Understanding the Ultimate Strength of Steel

[BEggleton]

I have neither physics nor engineering backgrounds. I am trying to figure somethings out, and I need some basics cleared up. I will pose my questions as such:

1.) How would you go about rating the strength of steel?
2.) How would you go about testing the strength of steel? For example: is it tested in units of cubic inches?
3.) What is the strength of steel?

 

[uart]

Strength of materials like steels are usually measured in units of force per unit area. This is the same basic unit as pressure, so you may use pounds per square inch (PSI), but preferably use the SI unit of Pa (or MPa and GPa for strong materials like steel). In material science this "pressure" is referred to as "stress".

As you increase the stress on a material it deforms (for example in the case of tensile stress it elongates). Up to a certain point if you remove the stress the steel "springs" back to it's original length without suffering permanent deformation.

Two very important ways of categorising the strength of elastic materials like steel are,

1. The maximum stress you can apply without causing permanent deformation, this is called the "yield stress".

2. The maximum amount of stress you can apply before the sample breaks completely, called the "ultimate tensile strength".

For example, structural steel has a typical yield strength of about 250 Mpa and an ultimate tensile strength of about 400 MPa (corresponding to approx 36000 PSI and 58000 PSI respectively).

See: http://en.wikipedia.org/wiki/Tensile_strength

 

[FredGarvin]

1) The standard tensile test is the benchmark for basic strength properties.

2) See above.

3) There are so many alloys and ways of heat treating that there is no one answer to this. Looking at the bare bones, annealed state 1010 or 1018 plain carbon steel, you are looking at the values stated by uart. Understand that there is a lot of variation.

 

[minger]

2. The maximum amount of stress you can apply before the sample breaks completely, called the "ultimate tensile strength".

That's not true. The Ultimate Strength of the maximum amount of stress that can be applied to a material, period. That is different than rupture, which is the actual breaking point. We run parts above ultimate (carefully of course) for very limited life applications occasionally (usually just demonstrator engines as well).

 

[dr dodge]

They actually "verify" the calculated values of specific samples with machines that have mechanical or hydraulic presses on them. They will give (usually) readouts in force and distance. Some can be programed so they vibrate between force or displacement set points. The biggest one I ever cal'd was in San Diego at the old Convair plant. I think it was 500,000 full scale, stood about 25 ft tall (it was an old Tinius Olsen) when those machines break something its usually pretty loud, too
(I always envied the guys who got paid to break stuff)

dr

 

[Mech_Engineer]

That's not true. The Ultimate Strength of the maximum amount of stress that can be applied to a material, period. That is different than rupture, which is the actual breaking point. We run parts above ultimate (carefully of course) for very limited life applications occasionally (usually just demonstrator engines as well).

I'm not sure what you're arguing here, other than trivial semantics? The ultimate strength of a material is comonly known as the stress as which it will break...

 

[timmay]

As minger says, the ultimate tensile stress is the maximum stress reached in a test. For a ductile material that necks considerably, the UTS will be higher than the stress at failure or rupture as the cross sectional area begins to decrease. Note that this is only the case for engineering stress, as true stress is calculated with the instantaneous cross sectional area.

Incidentally, we still have a 2.5 MN Avery large frame testing machine. It doesn't get used much, but we're replacing it at some point with a slightly smaller Instron of the same capacity. There aren't many of those about to the best of my knowledge - the most impressive use I've heard of for it is aircraft landing gear.

 

[minger]

I'm not sure what you're arguing here, other than trivial semantics? The ultimate strength of a material is comonly known as the stress as which it will break...

It's not trivial semantics at all. There is a difference between limited life and breaking. If I say to take a bar and bend it in one load application until it breaks, well it's pretty clear when that breaking happens. You had one bar, now you have two.

That is completely different from reaching the ultimate stress limit (well, for ductile materials anyways). There is lifing (while very limited and unpredictable) at ultimate stress.