• Korean Journal of Materials Research
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• v.23 no.7
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• pp.373-378
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• 2013

In order to clarify the effect of C/Ti atom ratios(${\chi}$) on the deformation behavior of $TiC_{\chi}$ at high temperature, single crystals having a wide range of ${\chi}$, from 0.56 to 0.96, were deformed by compression test in a temperature range of 1183~2273 K and in a strain rate range of $1.9{\times}10^{-4}{\sim}5.9{\times}10^{-3}s^{-1}$. Before testing, $TiC_{\chi}$ single crystals were grown by the FZ method in a He atmosphere of 0.3MPa. The concentrations of combined carbon were determined by chemical analysis and the lattice parameters by the X-ray powder diffraction technique. It was found that the high temperature deformation behavior observed is the ${\chi}$-less dependent type, including the work softening phenomenon, the critical resolved shear stress, the transition temperature where the deformation mechanism changes, the stress exponent of strain rate and activation energy for deformation. The shape of stress-strain curves of $TiC_{0.96}$, $TiC_{0.85}$ and $TiC_{0.56}$ is seen to be less dependent on ${\chi}$, the work hardening rate after the softening is slightly higher in $TiC_{0.96}$ than in $TiC_{0.85}$ and $TiC_{0.56}$. As ${\chi}$ decreases the work softening becomes less evident and the transition temperature where the work softening disappears, shifts to a lower temperature. The ${\tau}_c$ decreases monotonously with decreasing ${\chi}$ in a range of ${\chi}$ from 0.86 to 0.96. The transition temperature where the deformation mechanism changes shifts to a lower temperature as ${\chi}$ decreases. The activation energy for deformation in the low temperature region also decreased monotonously as ${\chi}$ decreased. The deformation in this temperature region is thought to be governed by the Peierls mechanism.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.23 no.4
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• pp.353-360
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• 2010

In this paper, an efficient algorithm is established in order to estimate the plastic properties of power-law hardening bulk specimen materials with one simple spherical indentation impression test. This work is based on a new formulation of representative strain and, therefore, compare to the preceding approaches the fitting parameters are significantly reduced. Moreover, the new definition of representative strain endowed more physical meaning to the representative strain. In order to verify the reliability of the reverse analysis, we have studied a broad set of materials whose property ranges cover essentially all engineering metals and alloys. Based on the indentation force-displacement P-${\delta}$ curves obtained from numerical simulations, the characteristics of the indentation response and material elastoplastic properties are bridged via explicit functions. Next, through the procedure of reverse analysis the yield stress and power-law hardening exponent of bulk specimen materials can be determined. Finally, good agreement between the result from reverse analysis and initial input data from experiment can be observed.

• Journal of the Korean Society for Precision Engineering
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• v.21 no.1
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• pp.71-79
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• 2004

The deformed layers generated in face milling works were comparatively investigated to type 316L and nitrogen(N)-added type 316LN stainless steels. In order to characterize mechanical properties between type 316L and type 316LN, high-temperature tensile tests were conducted with different temperatures: R.T to $700^{\circ}C$. The cutting forces of three components, Fx, Fy and Fz were measured using a tool dynamometer through the face milling cutting tests. The deformed layers were measured by micro-hardness tests along deformed layers. The results of mechanical properties showed that type 316LN was superior to type 316L. The deformed layers of two steels were generated in the 1501m-3001m ranges, and type 316L was higher than type 316LN. The reason for this is due to the high strength properties by nitrogen effect. It was found that deformed structures were well observed for type 316L, but were minutely observed for type 316LN in this cutting conditions.

• 한국가스학회:학술대회논문집
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• 1997.09a
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• pp.78-85
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• 1997

Indentation and small punch tests are very powerful methods to monitor the materials reliability since they are very simple, easy and almost non-destructive. First, recently-developed continuous indentation test can provide the more material properties such as hardness, elastic modulus, yield strength, work-hardening exponent, etc., than the conventional hardness test. In our study, the true stress-strain curve was derived from the indentation load-depth curve for spherical indentation. In detail, the strain was able to be obtained from plastic depth/contact radius ratio, and the flow stress was from mean contact pressure through the analysis of elastic-plastic indentation stress field. Secondly, the small punch test was studied to evaluate the fracture toughness and defomation properties such as elastic modulus and yield strength. Like the indentation test, this test can be applied without severe damage of the target structure.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2000.10a
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• pp.33-36
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• 2000

The dynamic recrystallization (DRX) of medium carbon steels (SCM 440 and POSMA45) was studied with torsion test in the temperature range of $900-1100^{\circ}C$ and the strain rate range of $5.0x10^{-2}\;-\;5.0x10^0/sec$. To establish the quantitative equations for DRX, the evolution of flow stress curve with strain was analyzed. The critical strain (${\varepsilon}_c$) and strain for maximum softening rate ( ${\varepsilon}^{*}$) could be confirmed by the analysis of work hardening rate ($d{\sigma}/d{\varepsilon}\;=\; \theta$). The volume fraction of dynamic recrystallization ($X_{DRX}$) as a function of processing variables, such as strain rate ( $\dot{\varepsilon}$ ), temperature (T), and strain ( $\varepsilon$ ) were established using the ${\varepsilon}_c$ and ${\varepsilon}^{*}$. For the exact prediction, the ${\varepsilon}_c$, ${\varepsilon}^{*}$ and Avrami' exponent (m') were quantitatively expressed by dimensionless parameter, Z/A respectively. The transformation-effective strain-temperature curve for DRX could be composed. It was found that the calculated results were agreed with the experimental data for the steels at any deformation conditions.

• Transactions of Materials Processing
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• v.18 no.8
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• pp.640-645
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• 2009

Recently, boss and rib test based on backward extrusion process was proposed to quantitatively evaluate the interfacial friction condition in bulk forming process. In this test, the tube-shaped punch with hole pressurizes the workpiece so that the boss and rib are formed along the hole and outer surface of the punch. It was experimentally and numerically revealed that the height of boss is higher than that of the rib under the severe friction condition. This work is focused on the effect of the punch design and flow stress on deformation pattern in boss and rib test. From the boss and rib test simulations, it was found that there is slight variation in both the heights of boss and rib according to the length of punch land, nose radius, and face angle. However the hole diameter of the punch and the clearance between the punch and die have a significant influence on the calibration curves showing the heights of the boss and rib. In addition, the effect of flow stress on the calibration curves was investigated through FE simulations. It was found that there is no effect of strength coefficient of the workpiece on the calibration curves for estimation of friction condition. On the other hand, the strain-hardening exponent of the workpiece has a significant influence on the calibration curve.

• Proceedings of the KIEE Conference
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• 1997.11a
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• pp.703-708
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• 1997

Continuous indentation test is a very powerful method to monitor the materials reliability since it is very simple, easy and almost non-destructive. It can provide material properties such as elastic modulus, yield strength, work-hardening exponent, etc., than the conventional hardness test. In our study, the true stress-strain curve is derived from the indentation load-depth curve. For this, average indentation strain is defined and the flow stress is obtained from the analysis of the indentation stress field. The residual stress is analyzed from the variation of the indentation behavior with the applied residual stress. And the estimation of fracture characteristic is tried by considering the conventional fracture toughness modeling and the stress/strain state under the spherical indenter.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2001.10a
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• pp.293-296
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• 2001

The dynamic softening mechanisms of AISI 316, AISI 304 and AISI 430 stainless steels were studied with torsion test in the temperature range of $900 - 1200^{\circ}C$ and the strain rate range of $5.0x10^{-2}-5.0x10^0/sec$. The austenitic stainless steels, such as AISI 316 and AISI 304 were softened by dynamic recrystallization (DRX) during hot deformation. Also, the evolutions of flow stress and microstructure of AISI 430 ferritic stainless steel show the characteristics of continuous dynamic recrystallization (CDRX). To establish the quantitative equations for DRX of AISI 316 stainless steel, the evolution of flow stress curve with strain was analyzed. The critical strain (${\varepsilon}_c$) and strain for maximum softening rate (${\varepsilon}^{*}$) could be confirmed by the analysis of work hardening rate ($d{\sigma}/d{\varepsilon}={\theta}$). The volume fraction of dynamic recrystallization ($X_{DRX}$) as a function of processing variables, such as strain rate ( $\varepsilon$ ), temperature (T), and strain ( $\varepsilon$ ) were established using the ${\epsilon}_c$ and ${\varepsilon}^{*}$. For the exact prediction the ${\varepsilon}_c,\;{\varepsilon}^{*}$ and Avrami' exponent (m') were quantitatively expressed by dimensionless parameter, Z/A, respectively. It was found that the calculated results were agreed with the experimental data for the steels at my deformation conditions. Also, we can reasonably conclude that the DRX, CDRX and grain refinement of stainless steels can be achieved by large strain deformation at high Z parameter condition.

• Journal of the Korean Society for Precision Engineering
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• v.18 no.4
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• pp.142-150
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• 2001

Automotive manufactures have taken more interests in tailored sheet metals for improving the rigidity, weight reduction, crash durability, and cost savings so that their application to auto-bodies has been increased. However, since the tailored sheet metals do not behave like un-welded sheet metals in press forming operations, the stamping engineers no longer rely only on conventional forming techniques. Futhermore, there is no clear understanding of the characteristics of welded metal which influence the overall press formability of tailored sheet metals. Recently, the computer simulations are prevailing for the evaluation of the formability. Unfortunately, the mechanical property of tailored sheet metal has to be quantitatively defined in the simulation. In this study, the analytical equations are formulated in order to find the mechanical properties of the welded metal in the tailored sheet metal welded by co$_2$laser. Based on force distribution assumption, the constitutive behavior of the welded metal is investigated using uniaxial tensile test results of base metals and tailored sheet metal. Then, the strength coefficient, work-hardening exponent, and plastic strain ratio of laser-welded metal are calculate from those of base metals and tailored sheet metal. In addition, the existence of weld defects in the welded metal is indirectly detected by examining the slop of strength coefficient of the welded metal.

• Korean Journal of Materials Research
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• v.14 no.10
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• pp.713-718
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• 2004

To study the dynamic strain aging behavior of Zr-0.4Sn-1.5Nb-0.2Fe sample tube for nuclear fuel cladding in the range of pressurized water reactor (PWR) operation temperature, the tensile tests of the tube specimens, which had been finally heat-treated at $470^{\circ}C\;and\;510^{\circ}C$, had been carried out with the strain rate $1.67{\times}10^{-2}/s\;and\;8.33{\times}10^{-5}/s$ at the various temperatures from room temperature to $500^{\circ}C$. It was observed that the elongation of the specimens got shortened as the temperature increased from $200^{\circ}C\;to\;340^{\circ}C$. The specimens that were finally heat-treated at $470^{\circ}C$ showed a plateau more remarkably on the plot of yield strength-temperature than those heat-treated at $510^{\circ}C$. In the range of $310\sim400^{\circ}C$, the strain rate sensitivity of the specimens finally heat-treated at $510^{\circ}C$ was $30.4\%\sim33.7\%$ lower but the work hardening exponent index of the specimens was a little higher than that without dynamic strain aging effect.