List of Abstracts 2018: SESSION F - MODELING OF TECHNOLOGICAL PROCESSES
Date: November 15, 2018
Prediction of initiation and propagation of cracks may be an important issue in the assessment of ductile metallic structures in many practical engineering problems. It involves the development of accurate and robust constitutive models that can mimic the progressive material degradation due to nucleation, growth, and coalescence of microvoids until and subsequent to the formation of meso-cracks.
However, despite the underlying clever simplifications of the microstructural complexity provided by those models, the presence of a softening behaviour at the constitutive level is known to be a possible source of problems in their numerical implementation.
Regularized solutions may be adopted resorting to gradient theories in which non-local variables may be introduced with the recourse of a diffusive type equation relying on of a characteristic length associated with the size of the non-local support region. The more recent extension of the phase-field concept to ductile failure problems is a similar alternative approach of dealing with discontinuities within a continuum framework, sharing many resemblances with more traditional gradient models.
The similar nature of the mathematical coupled temperature-displacement problems and gradient and phase field models for material ductile failure may be explored by resorting to built-in thermo-mechanically coupled finite-element solutions in common commercial software codes, avoiding the necessity special user subroutines. Some benchmarking examples are used to validate this proposed methodology.
The paper presents the results of the analysis of computer simulation of unidirectional and alternating torsion of cylindrical samples in the software complex Deform 3D. The accumulated deformation degree and damage criterion were taken as compared parameters. Damage was calculated by two existing methods: Cockroft-Latham and Bogatov methods. The results of the analysis show that the proposed method for determining the damage criterion is more accurate when testing samples for alternating twisting than the most currently used method since it takes into account the number of loading stages and the plastic deformation amplitude.
In recent years the use of the Mohr-Coulomb type fracture criteria has become popular in ductile fracture prediction. In ductile fracture prediction, fracture criteria are often transformed into the mixed stress strain space formulation and applied to numerical simulations in this formulation. This paper will investigate the shape of the Mohr Coulomb fracture criterion in the mixed stress strain space as a function of the used hardening curve formulation, in particular the hardening curve exponent n that determines the slope of the hardening curve approximation for large strains. It is found that the shape of the fracture criterion significantly depends on the slope of the hardening curve. This makes a correct extrapolation of the hardening curve for large strain values essential for a correct ductile fracture prediction using the Mohr-Coulomb fracture model in its mixed stress strain space formulation.
In the presented study the hot deformation behavior of Ti-6Al-2Sn-4Zr-6Mo (Ti-6246) alloy was investigated by isothermal hot compression tests conducted on the Gleeble 3800 thermomechanical simulator in the temperature range of 800-1100 °C and in strain rate range of 0.01 - 100 s-1 to true strain of 1. For determining the unstable flow regions of the investigated alloy during hot deformation, the processing maps based on the Malas’s criterion of plastic flow stability were used. For characterization of the material flow during forming operations, the microstructure analysis of the processed alloy was performed and associated with the processing maps. Basing of this analysis, the results of predicting the flow instability were confirmed, what made possible to understand the mechanical behavior of Ti-6246 alloy in a wide range of strain rates and temperatures.
Date: November 15, 2018
Introducing IDIADA CZ a.s. includes development activities in the following areas:
- Technical calculations, - Construction, - Testing, - Surface modeling.
Heat is used for welding and during heat treatment. In both processes the material properties change due to temperature gradients and resulting microstructure change. A physically realistic simulation has to consider the interaction of process model and material model.
The simulation chain material - welding - heat treatment enables the prediction of the state and the characteristics of products. Additionally, it can also enable the design of products from scratch - without any physical test.
Design parameters would be the chemical composition of the alloy as well as the process parameters for welding and heat treatment. The practical use of the targeted complex simulations in industrial environments requires user-friendly pre- and post-processors.
Using state-of-the-art materials information and FEA-solvers as a starting point the authors developed solutions for different applications which facilitate the simulation of complex processes so that they can also be used by non finite element experts.
Residual stresses in components are an important issue in most manufacturing processes, as they influence the performance of the final part. Regarding hot forming processes there is a great potential of defining a targeted residual stress state, due to numerous adjustment parameters like deformation state or temperature profile. In order to ensure appropriate numerical modelling of resulting residual stresses in a thermomechanical process, comprehensive material data regarding phase transformation is required. This paper presents an experimental-numerical procedure to efficiently determine time-temperature-transformation diagrams for cooling simulations after hot forming. The transformation behaviour of the steel alloys 42CrMo4 and 100Cr6 is determined by experiments as well as FE-simulations. Finally, the simulation model is validated by dilatometric experiments and metallographic investigations.
Improving the quality of metal products by grinding of their structure is one of the main trends in materials science. However, the main problem of such materials is the small final size of the workpieces and low technological production. The paper presents data of the experimental production of ultra-fine grained structure in a long rod by radial-shear rolling. With a grain size of 300-600 nm, the mechanical properties and microhardness increase by more than 2 times while having a heterogeneity across the bar section.
Date: November 15, 2018
In the research work the results of hot extrusion process of aluminium alloy compacts conducted at the temperature of conventional hot forming as well as at the temperature range corresponding to the semi-solid state were presented. Initial material in uniaxial pressing process of Al17Si5Fe3Cu1,1Mg0,6Zr master alloy powder was prepared. Size of powder particles do not exceed 60 µm. Compacts in isothermal conditions in various temperatures were extruded. The extrusion process was carried out under isothermal conditions, temperature stability was strictly controlled. During hot deformation process the changes of extrusion forces in function of punch displacement were registered, which was used to demonstrate the effects associated with the appearance of the liquid phase in the volume of the material. After extrusion samples were cooled in water as well as at the room temperature. The influence of temperature of extrusion and the resulting fraction of the liquid phase on the state of microstructure and chosen mechanical properties was determined. Metallographic observations allows to estimate changes in microstructure due to various temperatures of extrusion process. Mechanical properties of the tested alloy after deformation were carried out by measuring the hardness with the Vickers method and the uniaxial compression test. Additionally one of compact was remelted and cast, which made it possible comparison of microstructure and chosen properties of the products. In results of investigations has been proved, that extrusion in semi-sold state results in reduction of forces necessary to complete deformation process and leads to obtain material with good mechanical properties and fine, favourable microstructure.
The development of high-speed railways (HSR) is an important task aimed at improving the transport system of today's global community. The most heavily-loaded element of HSR, governing their reliability and lifetime, is a contact wire. In this connection, the present study was focused on designing a new set of physico-mechanical properties of a wire made from the heat-treatable Cu-0.65Cr alloy with the use of a new procedure of continuous plastic processing, based on the principles of severe plastic deformation. Thus, an important difference from the common technical solutions used in the production of wires was the use of a combination of SPD processes - radial forging and ECAP-Conform, joined with the shape-forming of shaped sections of wire. In our study, we performed finite-element computer and physical modeling of the processes of plastic and heat treatment. Using computer modeling, we demonstrated that as a result of the implementation of the new procedure of continuous processing, a rather homogeneous strained state is formed in the workpiece, and the accumulated strain is in a range of е=6-7. At all stages of plastic processing, compressive stresses prevail in the deformation site. As a result of physical modeling, we produced laboratory samples of contact wire from the heat-treatable Cu-0.65Cr alloy with a cross-section area of 150 mm2. Metallographic studies reveal that a banded structure of a grain-subgrain type with a fragment size below 1 micron is formed in the laboratory samples of contact wire. The ultimate tensile strength of these samples after heat treatment is 550-560 MPa, the electrical conductility is 72-75% IACS, the ductility is 16-20%.
ACKNOWLEDGEMENTS: This research was supported by the Ministry of Science and Higher Education of the Russian Federation under grant agreement No. 14.586.21.0025 (unique project identifier RFMEFI58616X0025).
Abaqus is one of the world's leading Finite Element Codes. It is used across all industries, but most notably in automotive and aerospace. Abaqus provides two types of solver: implicit and explicit.
In past decades the finite element models have developed rapidly, leveraging the computation power of newer and more powerful computers. This model development most notably concerns the model size (number of DOFs) and number of contacts.
Now days hardware offers multiple CPUs and even specialized GPUs can be purchesd in order to speed up the solver. This can be very beneficial with respect to the solving-time and ultimately to the simulation cost.
However there is a potential issue with using High Performance Computing. Some tasks can be parallelized better, some not at all. Some tasks can benefit greatly from employing GPUs while others may even slow down.
This presentation provides the listener with basic ideas on this topic. An in-house study has been carried out in order to identify the influence of hardware on the solving time. All tasks used for this study were chosen in a way to cover models with small as well as large numbers of DOFs, solid and shell elements, contacts, static, dynamic, harmonic and creep tasks.
Date: November 16, 2018
The presentation deals with damage and failure behavior of ductile sheet-metals. In this context a continuum damage and failure model is introduced and new biaxial experiments have been performed.
The proposed continuum damage model is based on a damage criterion and a damage law both depending on stress triaxiality and the Lode parameter. Consequently, the continuum model can reflect different stress-state-dependent damage and failure mechanisms acting on the micro-level like growth of voids for more elevated stress triaxialities and formation of micro-shear-cracks for lower or negative stress triaxialities.
Recently, new specimens have been proposed for in-plane biaxial testing. These specimens can be applied under a big variety of loading conditions generating different stress states leading to different damage and fracture mechanisms. In addition, these specimens are also applicable to study the material behavior under non-proportional loading paths. In this context, new results are presented for the aluminum-alloy EN AW-6082 including strain fields reported by digital image correlation, fracture surfaces studied by scanning electron microscopy and images of the fractured specimens. These experimental data can be used to validate damage and fracture models for the investigated aluminum-alloy and, thus, to facilitate numerical simulations.
This contribution is concerned with determining the edge strain in edges with different finish qualities associated with crack formation in the free edge of a deep drawing blank. The sheared edge quality is of major importance, particularly in high-strength steels which are used for safety components. The purpose of this study was to determine for various parting processes for sheet metal the limit strain which leads to cracking in the free edge. The parameter which was varied in this investigation was the parting process that produced the edge: shearing and machining. A special fixture was employed for measuring the strain value. The values were verified using FE modelling with AutoForm software. The surface finish qualities of the edges were compared using a special test procedure, the Diabolo test, which had been previously described in publications by Held [1] and Liewald and Gall [2]. In this test, the sheared edge is subjected to loading under plane strain conditions. Another test of susceptibility to edge cracking is the hole expansion test [4], in which crack is induced in a circular hole in the centre of a part. The limit edge strain can be identified using a DIC optical measuring system which monitors strain increments in the course of the test. Thanks to these results, the factors which affect the susceptibility of the sheared edge to edge cracking material have been identified and validated in the DP1000 material.
The main goal of the present paper is investigation of the mechanical behaviour of high manganese content steel sheet which is well known in the literature as Twinning-Induced Plasticity (TWIP) under linear and nonlinear loading path. Contrary to Nakajima experiment, cruciform specimen under biaxial loading can provide linear and nonlinear strain path. This work aims to determine the numerical Forming Limit Curve (FLC) for TWIP sheet using cruciform specimen under biaxial loading. For this purpose, uniaxial tensile tests are performed on standard dog-bone samples which are extracted from rolling, transversal and diagonal directions to identify the flow curve as well as Lankford ratios. Data are used as input for numerical simulation of cruciform specimen. Results show the applicability of cruciform specimen for linear and nonlinear strain paths.
The best tool for characterizing the behaviour of sheet metal during press forming is numerical modelling based on the finite element method (FEM). Using this technique, the response of the press-formed part can already be predicted during the process design stage. The main input data for the process simulation include the tool geometry and materials properties and process conditions, such as temperature, pressing speeds, lubrication and others. The resultant shape of the press-formed part is governed predominantly by the shape of the tools and the workpiece material. The input information on the workpiece material has the form of a material model whose accuracy dictates the accuracy of results. Where high-strength steels are used in the automotive industry, one should focus on springback which alters the shape of press-formed parts. Springback is an undesirable change to the part’s shape caused by residual stresses being released after its removal from the tool. Tooling designers need to take springback into account by choosing an appropriate material model. Conventional hardening models, such as kinematic and isotropic hardening rules, are not adequate for modelling springback. Instead, more sophisticated models, such as the one introduced by Lemaitre and Chaboche are a better choice. In this paper, the options for measuring and FE modelling of the Bauschinger effect are discussed. The proposed idea was tested on a 1.5 mm sheet of DC01 deep-drawing steel.
Date: November 16, 2018
Aspects of emergence of the internal stretching tension in the cast irons depending on a form, the size and number of graphite inclusions are considered. It is theoretically proved from the point of view of the theory of elasticity that bigger preshrink able expansion in castings from high-strength cast iron, in comparison with gray cast iron, directly after crystallization under all other equal conditions, depends on distribution of the internal stretching tension in a casting body. It is also proved that in castings from high-strength cast iron with reduction of the amount of graphite inclusions and increase in their quantity preshrinkable expansion decreases. These conclusions can be used for regulation of dimensional accuracy of castings from cast iron and decrease in volume of profitable part of casting.
The engineering importance of the problem of interface delamination for graphene monolayer nanocomposites is of high level interest and is directly connected with the optimal choice of the structure geometry and a choice of the matrix material. In this work modelling and parametric analysis of cohesive interface delamination of graphene monolayer nanocomposites are presented, using shear lag method. The external excitation of the matrix is an axial displacement, controlling the interface delamination. The statement of the considered problem is given in displacements, the interface behavior is assumed to be a bi-linear cohesive one. In three detected zones along the interface the solutions of the system of two ordinary differential equations about the displacements of the monolayer of graphene and that one for the matrix are fitted by appropriate boundary, symmetry and contact conditions.
All analytical results about interface shear stress, lengths of the elastic, cohesive and delaminated zones are calculated for a given structure and are illustrated in figures. The parametric analysis and the influences of the interface length, applied load and the thickness of the host layer are performed. After detailed discussion some recommendations are given.
An investigation on the interfacial stress transfer in nanocomposite between a monolayer on graphene and a common size polyethylene terephthalate (PET) matrix under extensional traction applied to the matrix is presented. The constitutive law describing the behavior of the interface is assumed to be a bi-linear cohesive one. Three zones are detected along the interface: elastic, cohesive and delaminated. The system of ordinary differential shear lag equations together with boundary, symmetric and contact conditions is solved consequently at each zone and the respective solutions are fitted at common points. The values of these common points - the ends of the lengths of the elastic, cohesive and delaminated zones are determined by the aid of the solutions of two nonlinear algebraic system equations about them. All results are illustrated in figures and discussed in detail. A comparison of the results obtained with the model considered and experimental data of Guo & Zhu (2015) is done and confirms the results from Guo’s experiment that for the structure of a length 21.8x10-6 m the interface delamination is not observed. The implemented method offers a simple tool to understand the nature of the interface behavior and evaluation of the damaging of the nanostructure.
In this paper the results of the numerical and experimental test of Vacuum Packed Particles (VPP) will be presented. Vacuum packed Particles it s the structure compose of grains inside the plastomer coat. When the pressure inside the envelope is equal or higher than 0, systems behave like a liquid. Other wise it behave like a elasto-plastic solid. It is possible to change the mechanical properties of the structure by changing the vacuum inside the coat. Since the mechanical properties of the structure can be change in the real time, it is possible to include this material to smart material group.
Author will presented the influence of the strain rate on the behaviour of the VPP. Additionally the resulst of the behaviour in complex stress/strain state will be presented. At the end of this work the modeling metodology using FEA will be proposed.