Dynamic Time Analysis

[alvis]

Hello sim users

About once a year I get a requirement to perform a dynamic time analysis
and therefore I'm not an expert at this by any means.

I have an assembly which is subjected to a ramped type pressure load.
Starting at zero it then reaches peak load after 2 milliseconds and
stays there for a period of 4 milliseconds. After that it then ramps
down to zero. The whole load cycle is 20 milliseconds from start to
finish.

Not happy with the results I then compared the dynamic results to a
straight forward static load at the peak value load.

In this case, I expected to see higher stresses in the static loadcase,
which I'm not getting.

I haven't the experience to help me out here.....can you help?

So my question is ...given the loading described above do you agree or
disagree with my results expectation? (Dyn V Static)

I hope the above makes sense....not too easy for me to explain in words.

Andy Watson
AndrewWatson@AlvisVickers.co.uk



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[randyspeed]

Andy,

You have it backwards. You would expect to see higher stresses in the dynamic time results. Think about it. When time is introduced into the analysis, you are including things like velocity and acceleration. An object being quickly hit a force, pressure or otherwise, it will travel further than if it would with an equivalent static load. The displacement would be higher and, whence the stresses (directly related to displacements) would be higher.

Definitely not a primer, but best I can do in a paragraph.

Randy Speed, Principal
Speed Consulting, LLC
2871 Howard Road
Waxahachie, TX 75165
(972) 938-0490 ph (972) 937-2319 fax
www.speedconsulting.com

[martymac]

But it depends on the natural frequency of the system. You can imagine a
system where a very short pulse acts on a large body and has no effect in
terms of displacement. Wheras that same magnitude static load could
actually deflect the body.

So it depends.

Marty

[alvis]

Ok...I've read the comments below and I would like to take this a little
bit further....

I've been reading a few books in the meantime and they do appear to
tally with what you are saying.

So... if the stresses are higher in this particular dynamic analysis
than a static analysis, what happens to the UTS value? Does it remain
the same?

This is what I'm having trouble with. If the UTS remains the same value
then it means that it is possible to have a structure subjected to a
static load of "X" (passes fine), however when subjected to a dynamic
load of "X" for a short duration of time it would then fail due to high
stress.


Can anyone help me out with this?


Andy
AndrewWatson@AlvisVickers.co.uk

[martymac]

I think UTS (ultimate tensil strength) is just a material property,
independent of load condition.

I'm not sure why you say "static load of "X" (passes fine)", a static (one
time) load that does not yield is elastic by definition, so repeating that
load (say sinusoidally) brings the problem into the dynamic realm. So if
you have cyclic loading, then its a fatigue analysis you need to
investigate. Many books describe fatigue and how to estimate life time that
from the material properties and stresses developed from the loads, and
mechanica gives you most of what you need for that.

Marty