NASA-STD-7003A states wrt joint attenuation predictions that “….up to a maximum of three joints….” be used. I realize this maximum of 3-joint attenuation has been typically utilized in the industry.
Does anyone know the basis behind this guideline (i.e. what’s the history and/or data behind it)?
And does anyone have test or flight data that would support its enforcement or provide evidence to the contrary?
Here is a rough perspective…
Consider that joints should be more effective in attenuating high-frequency waves than low-frequency waves, especially near the source. The shock field beyond three joints may be a far-field where the response is dominated by low-frequency bending modes which are less sensitive to joint attenuation.
Yes, there may be exceptions and special cases.
Also, shock propagation is very sensitive to structural details. The structures that I have dealt with in the past have had cut-outs, bulkheads, forward or reverse Y-joints, attached cables & components, varying shell thickness and materials, etc., which would complicated any generalizations.
Development testing is needed to measure the shock propagation for each system configuration.
The well-known NASA Pyroshock document (NASA CR 116406) published in 1970 has caveats to this effect.
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Space Engineering: Mechanical shock design and verification handbook
14 July 2015
Adding attenuation factors is always dangerous as shock is not a linear phenomenon: As a general rule, and when applying the attenuation rules described in the following paragraphs 8.3.1 and 8.3.4, no more than 3 junctions should be cumulated along the loadpath.
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