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)

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

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

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

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

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:

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.

# 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

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

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.