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Determining the fatigue behaviour of metallic materials using standardised testing methods is costly and time-consuming. Therefore, several methods have been proposed to shorten the testing time and improve the fatigue optimisation of materials and components. This work presents a new fatigue testing method based on fatigue damage monitoring that allows determining the fatigue resistance in a short time and with few specimens. The presented method, named as the stiffness method, monitors the inelastic strains as an indicator of fatigue damage evolution. Strain measurements were carried out by digital image correlation techniques and showed to effectively follow damage evolution during fatigue tests. Results are convincing and more evident to obtain and discuss than other monitoring techniques, like temperature dissipation. In addition, the method overcomes the main limitations of the existing fast testing methods by avoiding the utilisation of complex apparatus, like infrared cameras or acoustic emission sensors. The approach has been validated in ten different metallic materials, as titanium and aluminium alloys, carbon steels, and stainless steels. The estimated fatigue limit was compared with values obtained following standardised tests, showing excellent agreement. Results allow pointing out the stiffness method as an efficient and effective tool for rapidly determining the fatigue behaviour of metallic materials.

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.

Press hardened components have become widespread in the automotive industry in structural and crash-resistant applications, thanks to the combination of the complex shapes and high mechanical properties obtained. However, the press hardening of coated boron steel results in severe adhesive-based wear, with tool maintenance being required in as few as 3000 cycles. The current industrial implementation of press hardening is defined to work around this phenomenon. While this aspect has been studied by different authors, most of the literature deals with laboratory-scale tribosimulators, leaving an open question into how this knowledge transfers to macroscopic effects on the industrial process. In this work, wear in press hardening is studied by comparing the results obtained in laboratory conditions with a pilot-scale line, and finally, with wear mechanisms observed on industrial tools. The aim of this study is to consolidate the current knowledge about the micro-mechanisms involved, and to understand to what extent the existing tests reproduce the actual mechanisms observed in the press floor. The results show how material transfer mainly happens as an accumulation of dust compacted into initial defects on the tool surface. Moreover, this mechanism is effectively reproduced in laboratory tribosimulators and pilot environments, showing a similar morphology to wear on industrial tools. The work sheds light on the underlying causes of wear, and its potential mitigation strategies.

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.