In-line measurement of fiber mass fraction using Terahertz spectroscopy for a function-oriented quality assurance of glass fiber sheet molding compound

Glass fiber sheet molding compounds are a popular fiber-reinforced polymer because of their good processability. Both fiber mass fraction (FMF) and fiber orientation distribution significantly determine the mechanical properties of manufactured components. Local random variations in both properties occur during prepreg production and molding. This contribution expands a non-destructive measurement model for local FMF using Terahertz spectroscopy towards multiple specimen thicknesses. A thorough measurement uncertainty analysis according to the Guide to the Expression of Uncertainty in Measurement (GUM) shows an uncertainty reduction by 70 % compared to the uncertainty resulting from the manufacturing process. Measurement times of 0.5 s qualify the methodology for an in-line application. Integrating the measurement results into a parametrized, component-specific finite element model reduces the root mean squared error between physical experiments and simulations by 33 and 54 % compared to simulations with nominal fiber mass fractions.