Role of Adhesive Layers on Crack Growth Resistance Under Mode‐I and Mode‐ <scp>II</scp> Loading in Co‐Cured Skin–Stiffener Joints

ABSTRACT Skin–stiffener interface is a key element in any advanced composites co‐curing technology. An adhesive layer is often used at this interface to improve out‐of‐plane characteristics. In this work, fracture toughness of skin–stiffener interface is examined under Mode‐I and Mode‐II loading conditions. The crack initiation and propagation characteristics are evaluated using specimens having multi‐angular stacking sequence with (Type‐2) and without (Type‐1) adhesive layer at the crack interface. The critical load and critical displacement are observed to be higher for Type‐2 (insert at adhesive‐CFRP layer interface) than that of Type‐1 (insert at CFRP‐CFRP layer interface) specimens. Results show that fracture toughness of Type‐2 specimens is 111% higher in Mode‐I and 170% higher in Mode‐II compared to Type‐1 specimens. Fractography analysis is carried out to understand the failure mechanism in both types of specimens. Type‐1 specimens predominantly showed fiber pull‐out, fiber‐bridging failure mechanism, whereas Type‐2 specimens showed reinforcement pull‐out and failure from adhesive layers along with fiber pull‐out and fiber failure.