Characterization of the sub-critical crack growth of low alloy steels in a hydrogen environment by means of a acoustic-emission

Slow strain rate tests (SSRT) have been carried out using precracked compact tension specimens of low alloy stells in purified hydrogen gas at ambient temperature. By the use of elastic-plastic fracture mechanics parameters, the influence of hydrogen gas on sub-critical crack growth has been investigated in terms of the J-value at the onset of crack extension and also from a view point of fracture modes. Acoustic-emission (AE) during the SSRT tests has been monitored to clarify the cracking behavior in hydrogen gas and by mechanical tearing. In a hydrogen gas environment, it is clearly shown that a crack initiates and propagates in a quasi-cleavage manner until applied the J value reaches J1c and after that the fracture mode changes to mixture of ductile dimple and quasi-cleavage. This result suggests that mechanical tearing occurs at J-values greater than J1c even in a hydrogen environment. In SA 533 B steels, the AE activity showed two energy peaks with respect to loading levels, whose origins are attributable to the onset of quasi-cleavage fracturing caused by the environment and to the onset of mechanical tearing. (author)