Creep and creep-fatigue cracking behaviour of two structural steels Original Research Article

The present study deals with crack initiation and crack growth, not only under creep and creep-fatigue conditions but also under more complex thermomechanical cyclic loadings, in both 316L and 1Cr-1Mo-0.25V steel. In these creep ductile materials, most studies have focused on the creep crack growth rates, da/dt and load-geometry parameter C∗ correlations. In this paper, the creep crack initiation time is defined as the time Ti necessary for a defect to grow by a small critical distance Xc (Xc ≈ 50 μm for example). This initiation stage may represent a large part of the rupture life of a cracked component. The importance of such studies is discussed in the first part. In the second part, an attempt is made to present a simplified method based on the fracture mechanics of creeping solids to define the relevant load-geometry parameters for crack initiation and crack growth under creep-fatigue loadings. In particular, it is shown that da/dN-ΔK correlations apply only when the hold time th is smaller than the transition time ttr between small-scale and large-scale viscoplasticity. Conversely, for long hold times, it is suggested that the Ti-C∗ correlation be used to predict the fatigue.