The photoelastic behaviour of optically heterogeneous fibre composites—II. The principal direction of the equivalent birefringence for optically heterogeneous unidirectional fibre composites

Abstract

This paper describes a study of the principal direction of the equivalent birefringence of optically heterogeneous unidirectional fibre reinforced composites. These materials have an equivalent value of birefringence which was discussed in a companion paper [1]. It was further found in the present work that these materials also have an equivalent birefringent direction. This conclusion was derived by examining the behaviour of α ∥4 c , which is the fourth phase angle of the Fourier expansion of the transmitted polarized light intensity vs azimuth curve of the optically heterogeneous composites under the bright field of plane polariscopes ( I ∥ c - θ curves). It was found that the fourth phase angle α ∥4 c , acts in the same way as that of the initial azimuth of the first principal birefringence direction in optically homogeneous materials. Under the zero external stress condition and under the normal-axis loading conditions, the fourth phase angle α ∥4 c , always equals the initial azimuth of the fibre longitudinal direction. On the off-axis loading conditions, the fourth angle α ∥4 c , is stress dependent. Its response to the change in stress level agrees with the theory of photo-orthotropicelasticity. The equivalent birefringence direction can be measured by the azimuth of the first principal direction β 1 c b . In general, the azimuth of the first principal direction β 1 c b is equal to the sum of the fourth angle α ∥4 c with the rotated angle θ in the I ∥ c - θ curve measurement of the optically heterogeneous unidirectional fibre reinforced composites.