Applications of nuclear magnetic resonance to nondestructive evaluation of advanced materials

Two applications of nuclear magnetic resonance to nondestructive evaluation are presented. The first application is NMR imaging of gases and liquids imbibed into porous ceramic matrix composites. This inspection technique takes advantage of the porosity of the ceramic composite material. A fluorocarbon liquid or gas is infiltrated into the pores of the material and NMR imaging is performed on the fluorine nuclei. The gases and liquids have more amenable relaxation times for imaging than the ceramic itself, enabling imaging to be performed on commercially available equipment. The ceramic composite restricts the diffusion rate of the gas and dramatically alters the spin relaxation times. These differences provide good contrast mechanisms for the NMR imaging techniques. Flaws such as delaminations and voids were easily imaged. The difference in diffusion rates can be observed and permits characterization of the pore size distribution.; The second application of nuclear magnetic resonance is to monitoring curing epoxies. Relaxation times were measured for several curing epoxy systems. It was found that the transverse relaxation time, T{dollar}sb2{dollar}, and the relaxation time of dipolar order, T{dollar}sb{lcub}rm 1D{rcub}{dollar}, were good indicators of the epoxy cure state. The relaxation times are easy to measure and are relatively insensitive to instrumental settings. We envision that such measurements would be feasible in a factory setting. Finally, the relaxation times of a graphite epoxy "pre-preg" system were measured and found to have a similar behavior. This technique would have applications in the aero-industry where epoxies are used for graphite reinforced composites.