A numberical study of local crack-tip fields for modeling cleavage fracture initiation

The prediction of cleavage crack initiation in large-scale wide plate (WP) specimens from small (CT) specimens utilizing linear elastic fracture mechanics methodology has not been effective. In the wide-plate tests conducted by the Heavy-Section Steel Technology Program at Oak Ridge National Laboratory, crack initiation has consistently occurred at K_I values ranging from two to four times those predicted by the CT specimens. Studies were initiated to develop crack-tip stress field criteria incorporating effects of geometry, size, and constraint that will lead to improved predictions of cleavage initiation in WP specimens from CT specimens. The work centers around nonlinear 2-D and 3-D finite element analyses of the crack-tip stress fields in these geometries. Analyses were conducted on CT and WP specimens for which cleavage initiation fracture had been measured in laboratory tests. The local crack-tip fields generated for these specimens were then used in the evaluation of fracture correlation parameters to augment the K_I parameter for predicting cleavage initiation. Parameters of hydrostatic constraint and of maximum principal stress, measured pointwise and volumetrically, are included in these evaluations. The results indicate a potential for correlating the local crack-tip fields with the cleavage initiation process via the maximum principal stress criterion based on achieving a critical cumulative area within a critical stress contour. This criterion has been successfully applied to correlate cleavage initiation in 2T-CT and WP specimen geometries