• Composites Research
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• v.13 no.4
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• pp.1-10
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• 2000

SiC particulate reinforced metal matrix composites(MMCs) are emerging as candidate materials for the automobile and aerospace industries due to their significant increase in elastic modulus and strength compared to conventional metallic materials. However, in order to make successful application of MMCs, it is very important to understand micro-failure mechanism under cyclic loading because failure mechanism of MMC is dominated by accumulation of micro-failure due to applied loading. In this study, ultrasonic Lamb wave and acoustic emission(AE) have been used to monitor microscopic damage accumulation under cyclic loading for SiC particulate reinforced metal matrix composite(SiCp/A356). It was found that the change in velocity and attenuation of ultrasonic Lamb wave due to the increase of loading cycles could be characterized by three different stages corresponding to the microscopic fracture processes. The characteristic of AE signal at each stage was analyzed and discussed by comparing with the change of ultrasonic characteristic in MMCs.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.39 no.7
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• pp.693-700
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• 2015

To estimate the fatigue crack propagation behavior of compact tension shear (CTS) specimen under mixed-mode loads, crack path prediction theories and Tanaka's equation were applied. The stress intensity factor at a newly created crack tip was calculated using a finite element method via ANSYS, and the crack path and crack increment were then obtained from the crack path prediction theories, Tanaka's equation, and the Paris' equation, which were preprogrammed in Microsoft Excel. A new method called the finite element crack tip updating method (FECTUM) was developed. In this method, the finite element method and Microsoft Excel are used to calculate the stress intensity factors and the crack path, respectively, at the crack tip per each crack increment. The developed FECTUM was applied to simulate the fatigue crack propagation of a single-edge notched bending (SENB) specimen under eccentric three-point bending loads. The results showed that the number of cycles to failure of the specimen obtained experimentally and numerically were in good agreement within an error range of less than 3%.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2017.05a
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• pp.245-247
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• 2017

A turbine wheel undergoes high heat flux and centrifugal force when a gas turbine starts. The temperature and stress of the turbine wheel increase rapidly, and the time point and rate of them may not coincident. The difference of heating and rotating rates influences the life of turbine wheel. We conducted thermo-mechanical fatigue analysis with finite element methods to study the influence. The low acceleration and deceleration of the wheel extends the life. If the turbine wheel decelerate faster than cooling, the life increases.

• Journal of the Korea institute for structural maintenance and inspection
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• v.18 no.2
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• pp.54-63
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• 2014

Corrugated steel plate structure, which is built by assembling corrugated steel plate segments with bolts on site and filling the surroundings with quality soil, is widely used for buried structures as a eco-corridors, small bridges, and closed conduits. This experimental study is dealt with the static and fatigue performance of bolt connected corrugated steel plates under flexural loading. The experimental variables to verify the fatigue performance are bolt diameters and detailing of connection such as washer and the corrugation dimension of specimens has a $400{\times}150$ mm. The experimental ultimate strength of specimens under static loading was higher than the theoretical strength and all specimen failed by a bearing and tearing failure of bolt hole of upper plate. Therefore, a fatigue tests of specimens had 6.0mm and 7.0mm thickness was conducted in which the load range was up to 209kN and 516kN, respectively. From the fatigue test, failure patterns are changed from plate bearing and tearing which is a typical failure pattern of static failure to a bearing failure of plate and shear failure of bolt, and experimental fatigue limit at $2{\times}10^6$cycles is about 85MPa.

• Journal of the KSME
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• v.37 no.10
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• pp.41-46
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• 1997

철강구조물 부재 내에 노치나 균열이 존재할 수 있고, 외부의 피로하중에 의하여 취약부에서 발생한 균열이 진전하여 전구조물의 최종파손을 야기시킬 수 있다. 부재를 보다 안전하게 사용하고 또한 신뢰성을 확보하기 위해서는 이미 손상된 부재에서 균열의 진전상태를 계측할 수 있는 방법이 확립되어져야 하고, 파괴역학적 파라미터를 이용한 사용재의 균열진전거동특성이 평가되어야 한다. 균열길이의 측정방법은 지금까지 많은 연구자들에 의하여 개발되어져 왔는데 크게 광학현미경을 이용하여 육안으로 직접 균열길이를 측정하는 방법과 컴플라이언스, 초음파, AE 또는 전기적 신호를 통하여 얻어진 결괄부터 균열길이로 환산하는 간접적인 방법으로 대별된다. 대부분의 균열길이의 측정방법은 많은 수작업이 요구되고, 특히 하한계응력확대계수영역의 미세한 균열진전량을 측정하기에는 어려움이 따르고 있다. 이에 대하여 전도체 시험편에 일정전류를 흐르게 하고 균열길이의 증가에 따라 변화하는 전위차로 이를 균열길이로 평가하는 전기적인 측정방법이 있다. 이 방법은 실험장치가 비교적 간단하고 미세한 균열길이의 측정이 용이하여 균열길이의 직접적인 측정이 곤란한 고온역 그리고 충격하중하에서의 균열길이 측정에 이용이 확대되고 있다. 이 글에서는 여러 균열길이 측정방법의 장.단점에 대하여 고찰하고, 그 중 많은 장점을 갖고 있는 직류전위차법의 실험방법을 소개한다.

• Journal of the Korean Society for Heat Treatment
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• v.8 no.4
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• pp.326-332
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• 1995

Using natural frequency measurement method, which is one of NDT method, natural frequency of the hot-rolled mild steel plate(specimen) under fatigue loading was measured. Between the degradation of the specimen under fatigue loading and the variation of the natural frequency of the specimen was investigated. As a result, the degradation of the specimen was described and monitored as variation of natural frequency of specimen. The natural frequency of specimen decreased gradually under fatigue loading. This means the variation of material properties of specimen. Especially. It means the variation of Young's modulus of specimen.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 1997.10a
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• pp.829-832
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• 1997

It is difficult to explain the effects of complex variable loading experienced by the machine and the structure only with the studies of the single-overload itself. Hence, it is thought that the variation of overload-holding time are required to explain the effects more clearly. The effects of the overload were analyzed by means of the crack retardation, and the fractography on retardation zone. A characteristic of the fractography on retardation zone was that striation distribution did not appear due to decreased crack driving force. Rotary bending fatigue tests were performed with the circular shaft which has two hole defects.

• Journal of the Korean Society for Precision Engineering
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• v.18 no.1
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• pp.201-212
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• 2001

If defects are located far apart, fatigue cracks are independently initiated from them and gradually approach other cracks so that the fatigue life becomes influenced by the crack growth behavior of those interacting cracks. In this study, the effect of the stress interaction between defects on the fatigue crack propagation behavior is investigated experimentally and these results are verified by finite element method. In addition, fatigue crack propagation behaviors under micro hole interaction field are studied.

• International Journal of Highway Engineering
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• v.11 no.2
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• pp.167-174
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• 2009

The simulation of dynamic load was conducted based on surface profile on asphalt concrete pavement, vehicle speeds, and suspension types using a truck simulation program. The results of the simulated dynamic load based on different surface profile, vehicle speeds, and suspension types are analyzed. As pavement roughness and vehicle speed are increased, the dynamic load was increased. Walking beam suspension produces greater dynamic load than air spring suspension. Pavement damage index is calculated based on covariance of dynamic load and Paris-Erdogan fracture parameter, n which is based on creep compliance tests of asphalt mixtures used in Korea. The higher covariance of dynamic load, confidence level, and fracture parameter are used, the greater pavement damage index is obtained. Specification of pavement roughness can be developed in various vehicle speeds and asphalt mixtures, and pay factor can be determined after constructing asphalt concrete pavement using pavement damage concepts.

• Journal of the Korean Society for Nondestructive Testing
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• v.33 no.1
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• pp.20-25
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• 2013

Ultrasonic methods are widely used to degradation assessment. Remaining-life cycle of metal can be estimated by ultrasonic parameters because ultrasonic velocity and attenuation are affected by change of material properties with accumulated fatigue in the metal. Therefore, in this study, we will estimate overall change of material properties by 2D C-scan image. Fatigued aluminum alloy 6061-T6 samples from 0 to 85% were prepared for evaluating fatigue life cycle. Also, degraded image of materials using attenuation is proposed to estimate degree of material degradation for determining degraded area of fatigued samples. Finally, we will predicts process pf degradation with measured attenuation of fatigued aluminum alloy 6061-T6 samples.