Global Models versus Local Models
When to use a global model, a local model, or a global-local submodelling approach — and the boundary condition implications of each strategy.
Principle
Use a global model to understand load paths and identify critical regions. Use a local model to obtain detailed stresses at those regions. The two serve different purposes and should not be conflated.
Why it matters
A global model captures the overall load path and boundary condition effects but is typically too coarse for local stress accuracy. A local model resolves local detail but is only as good as the boundary conditions transferred from the global model. Understanding this relationship is central to credible FEA practice.
Good practice
- Run a global model first to identify critical locations and extract interface loads or displacements
- Use submodelling (also called global-local analysis) to transfer displacements from the global model as boundary conditions on the local model
- Ensure the local model boundary is far enough from the region of interest that boundary-condition effects do not contaminate the result
- Check that the local model reaction forces match the global model interface forces — a mismatch indicates inconsistent boundary conditions
- When submodelling is not available, apply interface loads from the global model as balanced forces on the local model cut boundaries
Warning signs
- A local model is built with fixed boundary conditions at an arbitrary cut plane — this almost always over-constrains the model
- A global model is used to report peak stress at a feature where the mesh is too coarse to resolve the stress gradient
- The local model boundary is too close to the feature of interest — the Saint-Venant principle requires the boundary to be several characteristic lengths away
- The global and local models use inconsistent material properties or thicknesses
Verification checks
- Compare global model displacements at the submodel boundary with local model displacements — they should match
- Compare local model reaction forces with global model cut-section forces
- Check that moving the local model cut boundary does not change the result at the feature of interest
- Verify that the global model mesh is adequate to provide smooth displacement fields for submodel interpolation