Recently, it has been discovered that carbon fibers polarize unpolarized light, providing an opportunity to detect the fiber orientation on the surface of carbon fiber-reinforced polymer parts using polarization imaging. The method uses digital image processing of carbon fibers reflecting polarized light when illuminated with non-polarized light. The angle of polarization (AOP) can then be correlated to the fiber orientation angle. The method provides a cost effective way of directly photographing or filming the surface of a material/component and obtaining complex fiber orientation information in real–time without special preparation and with no restriction on the specimen size. The technology is seen as one of the key elements for online quality control and the digitalization of carbon fiber based polymer composite properties for the many different manufacturing routes available for CFRP components. In this work, the fiber orientation of carbon fiber-reinforced Sheet Molding Compound (C-SMC) semi-finished product is measured during continuous production, enabling real-time quality control and complete digitization of the material’s fiber orientation information.
Polarization Camera – Principle of Operation
Figure 1 (a) illustrates the basic working principle of the polarization effect by reflection. Light is termed non-polarized if the light wave has no defined angle of oscillation about its axis of propagation. Almost all natural and artificial light sources emit non-polarized light. If these light waves hit a polarizing object, the reflected or transmitted light waves will all be oriented at the same angle, as shown in the figure by the single reflected wave. If unpolarized light strikes the surface of a CFRP, linearly polarized light is reflected. The angle of the reflected wave about its axis of propagation corresponds to the fiber orientation at the point of reflection. The resin system does not affect the measurement, as many polymer resins in their natural (uncoloured) state are amorphous and therefore relatively transparent. A polarizing filter pixel sensor patented by the Fraunhofer IIS (Figure 1 (b)) allows three image components to be captured simultaneously by the camera (intensity, degree of linear polarization and angle of polarization) as shown in figure 1 (c) on a specimen consisting of randomly dispersed carbon fiber rovings.
Comprehensive digitalization during C-SMC material production
Using the principles described in the previous section, the fiber orientation of a carbon fiber-reinforced Sheet Molding Compound (C-SMC) semi-finished product is measured during continuous production. A polarization imaging system is used to analyze the fiber orientation distribution for a complete roll of manufactured C-SMC semi-finished material. Figure 2 shows the set-up of a standard SMC production line schematically, including the position of an integrated polarization imaging system. Initially, a resin system is applied to a carrier film (1) before passing through a cutting unit, which distributes chopped carbon fibers with a length of 25.4 mm onto the resin and carrier film (2). The polarization camera is positioned directly behind the cutting unit, providing a clear planar field of view of the chopped carbon fibers passing beneath (3). The camera captures the material in this area at defined time intervals, creating an image stack. These images are then stitched to create a continuous image of the entire semi-finished material product roll (4). In addition to real-time material quality control, the fiber orientation information at any location of the roll can be extracted to create virtual-cutting and stacking plans for optimal usage of material from the roll for compression molding parts. These digital cut-outs can then be evaluated individually and the image data for each can be visualized or analyzed in form of colored FO images, FO distribution histograms, and tensor data (see figure 3). The digitization process involves several different software components, covering the acquisition of raw data by means of polarization imaging, to the processing of this raw data and the calculation and visualization of the material’s fiber orientation tensors (FOTs). The FOTs provide the exact input variables required to define the initial state of the fiber orientation in C-SMC compression molding simulations.
In this research work, a comprehensive digitalization pathway for C-SMC materials using polarization imaging has been demonstrated. The principle operation of the polarization effect has been explained and a polarization imaging system consisting of hardware and software components has been developed and implemented on an SMC production line. The system captures and processes image data, which is used for online process monitoring and to generate accurate fiber orientation inputs for simulation of the C-SMC compression molding process.
This project is funded by the Fraunhofer Institute for Industrial Mathematics (ITWM) within the framework of the High Performance Center Simulation and Software Based Innovation.