• 연구논문집
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• s.24
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• pp.81-96
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• 1994

To investigate failure mechanisms observed in carbon-phenolic thermal insulators, differential equations which govern the decomposition process in a deformable anisotropic porous solid are derived for three-dimensional axisymmetric constructions. The governing equations not only couple the material deformation with pore pressure, but also couple pressure and temperature, which means that heat convected by the pyrolysis gases is properly accounted for. Then the Bubnov-Galerkin finite element method is applied to these equations to transform them into a semidescrete finite element system. A thermal insulation liner in the cowl region under typical operating conditions is analyzed to find a mechanism for plylift. The results from the structural analysis show across-ply failure in the cowl zone. The mechanism for plylift is hypothesized as a sequential procedure : 1) the across-ply failure which is the precursor to plylift and 2) the local fiber buckling caused by generation of excessive in-plane compressive stress. To prevent plylift, the across-ply stress can be reduced by using appropriate material ply angles in cowl zone design.

• Bulletin of the Korean Chemical Society
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• v.7 no.5
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• pp.391-395
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• 1986

MNDO calculations were carried out to determine reactant complexes and transition states of the $S_N2$ reactions of $CH_3X\;+\;Y^-\;{\to}\;CH_3Y\;+\;X^-$ where X = F, Cl, CN and Y = CN, OH, F, Cl. The leaving group ability was found to vary inversely with the activation barrier, which in turn was mainly ascribable to the deformation energies accompanied with bond stretching of C-X bond and inversion of $CH_3$ group. The nucleophilicity was shown to be in the order $Cl^->F^->OH^->CN^-$ but the effect on the activation barrier was relatively small compared with that of the leaving group. The bond breaking and bond formation indices and energy decomposition analysis showed that the TS for the reaction of $CH_3$Cl occurs in the early stage of the reaction coordinate relative to that of $CH_3$F. It has been shown that the potential energy surface (PES) diagrams approach can only accommodate thermodynamic effects but fails to correlate intrinsic kinetic effects on the TS structure.

• Archives of Metallurgy and Materials
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• v.64 no.3
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• pp.863-867
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• 2019

The effects of carbon content on the austenite stability and strain-induced transformation of nanocrystalline Fe-11% Ni alloys were investigated using X-ray analysis and mechanical tests. The nanocrystalline FeNiC alloy samples were rapidly fabricated using spark plasma sintering because of the extremely short densification time, which not only helped attain the theoretical density value but also prevented grain growth. The increased austenite stability resulted from nanosized crystallites in the sintered alloys. Increasing compressive deformation increased the volume fraction of strain-induced martensite from austenite decomposition. The kinetics of the strain-induced martensite formation were evaluated using an empirical equation considering the austenite stability factor. As the carbon content increased, the austenite stability was enhanced, contributing to not only a higher volume fraction of austenite after sintering, but also to the suppression of its strain-induced martensite transformation.

• Korean Journal of Materials Research
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• v.10 no.2
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• pp.111-116
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• 2000

Fracture mode and carbide reactions of cast alloy 738LC during thermal exposure and creep at 816$^{\circ}C$/440MPa and 982$^{\circ}C$/152MPa were investigated. Crystallographic transgranular failure was observed in the specimen crept at 816$^{\circ}C$ due to shearing on the slip plane. Because selective oxidation at the grainboundaries which was exposed at the surface leads reduction in surface energy, however, early initiation of crack at the grainboundaries and intergranular failure were observed in the specimen crept at 982$^{\circ}C$/152MPa. As a result of decomposition of MC carbide at the tested temperatures, M(sub)23C(sub)6 carbide precipitated either on the grainboundaries or on the deformation band. The applied stress enhanced decomposition of MC. $\sigma$phase nucleated from Cr(sub)23C(sub)6 then grew to the ${\gamma}$+${\gamma}$\\` matrix. Precipitation of $\sigma$was accelerated by increasing temperature and applied stress.

• Journal of the Korean Ceramic Society
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• v.54 no.4
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• pp.340-348
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• 2017

$SiC_f/SiC$ composites were joined using a $60{\mu}m-thick$ $Ti_3AlC_2$ or $Ti_3SiC_2$ MAX phase tape. The filler tape was inserted between the $SiC_f/SiC$ composites containing a 12 wt.% $Al_2O_3-Y_2O_3$ sintering additive. The joining was performed to a butt-joint configuration at $1600^{\circ}C$ or $1750^{\circ}C$ in an Ar atmosphere by applying 3.5 MPa using a hot press. Microstructural and phase analyses at the joining interface confirmed the decomposition of $Ti_3AlC_2$ and $Ti_3SiC_2$, indicating the joining by solid-state diffusion. The results showed sound joining interface without the presence of cracks. Joining strengths higher than 150 MPa could be obtained for the joints using $Ti_3AlC_2$ or $Ti_3SiC_2$ at $1750^{\circ}C$, while those for joined at $1600^{\circ}C$ decreased to 100 MPa approximately without the deformation of the joining bodies. The thickness of initial filler tape was reduced significantly after joining because of the decomposition and migration of MAX phase owing to the plasticity at high temperatures.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.24 no.2
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• pp.185-191
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• 2011

Plastic gradient from geometrically necessary dislocation(GND) can affect material behavior significantly. In this research, mechanical behavior of polycrystalline solid is investigated using the finite element method incorporating plastic gradient from long range dislocation or GND effect. Plastic gradient effect is implemented in the analysis model by considering a long range strain term as well as elastic and plastic terms in the multiplicative decomposition. In the model, gradient hardness coefficient and length parameter are used to evaluate the effect of the long range strains and sensitive study is conducted for the parameters. It is confirmed that the GND amplifies hardening response of polycrystals compared with the single crystal.

• Advances in nano research
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• v.7 no.6
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• pp.405-417
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• 2019

This research is devoted to study post-buckling analysis of functionally graded carbon nanotubes reinforced composite (FG-CNTRC) micro plate with cut out subjected to magnetic field and resting on elastic medium. The basic formulation of plate is based on first order shear deformation theory (FSDT) and the material properties of FG-CNTRCs are presumed to be changed through the thickness direction, and are assumed based on rule of mixture; moreover, nonlocal Eringen's theory is applied to consider the size-dependent effect. It is considered that the system is embedded in elastic medium and subjected to longitudinal magnetic field. Energy approach, domain decomposition and Rayleigh-Ritz methods in conjunction with Newton-Raphson iterative technique are employed to trace the post-buckling paths of FG-CNTRC micro cut out plate. The influence of some important parameters such as small scale effect, cut out dimension, different types of FG distributions of CNTs, volume fraction of CNTs, aspect ratio of plate, magnitude of magnetic field, elastic medium and biaxial load on the post-buckling behavior of system are calculated. With respect to results, it is concluded that the aspect ratio and length of square cut out have negative effect on post-buckling response of micro composite plate. Furthermore, existence of CNTs in system causes improvement in the post-buckling behavior of plate and different distributions of CNTs in plate have diverse response. Meanwhile, nonlocal parameter and biaxial compression load on the plate has negative effect on post-buckling response. In addition, imposing magnetic field increases the post-buckling load of the microstructure.

• Structural Engineering and Mechanics
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• v.33 no.2
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• pp.215-238
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• 2009

This paper introduces an improved modal pushover analysis (IMPA) which can effectively evaluate the seismic response of multi-span continuous bridge structures on the basis of modal pushover analysis (MPA). Differently from previous modal pushover analyses which cause the numerical unstability because of the occurrence of reversed relation between the pushover load and displacement, the proposed method eliminates this numerical instability and, in advance the coupling effects induced from the direct application of modal decomposition by introducing an identical stiffness ratio for each dynamic mode at the post-yielding stage together with an approximate elastic deformation. In addition to these two introductions, the use of an effective seismic load, calculated from the modal spatial force and applied as the distributed load, makes it possible to predict the dynamic responses of all bridge structures through a simpler analysis procedure than those in conventional modal pushover analyses. Finally, in order to establish validity and applicability of the proposed method, correlation studies between a rigorous nonlinear time history analysis and the proposed method were conducted for multi-span continuous bridges.

• Korean Journal of Materials Research
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• v.26 no.5
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• pp.287-291
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• 2016

The effect of heat treatment on the microstructure and mechanical properties of cast Ti-6%Al-4%V alloy was investigated. Heat treatment of cast Ti-6Al-4V alloy was conducted by solution treatment at $950^{\circ}C$ for 30 min; this was followed by water quenching and then aging at $550^{\circ}C$ for 1 to 1440 min. The highest hardness of the heat-treated specimens was obtained by solution treatment and subsequent aging for 5 min due to precipitates of fine ${\alpha}$ that formed from retained ${\beta}$ phase. The tensile strength of this alloy increased without dramatic decrease of the ductility due to microstructural refinement resulting from the decomposition of ${\alpha}^{\prime}$ martensite into fine ${\alpha}$ and ${\beta}$ phases, and also due to the fine ${\alpha}$ phase formed from the retained ${\beta}$ phase by aging treatment for 5 min. In addition, this strengthening might be caused by the transformation induced plasticity (TRIP) effect, which is a strain-induced martensite transformation from the retained ${\beta}$ phase during deformation, and which occurs even after aging treatment at $550^{\circ}C$ for 5 min.

• Journal of the Korea Concrete Institute
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• v.18 no.6 s.96
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• pp.819-825
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• 2006

The objective of the present study is to verify the validity of a new truss model for evaluating the contribution by arch action to shear resistance in shear-critical reinforced concrete beams. The new truss model is based on the relationship between shear and bending moment in a beam subjected to combined shear and bending. The compatibility condition of the shear deformation that deviates from Bernoulli bending plane is formulated utilizing the smeared truss idealization with an inclined compression chord. The Modified Compression Filed Theory is employed to calculate the shear deformation of the web, and the relative axial displacements of the compression and the tension chord by the shear flow are also calculated. From this shear compatibility condition in a beam, the shear contribution by the arch action is numerically decoupled. Then the validity of the model is examined by applying the model to some selected test beams in literatures. On the basis of the analytical results, the contribution by the web to shear resistance can be constant and have an excellent linear correlation with the web reinforcement ratio. The present decoupling approach may provide a simple way for the assessment of the role of each parameter or mechanism that affects the ultimate shear behavior of reinforced concrete beams.