Interlaminar Stress Fundamentals

What Is It?

Interlaminar stresses are the through-thickness stresses that act at the interfaces between plies in a composite laminate. They include the through-thickness normal stress (peel stress, σ₃) and the interlaminar shear stresses (τ₁₃ and τ₂₃). These stresses are not captured by classical laminate theory (CLT), which assumes a state of plane stress. Interlaminar stresses are the drivers of delamination — the separation of plies at their interface. Understanding interlaminar stresses is essential for predicting delamination initiation.

Why It Matters

Delamination is one of the most critical failure modes in composite structures. It can reduce compressive strength by 30-60%, it can be caused by barely visible impact damage, and it can grow under fatigue loading. Interlaminar stresses are the drivers of delamination. Without understanding and modelling interlaminar stresses, the analysis cannot predict where and when delamination will initiate. This makes interlaminar stress analysis essential for any structure where delamination is a concern.

Interlaminar stresses drive delamination — one of the most critical composite failure modes. CLT does not capture these stresses. Without interlaminar stress analysis, delamination initiation cannot be predicted.

Through-Thickness Normal Stress

The through-thickness normal stress σ₃ (also called peel stress) is the stress that pulls the plies apart perpendicular to the laminate plane. It is driven by features that create through-thickness loading: pull-off loads, bending that creates through-thickness tension, curved geometries (L-brackets, T-stiffeners) where the load path changes direction, and free edges where Poisson mismatch creates peel. Peel stress is particularly dangerous because the through-thickness tensile strength of composites is very low — much lower than the in-plane strengths.

Through-thickness normal (peel) stress:

σ₃ = through-thickness normal stress

Drivers:
  Pull-off loads perpendicular to laminate
  Bending creating through-thickness tension
  Curved geometries (L-brackets, T-stiffeners)
  Free edges (Poisson mismatch)

Through-thickness tensile strength Z_T:
  Very LOW — typically 5-10% of fibre-direction strength
  Much lower than in-plane strengths
  σ₃ ≥ Z_T → delamination initiation

Interlaminar Shear

The interlaminar shear stresses τ₁₃ and τ₂₃ act at the ply interface, shearing adjacent plies relative to each other. They are driven by the mismatch in ply properties between adjacent plies — different orientations have different stiffness, so under the same laminate strain, adjacent plies want to deform differently. This mismatch creates interlaminar shear at the interface. Interlaminar shear is particularly significant at free edges, at ply drops, and at geometric discontinuities.

• Interlaminar shear: τ₁₃ and τ₂₃ — shearing adjacent plies relative to each other
• Driven by mismatch in ply properties between adjacent plies
• Different orientations have different stiffness → different deformation under same strain
• Particularly significant at free edges, ply drops, geometric discontinuities

Free Edges

Free edges are a classic source of interlaminar stress. At a free edge, the laminate boundary is unconstrained — the edge surface is stress-free. However, adjacent plies with different orientations have different Poisson ratios and different stiffness. Under in-plane loading, each ply wants to contract differently in the transverse direction. At the interior of the laminate, the plies are bonded and must deform together. At the free edge, the constraint is released, but the mismatch creates interlaminar shear and peel stresses that are concentrated near the edge. This is the free-edge effect.

Free edges create interlaminar stress concentrations. Adjacent plies with different orientations want to deform differently. At the interior, they are constrained; at the free edge, the constraint releases, creating shear and peel stress concentrations near the edge. This is the free-edge effect.

Ply Interfaces

Interlaminar stresses act at ply interfaces — the surfaces between adjacent plies. The interface is a thin resin-rich layer that bonds the plies. It is the weakest link in the laminate — the through-thickness strength and the interlaminar shear strength are determined by the matrix and the interface, not by the fibres. The interface is where delamination initiates and propagates. The stress state at the interface determines whether delamination will occur.

• Ply interfaces are where interlaminar stresses act
• Interface is a thin resin-rich layer — the weakest link
• Through-thickness and interlaminar shear strength determined by matrix, not fibre
• Interface is where delamination initiates and propagates

Geometric Discontinuities

Geometric discontinuities — ply drops, joints, flanges, radii, cut-outs — create interlaminar stress concentrations. At a ply drop, the laminate thickness changes, creating a step where the load must transfer from the dropped plies to the remaining plies. This load transfer occurs through interlaminar shear, which is concentrated at the ply drop. At a radius or curved geometry, the load path change creates through-thickness tension (peel). At a joint, the eccentricity and load transfer create both peel and shear. These geometric features are common delamination initiation sites.

Why CLT Alone Is Insufficient

Classical laminate theory (CLT) assumes a state of plane stress — only the in-plane stress components (σ₁, σ₂, τ₁₂) are computed. The through-thickness stresses (σ₃, τ₁₃, τ₂₃) are assumed to be zero. This assumption is reasonable for the interior of a thin laminate under in-plane loading, but it breaks down at free edges, geometric discontinuities and wherever through-thickness loading exists. CLT alone cannot predict delamination because it does not compute the stresses that drive it. A 3D analysis — either a 3D FEA model or an edge-stress solution — is needed to capture interlaminar stresses.

CLT assumes plane stress — it does not compute through-thickness stresses. It cannot predict delamination because it does not compute the stresses that drive it. For delamination assessment, 3D analysis or edge-stress solutions are needed. CLT alone is insufficient.

Analysis Methods

Several methods are available for computing interlaminar stresses. 3D FEA with solid or continuum-shell elements captures through-thickness stresses directly. Cohesive-zone elements at the interfaces capture both the stress state and the delamination behaviour. Analytical edge-stress solutions (e.g. Pipes-Pagano) provide approximate interlaminar stresses at free edges. The choice depends on the problem complexity, the required accuracy and the available tools. For delamination prediction, cohesive-zone modelling with 3D FEA is the most comprehensive approach.

• 3D FEA (solid or continuum-shell) — captures through-thickness stresses directly
• Cohesive-zone elements — captures both stress state and delamination behaviour
• Analytical edge-stress solutions (Pipes-Pagano) — approximate free-edge stresses
• For delamination prediction: cohesive-zone modelling with 3D FEA is most comprehensive

Key Takeaways

• Interlaminar stresses (σ₃, τ₁₃, τ₂₃) act at ply interfaces and drive delamination
• Through-thickness normal (peel) strength is very low — 5-10% of fibre-direction strength
• Free edges create interlaminar stress concentrations from Poisson mismatch between plies
• Geometric discontinuities (ply drops, radii, joints) are common delamination initiation sites
• CLT assumes plane stress and cannot predict delamination — 3D analysis or cohesive zones needed