21--THE MECHANICAL PROPERTIES OF WOOL FIBRES AND THE INFLUENCE OF FIBRE STRUCTURAL AND DIMENSIONAL VARIATION PART I: THE STRESS-STRAIN BEHAVIOUR AT VARIOUS HUMIDITIES.

The stress-strain behaviour (at constant rate of extension) for fibres at 0, 65, and 100% r.h. is compared. At 100% r.h., the material yield slope is close to zero, whereas a rapid stress relaxation at 65 % r.h. leads to a negative slope. This latter effect tends to be masked by fibre non-uniformity. The fibre-material yield slope at 0% r.h. is quite large, and the slow turnover from Hookean to yield is probably an interaction between this slope and fibre non-uniformity. Constant-rate-of-loading tests by others do not indicate a negative slope at 65% r.h., and the material yield slope at 0% r.h. appears to be significantly higher for this mode of testing. The variation in the stress at 15% extension in water from fibre to fibre appears to be mainly due to microfibril-proportion differences existing between fibres. A similar effect for the Hookean slope in water tends to be complicated by additional crimp effects. The post-yield-region variation decreases for lower humidities, and an explanation in terms of the series-zone model is suggested. Fibre non-uniformity greatly affects the observed breaking properties, and the actual material breaking properties are derived from the results. [ABSTRACT FROM AUTHOR]