Solid State Phenomena Vol. 388

Abstract: Advanced high-strength steels exhibit sensitivity to diffusible hydrogen content, mainly observed during tensile testing. Although the initial yield stress and ultimate tensile strength are not significantly affected, ductility decreases with increasing hydrogen content. This sensitivity to diffusible hydrogen depends on strain rate and stress concentrations. This study examines the influence of diffusible hydrogen content on the ductile fracture of DP780GI steel, in the form of 1 mm thick sheets. Samples were prepared with specific geometries, with notches and holes, to study different mechanical states, and fracture tests were performed to evaluate ductility as a function of hydrogen content and stress triaxiality. The local strain rate was around 1 × 10−4 s−1, which is lower than the value used in industrial applications, to enhance the hydrogen sensitivity. A hydrogen charging process was used, including zinc coating removal, electrochemical loading, and electrolytic deposition of a zinc layer to prevent hydrogen desorption. The hydrogen content was measured by thermal desorption analysis after the mechanical testing. It is observed that the maximum local elongation decreases with increasing hydrogen content, with a noticeable effect above 0.25 ppm. Cracks form in areas of maximum effective deformation, and their location varies depending on the geometry of the sample and the hydrogen content. The evolution of the maximum effective strain before fracture shows a significant decrease in ductility with increasing hydrogen content, regardless of the mechanical state.

Abstract: The flangeability of sheared edges in sheet metal forming is commonly evaluated using the ISO 16630 Hole Expansion Test (HET), in which fracture initiates at the edge under predominantly uniaxial tensile loading. For high-quality edges, this test can be interpreted as providing the strain to fracture under proportional uniaxial tension; however, the measured fracture strain is restricted to a single material-dependent sheet orientation. In this work, a novel experimental approach is proposed to directly measure the uniaxial tensile fracture strain in a predefined sheet orientation using digital image correlation (DIC). The method, termed the Asymmetric Hole Expansion Test (aHET), is derived from the standard HET through the introduction of a novel asymmetric punch geometry. This modification promotes accelerated edge stretching along a controlled direction, enabling orientation-specific characterization of fracture strain. The capability of the aHET to characterize direction-dependent strains to fracture under uniaxial tension is demonstrated on a DP450 dual-phase steel. The consistent fracture initiation at the edge along the predefined fracture direction, combined with the low scatter of the measured fracture strains across repeated tests for all three investigated sheet orientations, demonstrates that the aHET is well suited for identifying the strain to fracture under proportional uniaxial tensile loading for the calibration of fracture initiation models.

Abstract: This study investigates the effects of various cutting technologies on a 0.25 mm thick ferritic steel, a material widely used in packaging and other lightweight applications. The study provides a comprehensive comparison of four distinct cutting technologies: Laser Cutting, Milling, Electrical Discharge Machining (EDM), and Water Jet Cutting. The research focuses on the impact of these cutting processes on the material’s properties and its performance under uniaxial tension. X-ray diffraction is used to precisely measure the magnitude and distribution of residual stresses along the cut edge in order to correlate them with changes in the material's flow curve, which is critical for accurate mechanical characterization. Furthermore, a laser-scanning microscope was used for detailed morphological analysis of the cut edge and for roughness measurement. To quantify mechanical property changes, microindentation hardness testing was used to assess the degree of work hardening induced by each cutting method. Finally, Digital Imaging Correlation (DIC) was employed to track strain distribution and observe strain field variations.

Abstract: An oxygen-free copper has been utilized as a terminal material in the power transistors and their related electric system in the electric mobiles because of its high electric conductivity and excellent engineering durability in high current usage. The high ductility and its low mechanical strength cause large shear droop and increase of fractured surface. In this report, the shearing of oxygen-free copper was carried out using a punch with a mirror-finished surface roughness. Using the punch tip deflection as a parameter, a comparison of shearing characteristics was made between a punch with a nitrided tool surface and an untreated punch. The influence on the formation of the sheared surface was considered from an investigation of the shearing characteristics. When shearing oxygen-free copper with a thickness of 500 µm, it was shown that by providing a punch tip deflection of approximately one-tenth of the thickness in the punch stroke direction, the shear droop could be kept to 10 % or less of the plate thickness and a burnished surface ratio was approximately kept 90 %.

Abstract: The influence of the stress state on damage evolution, fracture behavior, and component performance is well established for proportional loading conditions. In contrast, many industrial sheet-forming processes involve non-proportional loading paths, which can significantly alter material hardening and fracture responses. Recent results have shown, that load direction changes affect damage evolution in the dual-phase steel DP800. This paper aims to investigate to what extend these results can be transferred to the aluminum alloy AA6082-T6. Therefore, specimens are first prestrained in uniaxial tension and subsequently reloaded either in the same direction or orthogonally, using additional tensile tests. Fracture strains during the subsequent tensile tests are determined by Aramis DIC. Orthogonal load direction changes lead to an increased fracture strain for DP800, but decreased fracture strain for AA6082. While the observed behavior of DP800 can be attributed to the void morphology, which is established during prestraining, the results of AA6082 indicate different damage mechanisms which cause this behavior.

Abstract: The determination of Forming Limit Curves (FLCs) remains challenging due to their strong dependence on test conditions, material properties, and measurement methods. Significant variability is observed even for identical specimens, limiting the reliability of FLCs as deterministic tools. The goal of this work is to investigate non-deterministic FLC prediction in order to illustrate the impact of uncertainties and provide a basis for quantification of failure risks. The proposed approach uses the probabilistic framework where a model of input uncertainties including dependence is inferred from literature data using a Gaussian copula. A prediction model based on the linear perturbation technique is described and used to propagateinput uncertainties using a Monte-Carlo approach. The obtained stochastic FLC is illustrated in terms of empirical confidence areas.

Abstract: As environmental issues arise, increasing battery efficiency is emerging as an important task. To manufacture more efficient lithium-ion batteries, the industry is striving to increase the depth of battery cell. A growing number of companies are focusing on developing pouch-type batteries using materials with high formability. To produce the battery cell with bigger depth, understanding of the formability of the material is important. Forming limit test can be considered. The material used in pouch-type batteries is an asymmetric aluminum-polymer laminate composite, which is consisted of four layers: polypropylene, aluminum, nylon, and PET. Because of good formability of polymer layers, forming limit curve cannot be obtained using typical forming limit test such as experiment using Nakazima specimen. Therefore, the modified Marciniak testing method is implemented for this research, which is helpful for making strain concentration in the middle of the specimen. In the modified Marciniak test, a dummy sheet is layered between the specimen and punch, to prevent the fracture around the region contacting with the fillet of the punch. In this research, different kinds of polymers were tried as the dummy sheet material. In addition, specimens of various designs were tested and the forming limit test result of Al-Polymer film with thickness of 153\mu m was converted using Polar Effective Plastic Strain (PEPS) approach.

Abstract: Edge cracking is an essential local formability phenomenon encountered in specific forming operations, such as stretch flanging, which is broadly employed in the automotive industry. However, the prediction of the edge cracks is challenging, and the hole expansion test is widely carried out to detect the edge cracking performance. This research analyzed quenched and partitioned steel (QP), one of the third-generation advanced high-strength steels widely adopted in the automotive industry to replace conventional high-strength steels due to its superior global formability features. However, its local formability has been a bottleneck due to its microstructure nature and shows strong anisotropy dependency. Therefore, a modeling framework is needed that consistently incorporates anisotropy in both plasticity and fracture. In this study, the hole expansion performance of QP1000 steel was evaluated through a fully anisotropic fracture model based on the Yld2004-18p anisotropic yield criterion and the DF2016 ductile fracture model. To this end, the standard uniaxial tensile tests in seven material orientations, the bulge test, and the tensile tests of different fracture samples were conducted. These samples were picked out to characterize the broad spectrum of loading conditions and cut in rolling, diagonal, and transverse directions to establish the fully anisotropic fracture model. It was seen that the edge cracking metrics, which are the hole expansion ratio and the fracture initiation zone, were accurately captured by the developed model.

Abstract: The present work investigates the global and local formability of two third-generation Advanced High Strength Steels (AHSSs), a quenching & partitioning (Q&P) steel and a Medium Mn (MMn) steel with 1GPa strength. Third-generation Q&P and MMn steels are designed to overcome the limitations of first-generation AHSS grades by enhancing formability while maintaining high mechanical strength, thus enabling more efficient structural design and improved crash performance. Understanding their forming behaviour is essential to ensure their reliable use in complex sheet metal forming operations. In this study, the forming performance of a Q&P and a MMn steel is analysed through experimental procedures involving both in-plane deformation under various loading paths and hole expansion tests with different hole edge qualities, to evaluate their global and local formability. A first-generation Dual Phase (DP) steel is included in the analysis for comparison. The results demonstrate that 3^rd Generation Q&P and MMn steels exhibit very good global formability, superior to conventional 1^st Generation AHSSs. However, local formability, as evaluated by hole expansion capacity, can be severely compromised by edge manufacturing process. These findings contribute to a deeper understanding of the distinction between global and local formability in third-generation AHSS, offering insights to improve process robustness and support the industrial implementation of these steels in high-performance automotive components.

Abstract: The transition between different shear fracture modes is still not well understood, especially in the case of mode II to mode III fracture. While transitions involving mode I have been investigated in previous studies, the interaction between in-plane and out-of-plane shear fracture remains largely unexplored. This work presents an ongoing study aimed at analysing this transition through a newly proposed formability test based on the compression of a thick sheet. The test is intended to investigate the transition region between the shear fracture forming limit (SFFL) and the out-of-plane shear fracture forming limit (OSFFL), associated with fracture mechanics modes II and III, respectively. A new specimen geometry was developed by combining features of existing configurations designed to activate each fracture mode separately. Finite element simulations were carried out to support the design of the specimen and to provide an initial analysis of the strain loading paths in the ligament region. Preliminary numerical results indicate that the proposed test can promote in-plane shear, out-of-plane shear, as well as mixed fracture mechanisms. The predicted fracture initiation sites and corresponding strain paths were examined in the effective strain–stress triaxiality space to provide initial insight into the role of stress state on fracture behaviour. The influence of notch orientation on the resulting fracture mode is discussed, highlighting the potential of the proposed approach for studying mixed shear fracture conditions.