The use of circumferentially notched tension (CNT) specimen for fracture toughness assessment of high strength steels

Abstract

The fracture toughness of high strength steels is commonly determined by standard methods using Compact tension (CT) or Single edge notched bend (SENB) specimens. In the past the Circumferentially Notched Tension (CNT) geometry has been reported as a potential candidate for determining the fracture toughness of highly constrained cracks, theoretically approaching plane strain conditions, even for small specimen dimensions. The goal of this study is to develop a more fundamental understanding of the CNT methodology and apply it to high strength S690QT steel. An alternative prefatiguing method was developed and a straightforward relation was established between the Crack Mouth Opening Displacement (CMOD) and the Crack Tip Opening Displacement (CTOD). With the new experimental aspects, it proved feasible to determine upper-shelf CTOD values for S690QT steel, using small CNT specimens (D = 12 mm), tested at room temperature with a relative high loading rate. Furthermore, CNT low temperature values were found comparable to those of conventional SENB tests. Hence, the research demonstrates that CNT geometry allows for small scale high loading rate specimen testing, resulting in simple, rapid and cost effective fracture toughness determination.