Kink-Band Failure

Kink-band breaks are easily recogni.sed by their sharp angular form. Fig. lOe. 

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Resistance to axial compressive deformation is another interesting property of the silk fibers. Based on microscopic evaluations of knotted single fibers, no evidence of kink-band failure on the compressive side of a knot curve has been observed (33,35). Synthetic high performance fibers fail by this mode even at relatively low strain levels. This is a principal limitation of synthetic fibers in some stmctural appHcations. 

Figure 7.38 Kink-band failure above lower clamp, specimen SC220-1 [22]. (With permission from Elsevier.)...

Kink-band failure occurred at 220°C. The mechanism that initiates kink-band failure was described by Argon [25] and Budiansky [26]. Similar to delamination. 

Bai, Y. and Keller, T. (2011) Delamination and kink-band failure of pultmded GFRP laminates under elevated temperatures and compression. Compos. Struct., 93, 843-849. 

Final in-plane failure under tension is caused by fiber breakage. Under compression, the dominating mechanism is kink-band formation and fiber microbuckling. 

Figure 15.2 Compression damage of fiber composites through microhuckling. (a) Undulations of buckled fibers (b) kink band local failure schematic and (c) micrograph of kink hand formation in a T800/924C carbon-fiber composite [6].

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 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. 

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