Sebastian Münstermann

A hybrid approach for modelling of plasticity and failure behaviour of advanced high-strength steel sheets:

The ductile damage mechanisms dominating in modern high-strength steels have emphasised the significance of the onset of damage and the subsequent damage evolution in sheet metal forming processes. This paper contributes to the modelling of the plasticity and ductile damage behaviour of a dual-phase steel sheet by proposing a new damage mechanics approach derived from the combination of different types of damage models. It addresses the influence of stress state on the plasticity behaviour and onset of damage of materials, and quantifies the microstructure degradation using a dissipation-energy-based damage evolution law. The model is implemented into ABAQUS/Explicit by means of a user material subroutine (VUMAT) and applied to the subsequent numerical simulations. A hybrid experimental and numerical approach is employed to calibrate the material parameters, and the detailed program is demonstrated. The calibrated parameters and the model are then verified by experiments at different levels, and a good agreement between the experimental and numerical results is achieved.

Evaluation of the cold formability of high-strength low-alloy steel plates with the modified Bai–Wierzbicki damage model

While strength and toughness properties of construction steels are major mechanical properties with respect to the safety assessment of components, increasingly often requirements are defined on the processing properties of materials, in particular cold formability properties. So far, the cold formability of structural steels is characterized in terms of three-point bending tests, which result in very expensive experimental effort and limited understanding of the material behaviour under various stress states. In this paper, the cold formability of a structural steel S355J2 + N is characterized under various stress states in a laboratory scale by a recently proposed hybrid damage mechanics approach. A damage initiation criterion that considers the influence of stress triaxiality and Lode angle is used to describe the onset of material degradation in a microstructural scale in the approach. Subsequent damage evolution is followed to further quantify the accumulation of damage till the final fracture. A series of laboratory tests is designed to calibrate the model parameters as well as to verify the calibration. Good agreement of experimental and numerical force–displacement responses proves the predicative capability of the model. With the laboratory findings, the industrial scale three-point bending test is simulated to predict the forming limit. In addition, the numerical study reveals that the stress state of the critical element in three-point bending test coincides with plane-strain tension, which results in a simpler method to characterize the bendability to reduce the experimental effort.

Design of damage tolerance in high-strength steels

Abstract With the extensive application of advanced high-strength steels in various fields due to their excellent mechanical properties, industrial interest in the damage of advanced high-strength steels has increased in recent years. For these modern steels, due to their multiphase microstructure and different deformation mechanisms, the concept of damage must be re-examined. In this paper, the definition, length scale and different mechanisms of damage are introduced. Both the experimental evaluation of damage and the numerical damage models are briefly reviewed and compared. The approaches to improving damage tolerance are given in the framework of the damage tolerance design principle.

Modeling of Damage and Failure of Dual Phase Steel in Nakajima Test

For modern high strength steels, instead of metal instability, ductile damage triggered by the formation of microvoids or microcracks resulting from the complex material microstructure, has become the key factor responsible for the final failure in the forming process of such steels. The target of this study is to describe the initiation and evolution of damage in a dual-phase (DP) steel (DP600). By applying a newly proposed approach that is able to indicate the onset of damage in an engineering sense and quantify the subsequent damage evolution, to predict the forming limits for DP600 are predicted by simulating Nakajima test. Accordingly, two forming limit curves (FLC) are numerically computed to characterize two moments: when damage becomes pronounced and when the final failure is triggered by the accumulation of damage. Comparing with the conventional experimentally calibrated FLC at necking, the limit at crack initiation predicted by modeling gives a lower but defect-free forming boundary. The forming limit at final fracture is well captured by allowing the subsequent damage evolution to a critical value.

A Generalized Damage Model Accounting for Instability and Ductile Fracture for Sheet Metals

With the requirement of vehicle performance and fuel economy, dual-phase (DP) steels as one of the advanced high stress steels (AHSS) are increasingly used in the automotive industry due to the excellent combination of the tensile strength and ductility. On a microscale the ductile fracture is governed by the void nucleation, growth and coalescence mechanism. In the dual-phase steels this damage mechanism exhibits a rather complex situation: voids are generated by the debonding of the hard phase from the matrix and the inner cracking of the hard phase besides by inclusions. On a macroscale fracture of these materials is observed in the automotive industry with the absence of strain localization or minimal post-necking deformation. Consequently the failure during the forming process is caused by a competitive or combined mechanism of internal damage evolution and metal instability. In this study, the target is to develop a simple and generalized model for metal forming processes accounting for instability, damage and ductile fracture. Theoretical predictions of metal instability by the Hill–Swift necking criterion and the modified maximum force criterion are considered. The damage model is developed by the combination of the prediction of metal instability and ductile fracture of sheet metals. The model is developed in 3D triaxial stress state and the accumulation of damage is stress state dependent. Furthermore, the influence of the hardening curve effected by damage on the forming limit curve is investigated.

Influencing parameters on elastic modulus of steels

Abstract An optimised balance between weight and stiffness of a car component is achieved when the selected material exhibits an increased ratio between elastic modulus and density. The elastic modulus of steel decreases progressively with increasing temperature due to the thermal activation of lattice vibrations. Since the elastic modulus of metals is related to their lattice constants, austenitic steels are expected to exhibit higher elastic moduli than ferritic steels because of their higher packing density. However, the coupling of electron spin moments due to the ferromagnetic character of bcc steels results in higher elastic modulus of ferritic steels at temperatures below the Curie temperature. An increase in elastic modulus in selected loading directions can be realised by properly adjusted crystallographic textures. However, no examples were found where the maximum elastic modulus exceeds 225 GPa in bcc steels with pronounced crystallographic textures. Alloying elements decrease the elastic modulus with increasing alloying content. Known exceptions from this rule are the elements Re, Co and Cr. The elastic modulus of steel can be increased significantly by particles of high elastic modulus. Finally, it is reduced by plastic deformation because of the increased dislocation density. This negative effect can be balanced by subsequent heat treatments. On obtient une balance optimisée entre le poids et la rigidité d′une piéce d′automobile lorsque le matériau choisi exhibe une augmentation du rapport entre le module d′élasticité et la densité. Le module d′élasticité de l′acier diminue progressivement avec l′augmentation de la température à cause de l′activation thermique des vibrations du réseau. Puisque le module d′élasticité des métaux est relié à leurs constantes de réseau, on s′attend à ce que les aciers austénitiques exhibent des modules d′élasticité plus élevés que ceux des aciers ferritiques parce qu′ils possédent une plus grande densité de tassement. Cependant, le couplage des moments du spin électronique, dû au caractére ferromagnétique des aciers bcc, résulte en un module d′élasticité plus élevé des aciers ferritiques à des températures plus basses que la température de Curie. On peut augmenter le module d′élasticité dans des directions choisies de charge en ajustant correctement les textures cristallographiques. Cependant, nous n′avons pas trouvé d′exemples où le module d′élasticité maximum excédait 225 GPa dans les aciers bcc ayant des textures cristallographiques prononcées. L′augmentation de la teneur en éléments d′alliage diminue le module d′élasticité. Les éléments Re, Co, et Cr font exception à cette règle. On peut augmenter significativement le module d′élasticité de l′acier à l′aide de particules à module d′élasticité élevé. Finalement, le module d′élasticité est réduit par déformation plastique à cause de l′augmentation de la densité de dislocation. On peut balancer cet effet négatif au moyen de traitements thermiques subséquents.

Einflussgrößen auf den Elastizitätsmodul von Stählen für den Karosseriebau

Kurzfassung Im Automobilbau wird seit einigen Jahren nachdrücklich das Ziel verfolgt, möglichst leichte Karosserien herzustellen. Aus diesem Grund werden in vielen Fällen die Blechdicken der einzelnen Karosseriebauteile reduziert. Die Tragfähigkeit eines Bauteils hängt aber von der Festigkeit des verwendeten Werkstoffs, von der Blechdicke und von der Bauteilform ab. Deshalb würde durch den Einsatz von dünneren Blechen die Tragfähigkeit der Karosserie herabgesetzt, wenn nicht gleichzeitig Stähle mit erhöhter Festigkeit verwendet würden. Infolgedessen haben sich in den letzten zehn Jahren die Liefermengen der höher-, hoch- und höchstfesten Stähle verfünffacht [1]. In erster Näherung hat sich der Elastizitätsmodul der genannten modernen Stähle mit erhöhter Festigkeit im Vergleich zu klassischen weichen Tiefziehstählen zwar nicht geändert. Im Automobilbau sollten aber die verwendeten Werkstoffe einen möglichst großen Elastizitätsmodul besitzen. In einer im Jahr 2002 durchgeführten Literaturstudie wurden die Einflussgrößen auf den Elastizitätsmodul von Stählen mit dem Ziel zusammengefasst, Möglichkeiten zur gezielten Einstellung eines besonders hohen Elastizitätsmoduls aufzuzeigen. Die Ergebnisse dieser Studie werden nachfolgend dargestellt.

Exploiting the Property Profile of High Strength Steels by Damage Mechanics Approaches

Abstract Although high strength low alloy steels offer the potential for lightweight design, their application is often rather limited due to existing design rules. A further development of these standards requires precise prediction of realistic limit states. The damage model of Bai and Wierzbicki has been extended in order to improve the description of the ductile failure behaviour of steels, especially when crack initiation in dynamic loading is to be simulated. Application examples of the extended model are given in order to show that it can be used for limit state analysis. An outlook presents the application of the model to derive improved safety factors for probabilistic safety concepts.

Der Kurzrisseffekt bei der bruchmechanischen Prüfung: A unified method to treat fatigue damage

Kurzfassung Unter dem ausgeprägten Kurzrisseffekt versteht man den Anstieg von zähbruchmechanischen Initiierungskennwerten bei reduzierter Ermüdungsrisslänge. Für Bruchmechanikproben mit niedrigem a/W-Verhältnis, also geringer Ermüdungsrisslänge, werden bei Vorliegen eines ausgeprägten Kurzrisseffekts demnach höhere Initiierungskennwerte ermittelt als für Proben mit größerem a/W-Verhältnis. In mehreren Forschungsarbeiten der Vergangenheit wurden teilweise unterschiedliche Ergebnisse zum Kurzrisseffekt bei Stahl erzielt. Ein im vergangenen Jahr durchgeführtes gemeinsames Forschungsprojekt der Materialprüfungsanstalt Uni Stuttgart (MPA) und des Instituts für Eisenhüttenkunde der RWTH Aachen (IEHK) sollte die Ursachen für die unterschiedlichen Ergebnisse klären.

Crystal plasticity assisted prediction on the yield locus evolution and forming limit curves

The aim of this study is to predict the plastic anisotropy evolution and its associated forming limit curves of bcc steels purely based on their microstructural features by establishing an integrated multiscale modelling approach. Crystal plasticity models are employed to describe the micro deformation mechanism and correlate the microstructure with mechanical behaviour on micro and mesoscale. Virtual laboratory is performed considering the statistical information of the microstructure, which serves as the input for the phenomenological plasticity model on the macroscale. For both scales, the microstructure evolution induced evolving features, such as the anisotropic hardening, r-value and yield locus evolution are seamlessly integrated. The predicted plasticity behaviour by the numerical simulations are compared with experiments. These evolutionary features of the material deformation behaviour are eventually considered for the prediction of formability.