• Steel and Composite Structures
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• 제33권3호
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• pp.433-444
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• 2019

Fatigue failure of a grid structure using bolt-sphere joints is liable to occur in a high-strength bolt due to the alternating and reciprocal actions of a suspension crane. In this study, variable amplitude fatigue tests were carried out on 20 40 Cr steel alloy M30 high-strength bolts using an MTS fatigue testing machine, and four cyclic stress amplitude loading patterns, Low-High, High-Low, Low-High-Low, and High-Low-High, were tested. The scanning electron microscope images of bolt fatigue failure due to variable amplitude stress were obtained, and the fractographic analysis of fatigue fractures was performed to investigate the fatigue failure mechanisms. Based on the available data from the constant amplitude fatigue tests, the variable amplitude fatigue life of an M30 high-strength bolt in a bolt-sphere joint was estimated using both Miner's rule and the Corten-Dolan model. Since both cumulative damage models gave similar predictions, Miner's rule is suggested for estimating the variable-amplitude fatigue life of M30 high-strength bolts in a grid structure with bolt-sphere joints; the S-N fatigue curve of the M30 high-strength bolts under variable amplitude loading was derived using equivalent stress amplitude as a design parameter.

• 대한기계학회논문집A
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• 제25권4호
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• pp.654-661
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• 2001

In this study, fatigue growth behavior of the transverse crack, which was the most dangerous damage among the various types of rail defects, was investigated using the notched keyhole specimen under constant amplitude loadings. Fatigue limit of smooth specimen in rail steel at R=0 was 110MPa, and the fatigue crack initiation life in the region of the low stress amplitude (ie. long life) occupied the major portion of the total fatigue life. The fatigue strength under variable amplitude loading was converted to the equivalent fatigue strength based upon. Miners rule, which was estimated approximately 9% lower than that under constant amplitude loading. Also, in the low ΔK(sub)rms region ($\leq$21MPa√m), fatigue crack growth rate (da/dN) under constant amplitude loading was higher than that under variable amplitude loading, whereas the tendency was reversed in the high ΔK(sub)rms region. It is believed that this behavior is due to the transition of fracture appearance.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 춘계학술대회
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• pp.261-266
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• 2003

The growth behavior of the transverse crack, which was one of the most dangerous damages of rail defects, was investigated under mode I and mixed mode loading in rail steel. In the case of variable amplitude loading, the fatigue crack growth behavior was discussed using characteristic stress intensity factor ranges ${\Delta}_{rms}$. In addition, characteristic comparative stress intensity factor ranges ${\Delta}_{V,rms}$ was proposed to evaluate the quantitative effects of the variable amplitude under mixed mode loading. As a result, crack growth rate under variable amplitude loading was faster than that under constant amplitude loading.

• International Journal of Railway
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• 제5권2호
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• pp.71-76
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• 2012

Recently, axle load, operating speed and traffic density on railroads have had a tendency to increase and thereby cause additional pressure applied on used track. These operating conditions frequently result in service failure due to wear caused by wheel-rail contact and fatigue damage under cyclic loading. Among rail defects, the transverse crack, which has been the most dangerous type of fatigue damages, is developed from the subsurface crack near the rail running face and grows perpendicular to the rail surface. Therefore, it is necessary to investigate systematically the growth behavior of transverse crack for rail steel under mixed mode. In this study, the fatigue crack growth behavior of the transverse crack in rail steel was experimentally investigated under mixed-mode variable amplitude loadings.

• Elastomers and Composites
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• 제42권4호
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• pp.209-216
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• 2007

가변진폭 하중에서 고무부품의 피로수명 예측방법에 대하여 연구하였다. 서로 다른 변위에서 변위제어 피로시험을 수행하였으며 피로손상변수로 최대 Green-Lagrange 변형률을 선정하였다. Green-Lagrange 변형률에 의한 고무의 피로수명 곡선은 3차원 덤벨시편의 비선형 유한요소법을 이용하여 결정하였다. 피로수명 예측을 위하여 가변진폭 하중이력으로 SAE의 하중이력을 이용하였다. 레이스트랙법과 단순화된 레인플로집계법을 이용하여 하중이력신호를 축약하였다. 누적손상피로를 계산하는 방법으로 수정Miner 법칙을 이용하였으며, 최종적으로 하중이력신호에서 최대 진폭의 30% 이하를 노이즈로 간주하여 예측하였을 경우의 피로수명은 실제 가변진폭 하중 하에서의 피로시험결과와 비교적 잘 일치하였다.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 2002년도 가을 학술발표회 논문집
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• pp.353-358
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• 2002

The purpose of this paper was to estimate the cumulative damage of pavement concrete by split tension fatigue test. The split tension fatigue test of variable amplitude loading were performed in two and three stages. The results of the fatigue test by variable amplitude loading showed that the sums of damage were greater than 1 in the increasing sequence loading tests, and less than 1 in the decreasing sequence loading tests. The remaining life estimated by equivalent damage theory was almost similar to that of experimental results.

• 대한토목학회논문집
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• 제6권4호
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• pp.79-88
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• 1986

본(本) 논문(論文)에서는 크기가 변(變)하는 변동진폭(變動振幅)의 반복하중이 콘크리트구조물(構造物)에 작용(作用)할 경우 피로파괴를 예측할 수 있는 비선형누적손상이론(非線型累積損傷理論)을 제안(提案)하였다. 이를 위하여 콘크리트보에 대한 피로실험이 수행되었으며, 피로하중은 2단계 및 3단계로 크기가 증가하는 경우와 감소하는 경우가 모두 고려되었다. 본(本) 연구(硏究)의 실험결과(實驗結果) 변동진폭(變動振幅)의 반복하중을 받는 콘크리트에 발생하는 누적손상은 피로하중의 크기와 피로하중의 작용순서에 따라 많은 영향을 받는 것으로 나타났으며, 따라서 Palmgren-Miner의 선형손상이론이 콘크리트의 경우 적합하지 않음을 보여주고 있다. 피로하중이 점진적으로 증가하는 경우 Miner의 손상합(損傷合)은 1 보다 크며, 피로하중이 점진적으로 감소하는 경우 이 손상합은 1 보다 작은 것으로 나타났다. 본(本) 논문(論文)에서 제안(提案)된 비선형(非線型) 누적손상이론(累積損傷理論)을 피로하중의 크기와 작용순서의 영향을 고려할 수 있는 보다 진보(進步)된 손상이론으로서, 콘크리트 구조용의 피로해석 및 설계에 있어 하나의 토대가 될 것으로 사료된다.

• 대한기계학회논문집A
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• 제24권8호
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• pp.2074-2082
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• 2000

In this study, to investigate and to predict the crack growth behavior under variable amplitude loading, crack growth tests are conducted on 7075-T6 aluminum alloy. The loading wave forms are generated by normal random number generator. All wave forms have same average and RMS(root mean square) value, but different standard deviation, which is to vary the maximum load in each wave. The modified Forman's equation is used as crack growth equation. Using the retardation coefficient D defined in previous study, the load interaction effect is considered. The variability in crack growth process is described by the random variable Z which was obtained from crack growth tests under constant amplitude loading in previous work. From these, a statistical model is developed. The curves predicted by the proposed model well describe the crack growth behavior under variable amplitude loading and agree with experimental data. In addition, this model well predicts the variability in crack growth process under variable amplitude loading.

• 한국철도학회:학술대회논문집
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• 한국철도학회 2009년도 춘계학술대회 논문집 특별세미나,특별/일반세션
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• pp.421-426
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• 2009

Recently, the design process for a railway bogie frame in Europe has been carried out according to the norm EN 13749. The activities in the norm EN shall demonstrate that the design of the bogie frame fulfills the acceptances of 4 steps of the program such as structural calculation, static tests, fatigue test and on-tracks tests. At the end of the acceptance program, the on-tracks tests have the aim to measure the real stress history generated in operation and to verify that they are reasonably next to those calculated and measured on the test rig. Therefore, in order to assure the safety of the railway vehicle, it is important to examined the durability of that under load histories measured from on-tracks tests. In this study, under variable amplitude loading based on the actual acceleration history, fatigue analysis of the welding bogie frame is investigated by using durability software. Moreover, the fatigue life of the frame under the loading in the norm EN fatigue test condition is evaluated and compared with the life under variable amplitude loading.

• 한국해양공학회지
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• 제6권1호
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• pp.96-104
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• 1992

Aircraft structures and engineering structures are always subject to variable amplitude loads. Variable amplitude loads include some kind of loading history; for example, constant amplitude load, single peak overload and block overload etc. Crack growth under variable amplitude loading exhibits retardation effect. In this study, the 4 point bending fatigue test was performed by hydrolic servo fatigue testing machine on 7075-T6 Al-alloy. The retardation effect of overload ratio and numbers of overload cycle was quantitatively studied. 1) Change of retardation effect against increment of overload ratio is more evident when the multiple overload is applied than single overload is done. 2) The number of overload cycle is very important factor about the crack growth retardation effect when the overload ratio is more than 1.75; that is not when the overload ratio is less than 1.75. 3) Overload affected zone size increased gradually by increment of crack growth retardation effect. 4) Crack driving force is more greatly reduced when the crack tip branched off two direction than it sloped to one direction.