Delaminations in flat and curved composite laminates subjected to compressive load

Delaminations reduce the compressive strength of composite laminates because they allow out of plane displacement of plies to occur more easily. This work is a study of the effect of delaminations in laminates with initial curvature. Initial curvature typically results in an increased buckling load and the hypothesis was that the reduction in compressive strength due to delamination would be less in a curved specimen than a flat one. Glass fibre reinforced plastic test laminates were manufactured containing artificial delaminations of different sizes and through thickness positions. The laminates were either flat or curved, where the plane of curvature was normal to the loading direction. Compressive load was applied to the laminates and measurements of out of plane displacements and failure loads were made. Contrary to expectations, the curved laminates gave lower failure loads than flat laminates, for the same delamination size and through thickness position. Finite element simulation was carried out to predict out of plane displacement and failure load, with failure being assessed using a maximum stress criterion. Good agreement between finite element predictions and experimental measurements of load versus deflection behaviour was observed for both flat and curved laminates. Ultimate failure loads were also well predicted for the flat laminates but relatively poorly for the curved laminates. The finite element predictions depend on modelling accurately the out of plane displacement behaviour, which was found to be very sensitive to the boundary conditions for the case of curved laminates.