The influence of processing on fiber orientation and creep in short carbon‐fiber reinforced low density polyethylene and polycarbonate

摘要

AbstractThis paper is concerned with the creep behavior of short fiber‐reinforced thermoplastics, especially with regard to the role of fiber orientation. Rectangular samples of low density polyethylene (LDPE) and polycarbonate (PC) containing varying amounts of carbon fibers were prepared by compression and injection molding. The materials were compounded using a technique producing a concentration independent fiber length distribution. The orientation distribution, on the other hand, was found to be strongly influenced by fiber concentration. The creep parameters were measured for both LDPE and PC. The contraction ratio was determined for the PC samples. In the case of LDPE reliable data could not be obtained due to the low modulus of the matrix. The creep properties of the PC and LDPE samples varied significantly with the orientation of the fibers. The creep strain was measured as a function of time for both polymers for different fiber concentrations and orientations. The predictions of the Halpin‐Tsai equation underestimated the experimental strain figures somewhat when the stress direction coincided with that of the fiber orientation. When the stress acted across the fibers the theoretical and experimental results showed satisfactory agreement. A plausible explanation is that incorporation of carbon fibers changes the morphology of the matrix material. We conclude that the contraction ratio is an important measure of the volume and its changes during deformation, especially with regard to its relation to the free volume and similar quantities. We have shown that the necessary data can be obtained, in spite of experimental difficulties even for anisotropic samp