Combining different interface features for enhanced performance in metal/CFRP joints

Abstract

The design of suitable bonding interfaces is crucial for the performance of joints between dissimilar materials in aerospace applications, e.g. metal/fiber-reinforced plastic (FRP). Adhesive bonding derives from the interaction of the polymer matrix of FRP with the joint surface of the metallic part and can be enhanced by surface pretreatments. Interface designs incorporating pins can change the damage mechanism of the joint from a one-stage to a two-stage failure behavior, allowing for a residual strength that provides a safety margin and allows damage detection and repair. We focused on the development of new metal /FRP interfaces exploiting the synergy of laser treatments with metal additively manufactured pin structures for structural bonding. The studies included laser surface pretreatments and the design and manufacturing of pin structures. Single lap shear samples with different pin configurations and a laser surface pretreatment were tested mechanically to obtain joint strengths as well as resistance to degradation by humidity. The tests were complemented by microanalytical surface and interface characterization. The hybrid joints behaved qualitatively similarly for thermoplastic FRP and epoxy FRP systems and showed that thicker pins lead to increased static strengths: a cohesive failure in the middle of the FRP laminates was found typically whereas shearing of the pins did not occur. The failure behavior was multi-stage and can be correlated with interface features through digital image correlation of the strain fields. While a laser treatment increased the bond strength further, its primary role was to ensure that aging only exerted a minor influence.