Magnus Ekh

Modeling of polycrystals with gradient crystal plasticity: A comparison of strategies

This paper treats the computational modeling of size dependence in microstructure models of metals. Different gradient crystal plasticity strategies are analyzed and compared. For the numerical implementation, a dual-mixed finite element formulation which is suitable for parallelization is suggested. The paper ends with a representative numerical example for polycrystals.

Models for Cyclic Ratchetting Plasticity—Integration and Calibration

Three well-known ratchetting models for metals with different hardening rules were calibrated using uniaxial experimental data from Bower (1989), and implemented in the FE code ABAQUS (Hibbitt et at., 1997) to predict ratchetting results for a tension-torsion specimen. The models were integrated numerically by the implicit Backward Euler rule, and the material parameters were calibrated via optimization for the uniaxial experimental data. The algorithmic tangent stiffnesses of the models were derived to obtain efficient FE implementations. The calculated results for an FE model of the tension-torsion specimen were compared to experimental results. The model proposed by Jiang and Sehitoglu (1995) showed the best agreement both for the uniaxial and the structural component case.

Geometry and Stiffness Optimization for Switches and Crossings, and Simulation of Material Degradation

A methodology for simulating wear, rolling contact fatigue, and plastic deformation for a mixed traffic situation in switches and crossings (S&C) has been developed. The methodology includes simulation of dynamic vehicle—track interaction considering stochastic variations in input data, simulation of wheel—rail contacts accounting for non-linear material properties and plasticity, and simulation of wear and plastic deformation in the rail during the life of the S&C component. To find means of improving the switch panel design, the geometry of a designed track gauge variation in the switch panel has been represented in a parametric way. For traffic in the facing and trailing moves of the through route, an optimum solution was identified and then validated by evaluating a wide set of simulation cases (using different wheel profiles). The optimum design includes a 12 mm maximum gauge widening. Several crossing geometries were investigated to find an optimal geometric design for the crossing nose and wing rails. The MaKüDe design showed the best performance for moderately worn wheel profiles in both running directions (facing and trailing moves). In connection with reduced support stiffness (e.g. elastic rail pads), this crossing design is predicted to lead to a significant reduction of impact loads and consequently provide a high potential of life-cycle cost reduction.

Modeling and Numerical Issues in Hyperelasto-plasticity with Anisotropy

In the paper we discuss modeling and numerical issues that arise in conjunction with anisotropic hyperelastic-plastic response. Both elastic and plastic anisotropy are included. A particular kinematic hardening rule is proposed and its predictive capability is investigated. From the numerical viewpoint, we are concerned with the algorithmic consequences of the loss of coaxiality that arises from anisotropy. The numerical investigation shows that significant truncation errors are introduced if commonly used linearizations of the (classical) exponential Backward Euler rule are utilized in the presence of non-coaxiality.

A class of thermo-hyperelastic-viscoplastic models for porous materials: theory and numerics

Abstract Modelling and numerical issues in conjunction with large strain analysis of thermohyperelastic–viscoplastic models are discussed with emphasis placed on porous materials (metal powders). In order to account for the proper temperature effect on the rate-dependent response of metal powders, the concept of “viscoplastic admissibility” is employed as part of a model framework that includes a dynamic yield surface. A generic class of pressure-sensitive models is considered; a particular model based on quasistatic and dynamic yield surfaces, that are elliptic in the meridian planes in the principal stress space, is used for the subsequent numerical evaluation. The generic model also includes kinematic hardening, thus introducing non-coaxiality between the deformation, the back-stress and the stress tensors (in any given reference configuration). Implicit (backward Euler) integration is used and the corresponding algorithmic tangent stiffness (ATS) tensor is established in a setting that is valid for complete noncoaxiality. For the prototype problem of simple shear (using constant strain approximation), quadratic convergence in the equilibrium iterations is demonstrated. The paper is concluded by an investigation of a Hot-Isostatic-Pressing (HIP) problem, whereby experimental results from the literature are used for calibration of the model parameters.

Anisotropic damage with the MCR effect coupled to plasticity

A model framework, presented in [Int. J. Numer. Meth. Engng., 54 (10) (2002) 1409] for anisotropic damage coupled to inelastic deformation is here extended to include the microcrack-closure-reopening (MCR) effect. Key ingredients are the concept of the effective undamaged configuration and the assumption of energy equivalence. The MCR effect is modelled with the aid of projection tensors that extract the positive part of the elastic strain. A prototype model based on the von Mises yield surface with linear mixed proportional and kinematic hardening is then elaborated on, and some typical results are shown for monotonic as well as cyclic loading.

Bifurcation results for plasticity coupled to damage with MCR-effect

Abstract The thermodynamic framework is outlined for a class of models, that involve the kinetic coupling of damage to inelastic deformation. This class includes effects of microcrack–closure–reopening (MCR). The general conditions for discontinuous bifurcations, which define the onset of band-shaped localization, are given for the considered class of models. Solutions for the band direction and the corresponding thermodynamic state are given for isotropic elastoplastic response at plane stress, whereby two different damage models that account for the MCR-effect in tension-compression, are employed.

Hierarchical and Heterogeneous Bioinspired Composites—Merging Molecular Self-Assembly with Additive Manufacturing

Biological composites display exceptional mechanical properties owing to a highly organized, heterogeneous architecture spanning several length scales. It is challenging to translate this ordered and multiscale structural organization in synthetic, bulk composites. Herein, a combination of top-down and bottom-up approach is demonstrated, to form a polymer-ceramic composite by macroscopically aligning the self-assembled nanostructure of polymerizable lyotropic liquid crystals via 3D printing. The polymer matrix is then uniformly reinforced with bone-like apatite via in situ biomimetic mineralization. The combinatorial method enables the formation of macrosized, heterogeneous composites where the nanostructure and chemical composition is locally tuned over microscopic distances. This enables precise control over the mechanics in specific directions and regions, with a unique intrinsic-extrinsic toughening mechanism. As a proof-of-concept, the method is used to form large-scale composites mimicking the local nanostructure, compositional gradients and directional mechanical properties of heterogeneous tissues like the bone-cartilage interface, for mechanically stable osteochondral plugs. This work demonstrates the possibility to create hierarchical and complex structured composites using weak starting components, thus opening new routes for efficient synthesis of high-performance materials ranging from biomaterials to structural nanocomposites.

Modelling of grey cast iron for application to brake discs for heavy vehicles

Cast iron brake discs are commonly used in the automotive industry, and efforts are being made to gain a better understanding of the thermal and mechanical phenomena occurring at braking. The high thermomechanical loading at braking arises from interaction between the brake disc and the brake pads. Frictional heating generates elevated temperatures with a non-uniform spatial distribution often in the form of banding or hot spotting. These phenomena contribute to material fatigue and wear and possibly also to cracking. The use of advanced calibrated material models is one important step towards a reliable analysis of the mechanical behaviour and the life of brake discs. In the present study, a material model of the Gurson–Tvergaard–Needleman type is adopted, which accounts for asymmetric yielding in tension and compression, kinematic hardening effects, viscoplastic response and temperature dependence. The material model is calibrated using specimens tested in uniaxial cyclic loading for six different temperatures ranging from room temperature to 650 °C. A special testing protocol is followed which is intended to activate the different features of the material model. Validation of the model is performed by using tensile tests and thermomechanical experiments. An application example is given where a 10° sector of a brake disc is analysed using the commercial finitie element code Abaqus under a uniformly applied heat flux on the two friction surfaces. The results indicate that the friction surface of the hat side and the neck can be critical areas with respect to fatigue for the uniform heating studied.

On the prediction of macroscopic yield surfaces for a pearlitic steel using computational homogenization

In the present work a multiscale modeling framework is used to predict yield surfaces for a pearlitic steel. On the mesoscale a model taking into account the features of the pearlite colonies, i.e. the crystallographic orientations of the ferrite and the cementite lamella orientation, is used. The microscale model includes both the ferrite matrix and the cementite lamellae and the interactions between these phases. The model is used to predict yield surfaces for both isotropic and deformed mesomodels.