• Title, Summary, Keyword: Surface Crack

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Reliability Assessment of Buried Pipelines with a Circumferential Surface Elliptical Crack under Axial Stress (축직각 표면타원균열이 존재하는 매설배관의 축방향응력에대한 건전성 평가)

  • Lee, Eok-Seop;Hwang, In-Hyeon
    • Journal of the Korean Society for Precision Engineering
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    • v.18 no.4
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    • pp.160-166
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    • 2001
  • The theoretical analyses for stresses induced in axial direction in the buried pipelines are reviewed. The influences of the axially directed stresses on the surface elliptical crack are studied in detail and thus some engineering technical informations are provided to use reliability assessment of buried pipelines. The change in temperature, the effect of inner pressure and soil friction in the buried pipeline constrained in axial direction are included to determine the axial stresses in the buried pipeline. Furthermore, the stress induced by the pipeline bending are also considered. The stress intensity factors calculated by two models such as a simple plane crack and an elliptical surface crack for a circumferential surface elliptical crack are compared.

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Finite Element Simulation of Surface Pitting due to Contact Fatigue (접촉피로에 의한 표면피팅의 유한요소 시뮬레이션)

  • Rhee, Hwan-Woo;Kim, Sung-Hun
    • Journal of the Korean Society for Precision Engineering
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    • v.27 no.3
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    • pp.80-88
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    • 2010
  • A simple computational model for modeling of subsurface crack growth under cyclic contact loading is presented. In this model, it is assumed that the initial fatigue crack will initiate in the region of the maximum equivalent stress at certain depth under the contacting surface. The position and magnitude of the maximum equivalent stress are determined by using the equivalent contact model, which is based on the Hertzian contact conditions with frictional forces. The virtual crack extension method is used for simulation of the fatigue crack growth from the initial crack up to the formation of the surface pit due to contact fatigue. The relationships between the stress intensity factor and crack length are then determined for various combinations of equivalent contact radii and loadings.

Development of the Advanced NDI Technique Using an Alternating Current : the Evaluation of surface crack and blind surface crack and the detection of defects in a field component (교류전류를 이용한 새로운 비파괴탐상법의 개발;표면결함과 이면결함의 평가 및 실기 부재의 결함 검출)

  • Kim. H.;Lim, J.K.
    • Journal of Welding and Joining
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    • v.13 no.2
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    • pp.42-52
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    • 1995
  • In the evaluation of aging degradation on the structural materials based on the fracture mechanics, the detection and size prediction of defect are very important. Aiming at nondestructive detection and size prediction ol defect with high accuracy and resolution, therefore, an lnduced Current Focusing Potential Drop(ICFPD) technique has been developed. The principle of this technique is to induce a focusing current at an exploratory region by an induction wire flowing an alternating current(AC) that is a constant ampere and frequency. Defects are assessed with the potential drops that are measured the induced current on the surface of metallic material by the potential pick-up pins. In this study, the lCFPD technique was applied for evaluating the location and size of the surface crack and blind crack made in plate specimens, and also for detecting the defects existing in valve, a field component, that were developed by SCC etc. during the service. The results of this present study show that surface crack and blind crack are able to defect with potential drop. these cracks are distinguished with the distribution of potential drop, and the crack depths can be estimated with each normalized potential drop that are parameters estimating the depth of each type crack. In the field component, the defects estimated by experiment result correspond with those in the cutting face of the measuring point within a higher sensitivity.

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Prevention of Exit Crack in Mirco-drilling of Soda-lime Glass (유리의 미세구멍 가공시 출구 크랙 발생 방지)

  • 박병진;최영준;주종남
    • Proceedings of the Korean Society of Precision Engineering Conference
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    • pp.1052-1055
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    • 2001
  • In micro-drilling of brittle materials including glass, cracks occur at the exit surface. In drilling glass, the main type of crack is cone crack. Cone crack is generated by thrust force acting at the bottom surface of the workpiece. Cone crack size could be reduced by changing cutting conditions, but cone crack still existed. Two methods were proposed to prevent crack formation and perfect hole shapes were obtained. One method is attaching two glass plates with water and the other method is constraining two glass plates. The proposed methods eliminated tensile stress acting on the exit surface of glass and prevented crack propagation.

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Fatigue Crack shape Variations by a Residual Stress and Fatigue Life Predition (잔류응력에 의한 피로균열면 형상변화 및 수명예측)

  • 강용구;서창민;박원종
    • Journal of Ocean Engineering and Technology
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    • v.7 no.2
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    • pp.68-78
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    • 1993
  • Fatigue crack shape variation by a residual stress during crack growth and life predition are studied. An analytical method is presented to predict the influence of a residual stress due to heattreatment on crack shape variations. Computer simulation results using this me thod are graphically shown that crack growth rate to surface direction are decreased due to compressive residual stress exisiting in surface area. These results are commpared with experimental results. The fatigue life is also predicted by computer simulation of crack aspect ratio variation which is based on the surface crack length increment per unit cycle calculated from a-N diagram. Predited life is about 12 percent lower than experimental life.

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Propagation Characteristics of a Surface Crack on a Semi-Infinite Body Due to Frictional Heating (마찰열에 의한 반무한체 표면균열의 전파특성)

  • Park, Jun-Ho;Park, Eun-Ho;Kim, Chae-Ho;Kim, Seock-Sam
    • Transactions of the Korean Society of Mechanical Engineers A
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    • v.20 no.10
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    • pp.3126-3134
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    • 1996
  • In this paper, to examine the propagation of inclined surface crack due to frictional heating, analytic model is considered as the semi-infinite elastic body subjected to the thermo-mechanical loading of an asperity moving with a high speed. Considering the moving of frictional heat source and convection on a semi-infinite surface having inclined crack, theoretical analysis was carried out to estimate the propagation characteristics of thermo-mechanical crack. Numerical results showed that stress intensity factor $K_\prod/P_0\sqrt{c}$ is increasing with increasing velocity and frictional coefficient, inclined degree, decreasing crack length and the maximum value of it is positioned at the trailing edge. So it is shown that the propagation probability of surface crack is high at the trailing edge of contact area as increasing velocity and frictional coefficient, inclined degree, as decreasing crack length.

The analysis of fracture stress using reflection coefficient of surface acoustic wave (탄성표면파의 반사계수를 이용한 파괴응력의 해석)

  • Shin, J.S.;Kim, J.K.;Jun, K.S.
    • The Journal of the Acoustical Society of Korea
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    • v.10 no.3
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    • pp.13-18
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    • 1991
  • In this study, the analysis technique of fracture stress using the reflection coefficient of SAW reflected from a brittle solid with surface crack has been studied. Fracture stress of brittle solid with surface crack has been obtained by the function of the critical stress intensity factor and the maximum normalized stress intensity factor of the crack in the body. And the maximum normalized stress intensity factor of a surface crack can be inferred from a measurement of reflection coefficient of SAW. In experiment, the surface cracks ranging from 0.5mm to 0.9mm in crack depth has been made at the center of each Pyrex disc, and the SAW wedge transducer has been set up for the pitch-catch mode. It has been compared the theoretical values of the fracture stress calculated from the reflection coefficient of SAW with the values of the fracture stress measured from UTM.

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Influence of crack geometry on fatigue crack growth behavior in 5083- H113 aluminium alloy (5083-H113 A1 합금의 피로균열진전거동에 미치는 균열형태의 영향)

  • 김정규;신용승;윤의박
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.12 no.4
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    • pp.781-789
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    • 1988
  • The fatigue crack growth and crack closure behavior of long through-thickness cracks and small half-penny shaped surface cracks were investigated in 5083-H113 Aluminum alloy under constant amplitude testing by the unloading elastic compliance method. It was found that, in the Region II, the crack growth behavior of both through-thickness and surface cracks exhibited the tri-linear form with two transitions and no concern with stress ratio R. In the Region I $I_{ab}$ and I $I_{b}$, through-thickness cracks grew faster than surface cracks in length direction, but at .DELTA. K .leq.4 MPa.root.m for R=0.1 the growth rates of surface cracks in depth direction, grew faster than those of through-thickness cracks. When the crack closure was considered, the growth rates of through-thickness cracks lay between the growth rates of depth direction and the growth rates of length direction in surface cracks. It is suspected that this was caused by the difference of crack closure at depth and length direction of surface cracks.s.

A Simplified Estimation of Stress Intensity Factor on the Hertzian Contact

  • Jin, Songbo;Kim, Seock-Sam
    • KSTLE International Journal
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    • v.1 no.1
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    • pp.8-11
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    • 2000
  • A surface crack in a semi-infinite body under Hertzian contact was considered. The simplified method used to estimate stress intensity factor K for specimen was extended to the model which is chosen in this paper. Very satisfactory results are obtained comparing with those known and it is proved that the method is more convenient than other methods. The results of the analysis show that due to the presence of $K_I$ for unlubricated condition, mode I fracture is active in the field below the surface and the maximum $K_{I}$ is obtained when the trailing edge of Hertzian contact reaches a position over the crack. The magnitudes of stress intensity factors $K_I$ and $K_Il$ increase with increasing friction forces. For a surface crack perpendicular to the contact surface, the stress intensity factor $K_I$ reaches its maximum value at a depth very close to the surface. Driving forve fer crack initiation and propagation is $K_I$ for unlubricated condition and $K_Il$ for both fluid and boundary lubricated condition.n.

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A Study on Physically small Surface Fatigue Crack Growth Behavior in 7075-T651 Aluminum Alloy (7075-T651 AI 합금에 있어서 물리적 미소 표면 피로균열 성장거동에 관한 연구)

  • Sin, Yong-Seung;Seo, Seong-Won;Yu, Heon-Il
    • Journal of the Korean Society for Precision Engineering
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    • v.9 no.1
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    • pp.106-117
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    • 1992
  • In this study, the propagation behaviour and the closure phenomena of physically small surface cracks were investigated by the techinque of the Kikukawa-unloading elastic compliance method using a back face strain gage. The surface cracks initiated and propagated from notched specimens under constant amplitude bending load. The crack shape (aspect ratio) with approximately semi-circular at the early stage was changed to semi-elliptical as the cracks grew larger. The crack depth (a) could be expressed uniquenly by the crack length (c). The dependence of the crack propagation rate on the stress ratio R was strongly related in the lower ${\Delta}K$ range. The deceleration of the surface crack propagation rate was prominent in lower R during the crack length was small. When the propagation rate was rearranged with the effective stress intensity factor range ${\Delta}$K_{eff} the dependence of the crack propagation rate on the stress ratio R was found to be diminshed. These were caused by the crack closure phenomena that was most prominent at the lower propagation rate. The mechanism of crack closure phenomena was dominated by the plasticity-induced mechanism.

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