PAPER 2 - J and CTOD estimation equations for shallow cracks in single edge notch bend specimens

Abstract

Fracture toughness values determined using shallow cracked single edge notch bend, SE(B), specimens of structural thickness are useful for structural integrity assessments. However, testing standards have not yet incorporated formulas that permit evaluation of J and CTOD for shallow cracks from experimentally measured quantities (i.e. load, crack mouth opening displacement (CMOD), and load line displacement (LLD)). Results from two dimensional plane strain finite - element analyses are used to develop J and CTOD estimation strategies appropriate for application to both shallow and deep crack SE(B) specimens. Crack depth to specimen width (a/W) ratios between 0.05 and 0.70 are modelled using Ramberg Osgood strain hardening exponents (n) between 4 and 50. The estimation formulas divide J and CTOD into small scale yielding (SSY) and large scale yielding (LSY) components. For each case, the SSY component is determined by the linear elastic stress intensity factor, KI. The formulas differ in evaluation of the LSY component. The techniques considered include: estimating J or CTOD from plastic work based on load line displacement (Apl|LLD), from plastic work based on crack mouth opening displacement (Apl|CMOD), and from the plastic component of crack mouth opening displacement (CMODpl). Apl|CMOD provides the most accurate J estimation possible. The finite - element results for all conditions investigated fall within 9% of the following formula:▪