Nonlinear compressive stiffness in impacted composite laminates determined by an inverse method

This paper presents use of an inverse method and non-contact optical measurements for determining the apparent compressive stiffness reduction in impact damage zones in composite laminates. The tensile stiffness distribution and nonlinearity is also briefly covered. The method is based on iterative updating of the material properties in a finite element model with the objective to match the predicted displacement fields to those measured optically in impacted specimens under load. To examine the effect of the damage on initial imperfections, strain and buckling, the displacement fields obtained experimentally by digital image correlation are demonstrated and discussed. Finally, the method is applied to the obtained full-field measurements and the influence of applied strain on the nonlinear tensile stiffness and apparent compressive stiffness of real impact damage zones is evaluated. Material nonlinearity in tension is found to increase towards the damage centre where fibre damage is more severe. Stiffness in compression can only be represented by a uniform apparent material nonlinearity, which is strongly linked to local buckling.