Calculating Bending Moment & Torque from Mohr's Strain Circle

[arghstar]

Hi, desperately need help with this assignment question as its in for 2moro!

"At a point on the surface of a shaft with diameter of 100mm, a 45deg rosette strain gauge is positioned with the middle gauge along the axis of the shaft. The gauges measure strains of a= -0.00015, b = 0.0003, c = 0.00005. Use Mohr's strain circle to determine the value of M(bending moment) and T(torque). E = 210GPa and u = 0.29."

I've drawn a mohr's circle to find principle strains of 0.00032 and -0.00042. From this i can get the principle stresses, but I do not know what formula to use to find the bending moment and torque?

Any help would be much appreciated thankyou!

 

[FredGarvin]

I didn't go back to check your numbers for the principal strains. Assuming that you made the correct circle when dealing with strains and you have backed out the principal stresses, then the rest is basic mechanics.

$$\sigma = \frac{M c}{I}$$

$$\tau = \frac{T c}{J}$$

 

[arghstar]

Yeh sorry that was basic. But which principle stress value would you use to find the bending moment? Iv calculated 45x10^6 and -75x10^6. Do you just use the positive value even though it is smaller?

 

[FredGarvin]

You'll want to use the larger value. The negative sign simply denotes a direction.