The limited knowledge of stiffness reductions is a major problem in reliably
predicting the post-impact strength of composite structures. This work describes
development and application of a non-destructive approach for evaluation of the inplane
stiffness of impact damage in composites.
The approach combines an inverse method linked to a finite element model and
non-contact full-field measurements. The material parameters of impact damage are
determined by iteratively matching the finite element model to displacement fields
measured optically during post-impact loading. A first order, gradient optimization
technique coupled with a modified quadratic algorithm is employed. The method is
validated on a reference finite element model with axisymmetric damage containing
several concentric zones having different properties, and the influence of measurement
noise is examined.
The approach is applied to in-house experiments with impacted carbon/epoxy
laminates to determine their quasi-isotropic mechanical properties in tension and
compression. The resulting stiffness distributions are presented and the corresponding
nonlinear behaviour of the damage is described. To examine the effect of the type of
damage on the mechanical properties a thorough fractographic analysis of the impacted
specimens was undertaken. The tensile stiffness is found to be mainly affected by fibre
fracture, while the compressive stiffness is strongly linked to delamination buckling.
The approach has further been extended for detection and evaluation of multiple
impact damage zones at arbitrary locations as well as for stiffness identification of the
damage in orthotropic laminates. The accuracy of both extensions is presented and
discussed. Finally, possible future applications of the approach are considered.