The object of this investigation was to determine the principal parameters which affect crack growth at elevated temperatures and under sustained loads. These results provide a better understanding for the development of crack growth models which improve the capability of predicting crack growth in materials which are used under extreme operating conditions such as those produced in gas turbine engines.
Crack growth behavior was investigated in a nickel-base superalloy (!NH)()) at 732°C in laboratory air under sustained load. The tests were conducted on side-grooved compact-type specimens. One of the purposes of the study was to evaluate the effect of the through-the-thickness stress distribution upon crack growth behavior in grooved specimens. Also in this study the effects of crack orientation and net thickness between the side grooves were evaluated.
The crack growth rates as a function of stress-intensity factor showed a slight dependence upon thickness and were independent of crack orientation. Also, the rates at the same stress-intensity level in the grooved specimens were faster than the rates in the smooth specimens, which indicated that the “plane-stress region” near the sides of the smooth specimen tends to retard the crack growth in the interior of the specimen. A model for the crack growth rates as a function of stress-intensity factor is presented. The model incorporates the dependence of growth rates upon initial loading conditions, and the effect of thickness and prior loading history can be taken into account in the model. The effect of overload ratios also is discussed.