NOTCH FRACTURE-TOUGHNESS OF A CAST DUPLEX STAINLESS-STEEL - MODELING OF EXPERIMENTAL SCATTER AND SIZE EFFECT

A cast duplex stainless steel containing about 20% ferrite and embritt led al 400 degrees C is studied. After ageing, the main damage mechani sm 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 speci mens (smooth and notched tensile bars, Charpy specimens). As the influ ence of the strain rate on the experimental results could be neglected , slow loading three point bending tests (instead of dynamic impact te sts) on standard Charpy U or V-notch specimens were especially investi gated. It was shown that the energy needed for crack propagation is al most constant, with low scatter. On the other hand, that needed for cr ack initiation depends on notch acuity and specimen thickness, with mu ch more significant scatter. A model based on local damage rates measu red by quantitative metallography and the use of the plastic criterion introduced by Gurson is proposed to predict fracture. Results of fini te element calculations, integrating random damage nucleation, show th at the model predicts both mean values and scatter of measured ductili ties (smooth and notched tensile bars) and crack initiation energies ( Charpy U-notch specimens). Size effects can also be accounted for. (C) 1997 Elsevier Science S.A.