Modeling and simulation of a metal-CFRP joint with 3D anchors

The integration of different materials into a single structure is revolutionizing structural design by exploiting the specific properties of each material reducing weight and mitigating environmental impact. Carbon fiber reinforced polymer (CFRP) composites contribute to this goal thanks to their high strength-to-weight ratio and anisotropic properties, allowing for optimized, lightweight components. CFRP is often required to be combined with metals that offer complementary isotropic properties and thermal tolerance. Hence effective joining techniques are essential for the durability of these hybrid structures. Traditional joining methods, such as adhesive bonding and mechanical fastening, have limitations. Adhesive bonding requires careful surface preparation, maintenance difficulties, and the use of hazardous chemicals, while mechanical fastening can damage fibers and create stress concentrations. One promising innovative method involves creating 3D anchors on the metal part to form a mechanical interlock with the fibers. This thesis focuses on the microscale FEM analysis of metal-CFRP joints with 3D anchors through the identification of a representative volume element (RVE). Several models of increasing complexity are developed, from a single-layer model to a complete 3D model of the anchor, aiming to predict complex failure phenomena through computationally efficient models. The objectives are to evaluate the effect of fiber wrapping on joint stiffness, understand the local stress state, predict potential failure mechanisms, and validate the model with experimental results from tensile tests. Findings indicate that closer proximity of the fiber to the pin in the wrapping results in greater joint stiffness. In the 3D model, stress measurements along an axis parallel to the pin, progressively distanced from the base, converge towards values comparable to those from the single-layer model, validating its effectiveness for an initial stress analysis. The highest stress regions are at the interface between the pin and matrix at the front and rear of the pin, where tensile stress can lead to mode I decohesion. Near the pin, the fibers experience a gradual reduction in tensile stress due to load transfer to the pin through the matrix. Instead, between two pins the fibers experience high tensile stress, potentially causing fiber breakage. These predictions align with experimental observations, where failed specimens show mode I decohesion and fiber breakage. These findings can help designers enhancing the load-bearing capacity and prevent premature failures in hybrid metal-CFRP joints. For instance, ensuring a tight wrapping to the anchor reduces the resin-rich zone, achieving a stiffer joint. The shape of anchors can be optimized to relieve stress concentrations at the interface to prevent mode I separation. It can also be designed to facilitate load transfer between fibers and the pin thus reducing fiber breakage. Moreover, comparing configurations with aligned longitudinal fibers and perpendicular fibers provides insights into the best layout to achieve the desired structural goals. Finally, technologists should prevent voids inside the matrix, especially at the anchor interface, where failure is more likely. This comprehensive approach provides a broad overview for understanding and improving the performance of metal-CFRP joints with 3d anchors, offering valuable insights into the mechanics of hybrid structures and guiding the development of more reliable and efficient joint designs.