Short answer
Delamination is the separation of adjacent plies in a laminated composite. The crack runs through the resin interlayer, where there are no fibres. Impact, free edges, holes, bending and manufacturing defects are the main causes. It is often invisible from the surface and reduces compressive strength, which makes it one of the governing failure modes in composite design.
How an interlaminar crack grows in a 2D laminate and in a 3D weave
2D laminate
The crack runs between plies
3D weave
Z-yarns bridge the crack
Fracture modes
Mode I: opening
Mode II: sliding shear
Mode III: tearing
Why delamination happens
A laminate is made by stacking fibre-reinforced plies and joining them with resin. The fibres lie only in the plane of each ply; tensile stresses through the thickness and interlaminar shear stresses are carried by the resin alone. Because resin has much lower strength and fracture toughness than fibre, the thin resin region between two plies becomes the weakest link in the structure.
Interlaminar stresses can arise even in a laminate that carries only in-plane loads. Pipes and Pagano (1970) showed that at the free edges of laminates made of plies in different orientations, the elastic mismatch between the plies produces local interlaminar stresses. For the same reason, hole edges, ply drops, curved sections and joints are common starting points for delamination.
Main causes
- Low-velocity impact: a dropped tool, a knock, a stone strike or a drone crash. An impact can leave a small mark on the front face while creating a large damage area between plies and on the back face (Abrate, 1998).
- Machining: when drilling, the drill peels up the top plies at the entry face and pushes out the bottom plies at the exit face, causing delamination. Precautions for cutting and drilling are covered in the carbon plate CNC cutting guide.
- Manufacturing defects: voids (porosity), dry areas without resin, foreign material trapped between plies or incomplete cure.
- Fatigue: under repeated loading, small defects can grow into delaminations.
- Moisture and temperature: moisture uptake by the resin and service temperatures approaching the glass transition temperature (Tg) reduce interlaminar strength.
Fracture modes
Fracture mechanics describes the growth of an interlaminar crack in terms of three basic modes:
- Mode I (opening): the crack faces move apart perpendicular to the crack plane.
- Mode II (sliding shear): the crack faces slide relative to each other in the direction of crack growth.
- Mode III (tearing): the crack faces slide in opposite directions parallel to the crack front.
In real structures, delamination usually grows under a combination of these modes, that is, in mixed mode. A material’s resistance to delamination is expressed by the critical energy release rate measured for each mode (GIc, GIIc). The higher the value, the more energy is needed to extend the crack by a unit area. In laminates, Mode I toughness is usually the lowest, which is why opening loads and through-thickness tension are especially critical.
Impact damage and compression after impact
The most common source of delamination in composite structures is low-velocity impact. Even when the impact energy is too low to break fibres, it can create overlapping delaminations between plies and matrix cracks. In aerospace, damage that leaves almost no trace on the surface is called barely visible impact damage (BVID).
Delamination is particularly dangerous under compression. Separated plies behave like independent thin plates, buckle locally and the damage grows. The effect of impact damage is therefore measured as compression-after-impact (CAI) strength: a plate is first given a controlled impact (ASTM D7136) and the damaged plate is then loaded to failure in compression (ASTM D7137). In aerospace structures, design values are generally set on the assumption that a structure containing barely visible impact damage must still carry the design loads.
How delamination is measured
Delamination resistance and impact damage tolerance are measured with standard test methods. The main ones are:
| Standard | Property measured | Specimen and loading |
|---|---|---|
| ASTM D5528 | Mode I interlaminar fracture toughness (GIc) | Double cantilever beam (DCB), opening load |
| ASTM D7905 | Mode II interlaminar fracture toughness (GIIc) | End-notched flexure (ENF), three-point bending |
| ASTM D6671 | Mixed-mode I/II fracture toughness | Mixed-mode bending (MMB) |
| ASTM D2344 | Short-beam strength (a comparative indicator of interlaminar shear) | Short beam, three-point bending |
| ASTM D7136 | Damage resistance to a drop-weight impact | Plate, controlled impact energy |
| ASTM D7137 | Compression after impact (CAI) | Plate damaged per D7136, compression |
In fracture toughness tests, a starter crack is created at the specimen mid-plane, usually by placing a thin non-stick film there during lay-up. In 3D woven composites the binder yarns prevent the crack from growing along a flat plane, so results from these tests need careful interpretation when compared directly with laminates.
Detection methods
Because delamination is often invisible from the surface, composite structures need non-destructive inspection. Common methods:
- Visual inspection and tap testing: quick and needs no special equipment, but shows only large separations near the surface.
- Ultrasonic inspection: sound waves reflect from the separated surfaces, mapping the position and area of the delamination (C-scan). This is the most widely used method for composites.
- Thermography: differences in how heat applied to the surface spreads reveal separations beneath it; it scans large areas quickly.
- Computed tomography: gives a three-dimensional image of the damage; usually used for laboratory-scale specimens.
How to prevent delamination
There are two basic ways to reduce delamination: make the resin between the plies tougher, or tie the plies together with fibre through the thickness.
Toughened resins (epoxies modified with thermoplastics or rubber) and tough interleaves placed between plies slow the crack by dissipating more energy at its tip. The crack still grows through resin, however, so the gain is limited by the toughness of the resin itself.
Through-thickness reinforcement works differently. In stitching, z-pinning, tufting and 3D weaving, yarns or rods passing between the plies tie the crack faces together. For the crack to open, these elements must pull out or break, which creates a bridging zone behind the crack tip and absorbs extra energy. The literature reports that through-thickness reinforcement markedly increases Mode I delamination toughness, reduces the impact damage area and raises compression-after-impact strength (Mouritz, 2007; Tong et al., 2002). The price, for some methods, is a reduction in in-plane properties.
| Method | How it works | Effect on in-plane properties | Practical note |
|---|---|---|---|
| Toughened resin or interleaf | Dissipates energy by plastic deformation at the crack tip | Generally small | Gain limited by resin toughness |
| Stitching | Dry plies are sewn together through the thickness | Can drop because of needle holes and fibre distortion | Applied to dry fabric preforms |
| Z-pinning | Thin composite or metal rods are inserted through prepreg | Fibre distortion and resin pockets around pins | Usually applied locally, at joints |
| 3D weaving | Binder yarns lock the layers during weaving | Depends on architecture; crimp can reduce them | Preform made in one piece; needs a special loom |
Design and manufacturing measures
Delamination risk can also be reduced without changing the material:
- Stacking sequence: large angle differences between adjacent plies raise interlaminar stresses at free edges. The stacking sequence is chosen with edge stresses in mind.
- Ply drops: where the thickness changes, plies are terminated gradually rather than along a single line.
- Hole and edge distance: holes are placed far enough from free edges and from each other, and washers are used at joints to spread the load.
- Through-thickness loads: details that pull the part perpendicular to its surface, such as fasteners loaded normal to the face and tight-radius curves, are avoided.
- Manufacturing quality: foreign material is kept out from between plies during lay-up, the cure cycle follows the resin maker’s recommendation and critical parts are ultrasonically inspected.
Delamination in 3D weaving
In 3D weaving, the layers are locked together during weaving by binder yarns that run through the thickness. Since there is no continuous interface where only resin holds the plies together, a separation that runs along the plies at low energy, as in a laminate, does not form. Even if a crack starts, it is bridged as soon as it reaches the first binder yarn, and it can only grow further if that yarn pulls out or breaks.
This does not mean that a 3D woven part cannot be damaged. Under excessive load, matrix cracks, fibre-matrix debonding and fibre breakage still occur. The difference lies in how the damage spreads: in a laminate it can spread over a large area between plies, while in a 3D weave it stays within a smaller region and the structure loses load-carrying capacity more gradually.
In practice
The OWC carbon plate is made from a 3D woven preform; through-thickness yarns tie its layers together. To try the material’s drilling, cutting and fastening behaviour in your own application, you can use the sample kit: it contains small rectangular pieces cut from a full plate.
Frequently asked questions
Can delamination be seen with the naked eye?
Can delamination be repaired?
Which tests measure delamination resistance?
Do 3D woven composites delaminate?
References
- ASTM D5528/D5528M. Standard Test Method for Mode I Interlaminar Fracture Toughness of Unidirectional Fiber-Reinforced Polymer Matrix Composites. ASTM International.
- ASTM D7905/D7905M. Standard Test Method for Determination of the Mode II Interlaminar Fracture Toughness of Unidirectional Fiber-Reinforced Polymer Matrix Composites. ASTM International.
- ASTM D6671/D6671M. Standard Test Method for Mixed Mode I-Mode II Interlaminar Fracture Toughness of Unidirectional Fiber Reinforced Polymer Matrix Composites. ASTM International.
- ASTM D2344/D2344M. Standard Test Method for Short-Beam Strength of Polymer Matrix Composite Materials and Their Laminates. ASTM International.
- ASTM D7136/D7136M. Standard Test Method for Measuring the Damage Resistance of a Fiber-Reinforced Polymer Matrix Composite to a Drop-Weight Impact Event. ASTM International.
- ASTM D7137/D7137M. Standard Test Method for Compressive Residual Strength Properties of Damaged Polymer Matrix Composite Plates. ASTM International.
- Pipes, R. B., Pagano, N. J. (1970). Interlaminar stresses in composite laminates under uniform axial extension. Journal of Composite Materials, 4(4), 538–548.
- Abrate, S. (1998). Impact on Composite Structures. Cambridge University Press.
- Mouritz, A. P. (2007). Review of z-pinned composite laminates. Composites Part A: Applied Science and Manufacturing, 38(12), 2383–2397.
- Tong, L., Mouritz, A. P., Bannister, M. K. (2002). 3D Fibre Reinforced Polymer Composites. Elsevier.
