How to Model Composite Layups in FEA

1. The Engineering Task

Define a composite laminate in an FE model such that the shell or solid elements correctly represent the stiffness, mass and failure behaviour of the physical laminate.

2. When to Use This Method

Every FE analysis involving composite structures. The laminate definition determines the model stiffness, the stress distribution across plies and the failure prediction.

3. What You Need Before Starting

• Unidirectional (UD) ply engineering constants: E₁, E₂, G₁₂, G₁₃, G₂₃, ν₁₂, density — from the material qualification data sheet
• The layup definition: ply orientations, thicknesses and materials
• The element type: conventional shell, continuum shell or solid
• The failure criterion to be used: Tsai-Wu, Puck, Hashin, or maximum stress

4. Step-by-Step Method

1. Input the UD ply engineering constants. Use test data from the actual manufacturing process and fibre volume fraction — generic textbook values introduce significant error
2. Define the laminate stack: ply orientations, thicknesses and materials. Convention: 0° aligns with the element reference direction. Positive angles are counter-clockwise from the top surface
3. Choose the element type: continuum shell (SC8R) for through-thickness stresses, conventional shell (S4R) for efficiency, solid elements for thick laminates and 3D effects near joints
4. Verify the ABD matrix: compute it analytically using Classical Laminate Theory and compare with the solver output. Check that B = 0 for symmetric laminates and that A₁₁/A₂₂ and D₁₁/D₂₂ are consistent with the ply angles
5. For temperature-dependent properties: define the ply properties as a function of temperature if the analysis spans a wide temperature range
6. Request ply-by-ply stress output in the ply material coordinate system, not the element coordinate system
7. Apply the failure criterion to each ply individually and report the failure index and reserve factor

5. What to Check

• Does the ABD matrix match the CLT prediction? Discrepancies indicate data entry errors
• Is the 0° direction correctly aligned with the primary load direction? A misalignment of 5° can significantly change the laminate stiffness
• For symmetric laminates: is the B matrix zero? Non-zero B indicates an unsymmetric layup or a modelling error
• Are the ply stresses extracted in the ply material coordinate system? Element coordinate stresses are not meaningful for composites
• Has interlaminar shear stress been checked at critical ply interfaces?

A [0/±45/90]₂s laminate is quasi-isotropic and is a common starting point for multi-directional loading. However, quasi-isotropic laminates are rarely optimal — tailoring the layup to the load path can save significant weight.

6. How to Interpret the Result

The FE model produces ply-level stresses and failure indices. The critical ply is the one with the highest failure index. For a well-designed laminate, the failure indices across plies should be reasonably balanced — if one ply is critical while others have very low indices, the layup is not optimised. Interlaminar shear stresses at ply interfaces should be checked for delamination risk, particularly near free edges and joints.

ABD matrix verification:
Aᵢⱼ  =  Σₖ  Q̄ᵢⱼ⁽ᵏ⁾ · tₖ

         1
Dᵢⱼ  =  ─  Σₖ  Q̄ᵢⱼ⁽ᵏ⁾ · (zₖ³ − zₖ₋₁³)
         3

For symmetric laminates: B = 0

ply-book

7. Common Mistakes

• Using generic textbook ply properties instead of test data from the actual manufacturing process
• Defining the 0° direction as the global x-axis rather than the element reference direction — these may differ for curved elements
• Not verifying the ABD matrix — the layup may have a data entry error that produces incorrect stiffness without obvious symptoms
• Extracting stresses in element coordinates instead of ply material coordinates
• Not checking interlaminar shear at free edges and bolt holes — delamination may be the critical failure mode

8. Further Reading

See the Composite Structures Knowledge category for laminate theory, failure criteria and composite FEA. See How to Interpret Composite Failure Criteria for the failure assessment workflow.

• Composite Ply Properties (Reference Library)
• Composite Laminate Calculator
• Composite Laminate Example (Worked Example)
• Composite Design Checklist