Projectile Impact + Plate Stress Check

[strainstress]

We have a projectile of mass m travelling at a velocity of v impacting in elastically to 1/2" plate. Assuming there is no local perforation of projectile, how do I determine that the global response of the plate is acceptable (i.e. some plate yielding is acceptable as long as it does not form a mechanism and fold/crumble under impact force). Also how do i determine the impact force.

Any pointers will be greatly appreciated.

Thanks

 

[IRstuff]

What is the purpose of the plate' is it supposed to act as armor? Often, armor must survive multiple impacts to be acceptable.

Impact force can be monitored via accelerometers; I'm surprised your company is not up on this.

TTFN
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[strainstress]

I have verify and calculate impact force analytically. The purpose of the plate is to act as a shield

 

[IRstuff]

Well, you can look at the myriad videos on YouTube to determine a reasonable impact time. Once you have time, you can determine force. Of course, for a less than rigid plate, the time will be somewhat different.

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[racookpe1978]

The "reason" there are so many videos of high-speed film showing impacts is because it is so extremely difficult to get the FEA "right enough" that it has any value at all.

Your "problem statement" of mass "m" BEGINS with too little information! Do you have a BB pellet hitting a matchbox car? Or a 16 inch cannonshell hitting a battleship?

How much mass?
How much mass is the target?
What shape impacting mass?
How is the target plate supported?
How thick is its maximum?
How many impactors? (As mentioned above).
What angle of impact?
What is your "success" criteria? One impact + "not too badly damaged"? Or "many machine gun bullets at close range" + "None get through even if plate is bent"?

 

[KENAT]

Similar questions have been asked before, maybe doing a search will get you some suggestions be it using energy methods or otherwise.

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[ornerynorsk]

The type of projectile will have a huge impact, no pun intended. A tungsten carbide penetrator is going to behave very differently than a solid lead mass which will deform radically and aid in deceleration. Composition of the plate has a bit of importance, as well. Research "terminal ballistics".

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[rstupplebeen]

For ballistic helmets the amount of back face deformation was also critical.

Rob Stupplebeen

http://www.optimaldevice.com

 

[drawoh]

The fact that the plate is deformed elastically makes the problem simpler. You are converting the kinetic energy of the projectile into strain energy of the plate.

Can you assume that the projectile is fairly soft?

--
JHG

 

[drawoh]

One more thing...

Can you assume that one projectile only hits the centre of your plate?

If all these conditions are valid, you can look up some plate equations in Roark, work out the spring rate, and work out the deflection when all of your kinetic energy is converted to strain energy.

Then, you can work out the stress on your plate and verify that you are within the yield stress of your material.

If my assumptions above are not valid, then this problems is as complicated as everybody says it is.

--
JHG

 

[strainstress]

I found a great reference to calculate the force due to impact, while accounting for the ductility of the material.

IMPACT EFFECT OF FRAGMENTS STRIKING STRUCTURAL ELEMENTS (

http://www.osti.gov/scitech/servlets/purl/4315907)

The force calculated by using equation 13 is way less then that calculated using an elastic energy conservation approach. The force obtained using equation 13 seems reasonable.

Any comments ?

 

[IRstuff]

That's a paper written in the 1960s!!

The force obtained in the paper is probably going to be way less than what will really happen, because the model is so simple. Note that calculators weren't even around then, so the level of verisimilitude is going to be relatively low. The model makes MANY assumptions that I don't think are true in a real impact.

TTFN
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[ornerynorsk]

Check out Terminal Ballistics by Rosenberg and Dekel. It's a very recent work and very in-depth.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[Compositepro]

The only thing you can do is apply the law of conservation of momentum. You will also probably have to apply some simplifying assumptions on how the plate reacts and is supported.

 

[tbuelna]

Very complicated problem. As others noted, the geometry and mechanical properties of the projectile can make a big difference. How the projectile impacts the plate can also make a difference. Think about how armored vehicles use slanted surfaces.