Degradation of Crack Shielding in Toughened Ceramics in Cyclic Fatigue

增韧陶瓷，包括相变增韧的氧化锆基陶瓷和晶须和纤维增强陶瓷复合材料，容易发生真正的循环疲劳现象。在循环加载中，相关的高亚临界裂纹扩展速率可能是由外在机制（裂纹屏蔽的退化）或内在机制（裂纹尖端损伤）引起的。本研究将通过利用罗马探针光谱直接测量裂纹屏蔽作用来分离外在和内在机制的贡献，从而研究铈部分稳定氧化锆/氧化铝和自增强氮化硅的循环疲劳的起源。将测量的屏蔽量与裂纹屏蔽理论进行严格的比较，并通过透射电镜和扫描电镜技术直接检查转换或桥接区，也是所提出研究的重要内容。本研究将采用罗马光谱学和电子显微镜对铈-部分稳定氧化锆/氧化铝和自增强氮化硅材料的循环疲劳起源进行研究。＊＊＊

9412691 Shetty Toughened ceramics, including transformation-toughened zirconia based ceramics and whisker-and fiber-reinforced ceramic composites, are susceptible to true cyclic fatigue phenomena. The associated high subcritical crack-growth rates in cyclic loading can arise from either extrinsic (degradation of crack shielding) or intrinsic (crack-tip damage) mechanisms. The research proposed will investigate the origins of cyclic fatigue in ceria partially stabilized zirconia/alumina and self-reinforced silicon nitride by separating the contributions of the extrinsic and the intrinsic mechanisms via direct measurements of crack shielding using Roman microprobe spectroscopy. Critical comparison of the measured shielding with the theories of crack shielding and direct examination of the transformation or bridging zones via Transmission and scanning electron microscopes techniques are also the important elements of the proposed research. %%% The proposed research will investigate the origin of cyclic fatigue in ceria-partially stabilized zirconia/alumina and self reinforced silicon nitride by means of Roman spectroscopy and electron microscope. ***