DO-160 static loading vs impact loading

[707olds]

Hello guys, this is my first post. I've enjoyed the site so far!

My question has to do with structural qualification for "crash safety." Is there a link between shock/impact loading and static loading? One of my friends has argued that equipment that has been tested for a 20-g shock load over x-number of milliseconds is probably also good for the 9-g forward static loading specified by FAR 25. It seems like this would work, but I assume that if this were true, there would be no need for separate static and impact load requirements. Anybody know of a link between shock test results and static load requirements?

 

[Sparweb]

No. Static load tests isolate the attachment structure from any effects related to the structure's own mass. If you shock load an object, the mass of the attachment structure will provide some resistance momentarily before the full static load can be applied.

If 20g's are applied for 10 ms, then the object of mass is only going to travel 3/8" in that time. In a structure like a seat, for example, the ultimate deflection under only 9 g's is more than that. By the time the shock load is release, the material hasn't had time to come to static equilibrium.

On the other hand, applying 9 g's for 3 seconds allows lots of time for all components of the structure to come into static equilibrium.

There's no formula to switch between the two.

Steven Fahey, CET

 

[andyfe]

DO160 section 7 not only contains the procedure for "shock/crash safety" test, which involve dynamic response from the structure, but also for "sustained" test, i.e. those for which the inertial load is applied as a "static" condition.
"Sustained" tests are basically acceleration tests, often performed in a centrifuge. Due to their "static" nature, compliance with these test conditions can often be easily demostrated by analysis instead of actual test.