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Purpose is the formulation, numerical implementation, identification and application of a material model for ductile damage and failure during cyclic and non-proportional loading. The authors combined a hyperelasticity-based elasto-plastic model for non-linear isotropic as well as kinematic hardening with a modified Gurson model. Evolution strategy helped identify the model parameters for the high-strength steel 10MnMoNi5-5. The simulation of ductile failure in fracture mechanics specimens verified the model with respect to cyclic loading at two temperatures. The simulation of additional fracture mechanics applications validated the model as to the development of residual stresses at the crack tip under cyclic loads. [ABSTRACT FROM AUTHOR]

The purpose of the current work is the investigation and comparison of aspects of the material behavior predicted by two models for anisotropic, and in particular cross, hardening in bcc sheet steels subject to non-proportional loading. The first model is the modified form (Wang et al., 2008) of that due to Teodosiu and Hu (1995, 1998). In this (modified) Teodosiu-Hu model (THM), cross hardening is assumed to affect the yield stress and the saturation value of the back stress. The second model is due to Levkovitch and Svendsen (2007) and Noman et al. (2010). In the Levkovitch-Svendsen model (LSM), cross hardening is assumed to affect the flow anisotropy. As clearly demonstrated in a number of works applying the THM (e.g., Boers et al., 2010; Bouvier et al., 2005, 2003; Hiwatashi et al., 1997; Li et al., 2003; Thuillier et al., 2010; Wang et al., 2008) and the LSM (e.g., Clausmeyer et al., 2014, 2011b; Noman et al., 2010), both of these are capable of predicting the effect of cross hardening on the stress-deformation behavior observed experimentally in sheet steels. As shown in the current work, however, these two models differ significantly in other aspects, in particular with respect to the development of the yield stress, the back stress, and the yield surface. For example, the THM predicts no change in the shape of the yield surface upon change of loading path, in contrast to the LSM and crystal plasticity modeling of bcc sheet steels (Peeters et al., 2002). On the other hand, the LSM predicts no hardening stagnation after cross hardening as observed in experiments, in contrast to the THM. Examples are given. [ABSTRACT FROM AUTHOR]

Abstract: The purpose of this paper is to introduce a homogenization method for the material behavior of two-phase composites characterized by a thin-layer-type microstructure. Such microstructures can be found for example in thermally-sprayed coating materials like WC/Fe in which the phase morphology takes the form of interpenetrating layers. The basic idea here is to idealize the thin-layered microstructure as a first-order laminate. Comparison of the methods with existing homogenization schemes as well as with the reference finite-element model for idealized composites demonstrates the advantage of the current approach for such microstructures. Further an extension of the approach to a variable interface orientation is presented. In the end the current method is compared to results based on FE-models of real micrographs. [Copyright &y& Elsevier]

Abstract: In this work, a thermodynamic approach to the modeling and simulation of induced elastic and inelastic material behaviour in the phenomenological realm as based on the concept of evolving structure tensors is discussed. From the constitutive point of view, these quantities determine the material symmetry properties. In addition, the stress and other dependent constitutive fields are isotropic functions of these by definition. The evolution of these during loading then results in an evolution of the anisotropy of the material. From an algorithmic point of view, the current approach leads to constitutive models which are quite amenable to numerical implementation. To demonstrate the applicability of the resulting constitutive formulation, we apply it to the cases of (i) metal plasticity with combined hardening involving both deformation- and permanently induced anisotropy relevant to the modeling of processes such as metal forming, and to (ii) deformation-induced anisotropy in an initially orthotropic pneumatic membrane consisting of a rubber matrix and nylon fibres. [Copyright &y& Elsevier]

The purpose of this work is the development and numerical implementation of a non-local extension of existing Gurson-based modeling for isotropic ductile damage and attendant crack growth. In the current work, this extension is based on a generalization of Gurson-based ductile damage modeling formally analogous to that of Needleman and Tvergaard as based on a (local) effective damage parameter f* which accounts in their case in an effective fashion for the effect of void coalescence on yield behaviour. Here, the corresponding generalized effective damage parameter ν is introduced in order to delocalize the model damage process. To this end, ν is modeled in this work as a scalar-valued continuum microstructural field or generalized phase field via a recent thermodynamic approach to the modeling of such fields. In the last part of the work, the complete model for coupled damage-deformation is implemented numerically using the finite element method and utilized to investigate the damage behaviour of an inhomogeneous steel block in simple tension. The corresponding simulation results demonstrate in particular that delocalization of the model damage process also leads to minimization of mesh-dependence. For simplicity, the current formulation is restricted to the case of small strain and isothermal conditions. [Copyright &y& Elsevier]

The present contribution deals with the development of a laminate-based model designed to study the single and polycrystalline tetragonal ferroelectric material behaviour. Laminate-based models are micromechanically motivated and consider the volume fraction of the distinct ferroelectric variants directly in their formulation. At first, a single crystal laminate-based model is established by considering the average strain and polarisation compatibility conditions. A suitable thermodynamic electric Gibbs energy and a rate-dependent dissipation equation are postulated to capture the dissipative hysteretic material response. The update of the inequality constrained volume fractions is solved by adopting a Fischer–Burmeister-type algorithm in combination with a Newton–Raphson scheme. Following the single crystal formulation, a homogenisation procedure based on random orientation of the individual grains in a polycrystalline aggregate is considered. The material properties and the polarisation switching response of the randomly oriented individual grains are averaged using a finite element framework in order to study the macroscopic polycrystalline behaviour. A parameter fitting procedure based on experimental data for single crystalline response, taken from the literature, is detailed and the material model as well as the algorithmic scheme are verified by solving representative boundary value problems. Moreover, the finite element based simulation results are compared with newly generated experimental hysteresis data for a barium titanate piezoceramic. [ABSTRACT FROM AUTHOR]

Crystal nucleation is among the most important processes in the synthesis of materials. Here we study the competitive and multistep nucleation process of body centered cubic (bcc) and face centered cubic (fcc) crystals using phase-field crystal simulations. The initial state is a non-equilibrium liquid. This transforms into an amorphous phase composed of clusters with short-range order (SRO) and medium-range order (MRO). Crystal nucleation begins with the formation of MRO clusters structurally similar to the subsequently nucleated crystal. The formation of bcc and fcc nuclei from MRO clusters involves the following steps: (1) formation of a small thin platelet with MRO; (2) its subsequent growth into 3D MRO clusters; (3) development of crystal embryos from MRO clusters; and (4) crystal embryos transformation into a stable crystal nuclei. In addition, the role of bcc precursors in the formation of fcc nuclei is clarified. In particular, {112} surfaces and steps on {110} surfaces of bcc precursors serve as energetically favorable sites for fcc nucleation. This is also reflected in the resulting orientation relationships, i.e., Pitsch, Nishiyama-Wassermann and Kurdjumov-Sachs, between the bcc precursor and fcc nucleus. [ABSTRACT FROM AUTHOR]

Abstract: The purpose of this work is comparing three strategies for dealing with inhomogeneities of non-elliptical shape in the context of homogenization methods. First, classical mean-field methods and two relatively new approaches, IDD and ESCS, are used in combination with analytical expressions for the Eshelby tensor based on its irreducible decomposition. The second strategy to be investigated is the Mori-Tanaka method in combination with the replacement tensor approach, which uses numerical models of dilute inhomogeneities embedded in large matrix regions. The third approach consists of the direct Finite Element discretization of microstructures. The elasticity tensors and directional Young’s moduli are first studied for arrangements of aligned inhomogeneities of three different shapes and of combinations of these shapes. Subsequently the three modeling strategies are applied to a real microstructure. Comparisons are not only carried out with respect to phase volume fractions, but also with respect to the contrast in the elastic phase properties. All calculations are restricted to plane strain conditions and to isotropic material behavior. [Copyright &y& Elsevier]

Alternative processing of the precursor protein pro-GIP results in endogenously produced GIP(1–30)NH 2 , that by DPP-4 cleavage in vivo results in the metabolite GIP(3–30)NH 2 . We showed previously that GIP(3–30)NH 2 is a high affinity antagonist of the human GIPR in vitro . Here we determine whether it is suitable for studies of GIP physiology in rats since effects of GIP agonists and antagonists are strictly species-dependent. Transiently transfected COS-7 cells were assessed for cAMP accumulation upon ligand stimulation or assayed in competition binding using human 125 I-GIP(1–42) as radioligand. In isolated perfused rat pancreata, insulin, glucagon, and somatostatin-releasing properties were evaluated. Competition binding demonstrated that on the rat GIP receptor (GIPR), rat GIP(3–30)NH 2 bound with high affinity (K i of 17 nM), in contrast to human GIP(3–30)NH 2 (K i of 250 nM). In cAMP studies, rat GIP(3–30)NH 2 inhibited GIP(1–42)-induced rat GIPR activation and schild-plot analysis showed competitive antagonism with a pA 2 of 13 nM and a slope of 0.9 ± 0.09. Alone, rat GIP(3–30)NH 2 displayed weak, low-potent partial agonistic properties (EC 50 > 1 μM) with an efficacy of 9.4% at 0.32 μM compared to GIP(1–42). In perfused rat pancreata, rat GIP(3–30)NH 2 efficiently antagonized rat GIP(1–42)-induced insulin, somatostatin, and glucagon secretion. In summary, rat GIP(3–30)NH 2 is a high affinity competitive GIPR antagonist and effectively antagonizes GIP-mediated G protein-signaling as well as pancreatic hormone release, while human GIP(3–30)NH 2 , despite a difference of only one amino acid between the two (arginine in position 18 in rat GIP(3–30)NH 2 ; histidine in human), is unsuitable in the rat system. This underlines the importance of species differences in the GIP system, and the limitations of testing human peptides in rodent systems. [ABSTRACT FROM AUTHOR]

A recent material model considering the evolution of plastic anisotropy in interstitial free steels is validated for the forming process of the channel die, a complex part. In the model the evolution of the intra-granular microstructure is represented by tensor-valued internal variables. The model accounts for the cross hardening behavior observed in rheological tests of interstitial free steels. A novel cross hardening indicator which is directly derived from the constitutive model is proposed. This cross hardening indicator is a quantitative measure for the occurrence of cross hardening in the forming process of complex parts. A correlation between the occurrence of cross hardening and larger values of the stored (elastic) energy is observed. The influence of cross hardening on the forming process is investigated, in particular, the drawing forces and the geometric deviations due to springback. The influence of cross hardening on the forming process of the channel die geometry is small. The influence of cross hardening on the more complex S-Rail geometry is larger due to larger plastic deformation and more severe loading path changes. The concept of the proposed transient hardening indicator should be applicable to other models for the evolution of plastic anisotropy. A possible use of the cross hardening indicator would be the efficient choice of the material model in the context of sheet metal forming simulations. [ABSTRACT FROM AUTHOR]

Abstract: The purpose of this work is the development, identification and validation of a model for the simulation of a thermomechanical multistage production process chain for sheet steels. The process of interest consists of forming (in particular deep-drawing) followed by cutting and heat treatment. For the forming and cutting stages, the complete model is based in particular on a model for thermoelastic, viscoplastic (i.e., rate-dependent) material behavior in sheet steels accounting for isotropic and anisotropic (i.e., kinematic and cross) hardening. This is combined with a model for thermally induced phase transformations in order to model heat treatment. The particular material modeled here is the sheet steel . This steel has an initially ferritic microstructure which is maintained during forming and cutting. Heating of the workpiece after forming and cutting during heat treatment phase results in transformation of ferrite to austenite. Subsequent air-cooling back to room temperature is accompanied by a second transformation from austenite to martensite. Model predictions for the workpiece behavior during forming and cutting show quite good agreement with corresponding experimental results. In contrast, small discrepancies between the model predictions and experimental results for the change in workpiece geometry during cooling imply that the phase transformation from austenite to martensite in is not purely volumetric in nature as assumed in the model. Rather, it results in change in the deviatoric state of stress in the material and a corresponding change in shape of the workpiece. [Copyright &y& Elsevier]

Abstract: The purpose of the present work is the experimental investigation of the nucleation of PLC deformation bands in the aluminium alloy AA5754. The PLC bands are investigated using both mechanical methods and infrared (IR) thermography. The latter employs a high-speed IR camera which captures local changes of radiated power resulting from mechanical dissipation and heating due to the nucleation of PLC bands. The resulting IR images are used to determine spatio-temporal power field variations via image subtraction. Furthermore, band trajectories obtained from the IR images are used to study possible correlations between the spatio-temporal evolution of stress and radiated power in the specimens and PLC band development. [Copyright &y& Elsevier]

Abstract: In this paper, a new method for analyzing the microstructure evolution of aluminum during deformation at elevated temperatures by extrusion is presented, which is entirely separated from secondary restoration effects viz. static recrystallization and grain growth. In order to observe the development of grains and their orientation under severe plastic deformation, a small-scale forward extrusion setup was designed which allows quenching the extrusion butt together with the die and the container immediately after extrusion to preserve the grain structure evolved during the deformation. The forming path and the forming history of a selected material point were calculated by numerical simulation. The evolution of the microstructure along the forming path was analyzed using electron backscatter diffraction. A database for the development of physically based phenomenological models for predicting and simulating the evolution of microstructure during the hot deformation of EN AW-6082 alloy is provided. [Copyright &y& Elsevier]

Abstract: Using thermal spraying various surface coatings consisting of different material compositions can be manufactured. Besides different solid phases the resulting coating microstructure often contains a non-negligible amount of pores. In this context a roller burnishing process with a hydrostatic ball-point-tool is examined to compact the thermally sprayed coating, thereby reducing porosity. The rolling process is performed by a robot on free-formed workpieces. A simulation concept for the prediction of forces in a robot-guided roller burnishing process based on a series of detailed ABAQUS simulations is presented. It is shown that, based on these test configurations, the process forces can be calculated much faster and with sufficient precision. Thereby an optimal rolling path, which requires the least amount of normal force to be applied, can be determined efficiently leading to the decision whether a specific robot is equipped to handle the path. Furthermore, the described approach may be used as a pattern to apply similar methods to other engineering problems where accurate simulative solutions exist, but cannot be applied to problems of realistic size due to their expenditure of time. [Copyright &y& Elsevier]

Abstract: The purpose of the current work is the development and application of a new experimental technique and testing device for investigating the complex behavior of sheet metals during non-proportional loading. The method is based on plain strain pure bending, enabling the investigation of large deformation cyclic reversed loading, orthogonal pure bending, as well as springback. The key feature of the pure bending experiment is the absence of contact forces, material slip and friction. Furthermore, during the pure bending test, the strain gradient through the thickness is kinematically prescribed because the specimen is subjected to a plane strain condition in de direction parallel to the rotational axis (), which allows for a straightforward comparison of the pure bending experiments and parallel simulations. The latter is used here via the identification of a recent model for directional hardening effects and arbitrary strain path changes, (). The current method facilitates experimental investigation of hardening stagnation after reverse loading and cross hardening going well beyond that which is possible with existing methods based on the cyclic shear or tension-shear of sheet metal strips (.), or pure and three-point bending (). [Copyright &y& Elsevier]

Abstract: The purpose of this work is the modeling and simulation of aluminum alloys during extrusion processes. In particular, attention is focused here on aluminum alloys of the 6000 series (Alph name="sbnd" /> Si) and 7000 series (Alyph name="sbnd" /> Mg). In the current paper, a number of aspects of the structural simulation as well as that of extrusion as a thermomechanical process are considered. These aspects include contact and adaptive mesh refinement, heat transfer inside the billet, heat transfer between the workpiece and the container, frictional dissipation, mechanical energy and surface radiation. The friction is considered to model the so called “dead material zone”. The radiation constant has been estimated so that the results are close to the experimental results. [Copyright &y& Elsevier]

Abstract: The purpose of the current work is the formulation and initial application of a phenomenological model for hardening effects in metals subject to non-proportional loading histories characterized by one or more loading-path changes. This model is closely related to the incremental model of Teodosiu and Hu [Teodosiu, C., Hu, Z., 1995. Evolution of the intragranular microstructure at moderate and large strains: modelling and computational significance. In: Shen, S.F., Dawson, P.R. (Eds.), Simulation of Materials Processing: Theory, Methods and Applications. Balkema, Rotterdam, pp. 173–182; Teodosiu, C., Hu, Z., 1998. Microstructure in the continuum modelling of plastic anisotropy. In: Proceedings of 19th Risø International Symposium on Material’s Science: Modelling of Structure and Mechanics of Materials from Microscale to Product. Risø National Laboratory, Roskilde, Denmark, pp. 149–168]. Like their model, the current model captures in particular hardening stagnation after a load reversal as well as cross-hardening after orthogonal loading-path changes. On the other hand, the two models predict qualitatively different behavior during loading-path changes which take place purely in the inelastic range. Such is the case for example during orthogonal loading-path changes from uniaxial tension to simple shear without release, or during monotonic simple shear, or during deep-drawing. As shown by the experimental results reported on in the current work for the mild steel DC06, significant cross-hardening can occur during continuous orthogonal loading-path changes. Beyond this, the current model accounts in an approximate way for the possible effects of texture development on the material behavior with the help of the plastic spin. After investigating the behavior of the current model for various ideal two-stage loading histories (e.g., tension-shear), the current work ends with a comparison of standard combined hardening and current approaches in the context of the simulation of internal stress development and residual stresses during deep-drawing and the resultant springback after ring-splitting. [Copyright &y& Elsevier]