How to calculate torsional stiffness?

[dpettersson]

I want to calculate the torsional stiffness of a cross member and compare two different materials. So i put Steel as material and enter the values:

Material 1

Tensile Yield Stress: 460 MPa

Tensile Ultimate Stress: 670 MPa

Material 2

Tensile Yield Stress: 550 MPa

Tensile Ultimate Stress: 760 MPa

After this, I do two simulations, one for each material. When I later open the Measures dialog box it shows that the "max_rot_x" is the same on both crossmembers (the rotation is in x-direction). This doesn't make any sense to me. The cross member with the better material quality should have a higher torsional stiffness than the exact same cross member with less quality, right?

Is there any other way to calculate the torsional stiffness?

See the pictures below

[JonathanHodgson]

Daniel,

Similarly to the reply to your other thread, the yield stress and UTS of the material do not affect its behaviour in the linear, elastic region - i.e., assuming that the load does not begin to fail the material.

These values only affect the maximum load that can be applied before the material begins to fail.

Only Young's Modulus will have a significant effect on the structure's stiffness.

[dschenken]

Next up he'll switch to titanium and wonder why it deflects even more than steel, when everyone knows titanium is always stronger than steel. Which titanium isn't; it is actually only as strong as some steels, but is about 2/3 the density, 2/3 the modulus**. Titanium also maintains its properties to higher temperatures than steels do, which is the main reason it is used to cover the SR-71.

It's a shame not knowing the engineering definition of the word stress. Where in the Google could such information be found?

** A plot of density vs Young's modulus shows that many metals fall nearly on a line; that is, the ratio of density to Young's modulus is a almost constant in comparison to non-metals. One standout material is Boron, which is exceptionally stiff for its density. However Boron is also extremely weak, hence subject to brittle failure.

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

Picture http://aluminium.matter.org.uk/content/html/eng/default.asp?catid=151&pageid=2144416669