• Journal of the Korea Institute of Information and Communication Engineering
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• v.26 no.1
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• pp.128-133
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• 2022

This paper deals with the graph in which the weights of edges are given the distances between two end vertices on a metric space. In particular, we will study about a path P with n vertices for these graphs. We obtain a new graph $\bar{P}$ by augmenting an edge to P. Then the length of the shortest path between two vertices on $\bar{P}$ is considered and we focus on the maximum of these lengths. This maximum is called the diameter of the graph $\bar{P}$. We wish to find the augmented edge to minimize the diameter of $\bar{P}$. Especially, for an arbitrary real number λ > 0, we should determine whether the diameter of $\bar{P}$ is less than or equal to λ and we propose an O(n)-time algorithm for this problem, which improves on the time complexity O(nlogn) previously known. Using this decision algorithm, for the length D of P, we provide an O(nlogD)-time algorithm to find the minimum of the diameter of $\bar{P}$.

• Structural Engineering and Mechanics
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• v.55 no.4
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• pp.857-869
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• 2015

The aluminum 7075-T6 is known as an alloy widely used in aircraft structural applications, which does not exhibit strain rate sensitivity during dynamic compressive tests. Despite mechanical importance of the material, there is not enough attention to determine appropriate sample dimensions such as a sample diameter relative to the device bar diameter and sample length to diameter (L/D) ratio for dynamic tests and how these two parameters can change mechanical behaviors of the sample under dynamic loading condition. In this study, various samples which have different diameters of 31.8, 25.4, 15.9, and 9.5 mm and sample L/D ratios of 2.0, 1.5, 1.0, 0.5, and 0.25 were tested using Split Hopkinson Pressure Bar (SHPB), as this testing device is proper to characterize mechanical behaviors of solid materials at high strain rates. The mechanical behavior of this alloy was examined under ${\sim}200-5,500s^{-1}$ dynamic strain rate. Aluminum samples of 2.0, 1.5 and 1.0 of L/D ratios were well fitted into the stress-strain curve, Madison and Green's diagram, regardless of the sample diameters. Also, the 0.5 and 0.25 L/D ratio samples having the diameter of 31.8 and 25.4 mm followed the stress-strain curve. As results, larger samples (31.8 and 25.4 mm) in diameters followed the stress-strain curve regardless of the L/D ratios, whereas the 0.5 and 0.25 L/D ratios of small diameter sample (15.9 and 9.5 mm) did not follow the stress-strain diagram but significantly deviate from the diagram. Our results indicate that the L/D ratio is important determinant in stress-strain responses under the SHPB test when the sample diameter is small relative to the test bar diameter (31.8 mm), but when sample diameter is close to the bar diameter, L/D ratio does not significantly affect the stress-strain responses. This suggests that the areal mismatch (non-contact area of the testing bar) between the sample and the bar can misrepresent mechanical behaviors of the aluminum 7075-T6 at the dynamic loading condition.

• Smart Structures and Systems
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• v.17 no.4
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• pp.593-610
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• 2016

In this study, an innovative and smart glass fiber-reinforced polymer (GFRP) hybrid bar was developed for stronger durability of concrete structures. As comparing with the conventional GFRP bar, the smart GFRP Hybrid bar can promise to enhance the modulus of elasticity so that it makes the cracking reduced than the case when the conventional GFRP bar is used. Besides, the GFRP Hybrid bar can effectively resist the corrosion of conventional steel bar by the GFRP outer surface on the steel bar. In order to verify the bond performance of the GFRP hybrid bar for structural reinforcement, uniaxial pull-out test was conducted. The variables were the bar diameter and the number of strands and pitch of the fiber ribs. Tensile tests showed a excellent increase in the modulus of elasticity, 152.1 GPa, as compared to that of the pure GFRP bar (50 GPa). The stress-strain curve was bi-linear, so that the ductile performance could be obtained. For the bond test, the entire GFRP hybrid bar test specimens failed in concrete splitting due to higher shear strength resulting in concrete crushing as a function of bar deformation. Investigation revealed that an increase in the number of strands of fiber ribs enhanced the bond strength, and the pitch guaranteed the bond strength of 19.1 mm diameter hybrid bar with 15.9 mm diameter of core section of deformed steel the ACI 440 1R-15 equation is regarded as more suitable for predicting the bond strength of GFRP hybrid bars, whereas the CSA S806-12 prediction is considered too conservative and is largely influenced by the bar diameter. For further study, various geometrical and material properties such as concrete cover, cross-sectional ratio, and surface treatment should be considered.

• Proceedings of the Korea Concrete Institute Conference
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• 2000.10b
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• pp.873-878
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• 2000

Tension stiffening effects of reinforced concrete member with large diameter bar, mainly used at reactor building of nuclear power plant, are studied by uniaxial structural tests. Bond length and stress of steel bar, size of steel bar, and compressive strength of concrete are evaluated to tension stiffening by uniaxial tests. Problems and solution during the uniaxial test are suggested. The prevent splitting cracks, concrete cover-to-bar diameter ratio $c/d_{b}$ is kept 2.6~2.8. Because the bond length is increased as the size of steel bar, the specimen length of the D35 steel bar is required at least 2.0 m. The specimen length must be decided with bond length as well as concrete cover-to-bar diameter ratio to prevent splitting crack.

• Proceedings of the Korea Concrete Institute Conference
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• 1998.04b
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• pp.621-626
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• 1998

An experimental investigation on the reinforcing bar corrosion and relationshid of reinforcing bar and concrete bond strength has been conducted to establish the allowable limit of rust in the construction field. The reinforcing bars used in this study were rusted before embedding in concrete. The first component of this experiment is to make rust of reinforcing bar rust artificially based on Faraday's theory at certain rates such as 2, 4, 6, 8 and 10% of reinforcing bar weight. For estimation of the amount of rust by weight, Clarke's solution and Shot blasting were adopted and compared. Parameters include 240 and 450kg/㎠ of compressive strengths and diameter of reinforcing bar (16, 19 and 25mm) corresponding development length for pull-ort test. And, pull-out tests were carried. out according to KSF 2441 and ASTMC 234 to investigate the effect of the corrosion rate on reinforcing bar-concrete bond behavior. It is found from the test results that the test techniques for corrosion of bar used in this study is relatively effective and correct test method. Results shows that up to 2% of rust increases the bond strength regardless of concrete strength and diameter of reinforcing bar like the existing data. It might be because of the roughness from rust. As expected, the bond strength increases as compressive strength of concrete increases and the diameter of bar decreases.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 1996.05a
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• pp.209-216
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• 1996

The shape variation of billet was investigated by numerical method and spray forming work with variation of average substrate withdrawal velocity$\bar{V}$, withdrawal velocity change interval $\Delta$t and velocity deviation from average velocity $V_{dev}$. The shape and diameter with large$\bar{V}$, $\Delta$t, $V_{dev}$ vary seriously. When $\bar{V}$, $\Delta$t, $V_{dev}$ are smaller, the shape of billet is more simillar to that of the billet with constant withdrawal velocity. The average diameter of billet is determined by only $\bar{V}$, independent of $\Delta$t, $V_{dev}$. With $\bar{V}$, : 0.2 mm/sec $\Delta$t: 200 sec and $V_{dev}$. : 0.2mm/sec billet of constant diameter 230mm$\times$ height 1000mm were manufactured.

• Kyungpook Mathematical Journal
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• v.61 no.3
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• pp.671-677
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• 2021

For a distance-regular graph with valency k, second largest eigenvalue r and diameter D, it is known that r ≥ $min\{\frac{{\lambda}+\sqrt{{\lambda}^2+4k}}{2},\;a_3\}$ if D = 3 and r ≥ $\frac{{\lambda}+\sqrt{{\lambda}^2+4k}}{2}$ if D ≥ 4, where λ = a₁. This result can be generalized to the class of edge-regular graphs. For an edge-regular graph with parameters (v, k, λ) and diameter D ≥ 4, we compare $\frac{{\lambda}+\sqrt{{\lambda}^2+4k}}{2}$ with the local valency λ to find a relationship between the second largest eigenvalue and the local valency. For an edge-regular graph with diameter 3, we look at the number $\frac{{\lambda}-\bar{\mu}+\sqrt{({\lambda}-\bar{\mu})^2+4(k-\bar{\mu})}}{2}$, where $\bar{\mu}=\frac{k(k-1-{\lambda})}{v-k-1}$, and compare this number with the local valency λ to give a relationship between the second largest eigenvalue and the local valency. Also, we apply these relationships to distance-regular graphs.

• Structural Engineering and Mechanics
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• v.19 no.6
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• pp.603-618
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• 2005

The split Hopkinson pressure bar (SHPB) technique is widely used to characterize the dynamic mechanical response of engineering materials at high strain rates. In this paper, attendant problems associated with testing 70 mm diameter concrete specimens are considered, analysed and resolved. An adaptation of a conventional solid circular striker bar, as a means of achieving reliable and repeatable SHPB tests, is then proposed. In the analysis, a pseudo one-dimensional model is used to analyse wave propagation in a non-uniform striker bar. The stress history of the incident wave is then obtained by using the finite difference method. Comparison was made between incident waves determined from the simplified model, finite element solution and experimental data. The results show that the simplified method is adequate for designing striker bar shapes to overcome difficulties commonly encountered in SHPB tests. Using two specifically designed striker bars, tests were conducted on 70 mm diameter steel fibre reinforced concrete specimens. The results are presented in the paper.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.38 no.3
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• pp.429-438
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• 2018

Cone and T-bar penetrometers have been frequently used to estimate the undrained shear strength of clay. For small-size model tests, miniature penetrometers should be used but their correlation factors have not been well published. In this study, a testing setup was developed to derive empirical factors of the miniature cone and T-bar penetrometers. A 350mm-diameter chamber and kaolin clay were utilized to prepare soil specimens consolidated under four different pre-consolidation pressures controlling undrained shear strength. Two miniature cones with two diameters of 10-mm and 16-mm and a T-bar penetrometer with 10-mm diameter were used to investigate boundary effect, penetration rate effect, and diameter and shape effect. Unconsolidated-undrained triaxial tests were carried out with samples taken from the specimens to measure undrained shear strength. Finally, empirical factors for the penetrometers were constructed to correlate tip resistance to undrained shear strength.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2019.11a
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• pp.110-111
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• 2019

In ACI 318-19 published recently, the conditions and development length equation to use the headed deformed bars were changed considerably. Although the use of the larger-diameter(No.14 and 18) headed deformed bars isn't yet permitted, the use of the high strength(80,000psi) headed deformed bars is permitted and the effect of bar-diameter($d_b$) on the development length is increased considerably. Therefore, structures using larger-diameter headed deformed bars will be expected to be affected by this code change. We will study the effect of the code change on the development design and find out the design optimization method to minimize the effect of the changed conditions and development length equation.