• 제목/요약/키워드: Crack Propagation Stage

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過大, 過小應力下에서의 疲勞크랙發생 傳播擧動 (II) - 탄소동재의 내부크랙을 중심으로- (Behavior of Initiation and Propagation of Fatigue Crack under Periodic Overstressing(II) - About the Inside Crack of the Caron Steel-)

  • 송삼홍;원시태
    • 대한기계학회논문집
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    • 제10권2호
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    • pp.188-197
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    • 1986
  • 본 연구에서는 응력이 변동되는 경우 내부크랙 전파특성을 중심으로 내부크랙 전파거동을 표면크랙 전파거동과 비교 검토하였다.

직접측정법을 이용한 혼합모드 하중 하에서 피로균열의 닫힘과 전파거동 (Fatigue Crack Closure and Propagation Behavior Under Mixed-Mode Loading Observed by the Direct Measuring Method)

  • 송삼홍;서기정;이정무
    • 한국정밀공학회지
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    • 제22권6호
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    • pp.152-158
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    • 2005
  • The stress conditions acting on the practical structure are complex, and thus most cracks existing in the practical structures are under mixed-mode loading conditions. The effect of shear load component of mixed-mode loading acts more greatly in the stage of crack initiation and initial propagation than crack propagation stage. Hence, research on the behavior in the stage of crack initiation and initial propagation need to be examined in order to evaluate behavior of mixed-mode fatigue cracks. In this study, the crack tip displacement(CTD) was measured by using the direct measuring method(DMM). We examined the behavior at crack tip by determining crack opening load$(P_{op})$. From the test results, the propagation behavior of mixed-mode fatigue cracks was evaluated by considering mixed-mode crack closure. Also, we examined the characteristic of crack propagation under mixed-mode loading with crack propagation direction.

다단계 피로하중 실험을 통한 균열 발생 및 전파단계에서 파괴모드 II 영향 분석 (Effects of Failure Mode II on Crack Initiation and Crack propagation Steps Using Multilevel Fatigue Loading Test)

  • 홍석표;박세민;김주희
    • 대한기계학회논문집A
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    • 제41권9호
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    • pp.853-860
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    • 2017
  • 본 연구는 균열 발생 및 전파단계에서 하중모드 II의 영향을 평가하기 위해 피로균열 하한계 영역의 혼합모드 I+II 하중을 통해 실험적으로 평가하였다. 균열 발생단계(Stage I)에서는 혼합모드상태에서 하중작용 각도(${\theta}$)가 증가할수록 모드 II 영향으로 인하여 낮은 하중에서 균열이 발생하고, 균열 전파단계 (Stage II)에서는 균열전파 속도는 감소하였다. 다단계 하중작용 각도변화에 따른 하중모드 II영향은 균열전파단계 실험을 통해 평가하였다. 혼합모드 I+II 하중 작용 시 작용각도 ($0^{\circ}{\rightarrow}{\theta}{\rightarrow}60^{\circ}$) 증가에 따라 피로균열전파속도는 감소하였으며 늦게 발생한 균열에서도 마찬가지로 감소하였다. 작용각도가 ${\theta}{\geq}75^{\circ}$ 범위에서는 하중작용각도 증가에 따라 피로균열전파속도가 증가하고 피로수명이 감소하는 것을 확인하였다.

분산분석법을 이용한 AZ31 합금의 피로균열성장에 미치는 시편두께 효과 평가 (Evaluating the Effect of Specimen Thickness on Fatigue Crack Growth in AZ31 Alloy Using ANOVA)

  • 최선순
    • 한국기계가공학회지
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    • 제19권6호
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    • pp.9-16
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    • 2020
  • This study aims to assess the effects of specimen thickness (ST) on fatigue crack growth in the early stages of crack propagation and near failure in magnesium alloys. The analysis of variance (ANOVA) method was adopted because fatigue crack propagation in magnesium alloys exhibits statistical behavior. The equality of variance test and residual diagnostics were performed on the grown cracks to confirm the validity of ANOVA by verifying the normal distribution and mutual independence of the residuals and their homoscedasticity. ANOVA confirmed that ST heavily impacts crack growth; i.e., when ST is smaller, cracks grow faster in the early crack propagation stage and break more quickly before the formation of larger cracks. We found that ST significantly affects fatigue crack growth in the early crack propagation stage and near the failure stage in magnesium alloys. The regression model was also used to predict crack formation near the failure stage.

압연강판(壓延鋼板)의 피로균열(被虜龜裂) 전파거동(傳播擧動)에 대(對)한 연구(硏究) (A Study on the Propagation Behaviour of the Fatigue Cracks in Rolled Steel Plates)

  • 강창수
    • 대한조선학회지
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    • 제12권2호
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    • pp.43-58
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    • 1975
  • There are many reports on fatigue crack of metallic materials but most of them relate crack propagation rate to stress intensity factor. The problem of crack propagation is not yet clarified, especially the bridge between micro and macro phenomena In this experiment rotating bending fatigue tests have been carried out with smoothed specimen of rolled steel plates including 0.2% carbon under application of three stress conditions to investigate the slip band and the crack propagation behaviour. The results obtained are as follows; 1) The length of cracks which have grown at initial crack tips can be expressed as follows; $l=Ae^{BNr}$(A,B: constant, $N_r$: cycle ratio) $\frac{dl}{dN}=\frac{AB}{N_f}{\cdot}e^{BNr}$($N_f$:fatigue life) 2) The ratio of slipped grain number to total grain number is $S_f=7{\sigma}-5.6$-5.6{\sigma}_c$($\sigma$: stress amplitude) (${\sigma}_c$: fatigue limit) 3) When the fatigue process transfers from Stage I to Stage II, the crack which propagates into specimen changes its direction from that of the maximum shear stress to the direction of perpendicular to principal stress and this is same in the circumferential direction of specimen. the crack propagation behaviors of both sides of a crack are different each other when they approach to the grain boundary.

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일반구조용 압연강재의 부식피로균열전파거동에 미치는 시험편 두께의 영향 (Effect of Specimen Thickness to Corrosion Fatigue Crack Propagation Behavior of Structure Rolled Steel)

  • 조약래
    • 수산해양기술연구
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    • 제28권3호
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    • pp.286-294
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    • 1992
  • In this paper, the corrosion fatigue crack propagation behavior of structure rolled steel (SWS 41C) was investigated by changing the thickness, and this experiment was done by the three point bending corrosion fatigue tester. The main results obtained are as follows: 1) As the thickness of specimen becomes thicker, the corrosion sensitivity to initial stage crack becomes some sensitive, and that the fatigue life becomes more sensitive. 2) The crack growth rate to initial stage crack (da/dN) was retarded as the thickness of specimen becomes thicker. But after initial stage crack, as the thickness of specimen is more thicker, da/dN is more rapid. 3) As the corrosion fatigue crack length grows, the accelerative factor of thick specimen (t=12mm) is more higher than that of thin specimen (t=6mm). 4) As the corrosion fatigue crack length grows, the corroson potential of both thick specimen and thin specimen becomes more less noble potential, however thick specimen (t=12mm) tends to more less noble potential than that of thin specimen(t=6mm).

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Cohesive modeling of dynamic fracture in reinforced concrete

  • Yu, Rena C.;Zhang, Xiaoxin;Ruiz, Gonzalo
    • Computers and Concrete
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    • 제5권4호
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    • pp.389-400
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    • 2008
  • In this work we simulate explicitly the dynamic fracture propagation in reinforced concrete beams. In particular, adopting cohesive theories of fracture with the direct simulation of fracture and fragmentation, we represent the concrete matrix, the steel re-bars and the interface between the two materials explicitly. Therefore the crack nucleation within the concrete matrix, through and along the re-bars, the deterioration of the concrete-steel interface are modeled explicitly. The numerical simulations are validated against experiments of three-point-bend beams loaded dynamically under various strain rates. By extracting the crack-tip positions and the crack mouth opening displacement history, a two-stage crack propagation, marked by the attainment of the peak load, is observed. The first stage corresponds to the stable crack advance, the second one, the unstable collapse of the beam.

압축잔류응력이 스프링강의 고온환경 피로균열 진전거동에 미치는 영향 (Effect of Compressive Residual Stress on the High Temperature fatigue Crack Propagation Behavior of Shot-peened Spring Steel)

  • 정찬기;박경동
    • 한국해양공학회지
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    • 제16권5호
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    • pp.73-79
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    • 2002
  • In this paper, the effect of the compressive residual stresses was obtained at the test conditions of the higher temperature than the ambient temperature. The examination was performed with the CT specimen result of the material(JISG SUP9) which is being commonly used for the marine engine parts and the ocean structures. As a result, the test conditions at the higher temperature were acquired considering the peak values of the compressive residual stresses of the specimens and the effect on the fatigue crack propagation speed da/dN in stage II and the threshold stress intensity factor range Δth in stage I. Also the material constant C and the fatigue crack propagation index m in the formula of Paris Law da/dN=C (ΔK)$^{m}$ were suggested to estimate the dependence on the test temperature.

Experimental study on propagation behavior of three-dimensional cracks influenced by intermediate principal stress

  • Sun, Xi Z.;Shen, B.;Zhang, Bao L.
    • Geomechanics and Engineering
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    • 제14권2호
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    • pp.195-202
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    • 2018
  • Many laboratory experiments on crack propagation under uniaxial loading and biaxial loading have been conducted in the past using transparent materials such as resin, polymethyl methacrylate (PMMA), etc. However, propagation behaviors of three-dimensional (3D) cracks in rock or rock-like materials under tri-axial loading are often considerably different. In this study, a series of true tri-axial loading tests on the rock-like material with two semi-ellipse pre-existing cracks were performed in laboratory to investigate the acoustic emission (AE) characteristics and propagation characteristics of 3D crack groups influenced by intermediate principal stress. Compared with previous experiments under uniaxial loading and biaxial loading, the tests under true tri-axial loading showed that shear cracks, anti-wing cracks and secondary cracks were the main failure mechanisms, and the initiation and propagation of tensile cracks were limited. Shear cracks propagated in the direction parallel to pre-existing crack plane. With the increase of intermediate principal stress, the critical stress of crack initiation increased gradually, and secondary shear cracks may no longer coalesce in the rock bridge. Crack aperture decreased with the increase of intermediate principal stress, and the failure is dominated by shear fracturing. There are two stages of fracture development: stable propagation stage and unstable failure stage. The AE events occurred in a zone parallel to pre-existing crack plane, and the AE zone increased gradually with the increase of intermediate principal stress, eventually forming obvious shear rupture planes. This shows that shear cracks initiated and propagated in the pre-existing crack direction, forming a shear rupture plane inside the specimens. The paths of fracturing inside the specimens were observed using the Computerized Tomography (CT) scanning and reconstruction.

살물선의 피로균열 전파해석과 피로강도 평가에 대한 연구 (A Study on Fatigue Crack Propagation Analysis and Fatigue Strength Evaluation for Bulk Carrier)

  • 엄동석;김충희
    • Journal of Welding and Joining
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    • 제11권4호
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    • pp.112-124
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    • 1993
  • It has been reported that fatigue damage sometimes occurred at the stress concentrated and dynamic loaded structural members of bulk carrier. In this paper, studies on fatigue strength of hull structures are reviewed, and the program for evaluating fatigue strength is developed. And the fatigue crack initiation and propagation on the end part of cargo hold frame of bulk carrier were calculated by FEM stress analysis and the fatigue strength evaluation program. These method can be applied not only to the crack initiation life but also to crack propagation life for the hull structural members at the hull design stage and be effective as the guideline to prevent the crack initiation or to estimate the fatigue strength for repairing of the fatigue damaged structures of real ships.

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