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Section 3.3: Control Section Up Section 3.3: Control Section Subsection 3.3.2: Time Stepper parameters

3.3.1 Control Parameters

The following parameters are common for all analysis. If not specified they are assigned default values, which are found in the last column. An asterisk (*) after the name indicates a required parameter. The numbers behind the description refer to the comments following the table.
 Parameter Description Default analysis Sets the analysis type (1) static time_steps* Total number of time steps. (= ntime)(2) (none) step_size* The initial time step size. (= dt) (2) (none) plot_level Sets the level of state dumps to the plot file (3) PLOT_MAJOR_ITRS plot_range Set the range of the states that will be stored to the plot file (4) 0,-1 plot_stride Set the stride of the states that will be stored to the plot file (4) 1 plot_zero_state Flag that controls whether the “zero” state will be written to the plot file, even if it is not defined in the range (4) 0 (false) print_level Sets the amount of output that is generated on screen (5) PRINT_MINOR_ITRS output_level Controls when to output data to file (6) OUTPUT_MAJOR_ITRS integration Set the integration rule for a particular element (7) N/A
1. The analysis element sets the analysis type. Currently, FEBio defines three analysis types: (quasi-)static, steady-state, and dynamic. In a quasi-static analysis, inertial effects are ignored and an equilibrium solution is sought. Note that in this analysis mode it is still possible to simulate time dependant effects such as viscoelasticity. In a dynamic analysis the inertial effects are included.
 Value Description static (quasi-) static analysis steady-state steady-state response of a transient (quasi-static) biphasic, biphasic-solute, multiphasic, fluid or fluid-FSI analysis dynamic dynamic analysis for solid, fluid and fluid-FSI analyses
2. The total running time of the analysis is determined by ntime * dt. Note that when the auto-time stepper is enabled (see below), the actual number of time steps and time step size may be different than specified in the input file. However, the total running time will always be determined by ntime * dt.
3. The plot_level allows the user to control exactly when the solution is to be saved to the plot file. The following values are allowed:
 Value Description PLOT_NEVER Don't save the solution PLOT_MAJOR_ITRS Save the solution after each converged timestep PLOT_MINOR_ITRS Save the solution for every quasi-Newton iteration PLOT_MUST_POINTS Only save the solution at the must points
The PLOT_MUST_POINTS option must be used in conjunction of a must-point curve. See the comments on the dtmax parameter for more information on must-point curves. When the plot_level option is set to PLOT_MUST_POINTS, only the time-points defined in the must-point curve are stored to the plotfile.
4. When using the fixed time stepper, several parameters control which time steps are stored to the plot file.
The plot_range parameter sets the range of the states that will be stored to the plot file. The range is defined by two values that specify the first and last time step that will be stored to the plot file. The value “0” refers to the initial time step, usually time zero, and negative values count backwards from the final time step (as defined by the time_steps parameter). For instance, the default values,
<plot_range>0,-1</plot_range>

store all time steps to the plot file, including the initial “zero” time step. As another example, to store only the last five time steps, set
<plot_range>-5,-1</plot_range>

By default, all time steps within the range will be stored to the plot file. Time steps can be skipped using the plot_stride parameter. For instance, to store only every 10 steps, set
<plot_stride>10</plot_stride>

Note that the first and last time step defined by the range will always be stored, regardless of the plot stride.
The “zero” time step refers to the initial state of the model, before any calculations are done. This state will only be stored to the plot file if the minimal value of the plot range is set to zero. To force storing this state to the plot file, set the plot_zero_state parameter to one.
<plot_zero_state>1</plot_zero_state>

This will store the zero time step to the plot file, even when it is not specified inside the plot range.
Again, the plot_range, plot_stride, and plot_zero_state parameters are only used by the fixed time stepper. Currently, these parameters are not used with the auto time stepper.
5. The print_level allows the user to control exactly how much output is written to the screen. The following values are allowed:
 Value Description PRINT_NEVER Don't generate any output PRINT_PROGRESS Only print a progress bar PRINT_MAJOR_ITRS Only print the converged solution PRINT_MINOR_ITRS Print convergence information during equilibrium iterations PRINT_MINOR_ITRS_EXP Print additional convergence info during equilib. iterations
6. The output_level can be used to control when FEBio outputs the data files. The following values are supported.
 Value Description OUTPUT_NEVER Don't generate any output OUTPUT_MUST_POINTS Only output at must points PRINT_MAJOR_ITRS Output at end of each time step PRINT_MINOR_ITRS Output at each iteration OUTPUT_FINAL Only output the data at the last converged time step.
7. You can override FEBio's default integration rule for specific element classes. For each element class, define a rule element in which you set the integration rule.
<integration>
<rule elem="<elem>">VALUE</rule>
<!-- repeat rules for other elements -->
</integration>

The elem attribute value defines for which element class you wish to override the default integration rule and can have any of the following values.
 elem Description hex8 8-node hexahedral element hex20 20-node quadratic hexahedral element tet4 4-node linear tetrahedral element tet10 10-node quadratic tetrahedral element tet15 15-node quadratic tetrahedral element penta15 15-node quadratic pentahedral element tri3 3-node linear triangles (e.g. for contact) tri6 6-node quadratic triangles
The values of the rule elements depend on the elem attribute. The tables below show the available integration rules for the different element types. The values marked with an asterisk (*) are the default.
• For the hex8 element, the following values are defined.
 hex8 Description GAUSS8* Gaussian integration using 2x2x2 integration points. POINT6 Alternative integration rule for bricks using 6 integration point
• For the hex20 element, the following values are defined.
 hex20 Description GAUSS8 Gaussian integration using 2x2x2 integration points. GAUSS27* Gaussian integration using 3x3x3 integration points.
• For the tet4 element, the following values are allowed.
 tet4 Description GAUSS4 Gaussian integration rule using 4 integration points. GAUSS1* Gaussian integration rule using one integration point. UT4 Nodally integrated tetrahedron. (1)
1. The UT4 is a special formulation for tetrahedral elements that uses a nodally averaged integration rule, as proposed by Gee et al [29]. This formulation requires additional parameters. To override the default values, use the following alternative syntax:
<rule elem="tet4" type="UT4">
<alpha>0.05</alpha>
<iso_stab>0</iso_stab>
</rule>

The alpha parameter defines the amount of “blending” between the regular tet-contribution and the nodally integrated contribution. The value must be between 0 and 1, where 0 means no contribution from the regular tet and 1 means no contribution from the nodally averaged tet. The iso_stab parameter is a flag that chooses between two slightly different formulations of the nodally integrated tet. When set to 0, the stabilization is applied to the entire virtual work, whereas when set to 1 the stabilization is applied only to the isochoric part. See the FEBio Theory Manual for a detailed description of this formulation.
• For the tet10 element, the following integration rules are supported.
 tet10 description GAUSS4* Gaussian integration rule using 4 integration points GAUSS8 Gaussian integration rule using 8 integration points LOBATTO11 Gauss-Lobatto integration rule using 11 integration points
Notes:
The Lobatto integration rule differs from a regular Gauss integration rule in that it includes the vertices of the tetrahedral element. The Lobatto11 integration rule uses the 10 tetrahedral nodes, plus one integration rule located at the center of the element.
• For the tet15 element, the following integration rules are defined.
 tet15 description GAUSS8 Gaussian integration rule using 8 integration points GAUSS11 Gaussian integration rule using 11 integration points GAUSS15 Gaussian integration rule using 15 integration points
• For the penta15 element, the following values are defined.
 penta15 Description GAUSS8 Gaussian integration using 2x2x2 integration points. GAUSS21* Gaussian integration using 3x7 integration points.
• For the tri3 element, the following integration rules are supported.
 tri3 description GAUSS1 Gaussian integration with one integration point GAUSS3* Guassian integration with three integration rules.
• For the tri6 element, the following integration rules are supported.
 tri6 description GAUSS3* Gaussian integration with 3 integration points GAUSS6 Gaussian integration with 6 integration points GAUSS4 Gaussian integration with 4 integration points GAUSS7 Gaussian integration with 7 integration points LOBATTO7 Gauss-Lobatto integration with 7 integration points.
Notes:
The GAUSS6 rule has only nonzero weights at the edge nodes, which effectively reduces this rule to a 3-node rule.
Section 3.3: Control Section Up Section 3.3: Control Section Subsection 3.3.2: Time Stepper parameters