Influence of gaseous environments on rates of near-threshold fatigue crack propagation in nicrmov steel

Influence de l'environnement sur la propagation des fissures de fatigue

“…Selon ces auteurs cependant, l'action de l'environnement au voisinage du seuil se cantonnerait à la seule réduction du facteur d'intensité de contrainte par le biais de l'accroissement du niveau de fermeture. Liaw et al [11,12] ont conduit une étude similaire, mais en mesurant l'intensité des effets de fermeture. Ils montrent que la fermeture induite par l'oxyde ne peut pas rendre compte à elle seule des différences de comportement observées entre atmosphères humides et milieu inerte.…”

Influence of dissolved hydrogen on the fatigue crack growth behaviour of AISI 4140 steel

“…They have reported that [80] the fatigue threshold stress intensity factor is higher in hydrogen than in air in low strength steels. In addition, it is also reported [81] that the fatigue crack growth rate is lower in hydrogen environment than in air in some low strength materials upto a ΔK of about 12 MPa√m. Hence, a combined effect of the lower strength of the 4140 alloy in the annealed condition and a diminished effect of hydrogen in the lower ΔK of about 12 MPa√m is the reason why the fatigue threshold is higher and the fatigue crack growth rate is lower for the 250 hrs hydrogen exposed specimen than in air as seen from the Figure 17 and Table 9.…”

“…This was hypothesized due to influence of environmental moisture surrounding the fatigue specimen which might have resulted in oxide products by the reaction of freshly exposed crack surface and moisture which might get wedged at the crack tip during the closing cycle of the fatigue test reducing the effective SIF experienced at the crack tip. In addition, some oxide debris might have also formed by plasticity induced crack closure due to fretting action between the crack tip surfaces owing to the low strength material (σ ys = 757 MPa for this material) as reported elsewhere [81]. The macro photos are shown in Figure 48.…”

“…Previous investigators [81] have reported that an increase in the strength of steel causes intense stress gradients at the cyclic plastic zone ahead of the crack tip region.…”

“…The lack of significant difference in the fatigue crack growth morphology from the fractographs, in terms of lack of IG facets in specimens with dissolved hydrogen, could be used to explain the relative closeness of the near threshold FCGRs in specimens with and without dissolved hydrogen in the annealed material. Though the fractographic evidence cannot be linked directly to the observed near threshold FCGRs [81], a qualitative mechanistic explanation using the cyclic plastic zone size effects calculated based on the fractographic observations can yield much information about the crack tip triaxiality encountered at near threshold CGRs with or without the influence of dissolved hydrogen. Noting the values of the cyclic plastic zone with or without dissolved hydrogen, it is clear that the dissolved hydrogen has not caused significant reduction of the cyclic plastic zone size which in turn is responsible for 1) the fatigue crack to propagate further on the cleavage fracture plane and 2) to setup a steep stress gradient to diffuse hydrogen from the matrix to the freshly created crack tip to continue this repetitive cyclic damage process.…”