A Study on Structural Cores for Lightweight Steel Sandwiches

Abstract: Lightweight materials and structures are essential building blocks for a future with sustainable transportation and automotive industries. Incorporating lightweight materials and structures in today's vehicles, reduces weight and energy consumption while maintaining, or even improving, necessary mechanical properties and behaviors. Due to this, the environmental footprint can be reduced through the incorporation of lightweight structures and materials. Awareness of the negative effects caused by pollution from emissions is ever increasing. Legislation, forced by authorities, drives industries to find better solutions with regard to the environmental impact. For the automotive industry, this implies more effective vehicles with respect to energy consumption. This can be achieved by introducing new, and improve current, methods of turning power into motion. An additional approach is reducing weight of the body in white (BIW) while maintaining crash worthiness to assure passenger safety. In addition to the structural integrity of the BIW, passenger safety is further increased through electrical systems integrated into the modern vehicle. Besides these safety systems, customers are also able to choose from a long list of gadgets to be fitted to the vehicle. As a result, the curb weight of vehicles are increasing, partly due to customer demands. In order to mitigate the increasing weights the BIW must be optimized with respect to weight, while maintaining its structural integrity and crash worthiness. To achieve this, new and innovative materials, geometries and structures are required, where the right material is used in the right place, resulting in a lightweight structure which can replace current configurations. A variety of approaches are available for achieving lightweight, one of them being the press-hardening method, in which a heated blank is formed and quenched in the same process step. The result of the process is a component with greatly enhanced properties as compared to those of mild steel. Due to the properties of press hardened components they can be used to reduce the weight of the body-in-white. The process also allows for manufacturing of components with tailored properties, allowing the right material properties in the right place. The present work aims to investigate, develop and in the end bring forth two types of light weight sandwiches; one intended for crash applications (Type I) and another for stiffness applications (Type II). Type I, based on press hardened boron steel, consists of a perforated core in between two face plates. To evaluate Type I's ability to absorb energy for crash applications a hat profile geometry is utilized. The hat profile is numerically subjected to loading from which the required energy to deform it can be found. These results are compared to those from a reference test, consisting of a hat profile based on solid steel and with an equivalent weight to that of the Type I hat profile. The aim is to minimize the weight of the core while maximizing the energy absorption. Type II consists of a bidirectional corrugated steel plate, placed in between two face plates. The geometry of the bidirectional core requires a large amount of finite elements for discretization causing a small time step and long simulation times. In order to reduce computational time a homogenization approach is suggested where the aim is to be able to predict stiffness of a planar sandwich at a reduced computational cost. The numerical results from Type I show that it is possible to obtain a higher energy absorption per unit weight by introducing perforated cores in sandwich panels. Typically, energy absorption of such a panels were 20% higher as compared to a solid hat profile of equivalent weight, making it an attractive choice for reducing weight while maintaining performance. However, these results are awaiting experimental validation. The results from Type II show that it is possible, by introducing a homogenization procedure, to predict stiffness at a reduced computational cost. Validation by experiments were carried out as a sandwich panel was subjected to a three point bend in the laboratory. Numerical and experimental results agreed quite well, showing the possibilities of incorporating such panels into larger structure for stiffness applications.