• Economic and Environmental Geology
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• v.42 no.3
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• pp.261-272
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• 2009

An experimental study of the accelerated weathering was performed to investigate the variations of physicomechanical properties of deteriorated rocks due to freeze-thaw weathering for the Jurassic granite specimens from Wonju, Gangwon-do. Each complete cycle of freeze and thaw was lasted 24 hours, comprising 2 hours saturating in vacuum chamber, 8 hours freezing at -20$\pm1^{\circ}C$ and 14 hours thawing at room temperature. Freeze-thaw cycles were implemented with measuring the index physical properties as well as geometries of microfractures. The seismic velocity was found to decrease with increasing freeze-thaw cycles. On the other hand, absorption tends to increase with freeze-thaw cycles. In the end, it was concluded that variations of the index properties of deteriorated specimen depend on its initial properties and flaws in rock. The size and density of the traces of the microfracture on slab specimen were changed continuously with increasing freeze-thaw weathering. The results obtained in this study show that the box fractal dimension($D_B$) has the strong capability of quantifying the combined effect of size and density of the microfractures.

• Journal of Welding and Joining
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• v.13 no.1
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• pp.78-88
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• 1995

This paper was performed to study the utility of the SP(small punch) test and the AE(acoustic emission) test in the evaluation of SCC(stress corrosion cracking) susceptibility for parent metal and bond line of HT80 steel-weld joint by SAW(submerged arc welding) with the various pH values. The loading rate used was 3*10$^{-4}$ mm/min and the corrosive environment used was synthetic sea water during the SP test and the AE test. According to the test results, the SCC susceptibility of the parent metal was increased in the order of pH6.0, pH8.2 and pH10.0. On the other hand, the bond line showed almost the same high SCC susceptibility in all pH concentrations. Synthetically, from the results of the SCC susceptibility, the macro- and micro-SEM observation, the microfracture behaviors by AE test and the relationship between SCC susceptibility and displacement at incipient failure, .delta.$_{i.f-AE}$, it can be concluded that the SP test and the AE test are the good test methods to evaluate the SCC susceptibility for parent metal and bond line of the weld joint with the change of environmental factors.

• Proceedings of the Korean Society For Composite Materials Conference
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• 2000.04a
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• pp.67-70
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• 2000

Mechanical properties of (10%$AI_20_3{\cdot}SiO_2$+5%Ni)/Al hybrid composites fabricated by the reaction squeeze casting were compared with those of (15%$AI_20_3{\cdot}SiO_2$)/Al composites. Intermetallic compound formed by reaction between molten aluminum and reinforcing powder was uniformly distributed in the Al matrix. These intermetallic compounds were identified as $Al_3$NI using EDS and X-ray diffraction analysis. Microhardness and flexural strength of hybrid composites were higher than that of (15%$AI_20_3{\cdot}SiO_2$)/Al Composite. In-Situ fracture tests were Conducted on (15%$AI_20_3{\cdot}SiO_2$)/Al Composites and (10%$AI_20_3{\cdot}SiO_2$+5%Ni)/Al hybrid composites to identify the microfracture process. It was identified from the in-situ fracture test of (10%$AI_20_3{\cdot}SiO_2$+5%Ni)/Al composites, microcracks were initiated mainly at the short fiber / matrix interfaces. As the loading was continued, the crack propagated mainly along the separated interfacial regions and the well developed shear bands. It was identified from the in-situ fracture test of (10%$AI_20_3{\cdot}SiO_2$+5%Ni)/Al hybrid composites, microcracks were initiated mainly by the short fiber/matrix interfacial debonding. The crack proceeded mainly through the intermetallic compound clusters

• Journal of Korean Foot and Ankle Society
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• v.24 no.2
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• pp.69-74
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• 2020

Osteochondral lesions of the talus (OLT) can heal and remain asymptomatic, or they can progress to deep ankle pain on weight bearing and the formation of subchondral cysts. Treatment varies from nonoperative treatment to open and arthroscopic procedures. Operative procedures include marrow stimulation techniques (abrasion chondroplasty, multiple drilling, microfracture), osteochondral autografts or allografts, and autologous chondrocyte implantation. Among these treatments, arthroscopic marrow stimulation techniques have been the preferred initial surgical treatment for most OLT. Despite these treatments, many patients complain of persistent pain even after surgery, and many surgeons face the challenge of determining a second line of treatments. This requires a thorough re-evaluation of the patient's symptoms as well as radiological measures. If the primary surgical treatment has failed, multiple operative treatments are available, and relatively more invasive methods can be administered. On the other hand, it is inappropriate to draw a firm conclusion in which methods are superior.

• Journal of the Korean Ceramic Society
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• v.33 no.5
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• pp.602-615
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• 1996

The purpose of the present study is to investigate the microstructural effect on the R-curve behavior in three aluminas with different grain size distributions by analyzing the bridging stress distribution. The crack opening displacement (COD) according to the distance behind the stationary crack tip was measured using an in situ SEM fracture method. The measured COD values in the fine-grained alumina agreed well with Wiederhorn's sollution while they deviated from Wiederhorn's solution in the two coarse-grained aluminas because of the increase of the crack closure due to the grain interface bridging in the crack wake. A numerical fitting procedure was conducted by the introduction of the power-law relation and the current theoretical model together with the measured COD's in order to obtain the bridging stress distribution. The results indicated that the bridging stress function and the R-curve computed by the current model were consistent with those computed by the power-law relation providing a reliable evidence for the bridging stress analysis of the current model. The strain-softening exponent in the power-law relation n, was calculated to be in the range from 2 to 3 and was closely related to the grain size distribution. Thus it was concluded from the current theoretical model that the grain size distribution affected greatly the bridging stress distribution thereby resulting in the quantitative analysis of microfracture of polycrystalline aluminas through correlating the local-fracture-cont-rolling microstructure.

• Journal of Korea Foundry Society
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• v.16 no.6
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• pp.537-549
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• 1996

This study aims at investigating the correlation of microstructure and mechanical properties of the AZ91 Mg/Al borate whisker composites fabricated by squeeze csting technique with a variation of applied pressure. Microstructure observation and in-situ fracture tests were conducted on the composites to identify the microfracture process. Detailed microstructural analyses indicated that the grain refinement could be achieved with applied pressure and the little change in volume fraction on reinforcing whiskers could be carried out. It was also found clearly from in-situ observation of crack initiation and propagation that in the composite processed by the lower applied pressure, microcracks were initiated earily at whisker/matrix interfaces, thereby resulting in the drop in strength. In the composite processed by the higher applied pressure, on the other hand, planar slip lines were well developed in the matrix, and then propagated through whiskers without whisker/matrix decohesion. Thus, the effect of the applied pressure on microstructure and mechanical properties can be explained by grain refinement, increased amounts of reinforcements, and improvement of whisker/matrix interfacial strength as the applied pressure in increased.

• Journal of Welding and Joining
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• v.12 no.4
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• pp.63-75
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• 1994

The conventional SCC(stress corrosion cracking) test methods have much difficulty in evaluating the SCC behaviors of the localized zone like HAZ, bond line and weld metal because of the specimen size. Accordingly, the purpose of this paper is to develop the new SCC test method of the welded zone by evaluating the SCC susceptibility on parent metal and various microstructures of the welded zone by SP(small punch) test method using miniaturized small specimen and SSRT(slow strain rate test) method(SP-SSRT). Besides, this study is to verify the efficiency of the SP-SSRT results through AE(acoustic emission) test which is a useful technique to monitor the microfracture processes of the material. From the results of SCC susceptibility, SEM observation and AE test, it can be concluded that the SP-SSRT test using miniaturized small specimen(10mm*10mm*0.5mm) will be a good test method to evaluate the SCC susceptibility on the local zone such as the welded zone.

• Journal of Dental Rehabilitation and Applied Science
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• v.17 no.3
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• pp.199-204
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• 2001

This study was to investigate the reused possibility of zirconia reinforced glass-ceramic(IPS Empress Cosmo ceramic) with sprue button in the flexure strength and fracture toughness. 40 disk-shaped ceramic specimens (20 specimens: as-pressed material; 20 specimens: reused material) with approximately 1.7 mm thickness and 15 mm diameter were prepared by "lost wax" technique. The remnants(sprue buttons) were used for repressing. The surface treatments for the discs were gradually abraded with 320, 800, 1200, and 2000 grit SiC sandpaper. The specimens were evaluated their flexure strength with the biaxial flexure jig(ball-on-three balls) and their fracture toughness with Vickers Indentation-microfracture test. The Weibull moduli were calculated for biaxial flexural strength. The mean flexure strength and fracture toughness of each group were $122.2{\pm}18.3MPa$, $1.00{\pm}0.09MPa{\cdot}m^{0.5}$ (as-pressed ceramics), and $122.2{\pm}20.3MPa$, $1.01{\pm}0.10MPa{\cdot}m^{0.5}$ (reused ceramics). There were no significant differences in the strength and the fracture toughness between the as-pressed and the reused IPS Empress Cosmo ceramic (P>0.05). This implied zirconia reinforced glass-ceramic(IPS Empress Cosmo ceramic) could be used one more time by reusing of sprue button in the flexure strength and fracture toughness.

• The Journal of Korean Academy of Prosthodontics
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• v.36 no.1
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• pp.183-198
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• 1998

This study was designed to investigate the peri-implant tissue reaction in ovariectomized osteoporotic female rats, and to evaluate effects of estrogen, calcitonin, parathyroid hormone on the bone - implant interface in osteoporotic rats. 120 Sprague - Dawley rats were used in this experiments. Osteoporosis was induced by bilateral ovariectomy. They were divided 5 groups : sham-operated control group(Sham), ovariectomized group (OVX), OVX and estrogen treated group (OVX+E), OVX and PTH treated group (OVX+PTH), and OVX and calcitonin treated group (OVX+CT). Eight weeks after ovariectomy, two titanium screw implants were inserted into the left tibia of each rat. Eight weeks after the insertion of the implants, the periotest values (PTV) of implant were examined, and the rats were sacrificed, and examined the reaction of bone tissue surrounding the implant both histologically and histomorphometrically. The bone density and ash weight of opposite right tibia were examined. Over 40 rats were fractured on left tibia that was implant inserted. On histologically finding, all groups were osseointegrated well, especially in OVX+PTH group. In OVX group, tibial cortical bone showed many large harversian canal and microfracture lines. The OVX+PTH group showed the lowest mean PTV (-2.33) (p<0.05), and the hightest mean bone - implant contact percentage (89%) (p>0.05). But the OVX+CT group showed the highest mean bone density ($5.45mg/cm^3$) and ash weight (56.12%) (p<0.05). The results indicate that PTH treatment enhances osseointegration of implant in OVX rats, and CT treatment depresses bone turnover and prevent the development of osteopenia in OVX rats.

• Journal of the Korean Arthroscopy Society
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• v.1 no.1
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• pp.112-115
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• 1997

While the cruciate ligament has a profuse vascular response following injury, spontaneous repair does not occur. This may result from the fact that synovial fluid dilution of the hematoma following injury prevents the formation of a fibrin clot and thus the initiation of the healing mechanism. Another theory suggests that the dynamic nature of the fascicles of the anterior cruciate ligament(ACL) through even small ranges of motion prohibits spontaneous union to these fibers. But we experienced two cases of spontaneous healing of partially injured ACL. Initially they showed more than grade II anterior instability. 6 mm difference by stress roentgenographs(pull view) and difference of 8 mm by KT 1000TM arthrometer between the ACL injured knee and normal side knee. Lax, nearly complete tear of ACL and synovial bleeding were noted during arthroscopic examination but the continuity of synovial membrane was seemed to be intact. These cases were treated by conservative management rather than reconstructive procedure. Postoperatively they showed excellent clinical results, no anterior instability and unlimited athletic activity. Based on our clinical experience. we think that cruciate ligament has the spontaneous healing potential in acute stage and middle aged patient. We consider the microfracture technique and initial immobilization for accelleration of healing response of the ACL.