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Numerical evaluation of lightweight ultra high strength steel sandwich for energy absorption
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.ORCID iD: 0000-0001-7895-1058
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.ORCID iD: 0000-0001-5206-6894
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.ORCID iD: 0000-0001-5218-396X
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.ORCID iD: 0000-0003-0910-7990
2020 (English)In: SN Applied Sciences, ISSN 2523-3963, E-ISSN 2523-3971, Vol. 2, no 11, article id 1876Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
Springer, 2020. Vol. 2, no 11, article id 1876
Keywords [en]
UHSS, Sandwich, Lightweight, Modeling, RVE
National Category
Applied Mechanics
Research subject
Solid Mechanics
Identifiers
URN: urn:nbn:se:ltu:diva-81231DOI: 10.1007/s42452-020-03724-9ISI: 000582471800002Scopus ID: 2-s2.0-85100829045OAI: oai:DiVA.org:ltu-81231DiVA, id: diva2:1479145
Note

Validerad;2020;Nivå 2;2020-11-16 (johcin)

Available from: 2020-10-26 Created: 2020-10-26 Last updated: 2023-09-05Bibliographically approved
In thesis
1. A Study on Sandwich Structures: Development, Mechanical Characterization and Numerical Modeling
Open this publication in new window or tab >>A Study on Sandwich Structures: Development, Mechanical Characterization and Numerical Modeling
2021 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Legislative demands force the automotive industry to reduce greenhouse gas (GHG) emissions. At the same time, crashworthiness must not be compromised. A ve-hicle’s GHG emissions, such as carbon dioxide, is dependent on its fuel consump-tion. Lowering the vehicle weight, reducing fuel consumption, will therefor reduce emissions. Thus, high performance lightweight materials and structures are on demand. Several methods for achieving high-performance lightweight components are available. One of the most successful approaches has been replacing mild steels with press-hardened steels, e.g. ultra high strength steels (UHSS). In the press-hardening process, a low-alloyed boron steel blank is austenitized followed by simultaneously forming and cooling. By controlling cooling rates, a martensitic microstructure can be obtained, resulting in components with superior properties compared to mild steels. Other methods of achieving lightweight components in-clude the usage of sandwich structures where stiff skins are bonded to a low-density core. In the present thesis, several types of sandwich structures are studied both numerically and experimentally. A UHSS sandwich with a bidirectionlly corru-gated core, intended for stiffness application, is manufactured and evaluated in three-point bending. Finite element models are utilized to recreate the three-point bend test. A large amount of finite elements are required for precise discretization of the core. The number of finite elements are reduced by replacing the sandwich with an homogeneous, equivalent model with input data obtained from analyzing representative volume elements (RVEs) of the core, subjected to periodic and ho-mogeneous boundary conditions. Good agreement is found between experiments and finite element models. A UHSS sandwich with a partly perforated core is evaluated numerically for energy absorption applications. Several hole configu-rations for the core are evaluated with respect to specific energy absorption. A fracture criterion is utilized for the sandwich skins. Computational time is re-duced by homogenization of the core using a stress-resultant based constitutive model. It is found that the sandwich concept allows for an increase in specific energy absorption and that the computational time can be reduced while still be-ing able to predict energy absorption. An experimental methodology is developed for mechanical characterization of micro-sandwich materials. Tools are developed for loading the micro-sandwich in out-of-plane tension and shear, where digital image correlation is used for measuring displacements fields and fracture of the micro-sandwich core. Statistical methods are adopted for analyzing the variation in the mechanical properties of the micro-sandwich from which statistical means may be obtained. The experimental data is used as input for constitutive models, simulating the micro-sandwich material subjected to peeling, using a T-peel test. The numerical models are validated against experiments, found to agree within one standard deviation, suggesting that the experimental methodology produces robust data.The present work has thus presented methods, further increasing the usability of UHSS with regard to lightweighting, and explored how such components may be simulated numerically with adequate accuracy and reasonable computation time. Furthermore, the present thesis contributes by presenting methods for character-izing micro-sandwich materials, including statistical methods for analyzing scatter in mechanical properties, and how such sandwich materials may be modeled, tak-ing elasto-plasticity and damage into account. These results opens up possibilities for further development and optimization of lightweight constructions.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2021. p. 50
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Keywords
Ultra-High Strength Steel, UHSS, Sandwich, Micro-sandwich, Hybrix, Modeling, Composite
National Category
Applied Mechanics
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:ltu:diva-85076 (URN)978-91-7790-876-0 (ISBN)978-91-7790-877-7 (ISBN)
Public defence
2021-09-24, E632, 09:00 (English)
Opponent
Supervisors
Available from: 2021-06-08 Created: 2021-06-08 Last updated: 2023-09-05Bibliographically approved

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Hammarberg, SamuelLarsson, SimonKajberg, JörgenJonsén, Pär

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