• 열처리공학회지
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• 제36권2호
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• pp.61-68
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• 2023

Ti and Ti alloys are used in the automobile and aerospace industries due to their high specific strength and excellent corrosion resistance. However their application is limited due to poor formability at room temperature and high unit cost. In order to overcome these issues, dissimilarly jointed materials, such as cladding materials, are widely investigated to utilize them in each industrial field because of an enhanced plasticity and relatively low cost. Among various dissimilar bonding processes, the rolled cladding process is widely used in Ti alloys, but has a disadvantage of low bonding strength. Although this problem can be solved through post-heat treatment, the mechanical properties at the bonded interface are deteriorated due to residual stress generated during post-heat treatment. Therefore, in this study, the microstructure change and residual stress trends at the interfaces of Ti/Al cladding materials were studied with increasing post-heat treatment temperature. As a result, compared to the as-rolled specimens, no difference in microstructure was observed in the specimens after postheat treatment at 300, 400, and 500℃. However, a new intermetallic compound layer was formed between Ti and Al when post-heat treatment was performed at a temperature of 600℃ or higher. Then, it was also confirmed that compressive residual stress with a large deviation was formed in Ti due to the difference in thermal expansion coefficient and modulus of elasticity between Ti Grade II and Al 1050.

• 센서학회지
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• 제32권1호
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• pp.44-50
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• 2023

In this study, a polydimethylsiloxane (PDMS)-based drag force-type flowmeter was fabricated using a graphite-carbon nanotube (CNT) composite as a piezoresistive material and evaluated. The device was in the form of a cantilever, which was composed of the soft material, PDMS, and fabricated using a mold manufactured by a three-dimensional printer. The cost-effective graphite was mixed with CNTs to serve as a piezoresistive material. The optimal mixing ratio was investigated, and the piezoresistive material formed using a graphite:PDMS:CNT ratio of 1.5:1:0.01 was adopted, which showed a stable output and a high sensitivity. Various forward and backward air flows in the range of 0-10 m/s were measured using the fabricated flowmeter, and both tensile and compression characteristics were evaluated. The measured results showed a stable output, with the resistance change gradually increasing with the air flow rate. Repeatability characteristics were also tested at a repeated air flow of 10 m/s, and the flowmeter responded to the applied air flow well. Consequently, the fabricated device has a high sensitivity and can be used as a flowmeter.

• 열처리공학회지
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• 제36권5호
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• pp.303-310
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• 2023

β titanium alloys are widely used in aerospace industry due to their excellent specific strength and corrosion resistance. In particular, mechanical properties of metastable β titanium can efficiently be controlled by various deformation mechanisms such as slip, twinning, and SIM (Stress-Induced Martensite Transformation), making it an ideal material for many industrial applications. In this study, Ti-5Mo-xFe (x=1, 2, 4 wt%) alloy was designed by adding a relatively inexpensive β element to ensure price competitiveness. Additionally, microstructural analysis was conducted using OM, SEM, and XRD, while mechanical properties were evaluated through hardness and compression tests to consider the deformation mechanisms based on the Fe content. SIMT occurred in all three alloys and was influenced by the presence of β_m (metastable beta) and beta stability. As the Fe content decreased, the α'' phase increased due to SIMT occurring within the β_m phase, resulting in softening. Conversely, as the Fe content increased, the strength of the alloy increased due to a reduction in α'' formation and the contributions of solid solution strengthening and grain strengthening. Moreover, unlike the other alloys, shear bands were observed only in the fracture of the Ti-5Mo-4Fe alloy, which was attributed to differences in texture and microstructure.

• Korean Chemical Engineering Research
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• 제61권1호
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• pp.19-25
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• 2023

PEM (Proton Exchange Membrane) 수전해의 성능향상에 대해 많은 연구개발이 진행되었으나, 내구성에 대한 연구는 아직 초기 단계라고 할 수 있다. 본 연구는 성능향상을 위해 PEM 수전해 구동 온도를 상승시켰을 때, 수전해 내구성에 미치는 영향에 대해 연구하였다. 50~80 ℃ 온도 범위에서 일정 전류 조건으로 구동하면서 전압변화, I-V, CV (Cyclic Voltammetry), LSV (Linear Sweep Voltammetry), Impedance, FER (Fluoride Emission Rate) 등을 측정했다. 운전온도가 상승할수록 열화속도가 증가했다. 50~65 ℃에서는 IrO₂ 전극 촉매 열화가 PEM 수전해 셀의 내구성에 주로 영향을 주었다. 80 ℃에서는 고분자 막과 전극 열화가 비슷하게 진행되어 short 저항이 1.0 kΩ·cm² 이하로 감소하면서 shorting 현상에 의해 구동한지 144시간 만에 성능이 초기의 약 1/3로 감소하였다.

• 한국지반공학회논문집
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• 제26권4호
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• pp.37-45
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• 2010

기후변화로 인해 물의 증발량이 많은 지역의 지반에서는 자연적인 소금의 고결화 현상이 발생한다. 이러한 소금의 고결화 분포는 모래지반에서 모세관력에 영향을 받는다. 이 연구의 목적은 콘 관입시험, 전기전도율 측정, 사진 이미지 촬영, 비파괴 이미지 분석 그리고 탄성파에 의한 프로세스 모니터링을 이용하여 소금의 고결화 현상에 대한 모세관력의 효과를 관찰하는 것이다. 실험은 입상재료를 모형화한 글라스비즈를 소금물에 포화시킨 후, 오븐에 넣어 시료를 건조시킴으로써 고결화를 발생시켰다. 실험결과, 고결된 소금의 농도는 시료입경이 작은 경우 최상부에서 높았고, 시료입경이 큰 경우 중간 또는 하부에서 높았다. 고결된 시료에서 높은 소금농도의 위치는 해석적 방법으로 산정된 모세관 높이와 유사하였다. 본 연구에서 수행한 5가지 실험은 모세관력이 소금이 고결된 흙과 같은 입상재료의 거동에 중요한 영향을 끼치는 것을 보여준다.

• 열처리공학회지
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• 제36권3호
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• pp.128-136
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• 2023

In this study, the effects of heat treatment on the nano-scale creep behavior of CoCrFeMnNi high-entropy alloy (HEA) processed by high-pressure torsion (HPT) was investigated through nanoindentation technique. Nanoindentation experiments with a Berkovich indenter were performed on HPT-processed alloy subjected to heat treatment at 450℃, revealing that the hardness of the HPT-processed alloy (HPT sample) significantly increased with the heat treatment time. The heat treatment-induced microstructural change in HPT-processed alloy was analyzed using transmission electron microscopy, which showed the nano-sized Cr-, NiMn-, and FeCo-rich phases were formed in the HPT-processed alloy subjected to 10 hours of heat treatment (HPT+10A sample). To compare the creep behavior of HPT and HPT+10A samples, constant load nanoindentation creep experiments were performed using spherical indentation indenters with two different radii. It was revealed that the predominant mechanism for creep highly depended on the applied stress level. At low stress level, both HPT and HPT+10A samples were dominated by Coble creep. At high stress level, however, the mechanism transformed to dislocation creep for HPT sample, but continued to be Coble creep for HPT+10A sample, leading to higher creep resistance in the HPT+10A sample.

• 수산해양기술연구
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• 제59권1호
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• pp.44-54
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• 2023

In this study, the second-order Nomoto's nonlinear expansion model was implemented as a Tagaki-Sugeno fuzzy model based on the heading angular velocity to design the automatic steering system of a ship considering nonlinear elements. A Tagaki-Sugeno fuzzy PID controller was designed using the applied fuzzy membership functions from the Tagaki-Sugeno fuzzy model. The linear models and fuzzy membership functions of each operating point of a given nonlinear expansion model were simultaneously tuned using a genetic algorithm. It was confirmed that the implemented Tagaki-Sugeno fuzzy model could accurately describe the given nonlinear expansion model through the Zig-Zag experiment. The optimal parameters of the sub-PID controller for each operating point of the Tagaki-Sugeno fuzzy model were searched using a genetic algorithm. The evaluation function for searching the optimal parameters considered the route extension due to course deviation and the resistance component of the ship by steering. By adding a penalty function to the evaluation function, the performance of the automatic steering system of the ship could be evaluated to track the set course without overshooting when changing the course. It was confirmed that the sub-PID controller for each operating point followed the set course to minimize the evaluation function without overshoot when changing the course. The outputs of the tuned sub-PID controllers were combined in a weighted average method using the membership functions of the Tagaki-Sugeno fuzzy model. The proposed Tagaki-Sugeno fuzzy PID controller was applied to the second-order Nomoto's nonlinear expansion model. As a result of examining the transient response characteristics for the set course change, it was confirmed that the set course tracking was satisfactorily performed.

• 한국프랜차이즈경영연구
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• 제3권2호
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• pp.79-102
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• 2012

본 연구는 대형마트, 기업 형 슈퍼마켓 (SSM) 에 대한 출점 및 영업규제를 2009년 이전과 2009년 이후로 구분하여 연구하였다. 대형마트, 기업 형 슈퍼마켓 입장에서 영업 및 출점규제 법률시행에 따른 향후영향 및 대응 방안과 대안을 제시 하고자한다. 특히 2009년 이후 기업 형 슈퍼마켓의 출점 증가로 중소상인 보호를 위하여 유통법이 개정되어 출점과 영업규제 법률이 통과되고 조례제정을 통하여 규제가 현실화되었다. 이러한 법 개정이 외국사례와 국제법과 비교해서 그 타당성의 여부를 확인하고 향후방향을 제시했으며 실질적인 영세 상인임에도 오히려 과잉규제 대상이 된 프랜차이즈 슈퍼마켓 가맹점의 선의에 피해에 대한 법률적 구제방안도 제안하고자 한다. 본격적인 규제시행으로 유통업계는 해외진출 가속화, 유통업자 상표개발확대, 해외소싱상품 개발 등 경쟁력 강화를 위한 방안을 강구하고 중소상인과 대기업간의 상생모델인 프랜차이즈 가맹사업으로 사업방향을 전환하여 실질적인 상생협업모델을 추진하고 상생협력도 구축하면서 지속적인 성장을 추구해야한다.

• Structural Engineering and Mechanics
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• 제84권4호
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• pp.547-560
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• 2022

By presetting various reinforcement diameters in topology optimization with the discrete model finite element analysis, an algorithm of bidirectional evolutionary structural optimization of multi-level reinforcement diameter is presented to obtain the optimal reinforcement topologies which describe the degree of stress of different parts. The results of a comparative study on different reinforcement feasible domain demonstrate that the more angle types of reinforcement are arranged in the initial domain, the higher utilization rate of reinforcement of the optimal topology becomes. According to the nonlinear finite element analysis of some deep beam examples, the ones designed with the optimization results have a certain advantage in ultimate bearing capacity, although their failure modes are greatly affected by the reinforcement feasible domain. Furthermore, the bearing capacity can be improved when constructional reinforcements are added in the subsequent design. However the adding would change the relative magnitude of the bearing capacity between the normal and inclined section, or the relative magnitude between the flexural and shear capacity within the inclined section, which affects the failure modes of components. Meanwhile, the adding would reduce the deformation capacity of the components as well. It is suggested that the inclined reinforcement and the constructional reinforcement should be added properly to ensure a desired ductile failure mode for components.

• 운동영양학회지
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• 제13권3호
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• pp.179-184
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• 2009

The purpose of this study was to evaluate a exercise combined with nutritional education for improving metabolic risk factor and dietary intakes in obese middle aged women. 35 obese (body mass index of at least ≥ 25 kg/m² or %body fat ≥ 30%) middle aged women were recruited from public health center. For intervention participants, their height, weight, percentage of body fat, waist circumference, blood pressure, lipid profiles, insulin resistance index, and nutrition intakes were measured at before and after post the intervention. And change of the total energy intakes per week was measured during intervention. The subjects underwent a 12 weeks educational program including nutritional education one day per week and, aerobic exercise 3 days per week (walking). After 12 weeks exercise program combined with nutritional education, body weight (p=0.002), percentage of body fat (p<0.001), BMI (p<0.001), waist circumference (p<0.001), WHR (p=0.004), TC (p=0.004), AI (p=0.006), DBP (p=0.010), MAP (p=0.013), glucose (p=0.018), insulin (p=0.003), HOMA-IR (p<0.001) were significantly decreased and cardiorespiratory fitness was significantly (p<0.001) increased. And energy intake was significantly (p<0.001) decreased and nutritional intake and intake-style were improved through 12 weeks exercise program combined with nutritional education.