I've come across this phenomenon several times and never found any real reason for this. When I consider a cylindrical object like a rod and I want to deform it in any way, I need to calculate several things, one of those things is the stress tensor. Mind you that I'm talking about a cylindrical coordinate system with $r,\theta$, and $z$ directions. Now, in many books and in my lecture notes there's always this one line that says:

$$ \frac{\partial\bullet}{\partial\theta}=0$$

I know that when compressing a steel rod (assuming there is little to no friction at the contact points) the steel rod should get compressed in the $z$-direction and expand in the radial direction. But I can't imagine why there is no stress in the circumferential direction.

I've looked in many places and I hope that I didn't accidentally miss the other SE question with respect to this specific topic.

Many thanks for any consideration and answer.