Fibre kinking

Figure 1.2 Fibre kinking induced by fibre instability or fibre microbuckling observed in a carbon fibre/epoxy laminate fibre diameter is about 6 pm [14].

Some modes may dominate for example, for large bending strains in a flexible structure, fibre fracture in tension and fibre kinking in compression wdl dominate near both surfaces. Matrix cracks can cause delamination when they reach a ply interface. If the structure is stiff enough to resist with a significant force, then local indentation damage, and shear-driven delamination in the interior, wdl occur. Figure 9.2 shows schematically the different modes of fadure in three zones of a laminate. The peanut shape deformations (3) have this shape because the compression under the impact force suppresses the delaminations. 

Yarn buckling tests were carried out in the FIBRE TETHERS 2000 (1994, 1995) Joint industry project. Failure due to axial compression fatigue was also studied in fibres from fatigued ropes in the study. As discussed below, the constraints on fibres within the yarns, especially if they were re.strained in a shrink-tube in the laboratory te.st or within ropes, causes very sharp fibre kinks to form. Kevlar, Vectran and Dyneema all showed kink-bands within fibres and breaks over short lengths. 

The compressive strength of composites is less than that in tension. Tills is because the fibres buckle or, more precisely, they kink - a sort of co-operative buckling, shown in Fig. 25.5. So wliile brittle ceramics are best in compression, composites are best in tension. 

Kilchherr, E., Hofmann, H., Steigemann, W., and Engel, J. (1985). Structural model of the collagen-like region of Clq comprising the kink region and the fibre-like packing of the six triple helices. / Mol. Biol. 186, 403-415. 

In compression, the mechanism of failure changes from fibre fracture to fibre buckling. Here, the modulus of the matrix is a critical factor in supporting the fibre and by prevention of kinking (of the fibres) in a shear band. Soutis [53] has discussed the modelling of these types of failure using Eqns (12.17) and (12.18) ... 

Moisture content, = 1.42%. Assumed initial fibre misalignnoent, = 1-75 kink band inclination angle, A = 15 the value of environmental test conditions. () estimates from Eqns (12.17) and (12.18) and the measured unidirectional compressive or shear strength properties. 

Fibre-optic cables look like steel wire armour cables (but of course they are lighter) and should be installed in the same way, and given the same level of protection, as SWA cables. Avoid tight-radius bends if possible and kinks at all costs. Cables are terminated in special joint boxes which ensure cable ends are cleanly cut and butted together to ensure the continuity of the light pulses. Fibre-optic cables are Band I circuits when used for data transmission and must therefore be segregated from other mains cables to satisfy the lET Regulations. 

Fi. 10. (a.b) Polye.sler fibres after a single bend, (c-e) Flex fatigue in polyester kink-band fracture, (f) Flex fatigue shear splitting in nylon, (g) Flex fatigue shear splitting in polyester. For further explanation. 

An early view of fracture of para-aramid fibres was given by Yang (1993, p. 97), who refers to three basic forms. The caption to his fig. 3.28 describes fracture morphology of Kevlar aramid fibre in tensile breaks as Type (a), pointed break type (b) fibrillated break type (c) kink-band break. The kink-band breaks, which extend over a length approximately equal to a fibre diameter can be attributed to fibres that have been weakened by axial compression and will be discussed in a later section. 

The agreement with experiment shown in Fig. 14 is fascinating, and with the vast increase in computer power since 1986, it would be valuable to follow up the approach pioneered by Termonia and Smith for models which included the possible defects in the structure. In HMPE fibres, it seems right to attribute creep to the movement of whole molecules past one another, which eventually leads to separation. However, the most likely mechanism would be the movement of defects such as those described by Reneker and Mazur (1983). A kink in a polyethylene chain due to an extra -GHj- group could move like a ripple in a carpet. 

The conditions for the formation of kink-bands within HM-HT fibres are the first part of the problem. The second part is what happens in repeated cycling. The axial compressive force causes the molecular buckling, and superficially the internal kink appears to be pulled out on retensioning. However, it seems likely that there is some residual structural disturbance, which becomes more severe after repeated cycling and leads to what appears to be crazing. Eventually failure occurs in the characteristic angled form of kink-band breaks, being the Achilles heel of HM-HT fibres. 

Carbon fibre reinforced composites, on the other hand, failed in a brittle manner with multiple transverse and 25 shear cracking (Fig. 3c). The failure mode for HP-PE/carbon hybrids was similar to that of each of its components. Shear cracking of the carbon part was here combined with kinking of the HP-PE components (Fig. 3d). 

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