Fig. 4 FCG for Epoxy at different R-ratios. a) in terms of ΔK and b) in terms of Kmax. c) ΔK-R plots at low crack growth rates. d) corresponding ΔK-Kmax plots with limiting ΔK and Kmaxvalues. e) The trajectory showing deviation from the pure fatigue line.
shows the corresponding ΔK-Kmax plots with characteristic L-shaped behavior. For this material, only one mechanism seems to operate for all R-values. Fig. 4e shows the FCG trajectory. Each point again corresponds to a crack growth rate starting from the threshold. The trajectory deviates from the pure fatigue line due to the superimposed Kmax-dependent process. Here the mechanism could be the superimposed monotonic fracture which depends on Kmax.
Fig.5 compares the crack growth trajectories of the three polymer materials tested by Osorio. Only Epoxy shows one crack growth mechanism governing FCG for all R values, while the other