Development of a material model suited for topology and shape optimization of crash structures made of thermoplastic fiber reinforced composites

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It is possible to design crash structures out of fiber reinforced plastics (FRP), which absorb kinetic energy on an adjustable and constant level of force. Provided that the design is suitable for FRP materials they achieve better results than conventional metallic solutions. This counts especially for FRP materials made out of thermoplastics because they have advantages regarding recycling and mechanical joining compared to the popular thermosets. Nevertheless the increased design complexity of FRP parts is challenging. For this purpose mathematical optimization tasks based on non-linear finite element calculations can be supportive and help to improve the design process. The execution of those optimization tasks need special requirements regarding the material model used. Within the scope of this research project a flexible and optimization compatible material model for thermoplastic unidirectional FRP materials will be developed. In the end this model can be used for automated topology and shape optimization tasks applied on crash structures. Therefore a hybrid optimization procedure is applied, which uses a rule based or heuristic approach to modify selected structure descriptions and controls the other structure descriptions by mathematical optimization algorithms. The geometry representation is realized through mathematical graphs, which is a flexible approach. Consequently an evaluable finite element crash model exits to each design.The development is carried out in three steps. In each of them the material model and heuristics are stated more precisely. Therefore the quality and development process of the material model is benchmarked within each step using structural optimization regarding given boundary conditions. The first step consists of the design of a material model that represents stiffness, strength and damage of a unidirectional thermoplastic FRP. The experimental evaluation of the material constants and functions is carried out by using different coupon-level test methods. By means of already defined heuristics and the generic material model topology optimizations are carried out simultaneously. This approach results in information about the sensitivity of the single parameters. The second step includes an upgrade of the material model in order to calculate progressive damage (post-damage/crushing). Specimens on component and coupon level are manufactured and tested for this purpose. Additionally different heuristics with given objectives are investigated: Generation of robust FRP structures, integration of FRP manufacturing constraints and management of the material model. Together with the results in step 1 und step 2 exemplary part optimizations (multifunctional support part) are performed in step 3. With help of experimental validation the quality and applicability of the developed material model for structural optimization can be verified. After all it can also be used for thermoplastic FRP materials.