Plastic forming of the doubly curved surfaces of sandwich plates with bi-directionally trapezoidal cores of different sizes

Plastic forming technology is a low-cost and high-efficiency way to produce doubly curved sandwich plate from a flat plate. However, there are a various potential forming defect modes which may affect the formability of sandwich plate. This paper is focused on the defect-free plastic forming of all -metal sandwich plates with bi-directionally trapezoidal cores, an approximate theoretical model for predicting the global buckling of sandwich plate was developed and the formability of sandwich plate with different geometric parameters were investigated analytically, experimentally and by numerical simulations. The results show that the forming defect mode which dominates the formability of a specified sandwich plate depends on the geometric parameter of the sandwich plate, the formability of the thin sandwich plate with small size core cells is limited by global plastic buckling, while that of the relative thick sandwich plate with relative large size core cells is limited by skin wrinkling and skin dimpling. Significant trends are found that the sandwich plates with thick skins have superior forming performance compared to those with thin ones, and that the limit forming radius is decreased greatly with the decrease of the characteristic sizes of core cell unit. Multi-point forming experiments on bi-directionally trapezoidal sandwich plates were performed, and the numerically simulated results and theoretically analyzed results on the formability were demonstrated finally.