A novel double‐image Fizeau system for accurate investigation of anisotropic polymer fibres

Summary This paper introduces a double‐image multiple‐beam Fizeau fringes system. The introduced system can dynamically determine the variations of the refractive indices for both parallel and perpendicular polarization simultaneously. This is achieved by the simultaneous capturing of two multiple‐beam interference patterns during the mechanical processing of isotactic polypropylene fibre. This parallel determination of the refractive indices of both polarization directions allowed us to determine the full‐field distribution of the stress vector, S. To accomplish this, a mathematical model was deduced to calculate the components of the stress vector, S, i.e. parallel stress component, S1, and perpendicular stress component, S2. Double‐image Fizeau fringes system and the deduced mathematical model were used to investigate the variation of the refractive index and stress components of the fibre during the stretching process and propagation of necked regions.