26—STRUCTURAL AND NON-STRUCTURAL EFFECTS IN THE OBSERVED STRESS-STRAIN CURVE FOR WET WOOL FIBRES

Abstract

This paper describes a detailed theoretical and experimental investigation into the relationship between wool-fibre non-uniformities and stress-strain behaviour. The shape of the stress-strain curve in the yield region is closely related to the fibre cumulative cross-sectional-area distribution. The fibre-material yield slope is found to be nearly zero, and the changes from Hookean to yield region and from yield to post-yield region for the material are quite sharp. The observed relative yield slope (relative to the stress at 15% extension) and the coefficient of variation of area interact in the manner predicted except that there are quantitative differences between the experimentally and theoretically derived relationships; these deviations are interpreted as structural effects due to fluctuations in the stress at 15% extension along the fibres, a variation that is relatively independent of the changes in cross-sectional area along the fibres. The mean effective coefficient of variation of area due to this structural variation is found to be of the same magnitude as the mean coefficient of variation of area for the wool-fibre types examined.

Previous results relating crimp to the cross-sectional area and the Hookean slope are confirmed; within the fibre groups, thicker fibres have a higher crimp level and a lower relative Hookean slope (relative to the stress at 15% extension), whereas between groups the thicker fibres have a lower crimp level and a higher relative Hookean slope than thinner fibres. The stress at 15% extension is significantly different for the different wool-fibre types, which indicates real structural differences between the fibre types. As the fibre mean cross-sectional area increases, the fibres show a tendency to be structurally ‘stronger’, the magnitude of this effect agreeing with values obtained by other investigators. Thinner fibres, as well as being structurally ‘weaker’, tend to be more variable in cross-sectional area and less elliptical in cross-section.

KEY WORDS:

• STRUCTURAL PROPERTIES
• STRESS-STRAIN PROPERTIES
• WOOL
• FIBRES
• WET MODULUS
• YIELD
• ELASTIC STRAIN
• MATHEMATICAL ANALYSIS
• COEFFICIENT OF VARIATION
• IRREGULARITY
• CROSS-SECTIONAL AREA
• FIBRE CRIMP
• MERINO WOOL
• LINCOLN WOOL
• CORRIEDALE WOOL