• Journal of the Korea institute for structural maintenance and inspection
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• v.19 no.2
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• pp.92-98
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• 2015

Recently, buildings tend to be large size, complex shape and functional. As the size of buildings is becoming massive, the need for structural health monitoring (SHM) technique is increasing. Various SHM techniques have been studied for buildings which have different dynamic characteristics and influenced by various external loads. "Abnormal behavior point" is a moment when the structure starts vibrating abnormally and this can be detected by comparing between before and after abnormal behavior point. In other words, anomalous behavior is a sign of damage on structures and estimating the abnormal behavior point can be directly related to the safety of structure. Abnormal behavior causes damage on structures and this leads to enormous economic damage as well as damage for humans. This study proposes an estimating technique to find abnormal behavior point using Hilber-Huang Transform which is a time-frequency signal analysis technique and the proposed algorithm has been examined through laboratory tests with a bridge model using a shaking table.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.16 no.7
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• pp.4398-4404
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• 2015

Stainless steel sheets are widely used as the structural material for dynamic machine structures, These kinds structures used stainless steel sheets are commonly fabricated by using the gas welding, For fatigue design of gas welded joints such as various type joint. It is necessary to obtain design information on stress distribution at the weldment as well as fatigue strength of gas welded joints. Thus in this paper, ${\Delta}P-N_f$ curves were obtained by fatigue tests. and, ${\Delta}P-N_f$ curves were rearranged in the ${\Delta}{\sigma}-N_f$ relation with the hot spot stresses at the gas welded joints. Using these results, the accelerated life test(ALT) is conducted. From the experiment results, an life prediction model is derived and factors are estimated. So it is intended to obtain the useful information for the fatigue lifetime of welded joints and data analysis by statistic reliability method, to save time and cost, and to develop optimum accelerated life prediction plans.

• Physical Therapy Rehabilitation Science
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• v.6 no.4
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• pp.182-188
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• 2017

Objective: The purpose of this study was to compare ankle function between adults with and without Down syndrome (DS). Design: Cross-sectional study. Methods: Ten adults with DS and 18 without participated in this study and underwent manual muscle test (MMT), range of motion (ROM) assessment, star excursion balance test (SEBT), and functional movement screen (FMS). The tests were demonstrated to increase their accuracy and the actual measurements were assessed after one or two demonstrations. To minimize the standby time and fatigue, the travelled distance and measuring order were adjusted. To remove the influence of shoes on the measurements, the shoes were taken off and only socks were worn. Results: Dorsal and plantar flexion MMTs of both ankles were significantly weaker and plantar flexion ROM of both ankles were significantly lower in adults with DS compared with those without (p<0.05). However, dorsal flexion ROM of both ankles were not significantly different between them. There were significant differences in distances measured in all the directions (anterior, anterolateral, lateral, posterolateral, posterior, posteromedial, medial, and anteromedial directions) of SEBT (p<0.05). Significant differences were also demonstrated in the scores of hurdle step, inline lunge, shoulder mobility, and rotary stability among the seven items of FMS (p<0.05). Conclusions: To enhance the dynamic stability of adults with DS, it is necessary to improve ankle stability by strengthening the ankle dorsal and plantar flexors.

• Journal of the Korea Convergence Society
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• v.8 no.12
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• pp.283-289
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• 2017

In this study, a general program has been developed to calculate the static design factor of a vehicle suspension system. The partial derivatives of Jacobians for constraint equations are calculated using the symbolic technique. In the commercial program, finite difference method is used to calculate the Jacobian matrix of Jacobian. But in this study, it is calculated by using the symbol calculation method to precisely consider it. The calculated Jacobian matrix for the system has proved its accuracy through the solution of the numerical example. A simulation was performed for a double wishbone suspension of a 1/4 vehicle. The result can be used to calculate the static design factor of the suspension, and also add a convergence module that can perform virtual tests.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.16 no.12
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• pp.8654-8664
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• 2015

Beam-type structures with non-uniform cross sections are widely used in mechanical, architectural, and civil engineering fields. This paper deals with dynamic characteristics and vibration problems. Governing equations are first derived by using local coordinates. Their solutions are then assumed by using Galerkin's mode summation method. Bisection method is also applied in solving the determinant of the matrix which can provide natural frequencies. Whereas finite element methods adopt admissible functions satisfying only geometric boundary condition, in this study we apply Galerkin's mode summation method which uses eigen-functions satisfying both governing equations and boundary conditions. Modal analysis and experimental tests are finally performed using simply-supported beams with four different non-uniform cross-sections. Our analytical results then show good agreement with experimental ones.

• Journal of the Korea Concrete Institute
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• v.19 no.6
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• pp.701-706
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• 2007

This study evaluated the durability of high-performance shotcrete mixed in the proper proportions using alkali-free and cement mineral accelerators as a permanent support that maintains its strength for the long term. Durability tests were performed the chloride permeability, repeated freezing and thawing, accelerated carbonation, and the effects of salt environments. Test results showed that all the shotcrete mixes included silica fume had low permeability. In addition, after 300 freeze/thaw cycles, the shotcrete mix had excellent freeze/thaw resistance more than the 85% relative dynamic modulus of elasticity. The accelerated carbonation test results were no effect of accelerator type but, the depth of carbonation was greater in the shotcrete mix containing silica fume. No damage was seen in a salt environments. Therefore, the high performance shotcrete mix proportions used in this study showed excellent durability.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.32 no.3
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• pp.30-36
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• 2004

As fiber reinforced composite materials are widely used in aircraft, space structures and robot arms, the study on the non-destructive testing methods of the composite materials has become an important research area for improving their reliability and safety. In this paper, the AE signal analyzer with the resonance circuit to extract the specified frequency of the acoustic emission signal were designed and fabricated. The noise levels of the fabricated AE signal analyzer by the disturbance such as impact or mechanical vibration had a very small value comparable to those of the conventional AE signal analyzer. Also, the fabricated AE signal analyzer was proved to have about the same crack detection capabilities with the conventional AE signal analyzer under the static and dynamic tensile tests of the composite materials.

• Bulletin of the Society of Naval Architects of Korea
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• v.27 no.2
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• pp.87-95
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• 1990

Conventional methods to estimate natural frequencies and damping ratios of structures from measured response time series obtained during impact tests are reviewed. Maximum Entropy Method and Least Square Prony Method are introduced to alleviate the inherent limitation of the conventional methods. The performance of the methods are explored through computer simulation. As an example of application, they are applied to the time series obtained from an anchor drop-and-snup test of a container ship and the result is compared to that of conventional FFT method. As a result of the computer simulation, it is found that Maximum Entropy Method is very efficient to estimate natural frequencies of structures when two neighboring natural frequencies are close enough and short data records are only available, but it is not a reliable estimator for damping ratios. And it is also found that Least Square Prony Method is efficient to estimate the natural frequencies and damping ratios of highly damped structural system, but the estimation efficiency of damping ratios is significantly deteriorated in the presence of significant additive noise.

• Journal of Advanced Research in Ocean Engineering
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• v.3 no.1
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• pp.41-52
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• 2017

This paper presents a numerical and experimental study of sloshing loads on liquefied natural gas (LNG) vessels. Conventional LNG carriers with membrane-type cargo systems have filling restrictions from 10% to 70% of tank height. The main reason for such restrictions is high sloshing loads around these filling depths. However, intermediate filling depths cannot be avoided for most LNG vessels except the LNG carrier. This study attempted to design a membrane-type LNG tank with a modified lower-chamfer shape that allows all filling operations. First, numerical sloshing analysis was carried out to find an efficient height of the lower-chamfer that can reduce sloshing pressure at partially filled conditions. The numerical sloshing analysis program SHI-SLOSH was used for numerical simulation; this program is based on SOLA-VOF. The effectiveness of the newly designed tanks was validated by 1:50-scale three-dimensional tank tests. A total of three different tanks were tested: a conventional tank and two modified tanks. As test conditions, various filling depths and wave periods were considered, and the same test conditions were applied to the three tanks. During the test, slosh-induced dynamic pressures were measured around the corners of the tank wall. The measured pressure data were post-processed and the pressures of the three different tanks were statistically compared in several ways. Experimental results show that the modified tanks were quite effective in reducing sloshing loads at low filling conditions. This study demonstrated the possibility of all filling operations for LNG cargo containment systems.

• Proceedings of the Korean Geotechical Society Conference
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• 2000.03b
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• pp.173-182
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• 2000

To find axial and lateral responses of impact-driven H piles in embankment(SM), the H piles are instrumented with electric strain gages, dynamic load test is performed during driving, and then the damage of strain gages is checked simultaneously. Axially and laterally static load tests are performed on the same piles after one to nine days as well. Then load-settlement behavior is measured. Furthermore, to find the set-up effect in H pile, No. 4, 16, 26, and R6 piles are restriked about 1, 2, and 14 days after driving. As results, ram height and pile capacity obtained from impact driving control method become 80cm and 210.3∼242.3ton, respectively. At 15 days after driving, allowable bearing capacity by CAPWAP analysis, which 2.5 of the factor of safety is applied for ultimate bearing capacity, increases 10.8%. Ultimate bearing capacity obtained from axially static load test is 306∼338ton. This capacity is 68.5∼75.7% at yield force of pile material and is 4∼4.5 times of design load. Allowable bearing capacity using 2 of the factor of safety is 153∼169ton. Initial stiffness response of the pile is 27.5ton/mm. As the lateral load increases, the horizontal load-settlement behaves linearly to which the lateral load reaches up to 17ton. This reason is filled with sand in the cavity formed between flange and web during pile driving. As the result of reading with electric strain gages, flange material of pile is yielded at 19ton in horizontal load. Thus allowable load of this pile material is 9.5ton when the factor of safety is 2.0. Allowable lateral displacement of this pile corresponding to this load is 23∼36mm in embankment.