Amount Of Twist Research Articles

Distortion of Twisted Orientation Patterns in Liquid Crystals by Magnetic Fields

Abstract At rest between parallel plates, cholesteric liquid crystals commonly exhibit a characteristic twisted orientation pattern, the axis of twist being perpendicular to the plates. Also, this orientation pattern appears possible in nematic liquid crystals, since it is consistent with continuum theory(1) for both types of liquid crystal. This paper discusses the influence upon such a twisted orientation pattern of a magnetic field perpendicular to the plates. If one employs a free energy of the form discussed by Frank,(2) the continuum theory equations have solutions relevant to this situation. For nematic liquid crystals, provided that the twist is not too large, our analysis suggests that no distortion of the orientation pattern occurs until the magnetic field strength exceeds a critical value which varies with the amount of twist. For cholesteric liquid crystals, there seems to be two possibilities depending upon the relative magnitudes of two Frank constants. Either distortion always occurs above a critical field strength, or does so only when the distance between the plates is sufficiently small.

10—THE EFFECT OF YARN TWIST ON FILAMENT PROPERTIES

It is shown that the twisting of a viscose rayon yarn followed by a subsequent de-twisting produces a reduction in tenacity and extensibility of the constituent filaments, and that the magnitude of this effect is influenced by the type of twisting machine, twisting tension, amount of twist, and the number of times the operation is repeated, but is unaffected by the time for which the yarn remains in the twisted state. It is further shown that this effect is caused by a form of filament-surface damage, which is produced by inter-filament contact.

80—CONTROL OF DRAFTING BY TWIST

A yarn may be drafted from a large mass of fibres by using twist to supply the nip force to select fibre leading ends. An account is given of an experimental machine using this principle in which withdrawal tension in the yarn was used to control the amount of twist being inserted. The coefficients of variation of the mass per unit length of a series of yarns produced by this machine are compared with a minimum value calculated assuming perfect operation. Some possible reasons for the shortcomings in levelness are discussed. Experiments in which the forces necessary to extract single fibres and groups of fibres from large masses are described and their relevance to tension control of twist is discussed.

Effects of torsion on the tensile stress-strain curve of aluminium

The work hardening of aluminium single crystals was investigated by alternate tensile and torsional deformation. The relative increase of tensile flow stress due to twisting was systematically investigated in relation to prior extension, crystal orientation and amount of twist. Crystallographic analysis of slip lines was carried out. This showed that several modes of torsional deformation occurred in different orientation regions. Some of these modes correspond to proposed dislocation mechanisms. Analysis of the azimuthal distributions of tensile slip lines on crystals which have been twisted and extended showed that dislocation barriers were formed in agreement with the Lomer-Cottrell theory of sessile dislocations. The relative increase of tensile flow stress and the change of hardening rate due to twisting are discussed in terms of dislocation theory.