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.

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.

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.

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.