• Computers and Concrete
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• 제15권6호
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• pp.903-919
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• 2015

The flow of freshly mixed cement-based material shows thixotropy, which implies some difficulties on robust measurement of its rheological properties: The flow curve of thixotropic materials depends on the used protocol. For examples, higher viscosity is obtained when the rate of shear strain is more quickly increased. Even though precise measurement and modelling of the concrete rheology needs to consider the thixotropic effect, engineers in the concrete field prefer considering as a non-thixotropic Herschel-Bulkley fluid, even more simply Bingham fluid. That is due to robustness of the measurement and application in casting process. In the aspect of simplification, this papers attempts to mimic the thixoropic flow by the non-thixotropic Herschel-Bulkley model. Disregarding the thixotropy of cement based materials allows us to adopt the rheological concept in the field. An optimized protocol to measure the Bingham parameters was finally found based on the accuracy and reproducibility test of cement paste samples, which minimizes the error of simulation stemming from the assumption of non-thixotropy.

• Transactions of the Korean Society of Mechanical Engineers A
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• 제22권2호
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• pp.416-427
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• 1998

Unidirectional commingled-yarn-based carbon fiber(CF)/polyamide(PA) 6 composite was fabricated under molding pressures of 0.4, 0.6 and 1.0 MPa to study its flexural deformation and fracture behavior. Fiber/matrix interfacial bonding area became larger with an increase of molding pressure from 0.4 to 0.6 MPa. For molding pressures .geq. 0.6 MPa, good flexural performance of similar magnitudes was attained. For the fracture test, four kinds of notch direction were adopted : edgewise notches parallel (L) and transverse (T) to the major direction of fiber bundles, and flatwise notches parallel(ZL) and perpendicular(ZT) to this direction. Nominal bend strength for L and ZL specimens exhibited high sensitivity to notching. ZL specimens revealed the lowest values of the critical stress intensity factor $K_c$ which was slightly superior to those of unfilled PA6 matrix. Enlargement of the compression area for T specimens was analyzed by means of the rigidity reduction resulting from the fracture occurrence.

• Journal of the Korea Institute of Military Science and Technology
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• 제4권1호
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• pp.20-29
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• 2001

We recently developed a friability test procedure, which will be used as one of substance tests for Extremely Insensitive Detonating Substances(EIDSs) in Test Series 7 of the United Nations. This test was intended to assess the susceptibility of insensitive explosives to the break-up as high strain rate and subsequent ignition characteristics of the deformed material. We designed an air gun system using the hydro code simulation. The projectile velocities of the gun were in good agreement with those predicted by the hydro code with an inert material. Three different types of explosives, melt castable Comp B, castable plastic bonded explosives(PBXs), and pressable PBXs were tested during the development of the test procedure. Two tastable PBXs, i.e. DXD-09 and DXD-10, which are currently under development as candidate formulations of EIDS were classified as EIDS, since test results with these formulations were far better than the criterion of the UN Test Series 7.

• Korean Journal of Veterinary Research
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• 제14권1호
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• pp.91-97
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• 1974

A total of 820 dairy cattle were examined formastitis by California mastitis test method and 181 dairy cattle were examined bacteriologically according to the method of Connecticut mastitis control program. The results obtained were as followings: 1. Milk samples from 1.231(38.1%) of 3,225 quarters and 568(69.3%) of 820 dairy cattle were positive for mastitis by California mastitis test method. 2. Among 181 cattle examined bacteriologically by the method of Connecticut mastitis control program 159(22.9%) quarters of 102(56.4%) cattle were infected. Clinical mastitis was found in 5 cattle. 3. Important causative organisms were found to de Staphylococcus aureus(44.8%), Streptococcus agalactiae(21.5%), and non-str, agalactiae Streptococcus(26.4%). Corynebacterium ssp., Pseudomonas spp., Coliforms and Yeasts were also occasionally found to be causative organisms. 4. Strains of Staphylococcus aureus isolated from bovine udders were tested for sensitivity to antibiotics. The rates of resistant strain were 13.7%, 11.8%, 8.8%, 5.9%, 100%, 4.9% and 4.9% for penicillin G, streptomycin, terramycin, erythromycin, chloramphenicol, ampicillin and kanamycin, respectively.

• Transactions of Materials Processing
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• 제16권4호
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• pp.304-308
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• 2007

Failure phenomena in uni-axial tension test were experimentally and numerically investigated for AA6111-T4, AA5083-H18 and DP-Steel, which were friction-stir welded with the same and different thicknesses. Forming limit diagram(FLD) was measured using hemispherical dome stretching tests for base materials and also predicted by Hill's bifurcation and M-K theories for welded areas. Finite element simulations well predicted hardening behaviors, failure locations as well as failure patterns for the uni-axial tension tests especially utilizing very fine meshes and FLD along with stress softening.

• Bulletin of the Society of Naval Architects of Korea
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• 제27권4호
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• pp.72-82
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• 1990

A simple numerical analysis procedure has been proposed to trace the response of unstiffened offshore tubular members subjected to lateral impacts and eventually to estimate the consequential extent of damage. In the procedure a tubular member is reduced to a spring-mass system having two degrees-of-freedom. one for local denting deformation and the other for that of overall bending. Results of impact tests have been correlated with those of numerical analysis in order to achieve an empirical representation of the strain-rate sensitivity and other dynamic effects upon the spring coefficient for bending deformation. The theoretical estimates of extents of damage correlate reasonably well with those obtained in experiments.

• Korean Journal of Microbiology
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• 제16권3호
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• pp.93-102
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• 1978

Saccharomyces cerevisiae strain M was cultured in a molasses-containing media with repeated transplantations of the yeasts from one culture to another to adapt to molasses. After that only different amounts of phosphorous and nitrogen sources were added to the media. And then some variations during the culture time and the effects of consituents of cell mass on the functional activity and sensitivity of the cell were investigated. The results obtained were summarized as follows : 1. In the same culture condition of yeasts, the carbohydrates and trehaloses contents were more remarkably increased when small amounts of phosphate and nitrogen sources were added, then when alrge amounts were added, but yield percentage on assimilated sugars was lower. 2. The content of trehalose in yeast cells was reduced remarkably at the early stage in the culture, but this increased remarkably at later stage. When small amounts of nitrogen and phosphate were added to the culture medium, the amount of thehalose in the cells increased greasly. 3. The more protein content was present in the yeast cells, the smaller the carbohydrate and trehalose content, but more amino-N, RNA and moisture content were present in the cells. And in this case fermentability of the cells was stronger, but sugar tolerance was lower. 4. During the preservation period of compressed yeast cells at different temperature, the higher the temperature was, the more rapidly the amount of trehalose in the cells decreased. And in the cell where the amount of trehalose in the cells decreased. And in the cell where the amount of trehalose(carbohydrate) was large and the amount of protein was small, the amount of trehalose decreased at a slower rate during the preservation period.

• Transactions of Materials Processing
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• 제14권6호
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• pp.502-508
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• 2005

Hypereutectic Al-Si alloys have been regarded attractive for automotive and aerospace application, due to high specific strength, good wear resistance, high thermal stability, low thermal expansion coefficient and good creep resistance. Spray casting of hypereutectic Al-Si alloy has been reported to provide distinct advantages over ingot metallurgy (IM) or rapid solidification/powder metallurgy (RS/PM) process in terms of microstructure refinement. In this study, hypereutectic Al-25Si-2.0Cu-1.0Mg alloy was prepared by OSPREY spray casting process. The change of strain rate sensitivity and Creep transition were analyzed by using the load relaxation test and constant creep test. High temperature deformation behavior of the hypereutectic Al-Si alloy has been investigated by applying the internal variable theory proposed by Chang et al. Especially, the creep resistance of spray casted hypereutectic Al-Si alloy can be enhanced considerably by the accumulation of prestrain.

• Journal of the Korean Society for Precision Engineering
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• 제17권1호
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• pp.204-208
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• 2000

An approximate similarity theory has been applied to predict the forming load of non-axisymmetric forging of aluminum alloys through model material tests. The approximate similarity theory is applicable when strain rate sensitivity, geometrical size, and die velocity of model materials are different from those of real materials. Actually, the forming load of yoke, which is an automobile part made of aluminum alloys(Al-6061), is predicted by using this approximate similarity theory. Firstly, upset forging tests are have been carried out to determine the flow curves of three model materials and aluminum alloy(Al-6061), and a suitable model material is selected for model material test of Al-6061. And then hot forging tests of aluminum yokes have been performed to verify the forming load predicted from the model material, which has been selected from above upset forging tests. The forming loads of aluminum yoke forging predicted by this approximate similarity theory are in good agreement with the experimental results of Al-6061 and the results of finite element analysis using DEFORM-3D.

• Korean Journal of Metals and Materials
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• 제48권9호
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• pp.791-797
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• 2010

The creep crack growth rate (da/dt) of the Cr-Mo steels tested by pre-crack and the voltage (or resistance) variables were related into fracture parameter (Ct), crack growth coefficient (H), and an exponent (q) in the parts of Base, weld and HAZ. The fracture parameter (Ct) has various variables relating to the specimen and crack shape, applied stress, and creep strain curve. The H and q was inferred by OLS regression (ordinary least square method), and the H values were solved in statistics and probability assessment, which were attained fromPDF's distributions (probability density function). The HAZ part has the highest value of q by OLS regression and the widest distribution of H by PDF of WEIBULL, which means that the crack sensitivity of HAZ should be cautioned against the creep crack growth and failure.