The effect of imperfect bonding on the pull-out behavior of non-crimp fabric Pi-shaped joints

The present work, investigates the effect of imperfect bonding, owing to partial lack of adhesive (voids), on the pull-out behavior of the novel non-crimp fabric (NCF) Pi-shaped joints. Investigation was conducted in two steps: inspection of bonding quality and prediction of joint’s mechanical behavior. Quality of the bondline was checked using ultrasonic testing. Based on the geometry of the Pi joint, different scan areas were defined in order to effectively capture the quality of the entire bonded area. C-scan images reveal a significant amount of voids in the specimens tested. The mechanical behavior of the joints was predicted using a mesomechanical model based on the FE method and progressive failure analysis. Implementation of the model is based on the homogenized behavior of the NCF material. In the frame of the current application, a simplified approach for evaluating the behavior of the NCF multi-layered material given the homogenized behavior of the constituent layers is presented. In the model, the inspected imperfectly bonded areas were given as input on the basis of a stiffness degradation scheme. Simulations reveal that the effect of imperfect bonding depends on the location of the voids and the loading condition. For the present case, a reduction in joint stiffness, which enlarges as the load increases, and a small reduction in pull-out strength have been predicted. Predicted failure mechanisms and failure load are in good agreement with relative experimental results.