Abstract
Macroscopic through-crack propagation in ductile materials under creep conditions has been described successfully by the creep J-integral, J. This paper extended the creep J-integral concept based on the power law creep to the case of shorter cracks, and limitation of the use of J was examined. The propagation of surface cracks from a small semi-circular notch of about 0.6mm diameter and that from the surface grain boundaries of the smooth specimen with 40μm grain diameter of 304 stainless steel were observed during the air and the vacuum tests. When the crack grew to the length (depth) of 0.1mm or longer, the J approach became applicable, resulting in the same correlation between dc/dt and J as that for the macro-through-crack. Cracks less than 0.1mm, or two or three grain boundary length, on the other hand, showed higher propagation rates than that estimated by the straight line extrapolation on log-log plots of dc/dt-J. The growth of creep cavities by grain boundary diffusion was considered to be a dominant mechanism for the microscopic crack propagation. The time needed to make one grain facet fracture (crack initiation) at several surface grain boundaries can be well predicted on the basis of the model of cavity growth and coalescence proposed by Chen and Argon.
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