• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 1997.03a
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• pp.89-93
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• 1997

A superlastic material, aluminum - lithium alloy 8090, were examined with uniaxial tensile test to investigate its thermomechanical behavior. The tests were carried out at the strain-rates ranging from 2${\times}$10-4 to 1${\times}$10-2 and at the temperatures from 48 0$^{\circ}C$ to 540$^{\circ}C$. The experiments produced force-displacement curves which converted to stress-strain curves. From the curves, several important superplastic factor such as strain-rate sensitivity, optimum strain-rate and strength coefficient were obtained.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2009.05a
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• pp.172-175
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• 2009

The purpose of this study is to predict the forming limit diagram (FLD) of strain-rate sensitive materials on the basis of the Marciniak and Kuczynski (M-K) theory. The strain-rate effect is taken into consideration in such a way that the stress-strain curves for various strain-rates are inputted into the formulation as point data, not as curve-fitted models such as power function. To solve the nonlinear system of equations derived from the equilibrium and constraints in the groove region and the safe zone, the Newton-Raphson method is used. The theoretical FLDs using four different yield criteria, that are von Mises, Hill (1948), Hill (1979), Logan and Hosford, are compared with the experimental, numerical (FEA) and other theoretical results. A new trial is made where a modified M-K model having n-step grooves is introduced to describe a real localized neck.

• Transactions of Materials Processing
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• v.15 no.2 s.83
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• pp.132-137
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• 2006

This paper is concerned with the thermo-mechanical behavior including temperature dependent strain-rate sensitivity of steel sheet for an auto-body. In order to identify the temperature dependent strain-rate sensitivity of SPRC35R and SPRC45E, uniaxial tension tests are performed with the variation of the strain-rates from 0.001 /sec to 200 /sec at environmental temperatures varied from $-40^{\circ}C\;to\;200^{\circ}C$. The thermo-mechanical response at the quasi-static state is obtained with the static tensile test and at the intermediate strain-rate is from the high speed tensile test. Experimental results show that the strain-rate sensitivity increases at low temperature. It represents that as the strain-rate increases, the variation of flow stress becomes sensitive on the temperature. The results indicate that the flow stress of SPRC35R is more dependent on the changes of strain-rate and temperature than those of SPRC45E.

• Proceedings of the KSME Conference
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• 2003.04a
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• pp.107-112
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• 2003

Mechanical properties of the materials used for transportations and industrial machinery under high strain rate loading conditions such as high impact loading are required to provide appropriate safety assessment to varying dynamically loaded mechanical structures. The Split Hopkinson Pressure Bar(SHPB) technique with a special experimental apparatus can be used to obtain the material behavior under high strain rate loading conditions. In this paper, the dynamic deformation behavior of a brass under both high strain rate compressive loading conditions has been determined using the SHPB technique.

• Journal of the Korean Society for Precision Engineering
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• v.20 no.12
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• pp.142-147
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• 2003

Mechanical properties of the materials used for transportations and industrial machinery under high strain rate loading conditions such as high impact loading are required to provide appropriate safety assessment to varying dynamically leaded mechanical structures. The Split Hopkinson Pressure Bar(SHPB) technique with a special experimental apparatus can be used to obtain the material behavior under high strain rate ]ending conditions. In this paper, the dynamic deformation behavior of a brass under both high strain rate compressive loading conditions has been determined using the SHPB technique.

• Nuclear Engineering and Technology
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• v.40 no.3
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• pp.225-232
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• 2008

To understand the effect of the cyclic strain rate on the environmentally assisted cracking behaviors of SA508 Gr.1a low alloy steel in deoxygenated water at $310^{\circ}C$, the fatigue surface and a sectioned area of specimens were observed after low cycle fatigue tests. On the fatigue surface of the specimen tested at a strain rate of 0.008 %/s, unclear ductile striations and a blunt crack tip were observed. Therefore, metal dissolution could be the main cracking mechanism of the material at this strain rate. On the other hand, on the fatigue surfaces of the specimens tested at strain rates of 0.04 and 0.4 %/s, brittle cracks and flat facets, which are evidences of the hydrogen induced cracking, were observed. In addition, a tendency of linkage between the main crack and the micro-cracks was observed on the sectioned area. Therefore, at higher strain rates, the main cracking mechanism could be hydrogen induced cracking. Additionally, evidence of the dissolved MnS inclusions was observed on the fatigue surface from energy dispersive x-ray spectrometer analyses. Thus, despite the low sulfur content of the test material, the sulfides seem to contribute to environmentally assisted cracking of SA508 Gr.1a low alloy steel in deoxygenated water at $310^{\circ}C$.

• Transactions of Materials Processing
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• v.33 no.5
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• pp.348-353
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• 2024

The S355NL steel has garnered attention as a structural material for applications in extremely challenging environments owing to its excellent mechanical properties. This study investigated the hot deformation behavior of S355NL steel through compression tests conducted in a temperature range of 900-1200℃ and a strain rate range of 10^-3-1 s^-1 to explore the optimal processing parameters. The flow behaviors consisted of an initial rapid increase and subsequent plateau with a marginal decrease in stress. This phenomenon was interpreted in terms of microstructural evolution, such as dislocation density and dynamic recrystallization. The efficiency of power dissipation and instability domains were derived using the dynamic material model based on the compression test dataset, providing a series of processing maps. In contrast to conventional processing maps plotted for a single strain value, this study has established ten maps at a strain interval of 0.1. This approach allowed for the consideration of continuously variable strain parameters, which is inherent to an actual metal-forming process. The efficiency of power dissipation was strongly governed by the high temperatures (≥ 1100℃). The strain rates barely affected the efficiency, but it primarily contributed to the instability domains. The application of high strain rates (≥ 10^-1s^-1) generated a region of negative instability due to the absence of dynamic recrystallization and the presence of cracks at grain boundaries.

• Korean Journal of Materials Research
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• v.26 no.4
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• pp.167-172
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• 2016

The high temperature deformation behavior of $Ni_3Al$ and $Ni_3(Al,Mo)$ single crystals that were oriented near <112> was investigated at low strain rates in the temperature range above the flow stress peak temperature. Three types of behavior were found under the present experimental conditions. In the relatively high strain rate region, the strain rate dependence of the flow stress is small, and the deformation may be controlled by the dislocation glide mainly on the {001} slip plane in both crystals. At low strain rates, the octahedral glide is still active in $Ni_3Al$ above the peak temperature, but the active slip system in $Ni_3(Al,Mo)$ changes from octahedral glide to cube glide at the peak temperature. These results suggest that the deformation rate controlling mechanism of $Ni_3Al$ is viscous glide of dislocations by the <110>{111} slip, whereas that of $Ni_3(Al,Mo)$ is a recovery process of dislocation climb in the substructures formed by the <110>{001} slip. The results of TEM observation show that the characteristics of dislocation structures are uniform distribution in $Ni_3Al$ and subboundary formation in $Ni_3(Al,Mo)$. Activation energies for deformation in $Ni_3Al$ and $Ni_3(Al,Mo)$ were obtained in the low strain rate region. The values of the activation energy are 360 kJ/mol for $Ni_3Al$ and 300 kJ/mol for $Ni_3(Al,Mo)$.

• Journal of Mechanical Science and Technology
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• v.16 no.11
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• pp.1412-1419
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• 2002

Dynamic shear tests for the tough-pitch copper at high strain and high strain rate was performed. The Split Hopkinson Pressure Bar (SHPB) compression test system was modified to yield a shear deformation in the specimen. Hat-shaped specimens for the tough-pitch copper were adopted to generate high strain of γ=3～4 and high strain-rate of γ= 10$^4$/s. The dynamic analysis by ABAQUS 5.5/EXPLICIT code verified that shear zone can be localized in hat-shaped specimens. A proper impact velocity and the axial length of the shear localization region wert determined through the elastic wave analysis. The displacement in a hat-shaped specimen is limited by a spacer ring which was installed between the specimen and the incident bar. The shear bands were obtained by measuring the direction of shear deformation and the width of deformed grain in the shear zone. The decrease of specimen length has been measured on the optical displacement transducer. Dynamic shear stress-strain relations in the tough-pitch copper were obtained at two strain-rates.

• Journal of Appropriate Technology
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• v.7 no.1
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• pp.2-25
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• 2021

This study evaluated gender preferences regarding the mechanical properties of polymers that are typically used as cooperative robot skin. Gender-based preferences of workers aged 20~30 and polydimethylsiloxane were examined according to the body parts which is most frequently in contact with the robot during operation. The factors influencing preference, i.e., stiffness and stickiness, as measured by strain rate and contact angle, respectively, were analyzed to compare gender-based differences. Female preferred stiffer materials with small strain rates while male preferred softer materials with large strain rates. As a result of evaluating mechanical properties of the materials to relate to gender-based preference, we found that female tended to prefer Dragon-skin with the lowest stickiness, and a low strain rate, during compressive creep tests. In contrast, male tended to prefer Ecoflex with high strain rate regardless of stickiness. Therefore, these results provide basis for material selection when considering cooperative robot skin.