Accelerate Stress Analysis in Large Assemblies with Ansys Mechanical Submodeling

Problem: Long solve times in large assemblies

When you need detailed stress results from a single region of a massive assembly—such as a bolted flange or a gear housing—running a full fine-mesh analysis can take hours or days. The bottleneck is the sheer number of degrees of freedom, not the physics.

Why traditional refinement is inefficient

Adding local mesh refinement to the entire model increases node count dramatically. Even if you refine only a small zone, the global solver still processes every element, so the runtime scales with the whole geometry rather than the region of interest.

Solution: Cut-boundary submodeling in Ansys Mechanical

Core idea

Submodeling creates two linked models: a coarse global model that solves the entire assembly, and one or more refined local submodels that focus on a region of interest. The global solution provides cut-boundary displacements at the interface, which Ansys interpolates onto the submodel as enforced boundary conditions. This avoids the need for a uniformly fine mesh across the whole part.

When to use submodeling

Submodeling shines for static or quasi-static stress extraction where the local region participates mainly in local loading. It is less suitable when the region influences global dynamic modes or when many load cases share the same global model, in which case substructuring (CMS) may be more efficient.

Practical requirements

Mesh and boundary considerations

• Displacement fidelity: The global mesh must represent the correct displacement field at the cut faces. Using quadratic (2nd-order) elements on the global model improves interpolation accuracy.
• Saint-Venant decay: Cut boundaries should be placed far enough from stress concentrations—typically 2–3 times the characteristic dimension of the region—to let stress errors dissipate before interpolation.
• Mesh compatibility: Global and submodel meshes do not need to match in element type, order, or density, but the submodel must receive consistent displacement constraints.

Worked example: bolted flange assembly

Scenario

A flange with 10 bolts and a central hole is modeled with a global mesh of ~50,000 2nd-order tetrahedra. The region around a single bolt hole (the stress concentrator) is defined as a submodel containing ~200,000 hexahedral elements.

Model	Nodes	Elements	Solve time (global only)	Peak von Mises error vs. full fine mesh
Global model	50,000	200,000	~4 hours	—
Submodel (cut-boundary)	200,000	800,000	~25 minutes (submodel solve)	~3% (within acceptance)

The submodel reduces total solve time from roughly 4 hours to under 30 minutes while matching the peak stress within 3% of a full fine-mesh reference. The global model still solves the entire assembly, but the expensive local refinement is performed only on the submodel.

Limitations

Linear superposition assumption

Submodeling assumes that the global displacement field can be linearly combined from the cut faces. It cannot capture nonlinear contact separation or plasticity that originates exactly at the cut boundary unless those effects are already included in the global solution.

Thermal-stress coupling

For thermomechanical problems, both temperature and displacement must be interpolated. Ensure the global thermal solution uses compatible convection or radiation boundary conditions on the cut faces.

Getting started

Step-by-step setup

1. Create the global model: Build the full assembly in Workbench, assign material properties, and mesh with a reasonable element size. Use 2nd-order elements if possible.
2. Define the submodel region: In the Project Tree, right-click the part or subassembly that contains the region of interest → Insert → Submodeling Analysis System. This automatically creates a linked submodel.
3. Specify cut boundaries: In the submodel's Analysis Settings, select the faces or edges that will serve as cut boundaries. These must be fully defined (no partial edges).
4. Link solution data: In the Workbench Solution cell, set Displacement as the source to be interpolated onto the submodel's cut boundaries. The system handles the interpolation automatically.
5. Run the solve: Click the green solve arrow. The global solver runs first, then the submodel solves using the interpolated displacements.

Verification checklist

• Open Help → Structural Analysis Guide → Submodeling to confirm that the workflow steps match your installed version (Mechanical ≥ 14.0).
• Run the built-in verification example: Help → Examples → Mechanical → Submodeling → Bolted Flange. Compare the reported stress error with the 3% benchmark.
• Overlay global and submodel displacement contours on the cut faces and inspect the Interpolation Error plot. Energy-norm error should be < 2%.
• Perform a single spot-check with a full fine-mesh model of the same region to validate the submodel results.
• Monitor license usage via the License Management Center; each concurrent submodel solve consumes an HPC pack increment.

Closing: take action now

If your current workflow spends excessive time on full-assembly solves, adopt cut-boundary submodeling to isolate high-stress zones. Start by creating a small test submodel on a representative part, verify the interpolation error, and then scale the approach to your full assembly. The payoff is dramatically faster turnaround without sacrificing the accuracy needed for design decisions.