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Press Hardening offers the possibility to obtain a wide range of mechanical properties through microstructural tailoring. This strategy has been successfully applied in thin sheet components, for instance, through differential cooling strategies. The application of these added value features to truck components implies adapting the process to the manufacture of thick sheet metal. This introduces an additional layer of complexity, but also opportunity, in a process where the final microstructure and, thus the mechanical performance is generated in the press shop. This work presents a study on optimizing the crash worthiness and impact energy absorption on a press hardened thick 22MnB5 steel sheet. Different microstructure design strategies have been studied, including ferrite-Pearlite (representative of a differential heating and austenitization strategy), in-die generated Bainite (representative of differential cooling) and Tempered Martensite (generated through laser tempering), keeping a fully hardened martensite as a reference condition. The material performance has been compared in terms of the monotonic properties, useful for anti-intrusion performance, and Essential Work of Fracture, a well-suited parameter to predict the crash failure behavior of high strength steels. The results show that laser tempering offers properties similar to Bainite-based microstructures and can be a successful replacement in components where the sheet thickness does not allow for the fine control of the in-die thermomechanical evolution., Godkänd;2023;Nivå 0;2023-03-20 (joosat);Konferensartikel i tidskriftFunder: Eureka-PID Worthtruck (PID, IDI-20171259); European Union’s Research Fund for Coal and Steel (No. 101034036–ToughSteel project)Part of special Issue: Hot Sheet Metal Forming of High Performance MaterialsLicens fullltext: CC BY License

This study presents an innovative approach for selecting high-strength materials for fatigue dimensioning parts, considering both fracture toughness and fatigue performance. Warm and hot forming processes enable the construction of high-strength parts above 1000 MPa with complex geometries, making them suitable for lightweight chassis in automotive and freight applications. This research reveals that high-strength steels can experience up to a 40% reduction in fatigue performance due to manufacturing defects introduced during punching and trimming. Fracture toughness has been proposed as a good indicator of notch sensitivity, with a strong correlation of 0.83 between fracture toughness and fatigue notch sensitivity. Therefore, by combining fracture toughness measurements and fatigue resistance obtained through the rapid fatigue test, it becomes possible to quickly identify the most fatigue-resistant materials to deal with defects. Among the nine materials analysed, warm-formed steels show promising characteristics for lightweight chassis construction, with high fatigue resistance and fracture toughness exceeding the proposed fracture threshold of 250 kJ/m2., Validerad;2023;Nivå 2;2023-08-10 (joosat);Licens fulltext: CC BY License

High-strength steels are widely used in vehicle body-in-white, offering a good balance between crashworthiness and lightweight design. The increased requirements of heavier electric vehicles, in terms of fatigue resistance and crashworthiness, highlight that chassis parts have remarkable lightweighting potential. However, applying these grades in chassis parts is not straightforward, as the forming processes, like trimming, may introduce surface defects that compromise the fatigue resistance of the component. This work presents a material selection strategy for the applicability of high-strength steels in chassis parts of electric vehicles. The proposed approach allows the evaluation of the key parameters of the chassis parts in a simple way. The crash performance is evaluated through fracture toughness using the essential work of fracture (EWF) methodology. The method is applied to thin high-strength steel sheets employing double-edge notched tensile specimens (DENT). On the other hand, fatigue performance is investigated in terms of fatigue resistance for both notched and unnotched specimens. The results for different complex-phase and dual-phase steels show a good agreement between the EWF and the fatigue notch factor. The method could help apply high-strength steel to chassis parts, as designers will have a tool to focus the expensive fatigue tests on the best material candidates., Funder: Catalan Government - ACCIÓ-Eurecat (Project PRIV - RapidFat)Licens fulltext: CC BY License

Most solutions for a lightweight design are based on the implementation of AHSS, Al alloys or CFRP. However, not all these strategies are susceptible to be applied to truck chassis parts. These components require high fatigue resistance and thick sheets. Additionally, they are usually trimmed and punched, which is known to affect fatigue resistance. This work addresses the lightweight con-struction of truck parts through the warm forming of steel grades tailored to show high formability and fatigue behaviour. The fatigue limit was evaluated for different edge conditions (polished and punched) and rationalized using the cracking resistance described by fracture toughness. The con-sideration of both mechanical properties, fatigue, and fracture toughness, gives an innovative and useful tool to develop and select materials for fatigue dimensioned parts., Funder: Research Fund for Coal and Steel RFCS (800649);ISBN för värdpublikation: 978-3-95735-150-0, RFCS programme - Warmlight project

The method of Radar Target Detection by Analysis and Statistical Classification of the Cellular Emission (DRACEC) can be functionally divided into three fundamental stages: Acquisition, Adaptation, and Detection. During Acquisition it is required to continuously exchange information between the hardware in FPGA and the software, without affecting the performance of the latter. This paper proposes the simplest communication interface that satisfies these requirements when DRACEC is applied to a small searching window. The solution is implemented on the serial port and ensures that samples stored in FPGA are available at the computer for the remaining processing stages. One fundamental characteristic of the proposal is a protocol designed to control the communication flow, which is implemented through a dedicated program thread. This allows software performance to not deteriorate during communication and lays the foundations for using multithreading techniques to develop the stages of DRACEC.

In the last decade, the favorable properties of the press hardening process for advanced high strength steel (AHSS) have increased the demands concerning passenger safety and lightweight design. AHSS show excellent mechanical properties from e.g. tensile test measurements, but it has previously been shown that results from tensile elongation or energy calculation of un-notched and smooth specimen are not appropriate to classify the crash behavior of steel grades. This is because they completely underestimate the post-uniform region from start of necking to failure. Another issue, the mechanical behavior of a notched or cracked component is different than a smooth and un-notched component. If the mechanical behavior in some loading is dominated by crack propagation, it should be rationalized in terms of the materials crack propagation resistance. Therefore, the evolution of the material property that controls crack propagation, i.e. the fracture toughness, is an interesting approach to evaluate loading and deformation of AHSS. Process modelling including fracture toughness depending properties gives valuable information and additional understanding of fracture behavior and crack propagation mechanisms in AHSS components. Fracture toughness in thin sheets can be readily measured through the application of the Essential Work of Fracture (EWF) methodology. The damage evolution law can be specified in terms of fracture energy (per unit area) or in terms of equivalent plastic failure strain as a function of triaxiality and lode angle. In this work, DENT test samples have been experimentally evaluated and finite element simulations of the DENT tests have been performed. By this approach the numerical study includes mechanical response of AHSS specimen including sharp cracks. In the numerical model, the J-integral was evaluated using the virtual crack-tip extension (VCE) method. From the comparison of the numerical and experimental results of load-displacement for different

During the last decades, the use of ultra-high strength steel (UHSS) has increased as its favorable ratio between strength and mass allows the design of lighter body-in-white while maintaining passenger safety. Modeling impact loads of components made of UHS steel requires reliable descriptions of the material deformation and fracture behavior. Traditional stress or strain based fracture criteria are used in finite element modeling. A different approach in modeling fracture in components uses the fracture energy as a model parameter. Fracture toughness is difficult to measure in thin sheets; a method termed Essential Work of Fracture (EWF) provides the possibility to determine the fracture toughness in sheet metal. With knowledge of the fracture toughness the understanding of fracture behavior and crack propagation in ultra-high strength steel can be increased. The obtained EWF is related to the fracture energy and can be used in numerical models as a material parameter. In the present work results from preliminary testing are shown and a discussion on cross-head speed and strain rate in the critical specimen cross section is given. The use of digital image correlation provides information about the displacement field in the vicinity of the notch and hence about the strain- and strain rate distribution. Furthermore, the difficulties in reliable measurement of force and elongation in high speed tensile testing machines are elucidated. Issues encountered during the development of the high-speed DENT specimen are not limited to the specific geometry presented in this paper. The present work aims at the development of a test specimen to obtain the Essential Work of Fracture (EWF) at high test speed. This work contributes to the overall goal to model fracture behavior and crack propagation, dependent on the strain rate. For the investigation, a high-speed tensile testing machine equipped with an in-house developed load cell and an optical elongation measurement system wa

Crashworthiness is a relevant engineering property for car parts. However it is not easy to measure at laboratory scale and complex impact tests have to be carried out to determine it. Crash resistance for high strength steel is commonly evaluated in terms of cracking pattern and energy absorption in crashed specimens. Accordingly, the material resistance to crack propagation, i.e. the fracture toughness, could be used to rank crashworthiness. It has been proved in a previous work by the authors, so the measure of fracture toughness, in the frame of fracture mechanics in small laboratory specimens, would allow determining the best microstructure for crash resistance parts. Press hardening offers the possibility to obtain a wide range of microstructural configurations, with different mechanical properties. So the aim of this work is to evaluate the fracture toughness following the essential work of fracture methodology for ferrite-pearlite, bainite, ferrite-bainite, martensite and martensite-bainite microstructures. Results showed that bainitic microstructures have high fracture toughness, similar to TWIP and CP steels, which allows pointing them as potential candidates for obtaining high crash resistance in parts manufactured by press hardening.