Impact Loading 1

[njm789]

Hi all,

What is the best way to determine the force/subsequent stress from an impact load. I have a project where a piston rams into/is retained by a housing. The material is glass filled nylon (which is anisotropic making it more challenging) and does not have a large elastic region. Would I go about this by using the kinetic energy of the piston and comparing it to the modulus of toughness of the housing? Any help is greatly appreciated.

 

[IRstuff]

You need some other piece of information. either stopping time or stopping distance.

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[njm789]

the stopping distance would be accounted for by the deflection in the elastic case correct?

 

[drawoh]

njm789,

Get out your mechanics of materials text and read up on strain energy.

--
JHG

 

[TED7]

I clearly don't exactly know what problem you're trying to solve, but I went through the Impact Load problem some years ago. I eventually decided to give in and just added a rubber bump stop and let time tell if it was going to work or not.

If you've got to justify something, or can't accept failure, I understand that won't be of much help though.

 

[rb1957]

"What is the best way to determine the force/subsequent stress from an impact load."

The best way to determine is a structure, particularly a brittle structure, will support an impact load it to test it.

The best way to mitigate the risk (of failing the test) is to run time-dependent load analysis, like LS-Dyna.

Hand calcs will either be super conservative or unconservative, because of the simplifying assumptions.

another day in paradise, or is paradise one day closer ?

 

[drawoh]

rb1957,

This is fairly easy is it not? We need to work out the elasticity of our structure(s) and of the piston. The primary simplifying assumption is that we stay within elastic stresses, but that probably is what we want. We need to work out the resulting stresses. The OP is using a piston, so I am guessing there is a mechanism that assures a predictable contact angle and position.

--
JHG

 

[FeX32]

Best way to determine the impact Force is to simulate the transient event. The impact force is dependent on the energy and the stiffness of the to components.
It is true that the lower the stiffness the lower the impact force, but this also increase the deflection. So to calculate the stress, you will need to compute the deflection during impact and correlate that to stress.
I have done this for different types of components in the past (including advanced polymers). rb1957 is correct, if the geometry if complex a CAE SW like LS-dyna (we use Radioss) is the way to go.

 

[handleman]

"Hand calcs will either be super conservative or unconservative..."

I gotta remember that one! What a competent-sounding way of saying, "We can do some hand calcs, but not only will they be way off, we won't even know which way they're off..."

 

[FeX32]

... "It's dangerous when you know just enough about a subject to think you are right, but just not enough to know you are wrong"

 

[rb1957]

"Hand calcs will either be super conservative or unconservative" …
I stand by it … so long as you add the last phrase … "because of the simplifying assumptions".

another day in paradise, or is paradise one day closer ?

 

[GregLocock]

Upper and lower bounds can be useful.

Cheers

Greg Locock


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[hendersdc]

njm789,

See my response in this thread for some useful equations from Roark's that attempt to convert static load stress and deformation to impact load values. [URL unfurl="true"]https://www.eng-tips.com/viewthread.cfm?qid=392778[/url]