The property and structure evolution with external stimuli in high performance fibers

This dissertation presents an investigation on the long-term behavior and the structure-property relation in high performance polymeric fibers. These fibers have wide applications in structural cable and ropes and reinforced composites. Therefore, the understanding of their structure-property relation and their behavior under external mechanical stress is very important.; The fatigue and creep behavior were studied. A general lifetime equation was derived to correlate the material lifetime with the applied stress level. The failure criterion of “critical strain” was identified. The failure mechanism was shown to be a deformation route. The fatigue lifetime was predicted from known material constants. Several material parameters were defined to characterize the fatigue and creep properties. The fatigue property is characterized by the fatigue strength and fatigue strength index; while the creep property is characterized by the creep strength and creep strength index. The fatigue strength index was used to show that fiber with a rigid chain structure, such as Kevlar®, Techanora ® and Vectran®, generally have better fatigue resistance than a flexible chain fiber, such as Spectra®.; Twisting was found to influence yarn properties. A model based on composite theory was developed to predict the change in the yarn modulus with the degree of twist.; The desire for nondestructive prediction of the residual lifetime of a used material initiated the efforts to find a signature of the stress and thermal history in Kevlar® fibers. To achieve this, a new testing methodology has been developed. An iso-strain force-temperature test (IFTT) was shown to yield quantitative information on the stress and thermal history as well as the information of relaxation and transitions in many materials. The IFTT behavior of a Kevlar® fiber maps well with its structure examined by X-ray diffraction. The IFTT was also used to screen the processing for property enhancement via post-treatment.; Different post-treatment routes have been investigated for Kevlar ® fiber. Tension, heat, hydrostatic pressure, and different media of water, mercury and silicon oil have been utilized to alter the materials. The evolution of its property and structure have been carefully examined. It is shown that the modulus of PPTA is determined by two structure parameters: misorientation and a paracrystalline parameter.