Tresca Yield Stress Question: Finding Maximum Compressive Stress

[stevo1978]

Homework Statement

Hi there if you have something that has 3 principle stresses on it, 200MPa and 50MPa and one is a compressive stress, what is the largest value of compressive stress using the tresca theory when the yield stress is 300MPa?
Many thanks

Homework Equations

The Attempt at a Solution

s1-s3=syield

so 200-s3=300

or s3=-100MPa

but max shear, t=(s1-s3)/2

so t=(200+100)/2=300/2=150MPa
I think i have done something wrong as i need a compressive stress which would be a negative answer

 

[nilesh_pat]

. Therefore the largest value of compressive stress using the tresca theory when the yield stress is 300MPa is -150MPa.