• Journal of the Korea Concrete Institute
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• v.23 no.2
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• pp.135-144
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• 2011

Recently, researches about fiber reinforced polymer (FRP) which has excellent durability, corrosion resistance, and tensile strength as a substitution material to steel tendon have been actively pursued. This study is performed to examine FRP tendon used prestressed beam's safety under service load. The specimen was a prestressed concrete beam with internal bonded FRP tendon. In order to compare the member fatigue capacity, a control specimen of a prestressed concrete beam with ordinary steel tendon was tested. A fatigue load was applied at a load range of 60%, 70%, and 80% of the 40% ultimate load, which was obtained though a static test. The fatigue load was applied as a 1~3 Hz sine wave with 4 point loading setup. Fatigue load with maximum 1 million cycles was applied. The specimen applied with a load ranging between 40~60% did not show a fatigue failure until 1 million cycles. However, it was found that horizontal cracks in the direction of tendons were found and bond force between the tendon and concrete was degraded as the load cycles increased. This fatigue study showed that the prestressed concrete beam using FRP tendon was safe under a fatigue load within a service load range. Fatigue strength of the specimen with FRP and steel tendon after 1 million cycles was 69.2% and 59.8% of the prestressed concrete beam's static strength, respectively.

• Architectural research
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• v.8 no.1
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• pp.47-56
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• 2006

In this research, the analysis of post-buckling behavior of tapered columns has been performed under a combined load of uniformly distributed axial load along the length and concentric axial load at free end by solving the nonlinear differential equation with the differential transformation technique. The buckling load at various slopes at free end of column is calculated and the results of the analysis using the differential transformation technique is verified with those of previous studies. It is also shown through the results that the buckling load of sinusoidal tapered columns is largest, the linear is second largest, and the parabolic is small in the all ranges of slopes at free end and the deflection of parabolic tapered columns in the x coordinates is largest, the sinusoidal is second largest, and the linear is smallest in the range of slope 0 to 140 degrees at free end. However, when the range of the slope is 160 to 176 degrees at the free end, the deflection of sinusoidal tapered columns in the x coordinates is largest, the linear is second largest, and the parabolic is smallest. In addition, for the linear tapered column, the buckling load increases along with the flexural stiffness ratio. Also, for the parabolic and the sinusoidal tapered column, the buckling loads increase and decrease as the flexural ratios increase in the range of flexural stiffness ratio n = 1.0 to n = 2.0. Through this research, it is verified that the differential transformation technique can be applied to solve the nonlinear differential equation problems, such as analysis of post-buckling behavior of tapered columns. It is also expected that the differential transformation technique apply to various more complicated problems in future.

• KSTLE International Journal
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• v.5 no.1
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• pp.1-6
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• 2004

Nano-scale studies on adhesion and friction were conducted in Si-wafer (100) using Atomic Force Microscopy (AFM). Glass (Borosilicate) balls of radii 0.32$\mu\textrm{m}$, 1.25$\mu\textrm{m}$, and 2.5$\mu\textrm{m}$, mounted on cantilever (Contact Mode type NPS) were used as tips. Adhesion and friction between Si-wafer and glass tips were measured at ambient temperature (24${\pm}$1$^{\circ}C$) and humidity (45${\pm}$5%). Friction was measured as a function of applied normal load in the range of 0-160 nN. Results showed that, both adhesion and friction increased with the tip radii. Also, friction increased linearly as a function of applied normal load. The effect of tip size on adhesion and friction was explained as the influence of the capillary force exerted by meniscus and that of the contact area on these parameters respectively. The coefficient of friction was estimated in two different ways, as the slope from the plot of friction force against the applied normal load and as the ratio between the friction force and the applied normal load. Both these estimates showed that the coefficient of friction increased with the tip size. Further, the influence of the adhesion force on the coefficient of friction was also discussed.

• Journal of Power Electronics
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• v.14 no.1
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• pp.105-114
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• 2014

The sliding mode control (SMC) strategy is applied to a permanent magnet synchronous machine vector control system in this study to improve system robustness amid parameter changes and disturbances. In view of the intrinsic chattering of SMC, a current sliding mode control method with a load sliding mode observer is proposed. In this method, a current sliding mode control law based on variable exponent reaching law is deduced to overcome the disadvantage of the regular exponent reaching law being incapable of approaching the origin. A load torque-sliding mode observer with an adaptive switching gain is introduced to observe load disturbance and increase the minimum switching gain with the increase in the range of load disturbance, which intensifies system chattering. The load disturbance observed value is then applied to the output side of the current sliding mode controller as feed-forward compensation. Simulation and experimental results show that the designed method enhances system robustness amid load disturbance and effectively alleviates system chattering.

• Structural Engineering and Mechanics
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• v.22 no.2
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• pp.197-222
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• 2006

In this paper, the non-linear time-dependent closed-form, discrete and combined solutions for the post-elastic response of a geometrically and physically non-linear suspended cable to a uniformly distributed load considering the creep effects, are presented. The time-dependent closed-form method for the particularly straightforward determination of a vertical uniformly distributed load applied over the entire span of a cable and the accompanying deflection at time t corresponding to the elastic limit and/or to the elastic region, post-elastic and failure range of a suspended cable is described. The actual stress-strain properties of steel cables as well as creep of cables and their rheological characteristics are considered. In this solution, applying the Irvine's theory, the direct use of experimental data, such as the actual stress-strain and strain-time properties of high-strength steel cables, is implemented. The results obtained by the closed-form solution, i.e., a load corresponding to the elastic limit, post-elastic and failure range at time t, enable the direct use in the discrete non-linear time-dependent post-elastic analysis of a suspended cable. This initial value of load is necessary for the non-linear time-dependent elastic and post-elastic discrete analysis, concerning incremental and iterative solution strategies with tangent modulus concept. At each time step, the suspended cable is analyzed under the applied load and imposed deformations originated due to creep. This combined time-dependent approach, based on the closed-form solution and on the FEM, allows a prediction of the required load that occurs in the post-elastic region. The application of the described methods and derived equations is illustrated by numerical examples.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.23 no.4
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• pp.265-272
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• 2011

The electrical load profiles of end-users must be analysed properly to introduce distributed generation system efficiently. In this study, numerical simulation for predicting a residential electrical load profile was developed to satisfy categorized electricity consumption range. We applied bottom-up approach to compose electrical load profile by using data from official reports and statistics. The electrical load profile produced from the simulation predicted peak times of public report accurately and agreed well with the standard residential electrical load profile of official reports within average error of 16.2%.

• Journal of the Korean Society for Nondestructive Testing
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• v.22 no.4
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• pp.333-346
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• 2002

The continuous indentation techniques are one of the most effective methods to nondestructively estimate mechanical properties. There are many applications in various dimensions of materials from macro-scale, through micro-scale, even to nano-scale range. The macro-range technology of kgf-load level is now focused on the evaluation of tensile properties and residual stress of bulk materials, for example, used in conventional load-bearing structures and in-use pipelines. The technology and the apparatus were successfully developed by a domestic research group. The micro-range technology of gf-load level can be applied to investigate some property-gradient materials such as weldment. Because it has better spatial resolution than the macro-range technology. The nano-range technology (called nanoindentation technique) of mgf-load level is basically used to evaluate hardness and modulus of micro- and nano-materials. Moreover, many researches are going on to measure tensile properties and residual stress. The nanoindentation technology is easy to be applied to the various fields, such as semiconductor devices, multiphase materials, and biomaterials, though other methods are too difficult to be applied due to dimensional or environmental limitations. On the basis of these accomplishments, the international and the domestic standards are being established.

• Proceedings of the KIEE Conference
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• 1991.07a
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• pp.354-357
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• 1991

In the case of system operation, a line overload cause damage to spread an whole range of power system. Of the theorems on load shedding, this study applied power distribution theorem and load reduction theorem which are local load shedding method, which are not affected by the magnitude of the power system and need not a large memory capacity and computation time. In this paper, we treat the problem of overload when power system occurred to fatal fault. Especially, there is the special case that local load shedding theorem is not always solved. Therefore, we introduce a solved device of the problem and construct the expert system of expanded local load shedding. Because proposed method uses the merits of expert system, in the case of system operation, the system operator don't embarrass to fatal fault and promptly deals with.

• Journal of the Korean Society for Precision Engineering
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• v.9 no.1
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• pp.126-136
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• 1992

In this paper, a simple structured feedback compensation scheme for a electro-hydraulic servo system to keep the response characteristics unchanged regardless of the load variation is proposed. In electro-hydraulic servo system, servovalve is most important control element. But the relation between input corrent and output flowrate of the servovalve has properties as follows; firstly, in spite of constant input current, output flowrate decreases as load pressure increases, secondly, according to frequency response of typical servovalve, the characteristics of gain and phase shift is something like 2'nd order system. Load pressure feedback compensation method has been applied to eliminate the first influence, the second influence has been improved by phase lead compensation method. As a result of above compensation methods, regardless of variation load condition, spring and inertia load, the compensation scheme has been verified to be effective within the range of frequency less than 25Hz by static response and dynamic response in time domain and frequency domain through experiments.

• The Korean Journal of Ceramics
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• v.6 no.3
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• pp.201-205
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• 2000

Fiber reinforced plastic (FRP) composites and ceramic matrix composites (CMC) which contain electrically conductive phases have been designed and fabricated to introduce the detection capability of damage/fracture detection into these materials. The composites were made electrically conductive by adding carbon and TiN particles into FRP and CMC, respectively. The resistance of the conductive FRP containing carbon particles showed almost linear response to strain and high sensitivity over a wide range of strains. After each load-unload cycle the FRP retained a residual resistance, which increased with applied maximum stress or strain. The FRP with carbon particles embedded in cement (mortar) specimens enabled micro-crack formation and propagation in the mortar to be detected in situ. The CMC materials exhibited not only sensitive response to the applied strain but also an increase in resistance with increasing number of load-unload cycles during cyclic load testing. These results show that it is possible to use these composites to detect and/or fracture in structural materials, which are required to monitor the healthiness or safety in industrial applications and public constructions.