Notch fracture toughness of a cast duplex stainless steel: modelling of experimental scatter and size effect

Abstract

A cast duplex stainless steel containing about 20% ferrite and embrittled at 400°C is studied. After ageing, the main damage mechanism is nucleation of cleavage cracks in the ferritic phase. This damage appears heterogeneously as millimetric clusters resulting in scatter and size effects in experimental results obtained for laboratory specimens (smooth and notched tensile bars, Charpy specimens). As the influence of the strain rate on the experimental results could be neglected, slow loading three point bending tests (instead of dynamic impact tests) on standard Charpy U or V-notch specimens were especially investigated. It was shown that the energy needed for crack propagation is almost constant, with low scatter. On the other hand, that needed for crack initiation depends on notch acuity and specimen thickness, with much more significant scatter. A model based on local damage rates measured by quantitative metallography and the use of the plastic criterion introduced by Gurson is proposed to predict fracture. Results of finite element calculations, integrating random damage nucleation, show that the model predicts both mean values and scatter of measured ductilities (smooth and notched tensile bars) and crack initiation energies (Charpy U-notch specimens). Size effects can also be accounted for.