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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

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)

slide2

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.

slide3

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

slide4
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

slide6

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
infamous fatigue failures
Infamous Fatigue Failures

War ship: S.S. Schenectady

slide8

Infamous Fatigue Failures

Offshore platform: Alexander Kielland

infamous fatigue failures1
Infamous Fatigue Failures

Aircraft : De Havilland Comet