Stress exponent and primary creep parameters using single specimen and strain relaxation and recovery test

Strain relaxation and recovery test (SRRT), requiring one specimen and viscous (permanent) strain, ɛv (≤0.001 per test) on full unloading during primary creep, is presented with results on gas-turbine engine materials: Ti-6246 at 600 °C, Discaloy at 500 °C, IN-738LC at 850 °C and Waspaloy at 732 °C. It is shown that a ‘steady-state’ in irreversible viscous flow develops during primary creep; the shape of the creep curve is controlled by time-dependent reversible delayed elastic (anelastic) response. The average viscous strain rate during primary-creep, ε˙v(av)(=εv/tSR) for load duration, tSR and corresponding ɛv can be used for the determination of the stress exponent, nv for viscous flow. It is shown that the value of nv for primary-creep is comparable to the stress exponent, nmin for minimum creep rate. Using a single specimen, SRRTs also allow determinations of Young's modulus, stress exponent, s for delayed-elasticity (anelasticity), about one-third to fourth of nv, and other parameters for the constitutive equation for primary creep—strictly before creep enhancement due to the onset of microcracking activities. Short-term and long-term SRRT data on Waspaloy indicated that the creep strain at minimum creep rate consists of a significant amount of recoverable strain (32% at 450 MPa and 38% at 650 MPa).