High temperature fatigue crack growth in Alloy 718 - Effect of tensile hold times

The present investigation aims to clarify the mechanisms behind hold-time fatigue crack growth in Alloy 718 by using well designed tests where the crack length is carefully monitored. The results indicate that there is a significant embrittlement in a zone ahead of the crack tip during the hold-time, which is cracked open on the next load reversal. This leads to a very large cyclic contribution to the total crack length increment during a cycle, orders of magnitude larger than expected from purely cyclic tests at higher frequencies. During the hold-time follows the growth determined from pure sustained load tests, with the exception of an initial transient after the opening of the embrittled zone. An attempt to model the crack growth rate was made using a superposition model where the crack growth increments from high frequency da/dN testing and sustained load tests were added. The predictions of the total crack growth rate are generally adequate, but when the predictions of the individual contributions are scrutinized, it is obvious that the simple model does not correlate with the physical reality. Therefore both inter- and extrapolations from such a model are uncertain. Further, the test results show a decreased sustained load crack growth immediately after unloading/reloading of the crack. This transient behavior can potentially explain the reduction in crack growth rates previously explained by overload effects. \ua9 2011 Published by Elsevier Ltd