• Title/Summary/Keyword: Fatigue crack

Search Result 1,961, Processing Time 0.029 seconds

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
    • /
    • v.5 no.1
    • /
    • pp.129-135
    • /
    • 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.

  • PDF

A Study on the Effect of a Single Overload on Fatigue Crack Retardation (단일과대하중이 피로균열성장에 미치는 영향에 관한 연구)

  • Kim, Kyung-Su;Kim, Sung-Cahn;Shin, Chun-Sik;Park, Jin-Young;Lee, Chang-Hwan
    • Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
    • /
    • 2002.10a
    • /
    • pp.73-78
    • /
    • 2002
  • Ships and ocean structures are generally under random loading. Various type of variable-amplitude loading affects fatigue crack growth and fatigue life. However interaction effects due to irregularity of loading including random loading have not explained exactly and it is difficult to examined fatigue crack growth behaviour and fatigue life for this reason. Therefore in this paper crack growth tests with constant-amplitude loading including a single overload were conducted to measure plastic zone size near crack tip of DENT specimen. And the observed plastic zone sized were discussed in terms of crack growth rate. As a result of this the effect the plastic rue size due to the overload is examined on the effect on crack growth rate and, consequently, fatigue life.

  • PDF

Fatigue Life Prediction of $SIC_w$/Al Composites by Using the Monte-Carlo Simulation (몬테카르로 시뮬레이션에 의한 $SIC_w$/Al 복합재료의 피로수명에측)

  • Ahn, Jeong-Ju;Kwon, Jae-Do;Kim, Sang-Tae
    • Transactions of the Korean Society of Mechanical Engineers A
    • /
    • v.20 no.5
    • /
    • pp.1552-1561
    • /
    • 1996
  • It requires uch time and cost to obtain the fatigue crack growth life and fatigue crack growth path morphlogy from the fatigue crack growth tests. In this study, the Monte-Carlo simulation program was developed to predict the fatigue crack growth lofe and fatigue crack growth path morphology of metal matrix composites. Fatigue crack growth lives of 5%, 10%, 15%, 20%, 25% and 30% $SiC_w$/Al composites were predicted by usign the Monte-Carlo Simulation. And the fatigue crack growth lives of 25% $SiC_w$/Al and Almatrix from Monte-carlo simulation were compared with fatigue life from experiments in order to verify the accuracy of Monte-Carlo Simulation program.

Fatigue Crack Initiation and Propagation From Two Micro Hole Defects (두개의 미소원공결함에서의 피로크랙발생과 전파에 관한 연구)

  • Song, Sam-Hong;Bae, Joon-Soo
    • Transactions of the Korean Society of Mechanical Engineers A
    • /
    • v.21 no.5
    • /
    • pp.842-849
    • /
    • 1997
  • The aim of this study is an investigation of the interaction of two micro hole defects affecting fatigue crack initation life and propagation behavior. The locatio of two micro hole defects was considered as an angle of alignment and the distance between the centers of two micro hole defects. The fatigue cracking behavior is experimented under bending. When micro defects are located close to each other, the fatigue crack initiation lives are varied with their relative locations. In the experiments, the area of local plastic strain strongly played a role in the fatigue crack initiation lives. Therefore we introduce a parameter which contains the plastic deformation area at stress concentrations and propose a fatigue crack initiation life prediction curve. In addition, the directions and propagation rates of fatigue cracks initiated at two micro hole defects are studied experimentally.

A model of fatigue crack growth based on plastic stretch at the crack tip (균열선단의 소성스트레치를 이용한 피로균열성장모델)

  • Ju, Yeong Sik;Kim, Jae Hun
    • Journal of the Korean Society for Aeronautical & Space Sciences
    • /
    • v.31 no.3
    • /
    • pp.15-22
    • /
    • 2003
  • The fatigue crack growth model is derived and the retardation model is proposed. The fatigue crack growth model considers the residual plastic stretch on the crack surface which results from the plastic deformation at the tip of fatigue crack. The fatigue crack growth rate is calculated by using the cumulative fatigue damage and plastic strain energy in the material elements at the crack tip. This model gives the crack growth rate in reasonable agreement with test data for aluminum alloy AL6061-T651 and 17-4PH casting steel. The fatigue crack growth retardation model is based on the residual plastic stretch produced from a tensile overload which reduced the plastic strain range of the following load cycles. A strip-yield model of a crack tip plasticity is used for the calculation of a plastic zone size. The proposed retardation model characterized the observed features and delayed retardation of the fatigue crack growth under tensile overload.

Crack Opening Behavior of Perpetrated Crack Under Fatigue Load

  • Nam, Ki-Woo;Ahn, Seok-Hwan
    • Journal of Mechanical Science and Technology
    • /
    • v.16 no.1
    • /
    • pp.24-31
    • /
    • 2002
  • The leak-before-break (LBB) behaviors of a structural component under high and low fatigue loads are an important problem in nuclear power plants, liquid nitrogen gas tankers and chemical plants. This paper is an experimental study to evaluate the crack opening behavior after penetration for plate and pipe specimens. Crack opening displacement after penetration under low fatigue load could be satisfactorily determined at the center of the plate thickness regardless of the specimen size. In the case of high fatigue load, it is shown that the crack opening displacement at the center of a penetrated crack carl be derived using the gross stress, $\sigma$/sug G/, and the front surface crack length, a$\_$s/, together with the back surface crack length, a$\_$b/.

Finite Element Analysis for Fatigue Crack Closure Behavior Using Reversed Plastic Zone Size (되풀이 소성영역 크기를 이용한 피로 균열 닫힘 거동의 유한요소해석)

  • Choi, Hyeon-Chang
    • Transactions of the Korean Society of Mechanical Engineers A
    • /
    • v.27 no.10
    • /
    • pp.1703-1711
    • /
    • 2003
  • An elastic-plastic finite element analysis is performed to investigate detailed closure behaviour of fatigue cracks in residual stress fields and the numerical results are compared with experimental results. The finite element analysis performed under plane stress using contact elements can predict fatigue crack closure behaviour. The mesh of constant element size along crack surface can not predict the opening level of fatigue crack. Specially, the mesh of element sizes depending upon the reversed plastic zone size included the effect of crack opening point can precisely predict the opening level. By using the concept of the mesh of element sizes depending upon the reversed plastic zone size included the effect of crack opening point, the opening level of fatigue crack can be determined very well.

Effects of Strain Hardening Exponents on the Retardation of Fatigue Crack Propagation (가공경화지수가 피로균열 지연거동에 끼치는 영향)

  • 김상철;강동명
    • Transactions of the Korean Society of Mechanical Engineers
    • /
    • v.14 no.5
    • /
    • pp.1193-1199
    • /
    • 1990
  • Effects of strain hardening exponents on the behavior of fatigue crack propagation are experimentally investigated. The retardation effect of fatigue crack propagation after single overloading is investigated in relation to strain hardening exponent and crack closure. A relationship between crack opening ratio and strain hardening exponents is inspected through an examination of the crack closure behavior. An empirical equation relating retardation effect of fatigue crack propagation after single overloading, percent peak load and strain hardening exponent of materials is proposed.

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
    • /
    • v.24 no.5 s.176
    • /
    • pp.1288-1296
    • /
    • 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.

A Study on the Fatigue Crack Growth Behavior in Ti-6Al-4V Alloy(I) (Ti-6Al-4V의 피로균열성장거동에 관한 연구(I))

  • 우흥식;한지원
    • Journal of the Korean Society of Safety
    • /
    • v.16 no.4
    • /
    • pp.52-57
    • /
    • 2001
  • Fatigue crack growth behaviour of Ti-6A-4V alloy is investigated in air and salt solution environment at room temperature and $200^{\circ}C$. Fatigue crack growth rate is blown to be fast for the formation of corrosive product in hot salt environment. For the effect on corrosion fatigue crack growth behaviour of region II. fatigue crack growth rate in atmosphere had a little gap to both case, $200^{\circ}C$ and room temperature. However, it showed very fast tendency in salt corrosive atmosphere, and it was remarkably accelerated in $200^{\circ}C$ temperature salt environment. When $\Delta$K was approximately 30MPa(equation omitted), fatigue crack growth rate had a little difference between at room temperature and at $200^{\circ}C$ high temperature, however in case of salt corrosive environment the room temperature was 3.5 times Inter and $200^{\circ}C$ high temperature for 16 times than air environment respectively.

  • PDF