Experimental and numerical investigation of the influence of imperfect bonding on the strength of NCF double-lap shear joints

The influence of imperfect bonding, owing to partial lack of adhesive, on the strength of composite non-crimp fabric (NCF) double-lap shear (DLS) joints was experimentally and numerically investigated. Fabrics were layered and compacted using a thermoplastic veil while infiltration of the preforms was done using the vacuum assisted process. Paste adhesive bonding was carried out by implementing the novel insertion squeeze flow process. Quality of adhesive bonding was tested using X-ray imaging and ultrasonic C-scan inspection. The tensile lap shear strength of the DLS joints was determined experimentally. Digital macrographs revealed that the specimens failed due to shear failure of the adhesive (debonding) and fracture of the composite boundary layer. As a second approach, a mesomechanical model based on the FE method and the (homogenized) progressive failure analysis method was developed. In the model, the areas without adhesion, as detected by the C-scans, were included. Numerical simulations of failure initiation and progression at the NCF joint and the adhesive indicated that it is possible to predict the strength and failure mechanisms of the imperfect bonded DLS joints.