# Bilaplacian and Kirchhoff-Love plate bricks¶

The following function

```
ind = add_bilaplacian_brick(md, mim, varname, dataname,
region = size_type(-1));
```

adds a bilaplacian brick on the variable varname and on the mesh region region. This represent a term \(\Delta(D \Delta u)\). where \(D(x)\) is a coefficient determined by dataname which could be constant or described on a f.e.m. The corresponding weak form is \(\int D(x)\Delta u(x) \Delta v(x) dx\).

For the Kirchhoff-Love plate model, the weak form is a bit different (and more stable than the previous one). the function to add that term is

```
ind = add_bilaplacian_brick_KL(md, mim, varname, dataname1, dataname2,
region = size_type(-1));
```

It adds a bilaplacian brick on the variable varname and on the mesh region region. This represent a term \(\Delta(D \Delta u)\) where \(D(x)\) is a the flexion modulus determined by dataname1. The term is integrated by part following a Kirchhoff-Love plate model with dataname2 the poisson ratio.

There is specific bricks to add appropriate boundary conditions for fourth order partial differential equations. The first one is

```
ind = add_normal_derivative_source_term_brick(md, mim, varname,
dataname, region);
```

which adds a normal derivative source term brick \(F = \int b.\partial_n v\) on the variable varname and on the mesh region region. It updates the right hand side of the linear system. dataname represents b and varname represents v.

A Neumann term can be added thanks to the following bricks

```
ind = add_Kirchhoff_Love_Neumann_term_brick(md, mim, varname,
dataname1, dataname2, region);
```

which adds a Neumann term brick for Kirchhoff-Love model on the variable varname and the mesh region region. dataname1 represents the bending moment tensor and dataname2 its divergence.

And a Dirichlet condition on the normal derivative can be prescribed thanks to the following bricks

```
ind = add_normal_derivative_Dirichlet_condition_with_multipliers
(md, mim, varname, multname, region, dataname = std::string(),
R_must_be_derivated = false);
ind = add_normal_derivative_Dirichlet_condition_with_multipliers
(md, mim, varname, mf_mult, region, dataname = std::string(),
R_must_be_derivated = false);
ind = add_normal_derivative_Dirichlet_condition_with_multipliers
(md, mim, varname, degree, region, dataname = std::string(),
R_must_be_derivated = false);
```

These bricks add a Dirichlet condition on the normal derivative of the variable varname and on the mesh region region (which should be a boundary). The general form is \(\int \partial_n u(x)v(x) = \int r(x)v(x) \forall v\) where \(r(x)\) is the right hand side for the Dirichlet condition (0 for homogeneous conditions) and \(v\) is in a space of multipliers defined by the variable multname (first version) or defined on the finite element method mf_mult (second version) or simply on a Lagrange finite element method of degree degree (third version) on the part of boundary determined by region. dataname is an optional parameter which represents the right hand side of the Dirichlet condition. If R_must_be_derivated is set to true then the normal derivative of dataname is considered.

The test program `bilaplacian.cc`

is a good example of the use of the previous bricks.