Time: Tuesday, 12/Sept/2023: 1:40pm - 3:20pm Session Chair: Knut Andreas Meyer Session Chair: Tobias Kaiser |
Location: EI8 |
A nonlocal model for damage-induced anisotropy in concrete
Technische Universität Braunschweig, Germany
A better understanding of the stress-deformation behavior of concrete structures under different loading and environmental conditions is inevitable to maintain the structural integrity and to avoid catastrophic failures. In the framework of continuum damage mechanics, several material models have been developed in the past to investigate the constitutive response of concrete under different conditions. It has been observed from the experimental studies that the elastic response and stiffness degradation of concrete are dependent on the orientation of micro-cracks and direction of applied loading. This necessitates the incorporation of damage-induced anisotropy[1] in material models for concrete.
In this regard, an anisotropic damage model that describes the softening response of concrete under different loading conditions is developed applying finite element formulations. A two dimensional damage effect tensor is employed to describe the anisotropic evolution of damage. Damage models that take account of the softening responses are known for their spurious mesh dependencies. An implicit gradient enhancement technique introducing an internal length scale is implemented to overcome the numerical difficulties due to damage localization[2]. The model is verified, calibrated and validated considering various experimental results from the literature including monotonic and cyclic loading cases with different load patterns.
[1] R. Desmorat, F. Gatuingt, F. Ragueneau. Nonlocal anisotropic damage model and related computational aspects for quasi-brittle materials. Engineering Fracture Mechanics, 74(10), 1539-1560, (2006).
[2] R.H.J. Peerlings, R. De Borst, W.A.M. Brekelmans and J.H.P. De Vree. Gradient enhanced damage for quasi-brittle materials. International Journal for Numerical Methods in Engineering, 39, 3391-3403 (1996).
An anisotropic crack initiation criterion for highly deformed R260 rail steel: experiments and numerical simulations
1Department of Industrial and Materials Science, Chalmers University of Technology, Sweden; 2Institute of Applied Mechanics,TU Braunschweig, Germany
Accumulation of plastic deformation in the surface layer of rails and wheels during many rolling contact loading cycles can result in fatigue crack initiation. The behavior and strength of this highly deformed and anisotropic layer are thus key properties of a rail or wheel material. Establishing crack initiation criteria that account for the properties of the material and are experimentally validated is of great importance in railway engineering.
In this contribution, test results from previously conducted axial-torsion experiments on pearlitic R260 steel specimens have been used to assess the accuracy of available crack initiation criteria as well as to suggest modified criteria. In the experiments, solid test bars were predeformed by torsion under different nominal axial stresses to replicate the anisotropic material in the surface layer of rails. Some of the predeformed specimens were re-machined into a thin-walled tubular shape and then subjected to further cyclic multiaxial loading.
Various crack initiation criteria for rolling-contact situations have been proposed in the literature. However, anisotropy has not been considered in many of them, or they are limited to a specific loading condition, or they are not based on experimental data. In this contribution, we predict the cyclic plasticity and anisotropy evolution during the tests by using a finite strain plasticity model and FEM. Then, by using the obtained stress and strain histories, several crack initiation criteria are evaluated as a post-processing step and further improved by considering the effect of anisotropy.
Experimental and simulative fatigue strength studies of laser beam welded copper connections based on the real geometry
1ZF Friedrichshafen AG; 2TU Dresden IFKM
The recording and evaluation of component life in electric drive systems is considerably complicated by the newly used materials and material compositions. Particularly critical are electrical subcomponents which have beam welded connections made of high-purity copper. Due to the strong coupling between stress and strength as well as the novel material properties, established methods of weld strength analysis cannot be applied without restriction. Therefore, an adapted procedure for the evaluation of the fatigue properties of these junctions is to be developed with the aim of a computational proof of service life under high-frequency vibration loads in the VHCF range.
The aim of the talk is to present the complex and thermally determined properties of the special welding spot and the inherent fatigue properties. On the one hand, the extensive and variable test program in relation to the investigated impact types as well as initial sheet configurations will be discussed. On the other hand, a self-contained methodology is to be presented, which guarantees the transferability of the simulatively determined strength between different welded joints. It is based on the NuMeSis method presented by KAFFENBERGER [1], which evaluates the specific, static notch stress situation based on real measured weld seam geometries of steel components. The transferability of the fatigue strength between different welded joints is then achieved by the combined consideration of the micro-support effect according to NEUBER [2] and the weakest link model according to WEIBULL [3]. The transfer of this method from statically loaded steel welds to high-frequency loaded copper welds requires both the embedding of the method in the methodologies of computational vibration fatigue as well as profound numerical changes of the method. This guarantees an efficient, automated evaluation and the consideration of the special properties of the high-purity copper material. Together with other influencing factors such as the presence of internal defects, this procedure leads to a self-contained evaluation concept for welded copper compounds.
[1] Kaffenberger, Vormwald: Considering size effects in the notch stress concept for fatigue assessment of welded joints, Computational Materials Science 64, 2012, S. 71-78
[2] Neuber: Über die Berücksichtigung der Spannungskonzentration bei Festigkeitsberechnungen, Konstruktion 20 Heft 7, 1968, S. 245-251
[3] Weibull: A statistical theory of the strength of materials, Royal Swedish Institute for Engineering Research, 1939
On the influence of microscale defects on electrical properties: nanoscale experiments and multiscale simulations
1Institute of Mechanics, TU Dortmund University, Germany; 2Mechanics of Materials Group, Eindhoven University of Technology, The Netherlands; 3Department of Materials Science and Engineering, Tel Aviv University, Israel; 4Erich Schmid Institute of Materials Science, Academy of Sciences, Austria; 5Max-Planck-Institut für Eisenforschung GmbH, Germany; 6Institute for Applied Materials, Karlsruher Institute of Technology, Germany; 7Division of Solid Mechanics, Lund University, Sweden
Computational multiscale methods are well-established tools to predict and analyse material behaviour across scales. They are applied so as to reveal the influence of the underlying microstructure on effective material properties and enable complex multi-physics interactions to be accounted for in simulations. Whereas multiscale approaches for thermo-mechanical problems and electro-active solids have been in the focus of intense research in the past decade, rather few works have so far focused on electrical conductors.
Based on the recent works [1,2] this material class and, in particular, the influence of mechanically-induced microscale defects on the effective conductivity is subject of the present contribution. At the outset of the developments, a quasi-stationary setting is assumed such that Maxwell’s equations reduce to the continuity equation for the electric charge and to Faraday’s law of induction. Scale-bridging relations for the kinematic- and flux-type quantities are established, their consistency with an extended Hill-Mandel condition is shown and a closed-form solution for the effective macroscale conductivity tensor based on the underlying microscale boundary value problem is provided.
In view of the experimental investigations [3,4] the effective conductivity tensor, as a fingerprint of the material microstructure, is of primary interest. To study the applicability of the proposed approach, focused ion beam milling is used in a first step to generate geometrically well-defined microstructures [4]. In a second step, focus is laid on mechanically-induced micro-cracks in metal thin films [3]. Both sets of microstructures are electrically characterised by means of four point probe resistance measurements and analysed by means of the proposed computational multiscale scheme. Good accordance between experiment and simulation is achieved which shows the applicability of the proposed multiscale formulation.
[1] T. Kaiser, A. Menzel, An electro-mechanically coupled computational multiscale formulation for electrical conductors, Archive of Applied Mechanics, 91, 1509–1526 (2021)
[2] T. Kaiser, A. Menzel, A finite deformation electro-mechanically coupled computational multiscale formulation for electrical conductors, Acta Mechanica, 232, 3939–3956 (2021)
[3] T. Kaiser, M.J. Cordill, C. Kirchlechner, A. Menzel, Electrical and mechanical behavior of metal thin films with deformation-induced cracks predicted by computational homogenisation, International Journal of Fracture 231, 233–242 (2021)
[4] T. Kaiser, G. Dehm, C. Kirchlechner, A. Menzel, H. Bishara, Probing porosity in metals by electrical conductivity: Nanoscale experiments and multiscale simulations, European Journal of Mechanics A/Solids, 97, 104777 (2023)
Prediction and compensation of shape deviations in internal traverse grinding
1Institute of Mechanics, TU Dortmund University, Germany; 2Institute of Machining Technology, TU Dortmund University, Germany; 3Division of Solid Mechanics, Department of Construction Sciences, Lund University, Sweden
Internal traverse grinding (ITG) with electroplated cBN tools and under high speed conditions if a highly efficient process for the machining of hardened steel components. In ITG, the grinding wheel consists of a conical roughing zone and a cylindrical finishing zone. The tool is fed in axial direction into a revolving workpiece, performing roughing and finishing in a single axial stroke. Due to the process kinematics, the process forces during ITG are dependent on the current material removal rate, which varies during the process. The mechanical compliance of the entire system, consisting of both tool- and workpiece spindle, the workpiece clamping device, and all other components in the flow of force, result in shape deviations of the workpieces after machining.
We recently proposed a multi-scale simulation framework to model ITG with electroplated CBN wheels numerically [1]. A digital grinding wheel, based on real grain geometries obtained from optical measurements, was implemented in a geometric physically-based simulation (GPS) to simulate the engagement of each individual grain during the process. The normal force contributions of each individual grain were modelled by a single-grain force model, which was calibrated against two-dimensional Finite Element Simulations of single grain cuts. By taking into account both the system compliance and the total normal force, the deflection between tool and workpiece was modelled in the GPS.
Based on the simulation results, different compensation strategies for the NC tool path were implemented and compared, and a significant reduction of the shape deviations was achieved.
[1] Tsagkir Dereli T, Schmidt N, Furlan T, Holtermann R, Biermann D, Menzel A. Simulation Based Prediction of Compliance Induced Shape Deviations in Internal Traverse Grinding. Journal of Manufacturing and Materials Processing. 2021; 5(2):60. https://doi.org/10.3390/jmmp5020060