Conversion of a Dynamic Load to a Quasi-Static Load 1

[davidlandis]

Hi,
I want to convert a triangular load with a peak of 2000 N over a time period of 0.064sec to a quasi-static load, so that I can apply it to my fea model in ANSYS. Can anyone suggest any methods or references. Any help would be appreciated.

Thanks,
David Landis

 

[rb1957]

i'd start with 2000N ...
but some details would help us understand the problem (as i read it the load increases to 2000N in 0.0645sec, and remains at that level).

 

[Transient1]

David,

It sounds like your describing a shock impulse. Steinberg's "Vibration Analysis" is an excellent resource for this type of analysis. You'll see that some structures will actual experience higher than that peak load due to its natural frequency. If I'm off on that, as rb1957 asked, please describe your problem more fully.

 

[ThomasH]

I would start by looking at the natural frequency and how it relates to the load. If you are unlucky, the structure will experiense significantly higher effects than from a 2000 N load.

My interpretation of the load is:

t = 0.000 sek, F = 0 N
t = 0.032 sek, F = 2000 N
t = 0.064 sek, F = 0 N

If the load is as rb1957 guessed, constant (2000) after 0.064 sek then it's a different type of problem. Basically a static analysis with a quickly applied load.

But I'm not sure I fully understand the problem either.

Regards

Thomas

 

[GregLocock]

Until you know the dynamic magnification of the structure then you can't figure out an equivalent load.

A pulse of length 64 ms will strongle excite all resonances in the frequency range 0-16 hz, approximately.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GBor]

If that's a 1/2 wave as ThomasH suggested, wouldn't it only affect up to about 8 Hz? I'm then accustomed to worrying about +/-20%, so rougly 6 - 10 Hz? Maybe I'm misunderstanding...certainly need more information...

 

[davidlandis]

Hi,
Thanks everyone for your input. The loading pulse is exactly like Thomas has described in his message i.e. a triangular pulse with loads at,
t = 0.000 sec, F = 0 N
t = 0.032 sec, F = 2000 N
t = 0.064 sec, F = 0 N
I cant seem to understand what the difference is between a static load and a quasi-static load and how can I convert it into a quasi-static load?

(Until you know the dynamic magnification of the structure then you can't figure out an equivalent load -GregLocock)......How do I find the dynamic magnification factor for this type of an impulse load and how can it be used to find an equivalent load?

Thanks,
David Landis

 

[ThomasH]

Let's say that your pulse fits perfectly into the half-sine of the structures natural frequency. Then it's bad news.

On the other hand, if there is a big difference between them, it's positive. The following extrem might give you a hint. A pulse of very short duration hits a structure that has a relatively long period. That means that the pulse will come-and-go but the structure will not react at all.
You probably don't have this case.

The dynamic amplification (as I know it) is based on the relation between your pulse time and the structures period (natural frequency).

Regards

Thomas

 

[Spirit]

davidlandis,

you have the applied load and its time history. Why don't you directly apply it to your structure in ANSYS and perform a dynamic transient analysis? This would avoid you to make (at best, simplifying) assumptions on an equivalent load.

This is just my opinion anyway.

Regards

Spirit



'Ability is 10% inspiration and 90% perspiration.'

 

[rb1957]

yeah, the key is the dynamic response of the structure

 

[GBor]

Static vs. "quasi-static" just helps determine whether you need to be using a dynamic analysis. If your situation is such that the dynamic effects on the structure are negligible (the positive situation that ThomasH described), then you can say that your loading, while dynamic, is quasi-static. However, considering the short duration of your pulse, this is unlikely.

Quasi-static also generally refers to very slow application of loads so that inertial effects are ignored. A car crash is dynamic. A box sitting on the floor is static. A person sitting down on a sofa is probably quasi-static.

Run a natural frequency check on your structure...whatever it is. See how low the fundamental frequency is. If it is up over 20 or so, you are probably fine without the dynamic analysis, but if you have dynamic analysis capability and this model isn't so large that it would take days to run, add your load as ThomasH suggested and let it go...

 

[davidlandis]

I will probably be solving the system as a spring mass damper equivalent and finding the max spring force since the dynamic effects will not be negligible. Now will this force I get from the dynamic simulation be a static or a quasi-static force? I would guess this would be a quasi-static load since I had to use a dynamic sim to get the load. What do you guys think?

Thanks,
David Landis

 

[rb1957]

if the load is applied constantly (in reality) then it's "static".

if you're applying a static load to represent a dynamic situation then its "quasi-static".