CYCLIC FATIGUE-CRACK PROPAGATION IN A SILICON-CARBIDE WHISKER-REINFORCED ALUMINA COMPOSITE - ROLE OF LOAD RATIO

The characteristics of subcritical crack growth by cyclic fatigue have been examined in a silicon carbide whisker-reinforced alumina composite, with specific reference to the role of load ratio (ratio of minimum to maximum applied stress intensity, R = K(min)/K(max)); results are compared with similar subcritical crack-growth data obtained under constant load conditions (static fatigue). Using compact-tension samples cycled at ambient temperatures, cyclic fatigue-crack growth has been measured over six orders of magnitude from approximately 10(-11)-10(-5) m cycle-1 at load ratios ranging from 0.05-0.5. Growth rates (da/dN) display an approximate Paris power-law dependence on the applied stress-intensity range (DELTAK), with an exponent varying between 33 and 50. Growth-rate behaviour is found to be strongly dependent upon load ratio; the fatigue threshold, DELTAK(TH), for example, is found to be increased by over 80% at R = 0.05 compared to R = 0.5. These results are rationalized in terms of a far greater dependency of growth rates on K(max) (da/dN is-proportional-to K(max)30 compared to DELTAK(da/dN is-proportional-to DELTAK5), in contrast to fatigue behaviour in metallic materials where generally the reverse is true. Micromechanisms of crack advance underlying such behaviour are discussed in terms of time-dependent crack bridging involving either matrix grains or unbroken whiskers.