Issue 35

E. Fessler et alii, Frattura ed Integrità Strutturale, 35 (2016) 223-231; DOI: 10.3221/IGF-ESIS.35.26 229 To confirm the assumption of an equivalent pure fatigue cycle at low ΔK, tests will be carried out with a continuous monitoring of the crack length during the hold time, using the DCPD method. This way, the crack growth occurring during the hold time could be isolated from the crack growth occurring during the cyclic part of the cycle. The confirmation would be achieved if no more crack growth during the hold time is detected at low ΔK. Figure 7 : BSE imaging of the crack tip region (ΔK≈13 MPa√m). Slip bands (indicated by black arrows) can be seen at the crack tip and the crack path appears partially transgranular. Test conditions: K-decreasing procedure under 10-300-10 loading at 600 °C. The framework of the analysis appears consistent with the observed transition from intergranular to transgranular fracture at low ΔK under hold time conditions. Characterizations carried out at the tip of an arrested crack at low ΔK tend to confirm the assumption of an equivalent pure fatigue cycle at low ΔK. However, this analysis does not take into account the embrittling mechanism of the environment occurring during the hold time. On the concept of an “effective oxygen partial pressure” at the crack tip It was demonstrated by Andrieu [15] that the transition pressure, described above, is accompanied by a transition in the fracture mode. Below the transition pressure, transgranular fracture is observed, while it is mainly intergranular at higher pressure. It is also demonstrated that oxidation at the crack tip is governed by the operating oxygen partial pressure. Above the transition pressure, Ni and Fe rich oxides will form first. This porous oxide layer leads to a reduced partial pressure at the metal-oxide interface, then leading to the build-up of a protective Cr 2 O 3 layer. At lower pressure, only the protective Cr 2 O 3 layer will form. The intergranular fracture, obtained at high partial pressure, could be explained by the deleterious effect Ni and Fe rich oxides may have on the grain boundaries fracture toughness ahead of the crack. The reduced grain boundaries fracture toughness will become lower than the applied mechanical load at the crack tip, inducing intergranular fracture. Regarding this result, one could explain the observed transition from intergranular to transgranular fracture in the low ΔK regime by the concept of an “effective oxygen partial pressure” at the crack tip. All tests presented in the first section were carried out in laboratory air. As K decreases, the effective stress intensity factor range ΔK eff decreases and consequently the crack tip opening displacement (CTOD) too. The lower the CTOD, the less oxygen would be available at the crack tip due to a trapping effect by the crack lip, resulting in a lower oxygen partial pressure at the crack tip. The effective oxygen partial pressure at the crack tip may decrease to the extent it will become lower than the transition pressure. The Ni and Fe rich oxides will not form at the crack, then preventing the grain boundaries fracture toughness decrease and subsequent intergranular fracture. Assuming this concept, one could expect to observe only Cr 2 O 3 on the transgranular fracture surface observed in the low ΔK regime. Moreover, as shown in Fig. 7, slip bands were observed at the tip of an arrested crack in the low ΔK regime (ΔK≈13 MPa√m), under hold time conditions. This is believed to be a sign of a transition from a time dependent to a cycle dependent crack growth mechanism. These slip bands can act as rapid diffusion paths for Cr [15], thus allowing a sustained Cr supply at the crack tip. This would lead to the preferential build-up of a protective Cr 2 O 3 layer. To confirm