POISSON'S RATIOS IN GLASS FIBRE REINFORCED PLASTICS.

The characterization of the mechanical properties of an orthotropic composite material generally requires nine interdependent elastic constants: three Young's moduli, three Poisson's ratios and three shear moduli. The Poisson's ratios, V//i//j, are obtained by dividing the strain response in the j-direction by the imposed strain along the i-direction, where i is at right angles to j and in the same plane. It is implicit in the symmetry of the stress-strain matrices that E//iV//j//i equals E//jV//i//j. This paper reports the results of experiments to measure Poisson's ratio at 45 degree and at ten-degree intervals within the plane for both a unidirectional fibreglass material and a 0 degree /90 degree woven roving laminate. The use of Huber's equation to determine the shear modulus required by classical laminate theory allowed the prediction of the Poisson's ratios, at each angle, from the elastic constants measured from tensile tests of in-plane specimens.