Issue 35

E. Fessler et alii, Frattura ed Integrità Strutturale, 35 (2016) 223-231; DOI: 10.3221/IGF-ESIS.35.26 228 decreases along the hold time, it was then concluded that the effect of the pressure cycle is related to the strain rates at the crack tip as stress relaxation occurs during the hold time. Figure 5 : Schematic illustration of the grain boundaries fracture toughness concept, applied at the crack tip. Figure 6 : Results on Inconel 718 at 650°C showing the part of the loading cycle where environment has an effect on crack propagation [13]. During the K decreasing procedure, the strain rate at the crack tip decreases. Consistently with the work of Molins [13], once the strain rates are lower than a critical value, the environment has no effect on the crack growth rates. As K decreases, it is assumed the embrittling effect of environment occurring during the hold time progressively vanishes. The hold time cycle can progressively be considered as an equivalent pure fatigue cycle of frequency leading to a cycle dependent crack growth mechanism, consistently with the observed transgranular fracture at low ΔK. BSE imaging was performed at the tip of an arrested crack at low ΔK (ΔK≈13 MPa√m), tested under hold time conditions (see Fig. 7). The crack path appears partially transgranular and slip bands are observed at the crack tip. This is usually observed under pure fatigue loading conditions and tends to indicate a crack propagation mechanism governed, at low ΔK under hold time conditions, by cyclic plastic strain at the crack tip.