Nonlinear Convergence Errors
Why the nonlinear solver fails to converge and how to fix it.
If you only have 2 minutes
Nonlinear convergence failures are almost always caused by missing boundary conditions, contact problems, load steps that are too large, or poor mesh quality.
Symptoms
- Error 75 (Newton iteration did not converge) on a static nonlinear solve
- Error 76 (
dt < minimum_dt) when the time increment is bisected down to its minimum and the step still fails - Residual forces don't drop below the convergence tolerance
Root causes
| Cause | Fix |
|---|---|
| Missing boundary condition (rigid body movement) | Add the missing support |
| Contact surfaces initially penetrating | Correct geometry so parts don't overlap at start |
| Load steps too large (nonlinear) | Increase number of load steps |
| Distorted mesh elements | Re-mesh with better quality settings |
Diagnostic approach
- Run a linear static solver first, if that also fails, the problem is a missing boundary condition (rigid body movement), not a nonlinearity issue
- Verify the boundary conditions
- For nonlinear: reduce load step size
- Refine the mesh in problem areas
- Check contact definitions for initial penetrations
Always start with the linear static solver. It's faster and diagnoses setup errors more clearly than the nonlinear solver, which can obscure the root cause behind convergence failure messages.
Good to know
Dr.Q may switch to a nonlinear solve on its own when it detects large deflection (Warning 1, "switched to nonlinear"). That is expected and usually needs no action; it just means the analysis is now iterating to equilibrium, so the guidance above applies.