Composite Structural Connections

As the polymer composites industry continues to grow, questions arise concerning connections between composites and between composites and other materials such as metals, wood, concrete or plastics. This column will briefly discuss the current industry methods for making these connections from the point of view of a registered design professional practicing in the pultrusion industry. It will focus on two common types of composite connections – mechanical connections and a combination of mechanical and bonded connections.

Mechanical Connections

Mechanical connections, which utilize traditional fasteners such as bolts, screws, rivets and pins, are currently the preferred connection method between composites, and between composites and other materials such as steel, concrete and wood. Published test data is readily available for the typical, hex-head bolt as well as screws, rivets and pins, all of which can be carbon steel, stainless steel or even polymer composites.  Nylon rivets and pins are available as well.

In designing these mechanical connections, engineers most often refer to them as bearing-type connections where the connector “bears” against the composite inside the drilled-hole; forces are transferred between the composite and the connector at the surface around the circumference of the drilled hole. Testing and calculations can be easily produced to determine strengths of these connections. In bearing-type connections, bolt shear seldom controls in pultruded composite connections since stainless steel bolts are the norm and have greater shear strength than the composite’s pin-bearing strength.

Mechanical connections include resistance to tensile and compressive forces, but what about the well-known rigid, or semi-rigid, steel connection (“moment connection”) that offers rotational strength, bearing and shear strength and stiffness in a mechanical connection?  Although the pultruded composites industry would benefit from having a semi-rigid composite connection method, it hasn’t yet been formally developed. While it can be achieved – and has been done in composite design and manufacturing – it is very difficult and isn’t common practice in our industry.

An additional example of a mechanical connection is integrating pultruded composites, or even traditional materials, into molded parts. This integration has been in practice for many years, and opinions vary on the best method. A mechanical connection between a pultruded shape and a molded shape can be achieved by fully encapsulating the pultruded composite in the molded part so that the connection will not solely rely on bonding one or more pultruded surfaces to the molded part during the lay-up process. Forces are ideally transferred between the parts, normal to the composites’ surfaces. Note that epoxy and resins are often an exception to this due to their bonding capabilities to many materials.