The influence of vacuum on fracture and fatigue behavior in a single aramid fiber

To perform micro-mechanical tests including fatigue in microelements, a specially designed fatigue testing machine has been developed. The testing machine was designed to perform fatigue tests in microelements under simple- or three-point bending as well as under uniaxial loading. The testing machine is equipped with an environmental chamber to control the testing environment, and thereby the testing can be done in controlled environments such as vacuum or (humid) gases. Using the system, quasi-static and fatigue tests on aramid single fibers (Kevlar 49®, diameter: about 12 μm) were performed. Kevlar 49 is the trademark of DuPont. The tests showed that the strength of the aramid fibers is strongly influenced by environment. Water absorption and vacuum conditioning decrease the quasi-static fracture strength. Under fatigue loading, the aramid fiber has excellent properties compared with metallic materials, showing gentle slopes in S–N curves, although large scatter bands exist. The fatigue strength in vacuum is higher than that in air. The fiber breaks with fiber splitting. The fiber surface damage induced by fatigue loading in air and in vacuum was examined by using an atomic force microscope.