Modeling Guide for Nonlinear Models

This guide is meant to help Abaqus users who want to use Tosca Structure for optimization with nonlinear behavior.

Topic Guideline Why? Examples and commands
Constraints Tie constraints:
  • When using tie constraints to tie two surfaces based on shell elements, it is best to exclude the shell section thickness and specify a position tolerance equal to the average thickness.
  • If possible, choose “Node to surface” as the discretization method rather than the default “surface to surface.”
  • In sizing optimization the thicknesses of the shell elements change, which could lead to problems that more elements get into contact due to the thickness growth or vice versa. In addition, the actual “surface” might divert from the nodal positions due to interpolation. This could lead to an unwanted amount of tie in the initial design.
  • This way of determining the tie nodes of the secondary surface is more efficient and more robust.
  • *Tie, no thickness, positiontolerance=<…> Or “Exclude shell/ membrane thickness” and specify the tolerance in Abaqus/CAE
  • *Tie, Type= Node to surface Or “Discretization method set to “Node to surface” in Abaqus/CAE.
Adjustment of secondary nodes:
  • When working with shell elements: Do not use the adjustment option because it will move nodes so that they are overlapping with their corresponding partner node without causing strain.
  • The geometry of the model will change and that is most often not wanted since the shell thicknesses will overlap.
  • *Tie, adjust=no Or Deactivate “Adjust secondary surface initial position” in Abaqus/CAE.
Loading
  • If possible, use displacement-controlled load instead of force-controlled.
  • This kind of load is more robust against unstable models such as buckling or with large plastic regions.
General Naming:
  • Choose characteristic and unique names for node and element sets.
  • A node and an element set might have identical names or the name of the design region might be a Tosca Structure default name. This would lead to problems in the optimization.
  • For example, choose “Design_Elements” instead of “Elements.”
Prerun:
  • First run your model with deactivated nonlinear features. Afterward, activate the nonlinear features and run the model again.
  • This helps to see whether there are problems with contacts or boundary conditions in the initial model.
  • Deactivate nonlinear geometry: *Step, NLGEOM=NO Deactivate plasticity: Delete the plastic material card.
Stabilization:
  • Only use the default settings of any stabilization unless you are experienced with the effects of any changes.
  • Not only will any kind of stabilization method affect the computation time, but also the accuracy of the simulation. In general, more stabilization leads to higher inaccuracy.
Plasticity Material law:
  • The values in the material card for plastic strain should cover all strain values appearing in the simulation. In addition to this, the "curve" of the material card should have a positive inclination for larger strains.
  • When exceeding the maximum plastic strain defined in the material card, Abaqus might extrapolate the data incorrectly, which might lead to convergence problems.
 
*Plastic
400.,      0.0
465., 0.00037
504., 0.02921
599., 0.10357
654., 0.18782
1000.,    1.34
Contact Contact normal:
  • Make sure that the contact pair is defined by two surfaces with their normal pointing toward each other.
  • When defining contacts properties, the penetration direction as well as the correct surface is important to get the wanted result.
Rigid body motion:
  • Do not use contact definition to constrain rigid body motion. Rather use tie constraints or beam connectors.
  • At the beginning of the optimization, Tosca might not start computing when two bodies are not connected properly. Such contacts may be ignored.