MSR/ressources/Layerwise Theories.md

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To [[Composite laminate models|model a composite laminate]] there are a few options, the most important are the [[Equivalent Single Layer Theories|equivalent single layer]] and the layerwise.
Those theories describe kinetically each layer. They can be used for very thick laminate, and useful to compute delamination. They predicts correct inter-laminar stresses and there is no need of shear correction factor.
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There is a unique displacement field per layer + interlaminar continuity of displacements (and sometimes of transverse stresses).
-> very computationally expensive, since the number of degrees of freedom increase proportionally with the number of layers.
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([[Context articles descriptions#First order Zig-Zag plate Theory|source]])
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## Stiffness
([[Videos descriptions#Composite materials course|video source]])
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When pulling on the material, strain is the same for every ply, but stress is not.
You need to average it along the whole thickness
$$
\left[\begin{array}{c}
\overline{\sigma_{xx}} \\
\overline{\sigma_{yy}} \\
\overline{\tau_{xy}} \\
\end{array}\right]
= \frac{1}{h}\int_{-h/2}^{h/2} \left[\begin{array}{c}
\sigma_{xx} \\
\sigma_{yy} \\
\tau_{xy} \\
\end{array}\right] dz \tag{1}$$
$Q$ is the stiffness matrix :
$$ \left[\begin{array}{c}
\sigma_{xx} \\
\sigma_{yy} \\
\tau_{xy}
\end{array}\right]
=
\left[\begin{array}{c}
Q_{11} & Q_{12} & Q_{13} \\
Q_{21} & Q_{22} & Q_{23} \\
Q_{31} & Q_{32} & Q_{33} \\
\end{array}\right]
\left[\begin{array}{c}
\varepsilon_{xx} \\
\varepsilon_{yy} \\
\gamma_{xy}
\end{array}\right] \tag{2}$$
Since the strain is the same for every ply, (1) & (2) gives us :
$$
\left[\begin{array}{c}
\overline{\sigma_{xx}} \\
\overline{\sigma_{yy}} \\
\overline{\tau_{xy}} \\
\end{array}\right]
= \frac{1}{h}\int_{-h/2}^{h/2}
\left[\begin{array}{c}
Q_{11} & Q_{12} & Q_{13} \\
Q_{21} & Q_{22} & Q_{23} \\
Q_{31} & Q_{32} & Q_{33} \\
\end{array}\right]dz
\left[\begin{array}{c}
\varepsilon_{xx} \\
\varepsilon_{yy} \\
\gamma_{xy}
\end{array}\right]
\tag{3}$$
and we can give a name to the mean matrix of the stiffness :
$$
\left[\begin{array}{c}
A \\
\end{array}\right]
= \frac{1}{h}\int_{-h/2}^{h/2}
\left[\begin{array}{c}
Q_{11} & Q_{12} & Q_{13} \\
Q_{21} & Q_{22} & Q_{23} \\
Q_{31} & Q_{32} & Q_{33} \\
\end{array}\right]dz
\tag{4}$$
or when the ply are continuous :
$$
\left[\begin{array}{c}
A \\
\end{array}\right]
= \frac{1}{h}\sum_{1}^{n\_ply}
\left[\begin{array}{c}
Q
\end{array}\right]_i *h_i
\tag{5}$$
to rotate the stiffness matrix :
$$[\overline{Q}]_\theta
= [C]_{|\theta}^T
\left[\begin{array}{c}
Q_{11} & Q_{12} & 0 \\
Q_{21} & Q_{22} & 0 \\
0 & 0 & Q_{33} \\
\end{array}\right]
[C]_{|\theta}
\tag{4}$$
where $[C]$ is the transformation matrix :
$$[C]_{|\theta} = \left[\begin{array}{c}
c^2 & s^2 & cs \\
s^2 & c^2 & -cs \\
-2sc & 2sc & (c^2-s^2)
\end{array}\right]$$
## Tsai-Hill criterion
$$
F.O.S = \frac{T_x}{\sqrt{\sigma_{xx}^2 - \sigma_{xx}\sigma_{yy} + \frac{T_x^2\sigma_{yy}^2}{T_y^2} + \frac{T_x^2\tau_{xy}^2}{S_{xy}^2}}}$$