An overview of the advantages and disadvantages of different joining methods for carbon fiber materials

There are various methods for joining composite materials, and the choice depends on factors such as the load characteristics of the specific joint area. Currently, there are five methods for joining carbon fiber composites: mechanical joining, adhesive bonding, stitching, Z-pin joining, and hybrid joining.

(At present, mechanical joining and adhesive bonding are the most widely used methods for load transfer in structural joints. Stitching and Z-pin joining are used primarily as auxiliary methods to enhance the joint’s resistance to peel stress.

Adhesive Bonding

Adhesive bonding involves joining two or more components together using an adhesive. Today, adhesive bonding is generally classified into three categories: co-curing, co-bonding, and secondary bonding. However, when we refer to “adhesive bonding” in everyday terms, we typically mean secondary bonding or co-bonding; the performance of co-curing preformed bonding is far superior to that of co-bonding.

Adhesive joints can be broadly classified into two main types: in-plane lap joints and orthogonal joints. In-plane lap joints primarily resist in-plane tensile loads, with the adhesive layer bearing shear forces; they are mostly used to connect plate-type components in aircraft structures. Orthogonal lap joints primarily resist out-of-plane tensile loads—commonly referred to as pull-out loads—and are mainly used to connect plate-type components to beams, ribs, and stringers.

Mechanical Connections

Mechanical connections involve using mechanical methods—typically bolts and rivets—to join two or more components together. Common mechanical connections in aircraft structures include bolted joints, riveted joints, blind fasteners, and ring-groove rivet connections. Mechanical connections in carbon fiber composite structures come in various forms. Based on the load-bearing characteristics of the fasteners, they can be classified into single-shear and double-shear types; based on whether a transition plate is used to facilitate the connection, they are categorized as lap joints and butt joints; each connection type corresponds to a specific chamfered joint configuration.

When selecting mechanical connection types for carbon fiber composite structures, the following should be noted: Single-shear lap joints are subject to bending deformation, which reduces joint strength and connection efficiency; therefore, double-shear connections are preferred in design. Due to bending stresses, for asymmetric single-lap joint structures, multi-row fastener connections should be used, and the spacing between rows should be increased; in multi-row fastener connections for carbon fiber resin-based composites, there is severe unevenness in load distribution, so care should be taken to avoid structural configurations with an excessive number of fastener rows.

In multi-fastener connection structures, to enhance structural connection strength—particularly fatigue strength—bolt holes should be arranged in parallel whenever possible, avoiding staggered arrangements; properly designed chamfered connections for plates with varying thicknesses can improve the load-bearing capacity of the connection structure, whereas improperly designed chamfered connections can reduce load-bearing capacity.

Advantages and Disadvantages of Adhesive Bonding

Advantages:

1. Prevents crack propagation and offers good safety against failure

2. No strength reduction caused by drilling holes

3. No corrosion issues; produces a smooth, aerodynamic surface

4. Excellent fatigue resistance, vibration damping, sealing, and insulation properties

5. No fasteners required; lightweight structure; low manufacturing cost

Disadvantages:

1. Requires special pretreatment of bonding surfaces; high precision is required between the parts to be bonded; requires heating and pressurization equipment for curing; repairs are difficult

2. Bonding creates a permanent connection; once bonded, the joint cannot be disassembled, making material recycling difficult

3. Strength distribution is uneven, peel strength is low, and it is difficult to bond thick structures or transmit large loads

4. Connection quality is difficult to control, resulting in poor reliability

Advantages and Disadvantages of Mechanical Connections

Advantages:

1. Can be repeatedly assembled and disassembled during manufacturing, maintenance, and replacement