Fracture behavior of surface cracked panels of HT-9 at low temperatures

Abstract

Surface cracked panels (SCP) of HT-9, a 12Cr tempered martensitic stainless steel, were tested in tension and three point bending at −73 and −100°C, corresponding to the lower shelf toughness, brittle quasi-cleavage fracture regime exhibited in standard static tests on compact tension specimens. Selected fracture surfaces were examined by scanning electron microscopy and confocal microscopy, and the confocal micrographs were used to evaluate the fracture process by fracture reconstruction methods. Test results showed that despite testing in the quasi-cleavage fracture regime, considerable ductility preceded final fracture, and the ductility increased with decreasing crack size. Conditions for crack initiation could be interpreted in terms of a critical combination of J and Q, as well as a critical stressed area criteria based on a micromechanical model for quasi-cleavage fracture. The combined results suggest that ferritic structures containing shallow, part through cracks may be relatively resistant to failure even in the quasi-cleavage fracture regime.