SUMMARY

This research details the theorization and development of a novel phased array based measurement technique for nonlinear wave mixing. This measurement technique can uniquely provide localized values of the acoustic nonlinearity parameter, β, which is scanned from multiple points in a material of interest. Under certain resonance conditions, cross interaction between two elastic waves can generate a scattered wave that’s amplitude is dependent upon the material at the point of mixing. A relative measure of β can then be used as an assessment of the local material state. Phased arrays are used in this work to generate the incident waves. In doing so, the material state of multiple localized volumes can be interrogated from the same experimental setup and contact conditions. The development of this technique is demonstrated from the various experimental iterations before arriving at a successful phased array based nonlinear wave mixing measurement method. Embedded ASR and fire damage are detected with single element nonlinear wave mixing in a concrete structure. A heterogeneous 316L AM specimen is characterized along a scanning path through different material states with phased arrays. The results from the phased array measurement show sensitivity to the change in lack-of-fusion porosity. This is the first known result showing the potential for the use of phased array for the nonlinear wave mixing to characterization a material microstructure.