Terahertz Inspection as a Unique Enabler for Characterizing Sustainably Derived Composites

The Sustainable Manufacturing of AirCraft (SUMAC) project at NASA is developing composites from naturally derived fibers and resins for aerospace applications, with a specific focus on advanced air mobility (AAM). Naturally derived composite materials have the potential to reduce carbon emissions in aviation manufacturing and create new markets. Naturally derived composites for AAM use can be made economically viable by using structural health monitoring (SHM) to reduce knockdown factors, to provide data for condition-based maintenance, and to increase the range and payload capacity for the AAM vehicles. Composites made from naturally derived fibers such as hemp and flax could also provide strong vibrational damping in vehicles and aircraft compared to carbon-based composites. However, the same vibrational damping also makes naturally derived composites difficult to inspect with ultrasonic testing (UT). Terahertz (THz) radiation sits between microwave and infrared frequencies in the electromagnetic spectrum, and it has been used to characterize and inspect lossy dielectrics like thermal foams and biological materials such as plants. Unlike traditional composites, naturally derived composites are non-conductive and are thus penetrable by THz radiation. THz inspection is not used as widely as more “traditional” inspection techniques like UT and X-ray computed tomography because of its relatively new availability on the market. However, it offers sub-millimeter resolution imaging and unique spectral information while being non-ionizing and low power emission. THz can give specific information about the chemical makeup of naturally derived composites while also measuring density and thickness. THz time-domain spectroscopy (THz- TDS) can be used to interrogate different layers of a material by analyzing the THz reflections arriving at different times. THz is highly attenuated by water and responds spectrally differently to liquid and vapor water. This sensitivity to water can enable THz-TDS inspection to be used as ground truth or a calibration point for SHM sensors for detecting moisture uptake in structures. Our study examining the response of natural composites to THz radiation could enable future integration of THz sensors into vehicles for SHM. The unique capabilities of THz for inspecting and characterizing novel naturally derived composites at NASA will be presented. THz inspections were explored on a variety of flax-reinforced thermoplastic composite samples and individual constituents. This work investigates the alignment between composite layers, moisture sensitivity and moisture uptake mechanisms in the composites, and mapping of the ratios of fibers and resins in the composites as a first step toward applying THz for periodic monitoring of natural composite structures in AAM vehicles.