In view of the complex nonlinear interaction mechanism between acoustic waves and damage in vibration sound modulation technology, this paper derives the kinematic equilibrium equation for linear elastic materials with cracks undergoing infinitesimal deformation using structural mechanics theory. The weak form of the equation is derived by applying the principle of virtual work to calculate the virtual work due to nonlinear changes in crack spacing. This paper also explains the physical origin of high harmonic and sideband signals in the system displacement solution. In addition, a three-dimensional contact model of micro-cracks is constructed to describe the nonlinear effect of contact sound on the crack surface caused by relevant displacement fields. To verify the correctness of the model, two indicators, the modulation index and the damage index, are used to evaluate the simulation results. The results indicate that the interface contact under micro-crack opening and closing motions causes additional nonlinear frequencies and that the nonlinear response increases with excitation amplitude while being relatively sensitive to micron-level cracks. Finally, experimental research is conducted, which confirms the theoretical derivation, and the reliability of the model has been verified.
In this paper, aiming at the problem that a multi-array focused ultrasonic transducer working in a high power range has a nonlinear influence on the measurement accuracy, a study of acoustic power measurement based on the near field cross-spectrum method was carried out. The focused acoustic field of a multi-array transducer was derived theoretically by combining the cross-spectrum method and the Westervelt nonlinear acoustic propagation equation. Finite element simulation was used to establish the model of the focused sound field of the multi-array transducer under different excitation conditions, and the influence law of each harmonic on the total sound power under specific excitation conditions was obtained. A cross-spectrum measurement system was built to scan the two near focusing planes under different excitation conditions. The total energy, each harmonic energy, and their proportion in the focusing region under the corresponding excitation were obtained through calculation and processing. The theoretical and simulation results were verified, and the harmonic energies were compensated in the calculation of the total ultrasonic power. The measurement results were compared with those obtained by radiation force balances. It was found that the maximum measurement deviation of compensated ultrasonic power was 7.89%, which met the requirements of acoustic measurement. The accuracy of the method and conclusion was verified under the power range of 10–60 W.
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