Breaking Iron (calling engineers)

Factual Questions
1

Okay, let say somebody is dumb enough to try to break a sheet of iron with his bare hand (see MPSIMS… I tried a couple of years ago).

Assume for a moment that the iron is 1/8" x 6" x 12". Assume that the area of impact is 2" x 2" (palm of the hand).

Now, somebody out there certainly has a list of the “strength” of iron. So, how many force/PSI or whatever measurement you want to use would said dummy have to exert to break the iron in two!

Now, can somebody also tell me how much of the same measurement would be required to break 6 concrete slabs (2"x6"x12") stacked with no space between them (the concrete is not baked to be brittle, nor reinforced with little bits of fibreglass, just standard concrete slabs).

I am curious as to the difference in force required between the two so I can gauge just how dumb I really am.

Thanks for your assistance.

What more could you expect from somebody who lets people kick him to the head?

2

Normally metals tend to deform rather than break. While I personally wouldn’t hit either iron or concrete with my hand, if I did so with a sledgehammer I’d expect the concrete to break whereas the iron would just bend.

3

Cast iron is brittle, rather than malleable, and will break instead of bend. Steel or wrought iron will break eventually but will deform considerably before doing so. I’ll see if I can remember enough of my strengths of materials class to do some sort of calculation on breaking a cast iron plate of the specified dimensions. No guarantees though – it’s been a long time.

“And comb London’s teeming millions for him? Had we but world enough and time.”
Dorothy L. Sayers
Murder Must Advertise

4

I can calculate the stresses easily enough. No big deal there. Unfortunately I don’t have strength data for the desired materials. I’ll see what I can round up, though. How about low-grade carbon steel? In the meantime, anyone have any ultimate tensile strength data for concrete and iron?

5

Here is a strengths table for you, don’t know how to do the figuring though…
http://ourworld.compuserve.com/homepages/MJVanVoorhis/T004.htm

6

All right, that pisses me off. I was 3/4 of the way through this and then my connection shut down. I’ll try again using Word as the text editor, dammit.

I’ll go ahead and check the required load to break the iron plate, assuming that it is ASTM no. 20 cast iron and using the link provided by funneefarmer. To calculate the loads for other materials, just multiply the calculated load by the ratio of the material ultimate tensile strengths.

Ultimate tensile strength, Ftu = 22000 psi

From Table 26, Case 1c of Formulas for Stress and Strain by Roark and Young, for a simply-supported rectangular plate,

<font face=“Symbol”>s</font> = <font face=“Symbol”>b</font>W/t[SUP]2[/SUP] = Ftu in this case

where

W = the impact load applied by Glitch’s fist

<font face=“Symbol”>b</font> = a function of the plate width, a, the plate height, b, the contact area width, a1, and the contact area height, b1. Referring to the <font face=“Symbol”>b</font> table for this load case and Glitch’s dimensions and performing a couple of interpolations gives a value of <font face=“Symbol”>b</font> = 1.3317

t = the plate thickness = 0.125"

Based on these numbers, I calculate an impact load of W = 258.13 lb. Feel free to check my math here.

Assumptions:

The <font face=“Symbol”>b</font> table assumes that Poisson’s ratio, <font face=“Symbol”>n</font> = 0.3. This is valid enough for iron but probably not for concrete. Unfortunately, I don’t have the text that the authors referenced to create the <font face=“Symbol”>b</font> table. Can any civil engineers help here?

This analysis assumes linear material properties (i.e. no plasticity). From what pluto was saying, this is probably close enough for iron.

The analysis assumes that the plate is supported right at the edge. Obviously in real life there would be some overlap.

7

Anyone ever see a chunk of cast iron as a simple slab? It tends to be used for more complex shapes, like fireplace tools or decorative objects. Unless it was some scavenged part from a potbelly stove, I think that slab was a lot less brittle than cast iron.

Glitch, can you tell us where you got the plate?

Laugh hard; it’s a long way to the bank.

8

Better yet Glitch,
Post a photo of the broken plate (and one of your knuckles afterwards) after you attempt the break. I’ve studied some martial arts myself and I’ve never heard of it being attempted. Frankly you’ve piqued my curiosity.

9

Dang it, I re-read glitch’s post and discovered that he tried this already. Not that he was contemplating an exotic break in the near future. After I went and got all excited…

Perhaps you could try thin walled cast iron drain pipe. I use to work with it and it breaks quite cleanly with the proper tools. Or not so cleanly with a claw hammer. Now that I think of it, I dont recall having to apply much more force than to break a brick, and Ive broken a few of those with my hands… The clanking noise as the pieces hit the floor would make for an impressive demonstration.

10

The plate was part of a ramp that was being thrown out. We took the ramp and cut out a piece from it with a acetylene torch.

I have a picture of my hand afterwords (in a cast) and I also have a picture of the plate (slightly dented/deformed but definitely not broken). I’ll see if I can get them scanned (I don’t own a scanner) and post them.

What more could you expect from somebody who lets people kick him to the head?

11

Interesting idea with the pipe. I’ll give that a shot and let you know how it goes.

Somebody call the ER and let them know I’m coming (see MPSIMS “Stupid things you, or I, did”. :slight_smile:

12

Good point AuraSeer. Thing is, that’s the only material data I had to go on at the time. Given the ductility (i.e. it dented but didn’t break), I would have to do a plastic analysis to determine what kind of impact load the mighty Glitch would have to apply in order to break through it. Anyone got a stress-strain curve for iron plate? I bet the materials guy here has one.

Incidentally, I think I screwed up the boundary conditions on this (although it may be a close enough appromixation). How did you support that thing, Glitch? Cinder block at each end?

13

Yes, but we put iron plates (from the same ramp) on top of the cinder blocks. We were concerned that if we put the iron sheet on the blocks directly we would either have to have a lot of overlap or the blocks would break at the edges. With the metal plates covering the top of the blocks the pressure on the cinder blocks would be more uniform. That part at least worked since the blocks survived quite nicely. IIRC, the overlap was between 1/4" & 1/3".

14

And here’s the data for concrete.

Ultimate Tensile Strength - concrete, Portland, 1 yr. old - 400psi from Machinery’s Handbook, 25th edition, 1996.

Dopeler effect:
The tendency of stupid ideas to seem smarter when they come at you rapidly.

15

BTW, they do make ductile cast iron, including cast iron pipe. So don’t assume that just because it’s cast, it’s brittle.

Don’t read this unless you care how they make it:

Cast iron has a lot of carbon in it. Most of the carbon is not dissolved in the iron but exists as flakes between the iron crystals. It is these flakes that make the iron brittle. Ductile cast iron has additives that persuade the carbon to form spheres rather than flakes. It will still break eventually but the spheres are much less likely to start a crack.

You can make steel brittle, as well. There’s a general relationship between hardness and ductility. Very hard steels (think of a file) tend to be brittle. They are a lot stronger than cast iron though.

You can even make a steel tool hard on the outside and ductile (i.e., tough) on the inside. That’s what “tempered” steel is. A steel punch for example. You want it to be harder than what you’re punching but you don’t want it to break. You do this by controlling the rate of cooling of the hot metal. Steel that cools slowly is softer and more ductile. The physical reason is based on the microscopic formation of the iron-carbon crystals that form.

“And comb London’s teeming millions for him? Had we but world enough and time.”
Dorothy L. Sayers
Murder Must Advertise

16

Whew!

17

Ok verdicts in. I found and old piece of cast iron drain pipe lying in the plumbing cast-offs. It was 4" diameter and about 20" long.

The first two attempts to break it were unsucsessful. (half hearted attempts I admit, I read the post in mpsims) So I went at it with my handy dremel too. I split the pipe lengthwise so I had 2 cove shaped pieces. Both broke with a resounding snap on the first attempt. (I used a palm heel, downward stroke.) I only received a small cut on my forearm while breaking the second.

If I had to compare it to something I would say it was a cross between ice and either tile or brick.

18

This all sounds like “shear” stupidity to me. So the units of the parameter Glitch is looking for are probably IQ points on a negative scale.

It seems to me, any kind of iron is going to have some ductility and thus you need to specify an impact, not just a force, i.e., how abrupt the force is applied. And I would assume you’d have to specify exactly how the impact is applied with respect to the particular mounting, in order to relate it to a pure shearing impact.

Ray (ex-EE. That ME stuff is just TOO crude. Now, let’s see; how many megavolts of lightning potential can I tantalize and jump out of the way of fast enough to survive? Oh, that was a past thread, sorry.)

19

Ray, that was uncalled for. Many people spend years developing skills that push the body to it’s limits. To just call us stupid outright for practicing centuries old artforms is, well… insulting.

Do you harbor these same feelings towards professional athletes? How about scientists who work with dangerous chemicals? Motorcyclists? Or are you just one of those people who see someone doing something they could never understand and dismiss them offhand as idiots?