In general the behavior of composite laminates can be represented through a set of orthotropic constitutive relations. In Orthotropic Elastic Model a set of such relationships are described which assume the material behavior remains elastic and the volumetric response linear. For more complicated material response a methodology was developed, under contract from ESA (European Space Agency) [1], which allows a nonlinear equation of state to be used in conjunction with an orthotropic stiffness matrix and is described in Equations of State. This is important when modeling applications such as hypervelocity impacts.
Some composite materials, such as Kevlar-epoxy, exhibit significant nonlinear stress-strain relationships (see Figure 3.1: Typical In-Plane Stress-Strain Behavior of Kevlar-Epoxy). In order to model such observed nonlinear behavior an orthotropic hardening model has been implemented. Again developed under contract from ESA [2] it uses an anisotropic plasticity based loading/failure surface and is described in Orthotropic Yield Strength and Hardening Model.
