Numerical evaluation of lightweight ultra high strength steel sandwich for energy absorption
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Numerical evaluation of lightweight ultra high strength steel sandwich for energy absorption Samuel Hammarberg1 · Simon Larsson1 · Jörgen Kajberg1 · Pär Jonsén1 Received: 29 May 2020 / Accepted: 15 October 2020 © The Author(s) 2020 OPEN
Abstract Legislation regarding greenhouse gas emissions forces automotive manufacturers to bring forth new and innovative materials and structures for weight reduction of the body-in-white. The present work evaluates a lightweight ultra high strength steel sandwich concept, with perforated cores, for energy absorption applications. Hat-profile geometries, subjected to crushing, are studied numerically to evaluate specific energy absorption for the sandwich concept and solid hat-profiles of equivalent weight. Precise discretization of the perforated core requires large computational power. In the present work, this is addressed by homogenization, replacing the perforated core with a homogeneous material with equivalent mechanical properties. Input data for the equivalent material is obtained by analyzing a representative volume element, subjected to in-plane loading and out-of-plane bending/twisting using periodic boundary conditions. The homogenized sandwich reduces the number of finite elements and thereby computational time with approximately 95%, while maintaining accuracy with respect to force–displacement response and energy absorption. It is found that specific energy absorption is increased with 8–17%, when comparing solid and sandwich hat profiles of equivalent weight, and that a weight saving of at least 6% is possible for equivalent performance. Keywords UHSS · Sandwich · Lightweight · Modeling · RVE
1 Introduction The automotive industry faces long-term goals regarding reduction of greenhouse gas (GHG) emissions. Effort is therefore put into finding new design solutions and material selections, reducing the life-cycle ecological footprint through energy efficient production and reduced weight of e.g. body-in-white parts (BIW). At the same time, innovative design and new materials have to fulfill the demand regarding final product performance, such as crashworthiness. Steel, a common material in many sectors, due to its high strength to cost ratio, has been utilized for manufacturing lightweight components by various methods of heat treatment. According to Kawajiri et al. [1], replacing mild steel with high strength steel has been one of
the most effective material substitutions with regard to lowering GHG emissions. In the 1970’s, press hardening was invented by Luleå university of technology and NJA (now SSAB) and industrialized by Plannja HardTech (now Gestamp Hardtech). In the press hardening process, a blank is austenitized and quenched, forming a mainly martensitic microstructure. The result is an ultra high strength steel (UHSS) with high shape accuracy, suitable for intrusion protection in vehicles. Due to the superior mechanical properties of UHSS, gauge thicknesses can be reduced, thus lowering the weight of the BIW. Karbasian and Tekkaya [2] describe the
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