Hello everyone, I would like to plot the equations of two variables written below where a = const, p = const while the independent variables are r e theta.
I would like to plot such sigmar = constant to vary from r e theta in a fixed interval .Thanks for any responses.

\[Sigma]r[r, \[Theta]] = 
 p/2 (1 - a^2/r^2) + p/2 (1 - (2 a/r)^2 + 3 (a/r)^4) Cos[2 \[Theta]]
\[Sigma]\[Theta][r, \[Theta]] = 
 p/2 (1 + a^2/r^2) - p/2 (1 + 3 (a/r)^4 ) Cos[2 \[Theta]]
\[Tau]r\[Theta][
  r, \[Theta]] = -p/2 (1 + (2 a/r)^2 - 3 (a/r)^4) Sin[2 \[Theta]]

Exemple: I want Sigma r = cost to vary from r and theta.

I'm going to draw the contour lines in the polar reference isotensional!!!