Carbon fiber prefabricated tubing is one of the most sophisticated structural products available today. It has mechanical properties that can be tailored precisely for your specific application. But even with the most ideal mechanical performance available in the tubing itself, the resulting structure is only as good as the bond strength at the joints. To realize the maximum performance benefits of the mechanical properties of carbon fiber tubing requires optimum execution of joint design and bonding.
The following discussion is not intended to provide load ratings but rather as an illustrative guide suggesting how structural design may be performed. Each application must be tested by the user to assure realization of the predicted performance.
Understand Tube Performance Characteristics:
Carbon fiber tubing can be fabricated in an unlimited number of fiber configurations to achieve strength and flexural characteristics specific to the needs of the intended structure. For the purposes of this discussion, we will select our standard carbon fiber tube which is unidirectional in its layup. The fibers selected are referred to as “general” which exhibit a modulus of elasticity of 38 million psi and tensile ultimate strength of 580,000 psi. The resulting epoxy laminated tubing exhibits a flexural modulus of 32 million psi and a tensile ultimate strength of about 200,000 psi.
For this example we will select a 1” diameter tube with a wall thickness of 0.060”. This tube can then carry a tensile load of about 37,680 pounds. With a safety factor of 3, the useful tensile design load would be about 12,560 pounds.
Adhesive Selection:
Clearly, there are literally dozens of adhesive formulations available that each give specific performance benefits depending upon the application.
For the purposes of this discussion we will select a commonly used, high strength epoxy adhesive made by Loctite/Hysol called “9430.” Loctite Hysol 9430 is a two-part modified epoxy structural adhesive that attains structural properties after a room temperature cure. It is formulated to give a combination of very high peel strength along with excellent shear strength.
Often used by custom builders for assembly of carbon fiber bicycle frames, Hysol 9430 is convenient to use and easy to cure while developing a tensile strength of 5,500 psi, a shear strength of 4,700 psi and a modulus of elasticity of 380,000 psi. It is a perfect choice for bonding dissimilar materials including aluminum, carbon fiber, fiberglass, steel, stainless, etc.
Of particular interest in the design of a bonded joint is adhesive shear strength measured in psi (pounds per square inch) and required film thickness which will define the necessary clearance in the joint between the carbon fiber tube and the ferrule.
Hysol 9430 requires a film thickness ranging from 0.010” to 0.020” to fully develop its inherent shear strength. Note that this film thickness will vary with adhesive selection.
Joint Design:
Knowing the structural strength characteristics of the adhesive to be used, we can now make more intelligent decisions regarding joint design. Structural tubing, by its very nature lends itself to truss frame construction. A basic truss structure is generally intended to place its structural members in purely a tensile or compressive load configuration through the use of the classic “pin-joints.” However, with rigid joints resulting from ferrules and adhesives, the tubes will also feel both bending moments and torsional loading.
Joining carbon fiber tubes requires the use of what are known as “tube clusters”. The easiest way to envision a tube cluster is to simply look at a typical tube frame welded up from aluminum, steel or chrome-moly tubing. Now imagine simply cutting off all of the tubes leaving about an inch or two after the welds. If we now add a sleeve that is machined to go inside each tube stub and extend out with a diameter appropriate to fit inside the carbon fiber tube, we have a cluster joint.
Alternatively, the joint can also be designed such that the carbon fiber tubing fits inside counterbored holes in a completely machined cluster made from a block of aluminum, for example.
Tube clusters can also be made as molded carbon fiber assemblies as sometimes found on modern bicycle frames.
Given a particular adhesive and its specific strength characteristics, the joint strength is completely dependent upon bond surface area. In the tubing selection we calculated the useful tube tensile load at 12,560 pounds.
To design a joint that would be consistent with this strength and given the shear strength of the adhesive of 4,700 psi, we can see that a bond area of about 2.7 square inches is required. Given the 1” tube diameter, this means that the penetration of the ferrule into the tube end would need to be about 0.85 inches. Let’s round it to 1” deep giving us a total bond area of 3.14 square inches and a joint load capacity therefore of 14,758 pounds. Adding our safety factor of 3, we get a useful joint design load of 4,800 pounds.
Remember also, to machine your ferrule diameters to assure they are a minimum of 0.020” smaller in diameter than the inside diameter of the carbon fiber tubes. Or, in the case of a machined cluster into which the tubes fit, make the counterbored holes 0.020” larger in diameter than the outside diameter of the tubing.
Surface Preparation:
Proper surface preparation is critical to achieving effective bond integrity. This preparation begins with surface abrasion. Surface abrasion increases the microscopic surface area and gives the adhesive a “tooth” that improves adhesion. This can be achieved by simply sanding the surfaces with emery paper or through grit blasting.
In the case of tube ferrules, you would abrade the inside surfaces of the tubing and in the case of the machined block clusters, you would abrade the outside surfaces of the tubing.
Grit Blasting:
This is the preferred method since it is easier to control uniformity. Both the carbon fiber tube ends and the ferrules of the tube cluster can be grit blasted with aluminum oxide, glass beads, etc. with air pressure set at about 100 psi. Surfaces not being abraded should be masked off to avoid cosmetic damage.
Manual Abrasion:
Alternatively, the surfaces can be roughed-up with sandpaper with a grit size of 180 to 200. You are trying to achieve a uniform degree of dullness, i.e., gloss removal.
Surface Cleaning:
Immediately prior to adhesive application, assure bonding surfaces are free of dust and fingerprint oils by wiping with acetone and a low lint shop towel. Rubber gloves are highly recommended both to avoid fingerprint oils and to protect your hands from the harsh chemicals in epoxy resins.
Adhesive Mixing:
Hysol 9430 is mixed in a 50/50 proportion of resin to hardener. The more precise you are in your measurements, the more consistent results you will achieve. Be careful in your mixing to assure a fully homogeneous blend.
