The Effect of Grain Size on Small Fatigue Crack Growth in Pure Titanium

Abstract

The growth behaviour of small fatigue cracks was studied on both fine-and coarse-grained materials of pure titanium under axial loading (R=-1), and the growth behaviour and its statistical properties on coarse-grained material were also investigated under rotating bending and were compared with the results of fine-grained material in a previous report. As the growth rate data are plotted in terms of an effective stress intensity range, small cracks still grow faster than large cracks in a small crack regime. Small cracks of coarse-grained material show higher growth rates than finegrained material due to a much smaller effect of microstructures such as the grain boundary and crack deflection. Stage-I facets are observed in all specimens and their depths are less than the maximum grain size estimated by the statistics of extreme values. However, the distribution of Stage-I facet depths corresponds to the maximum distribution of grain size of the materials. The growth rates of small cracks follow a log-normal distribution independent of grain size. The coefficient of variation of the crack growth rate in coarse-grained material is less than that in finegrained material. The coefficient of variation is significantly large at a/d <3 (a : crack depth, d : grain size), showing that the relative size of microstructurally small cracks is not dependent on grain size. © 1992, The Japan Society of Mechanical Engineers. All rights reserved.