Force to flex alumimum vs steel

[dknguyen]

Hello. I know steel is harder than aluminum, and I know that steel is able to flex more than aluminum before deformation and/or breaking, and the strength to permanently deform and break steel is higher than aluminum.

Steel is able to flex more than aluminum before breaking making aluminum more brittle by comparison, but does steel flex more easily than aluminum? For example, with fudged numbers, it might take 75kPa to permanently deform a steel bar, but it only takes 25kPa to deform an identical aluminum bar. My question is that if a force of 10kPa is appled to either bar (not enough to deform either material but enough to make it flex) which material will flex more given the same force?

Like, if I wanted a sheet metal robot leg (equal dimensions and thickness regardless of material), and I made it out of steel it would be "stronger" in the sense that it would be harder to permanently deform and outright break, but under forces that would not deform either material, would steel flex more than aluminum?

The problem is I am not sure of the name of this property like I am of tensile, yield, or shear strength, which all have to do with the forces to make the material pass deform or break. Is it called flexure? rigidity? stifness? I am not sure of the technical term to google.

Let's assume typical stainless steel 304 or 316 and typical aluminum 6061 or 6063, and remember we are talking about identical objects- not a steel object and an aluminum object designed to give the same strength (breaking and deformation) as steel, since this will cause the aluminum object to be stiffer than the steel object since aluminum is more brittle in the first place.

Intuitively, I think steel would bend less for a given force than aluminum, and at the same time could bend more wiithout breaking (best of both worlds, beats aluminum in both), but at the hobby store where I couldn't flex the sheet of 6061/6063aluminum but I could do so very easily with the 304 stainless steel (but it was in thinner sheets). But even when I stacked the sheets up to make the same thickness I could still flex it a bit (although I am not sure if this was because the sheets could slide against each other).

I have a sneaking suspicion that I may be separating "maximum possible flex before deformation" and " "ease of flex" into two different properties, when they may be the same thing- but that doesn't seem to make sense to me right now because it doesn't seem logical that the thing that can flex the farthest without deforming is always the one that is the hardest to flex.

 

[hjames]

Is this what you want?

http://en.wikipedia.org/wiki/Young's_modulus

You can use this along with moments of inertial to calcuate an effective spring constant...

The other thing that you are talking about tends to be called "toughness". There's something called a Izod notch test where they measure the amount of energy required to cause the failure of a test sample. Alternatively you can just integrate the force over distance to stretch a test specimen to failure.

James

 

[dknguyen]

Yeah, I just learned after typing the post that toughness is the difference in forces between yield (technical term for permanent deformation) and failure (breaking). Steel has a larger range so can be bent more before it breaks.

THe article you provided sounds about right. Thanks.

 

[hjames]

"toughness" is usually put in quotes because there isn't any agreed upon unit/condition. Just keep in mind toughness usually is determined by work - force over distance, so something that is tough requires a lot of energy (joules) to deform it - high force, short distance is equivalent to low force, long distance. The "toughest" materials are ones that require high force over a large distance to break - i.e. require the most joules to break. For example, I think Lexan is one of the tougher engineering plastics, but the stuff scratches (plastically deforms) very easy.

 

[mechalloy]

The property of "flexing" is referred to as Youngs Modulus, Modulus of Elasticity. or Bulk Modulus. Basically it is a measure of the stiffness of a material and can be modeled by assuming the atoms that make up a material are on springs, a stiffer material will have stiffer springs. Therefore a material with a higher modulus will be stiffer. Steel has a much higher modulus than aluminum so it is much stiffer, which means it will take more force for the same thickness to bend it the same amount. By the way the modulus refers to only the elastic movement of a material, which means the material bends but returns to its original shape, if you bend it too far you exceed what is called the Yield Strength and the material permanently changes shape. Hope this helps.

 

[dknguyen]

Dead thread! But yeah, I have somewhat better understanding of it now. The force required to deform something a certain amount and the amount of deformation before the structure becomes plastic are not related to each other.

So you could have something that takes a lot of force to deform, but if it deforms just a little bit it becomes plastic like a strong but brittle material. Or something that that takes a small amount of force to deform but can deform a lot, like a sheet of rubber or something like steel or CF which can deform a lot for the amount of force required. All unrelated.