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viscoelastic behavior and wet supercontraction

viscoelastic behavior and wet supercontraction - J exp Biol...

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J. exp. Biol. 118, 379-404 (1985) 379 Printed in Great Britain © The Company of Biologists Limited 1985 VISCOELASTIC BEHAVIOUR AND WET SUPERCONTRACTION OF MAJOR AMPULLATE SILK FIBRES OF CERTAIN ORB-WEB-BUILDING SPIDERS (ARANEAE) BY ROBERT W. WORK Fiber and Polymer Science Program, North Carolina State University, Raleigh, North Carolina 27695-8302, U.S.A. Accepted 4 April 1985 SUMMARY 1. The axial retractive stresses that cause the supercontraction of wetted major ampullate silk fibres and the stresses developed upon re- extension from the supercontracted condition are quantified and illus- trated. 2. The viscoelastic behaviour of major ampullate silk fibres, subjected to the amounts of elongation that would be produced by a spider on its dragline, is described and illustrated. 3. When major ampullate silk fibres are either wet elongated from supercontraction or when dry from initial lengths, viscoelastic stress relaxations are found to be functions of the logarithms of time. Regression curves illustrate these relationships and normalized results are subjected to statistical analyses. 4. Viscoelastic memory of major ampullate silk fibres is illustrated. 5. The characteristics of viscoelastic stress relaxation and viscoelastic memory of major ampullate silk fibres appear not to be associable with taxonomy. 6. Examples are suggested in which supercontraction and stress relaxa- tion act in the formation and placement of major apipullate silk fibres as structural elements of the orb web. INTRODUCTION This study of major ampullate silk fibres (MaAS, singular and plural) of some orb-web-building spiders has three primary objectives: (1) the quantification of stress-strain aspects of the phenomenon of supercontraction (supercontraction, supercontract and variants = SC) of MaAS, (2) the quantification of the stress relaxation behaviour of MaAS in the dry (dry = room-conditioned) and water- wetted states and (3) the application of the observations developed from items 1 and 2 to the placing of these fibres in orb webs. Key words: Spider, silk, ampullate.
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380 R. W. WORK Background When elastic solids, such as some metals and glass, are subjected to small deformations, Hooke's Law applies; stress is directly proportional to strain and independent of rate of strain. Simple liquids (not polymers or their solutions) obey Newton's Law; the stress being directly proportional to the rate of strain. Organic fibres, of which MaAS are typical, obey neither law, but their responses to deformation relate them to both. Hence, they are viscoelastic bodies. Although the application of this descriptive term to textile fibres is of technical origin, the lay public has always known that the force-elongation behaviour and dimensional recovery therefrom of fibrous compositions are time sensitive. Of the three forms of axial deformation of fibres under externally applied forces (these being extension, torsion and compression), the first has been studied widely, the second to a limited degree and the third virtually not at all.
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