• Geomechanics and Engineering
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• 제29권4호
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• pp.471-486
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• 2022

The effect of segmental joints is one of main importance for the segmental lining design when tunnels are excavated by a mechanized process. In this paper, segmental tunnel linings are analyzed by two numerical methods, namely the Beam-Spring Method (BSM) and the Solid-Interface Method (SIM). For this purpose, the Tehran Subway Line 6 Tunnel is considered to be the reference case. Comprehensive 2D numerical simulations are performed considering the soil's calibrated plastic hardening model (PH). Also, an advanced 3D numerical model was used to obtain the stress relaxation value. The SIM numerical model is conducted to calculate the average rotational stiffness of the longitudinal joints considering the joints bending moment distribution and joints openings. Then, based on the BSM, a sensitivity analysis was performed to investigate the influence of the ground rigidity, depth to diameter ratios, slippage between the segment and ground, segment thickness, number of segments and pattern of joints. The findings indicate that when the longitudinal joints are flexible, the soil-segment interaction effect is significant. The joint rotational stiffness effect becomes remarkable with increasing the segment thickness, segment number, and tunnel depth. The pattern of longitudinal joints, in addition to the joint stiffness ratio and number of segments, also depends on the placement of longitudinal joints of the key segment in the tunnel crown (similar to patterns B and B').

• 한국구조물진단유지관리공학회 논문집
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• 제25권1호
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• pp.85-92
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• 2021

본 연구는 액상폴리머인 에폭시수지를 이용한 고내구성 폴리머 시멘트계 재료를 개발할 목적으로 고로슬래그미분말 치환율을 20%로 고정하고, 4종류의 알칼리 활성화제를 이용한 경화제 무첨가 에폭시수지 혼입 폴리머 시멘트 모르타르를 제작하여, 강도 특성, 에폭시수지 경화도, 흡수율, 내황산성, 중성화 및 염화물이온 침투저항성을 검토한 것이다. 그 결과, 알칼리 자극제의 종류에 관계없이, 고로슬래그 미분말 20%를 사용한 경화제 무첨가 EMM의 압축강도는 폴리머 결합재비 10%에서 최댓값을 나타냈으며, 폴리머 결합재비 20%에서는 에폭시수지의 경화도 감소와 함께 압축강도 또한 감소하는 것으로 나타냈다. 휨강도는 압축강도와 마찬가지로 수산화칼슘을 첨가한 것이 가장 우수하게 나타났으며, 폴리머 결합재비의 증가와 함께 증가하였으며, 모두 9.0 MPa 이상의 휨강도를 발현하였다. 알칼리 자극제의 종류에 관계없이, 경화제 무첨가 EMM의 방수성, 중성화 및 염화물이온 침투저항성은 폴리머 결합재비의 증가와 함께 현저히 개선되었으며, 4종류의 알칼리 자극제 가운데 수산화칼슘을 첨가한 모르타르가 가장 우수한 것으로 나타났다. 한편, 알칼리 자극제의 종류에 관계없이, 경화제 무첨가 EMM의 내황산성은 폴리머 결합재비의 증가와 함께 떨어졌는데, 이것은 폴리머 결합재비의 증가에 따른 에폭시수지의 미경화 에폭시수지의 증가에 기인한 것으로 판단된다. 4가지 알칼리 자극제의 종류에 대하여 물성 및 내구성을 비교 검토한 결과, 수산화칼슘을 첨가한 것이 가장 우수한 것으로 확인되었다.

• 열처리공학회지
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• 제14권1호
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• pp.8-16
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• 2001

The effects of gas composition, pressure, temperature and time on the case thickness, hardness and nitride formation in the surface of tool steels(STD11 and STD61) have been studied by micro-pulse plasma nitriding. External compound layer and internal diffusion layer and the diffusion layer were observed in the nitrided case of tool steels. The relative amounts and kind of phases formed in the nitrided case changed with the change of nitriding conditions. Generally, only nitride phases such as ${\gamma}(Fe_4N)$, ${\varepsilon}(Fe_{2-3}N)$, or $Cr_{1.75}V_{0.25}N_2$ phases were detected in the compound layer, while nitride and carbide phases such as ${\varepsilon}-nitride(Fe_{2-3}N)$, $(Cr,Fe)_{\gamma}C_3$ or $Fe_3C$ were detected in the diffusion layer by XRD analysis. The thickness of compound layer increased with the increase of nitrogen content in the gas composition. Maximum case depth was obtained at gas pressure of 200Pa.

• 대한기계학회논문집A
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• 제26권8호
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• pp.1690-1697
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• 2002

The effects of re-peening on the fatigue damaged material are studied in this paper. The effects of re-peening process on surface hardness, surface roughness, surface compressive residual stress, and fatigue life are investigated. The results can be summarized in brief as follows: The depth of hardening layer was increased by re-peening process. There is no large variation of the surface roughness by re-peening process. The compressive residual stress of shot-peened specimen decreases under the fatigue loading and then increases again by re-peening process. Re-peening process increases the fatigue lifo of shot-peened and fatigue damaged specimen. The increase of fatigue lift under high stress level is much higher than under low stress level.

• 한국지반공학회:학술대회논문집
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• 한국지반공학회 2010년도 추계 학술발표회
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• pp.29-39
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• 2010

By understanding the construction process of sea dikes and the current state of the thickness and speed of fluid in the bottom layer protection work of final closure gaps, a construction method applicable for the blocking of bottom layer work will be selected. The three construction methods selected will be tested in site through various methods, and the reinforcement of bottom layer protection and impervious effect will be verified. The verification results are as follows: 1) The overall riprap layer were 0.5~1.0m thicker than planned so that the grouting depth and grout input amount increased 2) The applied construction methods permeability of riprap layers were improved from $\alpha{\times}10^{-2}cm/s$ before the construction to $\alpha{\times}10^{-4}cm/s$ after construction. 3) The results of core extraction in order to grossly verify the hardening time and durability allowed the identification of grout injection effect. The amount of filling of the injection was difficult to judge because the slime in many areas made the reading of borehole photography difficult.

• 열처리공학회지
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• 제19권1호
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• pp.3-9
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• 2006

The phase changes, nitride precipitation and hardness variations of 14%Cr-6.7Ni-0.65Mo-0.26Nb-0.05V-0.03C super martensitic stainless steel were investigated after nitrogen permeation heat treatment at a temperature range between $1050^{\circ}C$ and $1150^{\circ}C$. The nitrogen-permeated surface layer was transformed into austenite. The rectangular type NbN, NbCrN precipitates and fine round type precipitate were coexisted in the surface austenite layer, while the interior region that was free from nitrogen permeation kept the martensitic phase. The hardness of surface austenite showed 280 Hv, while the interior region of martensite phase represented 340 Hv. When tempering the nitrogen-permeated steel at $450^{\circ}C$, a maximum hardness of 433 Hv was appeared, probably this is attributed to the secondary hardening effect of the precipitates. The nitrogen concentration decreased gradually with increasing depth below the surface after showing a maximum of 0.3% at the outmost surface. The strong affinity between nitrogen and Cr enabled the substitutional element Cr to move from interiors to the surface when nitrogen diffuse form surface to the interior. Corrosion resistance of nitrogen permeated steel was superior to that of solution-anneaed steel in the solution of 1N $H_2SO_4$.

• Structural Engineering and Mechanics
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• 제38권4호
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• pp.385-403
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• 2011

Continuous deep girders which transmit the gravity load from the upper wall to the lower columns have frequently long end shear spans between the boundary of the upper wall and the face of the lower column. This paper presents the results of tests and analyses performed on three 1:2.5 scale specimens with long end shear spans, (the ratios of shear-span/total depth: 1.8 < a/h < 2.5): one designed by the conventional approach using the beam theory and two by the strut-and-tie approach. The conclusions are as follows: (1) the yielding strength of the continuous RC deep girders is controlled by the tensile yielding of the bottom longitudinal reinforcements, being much larger than the nominal strength predicted by using the section analysis of the girder section only or using the strut-and-tie model based on elastic-analysis stress distribution. (2) The ultimate strengths are 22% to 26% larger than the yielding strength. This additional strength derives from the strain hardening of yielded reinforcements and the shear resistance due to continuity with the adjacent span. (3) The pattern of shear force flow and failure mode in shear zone varies depending on the amount of vertical shear reinforcement. And (4) it is necessary to take into account the existence of the upper wall in the analysis and design of the deep continuous transfer girders that support the upper wall with a long end shear span.

• Journal of Welding and Joining
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• 제33권3호
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• pp.54-61
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• 2015

In this study, effect of laser power on hardness and microstructure of SKD61 Hot Die steel of which surface was melted and hardened with Yb:YAG disk laser was investigated. Beam speed was fixed at 70 mm/sec and distance between them was 0.8 mm about Laser surface melting. The only thing that was changed laser power. Laser powers were 2.0, 2.4 and 2.8 kW. No defect was found under all conditions. As the laser power increased, the penetration depth were deepened and the bead width was also widened. There was no hardness deviation of fusion zone at same laser power and it was higher than that of heat affected zone. In addition, the more laser power increased, the more hardness in fusion zone decreased. Fusion zone was macroscopically dendrite structure. However, core matric in dendrite was lath martensite of 100 nm size. There were $M_{23}C_6$ of 500 nm and the VC and $Mo_2C$ of a nano meters on boundary of dendrite.

• 여성건강간호학회지
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• 제6권2호
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• pp.203-217
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• 2000

The purpose of this study was done to explore the pregnant women's decision-making process about their infants feeding method. Data collection involved the in-depth unstructured interviews with 12 participants from January 1998 to January 1999. Data analysis was done by the grounded theory method. The 112 concepts, 29 sub-categories were confirmed in the analysis. The sub-categories were again grouped into 14 categories: expectation, situational condition, inevitability of breast-feeding, social recognition, self-awareness as mother, harmony, consideration, pursuit of ease, effect of external environments, lack of knowledge, hardening, the best choice, control, and bargain. " Adjustment through recognizing of motherhood" was the key category that was related to all categories. "Adjustment through recognizing of motherhood" was a process in which the mother became aware of mothering and sharing, and in which she considered herself or infant's needs and their priorities. This research will help nurse to understand mother's needs better. Therefore, nurse will be able to assist mother making the best decision for herself and her infant.

• 열처리공학회지
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• 제13권5호
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• pp.337-345
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• 2000

As a case hardening process for stainless steels, nitriding is more preferred and widely used than carburizing which deterioates corrosion resistance severely. In order to add the nitrogen into the stainless steels, passive film on the surface must be removed effectively before nitriding. Conventional gas nitriding process is performed in the temperature range of 500 to $600^{\circ}C$ with $NH_3$ gas, which often leads to sensitization of stainless steels. In this study, we tried to activate passive film of austenitic stainless steels by heating at low pressure. ($900^{\circ}C$, $5{\times}10^{-2}$ Torr.) Nitriding was performed at the solution treatment temperature of $1100^{\circ}C$ with nitrogen molecules instead of $NH_3$ gas. An attainable nitrogen content in a case depends on the nitrogen gas pressure at constant nitriding temperature. A case depth is proportional to the square root of solution time, which suggests that inward diffusion of nitrogen follows the Fick's 2nd law. Surface nitrogen atoms are dissolved as interstitial solutes, or precipitated in the form of MN, $M_2N$ nitrides, which increase the case hardeness. Dissolved nitrogen in the case enhances the cavitation resistance of austenitic stainless steels dramatically.