Loading rate effects on cleavage fracture of pre-cracked CVN specimens: 3-D studies

Abstract

Specimen size and loading rate effects on cleavage fracture of ferritic steels tested in the ductile-to-brittle transition region remain key issues for the application of pre-cracked Charpy specimens. This investigation employs 3-D, nonlinear finite element analyses to assess crack-front stress triaxiality in quasi-static and impact-loaded pre-cracked CVN specimens, with and without side grooves. Crack-front conditions are characterized in terms of the Weibull stress which reflects the statistical effects on cleavage fracture. These 3-D computations indicate that a less strict size/deformation limit, relative to the limits indicated by previous plane-strain analyses, is needed to maintain small-scale yielding conditions at fracture under quasi-static and impact 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 enables prediction of the distribution of quasi-static fracture toughness 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 1T-SE(B) specimens from impact-loaded, pre-cracked CVN specimens. These 3-D analyses also yield η-total values for use in impact testing to infer thickness average and mid-thickness J-values from measured work quantities.