13,15,16} One of the reasons for this lack of material property data is the lack of standardization of a suitable shear and tension test-piece configuration. However, more and more, the thick adherend shear test specimen is being accepted as the means to obtain shear properties because of the constancy of the shear across most of the length of the joint in this test configuration.^{11–13,15,16}