Particle Impact

[dagger440]

Hello. This is my first time on this site. I am hoping to utilize the experience on here. I have a scenario that I have tried to calculate, but I am not sure of my results. Here is what I have.
I have a material with the weight of .305 lbs/ in^2. Size is 25um. For calculation purposes, consider this the diameter of a sphere. The ultimate tensile strength is 144 ksi. This material will be at a velocity of 750 m/s hitting a surface with a tensile strength of 13,500 psi. The material hitting the surface will completely be deformed as needed. The surface it is hitting can not be considered to have a spring factor due to multiple particles at that velocity hitting it. There for my assumption will be no time to spring back. The force I calculate seems to be a bit higher than I would think it should be.

Thank you for your time.

 

[IRstuff]

I do recognize that that weight is a force. Nonetheless, when I get my "weight" of 160 lb on my bathroom scale is telling me that my mass is 72.6 kg, and not something else. Moreover, the US unit for weight as a force is a slug, i.e., 1 lb * 32 ft/s^2, which is substantially different.

Furthermore, we've pretty much narrowed the material to steel, which has density around 8 gm/cm^3, which is what you get when you convert the 0.305 lb/in^2 to metric by multiplying by 453.6 gm/lb and dividing by (2.54 cm/in)^3

TTFN
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[rb1957]

sorry but you won't convince me that something that weighs 1lb has a mass of 1lb. it has a mass (as you rightly say) of 1 slug ...

1lbf = 1slug*1ft/sec2 = 1lbm*32.2ft/sec2

1kgf = 1kg*9.8m/sec2 = 9.8N

"453.6 gm/lb" ... this is a force conversion ... 1kgf = 2.2lbf (a 1kg bag of sugar is pretty much equal to a 2lb bag of sugar).
"when I get my "weight" of 160 lb on my bathroom scale is telling me that my mass is 72.6 kg" ... no, it's telling you your weight is 72.6kgf; scales don't measure mass, they measure weight (force).

steel weighs 0.3lbf/in3.

Quando Omni Flunkus Moritati

 

[IRstuff]

Slug is the force unit in US units, just like Newton is the force unit in SI.

"1lbf = 1slug*1ft/sec2 = 1lbm*32.2ft/sec2"

Exactly as I said. All scales report 1 lb force, which is the force exerted by a 1 lb mass. Therefore, under colloquial usage, something that weighs "1 lb" has mass of 1 lbm

"sorry but you won't convince me that something that weighs 1lb has a mass of 1lb"

Your expression above does exactly that. 1 lbm (mass) --> 1 lbf (force)

"steel weighs 0.3lbf/in3."

By your own conversion above, steel's mass density is 0.3 lbm/in^3.

TTFN
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[ThomasH]

Hi

Here you have a link to a page regarding consistent units:

http://femci.gsfc.nasa.gov/units/index.html

As far as I know kgf (kilogram force) is not a unit in the SI system. In some cases a unit called kilopond was used in that context but that was ages ago. Today it is Newton that we (at least myself) use in the SI system. That does not mean that kgf can't be used but you have to be careful, as always, with the units.

Regards

Thomas

 

[rb1957]

why did i confuse things with kgf? i guess it seemed like a good idea at the time ...

V = 5E-10in3
wt = 1.5E-10lbf = 7E-10N
mass = 7E-10kg ... 100x my original number ! ('cause i divided kgf by 9.8 instead of multiplying)

or is it force = mass*accel ... mass = weight/g = 7E-11 ... sigh

@IR,
"Slug is the force unit in US units, just like Newton is the force unit in SI."

nope, lbf is the imperial force unit, slug is the native mass unit (mass moments of inertia in slug*ft2) ...
1 lbf = 1slug*1ft/sec2 = 1lbm*32.17ft/sec2
1 N = 1kg*1m/sec

Quando Omni Flunkus Moritati

 

[dagger440]

So to figure this out....
I will need to determine the amount of distance the particle will travel into the surface to ultimately equal the amount of compression strength the surface has. Meaning, the particle will indeed be embedded into the surface and the key will be how far. Too far being failure.
Or am I off here?

 

[rb1957]

ok, I'm stupid !

something that weighs 1kgf has a mass of 1kg, something that weighs 1lbf has a mass of 1 lbm.

particle weighs 7E-11kgf, mass = 7E-11kg, energy = 2E-5J

(still wonder about the 750m/s though)

"the amount of distance the particle will travel into the surface to ultimately equal the amount of compression strength the surface has" ... i don't think so; i think the KE of the particle is transformed into strain energy of the plate (and particle).
since this is an FE forum, i'd've thought that you'd be doing an impact analysis with MARC or similar.

Quando Omni Flunkus Moritati

 

[rb1957]

of course though, doing FE on something so small raises questions about the material of the plate. with a particle of 0.001" diameter grain boundaries are going to be important, probably the assumption of homogeneous material isn't valid.

Quando Omni Flunkus Moritati

 

[IRstuff]

re: slug

You're right, my bad; hopefully, that's my only senior moment this week. At least, Mathcad didn't let me down.

TTFN
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[GregLocock]

Back to the fundaments of the issue

Imagine a small lead pellet hitting a 12 inch thick case hardened steel plate. The end result is a thinnish disc of lead and no change to the plate.

No slowly increase the yield stress of the pellet, and decrease that of the plate. At some point you will start to see plastic behaviour of the plate. Hertz's formula will give you a good idea for that. Up until that point most of the energy in the collision is taken in plastic deformation of the pellet.

Cheers

Greg Locock


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