• Journal of the Korean Society of Industry Convergence
• /
• v.4 no.4
• /
• pp.379-386
• /
• 2001

The lower toughness or brittle materials for mechanical test lead to the additional requirement that applied displacements be controlled with high resolution. Biaxial tension and shear tester using stepper motor with harmonic driver is shown. The device had to be high resolution so that the crack initiation process of slow extension and steady growth could be examined, Grip plates were connected to a linear bearing and actuator. The actuators consisted of stepper motors with harmonic driver connected to pre-loaded ball screw and nut assemblies. The encoders and motor controllers were connected to a personal computer so that arbitrary displacements histories could he prescribed in normal and tangential directions. The linear bearings were used to react loads perpendicular to their axes while allowing low friction, parallel movement of the attached grips. Load cells measured the reactions normal and tangential. the loads measured the reactions were recorded by the computer.

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
• /
• 2001.11a
• /
• pp.88-95
• /
• 2001

Silicon nitride based ceramics reinforced with 3wt% Si$_{3}$N$_{4}$ whisker was prepared by tape casting to investigate the effect of microstructure on erosion behaviors. Hardness and fracture toughness were measured with prepared specimens. A gas blast type erosion tester was used to examine the erosion behavior of the specimens with different impact directions and angles. The erosion rate increases with increasing impact angle. Erosion rate of the silicon nitride ceramics also depends on the grain orientations, The erosion rate was lowered when impaction direction was parallel to the grain orientation This result was explained by the crack deflection and bridging due to the grain orientation.

• Proceedings of the Korea Concrete Institute Conference
• /
• 2004.11a
• /
• pp.635-638
• /
• 2004

External bonding of steel plates has been used to strengthen deficient reinforced-concrete structures since the 1960s. In recent years, fiber-reinforcde polymer(FRP) plates have been increasingly used to replace steel plates due to their superior properties. This paper is concerned with anchorage failure due to crack propagation parallel to the boned plated near or along the adhesive/concrete interface, staring from the critically stressed position toward the anchored end of the plates. Factor of bond-slip interface model is average bond stress, effective length, slip volume and fracture energy. The aim of the present paper is to provide a comprehensive assessment of bond-slip interface model with concrete and CFRP plates.

• Journal of the Korea Institute of Military Science and Technology
• /
• v.10 no.3
• /
• pp.173-180
• /
• 2007

In this paper, we have proposed a simple finite element model for separation mode analysis on the roadwheel and track assembly of main battle tank and established a contact stress-based mechanism which could explain the initiation and growth of separation defect occurred during the test of padreplacable track. It was proved that the longitudinal contact shear stress component on the pin hole region of the track shoe body which is parallel to the driving direction is consistent with the crack initiation at the bonding surface between track shoe and wheel-side rubber. The longitudinal shear stress increased locally near the separated region after the separation initiated. So we could assume that the local stress concentration accelerates the separation growth according to the shear mode.

• Korean Journal of Materials Research
• /
• v.7 no.3
• /
• pp.224-228
• /
• 1997

Rolling contact fatigue tests were performed for two types silicon nitrides using disk- type specimens. Materials showed a fatigue behaviour similar to that typically found in metallic materials From the fractographic and metallographic observations, it has been found that the crack initation in the silicon nitrides subjected to rolling contact fatigue is to be induced by cyclic subsurface shear stress, as is known in steel bearing. On the mid-sections of the specimens, many subsurface cracks which lay parallel to the contact surface can be found at a depth where fluctuation of the Herzian shear stress was the maximum.

• Journal of the Korean GEO-environmental Society
• /
• v.17 no.10
• /
• pp.23-31
• /
• 2016

Grout is generally operated with low viscous material similar to water, but grout for micro crack with high viscous materials and high injection pressure is gradually increased under the development of underground and subsea space. In order to estimate grouting injection performance considering crack width, viscosity of grouting materials, and injection pressure, there should be a reliable standard laboratory testing method. In this paper, theoretical injection mechanisms of grouting materials are presented as radial and linear flows, and laboratory testing apparatus are introduced to simulate each flow case. Radial flow is simulated by using acrylic disk plates which are able to spread grouting material radially from the center of the disk plates, and linear flow is simulated by using stainless parallel plane plates which are able to spread grouting material linearly. Apparatus are consist of upper and lower plates and industrial films with different thickness are placed between plates in order to simulate various crack widths. Laboratory verification tests with these apparatus were conducted with tap water (1cP at $20^{\circ}C$) as an injection material. Through the laboratory testing results, the best laboratory testing method is recommended in order to estimate grouting injection performance.

• Journal of Energy Engineering
• /
• v.21 no.3
• /
• pp.249-253
• /
• 2012

In this study, The report analysed the characteristics of power drop in solar cell through thermal shock test. The solar cells were tested 500 cycles in $-40^{\circ}C$ lowest temperature and $120^{\circ}C$ highest temperature by thermal shock test on ironbound conditions, that excerpted standard of PV Module(KS C IEC-61215). The result of the efficiency analysis through measure of I-V, efficiency of Cell decreased from 13.9% to 11.0% and decreasing rate was 20.9% after test. The result of the surface analysis through EL, solar cell has damage of gridfinger and ribbon joint. Cell cracks were founded in damage of cells through cross section of solar cells. Also, Fill factors were decreased from 72.3% to 62.0% after thermal shock test and decreasing rate is 11.8%. therefore, Yearly power drop is aggravated with facts that cell crack, damage of surface and power loss of cell by change of I-V characteristic curve with decreasing of parallel resistance.

• Journal of the Korean Ceramic Society
• /
• v.52 no.6
• /
• pp.410-416
• /
• 2015

Recently, a new $Y_2O_3$ coating deposited using the EB-PVD method has been developed for erosion resistant applications in fluorocarbon plasma environments. In this study, surface crack formation in the $Y_2O_3$ coating has been analyzed in terms of residual stress and elastic modulus. The coating, deposited on silicon substrate at temperatures higher than $600^{\circ}C$, showed itself to be sound, without surface cracks. When the residual stress of the coating was measured using the Stoney formula, it was found to be considerably lower than the value calculated using the elastic modulus and thermal expansion coefficient of bulk $Y_2O_3$. In addition, amorphous $SiO_2$ and crystalline $Al_2O_3$ coatings were similarly prepared and their residual stresses were compared to the calculated values. From nano-indentation measurement, the elastic modulus of the $Y_2O_3$ coating in the direction parallel to the coating surface was found to be lower than that in the normal direction. The lower modulus in the parallel direction was confirmed independently using the load-deflection curves of a micro-cantilever made of $Y_2O_3$ coating and from the average residual stress-temperature curve of the coated sample. The elastic modulus in these experiments was around 33 ~ 35 GPa, which is much lower than that of a sintered bulk sample. Thus, this low elastic modulus, which may come from the columnar feather-like structure of the coating, contributed to decreasing the average residual tensile stress. Finally, in terms of toughness and thermal cycling stability, the implications of the lowered elastic modulus are discussed.

• Journal of the Korean Ceramic Society
• /
• v.36 no.1
• /
• pp.43-49
• /
• 1999

Gas pressure sintered silicon nitride based composites with 0~5wt% $\beta$-Si3N4 whiskers were prepared. The whiskers were unidirectionally oriented by a modified tape casting technqiue and green bodies with various microstructure were formed by changing stacking sequences of sheets cut from the tape. Orientations of the large elongated grains of the sample after gas pressure sintering were the same as the those of the whiskers of green body, and the sintering shrinkage and mechanical properties of sintered sample were consistent with the microstructural characteristics. In case of unidirectional samples, the sintering shrinkage normal to whisker alignment direction was larger than that parallel to the direction. The shrinkage difference inceaed as the whiskercontent increaed. As whisker content increaed, the crack length normal to and parallel to tape casting direction became shorter and larger, respectively. Although the grain size increased by th whisker addition, the flexural strength of unidirectional samples was not lower than that of smaple without the whisker. In case of crossplied and 45$^{\circ}$rotated samples, the anisotropy of mechanical preoperties disappeared.

• Tunnel and Underground Space
• /
• v.27 no.5
• /
• pp.263-270
• /
• 2017

Hydraulic rock splitting is a technique which leads to failure of rockmass by means of water injection with a pressure higher than the tensile strength of rockmass, using straddle packer installed in boreholes drilled from free surface. Field tests were conducted in this study for several slopes of biotite granite according to various designs for borehole layout and water injection. Test results showed that new cracks were generated to connect to adjacent holes or that pre-existed cracks were propagated by injection, finally leading to failure. In particular, this study suggests the possibility of controlling the direction of generated cracks with guide slot, since new cracks were generated parallel to the guide slots carved on a borehole wall before injection. Various types of borehole layout and injection methods should be further developed for the practical uses, considering the factors influencing on crack generation.