FATIGUE-CRACK GROWTH-BEHAVIOR OF LARGE CR ACK IN SILICON-NITRIDE

Fatigue crack propagation under cyclic loading was observed in CT spec imens of silicon nitride by employing an electrical resistance techniq ue with crack gage made of platinum film. The crack growth rate was no t uniform even if the fatigue condition was constant. The crack growth behavior in maximum stress intensity factor K(Imax)-increasing condit ion was different from that in K(Imax)-decreasing condition. The effec t of stress ratio R (= sigma(min)/sigma(max)) was not significant when the minimum stress intensity factor (K(Imin)) was greater than crack opening stress intensity factor (K(open)). Fracture toughness measured by CT specimens after fatigue testing was smaller than that by SEPB m ethod. The experimental results, indicated that the unloading may caus e fretting and/or crashing of the asperities between the crack faces i n silicon nitride, where toughening is achieved by grain bridging. The refore, the cyclic loading may accelerate the static fatigue crack pro pagation caused by stress corrosion cracking.