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

Fatigue crack propagation under cyclic loading was observed in CT specimens of silicon nitride by employing an electrical resistance technique with crack gage made of platinum film. The crack growth rate was not uniform even if the fatigue condition was constant. The crack growth behavior in maximum stress intensity factor K(Imax)-increasing condition was different from that in K(Imax)-decreasing condition. The effect 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 method. The experimental results, indicated that the unloading may cause fretting and/or crashing of the asperities between the crack faces in silicon nitride, where toughening is achieved by grain bridging. Therefore, the cyclic loading may accelerate the static fatigue crack propagation caused by stress corrosion cracking.