Diagnosing Non-Convergence in Ansys Mechanical Thermal-Structural Coupling

Recognizable Failure Condition

After a temperature step the solver reports "Non-converged" or "Diverged". This indicates that the coupled thermal-structural iteration cannot reach a stable solution.

Cause / Diagnostic Table

Symptom	Likely Cause
Non-converged after temperature step	Material properties not defined for full temperature range
Diverged during iteration	Thermal load applied as body force conflicting with structural loads
Slow convergence or many iterations	Inadequate convergence settings (max iterations, under-relaxation)

Ordered Checks

1. Material temperature-dependent properties: Open the engineering data, verify that elastic modulus, thermal expansion coefficient, and any other temperature-dependent constants are entered for the entire temperature interval expected in the analysis. Missing values cause stiffness mismatch and lead to divergence.
2. Thermal load definition: Confirm that the heat source is set as a volumetric heat source or a surface flux. Do not use a body force that also applies a structural load; this creates contradictory stress states.
3. Convergence settings: In the solution branch, set max iterations to at least 200, under-relaxation to 0.4-0.6 for coupled physics, and enable automatic time stepping if temperature gradients are large.
4. Mesh refinement: For steep temperature gradients, refine the mesh locally or enable adaptive meshing. A coarse mesh can produce numerical instability.
5. Memory monitoring: On systems with less than 16 GB RAM, watch the memory consumption during the solve; close unnecessary result files to avoid out-of-memory failures.

Fixes Tied to Findings

• If material tables are incomplete, extend the temperature table in the engineering data editor and re-import.
• If a body force is used for heating, replace it with a volumetric heat source such as Heat Flux or Volumetric Heat Generation and delete the conflicting structural load.
• Adjust convergence parameters: increase max iterations, lower under-relaxation, and enable Coupled Solver diagnostics to watch residual history.
• Refine the mesh in regions where temperature changes rapidly, or activate adaptive time stepping to allow smaller steps when gradients are high.
• Reduce memory pressure by disabling unnecessary output requests or by running on a machine with more RAM.

Escalation Criteria

If after completing all ordered checks the solver still reports Non-converged or Diverged, consider the following escalation steps:

1. Run a simplified 1-D bar heated from one end and verify that the analytical thermal expansion matches the coupled result within 2 percent.
2. Enable the Coupled Solver diagnostics and confirm that no error messages appear in the log.
3. If residuals do not drop below 1e-5 after each iteration, contact Ansys support with the log files and model details; the issue may be a bug in the specific Ansys version.

Concrete Example (1-D Bar)

Create a steel bar 1 m long, 0.01 m^2 cross-section. Define a temperature-dependent material: at 25 C, E = 210 GPa, alpha = 12e-6 m/m/K; at 400 C, E = 190 GPa, alpha = 13e-6 m/m/K. Apply a surface heat flux of 10 kW/m^2 on the left end for 10 s. Set the solve type to steady-state thermal-structural, max iterations = 250, under-relaxation = 0.5. Run the solve.

# In Workbench -> Solution -> Set up:
solution = Mechanical
solution['solver'] = 'Coupled Thermal-Structural'
solution['max_iters'] = 250
solution['under_relaxation'] = 0.5
# Apply thermal load:
solution['thermal']['heat_flux'] = '10kW/m2'   # placeholder
# Run:
solution.solve()

After the run, open the results and compare the bar's final length to the analytical value delta L = alpha * L * delta T. The relative error should be no more than 2 percent. If the error exceeds this threshold, re-check material tables and mesh refinement.

Verification Steps

• Run the simple 1-D bar test described above; confirm that the coupled result deviates from the analytical expansion by no more than 2 percent.
• Inspect the convergence log: residuals should fall below 1e-5 after each iteration. If not, revisit the convergence settings.
• Enable Coupled Solver diagnostics; ensure that no error messages concerning mass-matrix coupling appear.

Risks and Limitations

Large temperature gradients may require finer meshes or adaptive time stepping to avoid numerical instability. Coupled solves consume more RAM; on machines with less than 16 GB, monitor memory usage and close unnecessary result files. The feature is only supported in Ansys Mechanical 2024 R2 and later; older releases require separate thermal and structural modules and do not support automatic coupling.