TRANSIENT FATIGUE-CRACK GROWTH-BEHAVIOR FOLLOWING VARIABLE-AMPLITUDE LOADING IN A MONOLITHIC SILICON-NITRIDE CERAMIC

Transient cyclic fatigue-crack growth behavior following variable-ampl itude loading sequences has been investigated in a hot-pressed, monoli thic Si3N4 ceramic (NTK EC-141), for which fracture toughness (resista nce-curve) and cyclic fatigue properties are well characterized. Follo wing rapid changes in the applied stress-intensity range, Delta K, dur ing both low-high and high-low block-loading sequences, transients wer e observed in the subsequent cyclic crack-growth rates, but only under specific conditions. When K-min was suddenly varied with K-max held c onstant, no transients were observed. However, when the load changes i nvolved both Delta K and K-max, subtle transients were detected in the form of retardations following high-low, and accelerations following low-high block-loading sequences. The transient growth rates were typi cally a factor of similar to 2 different from the steady-state (base-l ine) growth rates for load excursions of similar to 30-40%, and lasted for several hundred microns of crack extension before a steady-state growth rate was achieved. Such behavior is rationalized in terms of gr ain-bridging mechanisms active in the crack wake. (C) 1998 Elsevier Sc ience Ltd. All rights reserved.