What is torsional modulus, and where is it specified?

[jamesson]

TL;DR Summary

The attached site seems to use a quantity Torsional Modulus for which I cannot find neither the formal definition or a material spec

In the equation for minimum link length here

https://www.defproc.co.uk/analysis/lattice-hinge-design-choosing-torsional-stress/

he uses a quantity Torsional Modulus, denoted as G, which I cannot seem to find either a definition for or any kind of expected value. Shear modulus (also commonly denoted as G) is defined and published in multiple sources for the material he uses (acrylic). Google says it is 1.7GPa.

Now, from his description it seems fairly clear what he means by G and how he uses it. I tried to backsolve for G and this is what I got;

So, either my calculation is off or he's using some source I haven't found.

Thanks so much in advance for any assistance

Joe

PS - sorry if this belongs in materials, if so please let me know and I'll move it.

 

[FEAnalyst]

This must be the shear modulus. Your result is $2.08 \ GPa$ so not that far from value known from literature. Author of this article could have used slightly different shear modulus in his calculations than the value you’ve found in literature.

 

[jamesson]

Thanks so much. What's your source for shear modulus, please?

 

[gmreit]

It’s calculated through testing like young’s modules but instead of relating stress to strain the shear modules relates shear stress to shear strain through a torsion test. Most materials that are isotopic like steal had a young’s to shear modules relation, G=E/2(1+v) where v = poison ratio.