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[Noble Digger]

I ran into this very informative guide to the properties sought after in various metals for various purposes. This nicely explains what we look for in a metal or material for a particular purpose.

http://metalwebnews.org/machinist/ch2.html

"Rupture" or "Fracture" refer to permanent breaks in the crystal structure of the metal, across which forces are no longer shared and distributed. Basically, cracks in the metal which are permanent and reduce the strength of the piece even if they aren't complete breaks. Steel is made stronger by filling tiny gaps in the crystal structure of the Iron with Carbon atoms which reduce the "straight line" crystallization of the iron, which is very weak. Effectively the carbon atoms act to cushion the interior structure of the iron so that the atoms are less likely to disconnect from one another when the piece experiences stresses. This presumably also plays a role in preventing deep rusting of steel. Once oxidization of the surface of the steel creates a patina, there's really no way for the oxygen to access further iron atoms deeper in the crystal structure.

Add your thoughts, corrections, musings, etc, please!

[PMantix]

Basically, cracks in the metal which are permanent and reduce the strength of the piece even if they aren't complete breaks.

I get what you're saying here, but strictly speaking it is not correct.

A crack (or sharp edge / direction change) in an object does not reduce the strength, strictly speaking. What it does is create an area of high localized stress at the origin of the crack (or sharp edge).

So yes, an object with a crack is easier to break than a similar object without a crack. But not because the cracked object is actually now made from a weaker material, but because the crack focuses stress over a very small area.



Also, rust on the surface of steel or iron does not form a patina. Rust continues to propagate into the center of the object, eventually rusting through completely. Just look at any old car with rust holes that go straight through for an example.

However, you can make an alloy of steel with chromium (stainless steel) that is more resistant to corrosion by making a "shell" over the object that protects the material underneath.

[mendonca]

I really enjoyed doing a module on materials science at university. Unfortunately I wouldn't be that confident on the absolute detail of the subject, but a few interesting things I learnt and still stick with me (probably completely wrong, but this is how I remember it):

One of the first jet planes was called 'The Comet'. It had square windows. The corners of these windows were subject to reasonably high stress concentrations. As the plane was going high up in the atmosphere, the internal pressure vs. the external pressure caused the fuselage to 'expand' and 'contract' in the course of a flight. Eventually this caused a large number of failures due to 'fatigue' reducing the strength of the shell in this area and leading to fractures. Unfortunately the windows popped a number of times and it was quite disastrous. Thats why windows are small, and rounded on a plane.

Steel undergoes a 'Brittle-ductile transition' at low temperatures. At normal temperatures (above freezing) steel behaves in a ductile manner and is generally quite 'tough'. At a certain point, at cold temperatures (i forget the point, probably somewhere just below freezing) steel switches to behaving more like cast-iron at room temperature (i.e. brittle). Unfortunately this phenomenon was believed to be part of the downfall of the titanic. Also as the hull was fully welded, the brittle behaviour of the steel led to a huge rip all the way down the side (the 'wing' of the hull acted like a continuous sheet).

It's funny how these seemingly great design decisions seem stupid with the benefit of hindsight.

The brittle-ductile transition thing would be great in cold weather forts in DF. You think your boys are hard, as they are clad in steel, but the macegoblins (in copper armour) literally smash their armour in to tiny pieces. Leading to swift deaths.

[Footkerchief]

Basically, cracks in the metal which are permanent and reduce the strength of the piece even if they aren't complete breaks.

I get what you're saying here, but strictly speaking it is not correct.

A crack (or sharp edge / direction change) in an object does not reduce the strength, strictly speaking. What it does is create an area of high localized stress at the origin of the crack (or sharp edge).

So yes, an object with a crack is easier to break than a similar object without a crack. But not because the cracked object is actually now made from a weaker material, but because the crack focuses stress over a very small area.

He said "reduce the strength of the piece," not "reduce the strength of the material."  I think you're jumping to conclusions about what he was implying.  Anyway, for DF purposes, effects at that scale can and should be considered changes in material properties.

[PMantix]

He said "reduce the strength of the piece," not "reduce the strength of the material."  I think you're jumping to conclusions about what he was implying.

No, I agree with his conclusions..  the object will fail under a lower input force.

My point is that the word "strength" is being used in a naïve way here (no offense). Saying the strength changed at all is where the mistake was made.

The strength of the object or the material will not change if there is a crack, sharp edge, or other flaw. What is changing is the stress in the object...  because the stress is greatly increased the object fails at a lower input force.

The strength didn't change, the stress did.

This is a basic fundamental idea of mechanical engineering (which I happen to have a degree in, btw  )

Anyway, for DF purposes, effects at that scale can and should be considered changes in material properties.

That may give you the same end result for now, but it's a sloppy way to make things work and it definitely isn't an accurate model of the real world.

Are you suggesting that the material properties of the entire object be adjusted, or only locally?

If you mean the later, I might agree with that..  but that's still like making an object weigh less by changing the gravitational constant instead of changing it's mass. (In your case you suggest changing the object's material properties rather than applying a stress concentration factor). Both methods will have the same end results.. but only one makes any sense in the real world (and Toady seems to care about that.. err, at least most of the time )

[Noble Digger]

I consider this sort of thing interesting for the construction of bridges and the like, and support pylons, etc. Say, if this game ever goes into huge detail with weight distribution and collapse, and involves the requirement of buttressing tunnels with lathing under some circumstances or what have you. (apparently the body of the earth is not a cold and static thing, and a lot of mines have to have their walls shored up to prevent gradual collapse of the sides--leading to collapse of the tunnel--as mining equipment vibrates through repeatedly)

For example, suppose you buttress a 3-tile-wide horizontal mineshaft with untreated wood beams and this is sufficient to keep it from ever collapsing. But then the tunnel floods a bit, and the wood becomes more pliable as it soaks through. It now supports less weight and the tunnel collapses the next time the code checks for a collapse. Hilarity ensues, for I couldn't even imagine what would happen to the water at the entrance of the mine if the mountain collapsed on the tunnel and pressurized everything.

I used the word "strength" to refer to the general capacity of the object to resist breakage or other traumas which reduce its effectiveness. Regardless if the source of this strength--or loss thereof--is inherent to the material's innate properties or changes to those properties caused by thermal expansion\contraction, or caused by any type of stress or fracture or absence thereof in the finite crystal structure of the metal. Any of these things do, in fact, "reduce the strength of the piece", referring to the macroscopically observable and testable metal object in question. I agree with others that you read too far into my simple words, and I think you're quibbling. No offense.

Thanks for the bit about chromium, btw, I've also noticed how iron 'flowers' out as it rusts, flakes off, and exposes more iron. I've also seen chromed objects such as trailer hitches which have eventually pitted and then rusted through, just about leaving behind more chrome than anything else. But I assumed these were chrome-dipped cast or pig iron of some sort and that the chrome had just worn off. Of course mixing chromium with the iron in a smelter is different entirely, I've never seen a stainless steel object with more than the faintest haze of corrosion on it...

[Footkerchief]

My point is that the word "strength" is being used in a naïve way here (no offense). Saying the strength changed at all is where the mistake was made.

The strength of the object or the material will not change if there is a crack, sharp edge, or other flaw. What is changing is the stress in the object...  because the stress is greatly increased the object fails at a lower input force.

The strength didn't change, the stress did.

Huh.  If "reduce the strength of the piece" doesn't mean "the object fails at a lower input force," what does it mean, exactly?

That may give you the same end result for now, but it's a sloppy way to make things work and it definitely isn't an accurate model of the real world.

Are you suggesting that the material properties of the entire object be adjusted, or only locally?

Locally, if the item system becomes advanced enough to support it.

[Noble Digger]

That would be awesome with breath attacks, e.g. Blizzard Men, Ice Dragons, or contact attacks, e.g. Ice Golem. You'd want a more ductile material for your armor or else it'd just be useless.

[PMantix]

Huh.  If "reduce the strength of the piece" doesn't mean "the object fails at a lower input force," what does it mean, exactly?

The trouble is the word "strength" in this context means material strength. It's convenient to talk about an "object's strength" as the amount of stress an object can take before failure. But objects don't have strength, objects have materials that have strength. Talking about an object's strength is a convenience, but it doesn't have any physical meaning unless you are actually talking about the material.

For example, say we have two swords made of the same material. Both swords are the same shape, only one has a crack. It is convenient for us to say that the sword with a crack has "less strength" than the sword without a crack because it will fail under less load.

But strength isn't a property of an object's geometry. Strength is a property of a material. And since they are made of the same material, they therefore must have the same strength.

Which brings me back to why a cracked object will fail sooner...  not because it has lower strength (a material property), but because it has higher stress (in this case, a property dependent on the object's geometry).

Take a look at this pic to see why the geometry causes high stress:

Spoiler (click to show/hide)

This picture represents an object in tension. The red lines are 'force lines' showing where the object is supporting the tensile force. When the lines are bunched up, the stress in that area is very high.

As you can see, the crack acts as a discontinuity. It forces a large portion of the load to be carried near the crack's edges, which have a very small surface area. High force and low area = big stress.

The key here is that the material strength anywhere on the object is the same as it always has been (Not exactly true, of course.. a crack may have cold worked or corroded, etc.). But it will still fail because it has more stress in a smaller area..  failure will begin at the crack because it is under high stress, making the crack bigger, making the stress higher, making the crack bigger, etc.

All that has nothing to do with the material properties (strength) and everything to do with the geometry of the object (stress).

So to answer your question, a crack does not "reduce the strength of the piece", it greatly magnifies the stress, which causes "the object [to fail] at a lower input force,". Maybe that's a trivial difference in a mathematical model, but they definitely aren't the same thing.

[Footkerchief]

Huh.  If "reduce the strength of the piece" doesn't mean "the object fails at a lower input force," what does it mean, exactly?

The trouble is the word "strength" in this context means material strength. It's convenient to talk about an "object's strength" as the amount of stress an object can take before failure. But objects don't have strength, objects have materials that have strength. Talking about an object's strength is a convenience, but it doesn't have any physical meaning unless you are actually talking about the material.

It has a meaning in colloquial language.  This is like you walking up to someone speaking Portuguese and telling them their English sucks.

[Noble Digger]

And yet, you are still wrong.

[PMantix]

It has a meaning in colloquial language.  This is like you walking up to someone speaking Portuguese and telling them their English sucks.

The meaning you are referring to isn't correct in any language.

Are you suggesting that science in Portuguese follows different rules in than in English?

Hell, it's worth a shot...

Eu ter desbloqueado o segredo da fusão a frio aqui em Lisboa!

...well call me a monkey's uncle. 

[Noble Digger]

The image you attached shows really nicely how the material stress is affected by a fracture, that white area in the middle is entirely disconnected (by appearances, due to tensile forces, though since it's an intra-substance fracture rather than at the edges, I'm not sure) I spent a long time playing Bridge Construction Set and this really hits home just what monumental forces those steel girders are supporting. To you and I they appear to be immutable, indestructible things but when they are only links in a massive structure made of many hundreds of such beams, supporting rush hour traffic and cables and a roadway, it makes pure sense how a minor defect like that could send the whole bridge to the bottom of a river. :|

Since weaponry is commonly made of a non-homogeneous material (folded steel) my expectation is that the layers, having different levels of strength vs various types of destructive force, ought to fare poorly at any specific finite test compared to a weapon made of a homogeneous material specifically suited for that purpose (for example, a support cable ought to have tensile and shear strength whereas structural steel needs compression strength and whichever it is that prevents a beam from bowing out sideways due to compression forces)  --If I'm confusing you at this point, my question is basically "what combination of properties make folded steel better for weapons than homogeneous steels?" and does this hold true for all weapons or only swords and knives which must be ductile enough to bend slightly under stress without breaking while still being hard enough to hold an edge and win hardness competitions vs. armor materials?

It seems like e.g. a warhammer has only the virtue of its weight and sturdiness to worry about, so you'd pick a hard and dense material that is ductile enough that shattering it is impossible.

[eerr]

This thread dearly makes me want to bring back the "adamant" versus "adamantium" in dwarf fortress.

or maybe it was 'adamantium' vs adamantine.

The point being, your semantics are useless to us unless you give us a complete indoctrination.

Which you did not.

Useless.

[mendonca]

my question is basically "what combination of properties make folded steel better for weapons than homogeneous steels?" and does this hold true for all weapons or only swords and knives which must be ductile enough to bend slightly under stress without breaking while still being hard enough to hold an edge and win hardness competitions vs. armor materials?

It seems like e.g. a warhammer has only the virtue of its weight and sturdiness to worry about, so you'd pick a hard and dense material that is ductile enough that shattering it is impossible.

I suspect the folded steel issue is a real boon for things like swords, maybe not so necessary for hammers.

My take on it is that a single homogenous lump of metal is likely to crystallise in to something that is fairly weak and brittle.

It would be like if you tried to use a machined sand-cast sword, it would break in a second.

If you were to use a drop-forged sword, on the other hand, it would not be likely to break so quickly. This might still be a single lump, but the drop forging process (simply speaking, I may not be entirely correct here) 'lines-up' the structure of the metal. You then end up with something that instead of operating like a lump of spheres clinging together, ends up acting more like a collections of pieces of string.

Its far more resistant to microscopic defects, and stays together better under duress. I don't suppose it would be any harder just by the folding process, this would be more to do with how the metal is cooled I suppose.

If Dwarves could drop forge hammers, this would probably give them a serviceable weapon.

The 'Hatturi Hanzo' many-folded samurai sword though, comes in to it's own with a sword, as you have a long element prone to snapping with the tiniest imperfection (high moments as contact tends towards the end of the blade). The folding probably leads to a more careful imitation of the drop forging process, without needing a 6 tonne weight to smash down on to a hot bit of metal.

There's probably something about hardness control, and bashing out impurities as well, with the constant hammering under heat.

Not sure if this really makes sense or represents fact, but these are my musings. So take them as you will.