• Proceedings of the Korea Concrete Institute Conference
• /
• 2004.05a
• /
• pp.336-339
• /
• 2004

This paper presents an experimental investigation examining water-cement ratio effects on fiber-matrix interface properties and on matrix fracture properties, which are used for designing mix proportion suitable for achieving strain-hardening behavior at a composite level. A single fiber pullout test and a wedge splitting test were employed to measure the bond properties in a matrix and the fracture toughness of mortar matrix, respectively. Test results showed that the properties tended to increase with decreasing water-cement ratio. Composite design using these test results will be discussed in the follow-up paper.

• Journal of Powder Materials
• /
• v.12 no.4 s.51
• /
• pp.303-308
• /
• 2005

Various kinds of Mg-Zn-Ca base alloys were rapidly quenched via melt spinning process. The meltspun ternary and quaternary alloy ribbons were heat-treated, and then the effects of additional elements on age hardening behavior and phase change of precipitates were investigated using Vickers hardness tester, XRD, and TEM equipped with EDS system. In ternary alloys, age hardening was mostly due to the distribution of $Mg_6Ca_2Zn_3$ and $Mg_2Ca$. The stable phases of precipitates were varied according to the aging temperature and the alloy composition. With the increase of Ca content, $Mg_2Ca$ precipitates were detected more than $Mg_6Ca_2Zn_3$ precipitates. In quaternary alloys, the precipitates taken from Mg-Zn-Ca-Co were identified as new quaternary phase, whereas those taken from Mg-Zn-Ca-Zr as MgZnCa containing Zr. In general, the ternary alloy showed higher peak hardness and thermal stability than the quaternary considering the total amounts of the solutes. It implies that the structure of precipitate should be controlled to have the coherent interface with the Mg matrix.

• Proceedings of the Korean Society of Agricultural Engineers Conference
• /
• 2003.10a
• /
• pp.383-386
• /
• 2003

In this study, the MMA-modified paste mixed waste rubber powder, which has a small elastic modulus and a large modification, was produced by using the soft unsaturated polyester resin(UP) as a binder. Then the adhesive properties according to the matrices in both underwater and air-dry conditions and the hardening shrinkage according to the contents of shrinkage reducing agent(SRA) and of MMA were surveyed. The experimental results show that, regardless of humidity of matrices the adhesive strength of polymer concrete was larger than cement concrete. the adhesive strength of MMA content of 20% was larger than MMA content of 30%. regardless of matrix materials the adhesive strength in water condition were $20{\sim}30%$ comparing with the air-dry condition. The case of MMA content of 20% showed the largest adhesive strength. In the hardening shrinkage experiment, the hardening shrinkage reduced as MMA and SRA contents increased, and the decrease of the hardening shrinkage by SRA was larger.

• Proceedings of the Korean Society for Technology of Plasticity Conference
• /
• 2006.05a
• /
• pp.149-152
• /
• 2006

Dry sliding wear behavior of low carbon dual phase steel, of which microstructure consists of hard martensite in a ductile ferrite matrix, has been investigated. The wear characteristics of the dual phase steel was compared with that of a plain carbon steel which was normalized at $950^{\circ}C$ for 30min and then air-cooled. Dry sliding wear tests were carried out using a pin-on-disk type tester at various loads of 1N to 10N under a constant sliding speed condition of 0.2m/sec against an AISI 52100 bearing steel ball at room temperature. The sliding distance was fixed as 1000m for all wear tests. The wear rate was calculated by dividing the weight loss measured to the accuracy of $10^{-5}g$ by the specific gravity and sliding distance. The worn surfaces and wear debris were analyzed by SEM, EDS and a profilomter. Micro vickers hardness values of the cross section of worn surface were measured to analyze strain hardening behavior underneath the wearing surfaces. The were rate of the dual phase steel was lower than the plain carbon steel. Oxidation on the sliding surface and strain hardening were attributed for the higher wear resistance of the dual phase steel.

• Journal of Powder Materials
• /
• v.19 no.1
• /
• pp.13-18
• /
• 2012

This study has been performed on the full density sintering of Fe nanopowder and the surface hardening by plasma ion nitriding. The Fe sintered part was fabricated by pressureless sintering of the Fe nanopowder at $700^{\circ}C$ in which the nanopowder agglomerates were controlled to have 0.5-5 ${\mu}m$ sized agglomerates with 150 nm Fe nanopowders. The green compact with 46% theoretical density(T.D.) showed a homogeneous microstructure with fine pores below 1 ${\mu}m$. After sintering, the powder compact underwent full densification process with above 98%T.D. and uniform nanoscale microstructure. This enhanced sintering is thought to be basically due to the homogeneous microstructure in the green compact in which the large pores are removed by wet-milling. Plasma ion nitriding of the sintered part resulted in the formation of ${\gamma}$'-$Fe_4N$ equilibrium phase with about 12 ${\mu}m$ thickness, leading to the surface hardening of the sintered Fe part. The surface hardness was remarkably increased from 176 $H_v$ for the matrix to 365 $H_v$.

• Geomechanics and Engineering
• /
• v.36 no.4
• /
• pp.355-366
• /
• 2024

The original elastoplastic Hardening Soil model is formulated actually partly under hexagonal pyramidal Mohr-Coulomb failure criterion, and can be only used in specific stress paths. It must be completely generalized under Mohr-Coulomb criterion before its usage in engineering practice. A set of generalized constitutive equations under this criterion, including shear and volumetric yield surfaces and hardening laws, is proposed for Hardening Soil model in principal stress space. On the other hand, a Mohr-Coulumb type yield surface in principal stress space comprises six corners and an apex that make singularity for the normal integration approach of constitutive equations. With respect to the isotropic nature of the material, a technique for processing these singularities by means of Koiter's rule, along with a transforming approach between both stress spaces for both stress tensor and consistent stiffness matrix based on spectral decomposition method, is introduced to provide such an approach for developing generalized Hardening Soil model in finite element analysis code ABAQUS. The implemented model is verified in comparison with the results after the original simulations of oedometer and triaxial tests by means of this model, for volumetric and shear hardenings respectively. Results from the simulation of oedometer test show similar shape of primary loading curve to the original one, while maximum vertical strain is a little overestimated for about 0.5% probably due to the selection of relationships for cap parameters. In simulation of triaxial test, the stress-strain and dilation curves are both in very good agreement with the original curves as well as test data.

• Korean Journal of Materials Research
• /
• v.10 no.6
• /
• pp.409-414
• /
• 2000

Three different white cast irons alloyed with Cr, V, Mo and W were prepared in order to study their transformation behavior of matrix structures in heat-treated conditions. The specimens were produced using a 15kg-capacity high frequency induction furnace. Melts were super-heated to $1600^{\circ}C$, and poured at $1550^{\circ}C$ into Y-block pepset molds. Three combinations of the alloying elements were selected so as to obtain the different types of carbides and matrix structures : 3%C-10%Cr-5%Mo-5%W(alloy No. 1), 3%C-10%V-5%Mo-5%W(alloy No. 2) and 3%C-17%Cr-3%V(alloy No. 3). The heat-treatments were conducted as follows: frist of all, as-cast specimens were homogenized at $950^{\circ}C$ for 5h under the vacuum atmosphere. Then, they were austenitized at $1050^{\circ}C$ for 2h and followed by air-hardening in air. The air-hardened specimens were tempered at $300^{\circ}C$ for 3h. The observation of morphology of the matrix structures was carried out in the states of as-cast(AS), air-hardened(AHF) and tempered(AHFT). The matrix structures of each alloy were almost fully pearlitic in the as-cast state but it was transformed to martensite, tempered martensite and retained austenite by the heat-treatments such as air-hardening and tempering.

• Proceedings of the Korean Society for Technology of Plasticity Conference
• /
• 2003.10a
• /
• pp.301-301
• /
• 2003

In current study, Nanocomposites are reinforced with carbon nanofiber, carbon nanotube and SiC, etc. Since the nano reinforcements have the excellent mechanical, thermal and electrical properties compared with that of existing composites, it has lately attracted considerable attention in the various areas. Cu have been widely used as signal transmission materials for electrical electronic components owing to its high electrical conductivity. However, it's size have been limited to small ones due to its poor mechanical properties. Until now, strengthening of the copper alloy was obtained either by the solid solution and precipitation hardening by adding alloy elements or the work hardening by deformation process. Adding the alloy elements lead to reduction of electrical conductivity. In this aspect, if carbon nanofiber is used as reinforcement which have outstanding mechanical strength and electric conductivity, it is possible to develope Cu matrix nanocomposite having almost no loss of electric conductivity. It is expected to be innovative in electric conducting material market. The unidirectional alignment of carbon nanofiber is the most challenging task developing the cooer matrix composites of high strength and electric conductivity. In this study, the unidirectional alignment of carbon nanofibers which is used reinforced material are controlled by drawing process and align mechanism as well as optimized drawing process parameter are verified via numerical analysis. The materials used in this study were pure copper and the nanofibers of 150nm in diameter and of 10∼20$\mu\textrm{m}$ in length. The materials have been tested and the tensile strength was 75MPa with the elongation of 44% for the copper. it is assumed that carbon nanofiber behave like porous elasto-plastic materials. Compaction test was conducted to obtain constitutive properties of carbon nanofiber Optimal parameter for drawing process was obtained by analytical and numerical analysis considering the various drawing angles, reduction areas, friction coefficient, etc. The lower drawing angles and lower reduction areas provides the less rupture of co tube is noticed during the drawing process and the better alignment of carbon nanofiber is obtained.

• Journal of Pharmaceutical Investigation
• /
• v.19 no.4
• /
• pp.195-202
• /
• 1989

In attempt to improve the chemotherapeutic activity of adriamycin, adriamycin-entrapped HSA microspheres were prepared and investigated by the various in vitro experiments. The shape, surface characteristics and size distribution of HSA microspheres are observed by scanning electron microscopy. The in vitro drug release, albumin matrix degradation by protease of HSA microspheres were studied. The shape of HSA microspheres were spherical and the surface was smooth and compact. The size of HSA microspheres ranged from 0.4 to $2.5\;{\mu}m$ and have average diameters of 0.5 to $0.7\;{\mu}m$. The size distribution of HSA microspheres prepared by ultrasonication was mainly affected by albumin concentration and heating time in the process of hardening. In in vitro, almost all adriamycin was released from HSA microspheres for 8 hr. Analysis of the resulting adriamycin release profiles demonstrated that adriamycin is released from the microspheres in two distinct steps, a fast phase (until 30 min) followed by a much slower sustained release phase. Drug release, which is due to diffusion, was depended on the rate of matrix hydration. Drug release was largely affected by albumin concentration and heating temperature during the process of hardening. Albumin matrix degradation of HSA microspheres was affected by heating temperature and albumin concentration. Higher temperature and longer times generally produce harder, less porous, and slowly degradable microspheres.

• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.23 no.11 s.170
• /
• pp.1970-1981
• /
• 1999

The low-cycle fatigue behaviour of a SiC-particulate-reinforced Al-Si cast alloy with two different volume fractions has been investigated from a series of strain-control led fatigue tests with zero and nonzero tensile mean strains. The composites including the unreinforced matrix alloy, exhibited cyclic hardening behaviour, with more pronounced strain-hardening for the composites with a higher volume fraction of the SiC particles. For the tensile mean strain tests, the initial high tensile mean stress relaxed to zero for the ductile Al-Si alloy, resulting in no influence of the tensile mean strain on the fatigue life of the matrix alloy. However, tensile mean strain for the composite caused tensile mean stresses and reduced fatigue life. The pronounced effects of mean strain on the low-cycle fatigue life of the composite compared to the unreinforced matrix alloy were attributed to the initial large prestrain and non-relaxing high tensile mean stress in the composite with very limited ductility and Cyclic plasticity. Fatigue damage parameter using strain energy, density efficiently accounted for the mean stress effects. Predicted fatigue life using the damage parameter correlated fairly well with the experimental life within a factor of 3. Also, the fatigue damage parameter indicated the inferior life in the low-cycle regime and superior life in the high-cycle regime for the composite, compared to the unreinforced matrix alloy.