Vibration Testing Fixture is a very important component which determine the accuracy of vibration testing.
Vibration Testing Fixture has the function of transmitting the vibration excitation from the Vibration Exciter to the Test Specimen without any distortion.
How to evaluate your Vibration Testing Fixture?
MIL-STD-810G-Method 514.6, Chapter 220.127.116.11.a:
“Test fixture resonance: Prior to test, a test fixture survey should be conducted to ensure that the structural characteristics of the test fixture do not introduce uncontrollable resonances into the test setup. The survey may be experimental or analytical. If problematic resonances are identified, modifications should be made to the test fixture to shift the resonance beyond the frequency range of the test or to dampen the resonance in order to minimize the effect on the test.”
Part of MIL-STD-810G above shows how critical is a Vibration Test Fixture. Test fixture which is not properly designed will “over tested” or “under tested” product. Product which is over tested will be less cost effective. On the other hand, product which is under tested will have higher chances to fail in the actual service environment, causing even higher cost for “recall” and improving design.
MIL-STD-810G mentions the two ways of modification that shall be done to the test fixture:
- Shift the resonance beyond the frequency range
- Dampen the resonance minimize its effect
MIL-STD-810G aslo mentions that the test fixture evaluation can be performed by experimental and/or analytical.
How do you know you have the right design for your vibration test fixture?
From the perspective of test accuracy, the challenges of designing Vibration Testing Fixtures are:
- It has to be stiff enough in order to avoid resonance within the test frequency range
- It has to be as light as possible, in order to minimize the power required for performing the vibration test
How will your test fixture affect your result or even the test performance of your product or the operating performance of your shaker?
- During resonance, the test fixture will amplify the vibration excitation and the test specimen may become over-tested
- The resonance may come side by side with anti-resonance. During anti-resonance, the test fixture will attenuate the vibration excitation and the test specimen may become under-tested
- During resonance and/or anti-resonance, the total vibration system may become less stable and create distortion to the test result
Finite Element Modelling is a powerful tool to be used during the design phase of vibration test fixture. A valid FEM simulation will help test engineer to design an optimum test fixture, in terms of stiffness and weight and therefore may increase the test accuracy. Without FEM simulation, it may need “trial and error”, in order to produce a good vibration test fixture. This “trial and error” process may lead to delay on test schedule, longer product development cycle time, and finally delaying delivery of the final product.
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