Optical tunnelling through interfacial gaps in fibre-reinforced composites

Abstract

The formation of interfacial gaps between the fibre and matrix of a fibre-reinforced composite results in a degradation of its mechanical properties. Whether interfacial gaps arise as a consequence of moisture penetration or mechanical damage, they influence both the distribution and phase properties of transmitted light. For small gaps (≲ λ), it is shown how the transmission coefficients for TE and TM mode singly and multiply reflected waves may be calculated by consideration of optical tunnelling, and how the phase characteristics may be related to the interfacial gap width. Qualitative differences in attenuation between TE and TM modes are reported, and phase information is used to estimate quantitatively the gap width. It is also indicated how scattering theory could be used to explain some interference effects which are observed when an interfacial gap develops.