Composite Joining: Adhesive Pros and Cons

Carbon-fiber composites offer numerous benefits to automotive manufacturers, such as weight reduction. However, they pose a wide variety of joining challenges to engineers, who must choose between adhesive bonding or mechanical fastening. Each assembly process has many pros and cons that must be evaluated.

“Adhesives can bond almost any combination of materials and they produce a continuous bondline,” says George Ritter, principal material engineer at EWI, the leading North American authority on materials joining and applied technologies for the transportation industry. “This produces strong joints with good stress distribution.

“Spot welds and rivets produce point-source load management which requires thicker, heavier materials,” explains Ritter. “Adhesives also offer inherent galvanic corrosion protection when joining dissimilar materials—especially carbon-fiber composites, which can engage in galvanic corrosion in contact with metals.”

Acrylics, epoxies and urethanes can be used to assemble composite parts. According to Kevin Roslinski, senior application engineeratHenkel Corp., there are four main benefits to using adhesive. “Distribution of force over a larger area vs. point-by-point fasteners can often result in better impact resistance,” he explains. “Adhesive also offers better insulation of composites from metals if corrosion is a concern.”

Because adhesives seal, as well as adhere the interface, “there could be warranty benefits based on reduced corrosion,” adds Roslinski. “And, a more intimate bond line can reduce possible noise, [resulting in] better NVH (noise, vibration and harshness) performance.”

“[Structural achieve stress management] really well by spreading the load across large areas,” notes Manny Tesfaye, director of global technical services at Scigrip Smarter Adhesive Solutions. “Structural integrity of an assembly, whether under fatigue, impact or simple load, will depend on avoiding stress risers that can be a danger of using mechanical fasteners, drilled holes or badly designed joints.

“This challenge is more acute with high-modulus carbon-fiber composites under dynamic loading,” claims Tesfaye. “In many areas, it is vital to have a toughened adhesive that has good peel strength, is crack resistant while still exhibiting appropriate strengths, and moduli that engineers can work with.

“Traditionally, epoxies are popular [for carbon-fiber composite applications], but methacrylates have some important advantages of good multisurface adhesion and high toughness to give impact and fatigue resistance, particularly when joining very different substrates such as metal to composite,” Tesfaye points out. “Cold-curing adhesive [also offers advantages], depending on the process route. In choosing the [best] adhesive to use, you need to get a good balance between strength and toughness.”

While is impossible to weld composites, hybrid adhesive bonding processes may be an alternative. “The term ‘carbon-fiber composite’ is usually taken to mean the matrix resin is a thermoset material, such as epoxy,” says Ritter. “They cannot be welded, because thermoset resins don’t melt. They must be bonded or mechanically fastened.”

However, thermoplastic composites can be welded and can also be carbon-fiber reinforced. Welding methods include heated tools, ultrasonic welding, vibration welding, induction welding and microwave welding.

“The major hybrid process used in automotive assembly is weldbonding,” notes Ritter. “This is used universally with metal bodies and involves using adhesives and spot welding in the same operation. The adhesives provide higher strength and stiffness, and the welds provide higher peel performance. Combined, the two methods provide excellent fatigue life; higher than either method alone.

“Weldbonding has not been applied to thermoplastic composites to any great extent, but that is a possibility,” claims Ritter. “Another advantage to weldbonding is the welds hold the adhesive in place as it cures. This allows for real-time inline assembly because the welds provide the fixturing to rigidize the structure as the adhesive cures.

“Closely allied is the process of rivetbonding,” Ritter points out. “That has been used for automotive structures, especially in aluminum. Rivetbonding with composites would require holes, where the current aluminum structures have used self-piercing and clinch joining.”

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