• 한국가시화정보학회:학술대회논문집
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• 2002.11a
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• pp.97-99
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• 2002

Turbulent premixed flames were a subject of many researches for a number of decades. Especially, Borghi suggested a manificent diagram classifying turbulent combustion reasions and Lipatnikov and Chomiak modified this diagram. But this diagram has difficulties tn defining a flame thickness and velocity and measuring integral length scales In addition, recently experimental techniques are being developed, so we can accurately use PIV diagnostics measuring 2D velocity field instead of LDV and make good use of PLIF techniques for obtaining the flame information. In this study, according to developing techniques, suggest a new diagram replacing the existing Borghi diagram. Simultaneous PIV/OH PLIF measurements are used, which measure a shear strain rate and a location of flames, respectively. The shear strain rate represents turbulence and the OH signal indicates the flame information, but there is no geometric Information which is very important to flame quenching. Hence, to consider the geometric information, calculate fractal dimensions of the OH images. So the diagram suggested in this research has three axes which consist of strain rate, OH signal, and fractal dimension and can classify turbulent premixed flames.

• Transactions of Materials Processing
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• v.12 no.4
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• pp.358-363
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• 2003

Adiabatic shear bands have been observed in the serrated chip during high strain rate metal cutting process of medium carbon steel and titanium alloy The recent microscopic observations have shown that dynamic recrystallization occurs in the narrow adiabatic shear bands. However the conventional flow stress models such as the Zerilli-Armstrong model and the Johnson-Cook model, in general, do not predict the occurrence of dynamic recrystallization (DRX) in the shear bands and the thermal softening effects accompanied by DRX. In the present study, a strain hardening and thermal softening model is proposed to predict the adiabatic shear localized chip formation. The finite element analysis (FEA) with this proposed flow stress model shows that the temperature of the shear band during cutting process rises above 0.5Τ$_{m}$. The simulation shows that temperature rises to initiate dynamic recrystallization, dynamic recrystallization lowers the flow stress, and that adiabatic shear localized band and the serrated chip are formed. FEA is also used to predict and compare chip formations of two flow stress models in orthogonal metal cutting with AISI 1045. The predictions of the FEA agreed well with the experimental measurements.s.

• Journal of Ocean Engineering and Technology
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• v.25 no.2
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• pp.134-144
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• 2011

In this paper, the global study trends for material behaviors are investigated regarding the static and dynamic hardenings and final fractures of marine structural steels. In particular, after reviewing all of the papers published at the 4th and 5th ICCGS (International Conference on Collision and Grounding of Ship), the used hardening and fracture properties are summarized, explicitly presenting the material properties. Although some studies have attempted to employ new plasticity and fracture models, it is obvious that most still employed an ideal hardening rule such as perfect plastic or linear hardening and a simple shear fracture criterion with an assumed value of failure strain. HSE (2001) presented pioneering study results regarding the temperature dependency of material strain hardening at various levels of temperature, but did not show strain rate hardening at intermediate or high strain rate ranges. Nemat-Nasser and Guo (2003) carried out fully coupled tests for DH-36 steel: strain hardening, strain rate hardening, and temperature hardening and softening at multiple steps of strain rates and temperatures. The main goal of this paper is to provide the theoretical background for strain and strain rate hardening. In addition, it presents the procedure and methodology needed to derive the material constants for the static hardening constitutive equations of Ludwik, Hollomon, Swift, and Ramberg-Osgood and for the dynamic hardening constitutive equations of power from Cowper-Symonds and Johnson-Cook.

• Transactions of the Korean Society of Mechanical Engineers
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• v.19 no.3
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• pp.820-833
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• 1995

An analysis is performed to examine the equilibrium state and the stability of modeled Reynolds stress equations for homogeneous turbulent shear flows. The system of the governing equations consists of four coupled ordinary differential equations. The equilibrium states are found by the steady state solution of the governing equations. In order to investigate the stability of the system about its state in equilibrium, and eigenvalue problem is constructed. As a result, constraints for the coeffieients in the model equations are obtained by the stability condition of the equilibrium state as well as by their physically realizable bounds. It is observed that the models with pressure-strain rate correlation that are linear in the anisotropy tensor are stable and produce reasonable equilibrium tensor do not behave properly. Stability considerations about three most commonly used models are given in detail in the final section.

• Transactions of Materials Processing
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• v.17 no.3
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• pp.161-169
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• 2008

In the present work, the two-back stress model is proposed and continuum damage mechanics (CDM) is incorporated into the plastic constitutive relation in order to describe the plastic deformation localization and the damage evolution in a deforming continuum body. Coupling between damage mechanics and isothermal rate independent plasticity is performed using the kinematic hardening rule, which in turn is formulated by combining the nonlinear Armstrong-Frederick rule and the Phillips rule. The numerical analyses are carried out within h deformation theory. It is noted that the damage evolution within a work piece accelerates the plastic deformation localization such that the material with lower hardening exponent results in a rapid shear band formation. Moreover, the results from the numerical analysis reflected closely with the micro-structures around the fractured regime. The effects of the various hardening parameters on deformation localization are also investigated. As the nonlinear strain rate description in the back stress evolution becomes dominant, the strain localization becomes intensified as well as the damage evolution.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.25 no.12
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• pp.1933-1943
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• 2001

A micromechanical model is presented for superplasticity in which heterogeneous microstructures are coupled with deformation behavior. The effects of initial distributions of grain size, and their evolutions on the mechanical properties can be predicted by the model. Alternative stress rate models such as Jaumann rate and rotation incremental rate have been employed to analyze uniaxial loading and simple shear problems and the appropriate modeling was studied on the basis of hypoelasticity and elasto-viscoplasticity. The model has been implemented into finite element software so that full process simulation can be carried out. Tests have been conducted on Ti-6Al-4V alloy and the microstructural features such as grain size, distributions of grain size, and volume fraction of each phase were examined for the materials that were tested at different strain rates. The experimentally observed stress-strain behavior on a range of initial grain size distributions has been shown to be correctly predicted. In addition, the effect of volume fraction of the phases and concurrent grain growth were analyzed. The dependence of failure strain on strain rate has been explained in terms of the change in mechanism of grain growth that occurs with changing strain rate.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2007.05a
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• pp.272-275
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• 2007

This paper is concerned with modified integration algorithm on the strain-space for rate and temperature dependent elasto-plastic constitutive relations in order to obtain more accurate results in numerical implementation. The proposed algorithm is integrated analytically using integration by part and chain rule and then is applied to the 2-stage Lobatto IIIA with second-order accuracy. It has advantage that is able to consider the convective stress rates on the yield surface of the strain-space. Also this paper is carried out the iteration procedure using the Newton-Raphson method to enforce consistency at the end of the step. And the performance of the proposed algorithm for rate and temperature dependent constitutive relation is illustrated by means of analysis of adiabatic shear bands.

• Proceedings of the KSME Conference
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• 2007.05a
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• pp.762-767
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• 2007

As a macromolecule material, melted rubber flow shows characteristics of shear thinning fluid. The dynamic viscosity of this rubber fluid is influenced by temperature and shear strain rate. In this study, the numerical simulation of rubber extrusion forming process has been performed using commercial CFD code, Polyflow. Power-law model considering the effect of shear rate is used for the computer simulation of this non-Newyonian flow. Also Non-isothermal behavior is considered as Arrhenius-law model. Distributions of velocity and temperature are predicted through the simulation.

• Transactions of the Korean Society of Mechanical Engineers
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• v.19 no.9
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• pp.2297-2307
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• 1995

An experimental study on the structure of a separated shear layer downstream of the backward-facing step has been performed by examining mean flow and turbulent quantities in terms of free stream turbulence. When free stream turbulence exists, the entrainment rate of the separated shear layer and the flow rate in the recirculation region are enhanced, resulting in shorter reattachment length. The production and diffusion terms in the turbulent kinetic energy balance are shown to increase more than the dissipation term does. Rapid decrease of the pressure-strain term in the shear stress balance implies the enhancement of the three-dimensional motion by free stream turbulence.

• Korean Journal of Materials Research
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• v.24 no.11
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• pp.625-631
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• 2014

To investigate the deformation properties of TiC-(5-20) mol% Mo solid solution single crystals at high temperature by compression testing, single crystals of various compositions were grown by the radio frequency floating zone technique and were deformed by compression at temperature from 1250K to 2270K at strain rates from $5.1{\times}10^{-5}$ to $5.9{\times}10^{-3}/s$. The plastic flow property of solid solution single crystals was found to be clearly different among a three-temperature range (low, intermediate and high temperature ranges) whose boundaries were dependent on the strain rate. From the observed property, we conclude that the deformation in the low temperature range is controlled by the Peierls mechanism, in the intermediate temperature range by the dynamic strain aging and in the high temperature range by the solute atmosphere dragging mechanism. The work softening tends to become less evident with an increasing experimental temperature and with a decreasing strain rate. The temperature and strain rate dependence of the critical resolved shear stress is the strongest in the high temperature range. The curves are divided into three parts with different slopes by a transition temperature. The critical resolved shear stress (${\tau}_{0.2}$) at the high temperature range showed that Mo content dependence of ${\tau}_{0.2}$ with temperature and the dependence is very marked at lower temperature. In the higher temperature range, ${\tau}_{0.2}$ increases monotonously with an increasing Mo content.