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Composite Mesh
Generate through-thickness mesh for composite curing simulation. Invoke when user needs C3D8 solid elements, composite mesh, or through-thickness element layout.
Archived 2026-10-23 · Source updated 2026-09-30 · License: MIT. Check the original repository for the latest version.
SKILL.md instructions
# Composite Through-Thickness Mesh
This skill generates the through-thickness mesh for the composite (P8) part in curing
simulation. It covers element type selection, through-thickness discretization, in-plane
mesh layout, surface element set identification, and mesh quality considerations.
## Element Type
The composite part uses **C3D8** elements: 8-node linear brick elements with full
integration.
```
*Element, type=C3D8
1, <n1>, <n2>, <n3>, <n4>, <n5>, <n6>, <n7>, <n8>
```
### Why C3D8 (Not C3D8R)
| Property | C3D8 (Full Integration) | C3D8R (Reduced Integration) |
|------------------|--------------------------------|--------------------------------|
| Integration pts | 8 (2x2x2) | 1 (centroid) |
| Hourglassing | No | Yes (needs control) |
| Accuracy | Higher for bending | Lower for bending |
| Use case | Composite plies (this model) | Tool/mold (this model) |
Full integration (C3D8) is preferred for the composite because:
- Each ply is a single element layer through the thickness, so bending accuracy matters
- No hourglass control is needed
- Stress recovery is more accurate at the integration points
The tool/mold part uses C3D8R (reduced integration) because it is a bulk body where
hourglassing is less of a concern and computational efficiency matters more.
## Through-Thickness Direction
The through-thickness direction is along the **X-axis**, spanning from X = -1 to X = 0.
This means the composite thickness is 1 unit (1 mm in model units) in the X direction.
```
X = -1.0 +-----------------------------------+
| Ply-1 (element layer 1) |
X = -0.75+-----------------------------------+ (4-ply: 0.25 mm each)
| Ply-2 (element layer 2) |
X = -0.5 +-----------------------------------+
| Ply-3 (element layer 3) |
X = -0.25+-----------------------------------+
| Ply-4 (element layer 4) |
X = 0.0 +-----------------------------------+
```
The stack direction in `*Solid Section, composite` is **3**, which corresponds to this
X-axis through-thickness direction via the orientation definition.
## 4-Ply Mesh
The 4-ply mesh has 4 elements through the thickness, each 0.25 mm thick (total 1 mm).
### Mesh Statistics
| Property | Value |
|-----------------------------|---------|
| Through-thickness elements | 4 |
| Ply thickness | 0.25 mm |
| Total thickness | 1.0 mm |
| In-plane nodes per layer | 605 |
| Total elements | 2160 |
| Total nodes | 3025 |
### Node Layout
- **Through-thickness nodes**: 5 (4 elements + 1)
- **In-plane nodes per layer**: 605
- **Total nodes**: 605 x 5 = 3025
### Element Layout
- **In-plane elements per layer**: 540 (2160 total / 4 layers)
- **Through-thickness layers**: 4
- **Total elements**: 540 x 4 = 2160
## 8-Ply Mesh
The 8-ply mesh has 8 elements through the thickness, each 0.125 mm thick (total 1 mm).
### Mesh Statistics
| Property | Value |
|-----------------------------|---------|
| Through-thickness elements | 8 |
| Ply thickness | 0.125 mm|
| Total thickness | 1.0 mm |
| In-plane nodes per layer | 605 |
| Total elements | 4320 |
| Total nodes | 5445 |
### Node Layout
- **Through-thickness nodes**: 9 (8 elements + 1)
- **In-plane nodes per layer**: 605
- **Total nodes**: 605 x 9 = 5445
### Element Layout
- **In-plane elements per layer**: 540 (4320 total / 8 layers)
- **Through-thickness layers**: 8
- **Total elements**: 540 x 8 = 4320
## In-Plane Mesh
The in-plane mesh is shared between the 4-ply and 8-ply models. Both have **605 nodes per
through-thickness layer** and **540 elements per through-thickness layer**.
### In-Plane Layout
| Property | Value |
|-----------------------------|---------|
| Nodes per layer | 605 |
| Elements per layer | 540 |
| Shape | L-bracket |
| In-plane dimensions | 100 mm |
The in-plane mesh defines the L-bracket shape. The through-thickness discretization
(4 or 8 layers) is applied on top of this in-plane mesh by extruding nodes and elements
in the X direction.
## Element Connectivity Pattern
Each C3D8 element has 8 nodes. The connectivity follows the standard Abaqus brick element
node ordering:
```
8-------7
/| /|
5-------6 |
| | | |
| 4-----|-3
|/ |/
1-------2
```
### Node Ordering Convention
| Node | Position |
|------|-----------------------|
| 1 | Bottom face, corner 1 |
| 2 | Bottom face, corner 2 |
| 3 | Bottom face, corner 3 |
| 4 | Bottom face, corner 4 |
| 5 | Top face, corner 1 |
| 6 | Top face, corner 2 |
| 7 | Top face, corner 3 |
| 8 | Top face, corner 4 |
### Through-Thickness Element Assignment
Element N in the through-thickness direction has nodes at two consecutive X-coordinate
layers. For the 4-ply mesh:
- **Element layer 1** (Ply-1): nodes at X = -1.0 and X = -0.75
- **Element layer 2** (Ply-2): nodes at X = -0.75 and X = -0.5
- **Element layer 3** (Ply-3): nodes at X = -0.5 and X = -0.25
- **Element layer 4** (Ply-4): nodes at X = -0.25 and X = 0.0
For element N (where N is the global element number), the through-thickness layer is:
```python
layer = (N - 1) // in_plane_element_count
# in_plane_element_count = 540 for both 4-ply and 8-ply models
```
### Connectivity Example
For a 4-ply model with 540 in-plane elements per layer:
- Elements 1-540: layer 0 (Ply-1, X from -1.0 to -0.75)
- Elements 541-1080: layer 1 (Ply-2, X from -0.75 to -0.5)
- Elements 1081-1620: layer 2 (Ply-3, X from -0.5 to -0.25)
- Elements 1621-2160: layer 3 (Ply-4, X from -0.25 to 0.0)
Each element's 8 nodes consist of 4 nodes from the lower X layer and 4 nodes from the
upper X layer, with matching in-plane positions.
## Surface Element Sets
Two surface element sets are defined for the composite part. These sets identify the
elements on the outer and inner surfaces of the L-bracket, which are used for contact
and pressure application.
### _com-surface_S1 (Outer Surface)
```
*Elset, elset=_com-surface_S1, internal
<element_numbers>
```
- **Surface face**: S1
- **Location**: Outer surface of the composite (facing the mold)
- **Purpose**: Contact surface (slave) with the tool's `tool-surface`
- **Selection rule**: Every Nth element in the through-thickness direction at the outer
surface face
### __PickedSurf337_S2 (Inner Surface)
```
*Elset, elset=__PickedSurf337_S2, internal
<element_numbers>
```
- **Surface face**: S2
- **Location**: Inner surface of the composite (away from the mold)
- **Purpose**: Pressure application surface (`*Dsload _PickedSurf337, P, 0.6`)
- **Selection rule**: Every Nth element in the through-thickness direction at the inner
surface face
### Surface Selection Pattern
For the surface element sets, elements are selected from specific through-thickness
positions. The S1 face corresponds to one side of the through-thickness stack, and the
S2 face corresponds to the other side.
```python
# For a 4-ply model (4 through-thickness layers, 540 elements per layer)
# S1 (outer) elements: layer 0, specific in-plane elements
# S2 (inner) elements: layer 3, specific in-plane elements
# The surface elements are every Nth element where N depends on the
# in-plane mesh pattern at the surface face
```
## How to Identify Surface Elements from Connectivity
To identify which elements belong to the S1 or S2 surface, examine the element
connectivity and node coordinates.
### Step-by-Step Method
1. **Parse all elements**: Read the `*Element, type=C3D8` block to get all element
connectivities.
2. **Parse all nodes**: Read the `*Node` block to get all node coordinates.
3. **Identify surface nodes**: Find nodes that lie on the outer surface (S1) or inner
surface (S2) by checking their in-plane coordinates against the bracket boundary.
4. **Identify surface elements**: For each element, check if it has a face (4 nodes)
that lies entirely on the target surface. The face is identified by the S1 or S2
face label in the C3D8 element convention.
5. **Build element set**: Collect all element IDs that have a face on the target surface.
### Python Example
```python
def identify_surface_elements(elements, nodes, surface_face='S1'):
"""
Identify elements on a given surface face.
Args:
elements: Dict of {element_id: [node1, ..., node8]}
nodes: Dict of {node_id: (x, y, z)}
surface_face: 'S1' (outer) or 'S2' (inner)
Returns:
List of element IDs on the surface.
"""
# C3D8 face definitions: face -> node indices (0-based)
face_nodes = {
'S1': [0, 1, 2, 3], # Bottom face (nodes 1,2,3,4)
'S2': [4, 5, 6, 7], # Top face (nodes 5,6,7,8)
}
# Determine target X coordinate for the surface
if surface_face == 'S1':
target_x = -1.0 # Outer surface (X = -1)
else:
target_x = 0.0 # Inner surface (X = 0)
surface_elements = []
for elem_id, node_list in elements.items():
face_idx = face_nodes[surface_face]
face_node_ids = [node_list[i] for i in face_idx]
# Check if all face nodes are at the target X coordinate
all_on_surface = all(
abs(nodes[nid][0] - target_x) < 1e-6
for nid in face_node_ids
)
if all_on_surface:
surface_elements.append(elem_id)
return sorted(surface_elements)
```
### Face Label Reference
| Face | Node Indices (0-based) | Node Numbers (1-based) | Description |
|------|------------------------|------------------------|----------------------|
| S1 | 0, 1, 2, 3 | 1, 2, 3, 4 | Bottom face (X = -1) |
| S2 | 4, 5, 6, 7 | 5, 6, 7, 8 | Top face (X = 0) |
| S3 | 0, 1, 5, 4 | 1, 2, 6, 5 | Side face |
| S4 | 1, 2, 6, 5 | 2, 3, 7, 6 | Side face |
| S5 | 2, 3, 7, 6 | 3, 4, 8, 7 | Side face |
| S6 | 3, 0, 4, 7 | 4, 1, 5, 8 | Side face |
**Note**: The exact face-to-node mapping depends on the element node ordering in the INP
file. Always verify by checking node coordinates against the expected surface position.
## Mesh Quality Considerations for Curing Simulation
### Aspect Ratio
- Through-thickness element size: 0.25 mm (4-ply) or 0.125 mm (8-ply)
- In-plane element size: Should be comparable to avoid high aspect ratios
- Maximum aspect ratio: Keep below 10:1 for accurate stress recovery
### Through-Thickness Resolution
- **4 plies**: Minimum resolution for capturing ply-level behavior. Each ply is a single
element layer, so stress is constant through the ply thickness.
- **8 plies**: Better resolution for capturing through-thickness stress gradients. Each
ply is still a single element layer, but the ply thickness is halved.
### Contact Surface Quality
- The S1 surface (outer/contact surface) must have a smooth, continuous element layout
with no gaps or overlaps.
- Element faces on the contact surface should be as uniform as possible to ensure
accurate contact pressure distribution.
- Avoid highly distorted elements on the contact surface, as they can cause contact
convergence issues.
### Temperature Gradient Resolution
- During curing, temperature changes from 25C to 180C and back. The through-thickness
mesh must be fine enough to capture temperature gradients if they exist.
- For thin composites (1 mm total thickness), the temperature is approximately uniform
through the thickness, so 4 plies may be sufficient.
- For thicker composites, more through-thickness elements may be needed.
### Mesh Regeneration When Changing Ply Count
When changing from 4 plies to 8 plies (or vice versa), the entire through-thickness mesh
must be regenerated. This is because:
1. The number of through-thickness nodes changes (5 for 4-ply, 9 for 8-ply)
2. The element connectivity changes (different node pairings)
3. The total node and element counts change (3025/2160 for 4-ply, 5445/4320 for 8-ply)
4. All assembly-level sets referencing P8 nodes or elements must be updated
Use the merge strategy described in `modeling/composite-layup` to swap the P8 Part from
a pre-built model with the correct mesh.
## Common Pitfalls
1. **Element type mismatch**: The composite uses C3D8 (full integration), while the tool
uses C3D8R (reduced integration). Do not swap these.
2. **Through-thickness direction**: The thickness is along the X-axis (from -1 to 0), not
the Z-axis. This is set by the orientation and stack direction, not by the mesh alone.
3. **Surface face labels**: S1 is the outer/contact surface (X = -1), and S2 is the
inner/pressure surface (X = 0). Do not confuse these when defining surface element sets.
4. **Node count verification**: After mesh generation, verify the total node count matches
the expected value (3025 for 4-ply, 5445 for 8-ply). A mismatch indicates a mesh error.
5. **In-plane mesh sharing**: The in-plane mesh (605 nodes per layer, 540 elements per
layer) is the same for both 4-ply and 8-ply models. Only the through-thickness
discretization differs.
6. **Element numbering continuity**: Elements are numbered sequentially through the
through-thickness layers. Element layer boundaries occur at multiples of the in-plane
element count (540). Use this to assign ply labels correctly.