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A microstructurally short edge crack subjected to different fatigue load spectra has beenstudied by means of a dislocation technique. The crack was assumed to grow by emittingdiscrete dislocations that constitute the plastic deformation. The interaction between thedeveloping plasticity and grain boundaries has been investigated. The discrete events ofemission and annihilation of dislocations have been possible to study in detail for fatiguecrack growth rates of the order of a few Burgers vectors. The simulations showed that apositive overload resulted in crack retardation, or even arrest, for the cases where thespreading of the plastic deformation is not severely hindered by a grain boundary, whereasthe crack growth rate is not changed for the cases with dislocations piled up at the grainboundary. Single compressive overloads were found only to temporarily accelerate the crack.An increase in the mean load level resulted in an increase of the growth rate.