Font Size: 

According to a damage-tolerant approach, fatigue life becomes virtually infinite if the deltaK due to service loads is lower than the threshold for fatigue crack growth (FCG). For this motivation, a proper evaluaton of deltaKth is very important. On the other hand, for a given material this parameter appears to be strongly influenced by material microstructure and R=Kmin/Kmax ratio. At high propagation velocity this influence is explained by classical closure concept introduced by Elber, but this approach does not always work at threshold. Recently, new methods to account for microstructure and R-ratio effects at low propagation rates were proposed. The aim of this work is to evaluate the effectiveness of those methods testing very different materials: (i) a case hardened steel and (ii) an aluminium matrix particulate composite.