Composite FEA: Orientation, Ply Definition and Failure Criteria
Best practice for composite FEA — material orientation, shell normal direction, ply stack ordering, failure criteria selection and interlaminar stress limitations.
Principle
Composite FEA requires meticulous attention to material orientation, ply definition and failure criteria. A 90° orientation error or a reversed shell normal can change the result by orders of magnitude. These are not minor details — they are the model.
Why it matters
Unlike isotropic materials where orientation does not matter, composites are highly directional. The fibre direction determines stiffness, strength and failure mode. A model with correct geometry, mesh and loads but wrong ply orientation is completely wrong.
Good practice
- Verify material orientation visually: most pre-processors can display the fibre direction — check it against the drawing
- Verify shell normal direction: the ply stack is defined from the bottom to the top of the shell — a reversed normal reverses the stack
- Check ply drop-offs: ensure the ply sequence is correct at thickness transitions — a missing or extra ply changes the stiffness
- Use a failure criterion appropriate to the material and load: Tsai-Wu for general, Hashin for fibre/matrix discrimination, Puck for fibre/matrix interaction
- Check both fibre and matrix failure indices — the first-ply failure is often matrix (transverse) failure, not fibre failure
- Do not use solid-shell or solid composite elements without understanding through-thickness integration — interlaminar shear is not captured in shell models
- For bonded joints: model the adhesive layer explicitly or use cohesive elements for delamination
Interlaminar stress limitations
Shell-based composite models cannot capture through-thickness (interlaminar) stresses directly. These stresses are critical for delamination assessment at free edges, ply drop-offs and bolted joints. For delamination-critical assessment, use solid composite elements or a submodel with solid elements at the edge.
Warning signs
- Material orientation is not displayed or verified in the pre-processor
- Shell normal direction is not checked — the ply stack may be reversed
- A single failure criterion (e.g. Tsai-Wu) is used without checking whether fibre or matrix failure governs
- Interlaminar stresses are assessed from a shell model — shell models cannot capture through-thickness stress
- Ply drop-offs are not modelled — the stiffness transition is wrong
- The failure index is > 1.0 but the analyst does not identify which ply and which failure mode
Verification checks
- Run a single-element test in tension along the fibre direction — verify Ex matches E₁
- Run a single-element test in tension transverse to the fibre — verify Ey matches E₂
- Check the laminate effective modulus against a CLT hand calculation
- Verify the failure index for a simple load case matches a CLT calculation
- Display fibre direction arrows and check against the layup drawing