Originally Posted by Cooter
Those numbers are strength before breaking, they say nothing about bending, twisting, or shock loads.
I can tell you the larger the shaft diameter the less it will be.
Theta = TL/GJ
Theta = angle of twist
T = torque
L = lenght
G = modulus of elasticity
J = polar second moment of area^2 = (pi/2) X (r^4,0 - r^4,i)
Keeping the torque, lenght, and modlus (material property) constant the only thing to change is J. This value is dependent on diameter. The larger the diameter the larger J is. So all others constant a number divided by a larger number yeilds a smaller number, thus the angle of twist.
Also the way to test materials is the failure, this gives you the proper stress VS strain graph to get the modulus of elasticity, modulus of rigidity, as well as other material or part properties.