The same fundamental approach that has previously been found reasonabl
y accurate in predicting the tensile behaviour of bulky singles yarns
at small extensions of up to 10 % is used here to model yarn torsion.
The theoretical analysis is based on discrete-fibre-modelling principl
es, an energy method, and a 'shortest-path' hypothesis. It is applied
to the twisting behaviour of bulky singles yarns which may have non-un
iform fibre-packing-density across their cross-sections. The twisting
of a yam both under constant yarn length and under constant yarn tensi
on is modelled. The contributions to yarn torque due to fibre tension,
fibre torsion, and fibre-bending can all be calculated, Comparison of
the theoretical models with some experimental data will be presented
in Part II.