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

Abstract

A prominent critical problem with the use of adhesively bonded composite scarf joint in aircraft structures is their susceptibility to unpredictable failure. Any efficient and robust design approach for adhesively bonded scarf joints under fatigue loading should account for the damage mechanisms in the joint and this cannot be achieved without understanding damage development with initiation and propagation. Research into damage development using methodical approach were conducted on the stiffness of polymer adhesively bonded composite scarf joint since it is the prime physical parameter that can be measured most accurately. The research produced significant new knowledge of damage development under fatigue as strain hardening and a methodology to characterize damage initiation and propagation in adhesively bonded joints. Under uninterrupted fatigue, stiffness degradation and plasticity effects led to strain hardening of the joint adhesive until failure. But there was in-homogenous stiffness degradation in regions within the joint due to defects and anomalies so that the general stiffness degradation of the scarf joint could be either a stiffness increase or decrease to failure.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.