Papers by Keyword: Necking

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Authors: Jong Bong Kim, Jeong Whan Yoon
Abstract: Without initial imperfection and damage evolution model, it is difficult to analyze the necking behavior by finite element analysis with continuum theory. Moreover, the results are greatly dependent on the size of the initial imperfection. In order to predict necking phenomenon without geometric imperfection, in this study, a crystal plasticity model was introduced in the 3-dimensional finite element analysis of tensile test. Grains were modeled by an octahedron and different orientations were allocated to each grain. Damage model was also used to predict the sudden drop of load carrying capacity after necking and to reflect the void nucleation and growth on the severely deformed region. Well-known Cockcroft-Latham damage model was used. Void nucleation, growth and coalescence behavior during necking were predicted reasonably.
357
Authors: Isamu Riku, Koji Mimura
Abstract: To take advantage of the toughness mechanism of DN gels and explore the possibility for engineering application as the structural member, the information on the mechanical behaviour of DN gels under various loading conditions is indispensable. Therefore, in this paper, we at first constitute a model of DN gel by paralleling a slider element with a nonlinear rubber elasticity spring element based on the nonaffine molecular chain network model, where each element represents the first and the second network of DN gel respectively. The theoretical stress-strain relation of this model shows a strain softening and subsequent strain hardening response, which has been considered as an agent of the propagation of the necking during the simple tension of glassy polymer. Continuously, based on this model, we propose a constitutive equation for DN gel and a three-dimensional simple tension simulation is performed. The computational results show that the propagation of the necking together with the macroscopic mechanical response of DN gel can be reproduced by the proposed model very well.
193
Authors: Mohamed Mahmud Aish, Mikhail D. Starostenkov
Abstract: A many-body interatomic potential for metallic nanowires within the second-moment approximation of the tight-binding model (the Cleri-Rosato potential) was employed to carry out three dimensional molecular dynamics simulations. Molecular dynamics simulation results for metallic nanowires at various temperature are presented. The stress–time and stress length curves for nanowires are simulated. The breaking and yield stress of nanowires are dependent on the Volume and temperature. The necking, Plastic deformation, slipping domain, twins, clusters, microspores and break-up phenomena of nanowire are demonstrated. Stress decreases with increasing nanowire volume and temperature. The final breaking position occurs at the central part of the nanowire when it is short, as the nanowire length increases the breaking position gradually shifts to the ends.
277
Authors: Holger Aretz, Stefan Keller, Olaf Engler, Henk Jan Brinkman
Abstract: A modular ductile failure model is presented and applied to the forming of an AA5182 aluminium alloy sheet. A detailed description of the failure model and its calibration is provided. The final application of the calibrated failure model to the deep drawing of a cruciform cup reveals a good correlation with the experimental findings. Finally, a study on the influence of the r-value on formability is conducted.
47
Authors: Fethi Abbassi, Olivier Pantalé, Sébastien Mistou, Ali Zghal, Roger Rakotomalala
Abstract: The numerical simulation based on the Finite Element Method (FEM) is widely used in academic institutes and in the industry. It is a useful tool to predict many phenomena present in the classical manufacturing forming processes such as necking, fracture, springback, buckling and wrinkling. But, the results of such numerical model depend strongly on the parameters of the constitutive behavior model. In the first part of this work, we focus on the traditional identification of the constitutive law using oriented tensile tests (0°, 45°, and 90° with respect to the rolling direction). A Digital Image Correlation (DIC) method is used in order to measure the displacements on the surface of the specimen and to analyze the necking evolution and the instability along the shear band. Therefore, bulge tests involving a number of die shapes (circular and elliptic) were developed. In a second step, a mixed numerical–experimental method is used for the identification of the plastic behavior of the stainless steel metal sheet. The initial parameters of the inverse identification were extracted from a uniaxial tensile test. The optimization procedure uses a combination of a Monte-Carlo and a Levenberg-Marquardt algorithm. In the second part of this work, according to some results obtained by SEM (Scaning Electron Microscopy) of the crack zones on the tensile specimens, a Gurson Tvergaard Needleman (GTN) ductile model of damage has been selected for the numerical simulations. This model was introduced in order to give informations concerning crack initiations during hydroforming. At the end of the paper, experimental and numerical comparisons of sheet metal forming applications are presented and validate the proposed approach.
157
Authors: B. Hortigón, E.J. Nieto, F.F. Ancio, O. Hernández
Abstract: Necking process stress and strain analysis, which is key to determine the plastic flow evolution in finite deformation, has been widely studied and applied to a number of materials based on the theories established by Davidenkov-Spiridnova and Bridgman in the 40s decade. These theories envolve from the study of necking geometry in fracture. In this paper, we develop an exhaustive experimental analysis of the stress and strain field in the necking process, applied to concrete bars and mechanized samples with similar features, in order to compare the results with the ones given by the theories listed above and to look for the corrugation influence in the materials plastic behavior.
97
Authors: L.H. Martínez-Palmeth, Andrés Jesús Martínez-Donaire, C. Vallellano, G. Centeno, F.J. García-Lomas
Abstract: The present work discusses the effect of the punch radius on the formability of H240LA steel sheets of 1.2mm thickness. A series of hemispherical punch tests (Nakazima tests) and stretch-bending tests with cylindrical punches of different diameters have been carried out in order to characterize the influence of the strain gradient in the sheet failure. The limit strains have been obtained using a recently proposed time-dependent methodology, which is applicable not only in conventional Marciniak and Nakazima tests, but also in situations with a severe strain gradient through the sheet thickness. The results show that formability of H240LA steel sheets increases as the t0/R ratio decreases.
349
Authors: Zeng Tao Chen, Michael J. Worswick, David J. Lloyd
Abstract: In this paper, stretch flange forming experiments were performed on the AA5182 and AA5754 Al-Mg sheet materials. A triple-action servo-hydraulic press, developed at the University of Waterloo, was used in the experiments. A z-flange tooling, which incorporates mating drawbeads on the main and backup punches, was employed. Drawbeads are used in commercial stretch flange operations to control or limit the rate of cutout expansion. Of interest in the current research are the flange formability and the damage development induced by the bending-unbending of the sheet as it passes through the drawbeads. Both AA5182 and AA5754 were tested with thickness of 1.6 mm. Further tests were performed using 1.0 mm AA5182 to examine the effect of thickness. To examine the effect of cutout size on the formability, cutout radii in the range 88 to 98 mm in increments of 2 mm were tested to failure.
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