Linkage of Fracture Assessment Methodology With Fracture Toughness Testing Procedures

Abstract J-Integral, J, is a commonly accepted elastic-plastic fracture mechanics parameter defined as the amount of energy released per unit area of crack surface increase. J can be calculated numerically or measured experimentally using standard test specimen geometries and internationally accepted testing standards. The former is conventionally considered for structural components containing known or postulated defects and the latter for evaluating material fracture toughness. J for the structural component can subsequently be compared with the material fracture toughness J in order to assess structural integrity. R6 is an elastic-plastic fracture mechanics methodology that is inherently based on these two J evaluations. In terms of fracture toughness evaluation, despite J being originally developed to account for crack tip plasticity, it can be challenging to measure valid values of J using conveniently sized fracture toughness test specimens — even as large as B = 25mm Compact Tension (C(T)) — when testing modern, high toughness materials due to the extent of global plasticity generated during loading. Traditionally, fracture toughness testing procedures like ASTM E1820 have not been “linked” to structural integrity methodologies like R6 other than for the measured fracture toughness values to be “plugged” into the actual evaluations. However, in view of the fracture toughness evaluation challenges referred to above, a study has been undertaken to investigate how the validity limits specified in ASTM E1820 compare with the “plasticity” limit, specified in R6 and defined as Lrmax. The paper shows that C(T) B = 25mm specimens, tested/analysed according to ASTM E1820, produce JIc initiation toughness values residing in the Lrmax domain of the R6 Failure Assessment Diagram (FAD). This is despite the experimental JQ value being successfully qualified as JIc using the validity criteria in ASTM E1820 and J-R curves falling well below Jmax for the selected C(T) B = 25mm specimen size. This paper highlights the potential challenges associated with measuring valid and meaningful values of fracture toughness (J) for modern high toughness structural steels, without adopting inconveniently large specimen sizes. This further indicates that that there is a potential for reported JIc values to display a specimen size sensitivity due to the anticipated global plasticity experienced during testing.