Key Engineering Materials Vols. 651-653

Abstract: The use of finite element simulations has become one of the main tools of the mechanical engineer. The method is applied to the analysis and design of engineering structures, the study of manufacturing processes and even to perform virtual experiments. Traditionally, the constitutive laws chosen for finite element analysis have been as simple as possible, mainly due to the limitation imposed by the available computing power. However, the development of more powerful computers and more efficient methods is opening the possibility of using more elaborated (and, most often, more accurate) material models. In particular, polycrystal models capable of predicting not only the mechanical behaviour of the material, but also of the evolution of properties with increasing strain, are particularly well suited for the simulation of forming processes, for which a precise knowledge of the properties of the resulting product is of paramount importance.The present work studies how the Visco Plastic Self-Consistent model (VPSC) can be used in combination with the implicit finite element package Abaqus/Standard to simulate the behaviour of Ti-6Al-4V sheet, and compares it with the more common (and much simpler) Johnson-Cook model. More specifically, the goal of this study is to determine whether or not, with using similar experimental calibration data, the use of the much more complex polycrystal model, justifies the increased complexity and execution time. Using standard tensile experiments at different strain rates, the parameters of the VPSC and Johnson-Cook models are fitted using a minimization method. Then, both models are used in finite element simulations and the results given by both models are compared.

Abstract: The plastic deformation behavior of dual-phase (DP) steel is strongly affected by its underlying three-dimensional (3D) microstructural factors such as spatial distribution and morphology of ferrite and martensite phases. In this paper, we present a coupled simulation method by the multi-phase-field (MPF) model and the crystal plasticity fast Fourier transformation (CPFFT) model to investigate the 3D microstructure-dependent plastic deformation behavior of DP steel. The MPF model is employed to generate a 3D digital image of DP microstructure, which is utilized to create a 3D representative volume element (RVE). Furthermore, the CPFFT simulation of tensile deformation of DP steel is performed using the 3D RVE. Through the simulations, we demonstrate the stress and strain partitioning behaviors in DP steel depending on the 3D morphology of DP microstructure can be investigated consistently.

Abstract: The plastic behavior of the Ti-6Al-4V alloy includes several features as strength differential effect, anisotropy and yield strength sensitivity to temperature and strain rate. Monotonic tensions in the three orthogonal directions of the material are performed to identify the Hill '48 yield criterion. Monotonic compression and plane strain tensile tests are also included in the experimental campaign to identify the orthotropic yield criterion of CPB06. An assessment of the two models is done by comparing the yield loci and the experimental data points for different levels of plastic work. A first approach of the damage modelling of the Ti-6AL-4V alloy is investigated with an extended Gurson-Tvergaard-Needleman damage model based on Hill '48 yield criterion. Finite element simulations of the experiments are performed and numerical results allows checking force-displacement curves until rupture and local information like displacement and strain fields. The prediction ability of the Hill '48, CPB and extended Gurson models are assessed on simple shear and notched tensile tests until fracture.

Abstract: Development and validation of the micro scale cellular automata (CA) model of dynamic recrystallization (DRX) were the main goals of the present paper. Major assumptions of the developed CA DRX model, which is based on the Digital Material Representation (DMR) concept, are described. Parameters like neighborhood type, state and internal variables of the proposed model and their influence on final results are presented and discussed. Particular attention was put on description of the developed transition rules used to replicate mechanisms leading to dynamic recrystallization. Finally, obtained results in the form of flow stress curves are compared with the experimental predictions.

Abstract: The plastic response of an aluminium alloy type A6061 is modelled using different state parameter‐based approaches. Several of these models (one‐ and two‐parameter models) have recently been implemented into the thermo‐kinetic software package MatCalc. In the present work, a model based on the Kocks-Mecking-law is used to investigate the capabilities of one and two parameter approaches in order to model experimental data. The experimental work presented here is performed on a Gleeble 1500 thermo‐mechanical simulator for different natural ageing times. We demonstrate that one‐parameter models offer a ready‐to‐use and easy‐to‐calibrate solution for a rough correlation between flow‐curve data and microstructure. Such models describe the evolution of the average dislocation density in time. In many practical cases, a single state parameter is insufficient and multi‐parameter models must be utilized, for instance, with consideration of separate populations of dislocations in walls and subgrain interior. These approaches can consistently represent the deformation behaviour of alloys in a variety of conditions with respect to temperature and strain rates.

Abstract: The paper deals with the example of application of the two continuum rheological model of materials with microdefects, nanodefects and solute hydrogen for calculation of stress and strain in cylindrical specimen under periodic loading. The model suggested allows one to relate the mechanical characteristics with the hydrogen concentration.The stability analysis of the system metal-hydrogen is carried out. The influence of parameters of the mechanical loading, hydrogen concentration and parameters of sorption and desorption of hydrogen from the surface of the internal defects (traps) of various nature on the system stability is performed.It is shown that influence of hydrogen can be considered as parametric instability of a continuous medium under mechanical deformation.This can be important during forming or plastic deformation of materials and nanomaterials containing hydrogen.

Abstract: This paper shows that non-linearity of mechanical behaviour of metal in the elastic regime has an influence on the forming process. Discrete Dislocation Dynamics simulations show that pure elastic behaviour is altered when reversible dislocation displacements occur even in the very beginning of the elastic stage. The influence of such a non-linearity has an impact on the results of numerical simulations of industrial forming of very thin metal sheets of copper.

Abstract: A two-continua model is constructed which allows one to describe the kinetics of hydrogen in metals. The developed rheological model is appropriate for estimation of the hydrogen transition from mobile to bonded state depending on the stress-state relation and description of the localization of the connected hydrogen that results in the material fracture.

Abstract: Bending is an important forming process for ultra-high strength steel (UHSS) because it is cost-effective, fast and in many cases it can be used to replace welding in a part manufacturing processes. One major challenge in air bending of UHSS is to predict the limits for bendability since the traditional methods for failure prediction, such as forming limit diagram (FLD), cannot generally be applied to bending process. In this paper, 3D FE-modelling coupled with a CDM-damage model is used to simulate the air bending process and to determine the bendability limits for a hot-rolled 960MPa grade. Damage parameters for the CDM-model are determined by using optical strain measurements and inverse modelling of the tensile test. Three point bending tests with optical strain measuring were carried out to determine the deformation field of the outer bend in different bending angles and the results of the bending simulation are compared with the strain measurements of the bending tests. The damage model is then calibrated using the experimental results of the bending tests to adjust the crack occurrence in the simulation. A good agreement was found between simulations and experimental measurements.

Abstract: The mechanical and thermal properties of metallic materials are strongly related to theirmicrostructure. An accurate and quantitative prediction of microstructural evolutions is then crucialwhen it comes to optimize the forming process. Recently a new full field approach, based on a Level-Set (LS) description of interfaces in a finite element (FE) context has been introduced to model 2D and3D primary recrystallization (ReX), including the nucleation stage [1, 2], and has been extended to takeinto account the grain growth (GG) stage [3, 4]. The ability of this approach to model also the Zenerpinning (ZP) phenomenon without any assumption concerning the shape of second phase particleswas also demonstrated [5]. Moreover, recent developments have also illustrated the capability of thisapproach to take into account the characteristics of twin interfaces during grain boundary motion [6,7]. Current work concerns also the improvement of the numerical cost of this new approach [8]. Allthese developments are necessary to account for the microstructural complexity of ReX phenomenon.