Carbon fibers have the property of polarizing incident unpolarized light from natural or artificial light sources. Therefore, the orientation angle of the reflected polarized light reflects the orientation of the carbon fibers (Figure 1 (a)). With a suitable imaging technique, this effect can be used to measure the fiber orientation on CFRP components and semi-finished products in real time. Since neither cost-intensive equipment nor complex sample preparation is required, this method offers an efficient alternative to conventional measurement methods such as CT scans or eddy current measurements. In this project, carried out at the Leibniz Institute for Composite Materials (IVW), the measurement method using polarization is demonstrated using the example of a carbon fiber reinforced sheet molding compound (C-SMC) semi-finished product composed of one inch long carbon fibers and thermoset matrix. The fiber orientation of the semi-finished product is determined during the manufacturing process, which enables in-situ quality/process control, and digital representation of the fiber structure.
To facilitate such a measurement, a polarization camera equipped with a polarization filter pixel sensor patented by Fraunhofer IIS (Figure 1 (b)) is used. For a complete analysis of the fiber orientation, the method captures three image components, the light intensity, the degree of linear polarization (DOLP) and the angle of polarization (AOP) (Figure 1 (c)). In a downstream image processing step, these three image components are used to create a complete digital representation of the measurement zone.
Figure 2 shows the schematic structure of a standard production line for manufacturing a C-SMC semi-finished product available at the IVW. In the first step, a matrix system is applied to the carrier film (1) before a cutting unit (2) scatters chopped carbon fibers with a length of 25.4 mm onto the matrix and the carrier film. The polarization camera (3) is positioned directly behind the cutting unit, allowing a clear planar view of the passing carbon fiber C-SMC material. In order to capture a complete image of the semi-finished material roll (4), the camera creates a stack of images at predefined time intervals, which are then combined to form an overall image.
The data contained within the digital representation (Figure 3) can be used for documentation and quality control of the material as well as provide information of the fiber orientation at any position of the roll, which can be extracted to create virtual cutting and stacking plans. In order to determine the optimal use of the material from the roll during compression molding of CFRP components, these plans can serve as input information for a process simulation and, after an optimization process, as a template for the actual cut plan.
The digitization process for C-SMC semi-finished products developed at IVW together with Fraunhofer ITWM and IIS incorporates several different software components, ranging from the acquisition of raw data using polarization imaging to the processing of this raw data and the calculation and visualization of the fiber orientation tensors (FOTs) of the material.
This project is funded by Fraunhofer Institute for Industrial Mathematics (ITWM) within the framework of the High Performance Center Simulation and Software Based Innovation. Fraunhofer Institute for Integrated Circuits IIS is acknowledged for their support with the camera hardware.
Leibniz-Institut für Verbundwerkstoffe GmbH
Dr. Miro Duhovic
Phone: +49 631 2017-363
PD Dr.-Ing. habil. David May
Phone: +49 631 2017-400
Prof. Dr.-Ing. Joachim Hausmann
Phone: +49 631 2017-301
Prof. Dr.-Ing. Peter Mitschang
Phone: +49 631 2017-461
Fraunhofer-Institut für Techno- und Wirtschaftsmathematik ITWM
Dr. Konrad Steiner
Phone: +49 631 31600-4342
Miro Duhovica, Thomas Hoffmanna, Dominic Schommer a, David Maya, Jürgen Ernst b, Katja Schladitz c, Ali Moghisehc, Florian Gortner a, Joachim Hausmann a, Peter Mitschang a, Konrad Steiner c
a: Leibniz-Institut für Verbundwerkstoffe GmbH, 67663 Kaiserslautern, Germany
b: Fraunhofer IIS, Am Wolfsmantel 33, 91058 Erlangen, Germany
c: Fraunhofer ITWM, Fraunhofer-Platz 1, 67663 Kaiserslautern, Germany