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

点击翻译按钮获取中文摘要

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.