Modeling of CFRP hybrid lap joints via energy-based 2D framework

This paper presents an energy-based approach to develop a spring-based (semi-analytical) reduced-order model for the mechanical behavior (stiffness, load carrying capacity) of a hybrid (bonded/bolted) single-lap joint with carbon fiber-reinforced polymer (CFRP) laminates when subjected to tensile load. More clearly, the hybrid joint is modeled as an appropriate combination of springs, where their stiffnesses are determined with a deformation energy framework. The proposed model can predict the different failure modes in the hybrid joint with greater accuracy, starting with the disbond of the adhesive layer, followed by damage in CFRP laminates due to the bearing load via bolt, on subsequent loading. In this study, three CFRP ply orientations are considered, i.e., quasi-isotropic ([0 45 90-45]s), uni-directional ([0]8), and cross-ply ([0 90]2s). The damage modes in the adhesive are modeled using a bilinear cohesive law, and those in CFRP laminates are modeled using Hashin's damage initiation criteria. A linear degradation law is used to determine the degraded material properties of the CFRP laminate. The individual spring stiffnesses are solved by a developed 2D FE solver. The proposed framework is validated with commercial 3D FEA and experimental studies. Finally, certain design recommendations are provided for the hybrid joint based on the proposed model. The use of energy framework enables the model to be extended for fastened joints with complex geometries while not involving any empirical relations. Also, the generic nature of the model can aid in the modeling of various joint configurations, such as multi-bolted and hybrid-multi-bolted joint configurations.