Applicability of smaller than Charpy specimens for fracture toughness characterization with the VTT method

The term fracture toughness usually refers to the linear elastic fracture resistance parameter K_IC. In the case of structural steels, the estimation of K_IC is limited to the lower shelf of toughness or require extremely large specimens. This specimen size requirement has been one major obstacle for applying fracture mechanics in structural integrity assessment outside aviation, nuclear and off-shore industries. During the last decade, a statistical data treatment methodology, based on a micro-mechanistic cleavage fracture model, combined with elastic plastic finite element analysis has enabled the fracture toughness to be characterized with small specimens in the ductile-to-brittle transition region. The methodology is known as the VTT method or the Master Curve procedure. The development has led to a new testing standard for fracture toughness testing of ferritic steels in the transition range. Here, the premises for the methodology are described and its validity range is discussed. Presently the methodology has been validated for as small as 10.10 mm² bend specimens, but the use of even smaller specimens is under investigation. Specifically, results obtained with three different sub-Charpy specimen configurations are presented and discussed.