• Nuclear Engineering and Technology
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• 제33권5호
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• pp.526-538
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• 2001

Fatigue tests in air and in room temperature water were performed to obtain comparable data and stable crack measuring conditions. In air environment, fatigue crack growth rate was increased with increasing temperature due to an increase in crack tip oxidation rate. In room temperature water, the fatigue crack growth rate was faster than in air and crack path varied on loading conditions. In simulated light water reactor (LWR) conditions, there was little environmental effect on the fatigue crack growth rate (FCGR) at low dissolved oxygen or at high loading frequency conditions. While the FCGR was enhanced at high oxygen condition, and the enhancement of crack growth rate increased as loading frequency decreased to a critical value. In fractography, environmentally assisted cracks, such as semi-cleavage and secondary intergranular crack, were found near sulfide inclusions only at high dissolved oxygen and low loading frequency condition. The high crack growth rate was related to environmentally assisted crack. These results indicated that environmentally assisted crack could be formed by the Electrochemical effect in specific loading condition.

• Geomechanics and Engineering
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• 제21권5호
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• pp.433-445
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• 2020

The weakening and softening behavior of soft clay subjected to cyclic loading due to the build-up of excess pore water pressure is well-known. During the design stage of the foundation of highways and coastal high-rise buildings, it is important to study the mechanical behavior of marine soils under cyclic loading as they undergo greater settlement during cyclic loading than under static loading. Therefore, this research evaluates the cyclic stress-strain and shear strength of untreated and treated marine clay under the effects of wind, earthquake, and traffic loadings. A series of laboratory stress-controlled cyclic triaxial tests have been conducted on both untreated and treated marine clay using different effective confining pressures and a frequency of 0.5 and 1.0 Hz. In addition, treated samples were cured for 28 and 90 days and tested under a frequency of 2.0 Hz. The results revealed significant differences in the performance of treated marine clay samples than that of untreated samples under cyclic loading. The treated marine clay samples were able to stand up to 2000 loading cycles before failure, while untreated marine clay samples could not stand few loading cycles. The untreated marine clay displayed a higher permanent axial strain rate under cyclic loading than the treated clay due to the existence of new cementing compounds after the treatment with recycled tiles and low amount (2%) of cement. The effect of the effective confining pressure was found to be significant on untreated marine clay while its effect was not crucial for the treated samples cured for 90 days. Treated samples cured for 90 days performed better under cyclic loading than the ones cured for 28 days and this is due to the higher amount of cementitious compounds formed with time. The highest deformation was found at 0.5 Hz, which cannot be considered as a critical frequency since smaller frequencies were not used. Therefore, it is recommended to consider testing the treated marine clay using smaller frequencies than 0.5 Hz.

• 한국복합재료학회:학술대회논문집
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• 한국복합재료학회 2005년도 추계학술발표대회 논문집
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• pp.165-168
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• 2005

In this paper, the relation between the applied loading sizes and the natural frequency of vibration of some structural elements is presented. Many junior engineers get confused on such relations. It is hoped that this paper gives some guideline to such junior engineers.

• 한국통신학회논문지
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• 제15권5호
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• pp.406-412
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• 1990

본 논문에서는 리액터스 장하를 이용한 공진주파수 제어 안테나를 제안하고 공진주파수 제어의개념을 명확히 하고 있다. 또한 몇 가지의 유용한 응용에 관해서도 언급하고 있다.

• 한국정밀공학회지
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• 제25권1호
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• pp.145-150
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• 2008

One of the unresolved questions in articular cartilage biomechanics is the magnitude of the dynamic modulus and tissue compressive strains under physiological loading conditions. The objective of this study was to characterize the dynamic modulus and compressive strain magnitudes of bovine articular cartilage at physiological compressive stress level and loading frequency. Four bovine calf shoulder joints (ages 2-4 months) were loaded in Instron testing system under load control, with a load amplitude up to 800 N and loading frequency of 1 Hz, resulting in peak engineering stress amplitude of ${\sim}5.8\;MPa$. The corresponding peak deformation of the articular layer reached ${\sim}27%$ of its thickness. The effective dynamic modulus determined from the slope of stress versus strain curve was ${\sim}23\;MPa$, and the phase angle difference between the applied stress and measured strain which is equivalent to the area of the hystresis loop in the stress-strain response was ${\sim}8.3^{\circ}$. These results are representative of the functional properties of articular cartilage in a physiological loading environment. This study provides novel experimental findings on the physiological strain magnitudes and dynamic modulus achieved in intact articular layers under cyclical loading conditions.

• 한국자동차공학회논문집
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• 제11권2호
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• pp.190-195
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• 2003

We developed a loading sleeve for Automobile, which has the PCS and IMT-2000 frequency bands. The electric characteristics of the designed sleeve monopole has SWR<1.311 and 3.0dBi in average gain for that frequency bands. In this study, the designed sleeve monopole was installed at a roof and trunk lid of vehicle. The characteristics of designed sleeve monopole was investigated by measuring SWR, input impedance and radiation pattern. The experimental results show that the efficiency of the sleeve monopole is superior th that of domestic and foreign commercial PCS antenna.

• Structural Engineering and Mechanics
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• 제89권6호
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• pp.617-626
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• 2024

The study investigated the behavior of plain and fibered Ultra-High Performance Concrete (UHPC) beams under varying loading conditions using integrated analysis of the flexure and acoustic emission tests. The loading rate of testing is -0.25 -2 mm/min. It is observed that on increasing loading rate, flexural strength increases, and toughness decreases. The acoustic emission testing revealed that higher loading rates accelerate crack propagation. Fiber effect and matrix cracking are identified as significant contributors to the release of acoustic emission energy, with fiber rupture/failure and matrix cracking showing rate-dependent behavior. Crack classification analysis indicated that the rise angle (RA) value decreased under quasi-static loading. The average frequency (AF) value increased with the loading rate, but this trend reversed under rate-dependent conditions. K-means analysis identified distinct clusters of crack types with unique frequency and duration characteristics at different loading rates. Furthermore, the historic index and signal strength decreased with increasing loading rate after peak capacity, while the severity index increased in the post-peak zone, indicating more severe damage. The sudden rise in the historic index and cumulative signal strength indicates the possibility of several occurrences, such as the emergence of a significant crack, shifts in cracking modes, abrupt failure, or notable fiber debonding/pull-out. Moreover, there is a distinct rise in the number of AE knees corresponding to the increase in loading rate. The crack mapping from acoustic emission testing aligned with observed failure patterns, validating its use in structural health monitoring.

• 한국생산제조학회지
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• 제20권4호
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• pp.427-433
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• 2011

Chassis part in automotive body is affected by fatigue load at driving on the ground. Universal joint on this part is influenced extremely by the fatigue load. Fatigue life, damage and natural frequency are analyzed at universal joint under nonuniform fatigue load. The york part at universal joint is shown with the maximum equivalent stress and displacement of 60.755 MPa and 0.21086 mm as strength analysis. The possible life in use in case of 'SAE bracket' is the shortest among the fatigue loading lives of 'SAE bracket', 'SAE transmission' and 'Sine Wave'. The damage at loading life of 'SAE transmission' is the least among 3 types. The frequency of damage in case of 'Sine Wave' is 0.7 with the least among 3 fatigue loading life types but this case brings the most possible damage as 80% at the average stress of 0. Natural vibration at this model is analyzed with the orders of 1'st to 5'th and maximum frequency is shown as 701.73 Hz at 5'th order. As the result of this study is applied by the universal joint on chassis part, the prevention on fatigue damage in automotive body and its durability are predicted.

• 대한치과보철학회지
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• 제38권6호
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• pp.765-781
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• 2000

The purpose of this study was to compare the screw loosening characteristics of three avail-able cementation type abutments: one-piece cementation type abutment; two-piece cementation type abutment using titanium abutment screw; two-piece cementation type abutment using gold abutment screw. Two implant supported three-unit superstructures were fabricated using a pair of 3 kinds of abutments for each experimental model. Cyclic loading was applied on the specimen, and made to stop when the superstructure showed movement over threshold range. The loaded cycle was counted until the machine stopped. Frequency analysis was done to measure the change of natural frequency before and after the application of cyclic load and to find the effect of screw loosening on the change of natural frequency. The specimen assembly was modeled to perform the finite element analysis to see the distribution of the stress induced by the application of preload over the screw joint and to compare the pattern of the distribution of stress induced by the external force with the change of the preload condition. The following results were obtained: 1. The failure loading cycle of two-piece cementation type abutment using gold screw was significantly greater than those of the other groups. 2. One-piece cementation type abutment applied to multi-unit restoration case did not show greater resistance to screw loosening compared to two-piece cementation type abutments. 3. Frequency analysis showed decrease in natural frequency when screw loosening occured.

• Advances in nano research
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• 제10권2호
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• pp.175-189
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• 2021

In this article, frequency characteristics, and sensitivity analysis of a size-dependent laminated composite cylindrical nanoshell under bi-directional thermal loading using Nonlocal Strain-stress Gradient Theory (NSGT) are presented. The governing equations of the laminated composite cylindrical nanoshell in thermal environment are developed using Hamilton's principle. The thermodynamic equations of the laminated cylindrical nanoshell are obtained using First-order Shear Deformation Theory (FSDT) and Fourier-expansion based Generalized Differential Quadrature element Method (FGDQM) is implemented to solve these equations and obtain natural frequency and critical temperature of the presented model. The novelty of the current study is to consider the effects of bi-directional temperature loading and sensitivity parameter on the critical temperature and frequency characteristics of the laminated composite nanostructure. Apart from semi-numerical solution, a finite element model was presented using the finite element package to simulate the response of the laminated cylindrical shell. The results created from finite element simulation illustrates a close agreement with the semi-numerical method results. Finally, the influences of temperature difference, ply angle, length scale and nonlocal parameters on the critical temperature, sensitivity, and frequency of the laminated composite nanostructure are investigated, in details.