In plates there are three in-plane components of the stress tensor $$\sigma_{\alpha \beta}\{\sigma_{xx}, \sigma_{yy}, \sigma_{xy}\}$$. Replacing $$\sigma_{xx}$$ by $$\sigma_{\alpha \beta}$$ or $$\sigma_{z\alpha}$$ in Equations (2.2.16-2.2.18) the generalized forces and couples are defined
$M_{\alpha \beta} = \int_{-\frac{h}{2}}^{\frac{h}{2}} \sigma_{\alpha \beta}z dz \; [\mathrm{Nm/m}] = [\mathrm{N}] \label{2.3.1}$
$N_{\alpha \beta} = \int_{-\frac{h}{2}}^{\frac{h}{2}} \sigma_{\alpha \beta} dz \; [\mathrm{Nm/m}]$
$V_{\alpha} = \int_{-\frac{h}{2}}^{\frac{h}{2}} \sigma_{z\alpha} dz \; [N/m] \label{2.3.3}$