Transferability of Cleavage Fracture Parameters Between Notched and Cracked Geometries

Abstract

The present study investigates the temperature and specimen geometry dependence of cleavage fracture micromechanisms of a French pressure vessel steel (A508 Cl.3) over a temperature range which covers the lower shelf up to the DBT fracture toughness range. Notched tensile (NT) specimens with 3 different radii and CT specimens with two different thicknesses (12.5 and 25mm) were used to study the effect of temperature and geometry (constraint) on (i) the nature of the defects involved in the cleavage triggering, (ii) the mechanical parameters (stress and strain) values (iii) the Weibull parameters. The fractographic investigations showed that Manganese sulfide clusters and/or stringers were involved in the cleavage initiation process on the notched geometry at all temperatures from ¢ 130 £ C whereas they were never encountered in the cracked geometries. It is shown that only NT tests with a mean fracture strain lower than 25% have to be considered to make sure that the same nature of defects will be involved in the cleavage initiation. The mean value of the local stress was found to be approximately constant for NT and CT25 specimens over the temperature range [¢ 150. Also a unique set of Weibull parameters was found to describe all the NT tests over this temperature range.

Low-temperature fracture mechanisms in a spheroidised reactor pressure vessel steel

“…No microcracks were observed in the unfractured notch in specimens tested to catastrophic failure, and the reduced area fractions of voids in these notches suggest that a critical number density of voids is required for the fracture process to proceed from void nucleation to the microcrack nucleation stage. Recent studies using notched cylindrical uniaxial tensile test specimens of A508 RPV steel have also reported voids at the nucleation sites at low temperatures (Bouchet et al 2006;Hohe et al 2006). These studies and the results presented here strongly suggest that in the absence of a pre-crack, local ductile void-forming processes raise the stress of the material above the critical maximum principal stress required for the initiation and subsequent propagation of a microcrack (Bouchet et al 2006;Hohe et al 2006) even at very low temperatures.…”