常圧焼結窒化ケイ素のき裂進展挙動に及ぼすき裂寸法の影響と繰返し疲労機構の検証

抄録

In order to investigate the effect of crack size on the crack propagation behavior of a sintered silicon nitride, crack propagation tests were conducted under cyclic loads and static load. Two different sizes of small surface semielliptical cracks were introduced by Knoop indentation. By polishing the surface layer, the residual stress caused by Knoop indentation was perfectly removed. The crack propagation rates of small cracks (350μm, 200μm) were slower than that of a long crack obtained with CT specimen. Analyzing the crack opening displacement (COD) of the 350μm crack, we could determine the quantity of the true stress intensity factor K_Itip·K_Itip was expressed as K_Itip=K_Imax-K_s, where K_s is a component of the stress intensity factor shielded by bridging effects. Furthermore, we found that the COD after cyclic loading was larger than that before cyclic loading. It is evident that the increase in COD after cyclic loading was originated from the reduction of bridging force with cyclic fretting at the bridging sites under cyclic loads. It was concluded that the reduction of bridging force made the K_Itip increase and increased K_Itip accelerated the crack propagation rate.