Determination of initiation fracture toughness ( J i ) from the stretch zone parameters in low carbon austenitic stainless steel

Estimating the initiation fracture toughness ( J i ) of ductile strain-hardening materials is challenging because of extensive crack blunting. This study proposes a stretch zone (SZ)-based approach for estimating J i using stretch zone width (SZW) and height (SZH), determined by FE-SEM fractography and contact profilometry. Fracture toughness tests were performed on SS 316L(N) containing 0.07-0.22 wt.% nitrogen at room temperature and 550°C following ASTM E1820. The material exhibited a fully austenitic microstructure, with grain size decreasing from approximately 90 to 67 µm as the nitrogen content increased. A nearly constant SZH/SZW ratio (∼1.5 at room temperature and ∼1.3 at 550°C) resulted in revised blunting constants (M = 4 and 2.6, respectively), enabling more reliable and physically representative estimation of J i .