Well … thank you for those documents. I did not read the Wool and Mohair.
From the (badly written) article on cotton we have:
Under a microscope a cotton fiber appears as a very fine, regular fiber, looking like a twisted ribbon or a collapsed and twisted tube. These twists are called convolutions there are about sixty convolutions per centimeter. The convolutions give cotton an uneven fiber surface, which increases inter-fiber friction and enables fine cotton, yearns of squatted strength to be spun. The appearance of the cotton fiber’s cross sections is referred as being kidney-shaped.
So in the case of wool, the act of stretching a fibre would attempt to straighten-out the kinks. The inside surface of any kink would be put under greater stress because of the straightening, before any additional load is applied. Therefore any wool fibre is more likely to fracture at a kink site when stretched. Wool is not used as constructional material, it is used as a clothing material where the ‘kinks’ help trap air within the cloth produced therefrom and hence give you a superior warmth retention compared to some other fabrics.