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# Need help with some mechanical stress questions watch

1. Cant seem to get the correct answer for these two.

Question 7
A copper bar AB of length 2m is placed in position at room temperature with a gap of 0.40 mm between the end A and a rigid restraint.

Subsequently, the temperature of the bar is raised by 59 K.

For copper, take E = 120 GPa and coefficient of thermal expansion 17·10-6 (ºC)-1.

Calculate the normal stress in the bar in MPa taking into account the sign.

Compressive stresses are negative, and tensile stresses are positive.

Required accuracy is 0.1 MPa.

I tried using: Stress=120GPa*17·10-6*59 but this was incorrect.

Question 18

A circular bar of 12.4 mm diameter is subjected to tensile force 7.9 kN.

Calculate maximal shear stress in the bar (MPa). Required accuracy is 0.1 MPa.

I thought this would be a simple Stress=F/A initially then also tried shear stress=0 because it's a tensile force but that was also incorrect.

Thanks for any help
2. (Original post by Sljmaster)
Cant seem to get the correct answer for these two.

Question 7
A copper bar AB of length 2m is placed in position at room temperature with a gap of 0.40 mm between the end A and a rigid restraint.

Subsequently, the temperature of the bar is raised by 59 K.

For copper, take E = 120 GPa and coefficient of thermal expansion 17·10-6 (ºC)-1.

Calculate the normal stress in the bar in MPa taking into account the sign.

Compressive stresses are negative, and tensile stresses are positive.

Required accuracy is 0.1 MPa.
You need to compute by how much the bar *would* expand without the restraint, then find the amount of compression with constraint. Then you can find the stress.

Let stress be , original/expanded length of the bar be ,and be the compression of the bar. Then:

by definition, and , the expansion of the bar is:

where is the coeff. of linear expansion. Can you do the rest?

I tried using: Stress=120GPa*17·10-6*59 but this was incorrect.
Well, it's not even dimensionally consistent so that's not surprising.

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