A comprehensive finite element framework for the progressive damage analysis of fiber metal laminate bolted joints under static and cyclic loading: theory and experimental validation

To improve the robustness of fiber-reinforced plastics (FRP) in bolted joints, local metal-hybridization of the FRP laminate can be performed to obtain an FRP-based fiber metal laminate (FML) of increased bearing strength. This contribution presents the first modeling framework able to predict the entire fatigue degradation process of such FRP-based FML bolted joints from damage initiation until ultimate failure, including the safety-relevant bolted joint failure mode. For this purpose, each laminate constituent material (i.e. metallic inlays and FRP plies) is analyzed by an own physically motivated continuum damage model. To predict the fatigue crack initiation and growth in the metallic inlays, the framework deploys a newly developed continuum damage approach, which uses a cyclic elasto-plastic strain energy increment as primary fatigue metric. Based on its constitutive equations, the inlay damage model is able to predict a fatigue crack’s location and growth direction, both in the low- and the high-cycle fatigue regime. To account for the fatigue damage of the FRP plies, an energy-based fatigue damage approach for FRP composites, recently presented and validated by the authors, is applied. A comprehensive damage assessment is achieved as both fatigue models are accompanied by respective static damage models. The algorithms are implemented as one user-material (UMAT) subroutine in the commercial finite element software ABAQUS/Implicit. After the separate validation of the inlay damage model, the FML damage modeling framework is validated for open hole tension and T-bolt joint setups.