Bolts and Inserts in Carbon Fiber Parts
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Carbon fiber laminates are strong in the plane of the fibers and weak through the thickness. A bolt loads both: the shank bears on the side of the hole, and the clamp force squeezes the laminate. Get the details wrong and holes elongate, edges split, or the laminate crushes under the washer.
Edge distance and spacing
Keep holes further from edges than you would in metal. A common guideline for composites is at least three hole diameters from the hole center to the edge, against about two for metals, and four to five diameters between holes. That leaves enough material for the laminate to fail in bearing, a slow and visible failure, rather than split out suddenly.
Layup matters too. Bolted areas need fibers running in several directions. A laminate with all its fibers running one way splits easily at a hole.
Don't crush the laminate
- Use large washers, or a bonded metal plate, to spread the clamp load.
- For high clamp loads, fit a metal sleeve through the hole so the bolt clamps against metal, not the laminate.
- Specify a torque. Carbon parts are often over-tightened by people used to metal.
- Avoid deep countersinks in thin laminates. They leave a knife edge with little material to bear on.
Inserts
- Bonded inserts: a threaded metal insert bonded into a drilled hole or pocket. Simple and strong if the bond area is generous.
- Molded-in inserts: placed in the layup before curing so the fibers wrap around them. The strongest option, but they must be located precisely in the mold.
- Through bolts with nuts: the most reliable choice when you can reach the back side.
- Rivet nuts and self-tapping screws: generally a poor fit for thin laminates, which crack or strip.
Choose fastener materials carefully
Carbon conducts electricity and sits at the noble end of the galvanic scale. Titanium and stainless steel fasteners work well with it. Aluminum and plain or zinc-plated steel corrode when they touch carbon in wet conditions, so avoid them or isolate them with sealant, a glass fiber layer or coated washers.
Machine the holes properly
Hole quality matters as much as hole position, because a splintered exit is where cracks start. We cover tools and technique in how to drill and cut carbon fiber.
Joint design is one of the things we review when you send us a carbon fiber part.