• Journal of Conservation Science
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• v.26 no.2
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• pp.121-132
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• 2010

The Bukjiri Maaeyeoraejwasang (National Treasure No. 201) consists of two-mica granite in medium size, which was the simbol of power in the region of the Silla period. Magnetic susceptibility of the host rock was measured as 0.41(${\times}10^{-3}$ SI unit), which has the similar range with surrounding outcrop. The Buddha developed parallel discontinuous plane of NE to SW strike and damaged seriously by exfoliation, granular disintegration and brown discoloration as 41.5%, 16.7% and 40.0%, respectively. As a result of the ultrasonic velocity, which was relatively weak values as 1,629m/s (Buddha area) and 1,549m/s (surrounding outcrop), improved about 900m/s compared to last treatment. From the results of the evaluation for slope stability, identified the possibility of toppling failure in the Buddha, and planar and wedge failure in host rock. Therefore, we suggest for the safely conservation of the Buddha, continuance monitoring for understand behavior of discontinuity system of the surface, and necessitate foundation reinforcement method for the rock which has the danger of collapse.

• Journal of the Korea institute for structural maintenance and inspection
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• v.26 no.4
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• pp.48-56
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• 2022

Controlling the setting time of cementitious materials is one of the most important factors in securing early-age performance of concrete structures. Recently, the use of retarding admixtures, which enable the inhibition of some hydration products to control the securing time due to average temperature rise is suggested. Although various non-destructive evaluation methods have been proposed to evaluate cement hydration and hardening of cement-based materials to overcome the limitations of Vicat needle test, experimental research is still required to use the non-destructive evaluation method with added retarding admixtures. In this study, measurements of electrical resistivity and ultrasonic wave velocity in early-aged cement pastes were performed according to the addition of retarding admixture(tartaric acid). The setting time of the cement pastes was evaluated by obtained rising time of the both non-destructive measurements. As a result, the possibility of evaluating the setting delay in cement pastes was confirmed through comparative analysis with the initial and final setting times by Vicat test. In addition, X-ray diffraction results at the rising time of electrical resistivity showed a key hydration product affecting the setting delay.

• Composites Research
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• v.17 no.4
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• pp.53-60
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• 2004

This paper describes a method for a falling weight impact test to estimate the impact energy absorbing characteristics and impact strength of CFRP laminate plates based on considerations of stress wave propagation theory. The absorbed energy of T300 orthotropic composites is higher than that of quasi-isotropic specimen over impact energy 6.8J, but in case of using T700 fiber, much difference does not show. Also, absorbed energy of T300 orthotropic composites, which are composed of the same stacking number and orientation became more than that of T700 fiber specimen; however there was no big difference in case of quasi-isotropic specimens. The delamination areas of the impacted specimen were measured with the ultrasonic C-scanner to find correlation between impact energy and delamination area. The fracture surfaces were observed by using the SEM (scanning electron microscope) through a low-velocity impact test in order to confirm the fracture mechanism.

• Journal of the Korean Society for Nondestructive Testing
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• v.32 no.1
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• pp.41-46
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• 2012

As the development of MEMS/NEMS structure and application technology the demand for an assessment of the mechanical properties have increased. The mechanical properties are mainly evaluated by using tensile test or ultrasonic wave measurement. However, the new technology have been developed such as nano-indentation, guided wave method because they have a limitation in case of a thin plate and thin film. In the study, the guided wave velocities are measured by electromagnetic-acoustic transducer(EMAT), the material properties of thin metallic foils are obtained using optimization process of the theoretical and experimental group velocity of guided wave. The Young's modulus obtained by the optimization process(201.6 GPa), nano-indentation(207.0 GPa) and literature value(203.7 GPa) of a $50{\mu}m$ thick nickel thin plate shows good agreement within 3%.

• Tribology and Lubricants
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• v.34 no.6
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• pp.217-225
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• 2018

In this study, we spray a Ni-Cr composite powder onto AISI 4340 steel using the high velocity oxygen fuel method. We subsequently subject the Ni-Cr coating (as-sprayed) to ultrasonic nanocrystal surface modification (UNSM) process to improve the tribological performance. This study aims at increasing the wear resistance and durability of the Ni-Cr coating by altering the surface integrity and microstructure via the UNSM process. The UNSM process reduces the surface roughness of the as-sprayed coating by about 64%, which is explained by observing the elimination of high peaks and valleys and filling up micro-pores. Furthermore, a change in the microstructure of the coating due to continuous high-frequency strikes to the surface by a tip can lead to an increase in hardness from about 48 to 60 HRC. Furthermore, we investigate the characterization of the friction and wear behavior of Ni-Cr coating by a ball-on-disc tribometer in the dry conditions. We determine that after the UNSM process, there is a significant reduction in the friction coefficient of the as-sprayed coating from approximately 1.1 to 0.75. This is owing to the increased hardness and smoothed surface roughness. In addition, we investigate the surface morphology and wear track of the coatings before and after the UNSM process using a scanning electron microscope, energy dispersive spectrometer, and three-dimensional laser scanning microscope. We observe that the wear track of the Ni-Cr coating after the UNSM process is lower than that of the as-sprayed one. Thus, we confirm that the UNSM process has a significant influence on the improvement of the tribological performance of the Ni-Cr composite coating.

• Journal of the Korean Society of Propulsion Engineers
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• v.6 no.1
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• pp.55-62
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• 2002

A new off-surface visualization method of using the micro water droplet and laser beam sheet was presented. About a size of 5 to TEX>$10\mu\textrm{m}$ micro water droplet could be made from home-style ultrasonic humidifier, A 3 W Argon ion laser and cylindrical lens were used to generate a laser beam sheet, which interrogate specific cross section of the vortical flow field. Application of this new visualization method was conducted in KAFA small-sized low speed wind tunnel of having the test section of 0TEX>$0.9 m(W){\times}$0.9 m(H){\times}2.1 m(L)$$$. Visualization results show this method relatively easy and safe flow visualization method for wind tunnel testing. Moreover, this method is also make up for the disadvantage of smoke visualization, and can be applied to higher flow velocity range than that of smoke visualization.

• Computers and Concrete
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• v.34 no.2
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• pp.213-230
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• 2024

The use of waste tires and industrial wastes such as fly ash (FA) and ground granulated blast furnace slag (GGBS) in concrete is an important issue in terms of sustainability. In this study, the effect of parameters affecting the physical, mechanical and microstructural properties of FA/GGBS-based geopolymer concretes with waste rubber fiber was investigated. For this purpose, the effects of rubber fiber percentage (0.6%, 0.9%, 1.2%), binder (75FA25GGBS, 50FA50GGBS, 25FA75GGBS) and curing temperature (75 ℃, 90 ℃ and 105 ℃) were investigated. The Taguchi-Grey Relational Analysis (TGRA) method was used to obtain optimum parameter levels of rubber fiber geopolymer concrete (RFGC). The slump, fresh and hardened density, compressive strength, flexural strength, static and dynamic modulus of elasticity, ultrasonic pulse velocity (UPV) tests and scanning electron microscopy (SEM) analysis were performed on the produced concretes. The analysis of variance (ANOVA) method was used to statistically determine the effects of the parameters on the experimental results. A confirmation test was performed to test the accuracy of the optimum values found by the TGRA method. With the increase of GGBS percentage, the compressive strength of RFGC increased up to 196%. The increase in rubber fiber percentage and curing temperature adversely affected the mechanical properties of RFGC. As a result of TGRA, the optimum value was found to be A1B3C1. ANOVA results showed that the most effective parameter on the experimental results was the binder with 99% contribution percentage. It is understood from the SEM images that the optimum concrete had a denser microstructure and less capillary cracks and voids. For this study, the use of the TGRA method in multiple optimization has proven to provide very useful and reliable results. In cases where many factors are effective on its strength and durability, such as geopolymer concrete, using the TGRA method allows for finding the optimum value of the parameters by saving both time and cost.

• Computers and Concrete
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• v.21 no.3
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• pp.311-319
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• 2018

This study used a volumetric method for design. The control group used waste Liquid Crystal Displayplay (LCD) glass powder to replace cement (0%, 10%, 20%, 30%), and the PZT group used Pd-Zr-Ti piezoelectric (PZT) powder to replace 5% of the fine aggregate to make cement mortar. The engineering and the mechanical and electricity properties were tested; flow, compressive strength, ultrasonic pulse velocity (UPV), water absorption and resistivity (SSD and OD electricity at 50 V and 100 V) were determined; and the correlations were determined by linear regression. The compressive strength of the control group (29.5-31.8 MPa) was higher than that of the PZT group (25.1-29 MPa) by 2.8-4.4 MPa at the curing age of 28 days. A 20% waste LCD glass powder replacement (31.8 MPa) can fill up finer pores and accelerate hydration. The control group had a higher 50 V-SSD resistivity ($1870-3244{\Omega}.cm$), and the PZT group had a lower resistivity ($1419-3013{\Omega}.cm$), meaning that the resistivity increases with the replacement of waste LCD glass powder. This is because the waste LCD glass powder contains 62% $SiO_2$, which is a low dielectric material that is an insulator. Therefore, the resistivity increases with the $SiO_2$ content.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.10 no.9
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• pp.1718-1726
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• 2006

In this paper, we proposed a fuzzy controller and algorithm for efficiently obstacle avoidance in unknown space. The ultrasonic sensor is used for position and distance recognition of obstacle, and fuzzy controller is used for left and right wheels angular velocity control. The fuzzification is used singleton method and the control rule is each wheel forty-nine. The fuzzy inference is used simplified Mamdani's reasoning and defuzzification is used SCOG(Simplified Center Of Gravity). The computer simulation based on mobile robot modelling was performed for the capacity of fuzzy controller and the really applicable possibility revaluation of the proposed avoidance algorithm and fuzzy controller. As a result, mobile robot was exactly reached in target and it avoided obstacle efficiently.

• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.19 no.2
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• pp.198-203
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• 2010

In this paper, study on the mode characteristics analysis of the SH-EMAT (shear horizontal, electromagnetic acoustic transducer) waves for evaluating the thickness reduction in plates such as corrosion and friction is presented. Noncontact methods for ultrasonic wave generation and detection have been a great concern and highly demanded due to their capability of wave generation and reception on surface of high temperature or on rough surface. Mode identification of the SH-EMAT wave is carried out in an aluminum plate with thinning defects using time frequency analysis method such as wavelet transform, compared with theoretically calculated group velocity dispersion curve. The changes of various wave features such as the amplitude and the time-of-flight have been observed and the correlations with the thickness reduction have been investigated. Firstly, experiments have been conducted to confirm that it is possible to selectively generate and receive specific desired SH modes. These modes have then been analyzed to select the parameters that are sensitive to the thickness change. The results show that the mode cutoff and the time-of-flight changes are feasible as key parameters to evaluate the thickness reduction.