Ultrasonic horn contact-induced transient anharmonic resonance effect on vibro-thermography

Abstract

We present some new insights into the impact-separation contact dynamics induced spatial localization of anharmonic in a cavity-like feature. Origin of different nonlinear anharmonics is identified with the help of an inelastic/plastic collision signal model, a nonlinear Duffing oscillator model, and a detailed 3D finite element simulation, with increasing granularity and physical assumptions regarding contact dynamics. A rigid contact on the target elastic structure generates super-harmonics when the target structure resonates precisely with the excitation frequency. It generates sub-super harmonics when there is a slight mistuning in the motion of the target structure. The response amplitude and the energy in the primary excitation frequency reduce due to the energy partitioning over the different anharmonics generated. The ratio of Power Spectral Density (PSD) of primary harmonics to input excitation decreases almost linearly with decreasing separation distance between the actuator horn tip and the stationary target. The ratios of PSD of all anharmonics to that of primary excitation decreases quadratically with decreasing the relative amplitude of an elastic target structure moving in phase with the actuator. Different anharmonics further evolve as elastic waves in the target structure. The non-stationary vibration at the beginning of the excitation governs the evolving patterns of heat generation, which is of great interest in applications such as vibro-thermographic inspection. The effect of the initial transient state on time required to reach the steady-state excitation is analyzed, and related computational aspects are discussed. Spatio-temporal anharmonic modal distribution and the thermal signature obtained from numerical simulations are compared with the experimental results. The results strongly correlate the anharmonic modal contribution with the transient thermal signature due to a cavity-like feature in the target structure.