The photoelastic behaviour of optically heterogeneous fibre composites—I. The equivalence of birefringence in optically heterogeneous unidirectional fibre composites

Abstract

This paper describes a study of the stress induced polarizing optical effèct in optically heterogeneous unidirectional fibre reinforced composites. The fibre composites have a fibre volume content of about 60%, with mismatched refractive indices, and possess very poor transparency. The polarizing optical behaviour of these composites was studied in terms of the transmitted polarized light intensity vs azimuthal angle curve under the bright field of plane polariscopes ( I ∥- θ curve). This was done on the Laser Microscopic Analyser which was constructed in house. The stress induced optical effect was analysed by dividing the I ∥- θ curve into components of its Fourier expansion. It was found that only the fourth component ( j = 4) has a clearly sinusoidal response to the change in the principal stress difference over the entire experimental range. This response to stress is similar to that of homogeneous materials. The only difference between heterogeneous and homogeneous conditions is that the contribution of this stress optical effect takes only part of the amplitude of the fourth component. Normally for homogeneous conditions, the fourth amplitude is theoretically caused entirely by the birefringence effect. It is also found that the stress optical response for the zero'th component ( j = 0) is sometimes but not always noticeable. It is therefore recommended that for optically heterogeneous unidirectional fibre reinforced composites the value of the equivalence of birefringenece ( ΔN c ) should be determined by the fourth amplitude. The values thus obtained present a fine linear relationship with the principal stress difference. Finally, the equivalence of birefringence is considered to have a statistical meaning.