Modeling of Textile Composites
Modeling of Textile Composites: Theoretical Foundations
What are Woven Composites?
Professor, how are "woven composites" different from UD materials (unidirectional reinforced materials)?
In UD materials, all fibers are aligned in one direction, but woven composites have fibers that are woven (interlaced). They have weave structures like plain weave, twill weave, and satin weave.
What are the advantages of weaving?
- Strength in two directions simultaneously — Achieves the effect of a UD material's $[0/90]$ laminate in a single layer
- Drapability — Easier to form along curved surfaces
- Damage resistance — Stronger against delamination because fibers are interlaced
- Ease of handling — Can be handled as a single sheet
Types of Weaves
| Type | Structure | Characteristics |
|---|---|---|
| Plain Weave | One-over-one interlacing | Stable. Low drapability |
| Twill Weave | 2/1, 2/2, etc. patterns | High drapability |
| Satin Weave | 5HS, 8HS (long float) | Highest drapability. Fibers are nearly straight |
| NCF (Non-Crimp Fabric) | Fibers fixed by stitching without weaving | No crimp. Best mechanical properties |
What is "crimp"?
The waviness of fibers going over and under each other in a weave structure. Crimp causes fibers to be bent, resulting in a 10-20% reduction in tensile stiffness and strength compared to UD materials. NCF has no crimp, so it has performance close to UD materials.
FEM Modeling of Woven Composites
FEM modeling of woven composites has three levels:
| Level | Approach | Accuracy |
|---|---|---|
| Macro | Equivalent homogeneous shell (CLT-based) | Low (global behavior) |
| Meso | RVE (fiber tow + matrix) modeling | High (local stress) |
| Micro | Modeling individual fibers | Highest (for research) |
Is the mesoscale RVE model practical?
At the mesoscale, one unit of the weave pattern (Unit Cell) is modeled with solid elements, and equivalent properties are calculated using periodic boundary conditions. Specialized tools like TexGen and WiseTex automatically generate the Unit Cell geometry.
Summary
Let me organize the theory of woven composites.
Key points:
- Structure with interlaced fibers — Plain weave, twill weave, satin weave, NCF
- Stiffness and strength reduced by crimp — 10-20% lower than UD materials
- Excellent drapability and damage resistance — Suitable for forming on curved surfaces
- Three levels of modeling — Macro (CLT), Meso (RVE), Micro (individual fibers)
- Mesoscale RVE analysis is practical — Unit Cell generation with TexGen/WiseTex
Elastic Properties of Woven Composites and Unit Cell Theory
Woven composites are composed of repeating unit cells where warp and weft yarns intersect. Elastic properties can be calculated more accurately by FEM homogenization analysis of the unit cell, which better reflects the fiber bridging effect (crimp), compared to simple test values from ISO 527. A mere 5% crimp can reduce strength by 10-20%, and they offer better in-plane isotropy than unidirectional prepreg.
Computational Methods for Modeling of Textile Composites
Mesoscale RVE Analysis
Please explain the procedure for RVE (Representative Volume Element) analysis.
1. Generate Unit Cell geometry — 3D shape of the weave pattern using TexGen, WiseTex, etc.
2. Mesh generation — Mesh the Unit Cell with TET10
3. Apply periodic boundary conditions — Displacements on opposing faces have a linear relationship
4. Six load cases — Apply $\varepsilon_{11}, \varepsilon_{22}, \varepsilon_{33}, \gamma_{12}, \gamma_{23}, \gamma_{13}$ sequentially
5. Homogenization — Calculate equivalent elastic constants from the average stress of each load case
So the nine elastic constants (orthotropic) are determined from the six load cases.
Yes. You obtain $E_1, E_2, E_3, G_{12}, G_{23}, G_{13}, \nu_{12}, \nu_{23}, \nu_{13}$. These are used as material properties for macro-scale shell/solid elements.
Specialized Tools
| Tool | Features |
|---|---|
| TexGen | Unit Cell geometry generation. Open source (University of Nottingham) |
| WiseTex | Weave geometry + mechanics. Developed by KU Leuven |
| DIGIMAT | Multi-scale material modeling. eXstream/Hexagon |
| MicroMechanics | Fiber-matrix RVE analysis. MCT (Multi-Continuum Theory) |
It's great that TexGen is free.
TexGen automatically generates 3D geometry of weave structures and can directly output Abaqus input files. It's practically the standard tool for research on woven RVE analysis.
Handling Woven Composites at the Macro Scale
How are woven composites handled at the macro scale (regular FEM analysis)?
Treat them as equivalent homogeneous materials. Use the equivalent elastic constants obtained from RVE analysis as one layer in CLT and perform regular laminate analysis.
Points to note:
- Crimp effect — Verify if the equivalent properties include the influence of crimp
- Failure criteria — Applying UD material criteria like Tsai-Wu/Hashin directly can be inaccurate. Modifications for woven materials are needed
- Draping — Reflect changes in fiber angle during forming in the FEM
Summary
Let me organize the numerical methods for woven composites.
Key points:
- Calculate equivalent properties via RVE analysis — Nine elastic constants from six load cases
- Generate Unit Cell with TexGen (free) — Standard research tool
- Multi-scale coupling with DIGIMAT (commercial) — For industry
- Treat as equivalent homogeneous material at macro scale — One layer in CLT
- Do not use UD material failure criteria as-is — Modifications for woven materials are needed
Homogenization FEM Analysis of Woven Composites
Homogenization analysis of a unit cell proceeds in the order: ① 3D FEM construction of the cell, ② application of periodic boundary conditions, ③ calculation of equivalent elastic matrix from reaction forces under six-component unit loads. Computational cost is about solving one cell's FEM (~100k elements) six times, within a few hours. Specialized unit cell meshing tools like Software TexComp (KU Leuven), WiseTex, and TexGen are publicly available, automatically generating meshes just by inputting weave parameters (yarn width, thickness, crimp ratio).