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Project Goal
The goal for this project was to develop a robust, easily manufacturable, and repeatable method to bonding threaded aluminum inserts into carbon fiber tubes for use as suspension members. As well as determine an adequate wall thickness of commercially available carbon fiber tubes for the use of suspension members.
Constraints
Axle clearance
Maximum OD of 0.75"
ID of 0.5" to achieve this
Must accept a 1/4-28 rod end
Equivalent or greater strength than current steel members
Centering the insert
Suspension members can be assumed to all be two force members in a double A arm, pushrod or pullrod configuration. Being a two force member, it is important to keep the load aligned with the center axis to handle the load as efficient as possible. Traditionally, micro beads the same diameter as the bond gap are mixed with the adhesive. The mixture of adhesive and microbeads is applied on the outside of the insert, and then inserted into the tube. While this does work, it is messy and also decreases the strength of the bond as you are adding foreign debris into the adhesive. I chose to have two rings on either end of the insert that has a transition fit between the ring and the ID of the tube. Epoxy is then injected into the cavity between the two rings through a hole in the tube, air escapes through a corresponding hole on the other side of the tube.
Centering ring feature shown with arrows
Testing
To test the bond strength of the inserts, I conducted pullout testing of three different inserts with bond lengths of 0.5" 0.75" and 1". Each sample was prepared the same way as follows:
Prepare insert and tube surface with 220 grit sandpaper
Drill injection hole and air escape hole into tube
Both holes are aligned center in the bond cavity
Degrease insert and tube with acetone and lint free cloth
Fit insert and tube together
Inject epoxy until all air is evacuated from the opposite hole
Tape up holes to prevent leakage
Using flashbreaker tape
Prepared Inserts
Insert and tubes before bonding
Prepared Tube
Final samples
Bond Test Results
Three samples of each bond length were tested. After averaging failure load, it can be seen bond length and pullout strength are linear. For our load cases, a bond length of 0.75" was chosen.
Tube Selection
Commercially available carbon tubes come in various ID, and wall thicknesses. Due to constraints, 0.5" ID is used and testing of different wall thicknesses is conducted. This is compared to the current longest steel suspension member to prove equivalency.
Steel Member
1/16" Wall Carbon Member
Compression Test Results
1/16" wall 0.5" ID carbon tube is equivalent to its 0.5" 0.049" wall steel counterpart. You can also see how close Euler's column formula is. Math works!
Final Tube Insert Design
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