Evaluation of Mechanical Properties of Microelements and Environmental Strength under Dynamic Loading

研究了含单晶硅微区元素和芳纶纤维增强微细材料的断裂和疲劳行为。研制了专门设计的试验机，可对最小尺寸的微元器件进行准静态和疲劳试验。单晶硅微区元素发生弹性变形，直至最终失效，呈现脆性。断裂强度随试件尺寸的减小而增大，最大断裂强度约为7.7 GPA。然而，硅微结构元素对缺口敏感，聚焦离子束引入的亚微米深缺口降低了材料的断裂强度。至于疲劳载荷，即使用原子力显微镜对表面进行纳米显微镜检查，也没有观察到疲劳损伤。然而，在水中，疲劳寿命降低：动态载荷和水环境的协同作用促进了裂纹的形成。原子力显微镜能够对导致水中失效的纳米级裂纹进行成像。芳纶纤维的纤维强度受环境的强烈影响：吸水和真空处理会降低断裂强度。在疲劳性能方面，芳纶纤维具有优异的疲劳性能，与金属材料相比，虽然有较大的散射带，但在S-N曲线上呈现缓斜率。在真空效应方面，真空下的疲劳强度高于空气中的疲劳强度。用原子力显微镜观察了在空气和真空中疲劳载荷对纤维表面的损伤，并讨论了环境对疲劳的影响及损伤机理。

The fracture and fatigue behavior of micromaterials including single-crystal Si microelements and an advanced fiber reinforcement of aramid fiber have been performed. The specially designed testing machines are developed, which are capable of performing quasi-static and fatigue tests in mum sized microelements. Single crystal Si microelements deform elastically until final failure, giving a brittle nature. The fracture strength increases with a decrease in specimen size, and the maximum fracture strength is about 7.7 GPa. However, Si microelements are sensitive to notch, and sub-mum deep notch introduced by focused ion beam decreases the fracture strength. As for fatigue loading, no fatigue damage is observed even though the surface is nanoscopically examined by an atomic force microscope. However, in water, the fatigue lives are decreased : crack formation is promoted by a synergistic effect of dynamic loading and water environment. Atomic force microscopy is capable of imaging a nanoscopic crack, which causes the failure in water. The fiber strength of aramid fiber, Kevlar 49, is strongly influenced by an environment : water absorption and vacuum conditioning decrease the fracture strength. As for fatigue, aramid fibers have excellent fatigue properties, compared with metallic materials, showing gentle slope in s-N curves, although they have large scatter band. As for vacuum effects, the fatigue strength in vacuum is higher than that conducted in air. The fiber breaks with fiber splitting, and the fiber surface damage induced by fatigue loading in air and in vacuum was examined by using an atomic force microscope, and the influence of environment on fatigue and damage mechanisms are discussed.