Analysis for Impact vs Constant Force?

[tetwin11]

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!

 

[drawoh]

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

 

[tetwin11]

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

 

[Cockroach]

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

Regards,
Cockroach

 

[IRstuff]

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
faq731-376

 

[Cockroach]

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

 

[GregLocock]

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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[Twoballcane]

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”

 

[hydtools]

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

Ted

 

[FACS]

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

http://www.facsco.com

 

[FACS]

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

Charlie

http://www.facsco.com

 

[hydtools]

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

 

[GregLocock]

If you can work out some of the times and so on associated with the impact then you may be able to get a better handle on the forces eg

http://www.eng-tips.com/faqs.cfm?fid=1245

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[MotionGirl]

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.