Quantitative fiber orientation analysis of carbon fiber sheet molding compounds using polarization imaging and X-ray computed tomography

Fiber orientation critically influences the mechanical performance of carbon fiber sheet molding compounds, in particular their stiffness, strength, and manufacturability. Here, the potential of polarization imaging for the fiber orientation analysis of carbon fiber sheet molding compounds is explored. Cost-efficiency, large field of view, and fast data acquisition allowing for inline application render polarization imaging attractive for use in industrial production, in particular compared to computed tomography. We analyze fiber orientation distributions in eight samples of commercially available pressed carbon fiber sheet molding compounds by two methods: The first derived directly from angle of linear polarization images, the second by maximal response of anisotropic Gaussian filters in through-thickness planes of 3d images obtained by computed tomography. The results are visually very consistent, with mean local fiber orientations in well over half of the area deviating by less than 20°. However, quantitatively, global mean angular deviations at around 20° highlight remaining challenges in comparing results from the two methods. A particularly encouraging finding is, that the surface fiber orientations obtained from polarization images are representative of the fiber orientation throughout the entire specimen thickness. This proves applicability of this new method in typical carbon fiber sheet molding compound processing scenarios, where ply layers are aligned and placed in the same orientation.