Measurement of natural frequency and mechanical damping of thin brass diaphragm by pulsed laser generated vibrations

Abstract

Damped harmonic oscillator model based fitting of nanosecond pulsed laser induced amplitude variations of clamped vibrating circular plate is used to estimate the mechanical damping and natural frequency of the sample in current work. Laser Pulses of 50 mJ energy, 20 ns duration, and focused at a spot of 4 mm diameter at the center of the circular thin brass sheet of 100 µm thickness is used to generate vibrations in the target. Quadrature Michelson interferometer (QMI) with CW laser focused on the opposite side of the target surface is used to measure the amplitude of vibrations. Variations of fringe frequencies are identified in the frequency domain. Finite element based numerical modal analyses are also performed in ANSYS Workbench for the verification of experimental results for the same geometry and materials. Experimental frequencies of vibrations are found to match nearly 2 percent of FEM modes. Moreover, Elastic parameters are also found using the first two mode frequencies and a reasonable agreement is observed while comparing with the elastic parameter data of brass. Current work in itself is a unique attempt of getting mechanical parameters for the determination of elastic parameters in a single laser pulse impulse excited measurement for thin clamped targets.