| Nonlinear Ultrasonics has potential as a nondestructive evaluation technique for detecting defects such as dislocations. Understanding how the nonlinearity of metal responds to microstructural changes is essential in order to use nonlinearity for early predictions of structural failure. This work experimentally explored the behavior of the nonlinear ultrasonic parameter, β, with respect to the dislocation structures that caused it through processing and characterization of cold worked polycrystalline copper. Three sections classify the results of this work. First, methodology was examined to optimize repeatability and reliability of testing methods. The effects of experimental variations were examined, and couplant was found to have the greatest influence on repeatability. From the characterization of the couplant effect, criteria were developed to ensure experimental consistency. Second, the behavior of β for samples under various loading conditions was explored though tensile loading. Samples undergoing small stresses responded with a decrease in nonlinearity. Additional stress caused the nonlinearity to increase. This behavior was shown to correlate with theoretical dislocation models to explain the cyclic nonlinearity response to stress. Third, tensile samples with a monotonic increase in dislocation density had non-monotonic changes in nonlinearity. The microstructures of such samples were examined destructively and the formation of dislocation walls correlated to peaked nonlinearity curves. Models balance the effects of dislocation densities, substructures, and experimental conditions. This work concludes that nonlinear ultrasonic measurements are sensitive to test methods and microstructure. This develops understanding of fundamental causes of nonlinearity and of practical execution of nonlinearity, two key components required for future use as a nondestructive testing technique. |
