Research and Development of NMR Methods for the Nondestructive Characterization of Internal Stress and Strain in Nonferromagnetic Structural Materials

摘要

Two new nondestructive evaluation (NDE) methods for measuring residual stress in non-ferromagnetic metals have been investigated, namely, inductive nuclear magnetic resonance (NMR) and nuclear acoustic resonance (NAR). In metals, NMR is restricted to the surface by the electromagnetic skin effect while NAR is sensitive to the interior of the specimen as well as the surface. Results of NAR experiments on plastically deformed polycrystalline aluminum specimens were very encouraging. The NAR signature was found to vary with degree of plastic deformation in agreement with published NMR theory based on strain-induced quadrupole interactions. Shapes of the NAR signals from deformed aluminum were found to depend on the magnetic modulation frequency; it is suggested that this modulation dependence could form the basis for determining distributions of internal stress and strain in metals. Pulsed NMR experiments on bulk aluminum specimens were complicated by large electromagnetically generated acoustic transient signals not associated with nuclear resonance. Digital subtraction signal processing was used to discriminate against the spurious signals; however it did not separate NMR signals from the electromagnetically generated transients. Pulsed NMR signals observed from aluminum in a powdered specimen of the superalloy IN-100 provides a basis for considering NMR and NAR for nondestructive measurements in this important class of alloys. (Author)