Effect of temperature on the quasi-static strength and fatigue resistance of bonded composite double lap joints

Fibre reinforced polymer composites (FRP`s) are often used to reduce the we ight of a structure. Traditionally the composite parts are bolted together. however, increased weight savings can often be achieved by adhesive bondin g or co-curing the parts. The reason that these methods are often not used for structural applications is due to the lack of trusted design methods an d concerns about long-term performance. The authors have attempted to addre ss these issues by studying the effects of fatigue loading, test environmen t and pre-conditioning: on bonded composite joints. Previous work centered on the lap-strap joint which was representative of the long-overlap joints common in aerospace structures. However. it was recognised that in some app lications short-overlap joints will be used and these joints might behave q uite differently. In this work, double-lap joints were tested both quasi-st atically and in fatigue across the temperature range experienced by a jet a ircraft. Two variants on the double-lap joint sample were used for the test ing, one with multidirectional (MD) CFRP adherends and the other with unidi rectional (UD) CFRP adherends. Finite element analysis was used to analyse stresses in the joints. It was seen that as temperature increased both the quasi-static strength and fatigue resistance decreased. The MD joints were stronger at low temperatures and the UD joints stronger at high temperature s. It was proposed that this was because at low temperature the strength wa s determined by the peak stresses in the joints, whereas. at high temperatu res, strength is controlled by creep of the jointswhich is determined by th e minimum stresses in the joint. This argument was supported by the stress analysis.