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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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
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.
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
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.