• 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.

• 대한기계학회논문집A
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• 제23권7호
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• pp.1164-1172
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• 1999

In this study, to investigate the fatigue crack retardation behavior and the variability of retardation cycles, fatigue crack growth tests were conducted on 7075-T6 aluminum alloy under single tensile overload. A retardation coefficient, D was introduced to describe fatigue crack retardation behavior and a random variable, Z to describe the variability of fatigue crack growth. The retardation coefficient was separately formulated according to retardation behavior which is composed of delayed retardation part and retardation part. The random variable, Z was evaluated from experimental data which was obtained from fatigue crack growth tests under constant amplitude load. Using these variables, a probabilistic model was developed on the basis of the modified Forman's equation, and retardation behavior and cycles were predicted under certain overload condition. The predicted retardation curve well agrees with the trend of experimental crack retardation behavior. And this model well predicts the scatter of experimental retardation cycles.

• 한국해양공학회지
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• 제11권3호
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• pp.76-88
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• 1997

Constant .DELTA.K fatigue crack growth rate experiments were performed by applying an intermediate single and multiple overload for structural steel, SM45C. The purpose of the present study is to investigate the influence of multiple overloads at various stress intensity factor ranges and the effect of statistical variability of crack retardation behavior. The normalized delayed load cycle, delayed crack length and the minimum crack growth rate are increased with increasing baseline stress intensity factor range when the overload ratio and the number of overload application were constant. The crack retardation under low baseline stress intensity factor range increases by increasing the number of overload application, but the minimum crack growth rate decreases by increasing the number of overload application. A strong linear correlation exists between the minimum crack growth rate and the number of overload applications. And, it was observed that the variability in the crack growth retardation behavior are presented, the probability distribution functions of delayed load cycle, delayed crack length and crack growth life are 2-parameter Weibull. The coefficient of variation of delayed load cycle and delayed crack length for the number of 10 overload applications data are 14.8 and 9.2%, respectively.

• Journal of Welding and Joining
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• 제6권1호
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• pp.46-52
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• 1988

Retardation or delay in fatigue crack growth due to overloads are important for the accurate prediction of fatigue lives of structural materials. In this study, retardation of fatigue crack growth in Al 6061-T6 weldments and heat affected zones (HAZ) after single overload cycle had been investigated. Retardation in both weldments and HAZ was observed. It was concluded that retardation in both weldment and HAZ was greater than in base metal due to microstructural change and crack branching and crack closure were major governing factor in retardation.

• 대한기계학회논문집A
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• 제25권3호
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• pp.470-477
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• 2001

Constant ΔK fatigue crack growth tests were performed by applying an intermediate multiple overload for S45C steel. The purpose of the present study is to investigate effects of specimen thickness at various baseline stress intensity factor range levels (ΔK(sub)b), overload application position (a/W) and overload application frequency (OL(sub)HZ) on fatigue crack growth retardation behavior. The principal results are summarized as follows. The amount of retardation for a given ΔK(sub)b level is increased with increasing the baseline stress intensity factor range level for all specimen thickness. The normalized minimum crack growth rate is increased with increasing the specimen thickness, except for ΔK=45MPa√m. The retardation cycle is decreased with increasing a/W and increased with OL(sub)HZ.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2000년도 춘계학술대회논문집A
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• pp.55-60
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• 2000

Constant ${\Delta}K$ fatigue crack growth tests were performed applying an intermediate multiple overload for SM45C steel. The purpose of the present study is to investigate the effects of specimen thickness at various baseline stress intensity levels$({\Delta}K_b)$, overload application frequency(a/W) and overload application frequency$(OL_{HZ})$ on fatigue crack growth retardation behavior. The principal results are summarized as follows. The amount of retardation for a given ${\Delta}K_b$ level is increased with increasing the baseline stress intensity level in all specimen thickness. The normalized minimum crack growth rate is increased with increasing the specimen thickness, except for ${\Delta}K=45MPa \sqrt m$. The retardation cycle is decreased with increasing the overload application position and increased with the overload application frequency.

• 한국해양공학회:학술대회논문집
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• 한국해양공학회 2003년도 춘계학술대회 논문집
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• pp.306-311
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• 2003

A growing fatigue crack is known to be retarded on application of an overload cycle. The retardation may be characterized by the total number of cycles involved during retardation and the retarded crack length. The overload ratio plays an important role to influence the retardation behavior. The objective of the present investigation is to study the effect of different overload ratio on the retardation behavior. For DENT(double edge notched tension) specimens and ESET(eccentrically-loaded single edge crack tension) specimens, fatigue crack growth tests are conducted under cyclic constant-amplitude loading including a single tensile overloading with different overload ratios. The proposed crack retardation model predicts crack growth retardation due to a single tensile overloading. The predictions are put into comparison with the experimental results to confirm the reliability of this model.

• 한국항공우주학회지
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• 제31권3호
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• pp.15-22
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• 2003

피로균열성장모델을 유도하고 지연모델을 제안하였다. 피로균열성장모델은 피로균열선단의 소성변형으로 인하여 균열표면에 발생하는 잔류소성스트레치를 고려하고 있다. 균열 성장률은 균열선단 재료요소의 소성변형에너지와 누적피로손상으로부터 계산된다. 유도한 균열성장모델로부터 계산한 균열성장률은 AL6061-T651과 17-4PH 주강의 시험결과와 잘 일치하고 있다. 피로균열성장지연모델은 인장과대하중으로부터 생성된 잔류소성스트레치를 근거로 하고 있으며, 인장과대하중은 다음 하중 사이클의 소성변형률을 감소시킨다. Strip-yield모델을 이용하여 균열선단의 소성역을 계산하였다. 새로 제안된 지연모델은 인장과대하중하의 피로균열선장특성 및 지체지연 현상을 잘 기술하고 있다.

• 대한기계학회논문집
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• 제14권5호
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• pp.1186-1192
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• 1990

본 연구에서는 이와 같은 과대인장하중의 제하과정중에 발생하는 과대압축소 성역이 과대하중에 의한 피로균열의 진전지연에 미치는 영향을 중심으로 하여, 각 소 성역 치수를 매개변수로 하는 해석적 방법을 통하여 가속지연모델을 제안하고, 이를 2024-T3 Al 합금을 이용한 단일과대하중에 의한 지연시험결과 및 Wheeler 나 Willenb- org 등의 기존의 지연모델과 비교 고찰하고자 한다.

• 항공우주시스템공학회지
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• 제1권1호
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• pp.45-52
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• 2007

This paper analyzed the extension effects of fatigue life and the application of pre-indentation in aircraft structural material such as aluminum alloys. The test specimen used the thin sheet of aluminum alloy with a single-edged notch. The experiments were conducted after making the pre-crack under a constant amplitude loading. As the fatigue life extension technique, the pre-indentation making an indent on the predicted path of crack propagation was applied. The work presented here discussed about a proper mathematical relation between crack growth rate and the range of stress intensity factor and about the generalization of crack growth mechanism with large retardation effect. A technique to enhance the applicability of pre-indentation if also mentioned.