Papers by Keyword: Viscoplasticity

Abstract: In this study, macro/meso/micro elastic-viscoplastic analysis of plain-woven laminates is conducted based on a homogenization theory for nonlinear time-dependent composites. For this, a plain-woven laminate is modeled with respect to three scales by considering the laminate as a macrostructure, fiber bundles (yarns) and a matrix in the laminate as a mesostructure, and fibers and a matrix in the yarns as a microstructure. Then, an elastic-viscoplastic constitutive equation of the laminate is derived by dually applying the homogenization theory for nonlinear time-dependent composites to not only the meso/micro but also the macro/meso scales. Using the present method, the elastic-viscoplastic analysis of a plain-woven glass fiber/epoxy laminate subjected to on-and off-axis loading is performed. It is shown that the present method successfully takes into account the effects of viscoplasticity of the epoxy in yarns on the elastic-viscoplastic behavior of the plain-woven GFRP laminate. It is also shown that the results of analysis are in good agreement with experimental data.

Abstract: The presence of residual stresses in thermal oxide layers has been recognized for a long time. In the present work, the mechanical fields for chromium oxide are investigated. An extended model is established to take into account the effects of temperature and thermal cycling for the calculation of oxide stress. Numerical results are given in order to predict the influence of different parameters, especially the dependence of some material parameters with temperature. This enables to make comparison with experimental results.

Abstract: Bending of multilayered sheets like lightweight sandwich sheets or fiber reinforced thermoplastics is dominated by the mechanism of interply-slip. FE-analysis is performed to predict defects depending on this mechanism. The shear and damage behavior of the adhesive layer of sandwich sheets can be modeled by cohesive elements in Abaqus. Forming simulation of fiber reinforced thermoplastics requires coupled thermo-mechanical analysis methods due to temperature dependence. For this, alternative modeling strategies for the inner layer of adhesive or polymer matrices will be tested in this paper that are able to transfer heat. The layer will be presented by solid elements with enriched property definitions or viscoplasticity. Furthermore the thickness of the layer will be neglected and replaced by contact formulations or spring elements.

Abstract: Aircraft engine components are subjected, voluntarily or not, to the influence of residual stresses (RS). These RS may evolve in service conditions and may have an influence on fatigue life of the component. This paper presents a method to take into account the RS and their relaxation in a finite element calculation to obtain the fatigue life. This method is applied to a representative high-pressure turbine disk specimen made of N18 Nickel-based superalloy. Firstly, residual stresses are measured using X-Ray diffraction technique on the surface and the thickness of specimens. The influence of different surface finishing processes on the intensity and distribution of RS is compared to as-received specimen. Then, using the experimental profile as an initial state, a fatigue life analysis is performed (on fatigue specimen) by applying a multiaxial extension of the Smith-Watson-Topper model. Numerical and experimental results are discussed in detail and it appears that residual compressive stresses have almost no influence for high strain range but they improve the fatigue life for lower ranges.

Abstract: This paper presents the results of an experimental and numerical investigation of the tensile behaviour of glass fibre reinforced epoxy composite for incremental and cyclic creep-recovery load conditions. The proposed constitutive model incorporates linear viscoelasticity coupled with damage and viscoplasticity for prediction of the creep-recovery responses. The material parameters are determined by fitting the experimental results. Scanning electron microscopy (SEM) of composite fracture surfaces is performed to explain the failure mechanisms. A computational algorithm for the integration of the proposed constitutive model at the material point level is derived and implemented into the finite element code ABAQUS. The model predictions are found to be in good agreement with the experimental data.

Abstract: The present paper is concerned with an experimental research and numerical modelling of the viscoelastic-viscoplastic-damage behaviour of bovine cortical bone. A one-dimensional constitutive model is proposed to predict the experimental behaviour under creep-recovery load conditions. The material parameters are determined by fitting experimental results. The derived algorithm for the integration of the proposed constitutive model is implemented into finite element formulation. The computational algorithm shows an excellent capability to describe the tensile behaviour of bovine cortical bone for the specific mechanical conditions analyzed.

Abstract: Large deformation problem under dynamic loading has been an issue for mechanical engineer and researcher who study continuum mechanics because of its complex behavior. This study focuses on a large deformation problem based on nonlinear viscoplasticity. Therefore, the Johnson-Cook constitutive model and the Zerilli-Armstrong constitutive model are used to analyze and to understand this stretch example under dynamic loading. Numerical results for stretch example using both constitutive models were compared with the results from ABAQUS/Explicit that considered mesh sensitivity analysis, and were then applied to model failure processes of OFHC copper, with consideration of geometric nonlinearity based on concepts of progressive damage mechanics. This approach helps us to understand and to predict the behavior of large deformation and to perform damage failure analysis.

Abstract: In the field of sheet metal forming traditional forming processes are used. However, a quasi-static forming process combined with a high speed forming process can enhance the forming limits of a single one. In this paper, the investigation of the process chain quasi-static deep drawing – electromagnetic forming by means of a new coupled damage-viscoplasticity model for large deformations is performed. The finite strain constitutive model, used in the finite element simulation combines nonlinear kinematic and isotropic hardening and is derived in a thermodynamically consistent setting. The anisotropic viscoplastic model is based on the multiplicative decomposition of the deformation gradient in the context of hyperelasticity. The kinematic hardening component represents a continuum extension of the classical rheological model of Armstrong–Frederick kinematic hardening. Hill-type plastic anisotropy is modelled by expressing the yield surface as a function of second-order structure tensors as additional tensor-valued arguments. The coupling of damage and plasticity is carried out in a constitutive manner according to the effective stress concept. The constitutive equations of the material model are integrated in an explicit manner and implemented as a user material subroutine in the commercial finite element package of LS-Dyna with the electromagnetical modul. Aim of the work is to show the increasing formability of the sheet by combining quasi-static deep drawing processes with high speed electromagnetic forming process.

Abstract: This lecture presents constitutive modeling of the homogenized elastic-viscoplastic behavior of pore-pressurized anisotropic open-porous bodies. The base solids are assumed to be metallic materials at small strains and rotations. First, by describing micro-macro relations relevant to periodic unit cells of anisotropic open-porous bodies with pore pressure, constitutive features are discussed for the viscoplastic macrostrain rate in steady states. Second, on the basis of the constitutive features found, the viscoplastic macrostrain rate is represented as an anisotropic function of Terzaghi’s effective stress. Third, the resulting viscoplastic equation is used to simulate the homogenized elastic-viscoplastic behavior of an ultrafine plate-fin structure and a thick perforated plate subjected to macroscopic loading in the absence and presence of pore pressure. The corresponding FE homogenization analysis is performed for comparison to validate the developed viscoplastic equation.

Abstract: In this study, the elastic-viscoplastic properties of aluminum honeycomb sandwich panels are investigated using a homogenization theory for free edge analysis. For this, the mathematical homogenization theory is reconstructed for elastic-viscoplastic analysis of honeycomb sandwich panels by introducing a traction free boundary condition. Moreover, the domain of analysis is reduced to a quarter using point-symmetry of internal structures of honeycomb sandwich panels. The present method is then applied to the analysis of macroscopic elastic-viscoplastic behavior and microscopic stress distribution of an aluminum honeycomb sandwich panel subjected to in-plane uniaxial compression. It is shown that the stress concentration arises at face/core interfaces, especially at intersections of core walls.