Curvature Of Bent Steel

[EricMilfeld]

I was attempting to bend some drill rod yesterday that went well past my usual sticking point of 90 degrees before it ceased moving, when I noticed the rather large arc that had been created. You've probably noticed that some steel bends more in the shape of a sweeping "U", as opposed to a tight "V". Naturally, the thicker the steel (provided all othe factors are equal), the greater the diameter of the arc at the base of the "U". But there seems to be another factor at play in determining the "shape" of the bent steel. I was curious if harder steel tends to form a "U" or a "V". So when I arrived home tonight, I took a look at various pieces of steel I had bent of the same thickness. My conclusion is there is no direct correlation between the steel's strength and the breadth of the arc. I found in my collection of mangled nails and bolts, easy stuff in tight "V" formations as well as loose "U" formations. And with harder steel of the same dimensions I discovered an equally diverse array of formations. So perhaps a metallurgist can tell us what that other unknown (to me) factor is. It also dawned on me that we are probably loosing a great deal of leverage when a piece of steel bends in an excessively rounded-over shape, due to the legs of the steel being shortened as they become part of the sweeping arc. So not only can the strength of the steel itself, as well as it's length, affect the difficulty of a bend, but also the "shape" the steel assumes as it is being bent. I believe this is another reason the drill rod is so dang difficult.

[mobsterone]

Nope I didn't understand a word. Will any of this help me?

[mANVIL]

Eric, I've been thinking about that too. I remember reading a post recently (might have been on the ironhistory or natural strength boards) that talked about harder steel bending in a V pattern then some other people posted back that they had the opposite experiences. Do you think that maybe the V vs. U shape of the bend could have something to do with either your hand spacing or the quickness of the bend or at least the quickness of the kink (with a fast bend in a V shape and a slow bend more of a U). When you bend fast you have one small piece of the metal that stays "hot". A slow bend you are more or less starting and restarting which allows things to "cool off" giving you no single point in the bend but rather a general U'ing. (not sure if any of this is making sense).

[OldGuy]

I have noticed that my training partner Mmainlands who has bent hundreds of nails and bolts up to a 6'' grade 5, not only bends everything in a tight U but the sides of that U are perfectly equal. Only his grade 5 bolts come out V shaped. If he finishes a nail that I have failed to complete, it will come out with a tight U and sides equal, even if I bent it to 90 degres with one side 1/2'' longer than the other.

[Mikael Siversson]

My experience is that there is a fairly strong correlation between the strength of the steel and the shape of the arc. It is more V-shaped the harder the steel at a given diameter. Eric is right about the shortening of the ends on bars that make a broad U-shape. This is why that notorious 8mm HRS from David Horne is so hard to finish once you get down to 5 1/2'' and below. The steel is softish and easy to kink but the leverage drops off very quickly with decreasing length of the bar, as a result of the arc taking up so much of the length of the bar.

[EricMilfeld]

Steve, I'm still searching for a way to help you. I'll get back to you.

Matt, I'm not really sure about hand spacing affecting the outcome, as the steel I use for comparison was all bent wtih identical hand spacing. I think the speed with which the steel is bent could possibly be atleast one factor affecting the outcome.

OldGuy, that's just plain freaky about Mmainlands being able to take an asymmetrical bend of 90 degrees and bending into "even-leg" formation. Just exactly what kind of guy is this Mmainlands?

Mikael, yes, perhaps the strength of the steel is just one the factors that determines the tightness of the arc, but like OldGuy noted with the graded bolts, there are exceptions. Definitely an interesting point to ponder for us steel nerds.

[Thaibox]

Well Eric, you made me sit and stare at my wall o' bends wondering about this. The widest U was a very easy galv60. The sharpest V's seemed to be a grade 6" 5 and a dark 60common. All the challenge bars I've bent are u'd equally from the 212 to the 245.

I can't see a correlation between difficulty and shape of the bend. I would think its the material itself that would determine that. Just an uneducated guess on my part, but the only thing I can think of.

[EricMilfeld]

I can't see a correlation between difficulty and shape of the bend. I would think its the material itself that would determine that. Just an uneducated guess on my part, but the only thing I can think of.

This is kind of what I'm thinking. I wonder what it is about the steel itself, if it's not the strength, that creates these differences in arc. When you stop and think about it, there are a lot of little strange charactersistics with the various types of steel: some starts the bend tough and finishes easy, some does the opposite, others "give" throughout the bend with no flex, others "fight back" and are quite springy, and some steel seems to really harden-up with the least little hesitation in the bend, while others seem to be almost entirely unaffected by "cooling off". Such a beautiful and diverse world is that of steel. Oh, what do you know, I'm late for work.

[mmainlands]

Good term Eric - "steel nerds"! I think we should all embrace being one.

[AP]

Sometimes I can make a bigger "U" by moving my hands slightly during the bend. I like a bigger U as it seems to make the crushdown easier for me, don't know why, just my experience. Grade 5s don't seem to tolerate this nonsense though because they are so hard at the kink that I take abend anyway I can get it.

[mrzero]

After I changed my bending style (going from underhand at the waist with just wrist strength to underhand at face level) all my bends went from a V to a U with the puny 3/16ths I'm bending.

[MikeP]

I think it may have something to do with the width/thickness of the fulcrum. If you used a 2" piece of pipe to bend the rod against vs the sharp edge of a vice, you might get a different radius. I have bent some things wide but most are tight v's. I have some 5/16 HRS that always gives a nice gradual U and I bend it the same way as my others. I think it may also have to do with the orientation of the fibers in the metal. If they lay following the length of the steel it probably bends tighter but if they are eccentric rings running the length of the steel it probably gives a bigger U. I don't know much about steel, I just dreamed this up after watching a deal on the discovery channel and about how the Fins were making some high grade steel for some ship or something and they were testing how the metal itself was laid up and all - had effect on the bending/forming of the metal. Hmmm. You say it was drill rod? Maybe it was formed/extruded differently because it is designed to handle different forces. Bits are designed to handle torque but most steel would be made to handle bending. Sound reasonable?

[Clay Edgin]

Yup, soft nails make a wide U. Hard nails make a sharper V

[Dan Cenidoza]

Hey guys,

below is the response I got from a friend who is actually going for his PhD in metallurgy, or some related field. Interesting to say the least...

----------------------

After a lengthy scholarly discussion (we'll bill you at $100 an hour) the

concensus is that, in general, a softer material will bend into a tighter

bend, or V, than a harder material. This is because the radius of

curviture in the tight bend, as opposed to a U bend, is much smaller,

therefore the material must be able to strain to a larger degree to

conform to this smaller radius. When the strength/hardness of a material

is increased, its ductility generally decreases. Therefore the softer

material will be able to strain much easier to conform to the tight

bend/smaller radius (in tension on the outside of the bend and in

compression on the inside of the bend.

Of course, the largest contribution to the way the bar/mail/etc. will bend

in the manor in which it is loaded. If it is easier to bend a certain

geometry of material with the hands by grasping by the ends, then the

preference will be towards a U shape, whereas if the bar is gripped close

to the bend then a more V shape will be obtained. So, even if the tendency

is for a harder bar to be bent into a U shape, if it is easier for the

strongman to bend with the hands close to one another, a more V shape may

be obtained. It then follows that if the common manor of bending softer

stock is with the hands towards the end, then a U shape will be the

result.

In summary there is so much variation in the specimen (bar) geometry,

threaded vs. non-threaded (stress concentration), and bending styles that

a sweeping generalization can probably not be made.

[JohnOBrien]

I'm not a metallurgist, I'm only a chemist, but I do know that I have some galvinized 60d's that always give a nice U shape and that everytime I bend a G8, it bends into a nice, sharp V. I haven't videotaped myself doing each, but I would say my technique on each is very similar if not the same. To me, that says that tougher steel gives the V and easier steel give the U....

[Dan Cenidoza]

My experience has been the opposite John. I have HRS and SS both at 1/4" diameter and the SS is much harder to bend and it yeilds a more pronounced U shape.

[Clay Edgin]

Dan, stainless is a different animal altogether. It reacts differently to being worked by hand, both with and without the use of heat (torch). Having pounded more stainless sheet metal into forms with a hammer than I care to remember, I can tell you that it does act different.

[EricMilfeld]

Interesting. Thanks for the updates, guys! Dan's statement about no sweeping generalizations being possible sounds right.

[MikeP]

I attended the OK Scottish Clan Gathering and they had some blacksmiths there. I bugged em for 2 hours and their feeling was it has more to do with the amount of carbon in the steel than anything. He says, even though 2 pieces might bend with the same strength and are the same size, the one with more carbon will bend into a tighter "kink" as opposed to a gradual "U". He also felt that the majority of "old timers" (this is w/o any knowledge of old strongmen, just old steel) were probably bending rod iron and not actually steel as true steel was too expensive and hard to acquire. We need to obtain some rod iron and see what happens. He made me a ring handle out of 5/8" round and welded it right there. It was cool, turned out OK (the wind was blowing hard and cooling the piece too fast). He also had a 135lb (approx) anvil which I could nearly lift by the horn right but was way off left. I could pick it up both and get a good hesitation at the top with just the right. I tossed it overhead just for fun. He had some spring steel too that was interesting, bent it about 30deg and it just bounces back perfect. He makes knives out of it. They don't know where to get 14" rail spikes but could get antique ones (12/10/6"). They said there group has anvil collectors with huge collections. Aaaah, anvils.