ANALYSIS OF BENDING STIFFNESS REDUCTION IN LAMINATES DUE TO TRANSVERSE CRACKS AND DELAMINATIONS IN SURFACE LAYERS

In the present work an experimental study of bending stiffness reduction was conducted on carbon/epoxy cross-ply laminates with surface 90 degrees layers with different thicknesses. The initially undamaged laminate samples were statically loaded in 4-point bending by running loading-unloading steps with increasing maximum displacement levels, eventually leading to multiple transverse cracking in the surface 90 degrees layers as well as formation of delaminations at the interface between the surface 90 degrees layer and neighbouring 0 degrees layer. Density of transverse cracks was quantified and presence of delaminations was qualitatively inspected after each loading-unloading step using optical microscopy. Digital image correlation (DIC) system was used to measure the complete displacement distribution of the laminate in the middle region between the load application points ensuring accurate determination of the laminate mid-plane curvature and the bending stiffness. The experimental results of bending stiffness reduction and the mid-plane curvature were compared with 3-D FEM calculation results, where a 4-point bending test of a laminate with parametrically variable transverse crack density and delamination length was simulated. The experimental and numerical results were also compared with analytical model based on Classical Laminate Theory (CLT), where the damaged layer in the laminate is homogenized and replaced by an undamaged layer with effective (reduced) stiffness properties. In the analytical model the effective properties of a damaged layer at any given transverse crack density and delamination length are back-calculated using the in-plane stiffness properties of a representative volume element (RVE) with and without damage. The calculated effective properties of the damaged layer are then used in CLT to predict the reduction of laminate bending stiffness. The predictions of bending stiffness reduction obtained with the analytical CLT based model yield good agreement with experimental and 3-D FEM results for the tested cross-ply laminate configurations.