Simulation of Fiber Composites – An Assessment

Kurzfassung

Due to the complexity of fiber composites the number of models and tools for simulating the structural behavior is horrendous. It is by far not possible to value them all; rather, it is the intention to identify areas where, to the author’s opinion, there are still deficits or where there is a surplus of models but a lack of knowledge regarding suitable application. Micromechanics can be used to determine stresses between fibers and the matrix and to homogenize the constituents. All models suffer from the difficulties to measure fiber prop-erties. A representative volume element analyzed with finite elements allows calculating the internal state of stress. Homogenization models in general are well advanced, but de-termination of transverse and shear strength needs further attention. Slender structures from layered composites are reasonably analyzed by means of laminate theories. In case of smooth loading the classical lamination theory (CLT) or the first order shear de-formation theory (FSDT) with improved transverse shear stiffness can be regarded appro-priate. Transverse stresses should be determined by locally applying equilibrium condi-tions. So far, only limited knowledge is available with respect to the accuracy of layer-wise models. Engineering judgment is still needed when deciding upon the model for stress analysis of thicker layered structures. Depending on the conditions at hand it may rather be suitable to use 3D finite elements right away. For design and optimization the situa-tion is different because design is usually an ill-posed problem rather than a problem of optimization. In spite of advanced computer programs like ESAComp there seems to be a lack of knowledge for developing composites in the early design phase. On the other hand, there are obviously sufficient optimisation strategies and relevant computational tools available. Choosing the best strategy, however, needs experience and knowledge regarding the type of problem. Besides, optimization should be applied with caution since the optimal state can be a high-risk state and one should rather strive for a robust solu-tion. Simulation methods for buckling and vibration problems of composite laminates are generally well-developed. But further scientific investigation and the development of ade-quate models are needed regarding the behaviour in case of multiple delaminations and especially with respect to the propagation of short waves. In general one gets the impres-sion that there are more than enough failure criteria; urgently needed is a validation against reliable test data. In this respect the World-Wide Failure Exercise is of great value. Further, there is still an ongoing development in degradation models for progressive fail-ure of composites. For calculating the delamination progression the Virtual Crack Closure Technique has prevailed, but mixed mode failure still seems to be a matter of research. The effect of impact or crash loads on composite structures can be analyzed by means of explicit finite element codes like PAM CRASH or LS-DYNA. Simulation models which ac-curately describe the damage progression during fatigue loads are rare and not fully satis-fying; some models even lead to non-conservative results. Especially for very high cycle fatigue and the effect of temperature increase in epoxy resin reliable models do not yet exist. Draping is one task for simulating the manufacturing process. FEM-simulation seems to be the most promising technique in this field. In case of the resin transfer mould-ing technique several models have been proposed for preforming, preheating of the mould, filling and curing. Warping and the spring-in effect also have been successfully simulated. It seems desirable, however, to connect them to simulate the complete manu-facturing chain. It can be concluded, that in spite of intensive research there are still several gaps left. Special attention is needed for the model validation. To that end it is not sufficient to prove an adequate prediction in two or three test cases. Rather, extensive test series with rele-vant structural components should be run.