• Journal of the Korean Society of Safety
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• v.32 no.5
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• pp.62-68
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• 2017

It is necessary to analyze on the compressive strength among material properties of concrete for confirming damages of architectures due to large explosion. A non destructive test is known as the representative methods estimating compressive strength and ultrasonic pulse velocity, rebound hardness test are widely used because of their simplicity, convenience. But combined method supplementing two types is applied at now as they are affected by the characteristics of test specimen. In this research to check damages on the members of structure before and after explosion, the characteristics of compressive strength are compared and analyzed through a real explosion test prior to full scale structures. The test results showed that the larger the TNT powder and the shorter the distance, the greater the decrease in strength before and after the explosion and that the largest displacement and moment for the explosive load and the greatest decrease in the strength at the central part. Due to the surface condition and the thickness variation of the concrete specimens, the standard deviation value is the smallest in the combining method of fusion of the ultrasonic method and rebound hardness method. Thus, the combining method can be one of appropriate methods to evaluate the strength in the reinforced concrete structures damaged by the explosion.

• Journal of the Korean Society for Nondestructive Testing
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• v.20 no.5
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• pp.438-444
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• 2000

The prediction of mechanical properties for WC-Co alloys by evaluation of coercive force and magnetic saturation were studied in relation to their microstructure. The WC-8%Co alloys were prepared using different WC particle size, carbon content and various sintering temperature by PM process. The magnetic properties such as coercive force and magnetic saturation of sintered WC-Co alloys were critically dependent upon their final composition and microstructure. Slight changes of carbon contents and small variation of WC grain size result in marked changes of magnetic properties, hardness and transverse rupture strength of sintered WC-Co alloys. It was found that the coercive force and hardness were increased by fine WC grain size of sinterd alloys, and the coercive force was proportional to hardness. With decreasing total carbon content below the stoichiometric value in WC-8%Co alloys the volume fraction of $\eta$ phase increased steadily, while the magnetic saturation and transverse rupture strength decreased. The magnetic saturation was inversely proportional to the coercive force of WC-Co alloys.

• Journal of the Korean Society for Heat Treatment
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• v.10 no.4
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• pp.255-265
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• 1997

Precipitation and strengthening mechanisms in squeeze cast Mg-8.5wt%Li-4.5wt%Al have been investigated by differential scanning calorimetry(DSC), scanning electron microscopy(SEM), in-situ and ex-situ X-ray diffraction analysis and hardness measurement. Special emphasis was placed on the investigation of the precipitation behavior by the DSC technique. Microstructural and calorimetric analysis showed that ${\theta}$ and ${\delta}$ precipitates in the b.c.c. ${\beta}$ phase matrix, forming two exothermic peaks at the temperature ranges of $130^{\circ}C{\sim}180^{\circ}C$ and $236^{\circ}C{\sim}280^{\circ}C$. ${\theta}$ and ${\delta}$ dissolve into the matrix forming an endothermic peak at the temperature range of $280^{\circ}C{\sim}352^{\circ}C$. The as-cast microstructure consists of ${\alpha}$, ${\beta}$ and ${\delta}$. Peak strength was obtained after aging for 1 hour at $50^{\circ}C$. The aging time required for the peak strength decreased as the aging temperature increases. The hardness decrease during overaging was due to the coarsening of ${\theta}$ precipitates. Microhardness measurement showed that variation of the hardness of ${\beta}$ matrix was more pronounced than that of the ${\alpha}$ phase, indicating that the ${\beta}$ phase is more responsible for the strengthening of the Mg-8.5wt%Li-4.5wt%Al alloy.

• Journal of the Korean Society for Heat Treatment
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• v.11 no.4
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• pp.333-341
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• 1998

The study was investigated on the effect of austenitizing and austempering conditions on retained austenite amount and carbon contents in retained austenite and simultaneously the effect of these variation on hardness, tensile and impact properties. A material of as-cast condition is composed of bull's eye structure with ferrite surrounding spheroidized graphite having about $5-10{\mu}m$ size and matrix structure of pearlite. Then, the contents of spheroidized graphite was about 5%. The retained austenite and carbon contents in the retained austenite were increased with the increasing of austenitizing and austempering temperatures, while the retained austenite showed the peak value and is decreased with increasing of austempering time. With increasing of austenitizing temperature, tensile strength, elongation and impact absorb energy increased and hardness was almost not changed, while with increasing of austempering temperature, tensile strength and hardness decreased, whereas elongation and impact absorb energy was increased. With increasing of retained austenite amount, the tensile strength is slowly decreased but elongation was increased with direct proportion. Also, Impact absorb energy is shown identity value untile about 18%, but rapidly increased above it. Elongation and Impact absorb energy are strongly controlled by the amount of retained austenite, but tensile strength is affected with various factors such as retained austenite amount and bainitic morphology.

• Korean Journal of Materials Research
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• v.30 no.9
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• pp.465-473
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• 2020

For surface hardening of a continuous casting mold component, a fundamental metallurgical investigation on dissimilar laser clads (Cu-NiCrBSi) is performed. In particular, variation behavior of microstructures and mechanical properties (hardness and wear resistance) of dissimilar clads during long-term service is clarified by performing high-temperature post-clad heat treatment (temperature range: 500 ~ 1,000 ℃ and isothermal holding time: 20 ~ 500 min). The microstructures of clad metals (as-clads) consist of fine dendrite morphologies and severe microsegregations of the alloying elements (Cr and Si); substrate material (Cu) is clearly confirmed. During the post-clad heat treatment, the microsegregations are totally homogenized, and secondary phases (Cr-based borides and carbides) precipitated during the short-term heat treatment are also almost dissolved, especially at the heat treatment conditions of 950 ℃ for 500 min. Owing to these microstructural homogenization behaviors, an opposite tendency of the surface mechanical properties can be confirmed. In other words, the wear resistance (wear rate) improves from 4.1 × 10^-2 ㎣/Nm (as-clad condition) to 1.4 × 10^-2 ㎣/Nm (heat-treated at 950 ℃ for 500 min), whereas the hardness decreases from 453 HV (as-clad condition) to 142 HV (heat-treated at 950 ℃ for 500 min).

• Korean Journal of Materials Research
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• v.27 no.1
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• pp.12-18
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• 2017

Transparent organic-inorganic hybrid hard coating films were prepared by the addition of $SiO_2$ or $ZrO_2$, as an inorganic filler to improve the hardness property, filler was highly dispersed in the acrylic resin. To improve the compatibility in the acrylic resin, $SiO_2$ or $ZrO_2$ is surface-modified using various silanes with variation of the modification time and silane content. Depending on the content and kind of the modified inorganic oxide, transparent modified inorganic sols were formulated in acryl resin. Then, the sols were bar coated and cured on PET films to investigate the optical and mechanical properties. The optimized film, which has a modified $ZrO_2$ content of 4 wt% markedly improved in terms of the hardness, haze, and transparency as compared to neat acrylate resin and acrylate resin containing modified $SiO_2$ content of 8 wt%. Meanwhile, the low transparency and high haze of these films slowly appeared at $SiO_2$ content above 10 wt% and $ZrO_2$ content of 5 wt%, but the hardness values were maintained at 2H and 3H, respectively, in comparison with the HB of neat acrylate resin.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.19 no.6
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• pp.579-585
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• 2006

We fabricated BSCCO-2212 (2212) rod by the melt casting process (MCP) and evaluated the effect of the melt flowing on the critical current ($I_c$) by using vertical and tilt casting. It was observed that the $2212-SrSO_4$ rod processed by the tilt casting method with homogeneous pre-heating temperature of the mold had a higher $I_c$ than that processed by the vertical casting method. We also evaluated the influence of the strontium sulfates ($SrSO_4$) addition on the texture, microstructure, critical current and temperature, and mechanical hardness of the $2212-SrSO_4$ rods. It was observed that the addition of $SrSO_4$ improved the critical current ($I_c$) and mechanical hardness of the 2212. The $I_c$ of the 2212 increased as the $SrSO_4$ content increased and reached a peak value (260 A at 77 K) at an $SrSO_4$ content of 6 wt.%. In addition, the addition of $SrSO_4$ had a beneficial effect on the mechanical hardness of the 2212. We studied the possible cause of the variation in the $I_c$ with the melt flowing and the $SrSO_4$ content based on the XRD, EPMA analysis and the microstructural observation.

• Journal of the Korean Society for Heat Treatment
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• v.29 no.5
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• pp.220-226
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• 2016

The mechanical properties of Ti-6Al-4V can be improved by microstructural control through the heat treatment in ${\alpha}+{\beta}$ region. The heat treatment was carried out with a variety of heat treatment temperatures and holding times to find the optimized heat treatment conditions and it was analyzed by linking the microstructural characteristics and mechanical properties. The part of ${\beta}$ phase with $10{\pm}2wt%$ vanadium was transformed into ${\alpha}^{{\prime}{\prime}}$ martensite phase after quenched, so the hardness and tensile properties were decreased below $900^{\circ}C$. The higher the heat treatment temperature is, the smaller is the vanadium-rich region, which leads to transformation into hcp ${\alpha}^{\prime}$ martensite above $900^{\circ}C$. The hardness and tensile properties were improved due to the hard ${\alpha}^{\prime}$ martensite. As the holding times were longer, the hardness and tensile properties decreased below $900^{\circ}C$ because of the softening effect by the grain growth. When varying the holding times above $900^{\circ}C$, the change of mechanical properties was slight because the softening effect of grain growth and the strengthening effect of ${\alpha}^{\prime}$ phase were counteractive. Therefore, the best conditions of heat treatment, which is in the range of $920{\sim}960^{\circ}C$, 40 min, WQ, can effectively improve the mechanical properties of Ti-6Al-4V.

• Geomechanics and Engineering
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• v.33 no.1
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• pp.29-40
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• 2023

Xanthan gum and starch compound biopolymer (XS), an environmentally friendly soil-binding material produced from natural resources, has been suggested as a slope protection material to enhance soil strength and erosion resistance. Insufficient wet strength and the consequent durability concerns remain, despite XS biopolymer-soil treatment showing high strength and erosion resistance in the dried state, even with a small dosage of soil mass. These concerns need to be solved to improve the field applicability and post-stability of this treatment. This study explored the utilization of an alkaline-based cation crosslinking method using calcium hydroxide and sodium hydroxide to induce non-thermal gelation, resulting in the enhancement of the wet strength and durability of biopolymer-treated soil. Laboratory experiments were conducted to assess the unconfined compressive strength and cyclic wetting-drying durability performance of the treated soil using a selected recipe based on a preliminary gel formation test. The results demonstrated that the uniformity of the gel structure and gelling time varied depending on the ratio of crosslinkers to biopolymer; consequently, the strength of the soil was affected. Subsequently, site soil treated with the recipe, which showed the best performance in indoor assessment, was implemented on the field slope at the bridge abutment via compaction and pressurized spraying methods to assess feasibility in field implementation. Moreover, the variation in surface soil hardness was monitored periodically for one year. Both slopes implemented by the two construction methods showed sufficient stability against detachment and scouring, with a higher soil hardness index than the natural slope for a year.

• Journal of Korean Society of Environmental Engineers
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• v.27 no.4
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• pp.362-368
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• 2005

The pH, alkalinity and calcium hardness could be adjusted by $CO_2$, $Ca(OH)_2$, and $Na_2CO_3$ addition in the water treatment process for corrosion protection of the water pipes. This research was performed to investigate the effect on the variation of water quality on the unit process by addition $CO_2$, $Ca(OH)_2$, and $Na_2CO_3$ in water treatment process. Carbon dioxide and lime were added before the coagulation basin and soda ash was added after the BAC process. pH and aklainity were increased at coagulation basin then after the water qualities had sustained similiarly to BAC process. There was no effect on turbidity and DOC removal efficiency during experimental period by addition\ $CO_2$, $Ca(OH)_2$, and $Na_2CO_3$ solution was added into clear well, the last process for optimum control of CCPP and is used mainly to control pH and alkalinity. In this research, average pH, alkalinity, and calcium hardness in treated water were 8.39, 61.4 mg/L as $CaCO_3$, 59.4 mg/L as $CaCO_3$, respectively and CCPP of treated water was higher than 29.5 mg/L to BAC process water, so adjusted water was expected to prevent internal corrosion of water pipe.