• Title, Summary, Keyword: Surface Crack

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An Experimental Study on the Growth Behavior of Multi-Surface-Cracks in Type 304 Stainless Steel at Elevated Temperature (고온하 304 스테인리스철의 표면규열의 성장거동에 관한 실험적 연구)

  • 서창민;신형섭;황남성;정대윤
    • Journal of Ocean Engineering and Technology
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    • v.9 no.1
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    • pp.63-72
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    • 1995
  • The crack which is discovered in various structures and machine elements is multi-cracks. Multi-cracks may cause serious problems because they grow individually, and coalesce into one and it leads to fracture. Fatigue tests have been carried out to study the growth and coalescence behavior of multi-surface-cracks initiated at the semicircular surface notch in type 304 stainless steel at elevated temperature. The results are as follows; When multi-surface-cracks are lying on the surface of material, the major surface crack has greater influence on the fatigue life than the subcracks. The aspect ratio of multi-surface-cracks is lower than that of single crack because of the interaction and coalescence of surface cracks. Crack growth shape turns to semiellipse from the semicircle notch. After coalescence, the surface crack length increases rapidly, and it leads to fracture. Further, the slope transition of Paris law was found in the da/dN-$\Delta$K$_1$ plots.

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Behavior of fatigue crack propagation for the deep non-through radial holed notch specimens (深孔 非貫通노치材의 疲勞크랙 傳播擧動에 관한 硏究)

  • 송삼홍;원시태
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.12 no.6
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    • pp.1327-1334
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    • 1988
  • In this study rotating bending fatigue tests have been carried out with the deep non-through radial holed notch specimens of low carbon steels(SM 22C). It is investigated that the behaviors of surface and interior fatigue crack propagation and the variations of the shape of the cracked surface on the magnitude of bending stresses. The Obtained results are summarized as follows. (1) The relations between [Crack length] and [Cycle ratio] are expressed by following eq. in the 0.1~0.6 range of N/ $N_{f}$ long[ crack length] = A + B [N/ $N_{f}$ ] In case of surface crack length, values of A and B are uniformed independent upon the magnitude of bending stress, but those are variable according to the magnitude of bending stress for interior crack length. (2) The following eq. is derived on the surface crack propagation rate, bending stress and surface crack length. (dl/dN)=(3.94*10$^{-12}$ ).sigma.$^{4.54}$l (3) Under small stress, interior crack propagation rate increase with the interior crack growth but it decrease for large stress. (4) The shape of cracked surface depends upon the magnitude of bending stress. Under small stress fatigue crack propagates as an semi-ellipse with semi-major axis of surface crack length with semi-major axis of interior crack length for large stress.s.

Surface crack propagation behavior and crack closure phenomena in 5083-H113 aluminum alloy (5083-H113 알루미늄合金의 表面균열進展擧動과 균열닫힘 現象)

  • 박영조;김정규;신용승;김영운
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.11 no.2
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    • pp.243-252
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    • 1987
  • The propagation and closure behavior of surface crack initiated at a sharply notched specimens were investigated in 5083-H113 aluminium alloy under constant amplitude of tension load by the unloading elastic compliance method. The crack shape (aspect ratio) was found to be approximately semicircular during the crack was being small and to be changed to semi-elliptical during it was being long. The propagation rate of a surface crack initiated from notch root decelerated with increasing crack length when the crack was small and then accelerated when it was large. The effect of stress ratio was large in lower .DELTA.K range, but the effective stress intensity factor range .DELTA.K$_{eff}$ was found to diminish the difference of the crack propagation rate. By considering the increase in crack closure stress with crack length and examining the microphotographs, plasticity-induced and roughness-induced crack closure mechanisms were predominant in the range of this study.y.

반타원 표면균열의 피로성장 거동에 관한 연구

  • 최용식;양원호;방시항
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.10 no.6
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    • pp.916-922
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    • 1986
  • This paper presents the preliminary results of an experimental study on surface crack growth under fatigue loadings. The objective of this paper is to assess the effect of the initial crack size on crack propagation behaviors. Transparent PMMA plate speciments with shallow circular arc notch were used. Crack growth behaviors were observed and measured in two directions by travelling microscopes. The fatigue crack initiated at the deepest part on the initial arc shaped notch and then propagated to depth direction as well as spreading gradually along the notch tip. A considerable number of cycles was needed until the depth crack spreaded to the surface notch tip. When the fatigue crack reached the surface notch tip the crack front became an approximate semi-ellipse, primary semi-elliptical crack. Test results suggest that the relationships between fatigue crack growth rate and stress intensity factor range in both directions can be expressed by power law (Paris) and that relationship in width direction depends upon the crack ratios a$_{1}$/b$_{1}$, of the primary semi-elliptical crack. The relationship between the nondimensional crack lengths in both directions can be represented as the formula: (a/t)$^{n}$ =B(2b/W+A) where n and A are constants and B is seems to be depended upon the crack ratio a$_{1}$/b$_{1}$.

Ultrasonic evaluation of small surface fatigue cracks initiating in residual stress zone (잔류응력 영역에서 발생한 작은 피로균열의 초음파 평가)

  • Kang Kae-Myung;Kim Jin-Yeon
    • Journal of the Korean Institute of Gas
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    • v.4 no.1
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    • pp.55-62
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    • 2000
  • A surface acoustic wave method for the evaluation of small fatigue crack initiated from a pit-type surface flaw is presented. In-situ ultrasonic experiments are performed for aluminum 2024-T3 alloy samples under the fatigue test. During the fatigue test, the surface acoustic wave reflection signal from the pit and crack is measured under different hold-stress levels. From the measured and predicted surface wave reflections the depths of fully and partially open cracks are determined and results are verified by comparing with SEM fractography The crack opening behavior of the fatigue crack is evaluated from the predicted effective crack depths. The method developed in this study can be applied to monitor and characterize crack initiation and propagation from pit-type surface flaws in the early stage of fatigue life.

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A study on the growth behaviors of surface fatigue crack initiated from a small-surface defect of 2024-T3 and brass (2024-T3 및 황동의 작은 표면결함재의 피로균열 성장특성에 관한 연구)

  • 서창민;오명석
    • Journal of Ocean Engineering and Technology
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    • v.10 no.1
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    • pp.53-64
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    • 1996
  • In this paper, rotating bending fatigue tests have been carried out to investigate the growth behabiors of surface fatigue crack initiated from a small artificial surface defect, that might exist in real structures, on 2024-T3 and 6:4 brass. The test results are analysed in the viewpoints of both strength of materials and fracture mechanics, it can be concluded as follows. The effect of a small artificial surface defect upon the fatigue strength is very large. The sensitivity of 2024-T3 on the defect is higher than that of 6:4 brass. The growth behavior of the surface fatigue crack of 2024-T3 is different from that of 6:4 brass. The growth rate of the surface fatigue crack of 2024-T3 is considerably rapid in the early stage of the fatigue life and apt to decrease in the later stage. It was impossible to establish a unifying approach in the analysis of crack growth begabior of 2024-T3 and 6:4 brass using the maximum stress intensity factor because of their dependence on stress level. But if the elastic strain and cyclic total strain intensity factor range were applied to obtain the growth rate of surface fatigue cracks of the materials, the data were found to be nearly coincided.

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Mutual Interference of Two Surface Cracks under Hertzian Contact Loading (Hertz 접촉하중하에서의 복수표면균열의 상호간섭)

  • Kim, Sang-Woo;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.3048-3057
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    • 1996
  • Analysis model containing two inclined surface cracks on semi-infinite elastic body is established and analyzed on the basis of linear fracture mechanics to examine mutual interference of two surface cracks. Muskhelishvili's complex stress functions are introduced and a set of singular integral equations is obtained for a dislocation density function. The stress intensity factors at crack tip are obtained by using the Gerasoulis'method. When two surface cracks are parallel and have the same length, the values of $K_1$and $\Delta K_11$(variation of $K_11$) for crack 1 and crack 2 decrease by the mutual interference of two surface cracks as the distance between the two surface cracks shortens. The effect of mutual interference is remarkable in high friction coefficient. In case that two surface cracks are parallel, the values of $K_1$and $\Delta K_11$for crack 2 decrease as the length ratio ot crack 2 to crack 1 becomes small. As the crack inclination angle rises, the value of $K_1$ and the mutual interference of $K_1$for crack 2 increase and the value of$\Delta K_11$ for crack 1 becomes smaller than that for crack 2.

The Effect of Stress Ratio on the Surface Crack Growth Behavior in 7075-T651 Aluminum Alloy (7075-T651 Al合金의 表面균열進展에 미치는 應力比의 影響)

  • 박영조;김정규;신용승;김성민
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.10 no.1
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    • pp.62-69
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    • 1986
  • Fatigue surface crack growth was studied in 7075-T651 aluminum alloy plates subjected largely to bending loads. The surface crack length and its depth were measurement by the unloading elastic compliance method. The surface crack growth rate dc/dN, on the surface and da/dN, in the depth direction were obtained by the secant method. The stress intensity factor range .DELTA.K was computed by means of Newman and Raju equation. The aspect ratio a/c was presented in form of a/c=0.815-0.853(a/T). The effect of the stress ratio on the stable surface crack growth rates under increasing .DELTA.T is larger in lower .DELTA.K, while the relation between dc/dN, da/dN and the effective stress intensity factor range .DELTA.K$_{eff}$ is weakly dependent on the stress ratio.o.

Life Prediction by Retardation Behavior of Fatigue Crack and its Nondestructive Evaluation (피로균열의 지연거동에 따른 수명예측 및 비파괴평가)

  • Nam, Ki-Woo;Kim, Seon-Jin
    • Journal of Ocean Engineering and Technology
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    • v.13 no.3
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    • pp.36-48
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    • 1999
  • Fatigue life and crack retardation behavior after penetration were experimentally examined using surface pre-cracked specimens of aluminium alloy 5083. The Wheeler model retardation parameter was used successfully to predict crack growth behavior after penetration. By using a crack propagation rule, the change in crack shape after penetration can be evaluated quantitatively. Advanced, waveform-based acoustic emission (AE) techniques have been successfully used to evaluate signal characteristics obtained form fatigue crack propagation and penetratin behavior in 6061 aluminum plate with surface crack under fatigue stress. Surface defects in the structural members are apt to be origins of fatigue crack growth, which may cause serious failure of the whole structure. The nondestructive analysis on the crack growth and penetration from these defects may, therefore, be one of the most important subjects on the reliability of the leak before break (LBB) design. The goal of the present study is to determine if different sources of the AE could be identified by characteristics of the waveforms produced from the crack growth and penetration. AE signals detected in four stages were found to have different signal per stage. With analysis of waveform and power spectrum in 6061 aluminum alloys with a surface crack, it is found to be capabilities on real-time monitoring for the crack propagation and penetration behavior of various damages and defects in structural members.

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The Fatigue Crack Initiation Life under the Rolling Contact of Rough Surfaces using Critical Plane Approach (임계평면법을 이용한 거친 표면의 구름접촉시 피로균열 시작수명)

  • 설광조;김태완;조용주
    • Tribology and Lubricants
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    • v.19 no.1
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    • pp.26-30
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    • 2003
  • The stresses of surface and substrate under the rough surface contact are irregular. Using rainflow counting method for irregular stresses, the fatigue surface crack initiation lift was calculated. With the surface generated by computer, this paper figures out the random load generated by contacting to the rough surface, analyzes the stress of its subsurface, and calculates the fatigue crack initiation life of the rough surface fatigue theory.