Off-axis stiffness characterisation of fibre reinforced plastics.
A new theoretical characterisation is developed for the off-axis stiffness of FRP materials. The theoretical model treats an off-axis unidirectional ply as an inhomogeneous material, and considers the effect of rigid body rotations of the fibres within the matrix material. Linear analytical, and nonlinear finite element solutions are developed for the model. The differences between the new model and the traditional homogenous orthotropic characterisation are functions of both the strain level, and the relative modulus ratio (Ef/Em) of the constituent materials. For relative constituent moduli typical of most common FRP materials, there are significant differences between the new Rigid Body Motion (RBM) model and homogenous orthotropic characterisations at strains greater than 1%. In a 30° case with Ef/Em = 100 and a strain level of 2%, the RBM theory predicts a longitudinal modulus 11% higher than the linear orthotropic theory. At small strain levels the RBM theory reduces to the homogenous orthotropic approximation.
A simple and reliable methodology is developed and verified for the experimental characterisation of off-axis tensile FRP specimens. The method applies a tensile load to a thin walled tubular specimen through a high strength, small diameter length of steel wire. The low torsional stiffness of the wire allows one end of the tube to rotate, thus preventing any torsional constraint. Analytical and experimental verifications both indicate that the required tensile load can be applied to tubular specimens without significant torsional constraint. The wire based testing method is used to measure the off-axis stiffness properties of carbon/epoxy tubular specimens at a range of fibre orientations.