• Composites Research
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• 제12권5호
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• pp.1-11
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• 1999

본 연구는 단섬유보강 금속복합재료의 피로 및 주기적 거동에 관한 연구로서, 가압용침법을 이용하여 제조된 Al6061과 이를 기지금속으로 사용한 단섬유보강 Al/$Al_2O_3$-15% 복합재료의 주기적변형(cyclic deformation)과 피로거동(fatigue behavior)에 대해서 실험적으로 고찰하였다. Al/$Al_2O_3$ 복합재로의 피로강도는 210MPa로 나타났고 기지금속은 170MPa로 나타났다. 영구소성변형에 의한 합변위는 피로수명 초기 5%이내에서 대부분 발생하며 파단시의 합변위는 복합재료와 기지금속 모두 축적된 소성변형으로 인하여 인장시험에서의 연신율에 비해 작은 값을 가진다. 금속복합재료의 주기적 변위는 보강재가 전위 전파의 저지역할을 함으로써 기지금속에 비하여 다소 감소하는 것으로 나타났다. 파면관찰결과 기지금속의 경우 금속복합재료에 비해 전체 시험편 단면에서 넓은 영역에서 균열전파 형태를 관찰할 수 있었으며, 또한 금속복합재료의 파면에서는 연속적인 피로하중에 의한 기지금속과 보강재간의 분리(debonding)등의 결과로 보강재가 뽑혀나간 흔적이 관찰되고 있다.

• 한국해양공학회지
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• 제23권2호
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• pp.74-80
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• 2009

Surface hardening treatments, such as using the high-frequency induction hardening method, are widely used to increase the fatigue life and prevent the failure of materials by locally increasing the surface hardness. This method, in particular, brings an improvement in static strength by compressive residual surface stress due to the hardening. In this study, the mechanical properties of high-frequency induction hardened SCM440 steel were investigated. These results were also compared with those for base metal and a Q/T (tempering after quenching) treatment specimen. The test results showed that partially high-frequency induction hardened SCM440 steel specimens were more improved in static strength, surface hardness, fatigue limit, and anti-wear than the base metal and Q/T treatment specimens. In particular, the fatigue limit of the high-frequency induction hardened SCM440 steel increased by more than about 52% compared to that of base metal and by about 25% compared to that of the Q/T specimen.

• 한국자동차공학회논문집
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• 제11권6호
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• pp.101-107
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• 2003

Since stainless steel sheets have good mechanical properties, weldability, appearance and corrosion resistance, they are commonly used as one of the structural materials of the railroad cars or the commercial vehicles which are manufactured by the spat welding. Among the many kinds of spot welded lap joints, it can be found that multi-lap joints are employed in their body structure. But, fatigue strength of these joints is lower than that of base metal due to high stress concentration at the nugget edge of spot weld and is considerably influenced by welding conditions as well as the mechanical and geometrical factors. Thus, it is necessary to establish a reasonable and systematic design criterion for the long life design of the spot-welded body structures. In this paper, the stress distribution and deformation around the spot-welded multi-lap joints subjected to tensile shear load was numerically analyzed. Also, the $\Delta$P-Nf curve was obtained by fatigue tests. Using these results, $\Delta$P-Nf curves were rearranged in to the ${\Delta}{\sigma}$-Nf relation with the maximum stress at nugget edge of spot weld.

• 한국철도학회논문집
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• 제17권3호
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• pp.193-200
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• 2014

본 연구에서는 도시철도 콘크리트궤도 장대레일에 대한 실내피로시험을 수행하였고, 장대레일 잔존수명을 표현한 파괴확률 50% S-N 선도는 적은 실험데이터에 대한 가중치 확률 해석기법을 사용하여 도출하였다. 여기서 피로시험에 사용된 레일들이 누적통과톤수가 서로 다르기 때문에 누적통과톤수를 평균하여 반복횟수를 수정하였다. 또한, 레일표면요철 및 열차속도를 고려한 레일 저부 휨응력은 기존 연구결과 도출된 레일휨응력 예측식을 사용하여 콘크리트궤도 장대레일의 잔존수명을 평가하였다. 레일 피로수명 평가결과, 레일 피로수명이 기준치에 비해 약 2억톤이상 높았다. 또한, 자갈궤도에 비해 콘크리트궤도 레일의 피로수명이 약 3억톤이상 높은 것으로 분석되었다. 따라서 도시철도에서 레일교체기준을 자갈궤도와 콘크리트궤도로 구분할 필요가 있으며, 레일연마를 통한 레일관리가 이루어진다면 기준치가 아닌 목표치로 관리할 수 있을 것으로 판단되었다.

• 한국자동차공학회논문집
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• 제25권2호
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• pp.250-256
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• 2017

Automatic transmission(A/T) is one of a car's core components. The durability of A/T has an effect on the durability of the car. The input shaft of A/T passes the power directly from the engine, so the durability of the input shaft for A/T is likewise vital. In this study, the test method of the input shaft is considered and the load spectrum of A/T is analyzed to secure durability of the input shaft. The shape parameter, T-N curve of the input shaft torsional fatigue test are obtained for life evaluation and analysis. Results of this study can be used to get the basic background of the durability test for A/T components.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 1998년도 춘계학술대회논문집
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• pp.88-93
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• 1998

The service life of tools in metal forming process is to a large extent limited by wear, fatigue fracture and plastic deformation. In warm forging processes wear is the predominant factor for operating lives of tools. To predict tool life by wear, Archard's wear model is generally applied. Usually hardness of die is considered to be a function of temperature in Archard's wear model. But hardness of die is a function of not only temperature but also operating time of die. To consider softening of die by repeated operations, it is necessary to express hardness of dies by a function of temperatures and operating time. By experiment of reheating of dies, die softening curves were obtained. Finally modified Archard's wear model in which hardness of die was expressed as a function of main tempering curve was proposed.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 추계학술대회논문집A
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• pp.272-277
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• 2001

In order to evaluate variation of fatigue data of the LP steam turbine blade steel, it is important to estimate P - S - N curves to accurately define the probability distributions. In this study, new procedure is introduced to determine the expression of P - S - N curves. For this purpose, 3-parameter Weibull distribution was found to be most appropriate among assumed distributions when the probability distributions of the fatigue life were examined by the proposed analysis. Furthermore, parameter estimation for P - S - N curves was performed using various optimization to maximize the correlation coefficient. As a result of this, sequential linear programing method is used for estimation of P - S - N curves.

• 전력전자학회:학술대회논문집
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• 전력전자학회 2002년도 추계학술대회 논문집
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• pp.205-208
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• 2002

Thermal design is required with considering thermal stability to verify the reliability of electric power device with using IGBT. Numerical analysis is performed to analyzed the change in thermal resistance with respect to the various thermal density of heating element. Correlations between thermal resistance and heat generation density are established. With using these correlations, performance curve is composed with respect to the change in thermal resistance of cooling conditions for natural convection and forced convection. Thermal fatigue is occurred at the Inside and outside of IGBT by repeated heat load. The crack is occurred between base plate and ceramic substrate for the inside. When the crack length is 4mm, the failure is occurred. Therefore, Thermal design method considering thermal density, thermal fatigue resistance is presented on this study and it is expected to thermal design with considering life prediction.

• 한국철도학회논문집
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• 제11권3호
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• pp.317-325
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• 2008

최근 연구결과 레일연마 및 장대레일화로 누적통과톤수에 의한 레일교체기준의 연장이 가능한 것으로 보고되었다. 본 연구에서는 현재 도시철도에서 사용 중인 대표 궤도구조에 대해 실운행 열차하중에 의한 궤도의 동적 응답을 분석하여 현 궤도의 상태평가를 하였으며 궤도의 상태 변화에 따른 레일의 부담력을 검토하였다. 또한, 측정된 응력파형을 Rainflow Count Method를 이용하여 응력히스토그램을 작성하고 등가응력을 상정하여 신규 레일 용접부의 S-N 선도에 적용함으로써 노후레일의 누적피로손상도 및 휨 피로수명을 산정하였다. 최종적으로 본 연구에서는 궤도구조 및 상태를 고려한 레일교체기준의 개정을 제안하고자 하였다.

• Journal of Welding and Joining
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• 제27권1호
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• pp.102-107
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• 2009

Weldability of Mg alloy was investigated using servo-actuated DC resistance spot welder. Due to its uncommon electrical and mechanical properties, lots of voids and cracks were observed inside nugget with conventional weld schedule. Lobe curve was proposed to clarify proper electrode force and weld current, which guarantee reasonable weld strength and fatigue life. Macro structure of sectioned specimen was examined to count total number of void and crack. Post weld schedule is also proposed to reduce micro victors hardness value of weld zone.