Development of New Dielectric NDE Techniques for Spaceflight Materials

Dielectric Spectroscopy is a relatively new technique for nondestructively measuring material properties. The goal for this project is to extend the use of state-of-the-art dielectric spectroscopy systems to measure the spectra of spaceflight materials and develop non-destructive evaluation (NDE) sensors based on dielectric material properties.  This work may lead to a new class of material density, moisture, temperature, and defect sensors providing structural integrity and health measurements for future spacecraft and launch structures.

Dielectric spectrometry will be performed on select spacecraft materials (i.e. heat shield materials, structural materials and insulating foams) to examine their capacitance and permittivity under controlled humidity. Three types of samples will be tested for each selected material, a dry, defect free sample, samples with known defects and moisture levels, and samples of unknown condition as a test of validity. Tests will be conducted from three microhertz to three gigahertz at temperatures ranging from -160°C to 400°C in order to obtain a full dielectric profile. The complex dielectric spectra acquired will be analyzed using computational modeling in order to determine the fit parameters associated with that material. These dielectric spectra and computational models will be used as the basis for proposals for new capacitance three-dimensional permittivity mapping sensor systems for monitoring structural health, extending a material’s lifetime in high humidity environments (like those of Florida) and in forming improved repair processes.

A Novocontrol Concept 80 Broadband Dielectric Spectrometry system will be used to measure the complex dielectric spectrum for select spaceflight materials.  Samples with known defects will be measured to serve as a comparison to known good samples.  Materials that are known to have problems with moisture absorption will be measured after exposure to different moisture levels to see how the complex permittivities vary with moisture content.

Analysis will be performed on the spectra to determine the extent to which damage and moisture can be quantified for each of the materials.  Concepts for new NDE techniques and sensors will be proposed based on the results of the analysis. Materials targeted to be tested include heat shield materials, such as Phenolic Impregnated Carbon Ablator (PICA), Avocat, and high performance polymers.