Intrinsic thresholds in polycrystalline Udimet 720

Abstract

The long crack threshold behaviour of polycrystalline Udimet 720 has been investigated. Faceted crack growth is seen near threshold when the monotonic crack tip plastic zone is contained within the coarsest grain size. At very high load ratios R (=P_min/P_max) it is possible for the monotonic crack tip plastic zone to exceed the coarsest grain size throughout the entire crack growth regime and non-faceted structure insensitive crack growth is then seen down to threshold. Intrinsic threshold values were obtained for non-faceted and faceted crack growth using a constant K_max, increasing K_min, computer controlled load shedding technique (K is stress intensity factor). Very high R values are obtained at threshold using this technique (0·75–0·95), eliminating closure effects, so the intrinsic resistance of the material to crack propagation is reflected in these values. The intrinsic non-faceted threshold value ∆K_th is lower (2·3 MN m^−3/2) than the intrinsic faceted ∆K_th value (4·8 MN m^−3/2). This is thought to reflect not only the effect of crack branching and deflection (in the faceted case) on the crack driving force, but also the inherent difference in resistance of the material to the two different crack propagation micromechanisms.

MST/1681