• Proceedings of the Korean Society of Precision Engineering Conference
• /
• 2005.06a
• /
• pp.485-488
• /
• 2005

Most metals are polycrystalline material whose deformation is dominated by the slip system. During the deformation process, orientation of slip systems is rearranged with preferred orientations, leading to deformation-induced crystallographic texture which is called deformation texture. Depending on the texture development, the property of material can be changed. The rate-independent crystal plasticity which is based on the Schmid law as a yield function causes a non-uniqueness in the choice of active slip systems. In this work, to avoid the slip system ambiguity problem, rate-independent crystal plasticity model based on the smooth yield surface with rounded-off corners is adopted. In order to simulate the polycrystalline material under plastic deformation, we employ the Taylor model of polycrystal behavior that all the grains are assumed to be subjected to the macroscopic velocity gradient. Rigid-plastic finite element program based on this rate-independent crystal plasticity is developed to predict the grain-level deformation behavior of FCC metals during metal forming processes. In the finite element calculation, one integration point is considered as a crystalline aggregate which has a number of crystals. Macroscopic behavior of material can be deduced from the behavior of aggregates. As applications, the extrusion processes are simulated and the changes of mechanical properties are predicted.

• Transactions of Materials Processing
• /
• v.5 no.1
• /
• pp.18-26
• /
• 1996

In order to investigate the effect of pre-cooling of ingot on void closure in hot plate forging the internal strain and stress distributions are examined quantitatively by using ABAQUS. Simula-tions are carried out on a large slab ingot having the same temperature and the temperature gradient induced by air-cooling. It is shown that pre-cooling produces little effect on the strain behavior but remarkable effect on the hydrostatic stress at the central zone of ingot. The main factors for crushing micro-voids are the effective strain and the time integral of hydrostatic stress in the region surrounding the voids. Based on regression analysis it was found that the distortion of void can be expressed as a polynomial function of the two factors.

• Transactions of Materials Processing
• /
• v.15 no.8 s.89
• /
• pp.591-596
• /
• 2006

Torsion testing was employed to investigate the deformation and recrystallization behavior of coarse-grained Alloy 718, and the results are compared with the compression testing results. Mechanical testing was conducted on bulk Alloy718 samples within the temperature ranges, $1000^{\circ}C{\sim}1100^{\circ}C$. The strain gradient formed in the torsion specimens resulted in a recrystallization behavior which varied along the radial direction from the center to the surface. The flow curves based on effective stress and effective strain as obtained by Fields and Backofen's isotropic deformation theory and the dynamic recrystallization within the compression tested samples and torsion tested samples are different. The different deformation and recrystallization behavior can be rationalized by the fact that the deformation in the coarse-grained torsion specimens is not uniform and thus the strain gradient within the specimens cannot be analytically predicted by FE simulation. Thus, the extent of recrystallization cannot be properly predicted by the established recrystallization equations based on compression tests.

• Transactions of Materials Processing
• /
• v.33 no.4
• /
• pp.285-290
• /
• 2024

Electropulsing treatment (EPT) has been developed as an alternative to furnace heat treatment (FHT) to exploit its engineering advantages in rapidly annealing metallic materials. Conventionally, the separation of thermal and athermal effects of EPT has been attempted by comparing EPT and FHT specimens processed under identical temperature and duration. However, this method inherently introduces experimental and measurement errors. This study proposes a novel approach to distinguish the thermal and athermal effects of EPT-processed metals using T-shaped specimen with two observation points, namely 'C' and 'D'. For verification, the thermal gradient of T-shaped Mg alloys was examined under various EPT conditions. The points C exhibited higher temperatures compared to those at points D at a given electric current density, because only the former received both thermal and athermal effects. It was confirmed from twelve specimens that the point C at an electric current density of 65 A·mm^-2 and point D at 70 A·mm^-2 exhibited similar temperatures. This developed method is expected to reduce measurement errors in distinguishing thermal and athermal effects, thus providing a deeper understanding of their quantitative contributions in future studies.

• Proceedings of the KWS Conference
• /
• 2003.11a
• /
• pp.156-158
• /
• 2003

An implicit finite element implementation for Leblond's transformation plasticity constitutive equations, which are widely used in welded steel structure is proposed in the framework of parallel computing. The implementation is based upon the multiplicative decomposition of deformation gradient and hyper elastic formulation. We examine the efficiency of parallel computation for the finite element analysis of a welded structure using domain-wise multi-frontal solver.

• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.36 no.2
• /
• pp.187-194
• /
• 2012

The strength of particle-reinforced metal matrix composites is, in general, known to be increased by the geometrically necessary dislocations punched around a particle that form during cooling after consolidation because of coefficient of thermal expansion (CTE) mismatch between the particle and the matrix. An additional strength increase may also be observed, since another type of geometrically necessary dislocation can be formed during extensive deformation as a result of the strain gradient plasticity due to the elastic-plastic mismatch between the particle and the matrix. In this paper, the magnitudes of these two types of dislocations are calculated based on the dislocation plasticity. The dislocations are then converted to the respective strengths and allocated hierarchically to the matrix around the particle in the axisymmetric finite-element unit cell model. The proposed method is shown to be very effective by performing finite-element strength analysis of $SiC_p$/Al2124-T4 composites that included ductile failure in the matrix and particlematrix decohesion. The predicted results for different particle sizes and volume fractions show that the length scale effect of the particle size obviously affects the strength and failure behavior of the particle-reinforced metal matrix composites.

• Proceedings of the KSME Conference
• /
• 2004.11a
• /
• pp.43-48
• /
• 2004

When structures are loaded by a combination of primary and secondary stresses, plasticity effects occur which cannot be evaluated by a simple linear addition of the effects resulting from the two independent stress systems. Thermal stress due to temperature gradient is classified as secondary stress. It is known that secondary stress is released as increase of plastic zone. In this paper, two and three dimensional elastic-plastic finite element analyses are performed for the cracked plates and pipes under combined thermal and mechanical loading. And V-factor is introduced to account for plasticity effect. The present results provide that V-factor is function of thermal factor and loading and is consistent regardless of geometry. We developed the prediction method of elastic-plastic fracture mechanics parameter under combined primary and secondary loading from the present results.

• Proceedings of the Korean Society for Technology of Plasticity Conference
• /
• 2005.10a
• /
• pp.231-234
• /
• 2005

The high temperature deformation behavior of AZ 31 Mg alloy was investigated by designing a back propagation neural network that uses a gradient descent-learning algorithm. A neural network modeling is an intelligent technique that can solve non-linear and complex problems by learning from the samples. Therefore, some experimental data have been firstly obtained from continuous compression tests performed on a thermo-mechanical simulator over a range of temperatures $(250-500^{\circ}C)$ with strain rates of $0.0001-100s^{-1}$ and true strains of 0.1 to 0.6. The inputs for neural network model are strain, strain rate, and temperature and the output is flow stress. It was found that the trained model could well predict the flow stress for some experimental data that have not been used in the training. Workability of a material can be evaluated by means of power dissipation map with respect to strain, strain rate and temperature. Power dissipation map was constructed using the flow stress predicted from the neural network model at finer Intervals of strain, strain rates and subsequently processing maps were developed for hot working processes for AZ 31 Mg alloy. The safe domains of hot working of AZ 31 Mg alloy were identified and validated through microstructural investigations.

• Transactions of Materials Processing
• /
• v.14 no.6 s.78
• /
• pp.496-501
• /
• 2005

Precipitates, present in most commercial alloys, can have a strong influence on strength and hardening behavior of a single crystal. The effect of thin precipitates on the anisotropy of initial slip resistance and hardening behavior of crystals is modeled in this article. For the convenience of the computational derivation and implementation, the material formulation is given in the unrelated intermediate configuration mapped by the plastic part of the deformation gradient. Material descriptions for the considered two phased aggregates consisting in lattice hardening as well as isotropic hardening and kinematic hardening are suggested. Numerical simulations of various loading cases are presented to discuss and assess the performance of the suggested model. From the results of the numerical simulation, it is found that the suggested model represents the initial plastic anisotropy at least qualitatively well and that it has an improved representation of various characteristic hardening behaviors in comparison with conventional hardening descriptions where the precipitate structure is not reflected.

• Interaction and multiscale mechanics
• /
• v.6 no.2
• /
• pp.237-255
• /
• 2013

In this paper, a meshfree-enriched finite element method (ME-FEM) is introduced for the large deformation analysis of nonlinear path-dependent problems involving contact. In linear ME-FEM, the element formulation is established by introducing a meshfree convex approximation into the linear triangular element in 2D and linear tetrahedron element in 3D along with an enriched meshfree node. In nonlinear formulation, the area-weighted smoothing scheme for deformation gradient is then developed in conjunction with the meshfree-enriched element interpolation functions to yield a discrete divergence-free property at the integration points, which is essential to enhance the stress calculation in the stage of plastic deformation. A modified variational formulation using the smoothed deformation gradient is developed for path-dependent material analysis. In the industrial metal forming problems, the mortar contact algorithm is implemented in the explicit formulation. Since the meshfree-enriched element shape functions are constructed using the meshfree convex approximation, they pose the desired Kronecker-delta property at the element edge thus requires no special treatments in the enforcement of essential boundary condition as well as the contact conditions. As a result, this approach can be easily incorporated into a conventional displacement-based finite element code. Two elasto-plastic problems are studied and the numerical results indicated that ME-FEM is capable of delivering a volumetric locking-free and pressure oscillation-free solutions for the large deformation problems in metal forming analysis.