Influence of Environment on Fracture Behavior of High-Strength, High-Modulus Fibers and Nanoscopic AFM Damage Evaluation

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This research project focuses the fracture behavior of high-strength and high elastic modulus fibers, i.e., ceramic precursor type fibers and aramid fibers. Special attention is paid to the influence of an testing environment on the fracture behavior, and nanometric surface damage is evaluated with atomic force microscopy (AFM). In order to conduct tensile and fatigue testing of fibers with small diameter of micrometers, a specially designed testing machine, which enables mechanical testing including fatigue of microelements such as fibers in a controlled environment was used. Loads are applied to a specimen by means of an electromagnetic actuator. The testing machine is equipped with a chamber which can control environments such as vacuum, wet and/or dry gases. The longitudinal elastic modulus of the ceramic fibers including gamma-alumina (Altex^<<O!>>), SiC (Nicalon), and Si-Ti-C-O (Tyranno^<<O!>>))were independent of testing environment, i.e., vacuum, air and deionized water. However, the quasi-static fiber strength in vacuum was the highest, and decreased in the order of in air, and in water : the fiber strength decreased with increasing water content in an environment, and with decreasing displacement(strain)rate. Similarly to ceramic fibers, the fracture strength of aramid fibers was sensitive to a testing environment : the mechanical properties of aramid fibers including Kevlar^<<O!>>) fibers and Technora^<<O!>>) fiber, longitudinal elastic modulus and fracture strength, decreased by water absorption. When tested in vacuum, the air-preconditioned fiber strength of Kevlar 49^<<O!>>) decreased from that conducted in air. As for fatigue, the Kevalr 49 fiber had superior fatigue resistance compared with metallic materials. However, the tension-tension fatigue strength conducted in vacuum was higher than that conducted in laboratory air. The fiber surface damage was examined by an AFM,and the degradation and fracture/fatigue mechanisms are discussed.