• Journal of Welding and Joining
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• v.6 no.4
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• pp.54-62
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• 1988

Major problems in welding Al-7020 include shrinkage, rpopositgy in welds and loss of strength in the heat affected zone. Thus it is important to examine the mechanical properties and reliability of welds. In this study, a series of experiments was carried out to determine the mechanical properties such as micro-hardness distribution, tensile strength, porosity and residual stress distribution of the Al-7020 weldment made by pulse-GMA welding. The resuts of the experiemnts are as folows. 1) The micro-hardness of weld metal and heat affected zone was lower than that of the base metal. 2) The tensile strength of the deposited metal was much lower than that of the base metal. 3) The porrosity in weld metal zone was negligible under the adopted conditsion of experiemnts. 4) The residual stress in the weld metal was lower than that of the heat affected zone, because the weld metal was softened. And the mciro-hardness distribution, the tensile strength and the residual stess distribution of the weldment in the as-welded condition were compared with those of the weldment after heat treatment.

• Journal of the Korean GEO-environmental Society
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• v.14 no.5
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• pp.41-47
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• 2013

Well-graded and dense soils have good engineering properties. Unsaturated soil properties such as shear strength, compressibility and permeability are closely related to the soil-water characteristic curve of the soil. Therefore it is important to study the effects of the coefficient of uniformity and the porosity on the soil-water characteristic curve of the sandy soils, which are also related to the grain size distribution and the density of the soil, respectively. In this study soil-water characteristic curves (SWCCs) for six sandy soil specimens were investigated using Tempe pressure cells. The test data were best-fitted to Fredlund and Xing equation. The obtained fitting parameters and the characteristic points of SWCCs were discussed and correlated with the porosity and the coefficient of uniformity of the specimens. The results show that the smaller the porosity of the specimen becomes, the larger the value of the residual matric suction becomes, whereas the larger the coefficient of uniformity of the specimen becomes, the larger the value of the residual matric suction becomes. Regardless of the coefficient of uniformity, the smaller the porosity of the specimen, the flatter the max. slope of SWCC.

• Advances in concrete construction
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• v.8 no.4
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• pp.311-320
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• 2019

In this study, the effect of cellulose nanocrystals (CNCs) on the fire resistance properties of fiber-reinforced cement composites was investigated. The main variables were CNCs content (0.4, 0.8 and 1.2vol.% compared with cement), steel fiber ratio, and exposure temperature (100, 200, 400, 600 and 800℃). The fire resistance properties, i.e., residual compressive strength, flexural strength, and porosity, were evaluated in relation with the exposure temperature of the specimens. The CNCs suspensions were prepared to composited dispersion method of magnetic stirring and ultra-sonication. CNCs are effective for increasing the compressive strength at high temperatures but CNCs do not seem to have a significant effect on flexural reinforcement. Porosity test result showed CNCs reduce the non-hydration area inside the cement and promote hydration.

• Journal of the Korean Society of Marine Environment & Safety
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• v.3 no.2
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• pp.13-21
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• 1997

This paper describes the effectiveness of laser pulse shaping in eliminating weld defects such as porosity, cracks and undercuts in pulsed Nd:YAG Laser welding. A large porosity was formed in a keyhole mode of deep penetration weld metal of any stainless steel. Solidification cracks were present in Type 303 with about 0.3%s. The conditions for the formation of porosity were determined in further detail in Type 316. With the objectives of obtaining a fundamental knowledge of formation and prevention of weld defects, the fusion and solidification behavior of a molten puddle was observed during laser spot welding of Type 310S. through high speed video photographing technique. It was deduced that cellular dendrite tips grew rapidly from the bottom to the surface, and consequently residual liquid remained at the grain boundaries in wide regions and enhanced the solidification cracking susceptibility. Several laser pulse shapes were investigated and optimum pulse shapes were proposed for the reduction and prevention of porosity and solidification cracking.

• Environmental Engineering Research
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• v.12 no.3
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• pp.109-117
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• 2007

The characteristics of carbonaceous particles collected from the combustion of Vacuum Residue (VR) in a test furnace have been investigated. The physical and chemical characterization includes particle size, scanning electron microscopy of the surface structure, measurement of porosity, surface area and density, EDX/XRD analyses and measurement of chemical composition. The studies show that the carbonaceous VR particles are very porous and spheroidal, and have many blow-holes on the surface. The particles become smaller and more sponge-like as the reaction proceeds. The present porosity of VR particles is similar to that of cenospheres from the combustion of heavy oil, and the majority of pores are distributed in macro-pores above $0.03\;{\mu}m$ in diameter. Measurements of pore distribution and surface area showed that the macro-pores contributed most to total pore volume, whereas the micro-pores contributed to total surface area.

• Geotechnical Engineering
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• v.13 no.5
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• pp.155-168
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• 1997

The phenomena of detachment and movement of One particles are one of the important mechanisms both in geotechnical and geoenvironmental engineering. In geoenvironmental engineering, in particular, movement of fine particles may facilitate the transport of contaminants since the particle surfaces absorb contaminants before movement. Weathered granitic residual soils, which are the most abundant in Korea. contain large quantities of fine particles up to 50%. The characteristics of fine particle movement of weathered granitic residual soils are investigated in this paper. Samples are obtained from Poiiong, Shinnaedong in Seoul and Andong in Kyungpook : each of the samples represents typical residual soil types in Korea. Laboratory experiments for the three adopted soil types are performed. It is found that effluent concentration of the samples is influenced by porosity, fine particle percentage and particle size distribution. The critical velocity decreases as the fine particle percentage increases and the rate of change of erosion rate increases as the porosity increases. And well-graded samples showed less effluent concentrations compared to poorly-graded samples. The governing equation on the physical mechanism of fine particle movement and its nomerical solution scheme are suggested on the basis of the test results.

• Journal of Advanced Marine Engineering and Technology
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• v.28 no.8
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• pp.1270-1278
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• 2004

This paper describes the effectiveness of laser pulse shaping in eliminating weld defects such as porosity, cracks and undercuts in pulsed Nd:YAG laser welding. A large porosity was formed in a keyhole mode of deeply penetrated weld metal of any stainless steel. Solidification cracks were present in STS 310S with above 0.017%P and undercuts were formed in STS 303 with about 0.3%S. The conditions for the formation of porosity were determined in further detail in STS 316. With the objectives of obtaining a fundamental knowledge of formation and prevention of weld defects, the fusion and solidification behavior of a molten puddle was observed during laser spot welding of STS 310S through a high speed video photographing technique. It was deduced that cellular dendrite tips grew rapidly from the bottom to the surface, and consequently residual liquid remained at the grain boundaries in wide regions and enhanced the solidification cracking susceptibility. Several laser pulse shapes were investigated and optimum pulse shapes were proposed for the reduction and prevention of porosity and solidification cracking.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 1996.06a
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• pp.35-46
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• 1996

The powder forging process offers beneficial material utilization as well as the minimization of finishing operations over that of conventionally forged rods. In the present work, the sintering behavior of Fe-2Cu-0.6C-0.35MnS, optimum preform design and forgeability of various forging conditions were investigated. This data were generated using a newly proposed sub-scaled con-rod specimen developed specifically to simulate the powder forging process. The results of present work, powder perform is so difficult to flow material into die cavity and mass flow has no effect on improving the strength. And, applied force to increase density of the specimen flowed material is greater than that of all repessing mode. On the contrary, the specimen flowed material became increased hardness of inside in contrast with all repressing mode, but the tensile strength were decreased with residual porosity in surface. Due to material flow characteristic of powder preform, the section of lower density in powder preform became also lower density in forged con-rod. So, preform design is very important in manufacturing powder forged connecting rod.

• Journal of the Korean Ceramic Society
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• v.44 no.7
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• pp.381-386
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• 2007

A porous reaction bonded silicon carbide (RBSC) was fabricated by a molten Si infiltration method. The porosity and flexural strength of porous RBSC fabricated in this study were dependent upon the amount of carbon source used in the SiC/carbon preform as well as the amount of Si infiltrated into the SiC/carbon preform. The porosity and flexural strength of porous RBSC were in the range of $20 vo1.{\sim}49 vo1.%$ and $38{\sim}61 MPa$, respectively. With increase of carbon contents and molten Si for infiltration, volume fraction of the pores was gradually decreased, and flexural strength was increased. The porous RBSCs fabricated with the same amount of molten Si show less residual Si around neck with increase of carbon source, as well as a new SiC was formed around neck which resulted in the decreased porosity and improvement of the flexural strength. In addition, decrease of the porosity and increase of the flexural strength were also obtained by increase of the amount of molten Si with the same amount of carbon source. However, it was found that the flexural strength of porous RBSC depends on the porosity rather than the amount of the newly formed SiC in neck phase between SiC particles used as a starting material.

• The Journal of Engineering Geology
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• v.15 no.1
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• pp.29-39
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• 2005

For detecting a ground contamination survey, soil sampling method have been used a drilling or coring technique in general. However these methods are very difficult to systematically real-time monitoring of variation of contamination degree in field. ]'n this research frequency Domain Reflectometry (FDR) system was suggested and carried out to experimental approaches for determination of oil contamination on surface and underground. Experimental method using FDR method was discussed with feasibility of measurement in the laboratory column test. It is determined to degree of oil contamination due to response of dielectric constant re-lated with volumetric water content(θ/sub w/) and volumetric oil content( θ/sub al/ ) of saturated and unsaturated soil media. And physical properties such as effective porosity and oil residual ratio of saturated soil media were also measured through real-time monitoring works using installed FDR measurement sensors, which are defected characteristics of oil movement in the saturated soil media under the soil column tests. In the results of these experiments, a range of effective porosity was estimated to about 0.35 compared with initial porosity 0.40 of manufactured saturated soil media, which is also calculated to about 87.5% to the ratio of initial porosity to effective porosity. Finally oil residual ratio which is compared with volumetric water content and volumetric oil content was calculated about 62.5%.