Page 303 - Fiber Fracture

FRACTURE OF HIGHLY ORIENTED, CHAIN-EXTENDED POLYMER FIBRES 285

Fig. 19. Axial compression failures, from Hearle et ai. (1998, chapter 39). (a) Broken and unbroken yam
segments in a Kevlar rope. (b) Buckling and break of a fibre in a Twaron rope.

will have high bending stiffness and the free sliding case low stiffness. In order to adapt
the analysis to the prediction of kinking within a fibre, it would be necessary to have
data on the bending properties of molecules, on the resistance to lateral displacement as
the kink forms, and the resistance to axial sliding of the molecules. This is an interesting
problem for a polymer theoretical physicist.
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.

REFERENCES
Allen, S.R. ( 1987) Tensile recoil measurements of compressive strength for polymeric high-performance
fibres. J. Mate,: Sci., 22: 853-859.
Black, W.B. and Preston, F. (Eds.) (1973) High-Modulus Aromatic Fibres. Marcel Dekker, New York.
Dobb, M.G., Johnson, D.J. and Saville, B.P. (1977) Supramolecular structure of high-modulus polyaromatic
fibre (Kevlar 49). J. Polym. Sci. Phys., 15: 2201-2211.
FIBRE TETHERS 2000 (1994) Axial Compression Studies for United States Navy. Joint Industry Study,
Phase IIA, Noble Denton, London.
FIBRE TETHERS 2000 (1995) High Technology Fibresfor Deepwater Tethers. Joint Industry Study, Noble
Denton, London.
Hearle, J.W.S. (1982). Polymers and Their Properties. Ellis Horwood, Chichester.