• Title/Summary/Keyword: fatigue crack initiation

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An Experimental Study on the Fatigue Behavior and Stress Interaction of Arbitrarily Located Defects (I) (불규칙하게 분포된 미소결함 사이의 응력간섭 및 피로균열 거동에 대한 실험적 연구 (1))

  • Song, Sam-Hong;Bae, Jun-Su;Choe, Byeong-Ho
    • Transactions of the Korean Society of Mechanical Engineers A
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    • v.24 no.5 s.176
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    • pp.1288-1296
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    • 2000
  • In this study, fatigue crack behavior between arbitrarily located defects was investigated by experiment. Especially, stress interaction between micro hole defects and fatigue cracking, and fatigue crack initiation life following the variation of location of micro hole defects were considered. In addition, crack initiation position by micro hole stress interaction and the relationship between stress concentration factor and fatigue initiation life are studied in detail.

Effect of Indentation Residual Stresses on the Fatigue Crack Initiation Life (피로균열 발생수명에 대한 압입 잔류응력의 영향)

  • 이환우;강태일
    • Journal of the Korean Society for Precision Engineering
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    • v.21 no.5
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    • pp.158-165
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    • 2004
  • Up to now, many crack repair techniques have been developed for inhibiting crack growth in structural components. However, the simplest way for inhibiting crack growth is to apply a indentation at the crack tip or at some distance ahead of the expected crack growth path so as to produce residual compressive stresses that can reduce the effective stresses around the crack tip. In spite of its importance to the aerospace industry, little attention has been devoted to evaluation of the indentation residual stress effect on the fatigue crack initiation life quantitatively. Therefore, in the present work, the magnitude and distribution of the indentation residual stresses were investigated in order to estimate the beneficial effect on fatigue crack initiation by using finite element method. Furthermore, to examine the validity of finite element analysis results, residual stress distribution in the indented specimen was measured by using X-ray diffraction technique, and fatigue crack behavior at fastener hole in aluminum alloy 7075-T6 before and after indentation processes was investigated.

Study on the Fatigue Crack Initiation Life under 3-Dimensional Rough Contact (3차원 거친 접촉하에서의 피로균열 시작수명에 관한 연구)

  • 이문주;구영필;조용주
    • Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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    • 2000.11a
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    • pp.72-79
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    • 2000
  • In case of rough contact fatigue, the accurate calculation of surface tractions is essential to the prediction of crack initiation life. Accurate Surface tractions influencing shear stress amplitude can be obtained by contact analysis based on tile morphology of contact surfaces. In this study, to simulate rough contact under sliding condition, gaussian rough surface generated numerically in the previous study was used and to calculate crack initiation life in the substrate, dislocation pileup theory was used.

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A Study on the Crack Initiation from Double Edge Notch Tips by Ultrasonic Testing (초음파탐상법을 이용한 Double Edge Notch 선단의 균열발생에 관한 연구)

  • 임창현;최용식
    • Journal of the Korean Society of Safety
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    • v.8 no.2
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    • pp.51-57
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    • 1993
  • This study was done by ultrasonic testing. The proves used in this study were 5Z10$\times$10A70. The useful fatigue life of structural components is determined by the sum of the elapsed load cycles required to initiate a fatigue crack and to propagate the crack from subcritical size to critical dimensions. Thus, to predict the service life of many steel structures and to establish safe inspection intervals, an understanding of the Fatigue Crack Initiation and Fatigue Crack Propagation behavior in the steel is required. In the present study, the Fatigue Crack Initiation Life in SPS 5A steel was investigated by testing specimens having widely varying notch acuities. The variation in the notch acuity covered the range from notch root raidus 0.5mm to 2.5mm, and Fatigue Crack Initiation Life data were obtained in that range.

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Fatigue Life Estimation of Fillet Welded Joints Considering Statistical Characteristics of Weld Toe's Shape and Multiple Collinear Surface Cracks (비드형상 및 복수 표면균열의 확률적 특성을 고려한 필릿 용접부 피로수명 평가)

  • Han Seung-Ho;Han Jeong-Woo
    • Journal of Welding and Joining
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    • v.23 no.3
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    • pp.68-75
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    • 2005
  • The fatigue life of welded joints is associated with crack initiation and propagation life. Theses cannot be easily separated, since the definition of crack initiation is vague due to the initiation of multiple cracks that are distributed randomly along the weld toes. In this paper a method involving a notch strain and fracture mechanical approach, which considers the characteristics of welded joints, e.g. welding residual stress and statistical characteristics of multiple cracks, is proposed, in an attempt to reasonably estimate these fatigue lives. The fatigue crack initiation life was evaluated statistically, e.g. the probability of failure occurrence in 2.3, 50 and $97.7\%$, in which the cyclic response of the local stress/strain hi the vicinity of the weld toes and notch factors derived by the irregular shape of the weld bead are taken into account. The fatigue crack propagation life was simulated by using Monte-Carlo method in consideration of the Ad-factor and the mechanical behavior of mutual interaction/coalescence between two adjacent cracks. The estimated total fatigue life, $(N_T)_{P50\%}$, as a sum of crack initiation and propagation life under the probability of failure occurrence in $50\%$ showed a good agreement with the experimental results. The developed technique for fatigue lift estimation enables to provide a quantitative proportion of crack initiation and propagation life in the total fatigue life due to the nominal stress range, ${\Delta}S$.

A Study on the Effect of Fatigue and Crack Propagation Behavior in Spot Weld of High Strength Steel( I ) - Experimental Examination - (고장력 강판의 점용접부에서 면내 굽힘 모멘트가 피로특성 및 균열 성장 거동에 미치는 영향에 관한 연구(I) - 실험적 검토 -)

  • 성기찬;장경복;정진우;김기순;강성수
    • Journal of Welding and Joining
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    • v.19 no.1
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    • pp.112-117
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    • 2001
  • The factor affecting on the fatigue strength of spot welded specimen have been studied. To analyze and predict crack initiation position and propagation directions on the spot welded area are very important for strength design of the automobile body structure. In fact, there are a various of loads in running automobile but, it is impossible to replay like an actual conditions in the laboratory. So, in this study tensile-shear type and in-plane bending type specimens were used in fatigue test and includes an analysis of fatigue crack initiation position and propagation directions about earth specimens. The results obtained in the present study are summarized as follows: 1. In tensile-shear type fatigue test, the region of fatigue crack initiation position was affected by out-of-plane bending deformation due to bending angle. 2 In in-plane bending type fatigue test, the behavior of fatigue crack initiation position and propagation derections due to angle between upper plate and lower plate was dominated by magnitude of in-plane bending moment.

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A Study on the Knee Point of Low-cycle Fatigue Life in High Formability Titanium Alloy SP-700 (티탄계 초소성합금 SP-700의 저사이클 피로수명곡선의 절곡현상에 대하여)

  • ;淸水 眞佐男
    • Transactions of the Korean Society of Automotive Engineers
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    • v.5 no.1
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    • pp.129-135
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    • 1997
  • Previous studies has shown that the curve of low-cycle fatigue life was not expressed with the single line subjected to Manson-Coffin's law type and bent to short life in low ${\Delta}{\varepsilon}_p$ region. The main cause of this phenomenon has been considered that the localization of plastic strain in the crack initiation process fosters the crack initiation. In this study, the low-cycle fatigue life was investigated for each specimens omitted crack initiation process and it was found that fatigue life curve in log(${\Delta}{\varepsilon}_p$)-log($N_f$)was bent in low ${\Delta}{\varepsilon}_p$ region as ever. Therefore, the main cause of appearance of knee point in fatigue life curve is not found in the crack initiation process but in the crack propagation process. In the crack propagation process, the localization of the plastic strain in the vicinity of crack tip and the influence of test environment on the crack propagation rate were observed and these inclinations were more remarkable in low ${\Delta}{\varepsilon}_p$ region. Hence, it was concluded that these two phenomena in the crack propagation process were proved to the main cause which accelerates the crack propagation in low ${\Delta}{\varepsilon}_p$ region and bent the fatigue life curve in result.

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Characteristics of Corrosion Fatigue of High Strength Steel for Marine Weld Structure

  • Choi, Seong-Dae;Kubo, Takeo;Misawa, Hiroshi;Lee, Jong-Hyung;Song, Dug-Jung
    • Journal of the Korean Society of Manufacturing Process Engineers
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    • v.2 no.4
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    • pp.53-60
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    • 2003
  • Large sized marine structures are used under corrosion environment of seawater and applied by severe service loading such as an ocean current, a billow and a tempest. Marine structures are usually constructed by lots of thick wall steel pipes joining welded joints. The thickness of such as steel pipes is usually more than 40mm. The such as steels are called "Thermo-Mechanical Control Process steel (TMCP steel)" strengthened by a heat treatment in process of steel manufactures. The failure, especially crack initiation, of marine structures was starting at weld joints under service condition. Then they should be designed by basis of the fatigue strength under seawater corrosion environment of weld joints. To clarity the fatigue crack initiation behavior is important more than to clarify the crack propagation behavior on the strength design of marine structures, because it is very difficult to find out the crack initiation and propagation phenomena and then even if it will be able to find out, it is considered that the refit of the damaged parts of welded joints have a technical difficulty under the sea. Therefore, it is most important to clarify the corrosion fatigue crack initiation behavior under the seawater condition. But, there is one big difficulty to make a test for thick plate specimen, for example thicker than 40mm. Because, it is need large capacity loading apparatus to test such as thick plate specimen. In this research, the new configuration specimen for fatigue crack initiation tests was proposed. Using this new specimen, it is easy to carry out the fatigue clack initiation tests with relatively low cyclic loading and to observe a fatigue crack initiation behavior.

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Fatigue Crack Initiation around a Hole under Out-of-phase Biaxial Loading (이상 이축 하중 하에서 구멍 주위에서의 피로 균열 발생)

  • Huh, Yong-Hak;Park, Pi-Lip;Kim, Dong-Jin
    • Transactions of the Korean Society of Mechanical Engineers A
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    • v.27 no.10
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    • pp.1695-1702
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    • 2003
  • Fatigue crack initiation around a hole subjected to biaxial fatigue loads with a phase difference was investigated. Axial and torsional biaxial fatigue loads with different phase differences and biaxiality of 1/√3 were applied to thin-walled tubular specimens. Five phase differences of 0, 45, 90, 145 and 180 degrees were selected. Directions of the fatigue crack initiation around the hole were found to approach to the circumferential direction of the specimen with increment of the phase difference for fatigue tests with phase differences less than 90$^{\circ}$. Whereas directions for tests with phase differences greater than 90$^{\circ}$ got away from the circumferential direction and those were symmetric to the directions for tests with phase difference less than 90. . Furthermore, it was shown that the fatigue initiation life decreased with increment of phase difference for fatigue tests with phase differences less than 90$^{\circ}$, but it increased for tests with phase difference greater than 90$^{\circ}$. The crack initiation direction can be successfully explained by using the direction of the maximum tangential stress range obtained around the hole and at far-field.

Study on the Fatigue Crack Initiation Life uncle]r 3-Dimensional Rough Contact (3차원 거친 접촉하에서의 피로균열 시작수명에 관한 연구)

  • 김태완;구영필;조용주
    • Tribology and Lubricants
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    • v.18 no.2
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    • pp.160-166
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    • 2002
  • In case of rough contact fatigue, the accurate calculation of surface tractions is essential to the prediction of crack initiation life. Accurate Surface tractions influencing shear stress amplitude can be obtained by contact analysis based on the morphology of contact surfaces. In this study, to simulate rough contact under sliding condition, gaussian rough surface generated numerically in the previous study was used and to calculate clack initiation life in the substrate, dislocation pileup theory was used.