Local refinement toward a feature – Coreform IGA for Abaqus

1 Description

In this problem we will locally refine an immersed Coreform IGA mesh toward a small geometric feature. We continue directly from the thin plate with a hole example — complete that example first, since we reuse its model, mesh, and job.

The plate’s stress concentration lives at the edge of the hole, and that is where mesh resolution pays off: refining the entire background mesh sharpens the answer but multiplies the element count everywhere, including the far field where the stress is essentially uniform.

Local refinement instead refines the background mesh only near the hole: you name the hole’s surface in Abaqus/CAE, and the refinecf utility marks every background cell that surface touches for hierarchical refinement, level by level, down to the resolution you request.

2 How local refinement works

The pieces fit together as follows:

In Abaqus/CAE you create a named assembly set containing the feature’s faces.
When the Coreform IGA job writes its <job>.cf file, the set is exported into the file’s geometry description.
You run refinecf on the .cf file which will discover the background cells the named surfaces touch, so each refinement level tightens around the feature, and writes the resulting refinement instructions back into the .cf file.
The Coreform IGA meshing step reads those instructions and builds a hierarchical spline basis.

Everything downstream (trimming, the Abaqus solve, visualization) then works exactly as it does for a uniform mesh.

3 Naming the feature in Abaqus/CAE

Open the model database from the thin plate with a hole example. The only adaptivity-specific modeling step happens in the Assembly module: create a set containing the hole’s inner cylindrical face.

In the model tree, expand Assembly and double-click Sets.
In the Create Set dialog, enter the name hole and click Continue.
In the viewport, select the hole’s inner cylindrical face, and click Done.

The equivalent scripting-interface command selects the hole face by a bounding box that contains the hole but stops short of the plate’s outer faces:

inst = mdb.models["PWH-Kt"].rootAssembly.instances["Plate-1"]
hole_faces = inst.faces.getByBoundingBox(
    xMin=-1.1, xMax=1.1,   # the hole has radius 1 at the origin;
    yMin=-1.1, yMax=1.1,   # the plate's outer faces extend to x = +/-5, y = +/-2.5
    zMin=-1.0, zMax=1.0,
)
mdb.models["PWH-Kt"].rootAssembly.Set(faces=hole_faces, name="hole")

Create the set on the assembly, not on the part. An assembly set is exported with its bare name (hole); a part-level set is prefixed with the instance name, and refinecf --surface-set=hole will not find it.

4 Regenerating the model file

The <job>.cf file written by the Coreform IGA job carries the geometry, the background-mesh definition, and the exported sets, and is the input to every step below. Because we just added the hole set, the Job-PWH-Kt.cf file from the original example run does not contain it — resubmit Job-PWH-Kt from the Coreform IGA for Abaqus Job Manager so the file is regenerated with the set included. The refreshed Job-PWH-Kt.cf is written to the Abaqus working directory.

5 Running refinecf

refinecf is installed alongside the other Coreform command-line tools. Its inputs are the .cf file, the set name, and the number of refinement levels:

refinecf Job-PWH-Kt.cf \
    --surface-set=hole \
    --max-level=2 \
    --surface-offset=0.1 \
    --balance \
    --apply

--surface-set=hole selects the predicate: refine cells that the named CAD surfaces touch.
--max-level=2 refines two levels deep. Each level halves the element size, so the in-plane element size at the hole edge goes from 0.3 to 0.075.
--surface-offset=0.1 widens the catch: cells within that distance of the surface refine too, not only the cells it passes through. About a third of a background cell works well — it keeps the refined band snug against the feature while avoiding lone coarse cells grazing the surface. Pass 0.0 for the exact-touch band, or a per-level list such as --surface-offset=0.6,0.3 to cast a wider net at the coarse level and taper toward the surface at the deepest level.
--balance enforces 2:1 admissibility: after the predicate-driven refinement, additional cells are refined so that no two face-adjacent elements differ by more than one level, keeping the element-size transition gradual.
--apply writes the refinement instructions directly into the .cf file. Without it, refinecf only writes a JSON description (--output, default refinement.json) and leaves the model file untouched.

The log reports how many elements were selected at each level, per part.

5.1 Other predicates

If your feature has no convenient named surface, refinecf also accepts purely geometric predicates:

--center=x,y,z --radius=R — refine inside a sphere (add --inner-radius for a spherical shell),
--bbox=xmin,ymin,zmin,xmax,ymax,zmax — refine inside an axis-aligned box.

Both accept per-level value lists (level 0 first) to shape the refinement gradation, exactly like --surface-offset.

6 Meshing and solving

Run the Coreform IGA meshing step on the refined model file:

coreform_iga_mesh Job-PWH-Kt.cf

The refinement instructions stored in the file are picked up automatically and the remaining pipeline (interop, the Abaqus solve, and post-processing) proceeds exactly as in the uniform-mesh workflow.

Submitting the job from Abaqus/CAE regenerates the .cf file without the refinement instructions. Any time the file is regenerated, re-run refinecf --apply before meshing again.

7 What to expect

The refined mesh concentrates its added degrees of freedom in a band around the hole: the level-2 elements resolve the stress gradient at the hole edge while the plate’s far field stays at the coarse background resolution.

Von Mises stress near the hole on the locally refined mesh. The element boundaries show the two balanced refinement levels hugging the hole, with the stress concentration at the top and bottom of the hole resolved by the finest elements.

Compare the maximum principal stress along the line probe from the original example against the uniform-mesh run — the refined mesh sharpens the peak at the hole edge for a small fraction of the element count a uniformly-fine mesh would need.