Mixed Finite Element Methods for Linear Viscoelasticity
Introduction
One classical approach to linear isotropic elasticity is the displacement-based discretization for
Typically $u$ is discretized using continuous piecewise vector polynomials. This method yields accurate approximation for $u$. However, in practice
- The stress is usually the quantity of primary physical interest and pure displacement methods will yield stress approximations of lower order accuracy.
- The method performs poorly in the incompressible and nearly incompressible case, i.e., $\lambda \rightarrow \infty$.
An alternative approach, the stress-based formulation, addresses this problem by considering a mixed formulation. However, the main obstacle is to construct a stable pair of finite element spaces for symmetric tensors.
Instead of imposing the symmetry condition on the finite element space directly, we enforce the condition weakly by adding an additional equation and an associated Lagrange multiplier.
Mathematical Formulation
Consider the linear viscoelastic Maxwell model
Here $\sigma$ is the stress tensor, and $A_0$, $A_1$ are fourth-order material tensors. We introduce the velocity vector
and the rotation of the velocity vector
Let $M$, $V$, and $K$ be the space of matrices, vectors, and skew symmetric matrices on $\Omega$, then the weak form for Eq. 1 is:
Find $(\sigma, v, \rho)\in H(\text{div}, \Omega; M) \times L^2(\Omega; V) \times L^2(\Omega; K)$, such that for all $(\tau, w, \eta)\in H(\text{div}, \Omega; M) \times L^2(\Omega; V) \times L^2(\Omega; K)$
Here $\Gamma_D$ is the Dirichlet boundary condition for the velocity $v$. Note that if we have traction boundary condition
The condition is part of the Dirichlet boundaries for $\sigma$.
After discretization, this leads to a DAE
where
This DAE can be solved using TR_BDF2
in ADCME.
Linear Viscoelasticity Model
We consider the 2D Maxwell material model here
We will convert Eq. 2 to the form in Eq. 1. To this end, consider two linear operator
In the Voigt notation, these two linear operators have the matrix form
The following formulas can be easily derived:
Therefore, Eq. 2 can be rewritten as
which leads to
Here
In AdFem, $(a\sigma + b T\sigma, \tau)$ can be calculated using compute_fem_bdm_mass_matrix
.