A finite element investigation of the mechanical effect of ply drops in blended laminates

Abstract

Weight reduction in vehicles offers many advantages. Especially in aircraft, one of the advantages of weight reduction less need for power which is translated into less fuel consumption and lower emissions. Composite materials are now a popular construction material in new passenger aircraft, making them lighter by taking advantage of their high specific properties. However, composite materials can offer even greater specific properties and as a result lighter structures, by utilizing composite laminate blending. Laminate blending is the local optimization of laminates while taking into account the continuity, structural integrity and manufacturability guidelines. The main project started by developing an algorithm that blends laminates with a large number of sections and then a blended laminate with 25 sections was manufactured using the algorithm mentioned above. The manufactured laminate showed improved stiffened driven buckling performance and out of plane displacement imperfections due to thermal stresses in the autoclave caused by the stiffness mismatch in the ply drop areas of the laminate.

This thesis project has to do with the investigation of the mechanical behavior in blended laminates under buckling loading, by building FEM models that accurately simulate the stresses and out of plane displacement as well as the thermal effects taking place during the cooling down in the autoclave. A global-local modelling technique is used to model the stresses, where the local ply drop section FEM models are simulated by 3D elements and the global laminate model by 2D elements. As far as the out of plane displacement is concerned, an algorithm that calculates the stiffnesses of the ply drop sections in order to transfer them back into the global model is built, that works for any blended laminate. Finally, a 2D thermal FEM model is built that simulates the imperfections of a blended laminate due the thermal stresses in the autoclave caused by the stiffness mismatch in the ply drop areas of the laminate. All in all, the results show that the stress and out of plane displacement results of the FEM models built in this thesis project can in many cases be substantially different from simple FEM models that do not take into account the ply drop sections. Furthermore, the thermal FEM model showed that imperfections due to thermal stresses can be accurately simulated.