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# FATIGUE CRACK INTIATION

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FATIGUE CRACK INTIATION Introduction Fatigue cracks are usually initiated at the free surface of metals, this occurs at the grains of the surface . Arguments that damage in then a polycrystalline ductile metal is associated with the grains having a free surface rather than those within the body of the metal are; a) Surface grains are in intimate contact with the atmosphere thus if environment is a factor in the damage, then grains at the surface are susceptible. b) Surface grain is the only part of a polycrystalline not wholly supported by adjoining grains, again having a free surface will be able to deform plastically more easily than a grain in the body of a metal which is surrounded by other grains c) It has been shown that if a fatigue test is stopped after some fraction of the expected life of the specimen, a thin layer of metal removed and test continued at the same stress level, the total life of the specimen is longer than the life of the original specimen. d) If the surface of a specimen is hardened, metal logically of by surface working, the fatigue strength of ...

... the stress corrosion fatigue curve. These effects were due to corrosion effects of moisture which condenses at the crack tip at low frequency. THE FATIGUE CRACK GROWTH RATE CURVE DA/DN -∆k. The characteristics signadal shape of a da/dn -∆K Fatigue crack growth rate curve. The curve can be divided into three regions ascending the shape of the curve. There is a threshold value, ∆kth, below which cracks do into propagate. the crack growth rate above this value increased relatively rapidly with increasing ∆K. this region is influenced by. a) Microstructure. b) Mean stress. c) Environment A higher and steady crack growth rates. There is often a linear log – log relationship between da/dn and ∆k. There have been many attempts to describe the crack growth rate curve by crack growth laws. Some are stated below da = C(∆K)M - The Paris equation. dn da = C( ∆K)M - the form an equation dn (1-R) KC - ∆K The region is largely influenced by a combination of environment, mean stress and frequency.##### Essay is in categories

##### Natural Sciences

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/ ##### Inorganic Chemistry

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The essay is about fatigue cracks.Fatigue cracks are initiated at the free surface of metals, this occurs at the grains of the surface.

language | english | |

wordcount | 6842 (cca 19 pages) | |

contextual quality | N/A | |

language level | N/A | |

price | free | |

sources | 0 |

Introduction 1

CRACK PROPAGATION 3

MICROMECHANISM OF FATIGUE CRACK GROWTH 5

CONCEPT OF THRESHOLD STRESS INVENTORY FACTOR ▲kth 7

PREDICTION OF FATIGUE GROWTH UNBER VARIABLES 9

EFFECTS OF STRES RATION AND CRACK TIP PLASTICITY: CRACK CLOSURE. 11

EFFECTS OF FRACTURE PATH AND MICROSTRUCTURE. 13

MIXED FRACTURE PATH. 14

FATIGUE IN CRACKED COMPONENTS 16

THE RELATIONSHIP FOR PREDICTING FATIGUE STRENGTH UNDER FLUCTUATIND STRESS. 17

THE EFFECT OF A MEAN TENSILE STRESS ON THE FATIGUE STREGTH OF OUTINE METALS. 18

THE EFFECT F A MEAN COMPRESSIVE STRESS ON DUCTILE ME TALS. 19

THE EFFECTS OF MEAN STRESS ON CAST IRON. 20

THE RESISTANCE OF MATERIALS OF FATIGUE UNDER COMPLEX STRESS. 20

CRITERIA CONVERGIGN FAAILURE UDNER COMPLEX STRESS. 20

THE EFFCETS OF FLUCTUATIION IN STRESS AMPLITUDE AND CUMULATIVE DAMAGE. 21

THE LINEAR DAMAGE LAW. 21

THE EFFECT OF UNDERSTANDIGN COAXING AND REST PERIODS 22

THE EFFECTS OF THE FREQUENCY OF THE STRES CYCLE. 24

EFFECTS OF MICROSTRUCTURE. 26

THE EFECTS OF CRACK TIP GEOMETRY. 27

CRACK BRANCH AND KINKING. 28

DIRECTIONACITY 28

EFFECTS OF INCLUSIONS. 29

EFFECTS OF STRESS RATION 30

EFFECT OF FREQUENCY 33

THE FATIGUE CRACK GROWTH RATE CURVE DA/DN -∆k. 33

... the stress corrosion fatigue curve. These effects were due to corrosion effects of moisture which condenses at the crack tip at low frequency. THE FATIGUE CRACK GROWTH RATE CURVE DA/DN -∆k. The characteristics signadal shape of a da/dn -∆K Fatigue crack growth rate curve. The curve can be divided into three regions ascending the shape of the curve. There is a threshold value, ∆kth, below which cracks do into propagate. the crack growth rate above this value increased relatively rapidly with increasing ∆K. this region is influenced by. a) Microstructure. b) Mean stress. c) Environment A higher and steady crack growth rates. There is often a linear log – log relationship between da/dn and ∆k. There have been many attempts to describe the crack growth rate curve by crack growth laws. Some are stated below da = C(∆K)M - The Paris equation. dn da = C( ∆K)M - the form an equation dn (1-R) KC - ∆K The region is largely influenced by a combination of environment, mean stress and frequency.

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