• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2004.10a
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• pp.180-183
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• 2004

Superplastic formability depends on flow parameters such as temperature, strain rate, strain and stress, microstructures. Usually, superplastic properties of materials are characterized with using a uni-axial tension testing. However, superplastic sheet is formed under mutiaxial loading condition in most forming practices. In the present study, superplastic characteristics of A15083 alloys were determined with using both a uni-axial and biaxial bulging tests. Specially, cone-type die was used to achieve constant strain rate under constant pressure condition. Even though constant strain rate under a certain pressure was achieved only approximately, a cone-type bulging test was found to be quite beneficial to get a multiaxial formability of superplastic materials.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.30 no.11 s.254
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• pp.1494-1501
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• 2006

Mechanical and physical properties of a copper alloy for a liquid rocket engine(LRE) combustion chamber liner application were tested at various temperatures. All test specimens were heat treated with the condition they might experience during actual fabrication process of the LRE combustion chamber. Physical properties measured include thermal conductivity, specific heat and thermal expansion data. Uniaxial tension tests were preformed to get mechanical properties at several temperatures ranging from room temperature to 600$^{\circ}C$. The result demonstrated that yield stress and ultimate tensile stress of the copper alloy decreases considerably and strain hardening increases as the result of the heat treatment. Since the LRE combustion chamber operates at higher temperature over 400$^{\circ}C$, the copper alloy can exhibit time-dependent behavior. Strain rate, creep and stress relaxation tests were performed to check the time-dependent behavior of the copper alloy. Strain rate tests revealed that strain rate effect is negligible up to 400$^{\circ}C$ while stress-strain curve is changed at 500$^{\circ}C$ as the strain rate is changed. Creep tests were conducted at 250$^{\circ}C$ and 500$^{\circ}C$ and the secondary creep rate was found to be very small at both temperatures implying that creep effect is negligible for the combustion chamber liner because its operating time is quite short.

• Journal of the Korean GEO-environmental Society
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• v.12 no.1
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• pp.27-33
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• 2011

In general, the shear strength of a clay specimen under the direct shear test and the triaxial compression test increases with an increase in the shear rate. This study investigates the effects of shear rate and silt content on the stress-strain behavior of remolded Gwangyang clay, by changing the shear rate and the silt content. Based on the results of the triaxial compression tests, the equi-strain line of remolded Gwangyang clay shows initially positive slope and then becomes flat at certain strain level. As the strain level where the equistrain becomes flat is different depending on the soil with different silt contents, this can be considered as the inherent property of soil.

• 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.21 no.12
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• pp.154-159
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• 2004

This study is aimed to diminish the errors which created during transforming from 3D shape created with CAD program to STL file formation. The length strain which created on X-Y axises at STL file transforming is diminished by using the relation between common shape error and shrinkage rate. As the result of study, we have confirmed the length strain in accordance with facetres value. Also, the shape error is compensated with shrinkage rate by error of length strain. so, we could diminish to shape error by several tens micrometer.

• 한국연소학회:학술대회논문집
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• 2014.11a
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• pp.243-244
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• 2014

The opposed flow flame in a mesoscale channel was constructed to observe the flame stabilization behaviors at low strain rate conditions (<$10s^{-1}$). The purpose of this study is to get the overall flame behaviors of partially premixed flames with oxygen enhanced conditions at low strain rates. The oxygen ratio in oxidizer was changed from 18 to 30 %. Conclusively, the flame extinction limit approached to about $1s^{-1}$, and divided into three representative regimes corresponding to self propagating flame, transitional flame, quenching flame regimes.

• Geomechanics and Engineering
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• v.17 no.4
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• pp.333-342
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• 2019

Geological dynamic hazards during coal mining can be caused by the failure of a composite system consisting of roof rock and coal layers, subject to different loading rates due to different advancing velocities in the working face. In this paper, the uniaxial compression test simulations on the composite rock-coal layers were performed using $PFC^{2D}$ software and especially the effects of loading rate on the stress-strain behavior, strength characteristics and crack nucleation, propagation and coalescence in a composite layer were analyzed. In addition, considering the composite layer, the mechanisms for the advanced bore decompression in coal to prevent the geological dynamic hazards at a rapid advancing velocity of working face were explored. The uniaxial compressive strength and peak strain are found to increase with the increase of loading rate. After post-peak point, the stress-strain curve shows a steep stepped drop at a low loading rate, while the stress-strain curve exhibits a slowly progressive decrease at a high loading rate. The cracking mainly occurs within coal, and no apparent cracking is observed for rock. While at a high loading rate, the rock near the bedding plane is damaged by rapid crack propagation in coal. The cracking pattern is not a single shear zone, but exhibits as two simultaneously propagating shear zones in a "X" shape. Following this, the coal breaks into many pieces and the fragment size and number increase with loading rate. Whereas a low loading rate promotes the development of tensile crack, the failure pattern shows a V-shaped hybrid shear and tensile failure. The shear failure becomes dominant with an increasing loading rate. Meanwhile, with the increase of loading rate, the width of the main shear failure zone increases. Moreover, the advanced bore decompression changes the physical property and energy accumulation conditions of the composite layer, which increases the strain energy dissipation, and the occurrence possibility of geological dynamic hazards is reduced at a rapid advancing velocity of working face.

• Geotechnical Engineering
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• v.14 no.4
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• pp.47-60
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• 1998

A series of conventional tests and CRS consolidation tests with different rates of strain were performed to investigate the consolidation characteristics of marine clay. Preconsolidation pressures were evaluated by applying previously proposed methods for both the conventional tests and CRS tests results in order to check the legitimacy of those methods. The effects of strain rate on effective consolidation stress strain relationship, porewater pressure, and preconsolidation pressure were also discussed It was found that the effective stress strain relationship and the preconsolidation pressure are a function of strain rate imposed during consolidation test, but compression index isn't. The preconsolidation pressure ratio ($a_2=\sigma'_{pCRS}/\sigma'_{pConv}$)of marine clay appears proportional to the logarithm of strain rate, with average values ranging from 1.11 to 1.30 for strain rates between $1\timesx10^{-4} %/sec\; and\; 4\times10 %/sec$. The porewater pressure ratio during CRS teats does not exceed 6.0% except when the strain rate is $6.67\times10^{-4} %/sec$. Coefficient of consolidation or coefficient of permeability at normally consolidated range was not affected by the type of consolidation tests and the strain rate. Typical values of compression index (C.), coefficient of consolidation(c.), and coefficient of permeability (k.) at normally consolidated range were 0.56-0.95, $0.56\times10^{-4}~3.0\times10^{-4}cm2/sec,\; and\; 2.0\times10^{-8}~7.0\time10^{-4}cm/sec,$ respectively.

• Computers and Concrete
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• v.33 no.5
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• pp.519-533
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• 2024

Many materials including cementitious concrete-type materials undergo material property changes during high-rate loading. There is a wealth of research regarding this phenomenon for concrete in compression and tension. However, there is minimal knowledge about how mortar material used in concrete masonry unit (CMU) construction behaves in high-rate shear loading. A series of experiments was conducted to examine the bond strength of mortar bonded to CMU units under high-rate shear loading. A novel experimental setup using a shock tube and dynamic ram were used to load specially constructed shear triplets in a double lap shear configuration with no pre-compression. The Finite Element Method was leveraged in conjunction with data from the experimental investigation to establish if the shear bond between concrete masonry units and mortar exhibits any rate dependency. An increase in shear bond strength was observed when loaded at a high strain rate. This data indicates that the CMU-mortar bond exhibits a rate dependent strength change and illustrates the need for further study of the CMU-mortar interface characteristics at high strain rates.

• Proceedings of the Korean Geotechical Society Conference
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• 1998.03a
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• pp.135-142
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• 1998