Creep plastic zone and crack initiation in double edge notched specimens of 304 stainless steel at 650 °C

Creep tests of 304 stainless steel using double edge notched specimens (6 mm thickness, 8 mm net width, 0.3 mm notch width, notch depth a = 0.5, 1 and 2 mm) were carried out in air at 650 °C at a net stress of 88 MPa till crack initiation on the notch root surface. Creep crack initiation time: the time when ∼0.5 mm long crack is formed on the notch root surface, increased with increasing notch depth, i.e. from ∼3200 h for a = 0.5 mm, to ∼4000 h for a = 1 mm and to ∼5000 h for a = 2 mm. The creep crack initiation site was the notch root of near mid-thickness for a = 2 mm, while it was the notch root of near surface thickness for a = 0.5 mm. Development of the creep plastic zone was observed by the subsequent recrystallization thermal etching technique using multiple specimens which were crept to various times up to crack initiation. shown that the difference of the crack initiation site is related whether creep plastic zone is subjected to gross yielding or not.