• Geomechanics and Engineering
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• v.13 no.4
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• pp.535-570
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• 2017

There are only few cases where cause and location of failure of a rock structure are limited to a single discontinuity. Usually several discontinuities of limited size interact and eventually form a combined shear plane where failure takes place. So, besides the discontinuities, the regions between adjacent discontinuities, which consist of strong rock and are called material or rock bridges, are of utmost importance for the shear strength of the compound failure plane. Shear behaviour of persistent and non-persistent joint are different from each other. Shear strength of rock mass containing non-persistent joints is highly affected by mechanical behavior and geometrical configuration of non-persistent joints located in a rock mass. Therefore investigation is essential to study the fundamental failures occurring in a rock bridge, for assessing anticipated and actual performances of the structures built on or in rock masses. The purpose of this review paper is to present techniques, progresses and the likely future development directions in experimental testing of non-persistent joint failure behaviour. Experimental results showed that the presence of rock bridges in not fully persistent natural discontinuity sets is a significant factor affecting the stability of rock structures. Compared with intact rocks, jointed rock masses are usually weaker, more deformable and highly anisotropic, depending upon the mechanical properties of each joint and the explicit joint positions. The joint spacing, joint persistency, number of rock joint, angle of rock joint, length of rock bridge, angle of rock bridge, normal load, scale effect and material mixture have important effect on the failure mechanism of a rock bridge.

• Journal of the Korean Society for Advanced Composite Structures
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• v.1 no.3
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• pp.24-34
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• 2010

This paper analyzed the partial differential equations of laminated composite shells of revolution by using the finite difference method. The proof that numerical results are reasonable and accurate is obtained through converge ratio analysis and commercial program LUSAS for the structural analysis. The purpose of this study is to examine closely the engineering advantages and to analyze the structural behaviors of the anisotropic shells of revolution. Thus, the relevant reinforcement and most suitable arrangement of fiber to produce the highest strength are proposed through the numerical results according to a variety of parameter study. Namely, the distribution of displacements and stress resultants are analyzed according to the change of meridian's curvature, the ratio of height-width of shell, subtended angle, fiber angle, and so on. Using these distribution, the most suitable shell may be proposed to produce the highest strength. Also, the configuration of the entire laminated composite conical shells is analysed, and a variety of the design criterion of circular conical shell are proposed and studied in engineering view points.

• Tunnel and Underground Space
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• v.9 no.4
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• pp.350-363
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• 1999

The precision cyclic shear test system was established to investigate the mechanical characteristics of rough rock joints under cyclic loading conditions. Laboratory cyclic shear tests were conducted for saw-cut joints and artificial rough rock joints using Hwangdeung granite and Yeosan marble. Surface roughness and aperture characteristics of specimens were examined by measuring surface topography using the laser profilometer. Peak shear strength, phase difference during loading and unloading, and anisotropic shear behavior were investigated throughout the cyclic shear test results. These features and their subsequent variations in each loading cycle are significantly dependent upon the second order asperities and the strength of intact rock. It was observed that degradation of asperities for rough rock joints under cyclic shear loading followed the exponential degradation laws of asperity angle and that the mechanism for asperity degradation would be different depending upon the normal stress level, roughness of joint surface and the loading stage.

• Journal of the Korean Ceramic Society
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• v.47 no.6
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• pp.498-502
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• 2010

Aluminium titanate($Al_2TiO_5$) has extremely anisotropic thermal expansion properties in single crystals, and polycrystalline material spontaneously microcracks in the cooling step after sintering process. These fine intergranular cracks limit the strength of the material, but provide an effective mechanism for absorbing strain energy during thermal shock and preventing catastrophic crack propagation. Furthermore, since machinable BN-ceramics used as an insulating substrate in current micro-electronic industry are very expensive, the development of new low-cost machinable substrate ceramics are consistently required. Therefore, cheap $Al_2TiO_5$-machinable ceramics was studied for the replacement of BN ceramics. $Al_2O_3-Al_2TiO_5$ ceramic composite was fabricated via in-situ reaction sintering. $Al_2O_3$ and $TiO_2$ powders were mixed with various mol-ratio and sintered at 1400 to $1600^{\circ}C$ for 1 h. Density, hardness and strength of sintered ceramics were systematically measured. Phase analysis and microstructures were observed by XRD and SEM, respectively. Machinability of each specimens was tested by micro-hole machining. The results of research showed that the $Al_2TiO_5$-composites could be used for low-cost machinable ceramics.

• Journal of the Korean Society of Safety
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• v.31 no.3
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• pp.16-21
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• 2016

Carbon Fiber Reinforced Plastic(CFRP) composite with a higher specific strength and rigidity is more excellent than conventional metallic materials or other organic polymer of FRP. It has been widely used in vehicles, aerospaces and high technology industries which are associated with nuclear power fields. However, CFRP laminated composite has several disadvantages as like a delamination, matrix brittleness and anisotropic fibers that are the weak points of the crack initiation. In this present work, the reinforced silicon carbide(SiC) particles were added to the interlayer of CFRP laminates in order to mitigate the physical vulnerability affecting the cracking and breaking of the matrix in the CFRP laminated composite because of excellent specific strength and thermal shock resistance characteristics of SiC. The 1wt% of SiC particles were spread into the CFRP prepreg by using a spray coating method. After that, CFRP prepregs were laminated for the specimen. Also, the twill woven type CFRP prepreg was used because it has excellent workability. Thus the mechanical and fracture behaviors of the twill woven CFRP laminated composite reinforced with SiC particles were investigated with the acoustic emission(AE) method under a fracture test. The results show that the SiC particles enhance the mechanical and fracture characteristics of the twill CFRP laminate composite.

• Proceedings of the Korea Association of Crystal Growth Conference
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• 2000.06a
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• pp.1-33
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• 2000

Silicon carbide ceramics with sintering additives from the system AlN-Y$_2$O$_3$ can be gas-pressure sintered to theoretical density. While commonly a combination of sesquioxides is used such as Al$_2$O$_3$-Y$_2$O$_3$, the oxynitrid additives offer the advantage that only a nitrogen atmosphere is require instead of a powder. By starting form a mixture of ${\beta}$-SiC and ${\alpha}$-SiC, and by performing dedicated heat treatments after densification, anisotropic grain growth is obtained which leads to a platelet microstructure showing enhance fracture toughness. In the present work, recent improvement of the mechanical behaviour of these materials at ambient and high temperatures is reported. By means of a surface oxidation treatment in air it is possible to obtain four-point bending strengths in excess of 1 GPa, and the strength retention at high temperatures is significantly improved.

• Transactions on Electrical and Electronic Materials
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• v.7 no.1
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• pp.30-35
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• 2006

We have studied the motion of charged micro-particles that are immersed in a nematic liquid crystal (LC) and controlled by in-plane field. The LC is an anisotropic liquid such that the viscosity of the LC depends on flow direction, phase of the LC, and temperature, which affects the motion of the charged particles under the influence of electric field. This study shows that the motion of charged particles mainly depends on the applied voltage and the LC phase, but does not show any significant influence from the initial alignment of LC, although one may expect directional difference in drag force due to interaction between LC and particle. The viscosity changes due to temperature variations in nematic phase also show no signification influence on particle velocity when compared to the effect from varying in-plane field strength.

• Structural Engineering and Mechanics
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• v.32 no.5
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• pp.649-665
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• 2009

This paper reports part of a comprehensive research study conducted at the University of Queensland on the ability of CFRP web-bonded systems in strengthening an exterior beam-column joint subjected to monotonic loads. One 1/2.2 scaled plain and four CFRP repaired/retrofitted joints subjected to monotonic loads were analysed using the nonlinear finite-element program ANSYS and the results were calibrated against experiments. The ANSYS model was employed in order to account for tension stiffening in concrete after cracking and a modified version of the Hognestad's model was used to model the concrete compressive strength. The stress-strain properties of main steel bars were modelled using multilinear isotropic hardening model and the FRPs were modelled as anisotropic materials. A perfect bond was assumed as nodes were shared between adjacent elements irrespective of their type. Good agreement between the numerical predictions and the experimental observation of the failure mechanisms for all specimens were observed. Closeness of these results proved that the numerical analysis can be used by design engineers for the analysis of web-bonded FRP strengthened beam-column joints with confidence.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2003.10a
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• pp.217-220
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• 2003

Due to the lighter weight and the higher freedom of design than metals plastics have been spot lighted in a wide number of applications. In the making plastic parts injection-molding process is one of the most general methods. During the injection molding process, filling-packing-cooling process, plastics have exposed to several external stresses and then plastic parts injected have molding effects which are known as anisotropic properties, orientation, and residual stress. Those molding effects are often shown as unexpected phenomena which are warpage, strength decrease, stiffness reduction, etc. In case of glass fiber filed plastics these effects are more significant than the ufilled ones. Therefore the molding effects have to be considered in the parts design using glass fiber reinforced plastics. We have developed the interface program in order to consider the molding effects in structural analyses of plastic parts using Heirarchical structural searching and layer handling in direction of thickness algorithm. The advantages of this program are the freedom of FE mesh between molding and structural analysis, the variable layer to the thickness direction of parts and the conveniences of data transferring and checking

• Macromolecular Research
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• v.15 no.1
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• pp.65-73
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• 2007

Chitosan, fibroin, and hydroxyapatite are natural biopolymers and bioceramics that are biocompatible, biodegradable, and resorb able for biomedical applications. The highly porous, chitosan-based, bioceramic hybrid composite, chitosanlfibroin-hydroxyapatite composite, was prepared by a novel method using thermally induced phase separation. The composite had a porosity of more than 94% and exhibited two continuous and different morphologies: an irregularly isotropic pore structure on the surface and a regularly anisotropic multilayered structure in the interior. In addition, the composite was composed of an interconnected open pore structure with a pore size below a few hundred microns. The chemical composition, pore morphology, microstructure, fluid absorptivity, protein permeability, and mechanical strength were investigated according to the composition rate of bioceramics to biopolymers for use in tissue engineering. The incorporation of hydroxyapatite improved the fluid absorptivity, protein permeability, and tenacity of the composite while maintaining high porosity and a suitable microstructure.