Fatigue life improvement mechanisms of CFRP/Al hybrid joints – Load sharing study using a digital image correlation technique

In comparison with an adhesive joint, a hybrid joint often provides a more effective and reliable way to meet heightened safety requirements in engineering like FAA AC 20-107B. Nevertheless, its fatigue failure characterization, life improvement mechanism and engineering design approach have been understudied as compared to the static loading condition. This study aims to characterize the failure mechanisms by use of the digital image correlation (DIC) technique for elucidating the load sharing phenomenon of hybrid joints in the course of fatigue failure. In the study, bonded, riveted and hybrid joints were tested under static and fatigue loading conditions. To explore different mechanical behaviors of hybrid joints, two types of adhesives with an evident difference in modulus were selected to fabricate the joints. The fatigue life of an adhesive layer and that of the hybrid joint were separately studied to explore the means for life improvement. It is shown that the fatigue life of the hybrid joint can be significantly improved as a result of load sharing by rivets. The improvement of the fatigue life could reach 5 to 13 folders for the hybrid joint due to the load sharing, whereas its improvement ratio was only 1 to 2 times when the adhesive and the rivet were independent. By considering the overlap area as an effective region for design of rivet group, an optimal stiffness can be obtained for the hybrid joint. This study clearly presents two completely different fatigue failure processes for hybrid joints and illustrates the improvement mechanisms of fatigue life, thereby providing useful guidance for optimal design of hybrid joints.