Delamination in composite materials is one of the most critical failure modes in fiber-reinforced polymer (FRP) structures. This article explains what delamination is, why it is dangerous for composite laminates, and how design, manufacturing, and service loads influence delamination growth.
Key takeaways
- Delamination occurs when composite layers separate, drastically reducing strength and stiffness.
- Common triggers include impact damage, manufacturing defects, and cyclic loading.
- Non-destructive testing (NDT) methods like ultrasound and thermography detect hidden delamination.
- Repairs range from resin injection for minor damage to scarf patching for structural restoration.
- Prevention relies on proper layup, curing, and design with toughened resins or 3D reinforcement.
What is Composite Delamination?
Delamination refers to the separation of adjacent plies in a laminated composite. These materials — typically carbon fiber, glass fiber, or aramid reinforced with epoxy or other resins — rely on strong interlaminar bonds to transfer shear stresses between layers.
Delamination of UD-CF sample during Mode-I testing. Courtesy: KAUST
When these bonds fail, layers detach, forming internal cracks or voids. This creates a “kissing bond” or full disbond, compromising the entire structure even if the surface appears intact.
Types of Delamination
- Interlaminar failure: Between two plies due to shear or peel stress.
- Translaminar propagation: Delamination growing from matrix cracks or edge stress.
- Barely visible impact damage (BVID): Internal delamination from low-energy impact with minimal surface indication.
Why is Delamination Dangerous?
Unlike metals, composites do not yield plastically. Delamination can reduce compressive strength by up to 60% without visible warning. In aircraft, this has led to in-flight failures; in wind turbine blades, it causes sudden collapse.
The damage is often internal and progressive, growing under cyclic loads until catastrophic failure occurs.
Causes of Delamination
Manufacturing-Induced
- Poor wetting of fibers during impregnation
- Incomplete cure or trapped volatiles
- Contaminants (release agents, moisture)
- Improper ply orientation or stacking sequence
In-Service Triggers
- Low-velocity impact (tool drop, hail, bird strike)
- Fatigue from repeated flexing or vibration
- Thermal cycling causing differential expansion
- Moisture ingress weakening resin-fiber interface
See our composite manufacturing defects checklist for quality control tips.
Detection Methods
Early detection is essential. Non-destructive testing (NDT) allows inspection without disassembly.
Ultrasonic Testing (UT)
Most widely used. Pulse-echo or through-transmission detects delamination depth and size via C-scan mapping.
Active Thermography
Heat is applied; infrared camera captures cooling anomalies. Ideal for large areas and near-surface defects.
Advanced NDT Options
- Shearography – full-field strain measurement under stress
- Acoustic emission – real-time crack growth monitoring
- Computed tomography (CT) – 3D internal visualization
Repair Techniques
Repair restores structural integrity. Method depends on damage size, location, and load requirements.
Scarf Patch Repair (Bonded)
- Remove damaged plies in tapered scarf (1:20 to 1:50 slope)
- Lay up matching prepreg plies in original orientation
- Vacuum bag and co-cure with film adhesive
- Finish and repaint
Resin Injection
For small, closed delaminations. Low-viscosity epoxy is injected under vacuum/pressure, then cured.
Bolted Patch (Mechanical)
External composite or metal patch fastened with hi-lok fasteners. Used in non-flight-critical or temporary repairs.
Follow OEM repair manuals or refer to our certified composite repair training.
Prevention Strategies
- Use toughened resin systems with higher GIC/GIIC
- Add z-pins, tufting, or 3D weaving for through-thickness strength
- Minimize ply drop-offs and free edges in design
- Control cure cycle with thermocouples and dwell times
- Apply edge sealing and protective coatings
- Perform regular NDT inspections in high-risk components
Suggested related posts (internal links)
- Barely Visible Impact Damage in Composites
- Ultrasonic C-Scan Testing Guide
- Scarf Repair vs. Bolted Patch: When to Use Each
Frequently asked questions
What is composite delamination?
Delamination is the separation of layers in a composite laminate, leading to loss of structural integrity due to failed interlaminar bonds.
Can delamination be detected visually?
Not usually. Most delamination is internal and requires NDT methods like ultrasound or thermography for detection.
Is delamination repairable in aircraft structures?
Yes, when performed by certified technicians using approved scarf or bonded patch methods per SRM (Structural Repair Manual).
How does moisture cause delamination?
Absorbed moisture weakens the resin-fiber interface and creates steam pressure during heating, forcing layers apart.
What industries are most affected by delamination?
Aerospace, wind energy, marine, and high-performance automotive sectors rely heavily on composites and face delamination risks.