FREQUENCY-DEPENDENCE OF FATIGUE-CRACK GROWTH IN A TIB2-SIC COMPOSITE AT ELEVATED-TEMPERATURE

The subcritical crack growth behavior under tension-tension cyclic loa ding was investigated in a 30 vol.% TiB2 particulate-reinforced beta-S IC matrix composite at 800 degrees C. The results clearly show that be ta-SiC ceramic is susceptible to cyclic fatigue at this moderate tempe rature, with the crack growth rate following a power-law dependence on the applied stress intensity range. An important characteristic of th e cyclic crack growth at the elevated temperature is that the growth r ate depends strongly on the cyclic frequency. At a given stress intens ity range, a significantly higher crack growth velocity was measured a t lower frequencies. Examination of the fracture surface indicates tha t cyclic crack growth occurs intergranularly. Based on a strain-contro lled damage accumulation criterion, a fatigue crack growth theory is d eveloped to explain the frequency dependence of cyclic crack growth, i ncorporating the viscoelastic response of the grain boundary phase. It is shown that analysis of the frequency response of the material usin g this theory permits the extrapolation of the constitutive properties of the grain boundary phase from cyclic crack growth experiments.