Counter-plying of two-ply yarn: inner structure changes and external response

The paper introduces a principle of iso-quantities for investigating the distribution of a number of fibers in a two-ply yarn cross-section in a regular grid. The main aim is to map the two-ply yarn inner structure and changes of this structure to various ply twists. So far, modifications of two-ply yarn in different ply twists have been explained only indirectly from a macroscopic point of view by observation of the process of two-ply yarn retraction. This paper applies cross-sections and the method of iso-quantities to find the causes of these macroscopic changes. The ratio of the twist coefficients of the twisted and single yarns αs/αj was used to indicate the internal changes of the two-ply yarn. Using the iso-quantities method and the x–y difference curves of the fiber distribution in the two perpendicular directions of the cross-section, it was shown that counter-plying causes a change in the fiber distribution in the cross-section, resulting in the occurrence of negative values of the retraction, insignificant changes in the external geometry of the two-ply yarn, and a constant breaking strain behavior. These manifestations appear only up to the values of the ratio αs/αj of 1.2 or less. After this limit, the extreme value of the x–y difference settles down, the retraction begins to take on positive values, and the breaking strain of the two-ply yarn begins to increase as the two-ply yarn twist increases.