High-Performance SMC with Carbon Fibers – Digitalization, Characterization and Simulation

By reducing the weight of components and using resource-saving production methods in the commercial vehicle and aviation industries, pathways towards CO2 neutrality can be achieved. The use of fiber-reinforced plastics (FRPs) contributes to this goal through the use of materials with high specific stiffness and tailor-made component properties. With FRPs in particular, local material properties (such as the composition of the resin or matrix system, pore content, fiber orientation and distribution) are of immense importance for the structural properties of a finished component. These local material properties are fundamentally influenced by the manufacturing processes used in the production of components. For this reason, a deeper understanding of the process and material behavior is necessary. Using the example of a carbon fiber-reinforced sheet molding compound (C-SMC), a thermoset compound material, several methods have been demonstrated at Leibniz-Institut für Verbundwerkstoffe (IVW) leading to the acquisition of this knowledge. Beginning with the recording of the fiber orientation distribution during the production of the C-SMC semi-finished product, the process follows on to the characterization of the material behavior during the compression molding process and the implementation of the knowledge gained in a process simulation model. The goal of the entire process is to be able to predict the final component properties.

Digitalization of fiber orientation

In a standard production line for SMC semi-finished products, carbon fiber filaments are cut by a cutting unit and then spread onto a resin-impregnated carrier film. By adjusting the cutting length and speed, this process significantly determines the fiber distribution within the semi-finished product and thus the subsequent compaction and flow behavior during compression molding. An optical imaging system known as polarization imaging is used at IVW to record the fiber orientation. This system exploits the polarizing property of the surface of carbon fibers to incident unpolarized light. A polarization camera captures the reflected polarized light and a subsequent digitization process interprets the fiber orientation from the raw data generated. To ensure a clear view of the chopped fibers, the polarization camera is positioned directly behind the cutting unit. The camera takes images at defined time intervals and thus creates an image stack. These images are combined to form a continuous overall image of the semi-finished roll. This creates a two-dimensional (2D) digital image of the surface fiber orientation. In order to also capture thickness information, the camera can also be pointed directly at the spreading area below the cutting unit for complete three-dimensional (3D) visualization. This enables all chopped fibers to be captured on first contact with the resin-impregnated carrier film. In the digitization process of the image data, it is then possible to separate the fiber orientation information layer by layer. The result is a complete 3D digital image of the fiber orientation within the semi-finished product.

Material and process characterization

The mechanical properties of a final component rely on local material properties such as fiber orientation, fiber content, air pores and weld lines. These local properties are already determined during the processing of the C-SMC during compression molding. For this reason, the material behavior and process properties are characterized at IVW using press rheometry. The press rheometry is a simple flat press tool with open or closed heated tooling platens. Closing the tooling initializes compaction and flow of the C-SMC specimen, which is predefined in terms of geometry and number of layers. An open, circular rheometer is used at IVW. This setup enables unhindered biaxial flow and offers the possibility of carrying out the characterization test in two configurations. In the so-called "constant area" configuration, the sample area is equal to or larger than the area of the rheometer plates. This means that the pressed area remains constant during the test and allows the compression stress-compaction behavior and, if desired, the viscosity of the material to be easily derived via the recorded pressing force and the tooling gap height. In the second configuration ("constant mass"), an initial sample area is selected that is smaller than the area of the rheometer plates. In this case, the entire material remains between the rheometer plates during the test. This configuration is mainly used to investigate the development of the flow front and the evolution of the fiber orientation and thus to evaluate the local anisotropy properties of the C-SMC material.

Material modeling in process simulation

A user-defined material model has been implemented in the explicit FEM solver LS-DYNA® in order to mathematically describe the compression molding properties of a C-SMC with a long fiber length and high fiber content as accurately as possible. The process-critical properties can be identified and incorporated into the mathematical descriptions of the material model from the knowledge gained from the digitization of the fiber orientation on the C-SMC semi-finished product and the process-related material characterization using the press rheometry. A direct comparison of the force response, flow front development and fiber distribution from the simulated and experimental characterization test enables the material model to be calibrated and verified. Based on the new material model, process simulations can be carried out to describe the production behavior of components with complex geometries. This allows determination of the flow behavior within the cavity during the compression molding and the estimation of the final component properties. The results of the process simulation can be used to adjust both process and material parameters, such as the press closing speed, tool temperature and the C-SMC stack configuration and position, in order to optimize final component properties.

Acknowledgements

This project is supported by Fraunhofer-Institut für Techno- und Wirtschaftsmathematik (ITWM) within the framework of the High Performance Center Simulation and Software Based Innovation.