Frequency dependence of fatigue crack growth in a TiB₂-SiC composite at elevated temperature

The subcritical crack growth behavior under tension-tension cyclic loading was investigated in a 30 vol.% TiB₂ particulate-reinforced β-Sic matrix composite at 800°C. The results clearly show that β-SiC ceramic is susceptible to cyclic fatigue at this moderate temperature, with the crack growth rate following a power-law dependence on the applied stress intensity range. An important characteristic of the cyclic crack growth at the elevated temperature is that the growth rate depends strongly on the cyclic frequency. At a given stress intensity range, a significantly higher crack growth velocity was measured at lower frequencies. Examination of the fracture surface indicates that cyclic crack growth occurs intergranularly. Based on a strain-controlled damage accumulation criterion, a fatigue crack growth theory is developed to explain the frequency dependence of cyclic crack growth, incorporating the viscoelastic response of the grain boundary phase. It is shown that analysis of the frequency response of the material using this theory permits the extrapolation of the constitutive properties of the grain boundary phase from cyclic crack growth experiments.