Modelling ductile fracture behaviour from deformation parameters in HSLA steels

Abstract

A B S T R A C T In this work, an attempt is made to model the ductile fracture behaviour of two Custrengthened high strength low alloy (HSLA) steels through the understanding of their deformation behaviour. The variations in deformation behaviour are imparted by prior deformation of steels to various predetermined strains. The variations in parameters such as yield strength and true uniform elongation with prior deformation is studied and was found to be analogous to that of initiation fracture toughness determined by independent method. A unique method is used to measure the crack tip deformation characterized by stretch zone depth that also depicted a similar trend. Fracture toughness values derived from the stretch zone depth measurements were found to vary in the same fashion as the experimental values. A semiempirical relationship for obtaining ductile fracture toughness from basic deformation parameters is derived and model is demonstrated to estimate initiation ductile fracture toughness accurately.Keywords deformation effect; ductile fracture toughness; HSLA steels; semiempirical model; stretch zone depth. N O M E N C L A T U R Ea, a = crack length, crack extension C = constant in the CTOD-ε relationship C 1 , C 2 = coefficients in the J -a relation COD, CTOD = crack opening displacement, crack tip opening displacement E = Young's modulus EPFM = elastic plastic fracture mechanics HSLA = high strength low alloy J, J i , J SZD = an energy based ductile fracture toughness characterizing parameter, initiation fracture toughness, initiation fracture toughness determined from stretch zone depth measurement k = coefficient in the Hollomon's stress-strain relation LEFM = linear elastic fracture mechanics L-T = longitudinal transverse m = constant in the J -δ relation n = strain-hardening exponent SZD = stretch zone depth SZW = stretch zone width t = projection of stretch zone width TPB = three point bend W = specimen width w = stretch zone width