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TRANSITION - Tool-Part-Interaction simulation process linked to laminate quality

Periodic Reporting for period 3 - TRANSITION (TRANSITION - Tool-Part-Interaction simulation process linked to laminate quality)

Reporting period: 2019-07-01 to 2019-12-31

Given that components made of carbon fiber reinforced composites (CFRP) become increasingly complex, the traditional trial-and-error approach of a tool design capable of producing complex highly integrated parts leads to high development times and costs. Additionally, manufacturing variability leads to variations in the final part quality as to geometric deviations and porosity, which bears the risk of producing unserviceable parts. The overall objective of the TRANSITION project is the reduction of the “Time to Market” to enable a competitive production of complex parts regarding time and costs in the aviation industry. These goals enable an increased share of CFRP parts in the aviation industry and consequently lead to further weight savings and, thus, decreased fuel consumption and CO2 emissions.
The goal for this project is the prediction of laminate quality after cure. The project TRANSITION focuses on using and enhancing process simulation tools to predict the final porosity content of the laminate after consolidation and cure under consideration of interacting process phenomena and tool part interaction.
The output of this project will be used to explore new structural wing architectures which is a key contributor to the aircraft efficiency.
The activities performed within the project “TRANSITION” can be divided into “experimental” and “simulation” studies.

Experiments on “prepreg“ samples have been performed to gain understanding of the material behavior during the manufacturing process in an autoclave. The compaction (i.e. change of the thickness) represents one of the most relevant deformation mechanisms of the material during the manufacturing process of prepregs. This occurs at high temperature and under external pressure to improve the quality of the final part. Therefore, the compaction behavior has been exhaustively investigated. Figure 1 illustrates the experimental setup used to measure the relation between the applied compaction force and the resulting thickness at various temperatures. Moreover, 3D scans using micro-CT technology were performed to study the quality of the laminate for various processing conditions (see Figure 2). The high resolution of such scans combined with the three-dimensional information enable accurate measurements of the amount of air bubbles (i.e. porosity) entrapped in the laminate. Such voids are considered as “defects” since it reduces the performance of the final part.

Based on the outcomes gained from the experiments, a material model has been developed to describe the behavior of the prepreg during the manufacturing process. It enables to model the deformation behavior of the laminate at various temperatures, compaction pressures and deformation rates. An extended validation of the simulation approach has been performed. To that end, the experiments were modeled and compared with the experimental results. Figure 3 shows an example of these simulation models. In addition, a numerical approach has been implemented to calculate the growth of air bubbles in the laminate depending on the temperature and pressure during manufacturing. Thus, prediction regarding the part quality by means of final shape and porosity is made possible.

Future activities will focus on the application of the simulation approach on an academic demonstrator. Comparison of the numerical results with experimental trials will help to validate the applicability of the developed simulation approach for complex geometries.
The main contributors to the progress beyond the state of the art can be summarized as follows:
• Experimental studies on the compaction behavior correlated with the identification of the change in the microstructure broaden the understanding in the relative weight of physical phenomena and their relevance for commercially used
aerospace prepreg materials.
• Enhancement of constitutive models for predicting void formation, growth and collapse, eventually modeling the porosity development.
• Simulation platform for virtual processing, i.e. consolidation and cure, forecasting the final part’s porosity content for arbitrary tooling configurations and processing conditions, validated for larger scale specimen and on an industrial use
case
• Reduction of efforts related to trial and error approaches for tool design in order to achieve sufficient laminate quality.
• By an enhanced use of process simulation, a reduction of the development and manufacturing cost per structure, i.e. the number of prototype toolings, can be reduced by 50%.

The technical content and outcome of this project are used for the education on several levels of detail: More general know-how on process simulation and optimization targets and restrictions generated within the project are transferred into lectures held by the LCC such as “Process simulation of composite structures”. More specific and detailed knowledge on the implementation and execution of certain tasks are transferred to individual students through the supervision of their thesis.
Simulation result of compaction test showing the stress in compaction direction
CT scans of laminates cured in autoclave. Left: Region of interest, Right: Visualization of pores.
left: Compaction test setup with UTM and heat chamber; right: compaction plates with thermocouple