• Journal of the Korean Society of Industry Convergence
• /
• v.26 no.6_2
• /
• pp.1011-1017
• /
• 2023

In this study, the harmless crack size (a_hml) by double shot peening (DSP) using shot balls with different diameters was evaluated on carburized, quenched-tempered SCM822H steel. The minimum crack size (a_NDI) detectable by non-destructive inspection was also evaluated. The relationship between the crack size (a_25,50) that reduces the fatigue limit by 25% and 50% and a_hml was evaluated. The residual stress of DSP was greater in SP_(0.6+0.08) than SP_(0.8+0.08) and appeared deeper in the depth direction. In addition, the hardness below the surface appeared larger. The fatigue limit of DSP increased 2.07 times and 1.95 times compared to non-SP. All a_hml of the DSP specimen was determined at the depth (a). The compressive residual stress distribution affects a_hml, and the a_hml of SP_(0.6+0.08), which has a large compressive residual stress and a high fatigue limit, appeared large. a_hml of SP_(0.6+0.08) introduced deeper than the residual stress of SP_(0.8+0.08) is larger in the range of As=1.0-0.3. Since the residual stress in the thickness direction has a greater effect on a_hml than the residual stress at the surface, it is necessary to introduce it more deeply. The relation of a_hml, a_25,50, and a_NDI were evaluated in the point for safety and reliability.

• Resources Recycling
• /
• v.12 no.6
• /
• pp.13-18
• /
• 2003

The initial NiCuZn synthetic ferrite were acquired from thermally decomposing the metal nitrates Fe($NO_3$)$_3$ㆍ$9H_2$O, Zn($NO_3$)$_2$ㆍ$6H_2$O, Ni($NO_3$3)$_2$ㆍ $6H_2$O and Cu(NO$_3$)$_2$ㆍ $3H_2$O at 1$50^{\circ}C$ for 24 hours and was calcined at $500^{\circ}C$. Each of those was pulverized for 3 and 9 hours in a steel ball mill and was sintered between $700^{\circ}C$ and $1,000^{\circ}C$ for 1 hour, and then their microstructures and magnetic properties were examined. We could make the initial specimens chemically bonded in liquid at the temperature as low as $150 ^{\circ}C$, by using the melting points less than $ 200^{\circ}C$ of the metal nitrates instead of the mechanical ball milling, then narrowed a distance between the particles into a molecular level, and thus lowed sintering temperature by at least $200 ^{\circ}C$ to $300^{\circ}C$ Their initial permeability was 50 to 490 and their saturation magnetic induction density and coercive force 2,400G and 0.3 Oe to 1.2 Oe each, which were similar to those of NiCuZn ferrite synthesized in the conventional process.

• Applied Chemistry for Engineering
• /
• v.8 no.5
• /
• pp.830-836
• /
• 1997

The surface modification of iron oxide particle produced from steel-pickled acid by sodium-contained materials was studied. The molar ratio of $SiO_2$ to $Na_2O$ of sodium silicate was 1, 2, 3.5, respectively. The dispersion stability of iron oxide suspension as functions of amount of silica and pH was evaluated by surface charge and sedimentation velocity of iron oxide particle. Then the amount of sodium silicate was determined to provide a dispersion stability of iron oxide particle above pH 7. Finally, the surface modification of iron oxide particle with sodium silicate as silica-contained materials was done by wet ball milling. In the results of study, the dispersion stability of silica modified iron oxide particle was largely depended on amount of silica and pH together. The untreated iron oxide was unstable at pH 8, i.e. isoelectric point, but, the surface modified iron oxide particle with 0.8wt% silica was stable above pH 5. The dispersion stability was enhanced with 0.2wt% of anionic polyelectrolyte.

• KCI Concrete Journal
• /
• v.13 no.2
• /
• pp.10-18
• /
• 2001

The spectral analysis of surface waves (SASW) method, which is an in-situ seismic technique, has mainly been developed and used for many years to determine the stiffness profile of layered media (such as asphalt concrete and layered soils) in an infinite half-space. This paper presents a modified experimental technique for nondestructive evaluation of in-place cement mortar compressive strength in single-layer concrete slabs of rather a finite thickness through a correlation to surface wave velocity. This correlation can be used in the quality control of early age cement mortar structures and in evaluating the integrity of structural members where the infinite half space condition is not met. In the proposed SASW field test, the surface of the structural members is subjected to an impact, using a 12 mm steel ball, to generate surface wave energy at various frequencies. Two accelerometer receivers detect the energy transmitted through the medium. By digitizing the analog receiver outputs, and recording the signals for spectral analysis, surface wave velocities can be identified. Modifications to the SASW method includes the reduction of boundary reflections as adopted on the surface waves before the point where the reflected compression waves reach the receivers. In this study, the correlation between the surface wave velocity and the compressive strength of cement mortar is developed using one 36"x36"x4"(91.44$\times$91.44$\times$91.44 cm) cement mortar slab of 2,000 psi (140 kgf/$\textrm{cm}^2$) and two 36"x36"x4"(91.44$\times$91.44$\times$91.44 cm) cement mortar slabs of 3,000 psi (210 kgf/$\textrm{cm}^2$).

• Proceedings of the Materials Research Society of Korea Conference
• /
• 2011.10a
• /
• pp.40.2-40.2
• /
• 2011

초경합금은 경도가 높은 재료를 말하며 일반적으로는 탄화텅스텐(WC)계 재료를 말한다. 국내 현재 초경합금 동향은 반도체 산업, 내마모성 공구, 절삭공구, 금형 등 많은 분야에 사용되어지고 있다. 또한 최근 들어 FSW (Friction Stir Welding, FSW)기술이 발전함에 따라 접합기술개발이 다양화되면서 FSW Tool의 고성능의 초경 재료가 요구되어지며 장수명의 Tool개발이 되어야 한다. 국내에서는 초경 합금 재료로 사용되어지고 있는 텅스텐 카바이드(WC)와 코발트(Co)를 이용하여 많은 연구가 진행되었다. 본 실험에서는 텅스텐 카바이드와 코발트 및 몰르브덴 카바이드를 혼합하여 소결체를 제조하였다. 실험에 사용된 텅스텐 카바이드는 높은 경도를 가지고 강한 취성을 나타내며, 소결에 어려운 단점이 있다. 이러한 단점을 코발트와 몰리브덴 카바이드를 첨가하여 소결온도를 낮춰주는 역할과 액상 소결시 텅스텐카바이드 입자사이에 침투하여 액상소결에 의한 치밀화가 가능하게 해주며 인성이 향상되어 고인성 재료를 만들 수 있었다. 본 실험에서는 합성과 치밀화가 동시에 진행되는 SPS (Spark Plasma Sintering:SPS) 장비를 이용하여 실험을 진행하였다. 이 방법은 방전플라즈마 소결 공법으로, 기존의 연소법과 열간 가압기술(Hot-press, HIP)을 결합한 방식으로 단 시간, 단일공정으로 치밀한 소결체를 얻을 수 있는 장점이 있다. 본 연구에서는 $WC-5Mo_2C$-5wt%Co 소결체 제조를 위해 원소 분말을 Horizontal ball milling 혼합하였다. 균일하게 혼합된 분말을 흑연다이에 충진하여 펄스전류와 기계적 압력을 동시에 가하여 $WC-5Mo_2C-5Co$ 복합재료를 제조하고 소결체의 밀도, 순도, 상변태, 미세조직 등을 분석 및 평가하였다. SPS공정 조건은 고진공하에서 $1,200^{\circ}C$-60MPa, 펄스비 12:1 조건으로 수행하였으며, 얻어진 $WC-5Mo_2C-5Co$ 소결체의 상대 밀도는 98%이상 이였다. 또한, 결정립 크기는 약 400 nm였으며, 경도는 $2,453kg/mm^2$를 나타내었다.

• Journal of KSNVE
• /
• v.5 no.4
• /
• pp.515-524
• /
• 1995

Tubes in heat exchanger of fuel rods in reactor core are supported at intemediate point by support p0lates or springs. Current practice is, in case of heat exchanger, to allow clearance between tube and support plate for design and manufacturing consideration. And in case of fuel rod the clearance in support point can be generated due to the support spring force relaxation. Flow-induced vibration of a tube can cause it to impact or rub against support plate or against adjacent tubes and can result in fretting-wear. The tube-to- support dynamic interaction is used to relate experimental wear data from single-span test rigs to real multi-span heat exchanger configurations. The dynamic interaction cna be measured during experimental wear tests. However, the dynamic interaction is difficult to measure in real heat exchangers and, therefore, analytical techniques are required to estimate this interaction. This paper describels the nonlinear impact model of DAGS(Dynamic Analysis of Gapped Structure) code which simulates the tube response to external sinusodial or step excitation and predicts tube motion and tube-to-support dynamic interaction. Three experimental measurements-two single span rods excited by sinusodial force and a two span rod impacted by a steel ball are compared from the simulation nonlinear model of DAGS code. The simulation results from DAGS code are in good agreement with measurements. Therefore, the developed model of DAGS code is good analytical tool for estimating tube-to-support dynamic interaction in real heat exchangers.

• Tribology and Lubricants
• /
• v.30 no.3
• /
• pp.146-155
• /
• 2014

In this study, we conduct fretting corrosion tests on tin-plated brass coupons to investigate the effect of temperature on fretting corrosion for various span amplitudes. We prepare a coupled fretting corrosion specimens using a tin-plated brass coupon with a thickness of $10{\mu}m$. One specimen is a flat coupon and the other specimen is a coupon with a protuberance in 1 mm radius, which is produced using 2 mm diameter steel ball. We conduct fretting corrosion tests at $25^{\circ}C$, $50^{\circ}C$, $75^{\circ}C$, $100^{\circ}C$ by rubbing the coupled coupons together at the contact between the flat and protuberance coupons. We measure electric resistance of the contact during the fretting corrosion test period. There is increase in resistance with fretting cycles. It is found that rate of increase in electric resistance becomes faster with increase in testing temperature. Magnitude of friction coefficient increases with fretting span amplitudes. And, change in friction coefficient becomes desensitized to the increment in span amplitude. Assuming that failure cycle is the cycle with an electric resistance of $0.01{\Omega}$, we find that failure lifetime ($N_f$) decreases with increase in testing temperature. Furthermore, based on the assumption that the damage rate of the connector is inversely related to the failure cycle, we calculate the activation energy for fretting damage to be 13.6 kJ/mole by using the Arrhenius equation. We propose a method to predict failure cycle at different temperatures for span amplitudes below $30{\mu}m$. Friction coefficients generally increase with increase in span amplitude and decrease in testing temperature.

• Journal of Powder Materials
• /
• v.20 no.4
• /
• pp.269-274
• /
• 2013

The present work investigated the dispersion behavior of $Y_2O_3$ particles into AISI 316L SS manufactured using laser cladding technology. The starting particles were produced by high energy ball milling in 10 min for prealloying, which has a trapping effect and homogeneous dispersion of $Y_2O_3$ particles, followed by laser cladding using $CO_2$ laser source. The phase and crystal structures of the cladded alloys were examined by XRD, and the cross section was characterized using SEM. The detailed microstructure was also studied through FE-TEM. The results clearly indicated that as the amount of $Y_2O_3$ increased, micro-sized defects consisted of coarse $Y_2O_3$ were increased. It was also revealed that homogeneously distributed spherical precipitates were amorphous silicon oxides containing yttrium. This study represents much to a new technology for the manufacture and maintenance of ODS alloys.

• International Journal of Highway Engineering
• /
• v.9 no.2 s.32
• /
• pp.89-99
• /
• 2007

The stiffness of the asphalt concrete is represented by the complex modulus $E^*$, which is very important properties in the mechanistic design of flexible pavement system. The moduli of asphalt concrete were generally determined by dynamic modulus test. However, the dynamic modulus testing method is too complex, expensive, and time consuming to be applicable on a production basis. The IR(Impact Resonance) method has been shown to be a truly simple nondestructive testing method which produces very repetitive, consistent results. The major object of this study was to estimate of the effects on IR tests for determining modulus of asphalt concrete including impact position, specimen support condition, impact steel ball size and sampling rate. The variations of IR test results with various testing conditions are within ${\pm}2.7%$.

• Journal of the Microelectronics and Packaging Society
• /
• v.24 no.1
• /
• pp.1-8
• /
• 2017

Fe-9.8Si-6.0Al mother alloy was manufactured using by Fe-3.5Si recycled scrap and Si powder. And then, soft magnetic alloy powder of $D_{50}$ size and sphere type were prepared by gas atomization process. To obtain the soft magnetic powder of a high aspect ratio, in the first, we conducted the ball milling process for 8 hours. And heat treatment was performed under $650^{\circ}C$, 2 hours and $N_2$ atmosphere condition for reducing the residual stress of the powder. Based on these process, we made around $50{\mu}m$ diameter Fe-9.8Si-6.0Al powder, which morphology and shape was a similar to the commercial Fe-Si-Al powder. Finally, the soft magnetic sheets were prepared by tape casting process using by those powders. The permeability of the tape casting sheet was measured, and we confirmed the possibility of reusing to the soft magnetic materials of Fe-Si electric sheet scrap.