Volume 9, Issue 4 (July 1981)
The Effects of Stress Dwells and Varying Mean Strain on Crack Growth During Thermal Mechanical Fatigue
Two types of test were used to evaluate the effects of creep and thermomechanical fatigue (TMF) on the crack growth rates of two nickel-base superalloys: directionally solidified MAR-M200 + hafnium and conventionally cast B-1900 + hafnium. In the test type designated simulated creep/TMF, the mean strain was incremented by a fixed amount at the maximum cycle temperature of each 20th TMF cycle. For the test type designated as creep/TMF, a constant stress was imposed at the maximum temperature for a fixed time period of each 20th TMF cycle. The crack growth rates for both test types were generally higher than the average rates of the baseline data from pure TMF tests. The fracture surfaces of the simulated creep/TMF specimens showed a considerable amount of plane stress behavior, that is, changes in crack planes and growth along steep angular planes. On the other hand, the fracture surfaces of the creep/TMF specimens were more typical of crack growth under plane strain conditions. A strong correlation was found for both alloys between baseline primary creep rates and cumulative creep rates of the interaction test specimens.