BuzzMac International, LLC

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Buzz-o-sonic use the Impulse Excitation Technique...

waveform

frequency spectrum

tap the sample

measure the vibrations

get the frequency (f)

Out-of-plane flexure

Also known as impulse excitation of vibration, resonant vibration, impact acoustic resonance, eigen frequency method, or ping test, is where an elastic solid is tapped lightly with a small impulse tool causing the solid to vibrate at its natural frequencies. Although an elastic solid can vibrate in several modes simultaneously, the sample is supported and struck in such a way, that only one mode of vibration is prevalent. A typical example is shown below.

A screen shot from the Buzz-o-sonic 6 is shown on the left.

The left graph shows the amplitude of the vibrations plotted against time (waveform). Note the yellow curve fitted to the waveform envelope. This is used to calculate the internal friction (discussed below).

The right graph shows the amplitude or power (amplitude squared) plotted against the frequency (frequency or power spectrum). The sample was struck and supported in such a way that a clean, single-peak spectrum was obtained.

Buzz-o-sonic has several settings to aid in obtaining a clean spectrum such as Window functions (Square shown in the example) and waveform time offset (34 ms shown on example).

The elastic constants can then be calculated from the dimensions and mass of the sample and from the frequency for a given mode of vibration.

Buzz-o-sonic has built-in algorithms based on ASTM standards E1876 and C1259, to calculate the elastic constants of bars, cylinders, and discs. We also provide methods for measuring other shapes such as grinding/cut-off wheels, annular plates, square plates, tubes, rings, and tensile bars. A Microsoft® Excel® Spreadsheet is also available. Contact us for more details.

Buzz-o-sonic, has been shown to be very precise and reproducible by E. Lara-Curzio, M. Radovic, and L. Riester at Oak Ridge High Temperature Materials Laboratory. On comparing the impulse excitation technique (IE) to some other techniques, it was found that IE gave superior precision and repeatability to nanoindentation and four-point bending. The results were published in Materials Science and Engineering, A368 56-70 (2004).

The peak amplitude of vibrations of an impulse-excited solid follows an exponential decay given by:

Where:

A = amplitude at time t

A0 = initial amplitude following impulse excitation

f_{n} = resonant frequency of interest

z = damping ratio which determines system damping capacity

Thus, the ratio between adjacent amplitudes in the waveform follows a logarithmic law:

Where:

d = logarithmic decrement = 2pz

A1 = amplitude at time t1

A2 = amplitude at time t2

The internal friction is then given by:

The internal friction is also a measure of the breadth of the resonance peak, given by:

Where:

fh and fl are defined as shown in the figure below.

It is important that when measuring internal friction that the sample be carefully supported at the nodes for the mode of vibration being used. The supports should also be as thin as possible to reduce external damping. Trial and error may be necessary to determine the best method for supporting a given sample. We find that measuring the internal friction from the longitudinal mode of vibration gives the most consistent results, because the sample can be balanced/clamped and supported on one knife edge, rather than the two knife edges required for flexural measurements. Where possible, we use thin fishing fluorocarbon monofilament to support the samples. We provide the special supports. Contact us for more information.