• Computers and Concrete
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• v.21 no.4
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• pp.367-376
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• 2018

This study reports the results of nondestructive monitoring of macro void developments in mortars manufactured with both ordinary Portland cement and sulfate resistant cement. Two types of curing were utilized; tap water curing and another curing environment that contains 5% $Na_2(SO_4)$ solution. Being the primary objective of this study, macro void developments of the mortar specimens were monitored by X-ray Medical Computerized Tomography. Compressive strength tests and water absorption tests were conducted on specimens that were kept in both curing environments for a duration of 560 days. Data analyses yielded consistent results among the three tests used in this experimental study. Macro void ratios of mortars decreased at the beginning of experiments for a certain period; afterwards, macro void ratios increased. The objective of this study was accomplished as anticipated since X-CT image analysis was able to nondestructively monitor macro void development process in cement mortars.

• Journal of the Korean Wood Science and Technology
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• v.36 no.6
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• pp.13-22
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• 2008

Internal voids affecting the mechanical properties of wood composite were classified into two catagories and characteristics of voids were examined according to density variation of strandboard. The void distribution and content of strandboard according to board density were measured by X-ray computed tomography system and analized using image processing software. Prior to investigation, the densities of strandboard were measured by densitometer and the results were showed high correlation with conventional oven drying method. Based on the image analysis conducted on captured images by X-ray tomography, low resolution can be used to capture the macro-voids (between strand) but not the micro-voids (within strands). Intermediate resolution can be used to capture both the macro and the micro-voids and high resolution can be successfully used to capture the majority of the micro-voids. The content of macro-void was measured and content of micro-void was computed by corresponding related equation. The macro-void distribution can be successfully understood and void content can be correctly estimated through the results.

• Journal of the Korean Ceramic Society
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• v.43 no.10 s.293
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• pp.607-612
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• 2006

In VARTM (Vacuum Assisted Resin Transfer Molding) process, the permeability generally controls the filling time of the resin and it also affects the void characteristics of the fiber composite. In this study, carbon and glass fiber inter-layered hybrid composites (carbon fiber centered stack) with an epoxy matrix were fabricated by VARTM process and evaluated the resin flow and macro void characteristics. The permeability of glass fiber was higher than that of carbon fiber used in this study. Using Darcy's equation, the permeability of hybrid composites could be predicted and experimentally confirmed. After curing, the macro void content of hybrid composites was investigated using image analyzer. The calculated filling time was well agreed with experimental result and the void content was significantly changed in hybrid composites.

• Transactions of Materials Processing
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• v.21 no.5
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• pp.319-323
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• 2012

In order to predict the internal void closing behavior in open die forging process, multiple scale modeling has been developed and applied. The huge size difference between ingot and inner void makes it almost impossible to simultaneously model the actual loading conditions and the void shape. Multiple scale modeling is designed to integrate macro- and micro- models effectively and efficiently. The void closing behavior was simulated at 39 different locations in a large ingot during upsetting and cogging. The correlation between the closing behavior and variables such as effective plastic strain and maximum compressive strain was studied in order to find an efficient measure for predicting the soundness of the forging.

• Geomechanics and Engineering
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• v.18 no.1
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• pp.49-58
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• 2019

The effect of microstructure on the permeability of two saturated marine clays was studied through a series of falling head permeability tests and mercury intrusion porosimetry (MIP) tests. The key findings from this experimental study include the following results: (1) The permeability of undisturbed specimens is larger than that of reconstituted specimens at the same void ratio due to different soil fabrics, i.e., the pore size distributions (PSDs), even though they have the similar variation law in the permeability versus void ratio. (2) Different permeabilities of undisturbed and reconstituted specimens at the same void ratio are mainly caused by the difference in void ratio of macro-pores based on the MIP test results. (3) A high relevant relation between $C_k$ ($C_k$ is the permeability change index) and $e*_{10}$, can be found by normalizing the measured data both on undisturbed or reconstituted specimens. Hence, the reference void ratio $e*_{10}$, can be used as a reasonable parameter to identify the effect of soil fabric on the permeability of saturated soft clays.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.17 no.12
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• pp.511-517
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• 2016

Current oil-type potential transformers for trains are filled with insulating oil, which could have problems like explosions due to rising inner pressure during train operation. Therefore, mold and dry-type potential transformers are being developed to prevent explosions. One problem in manufacturing mold-type transformers is preventing void formation around the coiled core inside the mold during epoxy filling, which could cause an electrical spark. Micro voids can remain in the resin after filling, and macro voids can occur due to the structure shape. A transformer that is being developed has a cavity at the junction of the core and the coil for better performance, and when highly viscous epoxy flows inside the cavity channel, macro voids can form inside it. Therefore, in this study, the free-surface flow of the mold filling procedure was analyzed numerically by applying the VOF method. The results were used to understand the phenomena of void formation inside the cavity and to modify the process conditions to reduce voids.

• Geomechanics and Engineering
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• v.17 no.1
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• pp.19-30
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• 2019

Recent research on the behavior of silty sand tends to advocate the use of equivalent skeleton void ratio to characterize the density state of this type of soil. This paper presents an investigation to explore the physical meaning of the equivalent skeleton void ratio by means of DEM simulations for assemblies of coarse and fine particles under biaxial shear. The simulations reveal that the distribution pattern of fine particles in the soil skeleton plays a crucial role in the overall macroscopic response: The contractive response observed at the macro scale is mainly caused by the movement of fine particles out of the force chains whereas the dilative response is mainly associated with the migration of fine particles into the force chains. In an assembly of coarse and fine particles, neither all of the fine particles nor all of the coarse ones participate in the force chains to carry the external loads, and therefore a more reasonable definition for equivalent skeleton void ratio is put forward in which a new parameter d is introduced to take into account the fraction of coarse particles absent from the force chains.

• Journal of the Korean Geotechnical Society
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• v.27 no.11
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• pp.83-92
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• 2011

The random and heterogeneous pore structure is a significant factor that dominates physical and mechanical behaviors of soils such as fluid flow and geomechanical responses driven by loading. The characterization method using non-destructive testing such as micro X-ray CT technique which has a high resolution with micrometer unit allows to observe internal structure of soils. However, the application has been limited to qualitatively observe 2D and 3D CT images and to obtain the void ratio at macro-scale although the CT images contain enormous information of materials of interests. In this study, we constructed the 3D particle and pore structures based on sequentially taken 2D images of glass beads and quantitatively defined complex pore structure with void cell and void channel. This approach was enabled by implementing image processing techniques that include coordinate transformation, binarization, Delaunay Triangulation, and Euclidean Distance Transform. It was confirmed that the suggested algorithm allows to quantitatively evaluate the distribution of void cells and their connectivity of heterogeneous pore structures for glass beads.

• Journal of the Korea Concrete Institute
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• v.17 no.5 s.89
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• pp.829-834
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• 2005

A series of reinforced G109 type specimens was fabricated and pended with a 15 weight percent NaCl solution. Mix design variables included 1) two cement alkalinities (equivalent alkalinities of 0.32 and 1.08), 2) w/c 0.50 and 3) two rebar surface conditions (as-received and wire-brushed). Potential and macro-cell current between top and bottom bars were monitored to determine corrosion initiation time. Once corrosion was initiated, the specimen was ultimately autopsied to perform visual inspection, and the procedure included determination of the number and size of air voids along the top half of the upper steel surface. This size determination was based upon a diameter measurement assuming the air voids to be half spheres or ellipse. The followings were reached based upon the visual inspection of G109 specimens that were autopsied to date. First, voids at the steel-concrete interface facilitated passive film breakdown and onset of localized corrosion. Based upon this, the initiation mechanism probably involved a concentration cell with contiguous concrete coated and bare steel serving as cathodes and anodes, respectively. Second, the corrosion tended to initiate at relatively large voids. Third, specimens with wire-brushed steel had a lower number of voids at the interface for both cement alkalinities, suggesting that air voids preferentially formed on the rough as-received surface compared to the smooth wire brushed one.

• Journal of Welding and Joining
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• v.22 no.4
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• pp.43-49
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• 2004

Conventional method for electrode force application in resistance spot welding(RSW) processes is to use pneumatic cylinder. However, due to its inherent problems in pneumatic power system such as compressibility of air and poor transient response characteristics, new electrode force system with servo control are recently introduced in RSW machine. This machine is called “servogun”. The purpose of this study is to evaluate performance of servogun in case of spot welding of aluminum alloy. Aluminum alloy(A5052) sheets are spot welded using pneumatic gun and servogun. Both results are compared by means of macro cross-section etching test and tensile shear strength test. Numerous previous research have reported nugget with many voids and cracks are not uncommon defects in spot welds with aluminum alloy. The experimental results show similar defects in case of pneumatic gun. In contrast, use of servogun considerably reduced generation of voids and cracks. In case of step-wise increased forging force at the end of welding cycle with servogun, crack-free and void-free nuggets have been observed. The performance of servogun has been also verified by series of tensile shear test. Higher strength values have been achieved with servogun in comparison to that of pneumatic gun.