• Structural Engineering and Mechanics
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• 제33권5호
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• pp.589-604
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• 2009

In the field of predicting structural safety and reliability the operating conditions play an essential role. Since the time and cost limitations are a significant factors in engineering it is important to predict the future operating conditions as close to the actual state as possible from small amount of available data. Because of the randomness of the environment the shape of measured load spectra can vary considerably and therefore simple distribution functions are frequently not sufficient for their modelling. Thus mixed distribution functions have to be used. In general their major weakness is the complicated calculation of unknown parameters. The scope of the paper is to investigate the load spectra growth for actual operating conditions and to investigate the modelling and extrapolation of load spectra with algorithm for mixed distribution estimation, REBMIX. The data obtained from the measurements of wheel forces and the braking moment on proving ground is used to generate load spectra.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 추계학술대회
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• pp.1045-1050
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• 2003

In this study, to investigate the effects of omitting low-amplitude cycles from a flight-simulation loading, crack growth tests are conducted on 2124-T851 aluminum alloy specimens. Three test spectra are generated by omitting small load ranges as counted by the rain-flow count method. The crack growth test results are compared with the data obtained from the flight-simulation loading. The experimental results show that omission of the load ranges below 5% of the maximum load does not significantly affect crack growth behavior, because these are below the initial stress intensity factor range. However, in the case of omitting the load ranges below 15% of the maximum load, crack growth rates decrease, and therefore crack growth curve deviates from the crack growth data under the flight-simulation loading. To optimize the load range that can be omitted, crack growth curves are simulated by the stochastic crack growth model. The prediction shows that the omission level can be extended to 8% of the maximum load and test time can be reduced by 59%.

• 동력기계공학회지
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• 제8권4호
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• pp.11-16
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• 2004

In this study, to investigate the effects of omitting low-amplitude cycles from a flight-simulation loading, crack growth tests were conducted on 2124-T851 aluminum alloy specimens. Three test spectra were generated by omitting small load ranges as counted by the rain-flow count method. The crack growth test results were compared with the data obtained from the flight-simulation loading. The experimental results show that the ranges equal to or smaller than 5% of the maximum load do not contribute to crack growth behavior because these are below the initial stress intensity factor range. Omitting these from the flight-simulation loading, test time can be reduced by 54%. However, in the case of omitting the load ranges below 15% of the maximum load, crack growth rates decreased, and crack growth curve deviated from the crack growth data under the flight-simulation loading because loading cycles above fatigue fracture toughness were omitted.

• 조명전기설비학회논문지
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• 제14권6호
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• pp.35-42
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• 2000

본 연구에서는 몰드형 전력변압기의 절연열화 상태를 진단하기 위해 에폭시 수지의 보이드방전에 의한 절연열 화의 상태를 트리의 길이에 따라 열화 초기, 중기 및 말기로 구분하고 이때 방출된 음향선호흘 분석하였다. 또한 보이드방전에 의한 음향선호를 측정할 때 몰드변압기의 철심에서 발생되는 자기적 음향 노이즈가 중첩됨으로 이를 구분하기 위해 용량 500[kVA]인 실용 몰드변압기를 배전 계통 22.9[kV]의 선로에 접촉하였을 때, 자화전류 및 부하전류에 의해 각 상에서 발생되는 음향 노이즈 신호의 주파수 스펙트럼올 분석하였다. 실행 결과, 에폭시수지의 보이드방전애 의해 발생된 음향방출신호의 주파수 스펙트럼은 열화상태에 따라 약 50-230[kHz] 범위에서 측정되었으나, 배잔계동에 접속된 실용 몰드변압기의 자화전류 및 부하전류에 의해 발생된 자기회로의 음향 노이즈에 대한 주파수 스펙드럼은 약 4O-120[kHz] 대역인 것으로 나타났다.

• 대한기계학회논문집A
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• 제29권6호
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• pp.822-827
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• 2005

The characteristics of elastic waves emanating from crack initiation and propagation in 6061 aluminum alloy subjected to fatigue loading with different stress ratio was investigated. The objective of this study is to determine the properties of the signals generated from each stage of fatigue crack growth. AS a crack propagates, substantial elastic wave occurred just prior to penetration. Then it decreased and the crack penetrated. The waveforms and their power spectra were found to be dependent on the different stress ratio associated with the signals. It is determined that high-frequency signal $0.5{\sim}0.75$ MHz is most likely emitted during crack propagation at peak load of fatigue cycle which release the highest energy. It is determined that 0.3 MHz is closely related to crack closure effect. The frequency peaks below 0.25 MHz may be attributed to fretting or hydraulic noise.