$\newcommand{\Dev}{\operatorname{Dev}}$ $\newcommand{\dev}{\operatorname{dev}}$ $\newcommand{\grad}{\operatorname{grad}}$ $\newcommand{\Grad}{\operatorname{Grad}}$ $\newcommand{\divg}{\operatorname{div}}$ $\newcommand{\Ei}{\operatorname{Ei}}$ $\newcommand{\tr}{\operatorname{tr}}$ $\newcommand{\Divg}{\operatorname{Div}}$ $\newcommand{\cay}{\operatorname{cay}}$ $\newcommand{\rot}{\operatorname{rot}}$

For three-node triangular shell elements, the shape functions are given by $$\begin{equation} \begin{aligned}M_{1} & =1-r-s\\ M_{2} & =r\\ M_{3} & =s \end{aligned} \,.\label{eq400} \end{equation}$$ For six-node triangular shell elements they are $$\begin{equation} \begin{aligned}M_{1} & =r_{1}\left(2r_{1}-1\right) & M_{4} & =4r_{1}r_{2}\\ M_{2} & =r_{2}\left(2r_{2}-1\right) & M_{5} & =4r_{2}r_{3}\\ M_{3} & =r_{3}\left(2r_{3}-1\right) & M_{6} & =4r_{3}r_{1}\\ r_{1} & =1-r-s & r_{2} & =r & r_{3} & =s \end{aligned} \,.\label{eq401} \end{equation}$$