## Low Frequency Structural Loads for Secondary Structures & Components

May 20, 2014 by tomirvine999

Method 1

Secondary & Component loads can be specified in the form of a Mass-Acceleration curve as described in:

NASA-STD-5002, Paragraphs 4.2.1.2, 5.3.2, 5.4.

The Mass-Acceleration curve can be derived from measured flight or test data, or analytical loads. Care must be taken to avoid “double dipping,” if two or more sources are used.

This method effectively assumes that the component’s fundamental frequency is much higher than the excitation frequencies. The resulting analysis becomes a rigid-body analysis.

Method 2

Component loads can be expressed as a Vibration Response Spectrum (VRS).

This assumes that the component responds as a single-degree-of-freedom (SDOF) system subjected to base acceleration.

The Y-axis for this function is peak response acceleration. The X-axis is natural frequency (Hz). The amplification factor Q must also be noted. A family of curves for various Q factors can be included in a single plot.

The VRS curve(s) can be derived from measured flight or test data, or analytical loads. Again, care must be taken to avoid “double dipping,” if two or more sources are used.

The resulting VRS can then be used for design purposes by picking off the peak response acceleration value for a given natural frequency and Q.

A similar VRS can be derived for relative displacement. This is important for cases where clearance, sway space, alignment, or isolator deflection are concerns.

My colleagues and I used the VRS method at my previous workplace, Orbital Sciences Corporation, Chandler, AZ.

I have posted references for this method at: VRS Link

Method 3

A measured or synthesized acceleration time history can be used to base drive a finite element model of the component, as a modal transient analysis.

The time history can be derived from measured flight or test data, or analytical loads. Again, care must be taken to avoid “double dipping,” if two or more sources are used.

I also occasionally used this method at Orbital Sciences Corporation.

Shaker Table Testing

Avionics components are typically tested over the frequency domain 20 to 2000 Hz for random vibration.

Components may also need to withstand transportation vibration below 20 Hz.

See SMC-TR-06-11, sections 3.24 – 3.26

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See also:

Launch Vehicle Coupled Loads Analysis (CLA) Upper Frequencies

Equivalent Static Loads for Random Vibration

Mass Acceleration Curves

Effective Modal Mass

JPL D-5882 Trubert

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– Tom Irvine

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*Related*

on May 20, 2014 at 2:28 pm |Launch Vehicle Coupled Loads Analysis (CLA) Upper Frequencies | Vibrationdata[…] Low Frequency Structural Loads for Secondary Structures & Components […]

on June 3, 2014 at 4:28 pm |COLIN BrunoTom,

Thank you for your mail concerning this tree method. But it is impossible to telecharge “SMC-TR-06-11” document. Can you send it in pdf format by mail ?

Thank you for your Help.

Bets regards,

Bruno COLIN