Grain size effects on the critical stretch zone width of charpy impact specimens

Abstract

The elastic-plastic fracture process from a pre-existing crack in a monotonically loaded test specimen or structure consists of several stages: crack tip blunting, plastic zone formation at crack tip, nucleation and growth of voids ahead of crack tip, coalescence of voids, i.e., crack growth, along with the development of plastic wake, and final rupture. Just before crack initiation, which corresponds to the coalescence of the original crack tip with the nearest void, crack stretching is an important stage in the fracture phenomenon [1–6]. Observed on the microscope, this particular region exhibits a typical featureless appearance, which in most cases can be distinguished from the prior precrack surface and the subsequent main fracture by ductile tearing. Since the critical stretch zone width (SZWC) is indicative of the extent of the plastic blunting of the crack tip, it offers an alternative method for determining a fracture toughness parameter that can be correlated with the critical values of energetic and geometric-based fracture mechanics parameters JIC and CTODC, respectively. The main advantage of SZWC over other fracture toughness indexes is that no load, displacement or any other operational parameters need to be known. This is particularly important in post-mortem failure assessment or when load and displacement are absent or difficult to determine, which include the case of fracture toughness determination under dynamic loading conditions, where it is not possible or at least feasible to apply the standard J-R curve approach, e.g. by compliance or potential drop measurements, in order to determine the onset of crack growth. Furthermore, it does seem interesting to have a permanent record that can be verified independently in various laboratories, in round robin programs, so long as the permanent plastic strains on the fracture surface are a record of the failure process. In this regard, the stretch zone width has been claimed to be an effective back-up technique for characterization of the fracture toughness of metals and weldments. In the present study, a thermally embrittled nuclear grade steel was impacted in a Charpy testing machine and the SZWC of the precracked specimens was measured and tentatively correlated to the representative cell size of the tested microstructures, which have been fully characterized elsewhere [7, 8]. Basically, the A508 Class 3A steel is an exceptionally high-toughness