Composite structures: damage and repair

Damage bigger than it looks

Composite materials have transformed modern aircraft design by providing high strength, low weight, excellent fatigue resistance and outstanding corrosion performance. Aircraft such as the Boeing 787 and Airbus A350 use composites extensively in wings, fuselage sections, control surfaces, fairings and structural components. However, composites behave very differently from metals, and damage that would be obvious on an aluminum structure may be hidden deep within a composite laminate.

For maintenance personnel, one of the most important principles is that composite damage is often larger than it appears on the surface. This makes proper inspection and repair techniques critical to maintaining structural integrity.

What is a composite material?

A composite combines two or more materials to produce properties superior to either material alone. In aircraft structures, composites typically consist of high-strength fibers and a resin matrix. The fibers provide strength and stiffness, while the resin binds the fibers together and transfers loads between them.

Common aerospace fibers

• Carbon fiber: the most widely used structural composite material. It offers very high strength, high stiffness, low weight and excellent fatigue resistance, and is common in wings, fuselage structures and control surfaces.
• Glass fiber: less expensive than carbon fiber and commonly used in fairings, radomes and secondary structures. Its advantages include impact resistance and good electrical properties.
• Aramid fiber (Kevlar®): known for impact resistance, toughness and low density. It is often found in areas requiring damage resistance rather than high stiffness.

The resin matrix

The fibers are embedded within a resin system, typically epoxy, phenolic, bismaleimide (BMI) or other high-temperature aerospace resins. Without the resin, the fibers alone would have little structural value. The matrix:

• Holds the fibers in place
• Transfers loads
• Protects the fibers
• Determines environmental resistance

Laminate construction

Composite structures are built from multiple layers called plies. Each ply contains fibers oriented in a specific direction, typically 0°, ±45° or 90°. Different orientations allow engineers to tailor the structure to carry loads in multiple directions.

Why ply orientation matters

Composite strength is directional. A ply aligned with the load direction is extremely strong, while the same ply may be considerably weaker in another direction. Ply orientation is therefore a critical part of every repair. By combining multiple orientations, engineers achieve:

• Multi-directional strength
• Improved stiffness
• Fatigue resistance
• Torsional rigidity

Sandwich structures

Many composite aircraft components use sandwich construction. A sandwich panel consists of an outer skin, a lightweight core and an inner skin.

• Honeycomb core: the most common aerospace sandwich core. Materials may include Nomex®, aluminum and composite cores. Its advantages are extremely light weight, high stiffness and excellent buckling resistance.
• Foam core: some structures use foam cores instead of honeycomb. Benefits include simpler repairs, moisture resistance and lower manufacturing cost.

Why sandwich structures work

The principle is similar to an I-beam. The skins carry tension and compression, while the core maintains spacing between them and resists shear loads. This creates very high stiffness with minimal weight.

Why composite damage is different

Metal structures often reveal damage clearly through dents, buckling and cracks. Composite structures can suffer significant internal damage while appearing largely undamaged externally. This makes inspection particularly important.

Barely visible impact damage (BVID)

One of the most significant composite hazards is barely visible impact damage. A dropped tool, bird strike, hail impact or maintenance accident may leave only a small surface mark. The visible damage can represent only a small fraction of the actual affected area. Beneath the surface, the structure may contain:

• Fiber breakage
• Delamination
• Core crushing

Delamination

Delamination occurs when layers within the laminate separate from one another. Instead of behaving as a single structure, the plies begin acting independently. Delamination often spreads far beyond the visible damage area.

• Causes: impact, manufacturing defects, fatigue and overload events.
• Effects: reduced compressive strength, reduced stiffness and reduced load-carrying capability.

Disbonding

A disbond occurs when two intended bonded surfaces separate. Unlike delamination, which occurs within the laminate, a disbond occurs at a bonded interface. Examples include:

• Skin-to-core separation
• Skin-to-stiffener separation
• Bonded fitting separation

Why disbonds matter

A sandwich panel depends on the bond between skin and core. If that bond fails, structural stiffness decreases dramatically. Even a relatively small disbond can significantly affect performance.

Core damage

Impact loads often crush honeycomb or foam cores beneath apparently intact skins. Because the skin may spring back after impact, this damage can remain hidden. Core damage may include:

• Crushing
• Cell wall collapse
• Separation
• Moisture retention

Moisture ingress

Moisture can enter composite structures through damaged surfaces, fastener holes, disbonds and open honeycomb cells. Honeycomb panels are particularly vulnerable because moisture may spread internally across multiple cells. Moisture accumulation can lead to:

• Increased weight
• Freeze expansion damage
• Corrosion of metallic components
• Reduced bond quality

Heat damage

Composite structures have temperature limits. Excessive heat can damage resin systems, adhesive bonds and core materials. Unlike metals, composites may lose significant strength before obvious distortion appears. Sources include:

• Engine exhaust
• Bleed-air leaks
• Electrical faults
• Improper repair procedures

Lightning strike damage

Carbon-fiber composites conduct electricity far less well than aluminum, so lightning protection systems are incorporated into many composite structures. These often include metallic mesh, expanded metal foil and conductive layers. Even apparently minor strikes require careful inspection. A lightning strike may damage:

• Surface plies
• Conductive mesh
• Bonded joints
• Internal structure

Detecting composite damage

Visual inspection alone is rarely sufficient. Composite inspections often require additional methods to determine the true extent of damage.

Visual inspection

Visual inspection remains the first step. However, the visible area often understates the actual damage. Inspectors look for:

• Cracks
• Surface dents
• Paint damage
• Impact marks
• Distortion
• Moisture indications

Tap testing

Tap testing is one of the simplest composite inspection techniques. A coin or specialized hammer lightly taps the structure. A sharp sound generally means sound structure, and a dull sound means a possible disbond or delamination.

Tap testing is quick and inexpensive but depends heavily on inspector experience.

Ultrasonic inspection

Ultrasonic inspection is one of the most effective methods for evaluating composite structures. It can detect delamination, core damage, disbonds and internal defects. Ultrasonic mapping often reveals damage significantly larger than the visible impact area.

Thermography

Infrared thermography detects differences in heat flow. Defects such as delaminations, disbonds and moisture intrusion affect thermal conductivity and become visible on thermal images. Thermography allows rapid inspection of large areas.

The composite repair philosophy

Composite repairs aim to restore strength, stiffness, aerodynamic smoothness, fatigue resistance and lightning protection. Unlike metallic repairs, composites often restore structure using bonded laminate replacement rather than mechanical fasteners.

Step 1: Damage assessment

Before repair, inspect visually, map the defect boundaries, determine the depth and evaluate the core condition. The full extent of the damage must be known before repair design begins.

Step 2: Moisture removal

If moisture is present, it must be removed before repair, typically by controlled drying, heating procedures or vacuum-assisted drying. Trapped moisture can compromise bond quality and curing.

Step 3: Remove damaged material

Damaged plies are removed gradually rather than cutting a simple hole. Both common methods minimize stress concentrations and create efficient load transfer.

• Scarf repair: a tapered transition is created around the damage. The scarf allows loads to transfer smoothly between old and new material.
• Step repair: individual plies are removed in steps, and replacement plies rebuild the structure layer by layer.

Step 4: Lay up replacement plies

Replacement plies are installed according to the repair manual. This stage largely determines the success of the repair. Critical requirements include:

• Correct material
• Correct ply count
• Correct ply orientation
• Correct overlap dimensions

Why orientation matters in a repair

A 0° ply repaired with a 90° ply is not equivalent. The repaired structure may appear acceptable while losing significant strength. Always restore the original lay-up sequence exactly as specified.

Step 5: Vacuum bagging

Many repairs use vacuum bag technology. The repair area is sealed beneath a vacuum bag, creating a stronger and more uniform repair. Vacuum pressure:

• Removes trapped air
• Consolidates plies
• Improves bond quality
• Controls resin distribution

Step 6: Cure

The repair is heated according to a specified cure cycle, using heat blankets, ovens or, during manufacturing, autoclaves. The cure profile specifies ramp rates, temperatures, hold times and cooling rates. These parameters must be followed precisely.

Step 7: Restore lightning protection

If lightning protection was removed during the repair, it must be restored. This may include conductive mesh, expanded foil and bonding provisions. Failure to restore lightning protection can compromise future strike tolerance.

Step 8: Final inspection

The repair must meet all structural and quality requirements before returning the component to service. Following cure:

• Surface finish is restored.
• Dimensions are verified.
• NDI inspections may be performed.
• Documentation is completed.

Composite vs. metal repairs

Understanding these differences helps technicians avoid treating composites as if they were aluminum structures.

• Joining: composite repairs are often bonded. Metal repairs are usually fastened.
• Damage: internal damage is common in composites. In metal, damage is often visible.
• Critical design factor: in composites, ply orientation is critical. In metal, material thickness is usually the primary concern.
• Process: composites require a controlled cure. Metal requires mechanical fastening.
• Damage assessment: composite damage is often mapped with NDI. Metal damage is often confirmed visually and with NDI.

Key takeaways

• Composites consist of fibers embedded in a resin matrix.
• Carbon fiber, fiberglass and aramid fibers are common aerospace composites.
• Sandwich structures use lightweight cores to achieve high stiffness with low weight.
• Composite damage is often much larger internally than it appears externally.
• Common defects include delamination, disbonding, core crushing, moisture ingress, heat damage and lightning-strike damage.
• Visual inspection alone is usually insufficient.
• Tap testing, ultrasonic inspection and thermography are commonly used to identify hidden damage.
• Repairs require proper damage removal, moisture control, ply replacement, vacuum bagging and controlled curing.
• Ply orientation must match the original design.
• Lightning protection systems removed during repair must be restored.

The bottom line

Composite structures achieve exceptional strength-to-weight ratios but demand a different maintenance mindset than traditional metal aircraft structures. Damage can remain hidden beneath an almost undamaged surface, making inspection and nondestructive testing essential. Successful repairs depend on accurately identifying the full extent of the damage, restoring the original laminate design and cure characteristics, and returning the structure's strength, stiffness and lightning protection capability. In composite maintenance, what cannot be seen is often more important than what can.