Observation of Crack Retardation Resulting from Load Sequencing Characteristic of Military Gas Turbine Operation

Abstract

The nature of crack propagation resulting from flight loading representative of military gas turbine operation is investigated Mission stress profiles for turbine disks fabricated from the superalloys GATORIZED IN 100 and Waspaloy contain load sequences that produce synergistic effects on crack propagation. Major load throttle excursions, overloads, occur routinely during flight, and a retardation in subsequent crack propagation generally results. Such mission load interaction effects have been addressed in crack propagation testing employing repetitive overload-fatigue sequences. The influences of overload ratio (Poverload/Pmax) and the number of fatigue cycles between overloads have been investigated for crack propagation at 649°C (1200°F), and an interpolative model of these effects is presented. A determination of the instantaneous crack retardation following a mission major load excursion is accomplished with an unconventional method. The existence of a deceleration in crack growth rate, delayed retardation, following a mission overload is verified. Typically, this period is greater than the total number of baseline fatigue cycles applied between engine mission overloads, and delayed retardation is largely, if not entirely, responsible for the beneficial effects of the overloading.