Creep crack initiation and propagation on type 316 stainless steel

The investigations carried out on type 316 stainless steel plate for high temperature applications allowed to assess creep crack growth (CCG) properties of base material with respect to initiation and propagation phases. It has been found (Bell 1986) that for high ductility materials, as type 316 stainless steel, incubation time is an overwhelming fraction of total propagation time of cracks in structures operating in the creep range. In order to avoid an over estimation of the different metallurgical and mechanical contributions to creep crack growth, which would cause undue safety margins, and to permit the development of more confident defect assessments, it is necessary to reach: i) Improved experimental testing methodologies, ii) implemented data processing techniques, iii) a suitable selection of proper Fracture Mechanics parameters. Jointly with the presentation of additional CGG results (D'Angelo 1985) we will try here to analyse and to discuss, into some extend, experimental and theoretical issues strictly connected to CCG initiation and propagation, with a view to identify the application range of different Fracture Mechanic parameters, as supported by a detailed investigation of fractography and microstructural feature of the crack tip region. Possible mechanisms of defect initiation and propagation will be consequently introduced. (orig./GL)