• Transactions of Materials Processing
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• v.15 no.8 s.89
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• pp.568-573
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• 2006

In order to numerically evaluate automotive hydro-formed DP-steel tubes on crash performance considering welding heat effects, the finite element simulations of crash behavior were performed for hydro-formed tubes with and without heat treatment effects. This work involves the mechanical characterization of the base material and the HAG-welded zone as well as finite element simulations of the crash test of hydro-formed tubes with welded brackets and hydro-forming of tubes. The welding heat effects on the crash performance are evaluated in efforts to improve the process optimization procedure of the engine cradle in the design stage. In particular, FEM simulations on indentations have been performed and experimentally verified for material properties of weld zone and heat affected zone.

• Transactions of Materials Processing
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• v.12 no.4
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• pp.314-319
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• 2003

In the micro hole punching, the size and shape of burr and burnish zone are very important factors to evaluate quality of micro holes which depend on punch-die clearance, strain rate, workpiece material and etc. To get micro holes with small burr and wide burnish zone for industrial demands, not only the parametric study but also a study on fracture behavior in shear band are necessary. In this study, 100 $\mu$m, 25 $\mu$m micro holes in diameter were fabricated on brass (Cu63/Zn37) and SUS 316 foils as aspect ratio 1:1, and the characteristics of micro holes was investigated comparing with those of macro holes over several mm by scanning electron microscopic views and section views. Like macro hole, micro hole is also composed of 4 portions, rollover. burnish zone, fracture zone and burr, and it shows similar fracture behavior in shear band. But by high strain rate (10$^2$∼10$^3$s$^{-1}$ ) condition unlike that of macro hole fabrication and by the increment of relative grain size in the direction of the workpiece thickness, fracture zone is not observed.

• Structural Engineering and Mechanics
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• v.17 no.1
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• pp.51-68
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• 2004

Since the linear elastic fracture analysis has been proved to be insufficient in predicting the failure of strain hardening materials, a number of fracture concepts have been studied which remain applicable in the presence of plasticity near a crack tip. This work thereby presents a new finite element model to predict the elastic-plastic crack-tip field and fatigue life of center-cracked panels(CCP) with ductile fracture under large-scale yielding conditions. Also, this study has been carried out to investigate the path-dependence of J-integral within the plastic zone for elastic-perfectly plastic, bilinear elastic-plastic, and nonlinear elastic-plastic materials. Based on the incremental theory of plasticity, the p-version finite element is employed to account for the accurate values of J-integral, the most dominant fracture parameter, and the shape of plastic zone near a crack tip by using the J-integral method. To predict the fatigue life, the conventional Paris law has been modified by substituting the range of J-value denoted by ${\Delta}J$ for ${\Delta}K$. The experimental fatigue test is conducted with five CCP specimens to validate the accuracy of the proposed model. It is noted that the relationship between the crack length a and ${\Delta}K$ in LEFM analysis shows a strong linearity, on the other hand, the nonlinear relationship between a and ${\Delta}J$ is detected in EPFM analysis. Therefore, this trend will be depended especially in the case of large scale yielding. The numerical results by the proposed model are compared with the theoretical solutions in literatures, experimental results, and the numerical solutions by the conventional h-version of the finite element method.

• Journal of the Korean Society of Hazard Mitigation
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• v.1 no.1 s.1
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• pp.117-125
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• 2001

This paper presented the analysis of stress regimes in and around inactive and active fault zones. The stress regime in the vicinity of an existing inactive fault zone is dependent on the orientation of the fault with respect to the current stress field and the contrast between the elastic properties of the faulted rock and those of the surrounding rock. In the analysis of stress regimes around an active fault zone, if the yielding stress is exceeded during loading, the localized shearing in a fault zone will result in weakness with mean stresses in the fault becoming lower than those in the surrounding rock. It can be expected that such stress gradients will induce fluid flow towards the faults zone.

• Interaction and multiscale mechanics
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• v.4 no.3
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• pp.187-206
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• 2011

Accurate estimations of pre-failure deformations and post-failure responses of geostructures require that the simulation tool possesses at least three main ingredients: 1) a constitutive model that is able to describe the macroscopic stress-strain-strength behavior of soils subjected to complex stress/strain paths over a wide range of confining pressures and densities, 2) an embedded length scale that accounts for the intricate physical phenomena that occur at the grain size scale in the soil, and 3) a computational platform that allows the analysis to be carried out beyond the development of an initially "contained" failure zone in the soil. In this paper, a two-scale micropolar plasticity model will be used to incorporate all these ingredients. The model is implemented in a finite element platform that is based on the mechanics of micropolar continua. Appropriate finite elements are developed to couple displacement, micro-rotations, and pore-water pressure in form of $u_n-{\phi}_m$ and $u_n-p_m-{\phi}_m$ (n > m) elements for analysis of dry and saturated soils. Performance of the model is assessed in a biaxial compression test on a slightly heterogeneous specimen of sand. The role of micropolar component of the model on capturing the post-failure response of the soil is demonstrated.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2007.05a
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• pp.111-113
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• 2007

The Mechanical properties of steel in hot strip mill were associated with the alloy composition, plastic deformation, cooling history and so on. In the case of the same alloy composition and deformation conditions, cooling history on ROT (run out table) is the main factor in affecting mechanical properties of steel, especially, in carbon steel. On ROT, the steel undergoes under various kinds of cooling conditions such as radiation, convection by air, water and wetting zone. The coiling temperature (CT) of the steel is also important factor in affecting mechanical properties. But with the same CT, the mechanical properties of steel can be different because the temperature history of cooling is more important factor than CT itself. In this study, we have studied the relations between temperature history and mechanical properties of steel and then the predicted mechanical properties have compared with the measured values.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2008.10a
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• pp.304-307
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• 2008

In order to evaluate the effect of permanent softening behavior on springback prediction, 2D-draw bending simulations were compared with experiments for friction stir welded DP590 steel sheets. To account fur the nonlinear hardening behavior, the combined isotropic-kinematic hardening law was utilized with and without considering the permanent softening behavior during reverse loading. Also, the non-quadratic orthotropic yield function, Yld2000-2d, was used to describe the anisotropic initial-yielding behavior of the base sheet while anisotropic properties of the weld zone were ignored for simplicity.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2008.10a
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• pp.142-145
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• 2008

A finite element analysis has been conducted to simulate direct/indirect extrusion process for AZ31 Mg alloy at various ram and die speeds. Uniaxial compression test on AZ31 Mg alloy was carried out at various strain rates and temperatures and the result was used as input data fur finite element analysis. It was found that ram speed affects the distribution of dead zone area during direct extrusion. The inhomogeneous temperature and strain distributions through the thickness direction can be simulated under the various extrusion process conditions.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2008.10a
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• pp.354-357
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• 2008

In this study, the casting process using forged insert was applied to manufacturing a knuckle, in order to prove that application of casting process using forged insert is likely to get the effect of light weight and superior mechanical characteristic compared with existing casting products. Firstly, in the forging experiment, it was confirmed that the optimal configuration of the forged insert could be predicted by FE analysis. And by using FVM (Finite Volume Method) software, it was verified that a proposed casting design was available. To identify the mechanical characteristic of the final casting product, the microstructual observation and hardness test were performed at the boundary zone between forging and casting part.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2001.10a
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• pp.44-49
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• 2001

Tube hydroforming is a relatively new technology in comparison with conventional stamping process. Thus, there is no large knowledge base to assist the product and process designers, especially from the friction point of view. This paper covers the topic f friction and lubrication with regard to tube hydroforming. It presents the fact that frictional characteristic can have an effect on the formability of specific components. It also presents the results of test about several lubricants which are used in practical press process. The presented concept describes the equipment which is required to determine the friction coefficient.