Measuring Strand Orientation in Carbon Fiber Reinforced Plastics (CFRP) with Polarization

Carbon Fiber Reinforced Plastics (CFRP) are increasingly popular for lightweight but strong components, for example in aircraft or automotive applications. These materials come with the advantage of a large design space of the resulting material properties. A structural component can be tailor made for a specific load while minimizing the total weight of the component. However, this requires a non-uniform material where the strength and stiffness is controlled by defining the placement and orientation of individual fiber strands within the composite. For obtaining the desired properties of a component, a careful manual lay-up of fiber material in the correct orientation needs to be carried out. This poses a high risk that an assembly error could cause a failure where the structural component will not withstand the designed loads. We have developed a digital camera that can assist in the assembly process by measuring and visualizing the carbon fiber orientation. The used principle is as follows: The carbon fiber strands influence the incident light and mainly reflect light of a certain polarization orientation. With regular unpolarized illumination (as present in typical working environments), the polarization of the reflected light can be measured and directly gives the orientation of the fibers. This is possible for the uppermost layer of fibers, even with resin being applied. The degree of polarization shows areas where fibers are present. Other material like paper reflects unpolarized light which allows for an easy distinction. The developed camera operates with a single image sensor where adjacent pixels are sensitive to different angles of polarization. This way, the camera captures the orientation of polarized light in a single image. The PC software then processes the raw data and displays intensity, the degree of polarization and the angle of polarization which directly corresponds to fiber orientation. The easy to use system operates live which enables direct user feedback but can also be used for storing photos for documenting the lay-up process. More info about this article: http://ndt.net/?id=19678