Review of PTB Measurements on Emissivity, Reflectivity and Transmissivity of Semitransparent Fiber-Reinforced Plastic Composites

Due to several excellent material properties, fiber-reinforced plastic (FRP) composites are expediently used in many applications, e.g., in the field of renewable energy and in oil, gas, and transportation applications. They show excellent mechanical stability, low weight and fatigue, and high corrosion resistance. However, their full potential for exploitation, as well as the lifetime of FRP-based structures, is limited due to certain defects and damage mechanisms. One of the most important methods used to ensure the quality of FRPs is non-destructive testing and active thermography. A prerequisite for quantitative active thermography is accurate knowledge of the optical properties of the investigated material (i.e., its spectral emissivity, reflectivity, and transmissivity). The objective of PTB as a partner organization within the European EMRP project titled “Validated Inspection Techniques for Composites in Energy Applications” was to improve the state of the art of this technique. One of the goals in doing so was to significantly reduce the uncertainty of emissivity measurements of FRP materials in the visible and infrared wavelength ranges. Achieving a target value of lower than 0.01 of the emissivity of partially transmitting materials is very challenging, especially at temperatures close to room temperature. Different experimental setups at PTB were employed for these measurements: one setup for spectral emissivity measurements in air and the other for diffuse reflectivity and transmissivity measurements. In this paper, we give a review of PTB measurements on emissivity, reflectivity and transmissivity of semitransparent FRP composites. Part of this work has already been published in Adibekyan et al. (emissivity, reflectivity and transmissivity of semitransparent fiber-reinforced plastic composites. https://www.ndt.net/article/dgzfp-irt-2017/papers/17.pdf , 2018). Here, we present the complete set of data for seven technical relevant materials and compare the results. The directional spectral emissivity was determined at a nominal sample temperature of 40 °C, at angles of observation from 10° to 70° with respect to the surface normal and in a wavelength range from 5 µm to 25 µm. In addition, these spectrally and directionally resolved measurements allow to calculate the total directional emissivity and the hemispherical emissivity. For the determination of the directional–hemispherical spectral transmissivity and directional–hemispherical spectral reflectivity a gold-coated integrating sphere and a vacuum FTIR spectrometer were used. The directional–hemispherical spectral transmissivity was measured under an angle of incidence of 0°; the directional–hemispherical spectral reflectivity was measured under an angle of incidence of 10°. These investigations were performed in the spectral range from 1.0 µm to 16.7 µm and compared in the overlapping wavelength range; the emissivity measurements were performed using the setup for spectral emissivity.