Backscattered electron microscopy technique enhancing stretch zone width imaging for initiation fracture toughness measurements

Backscattered and secondary electron microscopy techniques have been compared in determining critical stretch zone widths (SZWC) formed in the original and thermally embrittled microstructures of a nuclear reactor pressure vessel steel tested under Charpy impact loading conditions. It has been shown that the former technique does improve fracture surface image quality, as compared to the widely used secondary electron imaging mode, and so accurate measurements of initiation fracture toughness have been achieved over the whole range of mechanical behavior studied. Hence, a Hall–Petch dependence of the SZWC on the representative cell size of the materials tested could be identified.