• Proceedings of the Korea Concrete Institute Conference
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• 1999.10a
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• pp.561-564
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• 1999

The uniform state of prestress in a fully prestressed concrete members shows reductions of more than half of the initial prestress in the construction and coupling joint vicinity. Especially, at a zone x/d=0.2 above the construction joint where at the edge of the member, a quite localized reduced stress state of 1/3 of uniform stress is encountered. Five full-scale test specimens were segmentally constructed and post-tensioned and analytic models is used to verify and validate measured test results. The encountered highly nonuniform stress/strain state in the coupling joint vicinity requires special design consideration for the successful application of tendon couplers.

• Journal of the Korean Society for Heat Treatment
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• v.20 no.1
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• pp.22-30
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• 2007

Effects of ferrite grain size on static and dynamic deformation behaviors of SCM415 stels were investigated in this study. Dynamic torsional test was conducted using torsional Kolsky bar with the strain rate of $1.6{\times}10^3/s$. Specimens with three different grain size of ferrite, $4.6{\mu}m$, $11{\mu}m$, $35.5{\mu}m$ were used. Dimple fracture mode of the dynamic test specimens showed adiabatic shear bands on the beneath of fracture surface. Increased uniform elongation and decreased non-uniform elongation appeared as grain size of ferrite decreased in dynamic torsional test. However, shear strength was independent on grain size of ferrite.

• Corrosion Science and Technology
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• v.5 no.1
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• pp.23-26
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• 2006

Recent development of ultrafine grained (UFG) low carbon steels by using equal channel angular pressing (ECAP) and their room temperature tensile properties are reviewed, focusing on the strategies overcoming their inherent mechanical drawbacks. In addition to ferrite grain refinement, when proper post heat treatments are imposed, carbon atom dissolution from pearlitic cementite during ECAP can be utilized for microstructural modification such as uniform distribution of nano-sized cementite particles or microalloying element carbides inside UFG ferrite grains and fabrication of UFG ferrite/martensite dual phase steel. The utilization of nano-sized particles is effective on improving thermal stability of UFG low carbon ferrite/pearlite steel but less effective on improving its tensile properties. By contrast, UFG ferrite/martensite dual phase steel exhibits an excellent combination of ultrahigh strength, large uniform elongation and extensive strain hardenability.

• Transactions of the Korean Society of Automotive Engineers
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• v.18 no.1
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• pp.127-130
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• 2010

This paper deals with the formability of DP590 steel considering the strain rate. The strain hardening coefficient, elongation and r-value were obtained from the static and dynamic tensile test. As strain rate increases from static to 100/s, the strain hardening coefficient and the uniform elongation decrease and the elongation at fracture and r-value decrease to 0.1/s and increase again to 100/s. The high speed forming limit tests with hemi-spherical punch were carried out using the high speed crash testing machine and high speed forming jig. The high speed forming limit of DP590(order of $10^2$/s) decreases compared to the static forming limit(order of $10^{-3}$/s) and the forming limit band in high speed forming test is narrower than that in the static forming test. This tendency may be due to the development of brittleness with increase of stain rate.

• 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 $Ni_3(Al,Mo)$.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2007.05a
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• pp.357-360
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• 2007

The profile ring rolling process of Ti-6Al-4V alloy was designed by finite element(FE) simulation and experimental analysis. The design includes geometry design and optimization of process variables. The geometry design such as initial billet and blank sizes, and final rolled ring shape was carried out with the calculation method based on the uniform deformation concept between the wall thickness and ring height. FEM simulation was used to calculate the state variables such as strain, strain rate and temperature and to predict the formation of forming defects during ring rolling process. Finally, the mechanical properties of profiled Ti-6Al-4V alloy ring product were analyzed with the evolution of microstructures during the ring rolling process.

• Structural Monitoring and Maintenance
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• v.7 no.2
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• pp.85-107
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• 2020

Dynamic responses of porous piezoelectric and metal foam nano-size plates have been examined via a four variables plate formulation. Diverse pore dispersions named uniform, symmetric and asymmetric have been selected. The piezoelectric nano-size plate is subjected to an external electrical voltage. Nonlocal strain gradient theory (NSGT) which includes two scale factors has been utilized to provide size-dependent model of foam nanoplate. The presented plate formulation verifies the shear deformations impacts and it gives fewer number of field components compared to first-order plate model. Hamilton's principle has been utilized for deriving the governing equations. Achieved results by differential quadrature (DQ) method have been verified with those reported in previous studies. The influences of nonlocal factor, strain gradients, electrical voltage, dynamical load frequency and pore type on forced responses of metal and piezoelectric foam nano-size plates have been researched.

• Transactions of the Korean Society of Mechanical Engineers
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• v.19 no.4
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• pp.896-907
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• 1995

Titanium 6242(.alpha. + .betha.) alloy has a good strength/weight ratio and is used for aircraft components such as engine disks and compressor blades. When this material is forged at an elevated temperature, the process parameters should be carefully controlled because the process window of this material is quite narrow. In the present investigation, a rigid-thermoviscoplastic finite element method is used to predict the deformation behavior and temperature/strain distributions in an engine disk during near-net shape hot forging. The purpose of the investigation is to obtain a proper ram speed profile, assuming the hydraulic press used in the forging is capable of varying ram speed during loading. In result, it was found that the ram speed at constant strain-rate of 0.5/sec shows a sound deformation behavior, a relatively uniform deformation and a good temperature distribution. This information is also valuable in predicting resulting microstructures in the disk.

• Journal of the korean Society of Automotive Engineers
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• v.13 no.2
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• pp.34-41
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• 1991

In this study, the equations of motion of the thin annular plate with uniform thickness were derived from the classical theory of the plate. In addition the distribution of the inplane stress and the natural frequency due to the change of the ratio of the outer radius to the inner radius was presented by the analytic method using the numerical analysis. Results were compared with those from the experiment. As a result, the strain of rotating circular plate increased as the radius and rpm became greater, and the strain of radial direction was two times greater than that of transverse direction. Besides, it was confirmed that the natural frequency increased according to the decrease of the radius keeping the thickness constant.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.28 no.9
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• pp.1337-1343
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• 2004

Shape optimization was performed to obtain a precise shape of cutouts including the internal shape of cutouts in a laminated composite plate by three dimensional modeling using solid element. Volume control of the growth-strain method was implemented and the distributed parameter chosen as Tsai-Hill fracture index for shape optimization. It makes Tsai-Hill failure index at each element uniform in laminated composites under the predetermined volume a designer requires. Shapes optimized by Tsai-Hill failure index were compared with those of the initial shapes for the various load conditions and cutouts. The following conclusions were obtained in this study; (1) It was found that growth-strain method was applied efficiently to shape optimization of three dimensional cutouts in a laminate composite, (2) The optimal shapes of the various load conditions and cutouts were obtained, (3) The maximum Tsai-Hill failure indices of the optimal shapes were remarkably reduced comparing with those of the initial shapes.