Joining Rivet Nuts

Joining automotive composite components when only one side of a hole is accessible can be challenging. When the blind rivet nuts currently used are inserted and tightened, their sleeve collapses and expands against the inside of the hole to hold it tight. This works fine for metal parts but not for composite parts.

With soft composites, the radial expansion of the sleeve may push the material aside so that the bolt expands inside the hole rather than behind it. With harder composites, the pressure of the expansion can cause microcracking and eventual deterioration of the hole, resulting in joint failure.

To overcome these problems, automakers may bond nuts or spacers to the parts before using the bolts. However, these take extra time to install and frequently fall off. Vehicle designers may insert steel brackets at the joining points or thicken the composite material near the hole to prevent microcracking. But both approaches add unwanted weight.

BBA Fasteners developed steel and aluminum rivet nuts with bulge control technology (BCT) as a lighter, faster and more effective alternative. “The BCT fastener has four holes at 90 degrees to each other. When the tool goes into the rivet nut and pulls upon it, it weakens the fastener so that it forms a bulge at a predetermined point,” says Tim Bartlett, CEO, BBA Fasteners. The bulge spreads out wider than a collar, reducing the risk of microcracking and hole failure.

Although BCTs are not commonly used in North America, Fiat and other European OEMs have used them in vehicles for years. In the U.S., electric vehicle companies are now incorporating them in their new vehicle designs.

Mary Lou Jay is a freelance writer based in Timonium, Md. Email comments to


The lattice structure for this beltline stiffener component is made of glass fiber and carbon fiber reinforced polypropylene unidirectional tapes, then laminated on either side of an unreinforced polypropylene sheet.

Photo Credit: WEAV3D

The BCT fastener is designed to form a bulge at a predetermined point, limiting microcracking around the hole in joined composite components.

Photo Credit: BBA Fasteners