As the cost of carbon fiber continues to decrease, the application of carbon fiber products is becoming more widespread across various industries. In the development of carbon fiber products, the method of connecting carbon fiber reinforced polymer (CFRP) components is extremely important. The connection methods for CFRP are crucial to its structural performance, lightweight effect, and reliability. Due to the anisotropy of CFRP, its low interlaminar strength, brittleness, and sensitivity to drilling, connection design is more complex compared to metals. The main connection methods are divided into three categories: mechanical fastening, adhesive bonding, and hybrid joining, each with its own advantages, disadvantages, and applicable scenarios.

1. Mechanical Fastening

This method primarily uses mechanical fasteners (such as bolts, rivets, screws, etc.) to connect CFRP components to metals or other composite material parts. Common fasteners include:

 

l  High-lock bolts/High-strength bolts: Most commonly used in the aerospace field, they require washers to prevent compression damage.

l  Pull-thru rivets (blind rivets): Suitable for single-side operation but limited in compressive strength on CFRP.

l  Threaded inserts: Pre-embedded or later inserted into CFRP holes to provide reliable threading, avoiding direct threading into composite materials.

l  Special fasteners: Such as low compression stress rivets for composite materials, Hi-Lite fasteners, etc.

Advantages:

Ø  High reliability and detectability: Connection status is easy to visually inspect or detect non-destructively.

Ø  Removable: Easy to maintain, replace, or inspect.

Ø  High load-bearing capacity: Especially suitable for applications involving high loads, concentrated loads, or frequent disassembly (such as the connection between the aircraft wing and fuselage).

Ø  Low surface treatment sensitivity: No need for the strict cleanliness required by adhesive bonding.

 

Disadvantages:

 

Ø  Severe stress concentration: Drilling damages the fiber continuity, creating high stress concentrations around the hole, which significantly weakens the strength (by up to 20-50%).

Ø  Increased weight: Fasteners and reinforcement plates (such as titanium alloy washers) add extra weight.

Ø  Difficult hole drilling: Requires special drill bits (diamond-coated, carbide), high speed, low feed rate, and strict control of hole diameter tolerance and hole wall quality (to avoid delamination, burrs, or tearing).

Ø  Risk of galvanic corrosion: CFRP-metal fastener contact requires insulation treatment (such as titanium alloy fasteners, sealants, or washers).

Key Design Considerations: Optimizing spacing/margin to avoid squeezing or pull-out, controlling compression strength (as CFRP's compression strength is typically lower than its tensile/compressive strength), layup design to increase the ±45° layer proportion around the hole for better compression strength, and adding local reinforcement or metal/composite patches in the connection area.

2. Adhesive Bonding

This method uses structural adhesives to bond CFRP components together. Common adhesives include:

l  Epoxy resin adhesives: The most commonly used, with good comprehensive performance (strength, heat resistance, environmental resistance).

l  Polyurethane adhesives: Good toughness, resistant to low-temperature impact.

l  Acrylic adhesives: Fast curing, good toughness.

l  Cyanoacrylate adhesives (instant adhesives)**: For temporary fixing, with limited strength, often used for temporary positioning.

l  High-temperature adhesives (BMI, polyimide)**: Used in high-temperature environments (>180°C).

 

Advantages:

Ø  No stress concentration: Continuous bonding ensures uniform stress distribution and better fatigue resistance.

Ø  Lightweight: No drilling or fasteners required, making it the lightest option.

Ø  Good sealing: Provides sealing and anti-corrosion functions simultaneously.

Ø  Adaptability to irregular shapes: Suitable for complex curved surfaces.

Ø  Electrical insulation and thermal insulation: Avoids galvanic corrosion and thermal bridges.

 

Disadvantages:

Ø  High surface treatment requirements: Requires strict degreasing, grinding (or sandblasting), plasma treatment, etc., to ensure surface bondability.

Ø  Significant strength dispersion: Performance is highly dependent on the process (adhesive application, curing pressure/temperature/time) and environmental conditions (humidity, temperature).

Ø  Difficult non-destructive testing: Internal defects in the adhesive layer (delamination, bubbles) are difficult to reliably detect (requires ultrasonic or infrared thermography).

Ø  Non-removable/difficult to repair: Destructive disassembly is required, and repairs involve removing old adhesive layers and reprocessing the surface.

Ø  Environmental aging needs verification: Moisture, heat, UV exposure may degrade the adhesive layer's performance.

Ø  Low peel strength: CFRP interlaminar strength is low, and adhesive bonds are sensitive to peel and fracture loads.

Key Joint Types:

Ø  Single lap: Simple but subject to eccentric loads and peel stress.

Ø  Double lap / Stepped lap: Reduces eccentricity and increases strength.

Ø  Scarf joint: Smoothest stress transfer, strength closest to the base material, but complex to prepare.

Ø  Butt joint with strap: Requires additional cover plate reinforcement.

3. Hybrid Joining

This method combines adhesive bonding and mechanical fastening to leverage the advantages of both methods and compensate for their respective drawbacks. Common forms include:

Bonded-bolted: The most common form, where the adhesive layer cures before drilling holes and installing fasteners.

Co-cured / Bonded with inserts: Inserts are embedded during adhesive curing.

Advantages:

Ø  Significantly improved reliability: Mechanical fasteners provide "failure insurance," so even if part of the adhesive layer fails, the structure can still bear loads.

Ø  Increased strength and stiffness: Synergistic effects outperform single connections.

Ø  Improved fatigue resistance: The adhesive layer reduces stress concentrations around holes, while fasteners suppress crack propagation in the adhesive layer.

Ø  Reduced sensitivity to adhesive process variability**: The mechanical part compensates for the variability in adhesive bonding.

Disadvantages:

Ø  More complex process: Both adhesive bonding and mechanical fastening process requirements need to be considered.

Ø  Higher cost: Involves additional fasteners and processing steps.

Ø  Hole drilling still required: Cannot completely avoid drilling damage.

4. Emerging Joining Methods

In addition to the commonly used connection methods, there are emerging connection techniques such as:

Stitching / 3D weaving: This connects multiple layers of fabric in the thickness direction using stitching lines or Z-direction fibers at the prefabrication stage, forming an integrated structure after curing. This fundamentally eliminates the interlaminar interface, greatly improving interlaminar toughness and damage tolerance. This method is suitable for manufacturing complex integral components (e.g., fuselage frames).

Ø  Fusion bonding (for thermoplastic CFRP): This method uses the thermoplastic matrix's melting property to connect CFRP components. Heating (via resistance, induction, laser, or ultrasound) is used to melt the resin at the interface, and the connection forms when it cools down.

Ø  Composite material-specific fastening structures: Development of integrated designs, such as lug joints and clip structures, reduces the number of parts and assembly stresses.

5. Key Considerations for Selecting Connection Methods for CFRP Products:

1. Load type and size: For high concentrated loads, mechanical or hybrid connections are preferred; for distributed loads or fatigue-sensitive structures, adhesive bonding is recommended.

2. Inspectability and maintainability: For components requiring frequent inspection or disassembly, mechanical connections are preferred.

3. Weight requirements: For extreme weight reduction, adhesive bonding is the best choice.

4. Cost and manufacturing process: Adhesive bonding is complex but reduces the number of parts; mechanical connections are simpler but require hole drilling and fasteners.

5. Environmental resistance: Wet, high-temperature, or corrosive environments require an evaluation of the compatibility of adhesives or metal fasteners.

6. Material type: Thermoset CFRP is primarily connected by adhesive bonding or mechanical fastening; thermoplastic CFRP can consider fusion bonding.

7. Structural location and criticality: Main load-bearing or safety-critical structures often use hybrid connections or advanced integration techniques.

Conclusion:

The connection of CFRP materials is a crucial aspect of "design determines performance." Mechanical connections are reliable but heavy and prone to hole damage; adhesive bonding is lightweight and continuous but difficult to inspect and repair; hybrid connections balance reliability and performance; and advanced integration techniques represent the future direction. Engineers need to comprehensively weigh performance, weight, cost, process, and reliability to select or innovate the optimal connection solution. For CFRP product design, you can contact us via sophiashi@yoo-better.com or sophiashi.cross@gmail.com