Effect of Notch Length on Fracture Toughness in Miniature Bend Specimens and Analysis by Net‐Section Mechanics

ABSTRACT Fracture toughness measurement using miniature specimens is of great interest in the context of small‐section applications, surveillance specimens for in ‐ situ radiation experiments, and for evaluating new alloys with limited volume of availability. In this study, and as part of a broader high‐temperature alloy design effort, the fracture toughness of two specialized refractory niobium (Nb) alloys was determined using miniature four‐point bend specimens. In particular, the question of how the initial notch length (or depth) affected the toughness of alloys with contrasting deformation behavior (ductile versus brittle) was explored in detail. In general, fracture toughness, as measured by maximum stress intensities at fracture, increased with notch length, reaching a maximum at a 0 / W of about 0.3, and then decreasing at large notch lengths. This behavior was found in both ductile and brittle alloys, suggesting that this is an intrinsic size effect arising from the mechanics of deformation of the net‐section. A new net‐section‐based stress analysis is used to model the fracture toughness behavior and explain the notch length dependence of fracture behavior. Based on the net‐section stress analysis, it is found that fracture occurred at a constant notch tip stress condition at all notch depths, yet creating a significant variation in fracture toughness with notch length in both brittle and ductile alloys. The implications of the findings on the measurement and interpretation of small specimen fracture toughness are discussed.