Elevated Temperature Cyclic Fatigue Behavior In Monolithic Ceramics

95-22134里奇 拟议的项目的重点是开发一个详细的理解和定量模型的物理和循环疲劳损伤的基本机制和高温下单片陶瓷裂纹前进。 本研究的主要目的是表征和模拟陶瓷在高同源温度下的循环疲劳行为，以研究在空气和真空环境中的可比损伤和裂纹推进机制。 该方法将利用高温（高达1800 ℃）蠕变和疲劳裂纹生长测试设备来检查单块氧化铝、氮化硅和碳化硅陶瓷中一系列微结构中的亚临界裂纹，选择这些微结构来改变晶粒尺寸和形态、晶界相的结晶度等，连同详细的微观结构表征（使用原位成像和故障前和故障后电子显微镜）的周期依赖性退化的屏蔽机制后面的裂纹尖端和累积的微观结构损伤的内在蠕变和疲劳机制之前的尖端。 随着科学的理解，这项工作的目的是提供指导方针的设计改进的微观结构和组成的耐高温陶瓷的低温韧性和高温耐亚临界疲劳裂纹生长的最佳组合。 ***

95-22134 Ritchie The proposed project is focused on developing a detailed understanding and quantitative models of the physics and fundamental mechanisms of cyclic fatigue damage and crack advance in monolithic ceramics at elevated temperatures. The central aim of the present study is to characterize and model the cyclic fatigue behavior of ceramics at high homologous temperatures to examine the comparable damage and crack advance mechanisms in both air and vacuum environments. The approach will utilize ultrahigh temperatures (up to 1800C) creep and fatigue -crack growth testing facilities to examine subcritical cracking in a range of microstructures in monolithic alumina, silicon nitride and silicon carbide ceramics, chosen to vary grain size and morphology, the crystallinity of the grain-boundary phase, etc., together with detailed microstructural characterization (using both in situ imaging and pre- and post-failure electron microscopy) of the cycle-dependent degradation of shielding mechanisms behind the crack tip and the accumulated microstructural damage by intrinsic creep and fatigue mechanisms ahead of the tip. %%% With the scientific understanding gained, this work is intended to provide guidelines for the design of improved microstructures and compositions for flaw-tolerant ceramics with optimal combinations of low-temperature toughness and elevated-temperature resistance to subcritical fatigue-crack growth. ***