R-Curve approach to describe the fracture resistance of tool steels

This work addresses the events involved in the fracture of tool steels, aiming to understand the effect of primary carbides, inclusions, and the metallic matrix on their effective fracture toughness and strength. Microstructurally different steels were investigated. It is found that cracks nucleate on carbides or inclusions at stress values lower than the fracture resistance. It is experimentally evidenced that such cracks exhibit an increasing growth resistance as they progressively extend, i.e., R-curve behavior. Ingot cast steels present a rising R-curve, which implies that the effective toughness developed by small cracks is lower than that determined with long artificial cracks. On the other hand, cracks grow steadily in the powder metallurgy tool steel, yielding as a result a flat R-curve. Accordingly, effective toughness for this material is mostly independent of the crack size. Thus, differences in fracture toughness values measured using short and long cracks must be considered when assessing fracture resistance of tool steels, especially when tool performance is controlled by short cracks. Hence, material selection for tools or development of new steel grades should take into consideration R-curve concepts, in order to avoid unexpected tool failures or to optimize microstructural design of tool steels, respectively.Peer ReviewedPostprint (author's final draft

The role of adhesive forces and mechanical interaction on material transfer in hot forming of aluminium

In this work, the mechanisms resulting in transfer of aluminium on hot forming tools have been analysed by means of two separate laboratory tests. The influence of chemical affinity in aluminium adhesion has been studied in contact tests, measuring the force used in pulling at low velocity an aluminium ball pressed against a tool surface. The role of mechanical interaction has been investigated through ball-on-disc sliding tests at high temperature, using tool steel discs with different surface finish against an aluminium counterpart. These tests have been used for the evaluation of different strategies in adhesive wear reduction, including different tool steels and surface modification, and to study the effect of surface finish on the material transfer mechanisms observed.Peer ReviewedPostprint (author’s final draft

Journal of Physics: Conference Series

Lightweight designs and demanding safety requirements in automotive industry are increasingly promoting the use of Advanced High Strength Steel (AHSS) sheets. Such steels present higher strength (above 800 MPa) but lower ductility than conventional steels. Their great properties allow the reduction of the thickness of automobile structural components without compromising the safety, but also introduce new challenges to parts manufacturers. The fabrication of most cold formed components starts from shear cut blanks and, due to the lower ductility of AHSS, edge cracking problems can appear during forming operations, forcing the stop of the production and slowing down the industrial process. Forming Limit Diagrams (FLD) and FEM simulations are very useful tools to predict fracture problems in zones with high localized strain, but they are not able to predict edge cracking. It has been observed that the fracture toughness, measured through the Essential Work of Fracture (EWF) methodology, is a good indicator of the stretch flangeability in AHSS and can help to foresee this type of fractures. In this work, a serial production automotive component has been studied. The component showed cracks in some flanged edges when using a dual phase steel. It is shown that the conventional approach to explain formability, based on tensile tests and FLD, fails in the prediction of edge cracking. A new approach, based on fracture mechanics, help to solve the problem by selecting steel grades with higher fracture toughness, measured by means of EWF. Results confirmed that fracture toughness, in terms of EWF, can be readily used as a material parameter to rationalize cracking related problems and select AHSS with improved edge cracking resistance.Peer ReviewedPostprint (published version

Analysis of fracture resistance of tool steels by means of acoustic emission.

The usage of advanced high strength steels (AHSS) in structural automotive components has been broadened in the past few years to satisfy the strict specifications of the automotive industry. Besides showing excellent strength to weight rations, AHSS have several limitations due to the high loads required in cold forming and cutting tools, which decrease considerably the tooling performances. Therefore, these important forces of impact provoke unforeseen breakage of the dies. The aim of this research is to study the micromechanical behaviour and fracture mechanisms (nucleation and crack propagation) during fracture of tool steels using the acoustic emission (EA) technique. To do that, bending testing specimens of different tool steels were monitored in order to establish a relationship between AE signals and their mechanical behavior (carbide breakage, cracks emanating from them and crack propagation through the metallic matrix).Postprint (published version

Fatigue resistance evaluation of high Mn-TWIP steel through damage mechanics: A new method based on stiffness evolution

The work presented here deals with the implementation of a new methodology that allows fast and reliable determination of the fatigue strength. It is based on monitoring the specimen stiffness changes at different stress levels, as an indicator of the evolution of fatigue damage. This new rapid fatigue test uses techniques available in many laboratories, as the DIC (Digital Image Correlation) technique and common extensometers. Moreover, the obtained data are easier to handle than infrared cameras or acoustic emission systems data, and the experimental procedure to determine the fatigue limit is more evident than in the self-heating method. Experiments have been conducted in TWIP (Twinning Induced Plasticity) steel, a material used for lightweighting the structural parts of vehicles. With their excellent energy absorption capacity, TWIP steels can satisfy the part requirements in terms of crash performance, while their high tensile strength can deal with the cyclic loads acting on chassis parts. Therefore, many efforts focus on improving the fatigue strength of TWIP steels through pre-straining and/or surface treatments. However, finding the best way to improve the fatigue resistance requires time and resources that often hinder the development of the material. For this reason, a TWIP steel has been selected to check the new rapid fatigue test. The prediction made using the proposed approach is validated by comparison with conventional staircase results and fatigue crack growth standardised tests. The good agreement allows proposing the new method as a fast and efficient way to determine the fatigue resistance in metals.Peer ReviewedPostprint (published version

Fracture toughness to assess the effect of trimming on the fatigue behaviour of high-strength steels for chassis parts

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.Postprint (published version

Understanding the fatigue notch sensitivity of high-strength steels through fracture toughness

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.Peer ReviewedPostprint (published version

Effect of sandblasting on low and high-cycle fatigue behaviour after mechanical cutting of a twinning-induced plasticity steel

In the last years, car bodies are increasingly made with new advanced high-strength steels, for both lightweighting and safety purposes. Among these new steels, high-manganese or TWIP steels exhibit a promising combination of strength and toughness, arising from the austenitic structure, strengthened by C, and from the twinning induced plasticity effect. Mechanical cutting such as punching or shearing is widely used for the manufacturing of car body components. This method is known to bring about a very clear plastic deformation and therefore causes a significant increase of mechanical stress and micro-hardness in the zone adjacent to the cut edge. To improve the cut edge quality, surface treatments, such as sandblasting, are often used. This surface treatment generates a compressive residual stress layer in the subsurface region. The monotonic tensile properties and deformation mechanisms of these steels have been extensively studied, as well as the effect of grain size and distribution and chemical composition on fatigue behaviour; however, there is not so much documentation about the fatigue performance of these steels cut using different strategies. Thus, the aim of this work is to analyse the fatigue behaviour of a TWIP steel after mechanical cutting with and without sandblasting in Low and High-Cycle Fatigue regimes. The fatigue behaviour has been determined at room temperature with tensile samples tested with a load ratio of 0.1 and load amplitude control to analyse High-Cycle Fatigue behaviour; and a load ratio of -1 and strain amplitude control to determine the Low-Cycle Fatigue behaviour. Samples were cut by shearing with a clearance value of 5%. Afterwards, a part of the cut specimens were manually blasted using glass microspheres of 40 to 95 microns of diameter as abrasive media. The results show a beneficial effect of the sandblasting process in fatigue behaviour in both regimes, load amplitude control (HCF) and strain amplitude control (LCF) tests, when these magnitudes are low, while no significant differences are observed with higher amplitudes. low-cycle fatigue, high-cycle fatigue, mechanical cutting, sandblasting, high manganese steel, TWIP steel. © The Authors, published by EDP Sciences, 2018.Postprint (published version

Simulación de la compactación de polvo metálico: efecto del estado tensional de la matriz de compactación

En este trabajo se presenta una primera simulación mediante el programa de elementos finitos ABAQUS de un proceso completo de compactación en frío de polvo metálico. El análisis comprende la colocación en la matriz de un inserto resistente al desgaste, la compactación del polvo y la posterior eyección del compacto. Se estudia la relevancia de la naturaleza del inserto sobre las características dimensionales del componente. Se analiza la problemática de utilizar un código implícito y la definición de contactos pieza-inserto.Postprint (published version

Modelización y simulación de la compactación de una pieza pulvimetalúrgica

En la industria del conformado, el diseño y el control de los procesos se han basado en la experiencia y, a menudo, se ha considerado un arte. La aplicación de los métodos numéricos reduce el coste del diseño y del propio proceso de elaboración y contribuye a mejorar la calidad del producto. En este trabajo, los autores se plantea la situación actual en el campo de la modelización del comportamiento mecánico de los materiales pulvimetalúrgicos y, mediante el modelo de plasticidad Drucker-Prager/CAP, y el programa de cálculo por elementos finitos ABAQUS, se simula el proceso de compactación y eyección de una pieza PM estructural.Postprint (published version