Parametric Finite Element Model of a CFRP Beam used in the SAMPE Bridge Contest

Kurzfassung

The Finite-Element-Method (FEM) is a frequently used approach to simulate and dimension structures. Especially structures tailor made from carbon fibre reinforced plastics (CFRP) have a large number of degrees of freedom that have to be determined by the engineer. These parameters are often identified by analytic calculations, expert knowledge, and manual variation. As the complexity of the structures increases, it is no longer possible to investigate all possible parameter combinations manually. Automated parameter variation and optimization algorithms can be used to find the optimal set of parameters for various load cases. In order to use optimization algorithms, automated parametric model analyses are required. This paper demonstrates how to create a parametric FE model of a CFRP beam used in the SAMPE bridge contest by application of the FEM software ANSYS. Parametric models have the advantage that they can be easily integrated in automated processes such as structural optimization. As an example, the beam model and the bending load case of the SAMPE Bridge Contest are used here. The case at hand is a CFRP beam with I-profile that is loaded between two bearings with a stamp. Within the bridge contest the maximum load at failure is the evaluation criterion for structural integrity. When multiple beams sustain the maximum load of 9,000 lbf, the mass of the beam will be used as secondary decision criterion for the identification of the best structural solution. For this reason, a high bending stiffness and low structural mass are the main objectives in this investigation. Hence, the parametric model output is deliberately truncated. The material used for the beam is the CFRP material Hexcel AS4/8552. The material definition is created with values of public sources and for material failure to be considered, a strain-based failure criterion is used. The proprietary scripting language ANSYS Parametric Design Language (APDL) is used to automate the pre-processing, solving and post-processing of the FE model. Under the use of APDL an automated FEM-script is created that calculates the predefined output values in dependency of variable input parameters. In the pre-processing step the FE model with FE mesh, composite properties, loads and boundary conditions is automatically created with user defined parameters. Variable input parameters are dimensions of the structure and material parameters, such as fiber direction and stacking sequence. After performing the calculation, the FE results are analysed by identifying the required values and writing them into an output file. This leads to a user independent process from input parameters to output values that is appropriate for the use in scripts. Furthermore, the FEM settings mesh size and element type are investigated to perform a convergence study. With the help of the automated convergence study suitable FEM settings are identified. The result of this investigation is a parametric FEM model in ANSYS with changeable parameters and a proposed set of output values. This analysis model can be comfortably integrated in automated processes like structural optimization. It can run completely in batch mode without any manual interventions. In addition, the paper illustrates how the parametric FE analysis is integrated into an automated process using a Python script. This script defines a set of input parameters, executes the FEM analysis, reads and evaluates the output parameters. This is the first step into an automated routine for structural optimization. The paper concludes with the evaluation of computational efficiency and time saving potential in the development process. As such it enables the identification of optimal structures in a repeatable and consistent way.