Size effects on toughness for fracture in the elastic plastic cleavage initiation region

A systematic investigation of the effects of specimen size on cleavage fracture toughness for a typical pressure vessel steel in the transition is reported. Size dependence of toughness arises from two basic mechanisms. The first is related to the total volume of material acted on by high stress fields near a blunting crack tip, which is a function of the crack front length (B). We call this the statistical sampling effect of B. The second is due to constraint loss, which depends on the ligament length (b) as well as B, and is characterized as deviation from small scale yielding (SSY) crack tip fields. Until now, it has not been possible to quantify the individual and combined effects of the statistical effect of B and constraint loss (or B versus b) size scaling laws, or even to verify fully the nature of the underlying mechanisms. In order to develop a single variable database on size effects, a complete B-b matrix of fracture specimens, fabricated from a single plate of steel from the Shoreham reactor pressure vessel, was tested at a common set of conditions. The B ranged from 8 to 254 mm and b ranged from 3.2 to 25.4 mm. The database was analyzed using 3-dimensional finite element simulations of the crack tip fields, calibrated to the local fracture properties of the Shoreham steel. The finite element based analysis shows that both the statistical effect of B, giving rise to a B−1/4 type scaling, as well as constraint loss, evaluated in terms of in-plane areas within critical stress contours, play a significant role in the size scaling of transition toughness. Similar results were found in applying a Weibull stress model of cleavage fracture. Applications of the size scaling models developed are presented, including temperature dependence of the Weibull stress model parameters, predictions of sub-size specimen behavior, and an analysis of the transition toughness behavior of a pressure vessel steel from standard specimen geometries.