Deformation band studies of axially compressed poly(p‐phenylene terephthalamide) fiber

Abstract

SynopsisDeformation mechanism of poly(p-phenylene terephthalamide) (PPTA) fiber during axial compression was studied. PPTA fibers were embedded in resin and axialy compressed in a tensile machine. PPTA fibers were then taken out from resin at various stages of compression. Kink band formation was examined by means of polarizing microscopy, X-ray diffraction (WAXD), electron diffraction (ED), and electron microscopy. WAXD pattern of seriously compressed PPTA fiber shows that (200) reflection spots and arcs appear on the equator and meridian respectively. On the other hand, (110) reflection spots are confined to the equator. PPTA fiber could be splitted tangentially and radially into two kinds of thin fibrillar fragments (I and 11) which reveal two distinct types of kink band and ED pattern. In fragment I obtained by tangential splitting, kink bands are formed at about 55O with respect to fibril axis, whereas in fragment I1 obtained by radial splitting kink bands are formed perpendicular to the fibril axis. These results were confirmed by ED studies. It was assumed that slip of (200) planes containing hydrogen bonded sheets as well as intermicrofibril slip plays an important role in the deformation of PPTA fiber during axial compression.

Liquid crystalline organic fibres and their mechanical behaviour

“…In the case of bending, yielding occurs at a relatively low strain of about 0.75% (Greenwood and Rose, 1974). Axial compression and severe bending may induce plastic deformation: Dobb et al (1981) and Takahashi et al (1983) showed the formation of kink bands at 55-60° to the fibre axis when the compressive strain reached about 0.5% (Greenwood and Rose, 1974), consistent with compressive yield stress. Figure 12.20 presents two micrographs, obtained with cross-polarized optical and scanning electron microscopes, of localized bands in the compressed region of a bent fibre.…”

A model for the compressive buckling of extended chain polymers

“…An example of kink banding in axially compressed Kevlar* fibers is shown in Figure 1. These compressive kink bands have been observed for well-oriented polymers based on both rigid rod [1][2][3][4][5][6] and flexible [7][8][9][10][11][12][13][14][15] chains. Most of these polymers exhibit nearly linear elastic behavior in compression until the onset of kink banding.…”

“…Analyses of the compressive strengths of oriented polymers have been focused on continuum mechanics treatments of anisotropic yield behavior [11], the analysis of deformation along preferential slip planes [4,7,9,11,13,15], or dislocation models for kink formation [34]. The bulk of this work indicates that failure in compression results from shear deformation or slippage between polymer chains.…”

Uniaxial Compressive Strength of High Modulus Fibers for Composites

“…It can be argued that the tensile strength in the organic fibers is controlled by flaws and covalent bonding, while in compression fibers having a microfibrilar morphology are susceptible to fibrilation and buckling due to weak lateral van der Waals forces. Compressive failure can also occur due to weak hydrogen bonding as has been observed [4] in Kevlar. Attempts to improve compressive strength should concentrate on utilizing the stronger covalent bonding and by minimizing the weaker van der Waals forces and hydrogen bonding: entangling and crosslinking of microfibrils is a step in that direction.…”

“…The factors which influence the compressive strength of fibers are discussed in this paper. directed [1][2][3][4][5][6][7][8][9] towards (i) understanding the compression behavior of these fibers, and (ii) influencing the structure and morphology of these fibers by varying processing and post-processing conditions in order to influence the compressive strength. Alterations in chemical structures [10] are also being pursued in order to achieve enhanced compressive strength.…”