Lucas Bretz, Benjamin Häfner, Gisela Lanza,
https://www.sciencedirect.com/science/article/abs/pii/S0263224120309222
Sheet Molding Compounds (SMC), a type of fiber-reinforced polymers, are popular lightweight material due to their handling and processability. The fiber mass content (FMC) as well as the fiber orientation distribution determine mechanical properties of manufactured components. However, local deviations from the intended FMC occur during the production of the semi-finished material as well as during the molding itself. In this contribution, SMC was investigated regarding its properties in the Terahertz (THz) spectrum to non-destructively quantify the local FMC. SMC with different FMC between 38 and 54 wt% was industrially produced and dry glass fibers were added to further increase the glass fiber content. Reference measurements of the FMC were conducted via thermogravimetric analysis and showed a mean deviation from the intended FMC of 2.64 wt%. The peak arrival time, the transmitted energy and the refractive index of the THz signal were determined and assessed regarding their correlation with the FMC. A multivariate regression analysis including backward elimination led to a low bias statistical model between the refractive index and the FMC with a cross-validated root mean square error (RMSE) of 1.33 wt%.
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