The propagation rate of distributed creep microcracks is influenced by the microstructure and the inhomogeneous local deformation, and it may not follow a definite law but depend on a probabilistic characteristic, i.e., the time and space scattering of crack initiation and the fluctuation of crack propagation rate. The crack density, n(t), and the distribution of crack length, f(c), which are closely related to the initiation and propagation of distributed creep microcracks, can be measured rather easily at any time. In this paper, a method for the estimation of the microcrack propagation, rate using n(t) and f(c) was proposed based on the supposition that no definite distribution of crack propagation rate exists. The crack propagation rate was estimated on 304 stainless steel in which surface microcracks of the size of more than one grain boundary facet were observed during creep in air and in vacuum at 650°C. The estimated propagation rate was close to the maximum crack propagation rate measured as far as the applied stress dependence was concerned. It is supposed that a better estimation is obtainable if the length of creep microcracks is measured more accurately at the early stage of the propagation.
