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Thermal Modulation of Nonlinear Ultrasonic Waves
Nonlinear acoustic/ultrasonic techniques show high sensitivity to microcracking damage in materials. However, these nonlinear test methods either need significant ultrasonic energy input to excite material's nonlinear response using an impact or a low-frequency pumping. It is difficult to generate a uniform strain field and measure the absolute value of nonlinear parameters due to difficulties in measuring the absolute strain or displacement. These limitations hinder the application of nonlinear ultrasonic methods to large scale concrete structures and in-situ monitoring of real structures. In this work, a new nonlinear ultrasonic technique, the thermal modulation of nonlinear ultrasonic waves, was proposed for characterizing material nonlinearity and damage levels. Temperature-induced thermal strain excites the nonlinear response of the material and modulates the ultrasonic wave propagating in it. A theoretical framework was presented. By using a quadratic polynomial to model the correlation between the relative velocity change of ultrasonic waves and the temperature change, the author derived the acoustic nonlinearity parameters and their relationships with the thermal modulation coefficients. The proposed theory was validated on classical and non-classical nonlinear materials. The thermal modulation method was used to characterize the nonlinearity of small concrete samples with different types of damage and different aggregate contents. Since the thermal modulation coefficients are the combinations of the nonlinear parameters alpha and beta, the coefficients can be used as to characterize nonlinearity and damage levels of concrete. The effects of temperature range and changing rate were also studied. A special nonlinear behavior, temperature-induced slow dynamics, was briefly discussed. The thermal modulation method was used for characterizing the Alkali-Silica Reaction (ASR) damage in concrete. The thermal modulation tests were conducted using the temperature change during the chamber shutdown and restart periods. The thermal modulation coefficients at different curing ages were extracted, and they showed good correlation with the ASR expansion. The nonlinear parameters were obtained from the thermal modulation tests with a close thermal cycle. An irreversible temperature effect was observed, which is related to the material hysteresis. Finally, the thermal modulation method was applied to three full-scale concrete mockup specimens with ASR damage (at University of Tennessee-Knoxville).
Sun, Hongbin, "Thermal Modulation of Nonlinear Ultrasonic Waves" (2020). ETD collection for University of Nebraska - Lincoln. AAI28085895.