• Bulletin of the Korean Mathematical Society
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• v.38 no.2
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• pp.285-292
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• 2001

The authors compute the Ricci curvature of the GRW space-time to obtain two conditions for the conjugate points which appear as the Timelike Convergence Condition(TCG) and the Jacobi inequality. Moreover, under such two conditions, we obtain a lower bound of the length of a unit timelike geodesic for focal points emanating form the immersed spacelike hypersurface, the graph over the fiber in the GRW space-time.

• Korean Journal of Mathematics
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• v.5 no.2
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• pp.107-112
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• 1997

For a ring R with 1, the higher K-theory of Quillen is defined by the higher homotopy groups of the plus construction of the general linear group of R. On the other hand, the Volodin K-theory is defined by the higher homotopy groups of the Volodin space. In this paper we show that these two K-theories are equivalent. We show that the Volodin space is a homotopy fiber of the acyclic map from BGL(R) to its plus construction.

• Communications of the Korean Mathematical Society
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• v.37 no.4
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• pp.1199-1207
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• 2022

In the present paper, a Riemannian submersion 𝜋 between Riemannian manifolds such that the total space of 𝜋 endowed with a torse-forming vector field 𝜈 is studied. Some remarkable results of such a submersion whose total space is Ricci soliton are given. Moreover, some characterizations about any fiber of 𝜋 or the base manifold B to be an almost quasi-Einstein are obtained.

• Journal of Korean Tunnelling and Underground Space Association
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• v.17 no.2
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• pp.107-115
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• 2015

As TBM construction method is widely applied to construct underground transmission lines, increasing attention has been drawn to SFRC(steel fiber reinforced concrete) segment with respect to structural characteristics. Health hazards of electromagnetic wave which the underground transmission lines emit has emerged, and there is a concern that electromagnetic wave can be amplified because a segment characteristic such as electrical conductivity is changed by steel fibers. In order to analyze correlation between steel fiber dosage and electromagnetic wave, the specimens were fabricated on three conditions to perform experimental tests. From the measured data, it is proven that there is no correlation between the electromagnetic wave and steel fiber reinforced concrete.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.30 no.1
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• pp.49-55
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• 2002

In this paper, we present the simulation monitoring of strain and temperature during and after the cure of unsymmetric composite laminate using fiber optic sensors. Fiber Bragg grating/extrinsic Fabry-Perot interferometric (FBG/EFPI) hybrid sensors are used to measure those measurands. The characteristic matrix of the sensor is analytically derived and measurements can be done without sensor calibration. A wavelength-swept fiber laser is utilised as a lighr source. Two FBG/EFPI sensors are embedded in a graphite/epoxy unsymmetric cross-ply composite laminate in different directions and different locations. We perform a real time monitoring of fabrication strains and temperatures at two points of the composite laminate during cure process in an autoclave. Also, the thermal strains and temperatures of the fabricated laminate are measured in a thermal chamber. Through these experiments, we can provide a basis for the efficient smart processing of composite and know the thermal behavior of unsymmetric cross-ply composite laminate.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.32 no.7
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• pp.43-50
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• 2004

For the simultaneous measurement of strain and damage signal a fiber Bragg grating sensor system with a dual demodulator was proposed. The dual demodulator is composed of a demodulator using a tunable Fabry-Perot filter measuring the low-frequency signal with large magnitude such as strain and the other using a passive Mach-Zehnder interferometer detecting the high-frequency signal with small amplitude such as impact or damage signal. Using the proposed fiber Bragg grating sensor system, both the strain and damage signals of a cross-ply laminated composite beam under tensile loading were simultaneously measured. The strain and damage signals detected by single fiber Bragg grating sensor showed that sudden strain shifts were accompanied with vibration at a maximum frequency of several hundreds of kilohertz at the instant of matrix crack propagation in the 90 degree layer in composite beam.

• Journal of Korean Tunnelling and Underground Space Association
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• v.13 no.2
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• pp.83-96
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• 2011

Currently, the commonly used tunneling method in Korea is NATM (New Austrian Tunneling Method). This method uses the rock bolt, shotcrete, and supporting system to maintain the strength of original soil and ensures the stability of tunnel by stabilizing the soil using the original strength of the soil in maximum after the excavation. In past years, wire-mesh reinforced shotcrete was common ones but currently steel-fiber reinforced shotcrete is being widely used for the tunnel construction site in Korea to save construction time with the advanced construction technology. The results further indicate that needs for the establishment of not only the specifications for shotcrete but the strengthening methods for the under reinforced shotcrete sections. Therefore, this study deals with the development of a new steel-fiber to ensure the stability of tunnels that are under reinforced with steel-fibers and to overcome the shortcomings of conventional method.

• The Journal of Advanced Prosthodontics
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• v.5 no.1
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• pp.16-20
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• 2013

PURPOSE. To assess the retention of glass fiber post cemented with self-adhesive resin cement into optimum and over-prepared root canals following obturation in the presence of either eugenol (EB) or calcium hydroxide (CB)-based sealers. MATERIALS AND METHODS. Roots of extracted premolars were endodontically-treated in 5 groups (n = 10). Roots of Group 1 (control) were left with no obturation and then optimally prepared to receive endodontic dowels. Other root canals were obturated with gutta-percha in the presence of either eugenol-based (Groups 2 and 4) or calcium hydroxide-based (Groups 3 and 5) sealer. Dowel spaces were prepared with optimal diameter in Groups 2 and 3, one size larger in Groups 4 and 5. Standardized fiber posts were luted to the prepared spaces using self-adhesive resin cement and its retention was then tested on an universal testing machine. Both one-way ANOVA and Tukey's HSD comparisons (${\alpha}$=0.05) were used to identify the significance of inter-group retention differences. Scanning electron microscopy (SEM) of both optimally and over-prepared dowel spaces was also considered to figure the nature of their interior out. RESULTS. The post retention was significantly higher to the non-obturated, optimally-prepared dowel spaces of Group 1 compared to the obturated, optimally-prepared ones of Groups 2 and 3. For each dowel space diameter, root canals obturated using CB of Groups 3 and 5 showed significantly higher dowel retention compared to those obturated using EB of Groups 2 and 4. Post retention to the over-prepared dowel spaces of Groups 4 and 5 was significantly higher than that recorded for the optimally-prepared ones of Groups 1-3. SEM images revealed traces of endodontic sealer and gutta-percha on the walls of the optimally-prepared dowel spaces. CONCLUSION. Despite the adverse effect of endodontic sealers on the retention of fiber posts, the over-preparation of dowel spaces helps to improve the retention.

• Journal of the Korean Institute of Landscape Architecture
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• v.29 no.6
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• pp.92-100
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• 2002

This study was carried out to obtain fundamental information on the growth response of interior landscape plants under fluorescent light, sunlight and optical fiber lighting indoors. Saintpaulia ‘Delaware’, ‘Kalanchoe blossfeldiana, Anthurium scherzerianum and Ardisia crenata were examined using light intensity of 5001ux and 1,0001ux of fluorescent light, sunlight and optical fiber lighting in an interior environment. Results of experiments are as follows; 1) Plant growth status showed the best results under optical fiber lighting compared with fluorescent light or sunlight. 2) Plant growth status was better under 1,0001ux light intensity than 5001ux light intensity and in cases of the same light intensity, the highest growth increase was under optical fiber lighting. while it was showed relatively different according to the different plant species between a fluorescent light and sunlight. 3) The deep pinkish red color of Saintpaulia ‘Delaware’flower was obtained first under an optical fiber lighting and a fluorescent light, a sun light in that order. 4) Regarding interred activity, photosynthetic rate and transpiration rate, intercellular CO, water absorption rate showed a similar tendency generally in spite of a little difference. Namely, transpiration rate and intercellular CO, $CO_2$ a absorption rate increased according to increase of photosynthetic rate. 5) Photosynthetic rate of test plants except Anthurium scherzerianum increased according to increase of light intensity and increased highest under optical fiber lighting in the same light intensity condition. Increases differed under fluorescent light and sun light. That of Saineaulia ‘Delaware’and Anthurium scherzerianum increased in the order of optical fiber, fluorescent light and sun light, but that of Kalanchoe blossfeldiana and Ardisia pusilla increased in the order of optical fiber lighting, sun light and fluorescent light. Summing up these results, In visual value or internal health status of all experimental plants we obtained the highest result under an optical fiber lighting. Finally, we need to introduce an optical fiber lighting in interior landscape space as main light source.

• Resources Recycling
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• v.23 no.4
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• pp.47-57
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• 2014

The research was conducted to analyze workability and fiber distributions of amorphous steel fiber reinforced concrete by changing fiber length and fiber addition ratio. The inverted slump cone and vebe tests as well as slump test was performed to understand the fluidity of amorphous steel fibers which have quite different appearance compared to conventional steel fibers. Test results showed that thin plate type of amorphous steel fibers required different test approach to figure out workability since the reduction of workability from slump test was different that from inverted slump cone and vebe tests. In conclusion, fluidity of amorphous steel fibers to concrete was significantly degraded as fiber length and addition ratio increase. Also, fibers space in cement matrix was apparently reduced as the increase of fiber length and addition ratios without fiber balling.