Fatigue crack growth in a type 316 stainless steel and a 20 pet Cr/25 pet Ni/Nb stainless steel at elevated temperature

Fatigue failure in metals and alloys occurs by the nucleation and controlled propagation of a surface crack. At ambient temperature propagation is transgranular and is controlled, to a large extent, by continuum plasticity effects at the crack tip. At elevated temperatures this simple process might be affected by oxidation and the tendency towards intergranular propagation. The elevated temperature fatigue crack propagation behavior of a 20/25/Nb stainless steel and a type 316 stainless steel is studied by optical measurement of the crack growth rate of artificially-induced notches under conditions where gross plastic straining is present in the bulk of the material. Tests conducted at ambient temperature are included for comparison with elevated temperature behavior. By reference to fatigue life data for smooth unnotched specimens, tested under identical conditions to the crack growth tests, an attempt is made to rationalize the roles of crack initiation and propagation in the fatigue process and indicate the relevance of crack growth data in predicting fatigue life data.