CFRP vs. GFRP — material comparison
CFRP (Carbon Fibre Reinforced Polymer) and GFRP (Glass Fibre Reinforced Polymer) are both fibre-reinforced composites — but differ significantly in stiffness, strength, cost, and failure behaviour.
| Property | CFRP (UD lamina) | GFRP (E-glass UD lamina) |
|---|---|---|
| Fibre modulus E1 (fibre direction) | 60–150 GPa | 40–50 GPa |
| Transverse modulus E2 | 8–12 GPa | 10–15 GPa |
| In-plane shear modulus G12 | 4–6 GPa | 4–5 GPa |
| Tensile strength (fibre direction) X_T | 1,200–2,000 MPa | 800–1,200 MPa |
| Strain to failure (fibre) | ~1.0–1.5% | ~3.0–4.0% |
| Density | 1,500–1,600 kg/m³ | 1,800–2,100 kg/m³ |
| Typical application | Aerospace primary structure, motorsport, wind turbine spar | Marine, wind blade skin, automotive body |
| Relative cost | High ($15–80/kg fibre) | Low ($2–5/kg fibre) |
FEA model setup for composites
Setting up a composite FEA model requires several steps beyond standard isotropic FEA:
- Define local material coordinate systems — fibre angle (θ) is measured relative to a local coordinate system, not global axes. Critical for correct stiffness and failure prediction.
- Define ply-by-ply laminate stack — each ply has a material, thickness, and fibre orientation angle (e.g., [0/45/-45/90]_s = symmetric quasi-isotropic layup).
- Select appropriate element type — shell for thin laminates; solid or continuum shell for thick sections or through-thickness effects.
- Define failure criteria — specify which failure theory to use. Most software supports Hashin, Tsai-Wu, and Maximum Stress/Strain.
- Configure output requests — failure index (FI), strength ratio (SR), ply-level stresses in material coordinates (S11, S22, S12).
Element type selection
Shell elements are the standard choice for thin composite laminates — giving accurate in-plane stresses with a fraction of the computational cost of solids. Solid elements are needed when through-thickness effects matter.
| Element type | Abaqus | Ansys | When to use |
|---|---|---|---|
| Conventional shell | S4R, S4, S8R | SHELL181, SHELL281 | Thin laminates (t/a < 0.1); membrane + bending; most typical composite FEA |
| Continuum shell | SC8R, SC6R | SOLSH190 | Moderate thickness; better through-thickness accuracy than conventional shell; still efficient |
| 3D solid | C3D8R, C3D20R | SOLID185, SOLID186 | Thick sections; bolt/joint regions; free edge delamination; interlaminar stresses |
Failure criteria for composite FEA
Maximum Stress / Maximum Strain
The simplest criteria — failure occurs when any stress or strain component exceeds its allowable value in the fibre or matrix direction. No interaction between components is assumed. Conservative for combined loading. Useful as a quick first estimate but not recommended for final design validation.
Tsai-Hill criterion
An energy-based interactive criterion derived from the Von Mises criterion for isotropic materials — adapted for anisotropic composites. Failure index: σ₁²/X² - σ₁σ₂/X² + σ₂²/Y² + τ₁₂²/S² ≥ 1. More physically meaningful than maximum stress for combined loading, but uses a single strength value per direction (not distinguishing tension from compression).
Tsai-Wu criterion
The most general polynomial failure criterion — includes linear stress terms to distinguish tensile and compressive strengths. Requires five material strength parameters: X_T, X_C, Y_T, Y_C, S12. Widely used in industry for strength prediction under complex combined loading states.
Hashin failure criterion (most recommended)
Physically-based failure criterion that separates failure into four distinct modes: fibre tensile failure, fibre compressive failure, matrix tensile failure, matrix compressive failure. Each mode has its own failure index and damage variable. Hashin is the most widely used criterion in aerospace and automotive FEA — implemented natively in Abaqus, Ansys ACP, and HyperWorks.
Progressive failure analysis
First-ply failure (FPF) is often overly conservative — many structures continue to carry load well beyond FPF through load redistribution. Progressive failure analysis (PFA) or Continuum Damage Mechanics (CDM) models the degradation of ply stiffness as damage accumulates, tracking how load redistributes as each ply progressively fails.
In Abaqus, the Hashin damage model (DAMAGEFT, DAMAGEF C, DAMAGEMT, DAMAGEMC damage variables) provides built-in CDM for plane-stress composite shells. For more complex damage scenarios, the VUMAT user subroutine interface allows implementation of custom CDM models — used for research-grade material characterisation programs.
Software tools for composite FEA
| Software | Best for | Key composites feature |
|---|---|---|
| Abaqus | Advanced nonlinear composite analysis, CDM, delamination | Built-in Hashin CDM, VUMAT for custom models, cohesive elements for delamination |
| ANSYS ACP (Composite PrepPost) | Ansys Mechanical composite workflow | Intuitive ply definition, rosette/orientation systems, automated failure post-processing |
| Altair HyperMesh + OptiStruct | Composite optimisation (ply orientation, thickness, stacking) | Composite optimisation module — fibre angle, ply count, and stacking optimisation |
| Siemens Femap + Nastran | Aerospace structural composite programs | PCOMP/PCOMPG cards, laminate post-processing, Tsai-Wu, Max Stress |
| MSC Patran + Nastran | Legacy aerospace composite programs | PCOMP laminate definition, Tsai-Wu/Max Stress failure, established post-processing |
Frequently asked questions
What is the difference between CFRP and GFRP?
CFRP has higher stiffness (E ≈ 60–150 GPa) and strength-to-weight ratio; GFRP is cheaper, more corrosion-resistant, and has higher strain to failure (~3–4% vs. ~1%). CFRP is used for primary aerospace/motorsport structures; GFRP for marine, wind blades, and cost-sensitive applications.
What failure criteria are used for composite FEA?
Main criteria: Maximum Stress/Strain (simple, no interaction), Tsai-Hill (energy-based), Tsai-Wu (most general tensor polynomial), and Hashin (physically-based, separate fibre/matrix failure modes). Hashin is the most common in modern aerospace and automotive FEA.
Should I use shell or solid elements for composite FEA?
Shell elements (S4R, SHELL181) for thin laminates — accurate in-plane stresses, low cost. Solid elements (C3D8R, continuum shell SC8R) for thick sections, bolted joints, or delamination analysis where through-thickness stresses matter.
What software is best for composite FEA?
Abaqus for advanced nonlinear and CDM analysis. ANSYS ACP for a clean pre/post workflow in the Ansys ecosystem. HyperMesh + OptiStruct for composite optimisation. Femap/Patran + Nastran for aerospace structural programs.
