Influence of the bonding method on interlaminar fracture toughness for composite material non-symmetric configurations

The DCB test is the most widely used method to assess the quality of composite bonded joints. However, in non-symmetric configurations, experimental testing has shown that cracks can propagate cohesively, adhesively, or migrate, leading to varying fracture toughness values. This study examines the influence of bonding methods (co-curing and secondary bonding) on fracture toughness in non-symmetric joints with unidirectional composite laminates. To achieve this, both an experimental campaign and numerical analysis were conducted. It is obtained that, as long as the thinner adherent is previously cured and the thickness ratio between adherents is less than 75 %, crack migration will occur, reducing the value of the energy necessary to make the crack propagate. Cohesive failure resulted in fracture toughness values about 100 % higher than those of adhesive failure. This fact raises questions about the applicability of the fracture toughness obtained from the DCB test to joints in real structures, as the strength and stiffness of the joint depend not only on the adhesive but also on the adherents. Results from the meso-mechanical analysis and experimental data allow for defining a cohesive law for the adhesive joint, suitable for implementation in macro-mechanical models.