A Review of Nondestructive Characterization of Composites Using NMR

Abstract

Nuclear Magnetic Resonance (NMR) is a nondestructive evaluation technique useful for characterizing organic matrix composites and other polymer based materials. NMR depends on the interaction between the nuclear magnetic moment and a magnetic field and thus it is sensitive to localized field variations caused by molecular motions, changes in molecular or crystal structure, and chemical composition differences. Application of NMR to composites and polymers involves measurement of the hydrogen nucleus (proton) NMR signal. Fortunately, the proton NMR signal is very strong and easily measured. Much of the physical and chemical information available through the use of NMR is associated with the relaxation characteristics of the nuclear magnetic moments, which can be measured using pulsed NMR techniques. The energy exchange between nuclear moments and the surrounding lattice is characterized by the spin-lattice relaxation time, T1, while the energy exchange among nudear magnetic moments is described by the spin-spin relaxation time, T2. These relaxation times are very sensitive to molecular motions and structural changes and can be used to provide both qualitative and quantitative information on the dynamic environment in which the nuclei are located. For application to composites and polymers, hydrogen NMR has been used to characterize water absorption, molecular diffusion, environmental degradation, aging, degree of cure, and modulus variations.