Compressive behavior of materials: Part II. High performance fibers
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Compressive behavior of materials: Part II. High performance fibers Victor V. Kozey, Hao Jiang, Vinay R. Mehta, and Satish Kumar School of Textile & Fiber Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0295 (Received 2 May 1994; accepted 28 November 1994)
The primary focus of this paper is on the axial compression behavior of high performance polymeric and carbon fibers. Seven test methods used for determining the compressive strength of single fibers have been reviewed. Various micromechanical models proposed in the literature to understand the compressive failure in single filaments and in other anisotropic systems have been discussed and analyzed. The results of various approaches to influence the compressive strength of polymeric fibers have been summarized. Possible reasons for the variation in the compressive strength of pitch and PAN-based carbon fibers have also been addressed.
I. INTRODUCTION Early development of high performance fibers focused on improving their tensile strength and modulus. However, the use of polymeric and high-modulus carbon fibers in composites brought attention to their poor axial compressive strength.1"3 Mechanical properties of various high performance fibers are listed in Table I. Despite the continuing interest and significant research efforts over the last decade, compressive behavior of highperformance fibers still remains a much misunderstood subject.2"4 In this paper various issues related to the compressive strength of high performance polymeric and carbon fibers have been reviewed. High performance polymeric fibers include extended chain fibers from flexible polymers (e.g., Spectra™ fiber from ultrahigh molecular weight polyethylene),5 thermotropic liquid-crystalline copolyester fibers (Vectran™),6 fibers from semiflexible lyotropic polymers such as poly(p-phenylene terephthalamide) (PPTA) (e.g., Kevlar™ and Terlon™),4'7 and fibers from rigid-rod lyotropic polymers (e.g., PABI, PBZT, and PBO).3'4'8'9 There are three main routes to process these fibers: (i) a gel-spinning technology for polyethylene fibers,5 (ii) a melt-spinning technology for thermotropic liquid crystalline copolyester fibers,6 and (iii) a dry-jet wet-spinning process for lyotropic liquid crystalline polymer solutions.3 In the category of polymeric fibers, most of the discussion on compressive strength is focused on semiflexible and rigid-rod polymer systems. Chemical structures of these fibers are shown in Fig. 1. Carbon fibers are commonly produced either from polyacrylonitrile (PAN) or from mesophase pitch.10'11 Table I lists compressive strength of various high1044 http://journals.cambridge.org
J. Mater. Res., Vol. 10, No. 4, Apr 1995
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performance fibers. Inorganic fibers such as alumina, boron, SiC, and glass fibers exhibit high compressive strength as compared to highly anisotropic polymeric as well as high-modulus PAN- and pitch-based carbon fibers.2 Therefore, compressive behavior of the highperformance polymeric and carbon fibers is of primary inter
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