Spring 2008, MET 415 - FEA Applications I

Prof. Dave Johnson, dhj1@psu.edu, Penn State - Erie, The Behrend College

Workbench: Using Simulation Features

Attaching Geometry:

Updating Simulation Model Geometry from CAD System

Virtual Topology (for meshing) – groups faces and edges into "virtual cells". Can help make mesh coarser by removing detail – simplifies complex geometry.

Coordinate Systems

• Used with springs, joints, loads, supports, and result probes
• Can be Cartesian or cylindrical  (polar)
• Can be associated with geometry (surfaces/edges) or not (fixed in space)
• Can translate, rotate, flip to make adjustments
• Are imported (along with geometry) from Pro/E, S-W, or WB-DM

Graphics

• Annotations:

• Use the mouse with the “Label” button to move BC tags on the model.
• “Environment” annotations, limited to 10 in legend.
• “Solution” annotations
• Include MAX / MIN / Probe
• Probe annotations can be deleted – use “Label” button, select probe tag, press delete, VIOLA !
• Probe annotations are also removed at the next solve; they are not stored with the model on the DB file
• “Message” annotations – used to show problem geometry when meshing failure occurs.
• Section Planes / Section Plots
  • Display geometry or results on a plane which cuts through the model
  • Can show section plane and all geometry behind the cutting plane (capped)

Analysis Settings

• TIME:

  • In a transient analysis time represents actual, chronological time in seconds, minutes, or hours.
  • In a static analysis, however, time simply becomes a counter that identifies steps and substeps
• Steps and Step Controls:
• Steps (or LOAD STEPS)
• When do you need steps ?
• To change analysis settings
• To change loads: delete or add
• To separate load cases/scenarios
• To establish initial conditions for transient simulation
• What are substeps and equilibrium iterations ?
• Nonlinear problems require gradual loading which is solving the problem at intermediate load values (SUBSTEPS)
• At each substep – equilibrium iterations are performed to arrive at “convergence” (equilibrium)
• Automatic Time Stepping (aka. Time Step Optimization)

• Activate Automatic Time Stepping
• Set: Initial / Min. / Max. number of substeps
• While the solution is running, step size is adjusted
• If convergence is NOT reached, the program can BISECT (retreat to the previous “good” solution and take a smaller increment of load)
• Some dynamic problems do NOT perform better with Automatic Time Stepping, e.g., seismic, kinematics, etc.
• In dynamic simulations: a proper integration time step size can be estimated based on expected response and model characteristics.
• Step Controls – are set in the “Details Area” for Analysis Settings

• The “Program Chosen” step controls are listed
• Can set either increments of TIME or number of SUBSTEPS
• “Carry Over” (w/multiple load steps) means: 1^st substep in next load step is the same size as the last substep in the previous load step  
  (To avoid sudden change/instability)
• Time Integration: applies to transient simulations
• Some loads may be activated / deactivated at load step intervals

• Solver Controls
• Solver Type
• Direct, good for thin, flexible models
• Iterative, for bulky solid models
• Program Controlled
• Weak Springs
• Can prevent instability
• Stiffness: user or auto control
• Should NOT show significant reaction forces, if used properly
• Large Deflection Effects
• Includes large deflection, large rotation, and large strain
• Used for hyperelasticity
• Used for contact/target pairs
• Inertia Relief – a special calculation to determine the accelerations needed to balance all applied loads

• Nonlinear Controls – convergence is a tolerance for being close to equilibrium and little change in displacement or rotation in consecutive iterations
• Force Convergence
• Moment Convergence
• Displacement Convergence
• Rotation Convergence
• Line Search – a tool which can significantly improve convergence in
• “Flimsy” structures that may stiffen as stress rises
• Force-loaded systems (rather than displacement-loaded)
• Any response which oscillates above/below equilibrium
• Output Controls
• Can reduce the amount of data calculated and stored
• Can suppress stress and/or strain data
• Can choose to compute results less frequently than every iteration
• Analysis Data Management
• Defines solver files directory.  You can change the default location under Options > Analysis Settings and Solution
• Future Analysis: indicates that the results of the current analysis will be used as initial conditions for a subsequent simulation
• Save ANSYS db: requests a DB file written (can be opened in ANSYS)
• Delete Unneeded Files: You can override removal of unnecessary files, if desired
• Nonlinear Solution (not a switch), simply reports IF the model has any nonlinear behavior