LOADING RATE EFFECTS ON CLEAVAGE FRACTURE OF PRECRACKED CVN SPECIMENS- 3-D STUDIES

Specimen size and loading rate effects on cleavage fracture of ferriti c steels tested in the ductile-to-brittle transition region remain key issues for the application of pre-cracked Charpy specimens. This inve stigation employs 3-D, nonlinear finite element analyses to assess cra ck-front stress triaxiality in quasi-static and impact-loaded pre-crac ked CVN specimens, with and without side grooves. Crack-front conditio ns are characterized in terms of the Weibull stress which reflects the statistical effects on cleavage fracture. These 3-D computations indi cate that a less strict size/deformation limit, relative to the limits indicated by previous plane-strain analyses, is needed to maintain sm all-scale yielding conditions at fracture under quasi-static and impac t loading conditions. For impact toughness values which violate these size/deformation limits, a toughness scaling methodology is described to remove the effects of constraint loss. The new scaling model also e nables prediction of the distribution of quasi-static fracture toughne ss values from a measured distribution of impact toughness values (and vice versa). This procedure is applied to experimental data obtained from a Cr-Ni-Mo-V pressure vessel steel and accurately predicts quasi- static fracture toughness values in IT-SE(B) specimens from impact-loa ded, pre-cracked CVN specimens. These 3-D analyses also yield eta-tota l values for use in impact testing to infer thickness average and mid- thickness J-values from measured work quantities. (C) 1997 Elsevier Sc ience Ltd.