Quick Start: Simple Triangle Mesh#

This page shows you how to create a simple StructureGene from scratch with a minimal working example.

Goal#

Create a square plate with 2 triangular elements and write it to SwiftComp format.

Square divided into 2 triangles 
Node Layout:
3 -------- 2
|        / |
|      /   |
|    /     |
|  /       |
0 -------- 1

Complete Example#

Here’s a complete, working example you can copy and run:

import numpy as np
from meshio import Mesh, CellBlock
from sgio import StructureGene
from sgio.model import CauchyContinuumModel
import sgio

# Step 1: Create StructureGene with configuration
sg = StructureGene(
    name='simple_plate',
    sgdim=2,  # 2D plate analysis
    smdim=2   # 2D structural model
)

# Step 2: Define material
steel = CauchyContinuumModel(
    name='Steel',
    isotropy=2,      # 2 = isotropic
    e=200e9,         # Young's modulus (Pa)
    nu=0.3,          # Poisson's ratio
    density=7850     # Density (kg/m³)
)
sg.materials['Steel'] = steel

# Step 3: Create material-orientation combination
sg.mocombos = {
    1: ('Steel', 0.0),  # Property ID 1 -> Steel at 0°
}

# Step 4: Define nodes (4 corners of square)
points = np.array([
    [0.0, 0.0, 0.0],  # Node 0 (bottom-left)
    [1.0, 0.0, 0.0],  # Node 1 (bottom-right)
    [1.0, 1.0, 0.0],  # Node 2 (top-right)
    [0.0, 1.0, 0.0],  # Node 3 (top-left)
])

# Step 5: Define elements (2 triangles)
cells = [
    CellBlock(
        type='triangle',
        data=np.array([
            [0, 1, 2],  # Triangle 1 (bottom-right half)
            [0, 2, 3],  # Triangle 2 (top-left half)
        ])
    )
]

# Step 6: Assign properties to elements
cell_data = {
    'property_id': [
        np.array([1, 1])  # Both elements use property 1 (Steel)
    ]
}

# Step 7: Assemble mesh
sg.mesh = Mesh(
    points=points,
    cells=cells,
    cell_data=cell_data
)

# Step 8: Verify
print(f"Created: {sg.name}")
print(f"  Nodes: {sg.nnodes}")
print(f"  Elements: {sg.nelems}")
print(f"  Materials: {list(sg.materials.keys())}")

# Step 9: Write to SwiftComp format
sgio.write(
    sg=sg,
    fn='simple_plate.sc',
    file_format='swiftcomp',
    format_version='2.1',
    model_type='PL1'  # 2D plate, classical model
)

print("✓ Written to simple_plate.sc")

Understanding the Example#

Step 1-2: Setup

Create the StructureGene container and define your material. For a 2D plate, use sgdim=2.

Step 3: Mocombos

Create the material-orientation combination. Property ID 1 maps to Steel at 0 degrees.

Step 4: Nodes

Define node coordinates. Always use 3 columns (x, y, z) even for 2D problems. Use z=0 for planar structures.

Step 5: Elements

Define elements by listing node indices. CRITICAL: Use 0-based indices (0, 1, 2, 3), not your original node IDs!

Step 6: Property Assignment

Each element gets a property ID that references a mocombo. Here both elements use property 1.

Step 7-9: Finish

Assemble the mesh, verify the structure, and write to file.

Key Points to Remember#

  1. Indices are 0-based: In the connectivity array, use indices 0, 1, 2, 3… not your original node IDs

  2. Property chain: Element → property_id → mocombo → (material, angle)

  3. Material names must match: The name in mocombos must exactly match a key in sg.materials

  4. Always 3D coordinates: Use [x, y, 0.0] for 2D problems

What’s Next?#

3D Solid Quick Example#

For a 3D structure, just change sgdim=3 and use 3D element types:

sg = StructureGene(name='cube', sgdim=3, smdim=3)

# ... define material and mocombos ...

cells = [
    CellBlock(
        type='tetra',  # Tetrahedral element
        data=np.array([[0, 1, 2, 3]])  # 4 nodes per tetrahedron
    )
]

# Write with SD1 (3D solid) model type
sgio.write(sg=sg, fn='cube.sc', file_format='swiftcomp',
           format_version='2.1', model_type='SD1')

Model Types#

When writing files, choose the appropriate model type:

  • Beams (1D): 'BM1' (Euler-Bernoulli), 'BM2' (Timoshenko)

  • Plates/Shells (2D): 'PL1' (Kirchhoff-Love), 'PL2' (Reissner-Mindlin)

  • Solids (3D): 'SD1' (classical), 'SD2' (refined)