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A Study on Sandwich Structures: Development, Mechanical Characterization and Numerical Modeling
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Solid Mechanics.ORCID iD: 0000-0001-7895-1058
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 [en]
Ultra-High Strength Steel, UHSS, Sandwich, Micro-sandwich, Hybrix, Modeling, Composite
National Category
Applied Mechanics
Research subject
Solid Mechanics
Identifiers
URN: urn:nbn:se:ltu:diva-85076ISBN: 978-91-7790-876-0 (print)ISBN: 978-91-7790-877-7 (electronic)OAI: oai:DiVA.org:ltu-85076DiVA, id: diva2:1562028
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
List of papers
1. Ultra high strength steel sandwich for lightweight applications
Open this publication in new window or tab >>Ultra high strength steel sandwich for lightweight applications
2020 (English)In: SN Applied Sciences, ISSN 2523-3963, E-ISSN 2523-3971, Vol. 2, no 6, article id 1040Article in journal (Refereed) Published
Abstract [en]

Methods for reducing weight of structural elements are important for a sustainable society. Over the recent years ultra high strength steel (UHSS) has been a successful material for designing light and strong components. Sandwich panels are interesting structural components to further explore areas where the benefits of UHSS can be utilized. The specific properties of sandwich panels make them suitable for stiffness applications and various cores have been studied extensively. In the present work, bidirectionally corrugated UHSS cores are studied experimentally and numerically. A UHSS core is manufactured by cold rolling and bonded to the skins by welding. Stiffness is evaluated experimentally in three-point bending. The tests are virtually reproduced using the finite element method. Precise discretization of the core requires large amounts of computational power, prolonging lead times for sandwich component development, which in the present work is addressed by homogenization, using an equivalent material formulation. Input data for the equivalent models is obtained by characterizing representative volume elements of the periodic cores under periodic boundary conditions. The homogenized panel reduces the number of finite elements and thus the computational time while maintaining accuracy. Numerical results are validated and agree well with experimental testing. Important findings from experimental and simulation results show that the suggested panels provide superior specific bending stiffness as compared to solid panels. This work shows that lightweight UHSS sandwiches with excellent stiffness properties can be manufactured and modeled efficiently. The concept of manufacturing a UHSS sandwich panel expands the usability of UHSS to new areas.

Place, publisher, year, edition, pages
Springer Nature, 2020
Keywords
UHSS, Sandwich, Lightweight, Modeling, Bidirectional core, Representative volume element (RVE)
National Category
Applied Mechanics
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:ltu:diva-78844 (URN)10.1007/s42452-020-2773-5 (DOI)000538087000044 ()2-s2.0-85100731139 (Scopus ID)
Note

Validerad;2020;Nivå 2;2020-05-14 (alebob)

Available from: 2020-05-11 Created: 2020-05-11 Last updated: 2023-09-05Bibliographically approved
2. Numerical evaluation of lightweight ultra high strength steel sandwich for energy absorption
Open this publication in new window or tab >>Numerical evaluation of lightweight ultra high strength steel sandwich for energy absorption
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
Keywords
UHSS, Sandwich, Lightweight, Modeling, RVE
National Category
Applied Mechanics
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:ltu:diva-81231 (URN)10.1007/s42452-020-03724-9 (DOI)000582471800002 ()2-s2.0-85100829045 (Scopus ID)
Note

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

Available from: 2020-10-26 Created: 2020-10-26 Last updated: 2023-09-05Bibliographically approved
3. Novel Methodology for Experimental Characterization of Micro-Sandwich Materials
Open this publication in new window or tab >>Novel Methodology for Experimental Characterization of Micro-Sandwich Materials
Show others...
2021 (English)In: Materials, ISSN 1996-1944, E-ISSN 1996-1944, Vol. 14, no 16, article id 4396Article in journal (Refereed) Published
Abstract [en]

Lightweight components are in demand from the automotive industry, due to legislation regulating greenhouse gas emissions, e.g., CO2. Traditionally, lightweighting has been done by replacing mild steels with ultra-high strength steel. The development of micro-sandwich materials has received increasing attention due to their formability and potential for replacing steel sheets in automotive bodies. A fundamental requirement for micro-sandwich materials to gain significant market share within the automotive industry is the possibility to simulate manufacturing of components, e.g., cold forming. Thus, reliable methods for characterizing the mechanical properties of the micro-sandwich materials, and in particular their cores, are necessary. In the present work, a novel method for obtaining the out-of-plane properties of micro-sandwich cores is presented. In particular, the out-of-plane properties, i.e., transverse tension/compression and out-of-plane shear are characterized. Test tools are designed and developed for subjecting micro-sandwich specimens to the desired loading conditions and digital image correlation is used to qualitatively analyze displacement fields and fracture of the core. A variation of the response from the material tests is observed, analyzed using statistical methods, i.e., the Weibull distribution. It is found that the suggested method produces reliable and repeatable results, providing a better understanding of micro-sandwich materials. The results produced in the present work may be used as input data for constitutive models, but also for validation of numerical models.

Place, publisher, year, edition, pages
MDPI, 2021
Keywords
composite, Hybrix, micro-sandwich, lightweight, characterization, digital image correlation (DIC)
National Category
Composite Science and Engineering
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:ltu:diva-85073 (URN)10.3390/ma14164396 (DOI)000689479900001 ()34442919 (PubMedID)2-s2.0-85112312847 (Scopus ID)
Funder
EU, Horizon 2020, 814517
Note

Validerad;2021;Nivå 2;2021-08-13 (alebob);

Artikeln har tidigare förekommit som manuskript i avhandling

Available from: 2021-06-08 Created: 2021-06-08 Last updated: 2023-09-05Bibliographically approved
4. Calibration of orthotropic plasticity- and damage models for micro-sandwich materials
Open this publication in new window or tab >>Calibration of orthotropic plasticity- and damage models for micro-sandwich materials
Show others...
2022 (English)In: SN Applied Sciences, ISSN 2523-3963, E-ISSN 2523-3971, Vol. 4, no 6, article id 182Article in journal (Refereed) Published
Abstract [en]

Sandwich structures are commonly used to increase bending-stiffness without significantly increasing weight. In particular, micro-sandwich materials have been developed with the automotive industry in mind, being thin and formable. In the present work, it is investigated if micro-sandwich materials may be modeled using commercially available material models, accounting for both elasto-plasticity and fracture. A methodology for calibration of both the constitutive- and the damage model of micro-sandwich materials is presented. To validate the models, an experimental T-peel test is performed on the micro-sandwich material and compared with the numerical models. The models are found to be in agreement with the experimental data, being able to recreate the force response as well as the fracture of the micro-sandwich core.

Place, publisher, year, edition, pages
Springer Nature, 2022
Keywords
Micro sandwich, Hybrix, Lightweight, Modeling, T-peel test
National Category
Composite Science and Engineering
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:ltu:diva-85075 (URN)10.1007/s42452-022-05060-6 (DOI)000798485800004 ()2-s2.0-85130368530 (Scopus ID)
Funder
EU, Horizon 2020, 814517 Form-Planet
Note

Validerad;2022;Nivå 2;2022-06-03 (hanlid)

Available from: 2021-06-08 Created: 2021-06-08 Last updated: 2023-09-05Bibliographically approved

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Hammarberg, Samuel

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