The present thesis is concerned with the simulation of laminates made from fiber reinforced polymers (FRP), as they are nowadays increasingly used in structural components. These laminates are stacks of layers of a matrix material reinforced by uni-directional fibers. The objective of this work is to improve predictions of the material behavior of such laminates by developing new tools for numerical simulation which can also be employed in structural analysis. To this end, material laws are formulated on the ply level which are focused on reflecting the typical failure mechanisms observed in FRP laminate plies. After a general introduction to modeling approaches of FRP laminates and a summary of failure mechanisms observed experimentally, the main portion of the thesis is concerned with two fields of laminate modeling. In chapter 2 the prediction of laminate failure is treated within the framework of the `first ply failure' (FPF) concept, while the simulation of progressive damage, which leads to a gradual change of material properties, is considered in chapter 3. In the beginning of chapter 2 the state of the art in FPF modeling is reviewed. Subsequently, a method for evaluating combined stress states is adopted and implemented as a `stand alone' tool as well as a post-processing tool combined with a finite element program. As one of the currently most promising failure criteria the Puck FPF criterion, which is based on physical failure mechanisms and Mohr's fracture hypothesis for brittle materials, is briefly introduced. The application of the developed program in structural analysis is demonstrated by some example problems. As a typical example for combined load cases, the influence of production related stresses superimposed on mechanical service loads is studied. The simulation of progressive damage in chapter 3 is based on continuum damage mechanics. Several existing damage models for FRP laminates are discussed and compared in an extensive literature review. Based on the failure mechanisms postulated by Puck a new damage model is developed.
The objective is to derive a thermodynamically consistent relation that is able to describe the change of the complete elasticity tensor as a function of damage capturing the non-isotropic nature of damage in FRPs. In view of its practical application, the model is designed such that only a relatively small number of parameters identifiable from standard test data is required. The damage model is implemented in two versions. First, it is combined with classical lamination theory for studying the damage behavior of laminates. In this version, the model is restricted to radial loading paths if the stress state is dominated by transverse compression. Furthermore, the model is adapted to arbitrary loading paths and implemented as constitutive law in a finite element program to enable analyses of complex structures. The application of both versions is demonstrated by examples, some of which are compared to experimental results from the literature. Based on correlations between simulations and experiments, the validity the fundamental assumptions of the damage model are discussed.