Failure modes of metallic sandwich plates with pyramidal truss cores during U-bending

Metallic sandwich plates are advanced lightweight materials characterized by their high bending stiffness and strength. Due to their low formability compared with commercial sheets, the range of application fields for these sandwich plates is narrow. General applications require the development and design of deformable sandwich plates. In this study, failure modes are experimentally defined and bendable sandwich plates are numerically designed. Sandwich plates with pyramidal truss cores as specimens of U-bending tests are fabricated. Expanded metals in the form of elongated diamond perforated sheets were bent to make regular pyramidal truss cores that were bonded with face sheets in a copper-based brazing technique. U-bending tests were carried out to observe the failure mode and bending mechanism of these sandwich plates with pyramidal truss cores. The results show that face buckling, core buckling and de-bonding are the main failure modes. The geometric shape and parameters of the core determine the formability of the sandwich plate. U-bending processes were simulated for sandwich plates that utilized pyramidal truss cores with respect to various ranges of geometric parameters. In the simulations, ideal bonding conditions between the cores and the face sheets were assumed. The results of the simulations showed that face fractures constitute an additional failure mode during U-bending. The existence of a gap between attachment points is an important geometric parameter in the control of the face buckling mode. The thickness of the core element represents a control parameter for core shear failures including core fractures (or core thinning) and core buckling. Face fractures or face thinning do not occur after the onset of face buckling or a core shear failure. The occurrence of a face fracture depends on the die radius and the properties of the face sheets. Safe design regions for bendable sandwich plates in the forming window were also determined. The main results of this study are that likely failure modes were determined and numerical design procedures for bendable sandwich plates with pyramidal truss cores were constructed to prevent the defined failure modes during U-bending.