Define plastic moment

The plastic moment $M_p$ is the moment of resistance of a cross-section when the whole section has yielded, that is, when every fibre has reached the yield stress $\sigma_y$ (tension on one side of the plastic neutral axis and compression on the other). It is the maximum moment the section can carry:

$$M_p = \sigma_y\,Z_p$$

where $Z_p$ is the plastic section modulus. For a rectangular section $M_p = \sigma_y\dfrac{bd^2}{4}$ (and the yield moment is $M_y = \sigma_y\dfrac{bd^2}{6}$, so $M_p/M_y = 1.5$).