STRUCTURAL MECHANIC APPROACH FOR PREDICTION OF ELECTRICAL CONDUCTIVITY OF GLASS FIBRE REINFORCED COMPOSITE WITH CNT-MODIFIED EPOXY MATRIX

The electrical conductivity of GFRP cross-ply laminates with CNT-modified epoxy matrix was studied. The effective electrical conductivity of the matrix and composites were measured experimentally. Anisotropy due to orientation of non-conductive fibers was taken into account by introducing the corresponding anisotropic conductivity tensor for each ply. Measurements of electrical conductivity were made for unidirectional singleand multi-ply composites cut on various angles, as well as for orthotropic cross-ply GFRP laminates. The experimental and calculated data are in reasonable agreement. Damage detection in GFRP with electrically conductive matrix was realized measuring the voltage distribution of the samples before and after damage and the analysis of damage correlated relative voltage changes. The results indicate that even a relatively large spaced electrode network is capable of determining location and quantification of damage. INTRODUCTION Incorporation of electrically conductive fillers, e.g. carbon nanotubes (CNT), into polymer matrixes of advanced fibre reinforced plastics allows one to get structural composites with the improved mechanical performance and tailored electrical conductivity. Glass fibre reinforced plastics (GFRP) being originally electrical insulators get additional functionality that opens a prospective for use in a wide range of multifunctional and high-performance applications, e.g. civil engineering, aerospace, and automotive industries [1], [2]. By adding of less than 0.3-0.7 wt.% of CNT into the polymer matrix, these composites possess ability to strain and damage monitoring via control of electrical conductivity [3]-[9]. CNT-modified matrix is usually considered as an isotropic material and variety of mixing and micromechanical models are applied for estimation of its electrical conductivity depending on the content of conductive fillers [10]-[12]. Laminated composites have an anisotropic electric conductivity, and the electric current density distribution is characterised by rather complicated configuration depending on the stacking sequence of the laminated plate. Finite element methods are usually applied for calculation of the electric current distribution in each ply, although often at a high computational cost. Alternative methods are based on electromechanical modelling [13] or analytical calculation of the electric potential function that can be simplified for the case of orthotropic materials [6], [8]. Despite on high interest on using CNT-doped GFRP as multifunctional damage-sensing structural composites, reliable estimation of electrical conductivity in these anisotropic composites is still an issue. The main aim of the study is to check applicability of structural mechanic approach to prediction of electrical conductivity of GFRP with epoxy matrix modified by CNT and to check a possibility for damage detection of the composite using in-plane electrical voltage measurements. It is supposed that epoxy matrix and composite on different structural levels obeys the Ohm’s law and the electric flux density j is proportional to the electrical field E