Study on the mechanism of cumulative fatigue damage of advanced engineering materials under service loadings

本课题研究了钛合金、粉末冶金（P/M）铝合金、金属基复合材料（MMC）和陶瓷等先进工程材料在恒幅和变幅载荷下的疲劳裂纹萌生和扩展行为，发现钛合金在变幅载荷下的裂纹张开行为与材料的微观结构有关，这导致了不同的裂纹扩展速率对相同的应力强度因子取决于材料。在钛合金和高强度结构钢上，载荷降低对疲劳小裂纹的扩展有加速作用，而对常规疲劳长裂纹的扩展有抑制作用。对于SiC颗粒增强铝基复合材料，即SiC颗粒增强粉末冶金复合材料和SiC晶须增强铸造复合材料，疲劳裂纹的萌生和扩展机制因SiC的几何形状和生产工艺而异。疲劳裂纹在粉末冶金复合材料中的SiC颗粒附近萌生，并以单个或多个裂纹扩展直至断裂。另一方面，在铸造复合材料中，疲劳裂纹容易在晶须/基体界面处或在SiC晶须被拔出的位置处萌生，并且发现一些微裂纹合并形成主裂纹。通过对3%硅铁在单次过载下疲劳裂纹扩展行为的直接观察，发现SiC颗粒增强的疲劳裂纹扩展阻滞行为与裂纹的偏转和分叉密切相关，SiC颗粒增强的疲劳裂纹扩展阻滞行为与裂纹的偏转和分叉密切相关，SiC颗粒增强的疲劳裂纹扩展阻滞行为与裂纹的偏转和分叉密切相关。在烧结氮化硅上发现，在重复的两步加载下，裂纹生长加速，这与通常在金属材料上观察到的延迟行为相反。

In this project, fatigue crack initiation and propagation behavior undre constant and variable amplitude loadings were investigated on advanced engineering materials such as titanium alloys, powder metallurgy (P/M) aluminum alloys, metal matrix composites (MMC) and ceramics.Crack opening behavior of titanium alloys under variable amplitude loadings was found to depend on the microstructure of materials, which resulted in the different crack growth rates against the identical stress intensity factor depending on the materials. Acceleration of crack growth by load reduction was observed for the small fatigue cracks, while retardation was found for conventional long fatigue cracks on the titanium alloys as well as high strength structural steels.As to the SiC reinforced aluminum matrix composites, i.e.SiC particle reinforced P/M composite and SiC whisker reinforced casting composite, fatigue crack initiation and growth mechanisms were found to be defferent depending on the geometry of SiC and production processes. A fatigue crack initiated in the vicinity of SiC particle in the P/M composite and a single or a few cracks propagated until fracture. On the other hands, in the casting composite, fatigue cracks easily initiated at the whisker/matrix interfaces or at the sites where SiC whisker was pulled out and some microcracks were found to coalesce to form a main crack. Furthermore, SiC reinforcement irrespective of particle or whisker was found to be benefit for fatigue characteristics of MMC due to the increase of fatigue crack growth resistance.Through the direct observation of fatigue crack growth behavior under a single overload on a 3% silicon iron, it was found that the retardation behavior was closely correlated with deflected or branched cracks.Finally, crack growth acceleration under repeated two step loadings was found on a sintered silicon nitride, to the contrary of the retardation behavior usually observed on the metallic materials.