Mechanism-Based Modeling of Failure and Damage in Thick Multi-Layered Composite Structures

Since the last few years, the usage of unidirectionally fiber-reinforced composite (UD FRC) materials for structural components in the automotive industry is increasing due to definite advantages in comparison to traditional metallic materials. The most important advantage is the weight of this material based on its low density with an accompanying high specific modulus and high a specific strength as well as the adaptability to specific applications. Also, their layer-wise processing into laminates enables the realization of complex geometries with locally strongly differing properties. Also, the dimensioning of fiber-reinforced composite (FRC) laminates in terms of stiffness and strength is being included into efficient, computeraided engineering processes. In the current work, composites made of filament winding are focused on. This has become a popular construction technique in a wide variety of industries for creating composite structures including high pressure fuel storage tanks for hydrogen powered automobiles. This research will develop impact and crash simulations of the next generation fuel-cell vehicles with high pressure hydrogen storage vessels made of carbon fiber-reinforced plastic (CFRP) material. In the present work, a computer aided engineering (CAE) process chain is developed which consists of the virtual composite vessel generation, three-dimensional explicit finite element analysis of the composite vessel with multi-layered solid elements and the constitutive model. The major focus is on the development and implementation of material equations for the CFRP to capture the intralaminar failure and postcritical behaviour. Presently, commercial finite element (FE) tools do not offer the possibility to represent a stack sequence of a composite material in solid elements. The goal of this thesis is to develop a multi-layered solid element to represent several plies with their winding angle in one solid element. The above new element formulation is presented in this thesis describing simulation results for different patches and analytical solutions. Also, this thesis presents a three-dimensional constitutive model for anisotropic damage to describe the elastic-brittle behavior of unidirectional fiber-reinforced laminated composites. The primary objective of the thesis focuses on the threedimensional relationship between damage of the material and the effective elastic properties for the purpose of stress analysis of composite structures. Damage initiation criteria are based on the Puck failure criterion for first ply failure and the progressive micro-crack propagation is based on the idea of continuum damage evolution. Internal variables are introduced to describe the evolution of the damage state under loading and as a consequence the degradation of the material stiffness. In order to assess the predictive capabilities of the proposed multi-layer solid element and the constitutive model, nonlinear finite element simulations are conducted. Thereby, different material systems, various laminate layouts, complex loading scenarios and the structural response are considered. The predictions are discussed in detail and compared to experimental results in order to validate the computational method.