Small Angle X-Ray Scattering of Carbon and Kevlar Fibers Under Load

Small angle X-ray scattering (SAXS) was used as the primary tool for investigating the microporous structure of Kevlar and mesophase pitch based carbon fibers. The orientation present in the scattering entities was determined by extending Porod's invariant into a pseudo-invariant for one-dimensional data slices. By comparing the pseudo-invariants at different azimuthal angles the extent of the orientation present in the scattering system was determined. The orientation of the microvoids was found to correlate best with the modulus of the carbon fibers. Taking advantage of the experimental and computational facilities currently available, distance distribution functions were also determined for the fibers. For the carbon fibers the average size of the voids and the breadth of the distribution was found to increase with both fiber modulus and strength, although the best correlation was seen with strength. In addition, the distance distribution curves were able to distinguish between fibers spun under different conditions. These results were found to mirror the enhancement of crystalline orientation and the development of graphitic phases in the fibers. Studies were also conducted to determine the response of the microvoids to an applied tensile load. Both Kevlar and the carbon fibers indicated a loss of outer fiber surface, with the most dramatic loss being seen for Kevlar. Experiments were conducted which compared both as received fibers to those which had been strained to failure. Also additional experiments were done with the fibers being strained while in the SAXS camera. For the Kevlar fibers the loss of its skin layer was indicated by less anisotropic scattering patterns of the fractured fibers. The dynamic studies indicated that initially void orientation increases which is accompanied the formation of new voids and a drop in average void size and breadth of the distribution. However these trends are reversed as the fiber tow approaches and reaches fracture. The loss of the skin region was confirmed by both laser backscattering and optical microscopy. The response of the microvoid phase of Kevlar to loading is interpreted in light of Morgan's chain end model. The carbon fibers proved to be more intransigent to loading with most of the changes in structure occurring upon fracture. The loss of fiber surface was primarily seen through a loss of scattering power, and optical microscopy.