2014-09-18

Inge-Innova Aeronautics delivers a fixture for

vibration testing of aeronautical components

Inge-Innova Aeronautics develops a turnkey project designing and

manufacturing a fixture for vibration testing of aeronautical components

Vibration test fixtures are employed as an ancillary element to facilitate the fixation of

the test specimen to the shaker table to properly characterize the specimen, be it a

system, subsystem or plainly a unique component of the aircraft.

The design of the fitting is critical in order to avoid errors induced in the prototype

dynamic response. The testing machine should transmit the vibration energy regularly

and gently through the mass of the auxiliary frame with the slightest energy loss,

minimizing the mechanical impedance introduced by an additional element on the

system trying to simulate the flight conditions on the specimen. These circumstances

are unknown on most of the occasions therefore the design criteria are based on the

theory of vibration.

At the design stage the material and dimensions should be chosen taking into account

the test restrictions. Parameters such as mass, stiffness, center of gravity and modes

and natural frequencies of the fixture will be evaluated and some design guidelines

should be followed.

Material selection

These sort of fixtures are made of an aluminum alloy, widely used in aeronautics, that

will be selected according to its weldability, more than the mechanical properties

because they will deteriorate after welding (HAZ: heat affected zone).

Among other characteristics aluminum alloys are resistant and light at the same time

which benefits the equipment performance.

Design

The first consideration is to manufacture a welded fixture, not a bolted one, on account

of its rigidity.

Bearing in mind the dimension restrictions, weight limits, dynamic clearance (between

specimen and fixture) a coarse model is designed to perform an initial modal analysis

and obtain a rough reckoning of the specifications deemed fundamental by the

customer.

Once this preliminary prototype is figured out, the design may move on to a second

stage in which more details are defined up to the definitive model: specimen-fixture

interfaces, tolerances, surface finishing, etc.

Another important part of the design is the connection between the base of the fixture

and the shaker table deciding the pattern, dimension and number of fixations so as to

allow the energy to be transmitted uniformly without any loss.

So important is also to consider the handling of the fixture just in case lifting lugs were

needed or, merely, strapping and securing through the fixture itself could be enough.

Fabrication

An adequate welding method should be chosen and special attention will be paid to the

welding sequence so as to prevent distortions that usually occur in every material and,

particularly, in aluminum.

A proper welding which involves process and parameters, the welder qualification,

welding preparation and tips expressly for aluminum alloys welding will avoid defects

leading to an unexpected breaking of the mounting structure.

Should it be necessary, a complete heat treatment would be applied. Distortions may

happen after quenching and the part should be straightened before ageing.

Conclusions

Delivering a tailored fixture to the customer according to his specifications, reveals the

importance and advantages of the knowledge, not only know but do, combining theory

and expertise besides the fact of its applicability in industry.

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