Terahertz spectroscopy to study the orientation of glass fibres in reinforced plastics

The prediction was bad with three different direction angles because the variations in refractive index and absorption coefficient acquired in this study were significantly bigger than in the prior result due to large variations of moisture content and density.

“The reason why the prediction was bad when using three different direction angles is that the variations in the refractive index and absorption coefficient acquired in this study were significantly bigger compared to their result due to big variations of MC and density.”

The result that β = 40° gave almost the same accuracy is consistent with the previous study which stated that accuracy was highest when the interval was 45°.

“This result is consistent with the previous study [19] which stated that the accuracy was highest when the interval was 45°.”

THz measurements at the same spatial position with more than three differing orientation angles of THz polarization with respect to the sample edge must be performed to identify the direction angle.

“Jordens et al. [19] showed that the THz measurements at the same spatial position with more than three differing orientation angles of the THz polarization with respect to the edge of the sample must be performed to identify the direction angle.”

Schajer and Orhan successfully predicted grain angles for hemlock and Douglas fir with standard errors of 0.9° and 2.5°, moisture content of 1.2 and 1.9%, and dry density of 0.16 and 0.30 g/cm3.

“Schajer and Orhan [19] successfully predicted the grain angles for hemlock and Douglas fir with standard errors of 0.9° and 2.5° (measured range from -90 to + 90°), MC of 1.2 and 1.9% (measured range from 7 to 28%), and dry density of 0.16 and 0.30 g/cm 3 (measured range from 0.325 to 0.625 g/cm 3 ) [20].”

Four different direction angles were employed to predict fiber direction because prediction of fiber orientation, moisture content, and density was not stable with only three different direction angles.

“However, we employed four different direction angles to predict the fiber direction of the wood samples, because the prediction of fiber orientation and MC and density was not stable when we employed only three different direction angles for the fitting like Jorderns et al.”

The three parameters, fiber direction, density, and moisture content, can also be simultaneously determined at microwave frequencies.

“The three parameters (i.e., fiber direction, density, and MC) can also be simultaneously determined at microwave frequencies.”