Description
This is an orthotropic composite model.
The stress can be expressed as:
$\mathbf{\sigma} = \mathbf{L} : \mathbf{\epsilon} + \displaystyle{ \frac{c}{c_{dec}} \int_0^t \dot{\mathbf{\epsilon}}(\tau)
\cdot \mathrm{e}^{(\tau-t)/c_{dec}} \mathrm{d}\tau }$
where $\mathbf{L}$ is the tangential stiffness of the material (fourth order tensor). There are four different failure criteria.
Fiber tension/shear if $\sigma_{11} \gt 0 $:
$D_1 = (\sigma_{11}/X_t)^2 + \beta(\vert \tau_{12} \vert / S)$
Fiber compression if $\sigma_{11} \lt 0$:
$D_2 = (\sigma_{11}/X_c)^2$
Matrix tension if $\sigma_{22} \gt 0$:
$D_3 = (\sigma_{22}/Y_t)^2 + \beta(\vert \tau_{12} \vert / S)$
Matrix compression/shear if $\sigma_{22} \lt 0$:
$D_4 = (\sigma_{11}/X_c)^2 + ( (Y_c/2S)^2 - 1) (\sigma_{22}/Y_c) + (\tau_{12}/S)^2$
All stresses are reduced with the factor $res$ at fiber failure, i.e. if $D_1 \ge 1$.
$D_2 \ge 1$ indicates fiber buckling whereby
compressive fiber stresses $(\sigma_{11})$ and in plane shear stresses $(\tau_{12})$ are reduced with the factor $res$.
$D_3 \ge 1$ or $D_4 \ge 1$ indicates matrix failure and $\sigma_{22}$ and $\tau_{12}$ are reduced with the factor $res$.
Initial fiber directions need to be defined using either INITIAL_MATERIAL_DIRECTION, INITIAL_MATERIAL_DIRECTION_VECTOR or INITIAL_MATERIAL_DIRECTION_WRAP.
