Tensile Strength

[Dude]

Spoke with my friend's father today about steel and bending materials. He was trying to explain to me the tensile (sp?) strengths of steel, bolts, and stuff. He's a machinist so he knows that stuff like nothing but I was wondering how tensile strengths could relate to the yield strengths that we follow? He kept referring to Rockwell chart or something like that for the tensile strength. Seems like that could be a fairly accurate gauge of stock strength.

He was also telling me about the different types of steel above CRS, like Tool Grade steel (i think that was what it was). Anyone ever have a chance to try any of that?

[gripmaniac]

A while back I found the following links were really helpful in understanding the difference: Tensile and Yield

Obviously Yield Strength is the way to quantify how hard a particular piece of steel is to bend. . . the problem is most steel manufacturers rate in terms of Tensile Strength

I haven't tried Tool Steel yet. . . but I think there are a few board members who have. Tuff stuff apparently. . . .

Dave

[supermagnamon]

i'm lost for words.

[steelbiceps]

The Rockwell hardness test involves indenting a piece of metal with a diamond-pointed pressure device. It's very useful for measuring the strength of, say, knife blades, which have to resist high point pressures, but not much use for measuring bending.

Bending geometry isn't so simple. The strength of a bar depends quite specifically on how it is loaded. Overhand and underhand benders apply force to the bars very differently, and an engineer modeling the deformation of a bar would use very different formulas for the two situations. Also, a bar's strength changes as it bends, and different materials and beam shapes will change differently as they distort.

For gauging how hard it is to bend a material, yields strength is the key criterion. It measures how much stress a metal can take before it bends plastically (doesn't bend back all the way after stress is removed).

For measuring a bolt's tensile yield strength, they simply use machines to stretch a bolt until it gets longer plastically (doesn't return to normal length after releasing tension).

It so happens that metals have the same tensile yield strength as compressile yield strength, which is convenient for analyzing bending. When you bend a bar, you put the outside of the bend in tension and the inside of the bend in compression. For a basic bar, the tension on the outside of the bend is equal to the compression on the inside of the bend. The mid-line of the bar, called the neutral axis, is niether in tension nor compression.

Although the geometry is where the equations get ugly, bending strength is directly proportional to yield strength, so if you have two bars of the same dimensions but one with double the yield strength of metal, that one will take double the force to bend, no matter what technique you use to bend it.

As for measuring metal strength, you need to decide what to measure. The below web page has a typical stress-strain curve graphed. Stress is the force per cross sectional area, and strain is the amount the metal stretches. At first, the strain increases proportionally to the stress. At a certain yield stress, the strain becomes plastic and the metal has a permanent stretch. You can keep stretching it, and it will keep getting harder to stretch, but every bit further you stretch it is a little more that it is permanently stretched. At the ultimate tensile strength, the metal is at its peak resistance to stress. If you stretch it further, it will start to get weaker and weaker until at the breaking strength it snaps.

http://www.ndt-ed.org/EducationResources/C...cal/Tensile.htm

Hope this ain't overkill. I'm a mechanical engineer. But I'd be happy to show you some bending calculations from my textbooks if you're that interested. Incidentally, the gold standard reference for beam strength calculations is called "Machinery's Handbook". You can find used copies at bookstores or online (expensive new). Your friend's father might even have a copy at the shop. Look up "Bending, stress in beams" in the index and you'll be able to see diagrams for lots of loading geometries for beam bending, and you'll see what variables go into those formulas. (It will be the young's modulus, or stiffness, and the yield strength.)

[GatorGrip]

steelbiceps

This was a very enlightening post you have written and I value the knowledge you took the time to share.

I anticipate your future posts will be valuable to us benders and gripsters but most of all have fun on the board.

GATOR

[gazza]

Thanks Steelbiceps.

[Dude]

Stan great post man!

What you basically stated in that post was what we was explaining to me. But he was saying stuff so fast I couldnt keep up with him.

Would the Rockwell testing have any relationship at all to bending? He gave me the numbers of like a G8 bolt around 38-42 (I think, but just giving an example) and I thought he told me that you could be somewhat accurate at which a bolt, metal, etc... would snap because it would become brittle.

I didn't realize it but he was explaining to me how a lot of hardware store hardware is essentially "scrap" metal alloyed together, kind of like scraps thrown together in a hotdog, lol. He has to order his steel from a certain place so he knows exactly what its composition is, so he's not drilling and hit a weak/hard spot, etc... This would explain how varied a lot of bending materials are, like 60ds feeling easy and then feeling like a g5.

Is there any information to reference when buying steel to look for to know its held to certain standards as far as being consistent? When I went to the Metal Supermarkets they copied me a sheet of a bunch of information about the tensile strength of the steel I bought but I threw it away because it resembled Hebrew to me, lol. But maybe there was some info in there I could've referenced to tell? If thats possible I would see it being possible to research a certain company's bolts/steel and have constant strength to them.

Eric should be in here soon, he's good with the yields stuff as well:)

[steelbiceps]

Rockwell hardness numbers will loosely correlate with bending strength, mostly because a higher quality bolt will probably be stronger in all ways. The Rockwell number, in particular, is probably most valuable for bolts for telling how resistant the threads are to being over-compressed or marred. Good bolts should also come with tensile yield criteria, ultimate tensile strength criteria, and shear criteria, and stiffness criteria.

Plus, machinists pay for other qualities in a bolt that benders don't care about -- non-corroding metals, lighter weight bolts for racecars and aircraft, higher dimensional tolerance bolts to fit high-tolerance holes snugly, temperature resistance, fatigue resistance... High quality bolts have features such as a rounded transition from the shank to the head. Wherever there is a sharp edge, a crack is much more likely to start, so high-quality bolts are smooth if you look at them under a magnifying glass. These bolts can't be modified. If you buy an expensive 6" bolt for your racecar and, deciding you want a 5" bolt instead, cut it shorter, you've ruined the bolt by damaging the smooth finish, heating it with a saw, or deforming it with bolt-cutters.

The sheet they gave you may have had info on manufacturing consistency, which sounds like it's of interest to you. If you can get your hands on another of those sheets, I'll help interpret it.

[Dude]

When I go back there soon I'll try and scan and post it up