The plasticity of wool

1. Shorter (1) has shown that the behaviour of wool under tension can be simulated by the Poynting and Thomson (2) model, consisting of two springs connected in series, one spring being free and the other damped by immersion in a viscous medium. The same model has been used more recently by Poole (3) to illustrate the behaviour of gelatin gels. Thus a wool fibre,. when subjected to tension, shows an immediate extension, followed by slow creep, and Shorter considers that “the explanation of this is not, as migbt be supposed, that the elastic; elements are showing plastic yield, but that the fibre contains elastic elements with very different degrees of damping.” Similarly, the elastic relaxation which occurs when a fibre is held stretched to a definite length is regarded, not as the disappearance of strain owing to molecular re-adjustment, but as “the transference of a state of strain from lightly damped to highly damped elements.” (1) When dry, extended wool fibres tend to return only very slowly to their original length, but immersion in water produces rapid and complete recovery. In terms of the model, this observation is explained by assuming the viscosity of the colloid medium to be reduced by water absorption. Its viscosity is also reduced by the mechanical disturbance associated with extension and contraction, but if a period of rest be allowed between two successive extensions, the fibre is found to be completely unaltered by extension. Shorter's observations were carried out at 68 per cent, relative humidity, and it is clear that under these conditions wool is perfectly elastic.