Issue 35

E. Fessler et alii, Frattura ed Integrità Strutturale, 35 (2016) 223-231; DOI: 10.3221/IGF-ESIS.35.26 227 Fig. 2-c). EBSD analyses revealed a straight crack path and cut grains beneath the crack path, associated with transgranular fracture, in the low ΔK regime under hold time conditions. At higher ΔK, under the same loading conditions, the crack path appears rough with full grains beneath the crack path, indicating an intergranular fracture. Characterizations of the fracture surfaces and the crack paths in the low ΔK regime revealed a transition from fully intergranular to transgranular as ΔK decreases, while cycling under hold time conditions. This has been observed at 650 °C for hold times of 300 s and 1200 s and at 600 °C for a hold time of 300 s. Below 600 °C, no transition was clearly observed. This transition is believed to be the sign of a progressive extinction of the embrittling effect of environment occurring during the hold time at maximum load, as K decreases. This transition is discussed in the next session, regarding the effect of environment and the mechanical state at the crack tip. D ISCUSSION Introduction t is well established, in Inconel 718 and other nickel-based superalloys, that the application of a hold time at maximum load has a deleterious effect on the crack growth behaviour (see for e.g. [1-3]). Under these conditions, the increased crack growth rates are accompanied with an intergranular fracture and the crack growth mechanism is time dependent. Without hold time at maximum load, and at higher frequencies, fracture is transgranular, lower crack growth rates are measured and the crack growth mechanism is cycle dependent. The exact mechanism responsible for the embrittling effect of environment under hold time conditions is still not clearly understood but there exists two main theories. The stress assisted grain boundaries oxidation theory (SAGBO) [11] involves grain boundaries oxidation ahead of the crack tip and subsequent failure of these grain boundaries, of reduced fracture toughness, during the next loading. The dynamic embrittlement (DE) [12], instead, involves short distance oxygen diffusion in grain boundaries ahead of the crack tip. The embrittled grain boundaries will break during the next loading, exposing new surfaces to subsequent oxidation. In both cases, the embrittling effect of environment can be assumed as a process leading to reduced grain boundaries fracture toughness ahead of the crack tip, thus inducing intergranular fracture. An explanation to the observed transgranular fracture under hold time conditions at low ΔK is proposed here, based on the coupled effect of oxidation and the mechanical state at the crack tip. On the concept of grain boundaries fracture toughness At a microscopic scale, the crack front can be considered as a population of grain boundaries interfaces with its respective orientation regarding the load axis. These interfaces will not break unless the applied loading excesses the grain boundaries fracture toughness. Whatever the damaging process occurring during hold time is, it can be assumed as a short distance embrittling process, consistently with the work of Molins [13], leading to reduced grain boundaries fracture toughness. This reduced grain boundaries fracture toughness may be due to the formation of oxides or the diffusion of embrittling species along the grain boundaries ahead of the crack tip. At a given temperature, and in the absence of any applied load, the grain boundaries fracture toughness can be assumed as an intrinsic property of the material. The grain boundaries fracture toughness may decrease during a sustained loading, due to the continuous embrittling effect of environment and the material ability to relax stresses. Then, it could be expected that temperature has a deleterious effect on the grain boundaries fracture toughness. This concept is illustrated in Fig. 5. Under hold time conditions, the grain boundaries fracture toughness will be reduced due to the embrittling effect of the environment, thus inducing intergranular fracture if the applied load excesses this fracture toughness. This concept could explain the transgranular fracture observed in the low ΔK regime under hold time conditions. As K decreases, the applied mechanical load will become lower than the reduced grain boundaries fracture toughness, thus inducing transgranular fracture at low ΔK. Discussion regarding the combined effect of the environment and the mechanical state at the crack tip It was shown by Molins [13] that the embrittling effect of the environment is a dynamic coupling of the oxidation process and the mechanical state at the crack tip, especially plastic strain rates. By performing crack growth tests on Inconel 718 at different oxygen partial pressures, the author has demonstrated the existence of a transition pressure below which the environment has no effect on crack growth rates. Such a transition was also evidenced on N18 superalloy [14]. This transition pressure (between 10 -2 and 1 Torr) is independent of the loading conditions. Further experiments were performed by superimposing a 60 s pressure cycle, above the transition pressure, over the mechanical cycle. This pressure cycle was applied at different instants of the hold time cycle (see Fig. 6). As the deleterious effect of the pressure cycle I