Random Vibration Test Fixtures

What Is It?

A random vibration test fixture is the mechanical interface between the shaker table and the test article. The fixture transmits the vibration from the shaker to the test article. A well-designed fixture is stiff, lightweight and free of resonances within the test frequency range. A poorly designed fixture can introduce its own dynamics, distort the input to the test article, and invalidate the test results.

Why It Matters

The fixture is an often-underestimated component of a vibration test. If the fixture has a resonance within the test frequency range, it amplifies or attenuates the input at that frequency, producing an input to the test article that does not match the specified PSD. The test article is then tested at the wrong levels — potentially over-testing at a fixture resonance or under-testing away from it. A poor fixture can make the test results meaningless.

The fixture transmits vibration from the shaker to the test article. If the fixture has resonances within the test frequency range, it distorts the input and invalidates the test. A well-designed fixture is stiff and resonance-free in the test band.

Fixture Stiffness

The fixture must be stiff enough to transmit the vibration without significant deformation. The fixture stiffness should be much higher than the test article stiffness at the interface — typically by a factor of 10 or more. A stiff fixture ensures that the motion at the shaker table is transmitted directly to the test article without attenuation or distortion. For large or heavy test articles, achieving adequate stiffness may require massive fixtures.

Fixture stiffness criterion:

k_fixture  >>  k_article  (at the interface)

Rule of thumb:  k_fixture  >  10 × k_article

This ensures the fixture does not significantly
affect the motion transmitted to the test article.

For large/heavy articles:
  May require massive fixtures to achieve stiffness

Fixture Modes

The fixture's own modes of vibration must be outside the test frequency range. If a fixture mode falls within the test band, the fixture resonates — it amplifies or attenuates the input at that frequency. This distorts the PSD seen by the test article. The first fixture mode should be above the upper test frequency — typically by a margin of at least 20-50%. If this cannot be achieved, the fixture must be redesigned or the test frequency range reduced.

• Fixture modes must be outside the test frequency range
• First fixture mode should be above the upper test frequency by 20-50%
• If a fixture mode is in the test band, the input to the article is distorted
• May require stiffer fixture, different design, or reduced test frequency range

Resonance

A fixture resonance within the test band can cause serious problems. At the resonant frequency, the fixture amplifies the input — the test article sees a much higher level than specified. Away from the resonance, the fixture may attenuate the input — the article sees a lower level. The control accelerometer may partially compensate, but only if it is at the right location. If the control accelerometer is on the shaker table (before the fixture), it cannot correct for fixture dynamics.

A fixture resonance in the test band distorts the input to the test article. The control accelerometer may not correct for this if it is on the shaker table rather than at the fixture-article interface. Always check for fixture resonances before testing.

Load Transfer

The fixture must transfer the shaker force to the test article without loss. The load path through the fixture should be direct and stiff. Indirect load paths (long moment arms, thin sections, bolted joints in the load path) introduce flexibility and potential resonances. The fixture should be designed with a direct, stiff load path from the shaker table to the test article interface.

Test Article Interface

The fixture interface must replicate the actual mounting interface of the test article. The bolt pattern, torque, surface flatness and stiffness at the interface should match the real installation. If the fixture interface is stiffer or more compliant than the real installation, the test does not represent the real condition. The interface is where the fixture and article interact — getting it right is essential.

• Fixture interface must replicate the actual mounting interface
• Bolt pattern, torque, surface flatness should match real installation
• Interface stiffness should match the real mounting
• If the fixture interface is different from the real installation, the test is not representative

Fixture Qualification

Before using a fixture for a qualification test, the fixture should be qualified — verified to be adequate. Fixture qualification typically involves a sine sweep or random vibration survey of the fixture (without the test article or with a dummy mass) to identify fixture modes. If the first mode is above the test frequency range with margin, the fixture is qualified. If not, the fixture must be redesigned.

Fixture qualification:

1. Mount fixture on shaker (with dummy mass
   representing the test article)
2. Run sine sweep or random survey
3. Identify fixture modes
4. Check: first fixture mode > f_max,test × 1.2

If first mode is above the test range
with margin → fixture is qualified
If not → redesign fixture

Qualify the fixture before the test — run a survey to identify fixture modes. The first fixture mode should be above the upper test frequency with margin. Do not use an unqualified fixture for a qualification test.

Fixture Design Considerations

Several design considerations help achieve a good fixture. The fixture should be as stiff and lightweight as possible — high stiffness-to-weight ratio. Materials with high stiffness-to-weight (aluminium, magnesium) are preferred. The design should avoid thin sections, long moment arms and flexible bolted joints in the load path. For complex articles, the fixture may need to be custom-designed and analysed by FEA.

• High stiffness-to-weight ratio — aluminium or magnesium
• Avoid thin sections and long moment arms
• Direct load path from shaker to article interface
• Custom design for complex articles — FEA of the fixture itself
• Consider the fixture-article system dynamics, not just the fixture alone

Key Takeaways

• The fixture transmits vibration from shaker to test article — it must not distort the input
• Fixture must be much stiffer than the article (k_fixture > 10 × k_article)
• Fixture modes must be outside the test frequency range — first mode above f_max with margin
• Fixture interface must replicate the real mounting — bolt pattern, torque, stiffness
• Qualify the fixture before the test — survey to identify modes, verify they are above the test band