• Title/Summary/Keyword: Fatigue crack

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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 of Stress ratio on the Fatigue Crack Growth Characteristics of Pressure Vessel SA516 Street at Low Temperature (저온 압력용기용 SA516강의 응력비에 따른 피로크랙 전파특성에 관한 연구)

  • 박경동;하경준
    • Proceedings of the KWS Conference
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    • 2001.05a
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    • pp.220-223
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    • 2001
  • In this study, CT specimens were prepared hem ASTM SA516 which was used for pressure vessel plates for room and low temperature service. And we got the following characteristics from fatigue crack growth test carried out in the environment of room and low temperature at $25^{\circ}C$, -3$0^{\circ}C$, -6$0^{\circ}C$, -8$0^{\circ}C$, -l$0^{\circ}C$ and -l2$0^{\circ}C$ and in the range of stress ratio of 0.1, 0.3 by means of opening mode displacement. At the constant stress ratio, the threshold stress intensity factor range ΔK$_{th}$ in the early stage of fatigue crack growth ( Region I ) and stress intensity factor range ΔK in the stable of fatigue crack growth ( Region II) was increased in proportion to descend temperature. It assumed that the fatigue resistance characteristics and fracture strength at low temperature is considerable higher than that of room temperature in the early stage and stable of fatigue crack growth region. The straight line slope relation of logarithm da/dN - ΔK in Region II, that is, the fatigue crack growth exponent m increased with descending temperature at the constant stress ratio. It assumed that the fatigue crack growth rate da/dN is rapid in proportion to descend temperature in Region H and the cryogenic-brittleness greatly affect a material with decreasing temperature.e.greatly affect a material with decreasing temperature.

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A Study on Evaluation of Crack Opening Point in High Strength Aluminum Alloy(I) (고강도 알미늄 합금재에 있어서 크랙열림점 평가에 관한 연구(I))

  • 최병기
    • Journal of the korean Society of Automotive Engineers
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    • v.15 no.1
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    • pp.100-106
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    • 1993
  • This paper aims to synthesize the research on fatigue fracture mechanisms of high strength aluminum alloys which are widely used in motorcars or airplanes to prevent accidents. To measure the data of crack opening ratio, the same materials and method are used for evaluating the fatigue crack propagation rate as an effective stress intensity factor. But, many researchers have brought different results. An exact crack opening ratio was, therefore, proposed for getting a more accurate fatigue crack propagation rate. The main conclusions obtained are as follows. (1) As a result of the fatigue test, the value of the crack opening ratio is the same regardless of the stress ratio. (2) The value of crack opening ratio is different according to the measuring point. After measuring the crack propagation rate by using an effective stress intensity factor, the crack opening ratio value measured at the crack mouth by a clip gage, or measured rear of the specimen by a strain gage is more accurate than that by any other measuring test.

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A Study on the Fatigue Behavior of Resistance Spot Welded Part of 5182 Aluminum Aloy Sheet (5182 Al합금판의 전기저항 점용접부 피로거동에 관한 연구)

  • 신현일;박용석;강성수
    • Journal of Welding and Joining
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    • v.16 no.2
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    • pp.84-92
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    • 1998
  • On this study, the variations of hardness and microstructure were observed at he spot-welded part of 5182 alminum alloy sheets with thickness of 1.2 mm. The hardness of spot-welded part of aluminum alloy indicated the lowest value at nugget center. Also, the position where fatigue crack exists was investigated by surveying microstructure of the spot-welded sections. Mean load-deformation diagrams were obtained from static tensile test. Fracture was occurred completely within 5 mm after transforming elastic into plastic area. Fatigue test was stopped when the specimens of fatigue test had the final displacement of 0.2mm and measured fatigue bending angle and crack length. This study utilized them, investigated the relations between fatigue bending angle and fatigue crack length and made a estimation of the fatigue fracture life of resistance spot welded part of 5182 aluminum alloy sheet. The relative equation o fatigue crack length and fatigue failure life can be represented by {TEX}$L_{C}${/TEX}=α{TEX}$N_{f}^ {β}${/TEX}.

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A Study on Corrosion Fatigue Crack Growth Behavior in Al-Alloy 7075-T651 (I) (Al-Alloy 7075-T651의 부식피로균열 성장거동에 관한 연구(I))

  • 김봉철;한지원;우흥식
    • Journal of the Korean Society of Safety
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    • v.13 no.4
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    • pp.113-120
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    • 1998
  • Fatigue crack growth rates(i.e. crack initiation and crack growth of short and long crack) are investigated using commercial plates of high strength Al alloy 7075-T651 for the transverse-longitudinal(T-L) direction in air, water and sea water. Also, the evaluation direct current potential drop(D.C.P.D) method and the fractographical analysis by SEM are carried out. Near threshold region, short crack growth rates were much faster than those of comparable long cracks, and these short crack growth rates actually decrease with increasing crack growth and eventually merge with long crack data. Fatigue crack propagation rates in aggressive media(i.e. sea water) increase noticeably over three times those in air. One of the most significant characters in this phenomenon as a corrosion-fatigue causes an acceleration in crack growth rates. Sea water environment, particularly Cl$^{[-10]}$ solution brings the most detrimental effects to aluminum alloy. The result of fractographical morphology in air, water and sea water by SEM shows obvious dimpled rupture and typical striation in air, but transgranular fracture surface in water and sea water.

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A Program Development of Life Prediction Simulation for Multi-Surface Cracks on the Finite Plate (무한 평면체에 존재하는 복수 표면균열의 성장에 대한 수명예측용 시뮬레이션 개발에 관한 연구)

  • 황남성;서창민;남승훈
    • Journal of Ocean Engineering and Technology
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    • v.11 no.4
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    • pp.61-75
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    • 1997
  • The social demand urges us to use some equipments and structures in high temperature environment. By this occasion, the necessity of studying the fatigue crack growth is an important aspect of new materials. However, the present situation is rarely to accumulate the fatigue data. Especially, 1Cr-1Mo-0.25V steel and 304 stainless steel have been increased to be used under the severe condition of high temperature. And so, the fatigue estimation of those materials is important and appropriate. Fatigue tests have been carried out to examine the crack initiation, growth behaviour for the small fatigue crack of 1Cr-1Mo-0.25V steel and 304 stainless steel at room temperature and 538^{\circ}C$. The remote measurement system which has many merits of checking and saving the image for detailed examination was applied to closely detect the crack length. Generally, the fatigue crack initiated in the form of multiple cracks and grew each other. And then it coalesced to become a major crack. The major crack governed the rest of the fatigue life. In the growing process, each peripheral cracks interact and grow for a certain period. After then, it coalesced and fractured. On the basis of the above experimental data for the small crack, a simulation program was developed to predict the residual life time and to estimate the integrity of machine elements and structures. At the same time, the simulation was extended to 1Cr-1Mo-0.25V steel. The simulation results have shown a good agreement to those of the experimental ones for both materials of 1Cr-1Mo-0.25V steel and 304 stainless steel with small cracks. The NASCRAC has applied to compare the fatigue life with the experimental results. And so, it can be said that the simulation program is valuable tools to the industrial fields.

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A Study on Fatigue Fracture Behavior of Laser Beam Welding and Steel with Different Materials ($CO_2$ 레이저 용접 이종재료강의 피로파괴거동에 관한 연구)

  • Han, M.S.;Suh, J.;Lee, J.H.;Kim, J.O.;Jeon, S.M.
    • Laser Solutions
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    • v.9 no.2
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    • pp.1-9
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    • 2006
  • In this paper, we investigated the characteristics of fatigue fracture on TB(Tailored Blank) weldment by comparing the fatigue crack propagation characteristics of base metal with those of TB welded sheet used for vehicle body panels. We also investigated the influence of center crack on the fatigue characteristic of laser weld sheet of same thickness. We conducted an experiment on fatigue crack propagation on the base metal specimen of 1.2mm thickness of cold-rolled metal sheet(SPCSD) and 2.0mm thickness of hot-rolled metal sheet(SAPH440) and 1.2+2.0mm TB specimen. We also made an experiment on fatigue crack propagation on 2.0+2.0mm and 1.2+1.2mm thickness TB specimen which had center crack. The characteristics of fatigue crack growth on the base metal were different from those on 1.2+2.0mm thickness TB specimen. The fatigue crack growth rate of the TB welded specimens is slower in low stress intensity factor range $({\Delta}K)$ region and faster in high${\Delta}K$ region than that of the base metal specimens. The slant crack angle slightly influenced the crack propagation of the TB specimen of 2.0+2.0mm thinkness.

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The Effects of the Microstructural Change of Dual Phase Steel on Fatigue Fracture Propagation (복합조직강의 미시조직변화가 피로파괴전파에 미치는 영향)

  • Oh, Sae-Wook;Kim, Ung-Jip
    • Journal of Ocean Engineering and Technology
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    • v.5 no.2
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    • pp.58-66
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    • 1991
  • Not only difference of fatigue crack growth and propagation behavior resulted from the grain size, the hardness ratio and volume fraction in M.E.F. dual phase steel composed of martensite in hard phase and ferrite in soft phase, but also the effects of the plastic constraint were investigated by fracture mechanics and microstructural method. The main results obtained are as follows: 1) The fatigue endurance of M.E.F. steel increases with decreasing the grain size, increasing the ratio of hardness and volume fraction. 2) The initiation of slip and crack occures faster as the stress level goes higher. These phenomena result from the plastic constraint effect of the second phase. 3) The crack propagation rate in the constant stress level is faster as the grain size gets larger, the ratio of hardness lower and volume fraction smaller.

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Reliability Engineering Approach to Fatigue Crack Growth Rate Under Random Loading Using DC Eletrical Potential Method (직류전위차법을 이용한 랜덤하중하의 피로균열 진전율에 대한 신뢰성 공학적 연구)

  • Bae, Sung-In
    • Transactions of the Korean Society of Mechanical Engineers A
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    • v.20 no.2
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    • pp.473-480
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    • 1996
  • Automatic fatigue crack length measuring system using DC electrical potential method and the system control program for automatic fatigue testing under random load condition were made in this study. And using these system and control program, fatigue tests were executed under constant and random load condition. As the result, the propagation of crack in random loading can be represented Paris equaiton and log normal probability function. But constant and random load test show different crack propagation properties.

A Stochastic Study on Fatigue Crack Propagation and Retardation Behavior of Pressure Vessel Steel (압력용기용강의 피로균열전파 및 지연거동에 관한 확률통계적 연구)

  • 김선진;남기우;김부안
    • Journal of Ocean Engineering and Technology
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    • v.9 no.1
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    • pp.132-141
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    • 1995
  • The purpose of the present study is to investigate the statistical characteristics of m and C in the fatigue crack propagation law, da/dN=C(.DELTA.K)/sup m/ and to studies on the randomness of fatigue crack propagation and retardation behavior. Fatigue tests were perfomed on 32 CT specimens of SPV50 steel under the same one condition. First, the value of m and C were determined for each specimen, and all the data were analyzed statistically. second, the material's resistance to fatigue crack propagation is modeled as a stchastic process, which varies randomly along the crack path. The statistical analysis of the material resistance is performed with the data obtained by constant load controlled tests. Finally, retardation behavior was examined experimentally by using a CT specimen, and a retardation parameters were analyzed statistically.

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