R. Piques

Crack initiation and growth under creep and fatigue loading of an austenitic stainless steel

Abstract Both the initiation and the propagation of macroscopic cracks have been studied in a creep ductile 316L type stainless steel at 575–650°C using various fracture mechanics specimens and a wide range of test conditions including steady load at constant or varying temperatures, varying loads at constant temperature. It is shown that, even for isothermal tests, the C ∗ parameter is unable to provide unique correlations for all the stages of both creep crack initiation and growth. A unique correlation nevertheless exists between C ∗ and the time to initiation, T i . Large differences – either conservative or not – from a simplified linear damage cumulation rule are found when the tests are performed at two successive temperatures or two loads. Very detrimental effects of creep-fatigue loadings are shown. A simplified global approach to creep crack initiation under isothermal conditions, based on reference stress and length concept is developed. A local approach to creep cracking, in which an intergranular physical damage law determined experimentally on notched bars, and stress-strain fields obtained by analytical results is shown to provide crack growth results in good agreement with experiment.

Creep and creep-fatigue cracking behaviour of two structural steels

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. n nIn 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. n nIn 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.

Crack initiation under creep and creep-fatigue on CT specimens of an austenitic stainless steel

Abstract In the nuclear industry, some methods for calculating the time of crack initiation from pre-existing defects are needed. For this purpose, a correlation is checked between Ti (initiation time) or Ni (initiation cycles) and local parameters at a characteristic finite distance d from the crack-tip (αd criterion) (D. Moulin, B. Drubay and D. Acker, PVP-Vol. 223 (1992), Pressure Vessel Fracture, Fatigue and Life Management, ASME, 1992.) For fatigue tests, relevance of σd criterion is observed when using finite element code for calculating axial stress range Δσd. For creep tests, experimental points corroborate σd criterion when no stress relaxation at the crack tip is supposed. Finite element code with plane strain or plane stress conditions gives a large value of axial stress at 50 μm from the crack tip. Crack initiation prediction is then conservative. Further, a remarkable T i -C h ∗ correlation was observed for creep tests. Both criterions are compared. For creep-fatigue tests, crack initiation prediction is conservative, but comparison of σd criterion with other approaches is still in progress. Fracture surface examinations show that both creep and creep-fatigue specimens revealed the same intergranular surface aspect, whereas pure fatigue specimens presented transgranular cracking.

Microstructural creep damage in welded joints of 316L stainless steel

The present work has been undertaken to study creep damage in welded joints. The complex dual phase microstructure of 316L welds are simulated by manually filling a mould with longitudinally deposited weld beads. Most of the moulded specimens were then aged for 2000 hours at 600°C. High resolution scanning electron microscopy was extensively used to examine the microstructure of the welded material before and after ageing. Columnar grains of austenite constitute a matrix in which thin dendrites of δ-ferrite can be found. The ageing generates the precipitation of carbides, resulting in less transformation in the material. Smooth and notched creep specimens were cut from the mould and tested at 600°C under different stress levels. The creep life of the simulated welded material is shown to be lower than that of the base material. Microstructural observations reveal that creep cavities are preferentially located along the austenite grain boundaries. This analysis of intergranular damage on test specimens is ...