Torsion and the modulus of rigidity

[Confusedbiomedeng]

Homework Statement

A specimen tested in a simple torsion testing machine , the specimen was 12.5mm in diameter and the angles were read over a gauge length of 0.75m Determine the modulus of rigidity of the material taking the torque T as 4.5Nm and the angle of twist as 4.0 degrees

Homework Equations

T/J=τMax/R=Gθ\L

The Attempt at a Solution

J=∏(0.025)⁴/32
J=3.8349x10^-8

4.0degress=0.0698132
4.5/3.8349x10^-8=G(0.0698132)/0.75
=>G=126x10^7
Wondering if this looks correct ??

 

[Dr.D]

Where does the factor (0.025)^4 come from?

Don't you want pi*r^4/2 for this?

 

[Confusedbiomedeng]

Where does the factor (0.025)^4 come from?

Don't you want pi*r^4/2 for this?

So I have for the polar moment of inertia to be ∏d^4/32
I did just see however I wrote 0.025 instead of 0.0125 which is the 12.5mm in m