Stress birefringence in polypropylene fibres

Abstract

Refractive indices and birefringence changes with strain produced by different stresses in undrawn polypropylene fibres have been measured dynamically by interferometry. A micro-strain device connected to a wedge interferometer has been constructed. Using this wedge interferometer, multiple-beam Fizeau fringes in transmission are made to cross the fibre perpendicular to its axis. As these fringes cross the fibre they suffer a shift, the area enclosed under which is considered to represent the optical path difference integrated across the fibre. It has been found that the relation between strain and birefringence is linear up to strains of 86%. For greater strain the rate of change of birefringence with strain is decreased. Poisson's ratio is calculated over three strain ranges. An empirical formula is suggested to represent the variation of cross-sectional area of polypropylene fibres with draw ratio, and the constants of this formula are determined. Micro-interferograms are given for illustration.