Experimental identification of a nonlinear model for composites using the grid technique coupled to the virtual fields method

The present paper shows some experimental results of the identification of the full set of parameters driving a nonlinear model for the in-plane behaviour of a unidirectional composite laminate. The strain field at the surface of a rectangular coupon submitted to a shear/bending loading is measured using the grid technique. The strain fields are then processed by the virtual fields method to retrieve the six constitutive parameters: the linear elastic orthotropic in-plane stiffnesses Qxx, Qyy, Qxy, Qss and softening parameters K   and εs0 driving the shear nonlinearity. It is shown that the shear response is correctly identified, with coefficients of variation similar to the ones of standard tests. The other parameters are identified with larger errors and higher coefficients of variation because their identification is affected by the lower normal strain levels in the specimen. It is therefore necessary to design new tests providing balanced strain levels for improving the identification procedure.