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.

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In this three-part series the compressive behavior of (i) glassy polymers, (ii) high performance polymeric and carbon fibers, and (iii) polymeric matrix composites has been addressed. The glassy polymers exhibit plastic yielding in compression. The dependence of compressive yield strength on factors such as tensile modulus, glass transition temperature, density, and free volume has been examined. Failure theories for yielding in glassy polymers have been reviewed. Compression behavior of high performance fibers and that of the composites is discussed in Parts II and III, respectively.

The effect of volume fraction and tensile strength of fibres, temperature and stress concentrators on the compression strength and fracture mode of unidirectional CFRP was studied. The cause of kinking is different for composites reinforced by low-(< 3 G Pa) and high-strength fibres. If fibre strength is high, the kink is initiated by composite splitting followed by fibre bend fracture in the tip of the split. In the case of low-strength fibres, kinking is initiated by compressive fracture of the fibres. The effect of stress concentrators on the CFRP compressive strength is described by linear fracture mechanics. In the presence of defects, fracture is a result of the emergence of splits near a hole. As the critical stress of splitting growth initiation reduces in proportion to the square root of the defect size, the Griffith criterion describes the composite compressive fracture. At elevated temperature, failure is caused by fibre buckling. The fracture band in this case is oriented perpendicular to the fibre direction. Carbon fibre compressive strength may be measured by the loop method. Bending a strand of carbon fibres glued to the elastic beam gives a fibre-controlled upper limit of the composite compressive strength.

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Transversal compression failure is a result of shear in a plane oriented at some angle to the load axis. Shear crack appears initially at an angle of 35 ~ to 45 ~ to the loading axis. In glass and carbon fibre-reinforced plastics the crack later turns along the load direction. As the failure mechanism at transversal compression and shear is the same, transversal compression may be used to measure composite shear strength. In aramid fibre-reinforced plastics shear yield lines appear exactly at an angle of 45 ~ to the loading direction. Each shear line consists of a number of perpendicular micro lines of ~ 10 gm length. The directions of the yield lines and shear macro cracks do not coincide. Transversal compression strength practically does not depend on fibre content, Vf. Normalized shear and transversal compression strengths plotted against square root of pore content are described by a single straight line.

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