Recent progress on damage mechanisms in polymeric adhesively bonded high-performance composite joints under fatigue

The changes in the rate of stiffness over the fatigue life of a scarf joint reveal three characteristic regions. Region I & II marked the damage initiation life of scarf joints and this was (66 ± 9)% of the fatigue life for specimens tested, and Region III marked the onset of damage propagation to failure. Mean strain effect was evident in some scarf joint under fatigue cycle. It is shown that the damage initiation & propagation life under variable and constant fatigue loading for scarf joints is independent of the adhesive thickness, the fatigue load application frequency, and the severity of the applied load. Damage propagation in adhesively bonded scarf joint was found to be a combination (1) strain hardening of the adhesive layer leading to brittle cracking of the adhesive layer (2) cracking of the adhesive layer at 45° or 90° to the axial loading, leading to break in stress distribution in the adhesive joint as it matures with strain hardening (3) leading to gradual quasi-static failure of the joint particularly since there was no evidence of direct residual fatigue pattern on the fracture surface either in this research work or from any existing literature for similar loading situation. In all, the paper addressed one of the most critical and important steps that will drive correct quality implementation of design for fatigue in polymer adhesively bonded joints towards addressing the current airworthiness limitations on use of these joints in the aerospace industry and further to expand the scope of application of adhesively bonded joints.