CHARACTERIZING THE CROSS-SECTIONAL GEOMETRY OF THIN, NONCYLINDRICAL, TWISTED FIBERS (SPIDER SILK)

Quantitative tensile property evaluation of fibres requires accurate c ross-sectional area measurement at a location close to the point of fa ilure. Laser diffraction was evaluated as a non-destructive technique for characterizing the cross-sectional geometry of translucent, non-cy lindrical, and/or twisted fibres with thicknesses in the range 2-5 mu m. Forcibly silked major ampullate fibres from Nephila clavipes spider s were used as specimens. Scanning electron microscopy was used to cal ibrate the extent to which laser diffraction over- or under-estimates fibre diameter. For the purpose of area measurement, elliptical or ova l cross-sections can be treated as though they were circular. We demon strate mathematically that the area can be obtained to within a few pe r cent of the true value, if (a) the circle is assigned an ''equivalen t diameter'' equal to the average of at least four measurements of app arent diameter, (b) the apparent diameter measurements are taken at eq ual intervals of fibre rotation through a 180 degrees range, and (c) t he axial ratio of the cross-section is less than 1.5. The extent to wh ich a non-cylindrical fibre is twisted can be deduced from the periodi city of bright and dark regions that alternate along the length of the fibre in reflected light. Values of cross-sectional ellipticity and a rea measured from a twisted fibre were smaller than the corresponding values obtained from a twist-free fibre. The apparent diameter of twis t-free major ampullate fibre was found to be highly variable - by as m uch as +/-20% relative to the mean within a 0.6 mm length. Therefore, local measurements of area, rather than a value derived from fibre den ier, should be used in tensile property evaluation of this material.