Resonance Frequency Measurement
Importance of Resonance FrequencyResonance frequency measurements are a major concern to materials, civil and aerospace engineers as well as others who work with structures. If a resonance frequency is too low, natural vibrations caused by wind or people walking can destroy the structure. The classic example of this is the Tacoma Narrows Bridge which was torn apart in a 42 mph wind storm. Space based structures such as the new International Space Station are particularly susceptible because they are small structures with many large, thin sections such as the radiator and photovoltaic panels. If the resonance frequency of the station is low enough, astronauts moving around in the station will start to vibrate the panels uncontrollably. They will eventually tear themselves apart just like the Tacoma Narrows Bridge.
Measuring Resonance Frequency Of A Material
Unless someone is actually trying to destroy an object by putting very large amounts of sonic energy into a tiny volume, all resonance measurements are nondestructive. Materials have a resonance frequency unique to a material that can be measured by vibrating a sample of the material and observing the response. Most resonance frequencies of interest occur between 0 Hz and 20,000 Hz or the audible range for humans. Audio microphones similar to those used in home stereo systems are used for much of the testing. Some methods also use voice coils similar to those found in speakers.
Excitation of the specimen can be done by a vibrating membrane, reed, tuning fork or by simply striking the specimen with a hammer. Sensitive microphones collect the waves (sound) generated within the specimen. At the resonance frequencies of a material, the amplitude of the measured sound waves will be greater than that of the supplied excitation. This is because the waves reinforce each other and essentially build up one upon another. As with harmonics in music, materials have resonance frequencies that go through a progression where the frequencies are progressively doubled, i.e., 100 Hz, 200 Hz, 400 Hz, 800 Hz, 1600 Hz, etc.
The resonance frequencies of materials actually reflect on the strength and distance between atoms in the material. Some elements with very low symmetry or outright different bonding have different resonance frequencies depending on what direction the sound is traveling. Take graphite for example. Graphite consists of covalently bonded carbon atoms tightly bound to one another arranged in sheets. These sheets are held together by very loose Van der Waals bonds. These weak bonds make graphite useful for pencil leads. The difference in bonds also means there is one set of resonance frequencies for sound waves traveling within the sheets along covalent bonds and another set for sound waves traveling perpendicular to the sheets along the Van der Waals bonds. Within the sheet the resonance frequencies are significantly higher than between the sheets.
Since the resonance frequencies reflect directly on the strength of the bonds of a material, it can be used to measure several elastic properties of the material. The dynamic Young's modulus and shear modulus are the two most common elastic properties measured. Young's modulus is essentially how stiff a material is when it deforms elastically (like an elastic band) when pulled or pressed uniaxially. Shear modulus is similar to the Young's modulus but is in the plane of atoms that gives the least resistance to deformation. It is a measure of rigidity or resistance to bending in torsion.
There are at least two standards for testing the resonance frequency of a material from the American Standard of Testing and Materials (ASTM). They are ASTM E1259 Standard Test Method for Dynamic Young's Modulus, Shear Modulus, and Poisson's Ratio for Advanced Ceramics by Impulse Excitation of Vibration and ASTM E 1876 Standard Test Method for Dynamic Young's Modulus, Shear Modulus, and Poisson's Ratio by Impulse Excitation of Vibration. These standards are available through your library and give full details on instrumentation and other aspects of testing materials as well as some of the uses of the results.
Measuring Resonance Frequency Of A Structure
Measuring the resonance frequency of a structure is more difficult but, as the Tacoma Narrows Bridge illustrates, very important. There are a variety of physics based computer simulations of varying complexity that can model the structure. One of the big problems is taking into account all the materials that go into a structure like a building (stone, steel, aluminum, carpeting, etc.). For relatively simple structures such as truss bridges made from steel and concrete there are very good models used by civil engineers to use. Considerable advancements in modeling the response of more complex structures such as skyscrapers to wind are also being made. The American Society of Civil Engineers is a good source for further information.
A Practical Use For Resonance FrequenciesIn materials science, electron microscopes are used to image details as fine as individual atoms. As you can well imagine, vibrations are a disaster, so the microscopes are isolated on air tables. A very good explanation of these devices is given at the Herzan Web site.
http://www.herzan.com/herz1.htmHope this helps.
