• Bulletin of the Society of Naval Architects of Korea
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• v.26 no.2
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• pp.41-48
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• 1989

This paper investigates the relationship between strain hardening exponents(n) of various marine materials and fatigue crack retardation effect after applying single overload. Using the four different sheet materials, following results are obtained. (1) The fatigue crack propagation rate after applying single overload was retarded and the effects of this retardation were closely related to the strain hardening exponent. (2) The larger the strain hardening exponents were, the more were the fatigue crack retardation effects after applying single overload. (3) The considerable crack closure with the applying of a overload was observed in matrals with large strain hardening exponent. When n is smaller than 0.1, the fatigue crack retardation effects are negligible. On the contrary, when n is larger than 0.2 the fatigue crack retardation effects are significant.

• Journal of Welding and Joining
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• v.14 no.2
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• pp.79-89
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• 1996

일정진폭하중과 과대하중비 2.5의 단일 인장과대하중에 의한 4140강 용접부 의 피로균열성장거동을 실온과 -45.deg.C의 저온에서 피로시험과 파면관찰을 통하여 고찰하였다. 이때, 용접부 미시조직의 영향을 평가하기 위해 모재(parent metal), 열영향부(as-welded HAZ), 열처리된 열영향부(PWHT HAZ)로 나누어 응력비 0과 0.5로 CT시험편을 이용하여 피로시험을 실시하였다. 피로균열성장거동은 재료의 미시조직과 온도변화보다는 응력비에 크게 영향을 받았으며, 단일 과대하중에 의한 피로균열성장 지연효과가 모든 재료에서 상당히 크게 나타났다. 전자현미경에 의한 피로파면 관찰 결과, 실온에서는 연성의 스트라이에이숀과 -45.deg.C에서는 의벽개파면과 같은 피로 균열성장거동을 나타내고 있다.

• Journal of the Korean Society for Nondestructive Testing
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• v.27 no.1
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• pp.38-47
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• 2007

The initiation of fatigue cracks in a simulated aircraft structure with a series of rivet holes was detected by acoustic emission(AE), then the crack length was determined by surface acoustic wave(SAW) technique. With the initiation and growth of fatigue cracks, AE events increased intermittently to form a stepwise incremental curve of cumulative AE events whereas the crack length increased more or less monotonically. With the SAW technique employed, the crack sizing for 13 different cracks including some short cracks was performed. With the reference to the measurement by traveling microscope, cracks in the range of $1{\sim}8mm$ long were reliably sized by the SAW technique. Although it was impossible to size the short fatigue cracks in the range shorter than 1 mm, the SAW technique still appeared practically useful for a range of crack lengths often found in aircraft structures.

• Journal of the Korean Society for Nondestructive Testing
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• v.20 no.1
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• pp.54-61
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• 2000

In order to determine the mode I stress intensity factor ($K_I$) by means of the alternating current potential drop(ACPD) technique, the change in potential drop due to load for a paramagnetic material containing a two-dimensional surface crack was examined. The cause of the change in potential drop and the effects of the magnetic flux and the demagnetization on the change in potential drop were clarified by using the measuring systems with and without removing the magnetic flux from the circumference of the specimen. The change in potential drop was linearly decreased with increasing the tensile load and was caused by the change in conductivity near the crack tip. The reason of decreasing the change in potential drop with increasing the tensile load was that the increase of the conductivity near the crack tip due to the tensile load caused the decreases of the resistance and internal inductance of the specimen. The relationship between the change in potential drop and the change in $K_I$ was not affected by demagnetization and was independent of the crack length.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2003.10a
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• pp.243-243
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• 2003

• Journal of the Korean Society for Nondestructive Testing
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• v.23 no.3
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• pp.246-253
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• 2003

The initiation and growth of short fatigue cracks in the simulated aircraft structure with a series of rivet holes was detected by acoustic emission (AE). The location and the size of short tracks were determined by AE source location techniques and the measurement with traveling microscope. AE events increased intermittently with the initiation and growth of short cracks to form a stepwise increment curve of cumulative AE events. For the precise determination of AE source locations, a region-of-interest (ROI) was set around the rivet holes based on the plastic zone size in fracture mechanics. Since the signal-to-noise ratio (SNR) was very low at this early stage of fatigue cracks, the accuracy of source location was also enhanced by the wavelet transform do-noising. In practice, the majority of AE signals detected within the ROI appeared to be noise from various origins. The results showed that the effort of structural geometry and SNR should be closely taken into consideration for the accurate evaluation of fatigue damage in the structure.

• Transactions of the Korean Society of Mechanical Engineers
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• v.14 no.5
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• pp.1193-1199
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• 1990

Effects of strain hardening exponents on the behavior of fatigue crack propagation are experimentally investigated. The retardation effect of fatigue crack propagation after single overloading is investigated in relation to strain hardening exponent and crack closure. A relationship between crack opening ratio and strain hardening exponents is inspected through an examination of the crack closure behavior. An empirical equation relating retardation effect of fatigue crack propagation after single overloading, percent peak load and strain hardening exponent of materials is proposed.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.23 no.5
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• pp.483-491
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• 2010

In this paper, ${\Delta}J$-integral is used to analyze fatigue crack growth of viscoelastic material. Using analytical integral value, the J-integral is calculated. So, reduction of calculation time and increase of accuracy are made possible. The stress intensity factor is calculated using the finite element method code. In difference with existed fatigue crack analysis using ${\Delta}K$, we were successfully able to analyze various load amplitude and the fatigue crack of load cycle only with two fatigue crack growth parameters and creep compliance. The analysis gives N-${\alpha}$ curve for simulation of crack growth, and the curve almost corresponds with test results.

• Journal of Advanced Marine Engineering and Technology
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• v.18 no.3
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• pp.52-62
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• 1994

기계나 구조물 파괴의 대부분은 노치부를 기점으로 하여 발생하기 때문에 첨단복합재료를 노지부 재로서 안전하면서도 경제적으로 사용하기 위해서는 각종 조건하에 있어서 강도특성을 명확히 하는 것은 대단히 중요하다. 본 연구에서는 노치를 갖는 복합재료를 이용하여 각종조건하에서 강도특성평가실험을 행하였으 며, 얻어진 결과를 종합하면 다음과 같다. (1) 첨단복합재료 노치재는 試驗片의 幾可學的 形狀과는 관계없이 노치반경 p만에 의해 결정되는 최대탄성응력 $\sigma_{max}$일정의 條件下에서 破t짧된다. (2) 破斷時 최소단면에서의 공칭응력 $\sigma_{c}$와 응력집중계수 $K_{t}$와의 관계에 있어서,$\sigma_{c}$의 값이 $K_{t}$의 증대와 더불어 떨어지고 있는 부분과, $K_{t}$와 관계없이 거의 일정하게 되고 있는 부분으로 나누어지는 現象은 노치재의 回轉굽힘 또는 인장압축파열에서 보여지는 현상과 外觀上 對應하고 있다. 즉, 정적파괴와 피로파괴는 파괴의 양상이 비슷하다 (3) PEN수지단체의 경우, 피로균열발생은 점발생적 피로균열이 최대탄성응력에 의해 지배되며, 노치에 만감하며,균열전파수명은전수명에 비해 상당히 짧다. (4) 단탄소섬유강화복합재료의 경우, 피로균열은 섬유端에 응력이 집중하기 때문에 일반적으로 섬유端에서 아주 빠른 시기에 발생하지만, 섬유가 피로균열진전에 대해 방해물로 작용하기때문에 아주 천천히 전파한다. (5) 短탄소鐵維는 피로균열발생에 대해서는 負의 강화작용 전수명의 극히 초기단계에 피로균열 발생을, 피로균열전파에 대해서는 正의 강화작용을 한다. (6) 단탄소섬유를 PEN에 강화함으로 인해 정적강도 보다 피로강도에 더 큰 강화효과를 초래했으며, 선형노치역학의 개녀은 첨단 복합재료의 강도평가에 대단히 유효했다.

• Journal of the Society of Naval Architects of Korea
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• v.28 no.2
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• pp.275-284
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• 1991

Fatigure crack growth from surface defects is one of the most important subjects for the evaluation and the assurance of safety in pressure vessels, piping systems, ship hulls and other various structures. This paper attempts to analysis some practical or general problems such as the estimation of crack growth life to penetrate the plate thickness, based on fatigure crack growth from a single surface flaw and the interaction of multiple flaws. An experiment on the coalescence of multiple undercuts was carried out under cyclic tension condition as a attempt to the analysis of multiple crack problems. It is noted that the fracture strength is characterized by the analogy to that in a single crack growth.