Step 1: Polar Section Modulus Formula.
The polar section modulus, \(Z_p\), for a circular shaft is:\[Z_p = \frac{\pi d^3}{16}\]This equation is based on the circular shaft's geometry and is used to calculate torsional stress.Step 2: Summary.
For a circular shaft with diameter \( d \), the polar section modulus is \( \frac{\pi d^3}{16} \). Final Answer: \[ \boxed{\frac{\pi d^3}{16}} \]