• Transactions of the Korean Society of Mechanical Engineers A
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• v.35 no.7
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• pp.745-751
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• 2011

This study proposes finite element modeling of dislocation punching at cooling after consolidation in order to calculate the strength of particle-reinforced aluminum composites. The Taylor dislocation model combined with strain gradient plasticity around the reinforced particle is adopted to take into account the size-dependency of different volume fractions of the particle. The strain gradients were obtained from the equivalent plastic strain calculated during the cooling of the spherical unit cell, when the dislocation punching due to CTE (Coefficient of Thermal Expansion) mismatch is activated. The enhanced yield stress was observed by including the strain gradients, in an average sense, over the punched zone. The tensile strength of the SiCp/Al 356-T6 composite was predicted through the finite element analysis of an axisymmetric unit cell for various sizes and volume fractions of the particle. The predicted strengths were found to be in good agreement with the experimental data. Further, the particle-size dependency was clearly established.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.33 no.5
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• pp.447-454
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• 2009

Rate-independent crystal plasticity model suffers from the non-uniqueness of activated slip systems and the determination of the shear slip rates on the active slip systems. In this paper, a time-integration algorithm which circumvents the problem of the multiplicity of the slip systems was developed and implemented into the user subroutine VUMAT of a commercial finite element program ABAQUS. The magnitude of the slip shears on the active slip systems in f.c.c Cu single crystal aligned with the specific crystallographic orientation was investigated to validate our solution procedure. Also, texture developments under various deformation modes such as simple compression, simple tension and plane strain compression were compared with the results of the rate-dependent model by using the rate-independent crystal plasticity model. The computation time employing the rate-independent model is much more reduced than the those of the rate-dependent model.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2000.10a
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• pp.25-28
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• 2000

The flow stress of 304 stainless steel during high during hot forming process were determined by conducting hot compression tests at the range of 1273 K-1423 K and 0.05 /s-2.0 /s as these are typical temperature and strain rate in hot forging operation. Based on the observed phenomena, a constitutive model of flow stress was assumed as a function of strain, strain rate, temperature. Dynamic recrystallization was found to be the major softening mechanism with this conditions as previous studies. A finite element analysis was performed to predict the recrystallized volume fraction and the mean grain size in hot compression of 304 stainless steel.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2003.10b
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• pp.22-25
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• 2003

The forming limits are studied for cold upsetting of high strength aluminium alloy in the present paper. Different geometry ratio and frictional conditions are investigated in the forgeability test to evaluate the forming limits and also to obtain the various strain paths. The critical fracture value can be obtained by integrating along the strain path till free surface crack initiation. To predict the damage evolution of cold upsetting, the computer-aided evaluation of forming limits is obtained by using the finite-element software DEFORM-3D and the modified Cockcroft-Latham criterion. The predicted theoretical limit strains agree quite well with the experimental results.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 1999.08a
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• pp.341-348
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• 1999

In the finite elemenet analysis of metal forming problems, the most critical input is the flow stress of workpiece. Conventionally, the flow stress of a metal at elevated temperatures is assumed to be a function of strain, strain rate and temperature, and obtained by experiment. However, if the workpiece is not continuously deformed as in mulit-pass rolling, the flow stress obtained by experiment is no longer valid because it does not consider the microstructure evolution occurring between deformations. In the present study, it was attemped that the flow stress of steel in the austenite region be obtained equations. It was applied to the prediction of flow stress variation at each stand during hot finishing rolling of steel.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2001.05a
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• pp.205-208
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• 2001

Localized shear band is investigated through the analysis of one-dimensional model for simple shearing deformation of thermally rate dependent material. Generally mesh size or interval of nodes play an important role in determining the overall flow behavior of the material. In order to observe these size effects we adapted non-local theory by including higher order strain gradients of the equivalent strain into the constitutive equation for the flow stress. for the ease of convergence and numerical stability the inplicit finite difference scheme is employed.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2007.10a
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• pp.84-86
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• 2007

Aluminum sheets were asymmetrically cold rolled without lubrication by using different roll velocities of upper and lower rolls in order to intensify the shear deformation. During asymmetrical cold rolling of aluminum sheets, a reduction per a rolling pass, initial sheet thickness, roll diameter, roll velocity ratio were varied to investigate the effect of rolling parameters. The formation of through thickness shear texture was related to the ratio of the contact length between the roll and sample($l_c$) to the sheet thickness(d). The strain states associated with asymmetrical rolling were investigated by the finite element method (FEM) simulation. FEM results indicated that the evolution of deformation texture in a thickness layer is strongly governed by integrated values of strain rates $\dot{\varepsilon}_{13}$ and $\dot{\varepsilon}_{11}$ along the streamline in the roll gap.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2008.05a
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• pp.404-407
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• 2008

In the present work, cross-roll rolling was carried out using a rolling mill in which the roll axis is tilted by $7.5^{\circ}$ towards the transverse direction of the rolled sample. The evolution of strain states during cross-roll rolling was investigated by three-dimensional finite element method (FEM) simulation. Parallel to cross-roll rolling, normal-rolling using a conventional rolling mill was also carried out in the same rolling condition for clarifying the effect of cross-roll rolling. It turned out that three shear rate components were all introduced to the rolled sample by the cross-roll rolling process, while only one shear rate component operated during normal-rolling.

• Transactions of Materials Processing
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• v.5 no.4
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• pp.343-352
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• 1996

One of the preferred methods that can be used to verify the results of finite element analysis is to measure surface strains of the deformed part for purpose of direct comparison with simulation results. Instead of using the usual manual method the vision-based measurement method is capable of determining surface geometry and strain from the deformed grid pattern automatically with the help of a computer. To obtain strain distribution over an area, the coordinates of such a surface grid are determined from the multiple video images by applying the photogrammetry principle. Methods to improve the overall accuracy of the vision-based strain measurement system are explored and the possible accuracies that can be attained by such a measurement method are discussed. A major emphasis is placed on the initial grid application method its accuracy and ease of subsequent image processing. Finite element analyses of limiting dome height(LDH) test are carried out and the results of them are compared with exsperimen-tal data.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 1990.04a
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• pp.70-74
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• 1990

The finite plasticity in strain space is viewed by formulating the consistency condition and the thermodynamic condition with respect to proposed state variables. The Naghi-Trapp work assumption is used to obtain a constraint equation, and the normality equation is formulated. Finally, an elastoplastic tangent modulus, which is based on the derived equations in strain space, is proposed.