Cause and Effect of Factors Affecting the triangle open K_(th) of Small Crack

摘要

The threshold stress intensity factor (triangle open K_(th)) of small crack is affected by various factors, especially by material hardness, stress ratio and crack size. Test results showed that harder material had higher triangle open K_(th) and smaller crack had lower triangle open K_(th). The crack closure measurement on a very small crack was done to make clear the root cause of those effects. Most of those effects could be explained by the peculiar behavior of crack closure for crack deeper than 100 mu m. However, everything could not be understood only by the crack closure behaviour. In addition to the difference in crack closure, (triangle open K_(eff))_(th) itself was also dependent on crack size when the crack depth was shallower than 100 mu m. Another remarkable phenomenon was experienced in this study. Unusual decrease in triangle open K_(th) was found in some case. The large decrease of triangle open K_(th) occurred under the conjunction of three factors, that is, extremely high stress ratio (if) higher than 0.8, small crack and hard material. This kind of large reduction in triangle open K_(th) in high if region is of much importance in turbo machinery that operates under high mean stress with small vibratory stress.