Effect of cyclic loading on the creep performance of silicon nitride

Tension-tension cyclic fatigue tests (triangular waveform, σ_max = 100 MPa, R = 0.1) were conducted on hot isostatically pressed (HIPed) silicon nitride at frequencies spanning several orders of magnitude (5.6×10^-6 to 0.1 Hz or 10^-3 MPa/s to 18 MPa/s) at 1370°C in air. The amount of cyclic creep strain was found to be a function of the frequency or stressing rate with greater strains to failure observed as the frequency or stressing rate decreased. The total strain was viewed as the sum of elastic, anelastic (or transient recoverable), and plastic (viscous or non-recoverable) strain contributions, after the empirical Pao and Marin model. The plastic strain was found to be the dominant component of the total creep and was unsatisfactorily represented by the Pao and Marin model. To circumvent this, a time exponent was introduced in the plastic strain term in the Pao and Marin model. This modification resulted in good correlation between model and experiment at the slower frequencies examined but over-predicted the cyclic creep strain at the faster frequencies. The utility of using the modified Pao and Marin model to predict cyclic creep response from static creep and strain relaxation tests is described.