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

Abstract

Strain relaxation and recovery test (SRRT), requiring one specimen and viscous (permanent) strain, ɛ v (≤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 ) ( = ε v / t SR ) for load duration, t SR and corresponding ɛ v can be used for the determination of the stress exponent, n v for viscous flow. It is shown that the value of n v for primary-creep is comparable to the stress exponent, n min 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 n v, 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).