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Fracture Mechanics in Fatigue Analysis

Fracture Mechanics in Fatigue Analysis. Mods for Factor “K”. The stress intensity factor K: a : the crack width and geometrical correction factor.

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Fracture Mechanics in Fatigue Analysis

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  1. Fracture Mechanics in Fatigue Analysis Mods for Factor “K” The stress intensity factor K: a: the crack width and geometrical correction factor. F: the product of a couple of factors (back crack shape factors, front face factor, finite thickness factor, finite width factor and stress gradient factor)

  2. Kmax= F.σmax(π.a)1/2 Kmin= F.σmin(π.a)1/2 Kmaxand Kminare the maximum and minimum values of the stress intensity factor, at the upper and lower limit stresses during a cyclic loading.

  3. Plastic Zone Size = Plastic Zone Characterized with K ∆K= Kmax-Kmin=F.∆σ(πa)1/2 ∆σ1=(σ1max-σ1min) Semi-Crack Lenght- Number of Cycles

  4. Three distinct regions are indicated: 1) the well-known thresholdregion2) intermediate region3) the failure region. Paris Law m: experimental constant m= 3 for steel m=3~4 for aluminum alloy

  5. Paris Law in Application

  6. Differences Between Classical Design and Fracture Mechanics Based Design • Classical desing predicts that the material under pressure is confident under yield criteria. • Classical desing ignores behaviours of material. (Ductile or Brittle) • Classical design does not consider crack in the material and the fracture roughness

  7. Infamous Fatigue Failures War ship: S.S. Schenectady

  8. Infamous Fatigue Failures Offshore platform: Alexander Kielland

  9. Infamous Fatigue Failures Aircraft : De Havilland Comet

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