Degradation of Strength by Cyclic Loading and its Mechanism in Fracture of Ceramic Materials

研究了几种陶瓷材料在恒应力强度因子下的慢裂纹扩展，特别注意了循环载荷的影响。氮化硅和氧化铝陶瓷的裂纹扩展速率随着裂纹的扩展而减小，最终阻止了裂纹的扩展。另一方面，硼硅酸盐玻璃没有表现出这样的行为。在恒定的DELTAKAPPA循环载荷下，陶瓷中的裂纹继续扩展，但在裂纹扩展的初始阶段，裂纹的扩展速率有所下降。在粗晶氮化硅和氧化铝中，裂纹扩展和止裂的延迟更为明显。扫描电子显微镜观察裂纹的张开和闭合情况表明，在粗晶处裂纹穿过晶界时，桥联效应导致了裂纹扩展的延迟和停止。粗晶的桥联效应依赖于裂纹扩展的长度。上述事实表明，陶瓷中裂纹扩展速率与Kappa之间的关系(所谓的Kappa-v关系)依赖于裂纹长度，不能被认为是材料本身的特性。得到了典型的氮化硅和氧化铝陶瓷的S-N图。断口和金相分析表明，裂纹萌生位置位于切应力幅值最大的接触面内侧。氮化硅在拉-压载荷作用下没有表现出循环疲劳行为。这表明脆性陶瓷材料在可压缩剪应力状态下存在循环疲劳的可能性。

Slow crack growth under the constant stress intensity factor was investigated for several ceramic materials with the particular attention of the effect of cyclic loading. The crack growth rate in silicon nitrides and alumina ceramics decreases with the crack extension and finally the crack extensions are arrested. On the otherhand, the Borosilicate glass doesn't show such behavior. Under constant DELTAKAPPA cyclic loading, the cracks in the ceramics continue to grow without being arrested, though some decrease in growth rate is observed in the initial stage of crack extension. The delay of crack growth and arrest were more significantly observed in coarse grained silicon nitride and alumina. From the observation of crack opening and closure in the SEM, it was shown that the bridging effect resulted the delay and arrest of the crack growth when the crack passed through the grain boundary at the coarse grain. The bridging effect due to the coarse grain depends on the length of crack extension. The fact above indicates that the relation between the crack growth rate and KAPPA in ceramics (so called the KAPPA-v relation) depends on the crack length and can not be considered to be the materials own properties.In this work, rolling contact fatigue of ceramics was also invetigated. Typical S-N diagrams were obtained for the silicon nitride and alumina ceramics. Fractographic and metallographic examinations showed the crack initiation sites located inside under the contact surface at which the shear stress amplitude took maximum value. The silicon nitride didn't show the cyclic fatigue behavior under the tension-compression loading. It indicates the possibility of the cyclic fatigue under the compressible shear stress state in brittle ceramic materials.