The Fracture of Wool Fibers

Abstract

Stress-strain curves up to the breaking point have been obtained for wook fibers under the following conditions: in water at 0°, 20°, 50°, and 100° C; at 20° C at 0%, 65%, and 100% RH; immersed (from 65% RH) in liquid nitrogen. Measurements were made under each set of conditions at rates of extension of 5%/mm and 500%/mm. Tests were also carried out on medullated fibers and on fibers with notches cut in the side. In water the tensile strength decreased with increasing temperature or with decreas ing rate of extension; the extensibility varied inversely with the tensile strength. At 20° C the effects of increasing the humidity were generally equivalent to increasing the temperature or decreasing the rate of exteasion. In these tests the tensile strength ranged from 3 × 10⁸ to 2 X 10 ⁹ dynes/cm², and the extensibility from 36% to 84%. The slopes of each of the three regions of the force-extension curve (Hookean, yield, and post-yield) all varied with temperature, humidity, and rate of extension, although the variation in slope of the post-yield region with humidity was small. The range of moduli observed was from 1.1 × 10¹⁰ to 3.9 × 10¹⁰ dynes cm² in the Hookean region ; from 0.19 × 10⁸ to 17.5 × 10⁸ dynes/cm² in the yield region ; and from 1.2 × 10⁹ to 4.8 × 10⁹ dynes/cm² in the post-yield region.

The fibers tested in liquid nitrogen behaved like glassy polymers with an extensibility of about 2.5%, a tensile strength of 2.3 × 10⁹ dynes cm², and an initial Young's modulus approaching 1011 dynes/cm².

Medullated fibers and tibers notched to a depth of as much as one-third of the fiber diameter showed surprisingly little reduction in strength. This is taken as an mdica tion of the presence of a few serious flaws in "normal" fibers.