Configuring a mass diffusion procedure

1. Display the Edit Step dialog box following the procedure outlined in Creating a step (Procedure type: General; Mass diffusion), or Editing a step.

2. On the Basic, Incrementation, and Other tabbed pages, configure settings such as steady-state or transient response and automatic or fixed incrementation as described in the following procedures.

1. In the Edit Step dialog box, display the Basic tabbed page.

2. In the Description field, enter a short description of the analysis step. Abaqus stores the text that you enter in the output database, and the text is displayed in the state block by the Visualization module.

3. Choose a Response option:

   • Choose Steady-state to specify that the analysis provide the steady-state solution directly. The rate of change of concentration with respect to time is omitted from the governing diffusion equation in a steady-state analysis. For more information, see Steady-state analysis.

   • Choose Transient to perform time integration with the backward Euler method. This method is unconditionally stable for linear problems. For more information, see Transient analysis.

   Note:

   After you have selected a Response option, a message appears informing you that Abaqus/Standard has selected the Default load variation with time option (located on the Other tabbed page) that corresponds to your Response selection. Click Dismiss to close the message dialog box.

4. In the Time period field, enter the time period of the step.

1. In the Edit Step dialog box, display the Incrementation tabbed page.

   (For information on displaying the Edit Step dialog box, see Creating a step, or Editing a step.)

2. Choose a Type option:

   • Choose Automatic if you want Abaqus/Standard to determine suitable time increment sizes.

   • Choose Fixed to specify direct user control of the incrementation. Abaqus/Standard uses an increment size that you specify as the constant increment size throughout the step.

3. In the Maximum number of increments field, enter the upper limit to the number of increments in the step. The analysis stops if this maximum is exceeded before Abaqus/Standard arrives at the complete solution for the step.

4. If you selected Automatic incrementation in Step 2, enter values for Increment size:

   1. In the Initial field, enter the initial time increment. Abaqus/Standard modifies this value as required throughout the step.
   2. In the Minimum field, enter the minimum time increment allowed. If Abaqus/Standard needs a smaller time increment than this value, it terminates the analysis.
   3. In the Maximum field, enter the maximum time increment allowed.

5. If you selected Fixed incrementation in Step 2, enter a value for the constant time increment in the Increment size field.

6. If you selected Automatic incrementation in Step 2 and Transient analysis on the Basic tabbed page, do the following:

   1. Enter a value in the End step when normalized concentration change is less than n field. The analysis will end when all nodal normalized concentrations are changing at a rate that is less than the rate that you enter.
   2. Enter a value in the Max. allowable normalized concentration change field. Abaqus/Standard restricts the time step to ensure that this value is not exceeded at any node (except nodes with boundary conditions) during any increment of the step.
   To configure settings on the Other tabbed page:
   1. In the Edit Step dialog box, display the Other tabbed page.

      (For information on displaying the Edit Step dialog box, see Creating a step, or Editing a step.)

   2. Accept the selection of the Unsymmetric matrix storage and solution scheme. This scheme is the only Matrix storage option that is valid for mass diffusion analyses. For more information on matrix storage, see Matrix storage and solution scheme in Abaqus/Standard.
   3. Click the arrow to the right of the Convert severe discontinuity iterations field, and select an option for dealing with severe discontinuities during nonlinear analysis:

      • Select Off to force a new iteration if severe discontinuities occur during an iteration, regardless of the magnitude of the penetration and force errors. This option also changes some time incrementation parameters and uses different criteria to determine whether to do another iteration or to make a new attempt with a smaller increment size.

      • Select On to use local convergence criteria to determine whether a new iteration is needed. Abaqus/Standard will determine the maximum penetration and estimated force errors associated with severe discontinuities and check whether these errors are within the tolerances. Hence, a solution may converge if the severe discontinuities are small.

      • Select Propagate from previous step to use the value specified in the previous general analysis step. This value appears in parentheses to the right of the field.

      For more information on severe discontinuities, see Severe discontinuities in Abaqus/Standard.

   4. Abaqus/Standard automatically selects the Default load variation with time option that corresponds to your Response selection on the Basic tabbed page. It is recommended that you leave the Default load variation with time selection unchanged.
   5. Click the arrow to the right of the Extrapolation of previous state at start of each increment field, and select a method for determining the first guess to the incremental solution:

      • Select Linear to indicate that the process is essentially monotonic and Abaqus/Standard should use a 100% linear extrapolation, in time, of the previous incremental solution to begin the nonlinear equation solution for the current increment.

      • Select Parabolic to indicate that the process should use a quadratic extrapolation, in time, of the previous two incremental solutions to begin the nonlinear equation solution for the current increment.

      • Select None to suppress any extrapolation.

      For more information, see Extrapolation of the solution.

When you have finished configuring settings for the step, click OK to close the Edit Step dialog box.