Dear Sirs,

Thanks for the reply sir. But my question how come tresca theory is applied for thermal & longitudinal stress both independently as dave sir quoted

"The diameter is easier to come by (S1-S3), so use that number and compare to double the allowable (using Sy rather than Sy/2 - another easy number - Sy). Simplification (for a slide rule)." I agree to the above statement.

However S1 is hoop & S3 is longitudinal stress so where comes the thermal stress.

If i have to evaluate combined three stress thermal hoop & longitudinal then is there any equation for tresca which i can apply.

As i can understand that calculated stress in thermal stress is much higher than actual but it is still at yield point. So what happens to shear thermal stress when both longitudinal & thermal or displacement stress is applied simultaneously. If tresca would have predicted theory of failure as it is applicable for primary stress only then there would have been no confusion. But i understood it is applicable to both independently but then what is tresca significance.