Inherent cohesive failure of epoxy adhesive in carbon-fiber-reinforced plastic composites revealed by micro-tensile testing and finite element analysis

Cohesive failure in the epoxy adhesive layers of carbon-fiber-reinforced plastic (CFRP) joints is incomprehensible because the adhesion interface with almost no covalent bonds is supposed to be the weakest in the joined structure. It is a long-standing unsolved issue that adhesive failure at the interface is a rare case. Cohesive failure in CFRP joints may be caused by micrometer-scale defects in their adhesive layers. In the aviation industry, one of the typical thermosetting molecular systems is a combination of the diglycidyl ether of bisphenol A (DGEBA) adhesive and the tetraglycidyl 4,4′-diaminodiphenylmethane (TGDDM) adherend, both of which are hardened with the 4,4′-diaminodiphenyl sulfone (DDS) curing agent. Eliminating the influence of microdefects, the fracture mechanism was studied using micro-tensile test specimens with gauge dimensions of 10 × 10 × 30 μm3, which were locally extracted from macroscale adhesion blocks. Two cohesive failure modes were predominantly observed with equal probabilities: 0.1–1 μm away from the interface and at the middle of the DGEBA section, except infrequent adhesive failure because of weak bonds. Cohesive failure is demonstrated to be an inherent behavior, which is consistent with the finite-element-method (FEM)-derived stress profiles, provided that the adhesion interface sustains the load stress. A phenomenological model for cohesive failure is proposed, which helps explain the fact that the elastically deformable adhesion interface with the physical bonding due to van der Waals forces is stronger than the adhesive and adherend hardened by both physical and chemical bonding. These findings lead to improvements in the reliability of CFRP components.