X‐ray modulus and strain distribution in single fibers of polyethylene

Abstract

AbstractSynchrotron x‐ray radiation was used to study structural changes during deformation of a single high‐modulus, high‐strength fiber of polyethylene (Spectra 1000, Allied Corp.) 20–40 μm in diameter. The high brightness of the synchrotron beam allowed x‐ray diffraction patterns to be obtained from 0.5 μg fiber samples in ca. 1 min. The (002) chain axis reflection shifted and broadened when the fiber was loaded in tension. When a model of crystals and disordered material in series was used for the fiber, the measured lattice strain at stresses up to 1 GPa gave a crystal modulus of 250 GPa. At stresses above 1 GPa the (002) reflection no longer shifted with increasing load. A noncrystalline part of the material deforms and takes up the load; this effect has been observed by Raman spectroscopy. The (002) peak also broadens under stress. When fiber bundles were used as samples, broadening could be due to uneven loading of fibers, but with a single fiber sample, one can be sure that the loading at the fiber level is uniform. Broadening of the (002) reflection then indicates inhomogeneities within the fiber. Deconvolution with the line profile of the fiber at zero stress should give the distribution of crystal strains. The strain distributions are symmetric and during loading the full width at half maximum (fwhm) is approximately equal to the mean strain. With an assumed constant crystal modulus this describes the stress distribution as extending from zero to twice the mean stress.