Nonlinear finite elements/Updated Lagrangian formulation

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Updated Lagrangian formulation[edit | edit source]

We will now derive the finite element equations for the updated Lagrangian formulation for three-dimensional problems in solid mechanics.

Updated Lagrangian governing equations[edit | edit source]

The updated Lagrangian equations are

The boundary conditions are

The initial conditions are

Dependent variables[edit | edit source]

Note that the dependent variables in this case are all expressed as functions of the Lagrangian (material) coordinates, i.e.,

  1. Velocity ()
  2. Cauchy stress ()
  3. Rate of deformation ()

Also note that instead of and , we may use and in our calculations and then transform to .

Weak form of the balance of linear momentum[edit | edit source]

Another name for the weak form is the Principle of Virtual Power (or Principle of Virtual Work if rates are not involved).

The strong form consists of the balance of linear momentum and the traction boundary conditions. That is,

where is the acceleration.

The weak form of this equation can be written as

where is a vector valued weighting function (also called a test function) that is zero at the points on the boundary where the essential boundary conditions on are applied. Therefore we can think of as a variation of .

Derivation of the weak form[edit | edit source]

We proceed in the usual manner to derive the weak form. We multiply the differential equation with the weighting function and integrate over the volume to get

Using the identity

and noting that (conservation of angular momentum), we have

From the divergence theorem we have

where is the outward unit normal to the boundary. So,

Now, on the boundary

On the part of the boundary where no tractions are applied, we have and hence . Therefore we can write

where is the part of the boundary where the tractions are applied.

Plugging this expression into the weighted average form of the balance of linear momentum we get

Rearranging terms we get the required weak form

Physical interpretation of terms[edit | edit source]

To provide a physical interpretation of the four terms in the weak form, we write the weighting function as a variation of the velocity, i.e., . Then we have the familiar principle of virtual power:

The first term on the left can then be identified as the virtual kinetic power, the second term on the left as the virtual internal power, and the sum of the two terms on the right as the virtual external power. In other words, we have


We can see why the second terms is identified as the virtual internal power as follows. Note that

where is the velocity gradient, is symmetric rate of deformation tensor, and is the skew symmetric spin tensor. Therefore, since the product of a symmetric and a skew symmetric tensor is zero, we have


This has the same form as the expression for internal power that we discussed earlier.

Finite element discretization[edit | edit source]

Let us go back to the original weak form that we derived, i.e.,

Trial function[edit | edit source]

To start the discretization process we have to first choose an approximate solution from the space of admissible finite element trial functions. These functions must satisfy at least continuity and the essential boundary condition on .

More formally, we choose where

If we discretize the body into a number of elements with nodes, then we can write the approximate solution as

The quantities are the shape functions which provide the required level of continuity and are chosen so that .

Test function[edit | edit source]

We also have to choose appropriate weighting functions () which are also known as test functions.

In Galerkin finite elements we choose these functions from the same space as the trial functions with the additional restriction that these functions go to zero at points on the boundary where essential boundary conditions are applied.

Formally, choose where

For finite element analysis, these weighting functions have the same form as the trial function, i.e.,

Motion[edit | edit source]

Instead of a trial function for the velocity, we can start with one for the motion and then take time derivatives. In this case, we choose the motion to be approximated by

where are the nodal shape functions (note that these are functions of ) and are the positions of nodes in the current configuration. Therefore, nodes remain coincident with material point labels at all times.

Therefore, in the reference configuration, we have

The shape functions are also chosen such that

Then the displacement can be approximated by

The velocity is given by the time derivative of keeping fixed. So we have

Note that this approximation has the same form as the trial function for the velocity that we started with.

The acceleration is given by the time derivative of . Hence

Velocity gradient and rate of deformation[edit | edit source]

The spatial velocity gradient is given by

Note that the derivatives are with respect to and not .

Therefore, the approximation that we seek has the form

Using the identity

we get

In index notation,

The rate of deformation is then given by

In index notation,

Approximate weak form[edit | edit source]

We can now substitute the trial and test functions into the weak form of the momentum equation to get (using the same procedure as in one-dimension)

We can write this approximate weak form as

where the first term represents the inertial force, the second term the internal force, and the third term the external force. Thus,

The mass matrix[edit | edit source]

The term

is the inertial force term.

If we substitute

into the inertial force terms, we get

We define the mass matrix as the quantity

Therefore, the inertial force can be expressed, in analogy with Newton's second law, as

Semi-discrete finite element equations[edit | edit source]

We can then write the semi-discrete version of the weak form in the shape of a matrix equation as

These equations are ordinary differential equations because we still have time derivatives on the left hand side - hence the system of equations is semi-discrete.

There are a number of ways in which the discretization in time can be effected. Some common approaches are the generalized midpoint rule, the Newmark method, or Runge-Kutta explicit integration schemes. There is a large literature on the best way of doing the discretization in time with the goal of conserving both momentum and energy as accurately as possible.

Taking derivatives and calculating integrals[edit | edit source]

At this stage we are faced with the problem of calculating derivatives and integrals with respect to of quantities that depend on . How can we proceed?

The standard way of resolving this issue is to do all our calculations with respect to a parent element and then map the results to the reference or current configurations as necessary.

To make things more concrete let us consider a single element and label the three domains as

  1. Parent element ().
  2. Reference element ().
  3. Current element ().

Let the maps that we need be

  1. Parent Reference : .
  2. Parent Current : .
  3. Reference Current : .

This situation is illustrated (for a two dimensional situation) in the following figure.

Maps from parent element to reference and current configurations

We define the map between the parent element and the element in the current configuration as


Let us now try to compute the spatial velocity gradient. We have

The quantity

is called the Jacobian of the motion and is a function of time.

Inverting, we get

But the spatial velocity gradient is


If we substitute the approximation

we get a formula for the velocity gradient

A similar procedure can be followed when other spatial gradients need to be calculated. The most frequently encountered situation is the calculation of the gradient of the shape functions. In that case, we have

For computing integrals, we observe that for a function on we can write


Similarly for a function over , we have


Recall that the internal force terms have the form

Expressed in terms of an integral over the parent element, we then have


Finite element implementation[edit | edit source]

Recall that the finite element system of equations can be written as


To get a feel for the structure of the equation, let us consider a four-noded plane element. If the components of the velocity in the two coordinate directions are , the acceleration vector has the form

The mass matrix has the form

In expanded form

Next, we consider the internal force term. Let the components of the internal force in the two coordinate directions be (). Then,

At this stage, recall that the velocity gradient is given by

In index notation,

We define


In matrix form

Expanded out

In more compact form, we can just write

Similarly, for the internal force term,

we have

In matrix form,

Expanded out,

In compact form, we have

We can express the external force term in a similar manner. Notice that we are not taking adavantage of the symmetry of the stress tensor in the above expressions.

A widely used alternative way of expressing the finite element system of equations is the Voigt notation. This notation can be found in discussions of introductory finite element analysis.

Algorithm for computing internal forces[edit | edit source]

The major complications and computational effort in the finite element implementation are encountered during the computation of internal forces.

The basic procedure for computing the internal force at the nodes of an element is given below.

  1. Set .
  2. For all quadrature points in the parent element ()
    1. Compute the gradient of the shape functions () for all nodes .
    2. Compute the velocity gradient .
    3. Compute the rate of deformation tensor.
    4. Compute the deformation gradient/ the Lagrangian Green tensor.
    5. Compute the Cauchy stress or the 2nd P-K stress using the constitutive equation.
    6. Update the nodal internal force () using where are the weights for Gaussian integration.

The mass matrix and integration in time will be discussed later.