Formability improvement and mechanism analysis of fiber metal laminates under extra normal stress

Fiber metal laminates (FMLs) are advanced super-hybrid sandwich materials. They have been increasingly used in the aerospace industry. However, the low formability of heterogeneous FMLs in the traditional forming process restricts its large-scale application. This paper proposed the idea of introducing extra normal stresses to improve the formability of FMLs by using flexible die forming. The carbon fiber reinforced aluminum alloy laminates (CARALL) were prepared. A flexible die forming finite element model was established, and a platform was constructed to investigate the effects of laminate layup structures and the flexible die hardness. The mechanism of formability improvement was analyzed. The results indicate that the formability of the FMLs is better compared with the carbon fiber sheets. The formability can be increased significantly with a larger flexible die hardness. The 3 + 2 type laminate showed the highest improvement in formability and ultimate limit states per unit thickness. It is formed by alternately stacking three layers of aluminum alloy and two layers of carbon fiber reinforced plastics (CFRP). The specific order of layers is AL/CFRP/AL/CFRP/AL. The matrix type of CFRP is thermosetting epoxy resin. The maximum improvement in formability and the load limit per unit thickness of the 3 + 2 type layup structures were 28.8% and 4.94 N/mm, respectively. The mechanism of formability improvement was analyzed. The mechanism analysis showed that the additional normal stresses through a flexible die forming reduced the spherical tensor of stress. The additional normal stresses can promote damage closure during plastic deformation. It also improved the material’s Lode parameter, increasing the formability. This study demonstrates that flexible die forming helps enhance the deformation capacity of laminates and is a potential method for preparing high-performance, high-plasticity super-hybrid material components.