Forming and shaping 3- and 5-layered metal/polymer/metal sandwich composites

Technically, a sandwich-structured composite is a special class of composite materials that is fabricated by attaching two thin but stiff skins to a lightweight but thick core layer. Noteworthy literature has recently highlighted the fundamental advantages and some applications of sandwich composites in conjunction with an in-depth review of the forming behavior mainly under deep drawing conditions of steel/polymer/steel sandwich composites (SPS). On one hand, the beneficial potentials of sandwich laminates compared to monolithic sheet in respect to the bending properties show that comparable bending stiffness of the sandwich systems can be reached with approximately 30–40% weight saving. On the other hand, they exhibit significantly different forming potentials, limits and forces compared to comparable monolithic materials due to their inhomogeneous properties through their layers thicknesses and adhesion strength. Just like in many other metallic materials, some types of defects are expected, such as cracking of the outer shell in the bending zone, when the composite sheet is subjected to bending. This aspect of metal/polymer/metal sandwich composites has over the years been widely researched. In fact, the springback behavior has been studied on such composites both experimentally and numerically. Further, a plethora of literature is available on various modeling and finite element simulations. However, there are some deficiencies and unresolved technical problems that affect the practical applicability of such sandwich composites. For instance, among other deficiencies, some of the proposed FEM approaches are not able to characterize the post-buckling behavior of sandwich structures using a von Mises-type viscoplasticity model.

Ideally, sheet metal bending is a fundamental forming process that is widely used in various application fields. However, in preceding works, i.e. Part 1, the forming behavior of SPS sandwich composites under deep drawing conditions was intensively investigated and introduced. Afterwards, the same research group from the Clausthal University of Technology in Germany namely Prof. Dr. Heinz Palkowski (Forming and Processing) and Prof. Dr. Stefan Hartmann (Technical Mechanics) investigated the bending behavior of multilayered SPS systems. Their goal was to describe the bending behavior in terms of the bending force, springback ratio and the failure conditions for different SPS configurations considering their thickness, number of layers and symmetry. Their work is currently published in the “Composite Structures” journal.

To realize this, the research team adopted a systematic investigation methodology based on an ascending scaling approach of the material combinations starting from the mono-materials, sheet metal laminate (steel/resin/steel) and ending with several symmetric and asymmetric arrangements of SPS. Specifically, the symmetric arrangement of the laminate sheets was investigated by combining skin sheets possessing the same mechanical properties and thickness. Overall, the team investigated the bending conditions of SPS sandwich composites and validated the same by analytical and numerical methods.

The authors reported that for thicker cores, a reduced springback could be stated. However, with thicker cores, the crack probability was seen to increase as a result of the increasing tensile stresses on the outer layer. Generally, a good agreement between the numerical and analytical results with the experimental ones was achieved.

In summary, the study presented a comprehensive investigation of the bending behavior of different combinations of steel/polymer/steel sandwich composites; experimentally, numerically and analytically. Altogether, the researchers recommended that for one to avoid possible defects when bending SPS sheets, it was advisable to bend with punch radii of about twice the total thickness of the composite. In a statement to Advances in Engineering, Prof. Dr. Heinz Palkowski, the lead author, pointed out that in addition to the guidelines mentioned here, their work outlined various conditions for bending faultless SPS sheets.