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Acoustic Resonance
A reliable alternative to conventional NDT methods
Acoustic Resonance Testing (ART) is a new, very upcoming, non-destructive testing method that allows rapid and inexpensive 100% testing of a wide range of work-pieces. It relies on the well-known physical effect that a body, after suitable excitation (e. g. through impact), oscillates at certain characteristic modes and frequencies (its natural or resonant frequencies). These oscillations are, so to speak, the specimen's "language" (or its "fingerprint"): they can be measured with a microphone (airborne sound) or a laser vibrometer (body sound).
The specimen's geometry determines the number of its natural frequencies: a rod has few whilst a complex work-piece has many such frequencies. Typically, the information that can be obtained by acoustic resonance analysis includes cracks, structural properties, cavities, layer separation, chipping, density fluctuations etc. Damping behaviour depends firstly on the material, and secondly, on how the specimen is positioned during its excitation. In order to achieve high frequency resolution, signal duration ("ringing duration") should be as long as possible (> 50 ms).
From the natural frequencies it is possible to calculate specimen-specific characteristics and assign them to quality attributes, e. g. pass / OK, cracked, material structure, hardness deviation / partly hardened etc.
Application
Acoustic resonance testing can be applied to all workpieces that "sound".
| Industry | Material | Products (examples) | Testing for |
| Casting | iron casting | calipers, steering knuckles | nodularity, cracks, inclusions, cementite |
| grey casting | camshafts, brake discs | perlite, cracks, natural frequencies | |
| malleable cast iron | strut plate | cracks, structure | |
| Die-casting | Aluminium | pedals, steering wheel | cracks |
| steering gear housing | |||
| Ingot mould | chassis parts, cylinder head | cracks, inclusions | |
| Forge | conrods, stub axles | cracks | |
| synchronous rings | cracks | ||
| Sintered metal | powder metal | chain wheels, drive pulleys | cracks |
| miscellaneous products | cracks | ||
| planetary carrier | cracks, welding | ||
| Metal forming | air bag parts | cracks ("smilies") | |
| Steel | iron | drive shafts | cracks, natural frequencies |
| steering racks | cracks, hardness | ||
| rims | cracks | ||
| Ceramics | clay | roof tiles, industry ceramics | cracks, surface, dimensional stability |
| catalytic converter | cracks | ||
| Glass | bottles | cracks | |
| plates | cracks |
Summary
Resonance analysis is a qualitative method, i.e. it can differentiate between defective and non-defective parts, so that it is especially suitable for quality assurance in the series production cycle. It compares the actual oscillatory situation with the target one derived from a learning base. This learning base is established by using defined standard parts. The number of self-resonant frequencies is determined by the geometry of the object under test. For instance a bar has few resonant frequencies, while a complex lattice-type object has many natural resonances. After a systematic engineering approach, it is possible to compensate the influence of the production scatter.
The user can benefit from the following predominant features of the resonance analysis:
- High reproducibility: you will get exactly the same frequencies when you hit the part again!
- High resolution down to less than 1 Hertz possible - linear in the whole frequency range (!)
- Non-destructive, dry and clean.
- Fast analysis (within a second!).
- Simple to automate in comparison to other methods like x-ray and ultrasonic.
- Entire part involved independent of the defect position.
- Documentation, storage and traceability of each or selected parts.
- No costly training and maintenance.
- Excellent cost-value ratio.
This method has the big advantage that it is based on the components' mechanical properties and not on a method-specific effect.
Instrument and Versions
The SR20AT forms the base electronic and testing instrument setup to analyze the acoustic properties of the specimen under test.
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| Mobile Setup | Manual Setup | Lab Setup | ||
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| Inline setups | Inline setups | Production shopfloor setup | ||
The SR20AT provides for a very graphic user interface (like Explorer), it is configurable, modular and available in versions that suit your application & testing needs, can work on an unlimited number of product database, provides for database archiving with export functions, several channels in parallel, PLC interface, remote diagnostics, network integration etc.