• Design & Manufacturing
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• 제2권6호
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• pp.43-48
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• 2008

Magnesium alloys have given high attention to the industry of light-weigh as automobile and electronics with aluminium, titanium and composite alloys due to their high strength, low specific density and good damping characteristics. But the magnesium contained structures under high temperature have the problems related to creep deformation and rupture life, which is a reason of developing the new material against creep deformation to use them safely. The purpose of this study is to predict the creep deformation mechanism and rupture time of AZ31 magnesium alloy. For this, creep tests of AZ31 magnesium alloy were done under constant creep load and temperature with the equipment including automatic temperature controller with acquisition computer. The apparent activation energy Qc, the applied stress exponent n and rupture life have been determined over the temperature range below 0.5Tm and stress range of 109~187MPa, respectively, in order to investigate the creep behavior. AZ31 Magnesium alloy identify the activation energy for creep deformation and the stress dependence to creep rate at below 0.5Tm, and then investigate the mechanism for creep deformation and creep rupture life of AZ31 Magnesium alloy.

• 한국생산제조학회지
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• 제8권1호
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• pp.81-88
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• 1999

This paper was carried out to know the influence of advanced austempered ductile cast iron (ADI) on the tool life and mechanical properties of drilling machinability. For manufactured method of ADI, the spheroidal graphite cast iron were austenized at 90$0^{\circ}C$ for 1 hour and then austempered for 2 hour at 37$0^{\circ}C$ in the salt bath. And interrelationship has been investigated between tool life and mechanical characteristics of specimen material on drilling condition when the ordinary and step-feed drilling are carried out to drill holes of specimens. Tensile strength and hardness of ADI decrease and elongation of ADI increases with the increase austempered temperature. It is known that about 2 times of tool life in the case of step-feed decreases compared with ordinary feed due to the high hardness of ADI and hardness ascribed to the fact that retained austenite became to martensite state due to cutting heat in drilling. Under the constant feed rate 0.1mm/rev relation between hardness and length of end tip after drilling can be formularized to Hv=$788.46L^{-0.096}$ for the cutting speed 6.1m/min.

• International Journal of Precision Engineering and Manufacturing
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• 제9권4호
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• pp.57-62
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• 2008

The aim of this investigation was to extract the fatigue properties at the designated fatigue life of copper foil and observe the mean stress and stress amplitude effects on both the fatigue life and the corresponding surface morphology. Tensile tests were performed to determine the baseline monotonic material properties of the proportional limit and ultimate tensile strength. Constant amplitude fatigue tests were carried out using a feedback-controlled fatigue testing machine. The mean stress and the stress amplitude were changed to obtain the complete nominal stress-life curves. An atomic force microscope was utilized to observe the relationship between the fatigue damage and the corresponding changes in surface morphology. A Basquin's exponent of-0.071 was obtained through the fatigue tests. An endurance limit of 122 MPa was inferred from a Haigh diagram. The specimen surface became rougher as the number of fatigue cycles increased, and there was a close relationship between the fatigue damage and the surface roughness evolution.

• 대한조선학회논문집
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• 제45권6호
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• pp.679-687
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• 2008

The fatigue life of ship structure under cyclic loading condition is made up of crack initiation and propagation stages. For a welding member in ship structure, the fatigue crack propagation life is more important than the fatigue crack initiation life. To calculate precisely the fatigue crack propagation life at the critical welding location, the knowledge of the residual stress sensitivity on the fatigue strength is necessary. In this study, thermo elastic-plastic analysis was conducted in order to examine the effect of residual stress on the fatigue crack propagation life. Also the fatigue crack propagation lives considering residual stress were calculated using fatigue crack growth code, AFGROW, on the basis of fracture mechanics. AFGROW is widely used for fatigue crack growth predictions under constant and variable amplitude loading. The reliability of AFGROW on the fatigue of ship structure was confirmed by the comparison of the estimated results with the fatigue propagation test results.

• 한국자동차공학회논문집
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• 제13권3호
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• pp.117-122
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• 2005

Titanium alloy has widely been used as material for glasses frame parts because it has high specific strength. It is also light and harmless to human body. However, we have little design data about the mechanical properties such as the creep behaviors of the alloy. Therefore, in this study, creep tests under four constant stress conditions have been conducted with four different temperature conditions. A series of creep tests had been performed to get the basic design data and life prediction of titanium products and we have gotten the following results. First, the stress exponents decrease as the test temperatures increased. Secondly, the creep activation energy gradually decrease as the stresses became bigger. Thirdly, the constant of Larson-Miller parameter on this alloy was estimated as about 2.5. Finally, the fractographs at the creep rupture showed the ductile fracture due to the intergranullar rupture and some dimples.

• 한국강구조학회 논문집
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• 제10권3호통권36호
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• pp.497-508
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• 1998

본 연구에서는 평면결함의 일종인 용입부족의 크기가 맞대기 용접부의 피로 특성에 미치는 영향을 정량적으로 규명하였다. 정적실험 결과 비드를 삭제하지 않은 맞대기 용접부의 인장강도와 항복강도는 용입부족 크기에 관계없이 일정하였다. 또한 피로실험 결과 완전용입된 맞대기 용접부의 피로강도는 국내 외의 시방서에서 규정하고 있는 피로 등급을 모두 만족하며, 용입부족 크기에 따른 피로강도의 저하는 용입부족 크기가 증가함에 따라 피로강도가 급격하게 감소하였다. 파단형태에서 완전용입 시험체와 용입부족 시험체의 파단형태가 서로 다른 형식을 취하고 있음을 알 수 있었는데 이는 용접지단부와 용접루트부의 기하학적 형상에 따른 응력집중계수의 변화에 기인하는 것을 유한요소해석에서 확인하였다. 따라서 강구조물에 대한 용접제작시 발생할 수 있는 용접결함 중 평면결함의 일종인 용입부족은 반드시 피해야만 강구조물의 공용수명을 충분히 확보할 수 있을 것이다.

• The Journal of Korean Physical Therapy
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• 제24권2호
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• pp.82-89
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• 2012

Purpose: In this study, the effects of stability exercise, extracorporeal shock wave therapy, and taping on pain and function in patients with myofascial pain syndrome of upper trapezius were compared. Methods: The subjects were divided into the stability exercise, ESWT and the taping treatment group and the clinical outcomes were evaluated by visual analog scale (VAS), pressure pain threshold (PPT) and a constant-murley scale (CMS) at pre-treatment and post-treatment. Paired t-test and ANOVA was used for statistical analysis. Results: All groups were statistical significance in the change in visual analog scale (p<0.05). The difference between the ESWT group and taping group was statistical significance in the change in pressure pain threshold (p<0.05) except for the taping group. Using the constant-murley scale, the stability exercise group showed a significant decrease in pain, and a significant increase in ROM, ADL, strength, total score of shoulder (p<0.05); however, the ESWT group showed no difference on ADL. In addition, there was no difference in strength for the taping group. The comparison of the effect between the stability exercise group, ESWT group and taping group in CMS showed a statistical significant difference in pain, ADL and ROM (p<0.05). Conclusion: These results indicate that stability exercise, ESWT and taping could be considered an effective and efficient treatment modality for myofascial pain syndrome of upper trapezius.

• 연구논문집
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• 통권27호
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• pp.57-73
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• 1997

The bogie between the track and the railway vehicle body, is one of the most important component in railroad vehicle. Its effects on the safety of both passengers and vehicle itself, and on the overall performance of the vehicle such as riding quality, noise and vibration are critical. The bogie is mainly consisted of the bogie frame, suspensions, wheels and axles, braking system, and transmission system. The complex shapes of the bogie frame and the complicate loading condition (both static and dynamic) induced in real operation make it difficult to design the bogie frame fulfilling all the requirements. The complicated loads applied to the bogie frame are i) static load due to the weight of the vehicle and passengers, ii) quasi-static load due to the rolling in curves iii) dynamic load due to the relative motion between the track, bogie, and vehicle body. In designing the real bogie frame, fatigue analysis based on the above complicated loading conditions is a must. In this study, stress analysis of the bogie frame has been performed for the various loading conditions according to the UIC Code 6 15-4. Magnitudes of the stress amplitude and mean stress were estimated based on the stress analysis results to simulate the operating loads encountered in service. Fatigue strength of the bogie frame was evaluated by using the constant life diagram of the material. 3-D surface modelling, finite element meshing, and finite element analysis were performed by Pro-Engineer, MSC/PATRAN, and MSC/NASTRAN, respectively.

• 대한기계학회논문집A
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• 제26권10호
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• pp.2018-2025
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• 2002

It is well known that the strength and the fatigue life of welded steel components are affected extensively by welding residual stresses distributed around their weldments under not only monotonic but also cyclic loads. The externally applied loads are to be superimposed with the welding residual stresses, so that unexpected deformations and failures of the components might occur. These residual stresses are not kept constant, but relaxed or redistributed during in service. Under monotonic loads the relaxation takes place when the sum of external and welding residual stress exceeds locally the yield stress of material used. By the way, it is shown that under cyclic loads the welding residual stress is considerably relieved by the first or the early cycles of loads, and then gradually relaxed with increasing loading cycles. Although many investigations in this field have been carried out, the phenomenon and mechanism of the stress relaxation are still not clear, and there are few comprehensive models to predict amount of relaxed welding residual stress. In this study, the characteristics of the welding residual stress relaxation under monotonic and cyclic loads were investigated, and a model to predict quantitatively amount of welding residual stress relaxation was proposed.

• 한국산학기술학회논문지
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• 제16권12호
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• pp.8637-8642
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• 2015

현재 군에 보급되고 있는 접착식 전투화용 고무의 대표적인 노화현상은 열에 의한 정기적인 스트레스 노화가 대표적이다. 본 연구에서는 전투화용 고무의 노화시험을 실시하고, 아레니우스 관계식을 이용하여 노화온도($60^{\circ}C$, $80^{\circ}C$, $100^{\circ}C$)에 따른 반응속도 상수 k를 구하였다. 제품의 수명한계를 인장강도는 30% 감소시점, 신장률은 50% 감소시점, 내마모도는 380%로 가정하였으며, ln($P/P_0$) 및 활성화 에너지와 상수를 고려한 수명예측을 하였다. 이를 통해 노화온도에 영향을 받은 전투화용 고무의 수명을 예측하였고, 그 결과 실온($20^{\circ}C$)에서 예상수명이 10년 이상임을 확인할 수 있었다.