Development of an innovative FEM simulation methodology for composite structures based on orthotropic multi-modulus stress auto-adaptive materials.

Composite materials are increasingly used in structural applications because they provide significant weight savings. The weight savings result from the combination of a lightweight, weak, and flexible matrix material with a strong and stiff reinforcing material in the form of fibers. Fiber-reinforced composites are used in a wide variety of applications, from laminated aircraft wings to sports equipment. One of the important properties of composites is that they are often characterized by different moduli of stiffness under tension and under compression. The difference between the moduli varies considerably depending on the material considered, but it is generally greater than 10%, therefore a bimodular approach seems to be a necessary development for a more accurate prediction of the mechanical behavior of this class of materials. In this thesis we propose a Finite Element approach to the bimodulus problem. Two FEM subroutines have been developed for the bimodulus analysis of orthotropic materials. Instead of using a new material model to account for the bimodular nature of composite materials, a multi-step, auto-adaptive material approach was adopted, where at each loading step the stress state is evaluated and stored for each finite element, so that when proceeding to the next loading step, the material properties of the finite elements are changed according to the current stress state. These subroutines were developed using ANSYS Parametric Design Language (APDL) and are for 3D and 2D geometries respectively, they can be used for linear and non-linear static structural analysis by attaching them to any APDL macro, provided that the user is able to properly choose the spacing between load steps according to the problem.