Prepare a part for FEA

This is what CAD Preview is for. The previous tutorials made geometry; this one turns geometry into something a finite-element solver can run: named regions, a graded tetrahedral mesh, and a Kratos MDPA file with those regions carried through as sub-model-parts.

It picks up the bracket from the first tutorial — build that first, or paste its operation list in.

It should take about 20 minutes.

Starting point

The finished bracket: an L-shaped solid with a filleted inside corner and two counterbored bolt holes, 18 faces in total. Its full operation list is repeated at the bottom of this page if you need it.

Step 1 — Name the regions you will apply boundary conditions to

A solver needs to know where things are fixed and where the load goes. In CAD Preview those are parts — named groups of entities, stored in a <model>.parts.json sidecar, never in the CAD file.

1. Open the Select ▾ menu in the toolbar, click Selection mode, and choose Surf.
2. Click the bracket's underside (the large face at z = −3).
3. In the Parts panel, click ＋ New, rename the part to FixedBase, and click the ＋ on its row to assign the selection.
4. Repeat for the wall's outer face (at y = −20), naming it LoadFace.
5. Repeat once more for the two bolt-hole walls — shift-click to select both — naming it BoltHoles.

Each part gets a colour, and the assigned faces recolour in the 3D view immediately. If you lose track of which face is which, the hover tooltip names the entity under the cursor.

Step 2 — Refine the mesh where it matters

Bolt holes are where stress concentrates, so they want smaller elements than the rest of the part.

1. In the Parts panel, find the BoltHoles row and set its mesh size field to 1.5.
2. The same field is mirrored in the FE Mesh panel's Part sizes section — either one works, and they stay in sync.

A per-part size becomes a Gmsh sizing field scoped to that part's entities. Where several overlap, the smallest requested size wins; everything unassigned keeps the global size from the next step.

Step 3 — Set the global mesh size and generate

1. Open the FE Mesh panel.
2. Leave Dimension at 3 (a volume mesh of tetrahedra).
3. Drag the size slider, or use the Coarse / Medium / Fine presets. For this part, a Size max of 4 is a reasonable starting point — the readout shows the resulting element estimate as you drag.
4. Click ▶ Generate.

The mesh appears as an overlay on top of the model, coloured per part — so you can see at a glance that your named regions ended up where you meant them.

With Size max = 4 and the bolt holes at 1.5, this bracket meshes to roughly 1200 nodes and 3600 elements in well under a second.

Step 4 — Read the quality summary

Under the node and element counts, the panel reports the mesh's minimum and mean element quality plus a histogram. The metric is Gmsh's minSICN, where 1 is an ideal element and 0 is degenerate.

For this bracket you should see a minimum around 0.2 and a mean around 0.75 — healthy for a tetrahedral mesh with a fillet and two holes in it.

If any elements fall below 0.2, a Worst toggle appears next to Clear and lights up automatically. It highlights those elements in red, drawn through the rest of the model so you can see a bad element buried inside the volume rather than only on the surface. Coarsening or refining usually clears them.

Step 5 — Advanced settings, if you need them

Expand Advanced settings for element order and shape.

• Element order 2 adds mid-side nodes — Tetrahedra3D10 instead of Tetrahedra3D4. The overlay still draws corner geometry only, so it looks the same; the node count roughly quadruples.
• Element shape switches between tetrahedra, hexahedra, and hex-dominant.

Kratos MDPA cannot represent a hex-dominant mesh

Hex-dominant meshing emits a tet/hex transition element that has no Kratos geometry equivalent. The export refuses with a message naming it, rather than writing a file that would fail to load. Gmsh's own formats and VTK handle it fine.

Step 6 — Export for Kratos

1. In the FE Mesh panel, choose Kratos MDPA — Elements + Conditions in the export dropdown (it is the default).
2. Leave the unit selector at mm unless your solver expects otherwise — it applies a real geometric scale, not a relabelling.
3. Click 📤 Export and choose where to save.

Open the resulting .mdpa in a text editor. Past the Nodes, Elements, and Conditions blocks you will find one sub-model-part per named part:

Begin SubModelPart FixedBase
    Begin SubModelPartNodes
    ...
    Begin SubModelPartElements
    Begin SubModelPartConditions

Those are the handles your Kratos case file references when it applies a fixity or a load. The two export modes differ in how they write the cells: Elements + Conditions writes Element* and Condition* blocks with a property id; Geometries writes a single Geometries container sharing one id space.

Full operation list

The bracket, for reference:

[
  { "op": "addBox", "center": [0, 0, 0], "size": [60, 40, 6] },
  { "op": "addBox", "center": [0, -17, 18], "size": [60, 6, 30] },
  { "op": "boolean", "kind": "union", "a": ["solid-1"], "b": ["solid-2"] },
  { "op": "fillet", "edges": ["edge-13"], "radius": 4 },
  {
    "op": "addCounterboreHole",
    "targets": ["solid-0"],
    "position": [-22, 10, 3],
    "axis": [0, 0, -1],
    "radius": 3,
    "depth": 6,
    "cbRadius": 5,
    "cbDepth": 2
  },
  {
    "op": "addCounterboreHole",
    "targets": ["solid-0"],
    "position": [22, 10, 3],
    "axis": [0, 0, -1],
    "radius": 3,
    "depth": 6,
    "cbRadius": 5,
    "cbDepth": 2
  }
]

The parts and mesh options live in their own sidecars (<model>.parts.json, <model>.mesh.json) rather than in the op list — they are not edits, they are metadata about the model.

What you practiced

• Parts as named entity groups, stored beside the CAD file and never in it.
• Per-part mesh sizing, and the smallest-size-wins rule where regions overlap.
• FE mesh generation, its quality summary, and the worst-element highlight.
• Kratos MDPA export, and how parts become SubModelPart blocks a case file can reference.

Next: build this same bracket with an AI agent instead of by hand.