CYCLIC FATIGUE OF A SILICON-NITRIDE - INFLUENCE OF FREQUENCY AND FATIGUE MECHANISM

An investigation has been carried out on the slow crack growth behavio ur-of an advanced Si3N4 ceramic material at room temperature at differ ent loading frequencies. The results clearly show a detrimental effect of cyclic loading on crack growth rate in terms of time and a reduced crack growth resistance with increasing cyclic frequency. Crack growt h rates can be described by the Paris power-raw expression for both st atic and cyclic loading, but the exponent n increases with decreasing loading frequency. Further support for the existence of mechanical fat igue in this material is provided from experiments involving alternate cyclic and static fatigue using the same specimen, which show substan tial differences in crack growth rate in terms of time. Removal of cra ck wakes resulted in an unchanged crack growth rate under sustained lo ad, which suggests that the crack wake does not play a key role in enh anced crack growth under cyclic loading. The likely crack growth mecha nism is discussed.