This brick represents a term

with

is the small strain tensor, is the stress tensor, and are the Lamé coefficients. This represents the system of linearized isotropic elasticity. It can also be used with together with the linear incompressible brick to build the Stokes problem.

Let us recall that the relation between the Lamé coefficients an Young modulus and Poisson ratio is

except for the plane stress approximation (2D model) where

The function which adds this brick to a model and parametrized with the Lamé coefficients is:

```
ind_brick = getfem::add_isotropic_linearized_elasticity_brick
(md, mim, varname, data_lambda, data_mu,
region = size_type(-1));
```

where `dataname_lambda` and `dataname_mu` are the data of the model
representing the Lamé coefficients.

The function which adds this brick to a model and parametrized with Young modulus and Poisson ratio is:

```
ind_brick = getfem::add_isotropic_linearized_elasticity_brick_pstrain
(md, mim, varname, data_E, data_nu, region = size_type(-1));
```

This brick represent a plane strain approximation when it is applied to a 2D mesh (and a standard model on a 3D mesh). In order to obtain a plane stress approximation for 2D meshes, one can use:

```
ind_brick = getfem::add_isotropic_linearized_elasticity_brick_pstress
(md, mim, varname, data_E, data_nu, region = size_type(-1));
```

For 3D meshes, the two previous bricks give the same result.

The function:

```
getfem::compute_isotropic_linearized_Von_Mises_or_Tresca
(md, varname, dataname_lambda, dataname_mu, mf_vm, VM, tresca_flag = false);
```

compute the Von Mises criterion (or Tresca if `tresca_flag` is set to true) on
the displacement field stored in `varname`. The stress is evaluated on the mesh_fem
`mf_vm` and stored in the vector `VM`. It is not valid for 2D plane stress approximation and is parametrized with Lamé coefficients. The functions:

```
getfem::compute_isotropic_linearized_Von_Mises
(md, varname, data_E, data_nu, mf_vm, VM);
getfem::compute_isotropic_linearized_Von_Mises
(md, varname, data_E, data_nu, mf_vm, VM);
```

compute the Von Mises stress, parametrized with Young modulus and Poisson ratio, the second one being valid for 2D plane stress approximation when it is applied on a 2D mesh (the two functions give the same result for 3D problems).

The program `tests/elastostatic.cc` can be taken as a model of use of a linearized isotropic elasticity brick.

This brick adds a linear incompressibility condition (or a nearly incompressible condition) in a problem of type:

This constraint is enforced with Lagrange multipliers representing the pressure, introduced in a mixed formulation.

The function adding this incompressibility condition is:

```
ind_brick = getfem::add_linear_incompressibility
(md, mim, varname, multname_pressure, region = size_type(-1),
dataexpr_penal_coeff = std::string());
```

where `varname` is the variable on which the incompressibility condition is
prescribed, `multname_pressure` is a variable which should be described on a
scalar fem representing the multiplier (the pressure) and `dataexpr_penal_coeff`
is an optional penalization coefficient for the nearly incompressible condition.

In nearly incompressible homogeneous linearized elasticity, one has where is one of the Lamé coefficient and the penalization coefficient.

For instance, the following program defines a Stokes problem with a source term
and an homogeneous Dirichlet condition on boundary 0. `mf_u`, `mf_data` and
`mf_p` have to be valid finite element description on the same mesh. `mim`
should be a valid integration method on the same mesh:

```
typedef std::vector<getfem::scalar_type> plain_vector;
size_type N = mf_u.linked_mesh().dim();
getfem::model Stokes_model;
laplacian_model.add_fem_variable("u", mf_u);
getfem::scalar_type mu = 1.0;
Stokes_model.add_initialized_data("lambda", plain_vector(1, 0.0));
Stokes_model.add_initialized_data("mu", plain_vector(1, mu));
getfem::add_isotropic_linearized_elasticity_brick(Stokes_model, mim,
"u", "lambda", "mu");
laplacian_model.add_fem_variable("p", mf_p);
getfem::add_linear_incompressibility(Stokes_model, mim, "u", "p");
plain_vector F(mf_data.nb_dof()*N);
for (int i = 0; i < mf_data.nb_dof()*N; ++i) F(i) = ...;
Stokes_model.add_initialized_fem_data("VolumicData", mf_data, F);
getfem::add_source_term_brick(Stokes_model, mim, "u", "VolumicData");
getfem::add_Dirichlet_condition_with_multipliers(Stokes_model, mim,
"u", mf_u, 1);
gmm::iteration iter(residual, 1, 40000);
getfem::standard_solve(Stokes_model, iter);
plain_vector U(mf_u.nb_dof());
gmm::copy(Stokes_model.real_variable("u"), U);
```

An example for a nearly incompressibility condition can be found in the program
`tests/elastostatic.cc`.