Size effect on tensile strength of carbon fibers

Abstract

Tensile strength of carbon fibers exhibits statistical Weibull type distribution and significant size dependence. In the present work, ten types of polyacrylonitrile based and mesophase-pitch based carbon fiber monofilaments were tested for two or three different gauge lengths. Size effect in both axial and radial directions were analyzed based on the two parameters Weibull statistics. It was found that the size effect in axial direction was almost similar for all fibers tested. This result suggests that the tensile strength obtained for a certain gauge length is a meaningful measure as a representative strength of the fiber strength. In radial direction, the size effect of the tensile strength was larger than that in axial direction. The tensile strength of the carbon fibers seemed to have unisotropic statistical characteristics. Size dependence in diameter was numerically simulated with an assumption of unisotropic distribution of Reynolds-Sharp type defects.

Introduction

Carbon fiber is one of the high performance fibers employed in the advanced composites. In the case of the composites reinforced by carbon fibers, continuous fibers are used and fiber volume fraction usually exceeds 40%. Therefore, characteristics of the fiber strength is the most influential factor on the strength of the composites. At the composites strength design, the fiber strengths evaluated from the fibers strands with a certain gauge length are usually used. Tensile strength of carbon fibers, however, shows a large scatter and remarkable size dependence according to the weakest link analogy 1, 2, 3. Although the statistical characteristic of fiber strength will give significant influences on the strength will give significant influences on the strength of composites, the statistical behaviors of fiber strength have never been systematically characterized yet. Supposing that, for instance, the size effect of the strength is different for each fiber, the tensile strength evaluated for a certain gauge length will be lacking in a generality as a measure of fiber strength. In the present work, the tensile tests of mono-filaments for various types of carbon fibers were performed, the statistical size effects of the tensile strength were investigated on the basis of the Weibull statistics.