Air-coupled acoustic method for testing and evaluation of microscale structures

A noncontact testing and characterization approach for microscale structures based on air-coupled acoustic excitation and optical sensing is proposed and demonstrated. Using an air-coupled transducer to externally excite and a laser Doppler vibrometer/interferometer to capture transient displacement wave forms, the experimental approach results in a technique to determine mechanical properties of microscale structural elements. The effectiveness of this method has been demonstrated on commercially available microcantilever beams and microscale rotational oscillators fabricated for this study. The resonance frequencies and mechanical properties (Young's modulus and stiffness) extracted from the transient displacement wave forms have been compared, with good agreement, to computational and simplified analytical models for each case. It is also shown that the technique could serve to diagnose stiction problems of microscale structures. Some potential advantages of the approach described include the simplicity of the test setup, functionality at room conditions, noncontact and nondestructive operations, and repeatability and rapid turn-around time for the evaluation of modal parameters and mechanical properties of microscale structures.