Abstract
In order to investigate the behavior of small crack initiation and growth in creep-fatigue at high temperature, c-p type fatigue (slow tension and fast compression) tests were carried out using smooth specimens of Type 304 stainless steel, 21/4Cr-1Mo steel and Ti-17 titanium alloy. Their average grain sizes were 50, 20 and 5μm, respectively. The results obtained are summarized as follows:
(1) Multiple small cracks initiated and grew along the grain boundaries on the specimen surface.
(2) The increasing rate of crack density showed stronger dependence on the strain range than the crack growth rate.
(3) Remarkable fluctuation of the crack growth rate, which is characteristic of microstructurally small cracks, was found for the cracks of the half length being smaller than 4 or 6 times of the average grain diameter.
(4) The average growth rate of the small cracks coincided with the rate extrapolated from the large crack growth law.
(5) The normalized relation between crack growth rate and crack length, being available for representing the region of microstructurally small cracks, was successfully derived on the basis of large crack growth law and average grain diameter of materials. This relation was independent of material and strain range.
