• Polymer(Korea)
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• v.36 no.1
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• pp.88-92
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• 2012

Long-term performance of polymer under constant sustained load has been the main research focus, which created a need for the accelerated test method providing proper lifetime assessment. Cycling fatigue loading is one of the accelerated test method and has been of great interest. Microstructure change of high density polyethylene under cyclic fatigue loading and creep was examined utilizing a tensile device specially designed for creep and fatigue test and also can be attachable to the X-ray diffractometer. In this way, the crystal morphology change of polyethylene under creep and cyclic fatigue load was successfully monitored and compared. Despite the marked differences in macroscopic deformation between the creep and cyclic fatigue tests, crystal morphology such as crystallinity, crystal size, and $d$-spacing was as nearly identical between the two test cases. Specimens pre-deformed to different strains, i.e., before yield point (BYP), at yield point (YP) and after yield point (AYP), however, showed markedly different changes in crystal morphology, especially between AYP and the other two specimens.

• Smart Structures and Systems
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• v.15 no.2
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• pp.335-353
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• 2015

Previous long-term measurements of the Uldolmok tidal current power plant showed that the structure's natural frequencies fluctuate with a constant cycle-i.e., twice a day with changes in tidal height and tidal current velocity. This study aims to improve structural health monitoring (SHM) techniques for offshore structures under a harsh tidal environment like the Uldolmok Strait. In this study, lab-scale experiments on a simplified offshore structure as a lab-scale test structure were conducted in a circulating water channel to thoroughly investigate the causes of fluctuation of the natural frequencies and to validate the displacement estimation method using multimetric data fusion. To this end, the numerical study was additionally carried out on the simplified offshore structure with damage scenarios, and the corresponding change in the natural frequency was analyzed to support the experimental results. In conclusion, (1) the damage that occurred at the foundation resulted in a more significant change in natural frequencies compared with the effect of added mass; moreover, the structural system became nonlinear when the damage was severe; (2) the proposed damage index was able to indicate an approximate level of damage and the nonlinearity of the lab-scale test structure; (3) displacement estimation using data fusion was valid compared with the reference displacement using the vision-based method.

• Structural Engineering and Mechanics
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• v.51 no.3
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• pp.447-470
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• 2014

Numerical simulation of the non-linear behavior of (RC) structural walls subjected to severe earthquake ground motions requires a reliable modeling approach that includes important material characteristics and behavioral response features. The objective of this paper is to optimize a simplified method for the assessment of the seismic response and damage development analyses of an RC structural wall building using macro-element model. The first stage of this study investigates effectiveness and ability of the macro-element model in predicting the flexural nonlinear response of the specimen based on previous experimental test results conducted in UCLA. The sensitivity of the predicted wall responses to changes in model parameters is also assessed. The macro-element model is next used to examine the dynamic behavior of the structural wall building-all the way from elastic behavior to global instability, by applying an approximate Incremental Dynamic Analysis (IDA), based on Uncoupled Modal Response History Analysis (UMRHA), setting up nonlinear single degree of freedom systems. Finally, the identification of the global stiffness decrease as a function of a damage variable is carried out by means of this simplified methodology. Responses are compared at various locations on the structural wall by conducting static and dynamic pushover analyses for accurate estimation of seismic performance of the structure using macro-element model. Results obtained with the numerical model for rectangular wall cross sections compare favorably with experimental responses for flexural capacity, stiffness, and deformability. Overall, the model is qualified for safety assessment and design of earthquake resistant structures with structural walls.

• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.11
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• pp.2033-2048
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• 1992

This paper presents the theoretical development and qualitative evaluation of a new concept in the mathematical modeling of dynamic structures. We use both test data and analytical approximations to identify the parameters of an incomplete model. The model has the capability of predicting the response of the points of interest on the structure over the frequency range of interest and can be used to predict the changes in natural frequencies and normal modes due to structural changes. The theory was tested by running simulated tests on a relatively simple structure, identifying the parameters of the incomplete model, and using this model to predict the effects on frequency and mode shapes of several mass and stiffness changes. The conditions of the tests were varied by selecting different numbers of points of measurement, varying the frequency range, and by including assumed measurement error. It is recommended that the theoretical development be continued and that applications to more complex structures be carried out in order to develop a better understanding of the limitations and capabilities of the method. A successful, more definitive sevaluation could lead to immediate practical applications.

• Journal of Korean Society of Transportation
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• v.29 no.1
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• pp.57-67
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• 2011

This study has been conducted to provide the basic materials about the occurrence structures, causes and choice of treatments of the traffic accident offenders' PTSD(Posttraumatic Stress Disorder). The results of this study can be summarized as follows. First, accident offenders', not victims', PTSD conditions, which has been only estimated until now, are measured by nationwide questioning survey. Second, accident offenders' changes of driving attitude after accidents could be typed by factors analysis in the 20 driving situations closely connected to the occurrence of traffic accidents. Third, by the t-test, the difference of driving attitudes in primary driving situations according to existence of PTSD is distinguished. Six driving situations, which are found significant by t-test, are applied to structural equation. So the variables related to PTSD and the models which analyze the attitude changes of driving are constructed. This study provides the basic materials to help the reduction and prevention of the occurrence of PTSD.

• East Asian mathematical journal
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• v.38 no.5
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• pp.617-641
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• 2022

In this study, we first present a brief overview of the Korean general insurance market. We then explore the characteristics of the loss ratios of the Korean general insurance industry and apply Markov regime-switching methodology to model the loss ratios of these insurance companies by line of business based on changes in economic regimes. This study applies a number of confirmatory tests such as Zivot-Andrews test (2002), the Chow (1960) test and the Bai and Perron (1998) to confirm the presence of structural breaks in the time series of the loss ratios by line of business. Then, we employ Markov regime-switching methodology to model these loss ratios. We find empirical evidence that the loss ratios reported by insurance companies in Korea is characterized by two distinct regimes; a regime with high volatility and a regime with low volatility, except for vehicle insurance. Our analyses suggest that macro-economic conditions have significant explanatory effect on loss ratios but the direction of effect differs based on the line of business and the regime. Unlike previous studies that have applied linear regressions or divided the samples into different periods and then apply linear regressions to model loss ratios, we argue for the application of Markov regime-switching methodology, which are able to automatically distinguish the different regimes that may be associated with the movements of loss ratios based on differing economic conditions and regulatory upheavals. This study provides a more in depth understanding of loss ratios in the general insurance industry and will be of value to insurance practitioners in modelling the loss ratios associated with their businesses to aid in their decision making. The results may also provide a basis for further studies in other markets apart from Korea as well as for shaping policy decisions related to loss ratios.

• Journal of the Korea institute for structural maintenance and inspection
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• v.12 no.6
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• pp.173-179
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• 2008

This paper aims to calculate age coefficient of ultrasonic pulse velocity by non-destructive test. When concrete compressive strength is measured by non-destructive test, rebound test hammer method is applied to estimate age coefficient depending on the course of time after concrete casting, but ultrasonic pulse velocity method is not applied in the process. Although it is necessary to consider age coefficient with change of ultrasonic pulse velocity of concrete depending on aging, there have been little attempts to apply that method. The experiments were conducted to calculate aging effects which will be applied to establish the formula of measuring concrete strength. As a result of experiments, it was found that ultrasonic pulse velocity showed radical changes depending on concrete hardening in comparison with initial standard values. So, it was concluded that age coefficient must be applied to calculate strength. In conclusion, age coefficient of ultrasonic pulse velocity of concrete was suggested on the basis of experimental results.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2005.05a
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• pp.184-187
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• 2005

Physical changes in a structural system may cause changes in mechanical impedance of the system. Due to the electro-mechanical coupling effect in piezoelectric materials, this change can be monitoring by the electrical impedance of the piezoelectric sensor. In this paper, the variability of electro-mechanical impedance caused by temperature effect is assessed to adjust impedance data used for EMI based damage detection in beams. First experiments on beams are described. Next, experiments were performed under the temperature varying condition, in the range of $3^{\circ}C\;to\;23^{\circ}C$. Finally, the relationship between temperatures and impedance signatures is analyzed empirically temperature-frequency patten for the test structure.

• Knowledge Management Research
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• v.15 no.1
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• pp.45-63
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• 2014

Due to advance in Internet technology, most tourists tend to search travel information in the Online Travel Communities(OTC). Given this new paradigm in terms of finding travel information, most of relevant studies in this area are still dealt with explaining tourists' behaviors regardless of the types of communications. Therefore, to overcome some limitations in previous studies, we attempt to examine the relationships between both formal and informal communications and tourists' behavioral changes in the OTC context. Specifically, we developed a research model by employing the PPM(push-pull-mooring) framework and tested it to understand why and how tourists' behaviors might be changed. Survey data collected from 323 online tourists were used to test the model the model using SEM(structural equation modeling). The implications of our empirical findings for both research and practice are discussed.

• Journal of Korean Association for Spatial Structures
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• v.13 no.4
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• pp.83-90
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• 2013

PTEF membranes are used for roofing materials of membrane structures. PTEF is the abbreviation of Poly-tetra Fluotide-ethylene. These materials are consisted of fiberglass weave and polyetrfluoroethylene coating. Also, PTEF membranes have some problems of structural capacity by wind or snow load, etc. In this study, sensor housings using lead switches are bonding in PTFE membranes, Monitoring to changes tension and tear damages are studied using radio frequency. If tension is received on edged membranes, bonded lead switches of sensor housings will be destroyed by changes tension, and these become to send signals of damages at the connected radio frequency system with increased tension. Study of these functional membrane materials will be contributed to prevent water leakage and long-term maintenance of membrane structures.