Direct Measurement of Fatigue Damage Accumulation in Ceramics and Elucidation of Its Mechanisms

目前已有“裂纹桥接退化模型”和“裂纹抵抗-再激活模型”（或“楔入微裂纹模型”）被提出来解释陶瓷材料的疲劳裂纹扩展行为。两种模型的显著区别在于，前者只在加载过程中裂纹扩展，而后者在加载和卸载过程中裂纹都扩展。针对这一点，我们试图阐明陶瓷疲劳损伤的机理。测量了氮化硅和氧化铝疲劳裂纹扩展的基本数据，从而明确了微观组织因素（晶粒尺寸和长径比）、应力比和裂纹闭合对裂纹扩展行为的影响。研究了非稳态裂纹扩展与微观组织的关系。然后，利用导电膜电位法精确测量了氮化硅在加载和卸载过程中的裂纹扩展。结果表明，裂纹扩展不仅发生在加载过程中，也发生在卸载过程中，且卸载过程中裂纹扩展的比例随频率的增加而增加。综上所示，在氮化硅陶瓷的循环疲劳中，“抗裂-再激活”或“楔形微裂纹”机制起主导作用。

There have been "Crain-bridging degradation model" and "Grack resisting-reactivating model" (or "Wedging microcracking model" ) proposed to explain the fatigue crack growth behavior in ceramic materials. The significant difference between the two models is that in the former the crack propagates during loading only but in the latter it should do both during loading and unloading. Paying attention to this point, we tried to elucidate the mechanism of fatigue damage in ceramics.The basic data on fatigue crack growth in silicon nitride and alumina were measured so that the effects of microstructural factors (grain size and aspect ratio), stress ratio and crack closure on crack growth behavior were made clear. Unsteady crack growth was examined in relation to the microstructure. Afterwards, the crack extension during loading and unloading processes in silicon nitride was precisely measured by using the conductive film-potential method. As a result, it has been found that the crack growth occurs not only during loading process but also during unloading process and the fraction of the crack advancement during unloading increases with increase of frequency. Judging from these results, it was concluded that the "Crack resisting-reactivating" or "Wedging microcracking" mechanism should be dominantly operative in the cyclic fatigue in silicon nitride ceramics.