418 CHAPTER 14 Fatigue
Fig.14.6
Plane strain plasticity.
14.5.3 Crack Propagation Rates
Having obtained values of the stress intensity factor and the coefficientα, fatigue crack propagation
ratesmaybeestimated.Fromthese,thelifeofastructurecontainingcracksorcrack-likedefectsmay
bedetermined;alternatively,theloadingconditionmaybemodifiedorinspectionperiodsarrangedso
thatthecrackwillbedetectedbeforefailure.
Underconstantamplitudeloading,therateofcrackpropagationmayberepresentedgraphicallyby
curvesdescribedingeneraltermsbythelaw
da
dN=f(R, K) (see[Ref.6]) (14.38)inwhich KisthestressintensityfactorrangeandR=Smin/Smax.IfEq.(14.30)isused,
K=(Smax−Smin)(πa)(^12)
α (14.39)
Equation(14.39)maybecorrectedforplasticityundercyclicloadingandbecomes
Kp=(Smax−Smin)(πa 1 )
1
(^2) α 1 (14.40)