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Analysis for Impact vs Constant Force?

Analysis for Impact vs Constant Force?

Analysis for Impact vs Constant Force?

(OP)
I know how to analyze a structural system for a constant applied force, but what about impact forces? Are there some general tips or rules for what a structure might be able to handle in impact when it has been rated for a constant load? i.e. If a structure is rated to withstand a constant 5kN, can we estimate what sort of impact forces  it would also withstand?
Thanks in advance!

RE: Analysis for Impact vs Constant Force?

tetwin11,

   Check your mechanics of materials textbook under Strain Energy.  This is where I was taught about impact loads.

   Under impact, your structure must absorb the kinetic energy, somehow.

               JHG

RE: Analysis for Impact vs Constant Force?

(OP)
Good tip, looks like a good place to start. Thanks, drawoh.

RE: Analysis for Impact vs Constant Force?

I use 3:1 as an initial rule.  Impact load imparts three times that of the static load under similar conditions.

Regards,
Cockroach

RE: Analysis for Impact vs Constant Force?

Impact forces differ in another aspect from static forces, which is the time element.  You cannot simply state that your structure can withstand an impact force, without specifying the duration and shape of the force.  

Thus, you personally could survive a 200-g impact if it was sufficiently short, but a long duration impact would squash you flat.  The difference is the time integral of the force, which is correlated with the total energy of the impact.

TTFN
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RE: Analysis for Impact vs Constant Force?

Don't get impact and vibration/resonances confused. Impact is just that, one sharp blow. What I'm saying is that the estimated load would be three times the static load thereof.

In the structure, TetWin11 would need to perform other basic calculations, of which we have no information. His general question got him a general answer. We have no idea of the collision, elastic or inelastic, material composition of bodies, deformation as a result thereof, impulse parameters, nothing. If the impact load was something on the order of 30 lbf in TetWin11 world, I would be considering his structure to statically withstand 90 lbf.

There would be other considerations of course. Just keeping it simple!
 

Regards,
Cockroach

RE: Analysis for Impact vs Constant Force?

The horrible old rule of thumb is that a suddenly applied load applies twice the stress as the static load.

It isn't actually a rule of thumb, it is a perfectly good piece of physics for a single degree of freedom system... that lazy engineers have misused.

 

 

Cheers

Greg Locock


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RE: Analysis for Impact vs Constant Force?

I think you may have to break out your kinetics and calc it out yourself.  I don't think there is a rule of thumb just because the velocity of the object just before impact will differ from one situation to the next.  I would think you will have a different load between me throwing a baseball and one being shot out of a cannon.  Look into the Work-Energy principals and you can work back to a deflection which will translate to a static load.

Tobalcane
"If you avoid failure, you also avoid success."
"Luck is where preparation meets opportunity"  

RE: Analysis for Impact vs Constant Force?

There is a classic textbook example of a suddenly applied vertical load applying a force twice its weight.

Ted

RE: Analysis for Impact vs Constant Force?

So by using the 3:1 rule of thumb, I can assume that the impact of dropping a TV 1-inch from the ground has an impact load of 3:1 to the static force of gravity..... as it sits on the TV stand.... and if I drop the TV from a 3-story building, it will have the same 3:1 impact load.

Is that right?

Charlie
www.facsco.com

RE: Analysis for Impact vs Constant Force?

Oh, I see, I read that wrong.
Now I need a new TV

Charlie
www.facsco.com

RE: Analysis for Impact vs Constant Force?

Once you get some velocity going you could use Impulse = change in momentum, F*t = m(v2 - v1).  F=force, acting during time,t.  m=mass.  v1=initial velocity.  v2=velocity at the moment of impact.  t is a challenging guess, how long does the impact act.

Ted

RE: Analysis for Impact vs Constant Force?

Just for information: a body released from zero height (ie its weight is totally supported but is touching the surface just so) on to an elastic system (say a cantilevered beam) will deliver no "impact" (ie no diference in velocities) but will initially deflect the beam at twice the amplitude to that it ends up at rest after the vibrations have settled down.

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