• Journal of the Korean Society of Radiology
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• v.13 no.5
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• pp.757-763
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• 2019

Recently, Following the recent development of flat panel detector with wide dynamic ranges, increasing numbers of healthcare providers have begun to use digital radiography. As a result, filter thickness standards should be reestablished, as current clinical practice requires the use of thicknesses recommended by the National Council on Radiation Protection and Measurements, which are based on information, acquired using conventional analog systems. Here we investigated the possibility of minimizing dose creep and optimizing patient dose using Al filters in digital radiography. The use of thicker Al filters resulted in a maximum 19.3% reduction in the entrance skin exposure dose when medical images with similar sharpness values were compared. However, resolution, which is a critical factor in imaging, had a significant change of 1.01 lp/mm. This change in resolution is thought to be due to the increased amount of scattered rays generated from the object due to the X-ray beam hardening effect. The increase in the number of scattered rays was verified using the scattering degradation factor. However, the FPD, which has recently been developed and is widely used in various areas, has greater response to radiation than analog devices and has a wide dynamic range. Therefore, the FPD is expected to maintain an appropriate level of resolution corresponding to the increase in the scattered-ray content ratio, which depends on filter thickness. Use of the FPD is also expected to minimize dose creep by reducing the exposure dose.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.27 no.3
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• pp.472-477
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• 2003

A piezo-actuator is an important component for an E-beam lithography system. But it is very difficult to model its characteristics due to nonlinearities such as hysteresis and creep, to the input voltage. In this paper, one-axis micro stage with a piezo-actuator is modeled including the nonlinear properties. Hysteresis and creep are modeled as the first order differential equation and a time-dependent logarithmic function, respectively. The dynamic motion of the stage is also modeled as a mass-spring-damper system and the parameters are determined by utilizing the system identification technique. The simulation tool for a micro stage is constructed using the commercial software and its simulation results are compared with the experimental data.

• Tribology and Lubricants
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• v.24 no.1
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• pp.27-33
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• 2008

A micro-impression creep machine was designed and developed, adopting a small punch in diameter of 150 um, displacement gage with an accuracy of sub-${\mu}m$ scale, and load-cell with an accuracy of mN scale in order to investigate creep behavior of small solder ball in diameter of less than 1 mm. Creep behavior of lead-free solder ball(Sn-3.0Ag-0.5Cu) in diameter of $760\;{\mu}m$ was investigated in the stress range of $8{\sim}60\;MPa$ and at $303\;K{\sim}393\;K$. The applied load became decreased slightly and continuously in the creep rate of $10^{-4}/s$ range during the current experiments. Also, the machine frame was so sensitive to the environmental temperature that nm scaled displacement recording was unstable according to the change in environmental temperature.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.29 no.4D
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• pp.491-497
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• 2009

This paper presents the results of accelerated pavement tests (APT) that simulate permanent deformation (rutting) of asphalt concrete pavements under different temperatures and loading courses. Also, finite element (FE) analysis has been conducted to predict the test results. Test section for APT is the same as one of test sections at Korea Expressway Corporation test road and is subjected to a constant moving dual tire wheel load of APT at three different temperatures: 30, 40, $50^{\circ}C$. The moving wheel is applied at different loading courses within a 75cm wide wheel path to account for traffic wandering. Also, the effect of wandering on permanent deformation development is investigated numerically with three wandering schemes. In this study, ABAQUS is adopted to model APT pavement section with plain stain elements and creep strain rate model is used to take into account viscoplastic stain of asphalt concrete mixtures, and elastic layer properties are back-calculated from FWD measurements. Plus, the effect of boundary condition and subgrade on FE permanent deformation predictions is investigated. A full FE model that accounted for subgrade provided more realistic rut depth predictions, indicating subgrade has contributed to surface rutting.

• Transactions of the Korean Society of Automotive Engineers
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• v.3 no.1
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• pp.1-12
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• 1995

Isothermal and thermal-mechanical fatigue characteristics of 12Cr heat resisting steel used for high temperature applications were investigated including hold time effects. Isothermal low cycle fatigue test at $600^{\circ}C$ and in-phase, out-of-phase thermal-mechanical fatigue test at 350 to $600^{\circ}C$ were conducted using smooth cylindrical hollow specimen under strain-control with total strain ranges from 0.006 to 0.015. Regardless of thermal-mechanical and isothermal fatigue tests, cyclic softening behavior was observed and much more pronounced in the thermal-mechanical fatigue tests with hold times due to the stress relaxation during the hold time. The phase difference between temperature and strain in thermal-mechanical fatigue tests resulted in significantly shorter fatigue life for out-of-phase compared to in-phase. The differences in fatigue lives were dependent upon the magnitudes of plastic strain ranges and mean stresses. During the hold time in the strain-controlled fatigue tests, the increase in the plastic strain range and the stress relaxation were observed. It appeared that the increase in plastic strain range per cycle and the introduction of creep damage made important contributions to the reduction of thermal-mechanical fatigue life with hold time, and the life reduction tendency was more remarkable in the in-phase than in the out-of-phase thermal-mechanical fatigue. Isothermal fatigue tests performed under the combination of fast and slow strain rates at $600^{\circ}C$ showed that the fatigue life decreased as the strain rate and frequency decreased,especially for the low strain ranges.

• Journal of Powder Materials
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• v.23 no.3
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• pp.247-253
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• 2016

In this study, solid solution heat treatment of consolidated nickel-based superalloy powders is carried out by hot isotactic pressing. The effects of the cooling rate of salt quenching, and air cooling on the microstructures and the mechanical properties of the specimens are analyzed. The specimen that is air cooled shows the formation of serrated grain boundaries due to their obstruction by the carbide particles. Moreover, the specimen that is salt quenched shows higher strength than the one that is air cooled due to the presence of fine and close-packed tertiary gamma prime phase. The tensile elongation at high temperatures improves due to the presence of grain boundary serrations in the specimen that is air cooled. On the contrary, the specimen that is salt quenched and consists of unserrated grain boundaries shows better creep properties than the air cooled specimen with the serrated grain boundaries, due to the negative creep phenomenon.

• Journal of Powder Materials
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• v.19 no.3
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• pp.215-219
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• 2012

An attempt was made to evaluate creep reliability of two commercial Ni-based superalloys by using ultrasonic wave. The materials include fine-grained PM alloy fabricated by mechanical alloying and subsequent hot isostatic pressing, and IN738LC cast alloy with a grain size of a few cm. Microstructural parameters (fraction of creep cavity and size of ${\gamma}^{\prime}$ precipitates) and ultrasonic parameters (velocity, attenuation) were measured to try to find relationships between them. Ultrasonic velocity decreased with creep cavity formation in PM alloy. On the other hand, no distinct changing trend of ultrasonic velocity was observed for IN738LC alloy. Ultrasonic attenuation was found to have a linear correlation with the size of ${\gamma}^{\prime}$ precipitates and was suggested as a potential parameter for monitoring creep reliability of IN738LC alloy.

• Korean Journal of Materials Research
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• v.9 no.10
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• pp.1018-1024
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• 1999

The effects of operating temperature and stress on degradation of components in high temperature steam generator were investigated. Several 2.25CrlMo tubes which had operated over 20 years and an unused 9CrlMoVNb tube were tested. For the former samples, the amount of $\textrm{M}_{6}\textrm{C}$ carbide and its size are increased with the aging or operating time. The precipitation behavior of carbides ($\textrm{M}_{2}\textrm{O}$, $\textrm{M}_{6}\textrm{C}$) is changed with the operating temperature of the tubes. However, unused 9CrlMoVNb samples show a different carbide precipitation process due to high chromium, vanadium, and niobium contents. The amount of Cr-rich $\textrm{M}_{23}\textrm{C}_{6}$ carbide is significantly increased with aging time, but that of $\textrm{M}_{6}\textrm{C}$ type carbide is rarely changed with aging time at elevated temperatures.

• Journal of Korea Foundry Society
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• v.38 no.5
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• pp.103-110
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• 2018

The effects of a heat treatment on the carbide formation behavior and mechanical properties of the cobalt-based superalloy X-45 were investigated here. Coarse primary carbides formed in the interdendritic region in the as-cast specimen, along with the precipitation of fine secondary carbides in the vicinity of the primary carbides. Most of the carbides formed in the interdendritic region were dissolved into the matrix by a solution treatment at $1274^{\circ}C$. Solutionizing at $1150^{\circ}C$ led to the dissolution of some carbides at the grain boundaries, though this also caused the precipitation of fine carbides in the vicinity of coarse primary carbides. A solution treatment followed by an aging treatment at $927^{\circ}C$ led to the precipitation of fine secondary carbides in the interdendritic region. Very fine carbides were precipitated in the dendritic region by an aging heat treatment at $927^{\circ}C$ and $982^{\circ}C$ without a solution treatment. The hardness value of the alloy solutionized at $1150^{\circ}C$ was somewhat higher than that in the as-cast condition; however, various aging treatments did not strongly influence the hardness value. The specimens as-cast and aged at $927^{\circ}C$ showed the highest hardness values, though they were not significantly affected by the aging time. The specimens aged only at $982^{\circ}C$ showed outstanding tensile and creep properties. Thermal exposure at high temperatures for 8000 hours led to the precipitation of carbide at the center of the dendrite region and an improvement of the creep rupture lifetimes.

• Proceedings of the KWS Conference
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• 2009.11a
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• pp.43-43
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• 2009

단련용 다결정 Ni기 초내열합금은 우수한 가공성, 내산화성, 고온특성 등으로 가스터빈 연소기, 디스크, 증기발생기 전열관 등 발전용 고온부품 소재에 널리 적용되고 있다. 최근 발전설비의 고효율화를 꾀하기 위해 작동 온도를 현격히 증가시키는 기술방향으로 발전하고 있고, 소재측면에서는 기존의 초내열합금 대비 고기능성을 확보할 수 있는 차세대 Ni기 초내열합금 개발이 유럽, 미국, 일본, 중국 등을 중심으로 활발히 이루어지고 있다. 이러한 소재의 고온강도 (온도수용성)를 향상시키기 위해서는 통상 규칙격자 금속간화합물인 $Ni_3(Al,Ti)-{\gamma}'$상의 분율을 증가시킬 수 있지만, ${\gamma}'$상분율이 증가할 경우 용접 및 후열처리 동안 용접열영향부 (HAZ)에서 액화균열이 발생할 가능성이 높아진다. 결정립계를 따라 발생하는 HAZ 액화균열은 입계특성에 의해 크게 영향을 받을 것으로 판단된다. 한편, 본 연구자들은 최근 입계 serration 현상을 단련용 합금에 도입시키는 특별한 열처리를 이론적 접근법을 통해 개발하였다. 형성된 파형입계는 결정학적인 관점에서 조밀 {111} 입계면을 갖도록 분해 (dissociation)되어 낮은 계면에너지를 갖게 됨을 확인하였으며, 입계형상 변화뿐만 아니라 탄화물 특성변화까지 유도하여 크리프 수명을 기존대비 약 40% 정도 향상시킴을 확인하였다. 이러한 직선형 입계 대비 'special boundary'로 간주되는 파형입계가 도입될 경우, HAZ 결정립크기 변화 및 액화거동에 미치는 영향을 고찰하고, 아울러 입계특성 제어가 용접성/용접부 품질 향상에 기여할 수 있는 가능성도 토의하고자 하였다. 본 연구에서는 재현 HAZ 열사이클 시험을 통해 미세구조를 정량적으로 비교하였다. 상대적으로 입계구조가 안정된 파형입계의 이동속도가 高계면 에너지를 갖는 직선형 입계보다 느려 HAZ 결정립 성장이 효과적으로 억제됨을 확인할 수 있었다. 입계 액화거동을 살펴보면, 두 시편 모두 $M_{23}C_6$, MC 등 입계탄화물 계면이 빠른 승온중 액화반응 (constitutional liquation)에 의해 입계가 액화되었으며, 이후 급냉에 의해 입계에 액상막이 존재한 흔적이 발견되었다. 최고온도별로 입계액화 폭/비율을 정량적으로 비교한 결과, 파형입계가 직선입계 대비 대체로 낮음을 확인할 수 있었으며, 때때로 액화되지 않고 잔존하는 입계 탄화물이 관찰되었다. 재현 HAZ 미세조직을 통해 Hot ductility 시험 결과를 유추하자면, 파형입계가 직선입계 보다 좁은 취성온도영역 (Brittle Temperature Range)을 나타낼 것으로 예상되어, 입계특성제어에 의해 Ni기 초내열합금의 용접성을 향상 가능성을 확인하였다.