Improving energy absorption and dissipation of composites through optimized tailoring

Fibre-reinforced and sandwich composites with laminated faces are the best candidate materials in many engineering fields by the viewpoint of the impact resistance, containment of explosions, protection against projection of fragments, survivability and noise and vibration suppression. Besides, they offer the possibility to be tailored to meet design requirements. A great amount of the incoming energy is absorbed through local failures. The most important energy dissipation mechanisms are the hysteretic damping in the matrix and in the fibers and the frictional damping at the fiber-matrix interface. The dissipation of the incoming energy also partly takes place as a not well understood dissipation at the cracks and delamination sites. As self-evident, the local damage accumulation mechanism on the one hand is helpful from the standpoint of energy absorption, on the other hand it can have detrimental effects. To date sophisticated computational models are available, by which the potential advantages of composites can be fully exploited. A large amount of research work has been oriented to improve the impact resistance, the dissipation of vibrations and to oppose the propagation of delamination. These goals can be obtained with incorporation of viscoelastic layers. Unfortunately this makes quite compliant the laminates and reduce their strength. Studies have been recently published that seeks to comply stiffness and energy dissipation. The existence of fiber orientations that are a good compromise between optimal stiffness and optimal absorption of the incoming energy can be supposed by the results of a number of published studies. In this paper, a variable spatial distribution of plate stiffnesses, as it can be obtained varying the orientation of the reinforcement fibres along the plate and their constituent materials, is defined by an optimization process, so to obtain a wanted specific structural behaviour. The key feature is an optimized strain energy transfer from different deformation modes, such as bending, in-plane and out-of-plane shears. Suited plate stiffness distributions which identically fulfil the thermodynamic and material constraints are found that make stationary the energy contributions and transfer energy between the modes as desired. An application to low velocity impacts and to blast pulse loads is presented. The use of the optimized layers with the same mean properties of the layers they substitute were shown to reduce deflection and the stresses that induce delamination. A new discrete layer element is developed in this study, to accurately account for the local effects. Characteristic feature, it is based on a C° in-plane approximation and a general representation across the thickness which can either represent the kinematics of conventional plate models or the piecewise variation of layerwise models