CF to Aluminum Bond Test (1st Iteration)
Factors to consider
Bond gap
Insert size
Insert length
Insert material
Filler material
What material
What percentage
Adhesive
Oven Curing
Surface Prep
Prep of carbon
Prep of aluminum
Variable Testing
Insert Length
We will increment by 1/4” since our math seems like we only need 0.5” of insert length. We will start at 0.5” and work our way up to 1.25”. We will see how much the length impacts the max force to test for redundancy.
Bond Gap
We are starting with a gap of 0.008” because research shows this is the ideal gap. We can decrease the gap to 0.006” and increase it to 0.010” and analyze the increase/decrease in strength.
Insert Material
We are going to start by using aluminum because of the low weight, however the strength is less than steel and may be scratched by steel rod end. We may even have to use a Helicoil to interface the two parts. We can change the insert material to steel to see if the strength increase and less work is worth the extra weight.
Adhesive
We are going to start with slathering adhesive on the insert and inner part of the CF tube. We will also test injecting adhesive straight into the tube with the insert already in the CF tube. Whichever method yields the best strength/if there is any noticeable difference, we will go with that method.
We will also test if oven curing is a necessary/viable option for our epoxy. We are going to start with room temperature curing for our control, and then test how heat curing affects the strength of the bond.
Filler
We are going to try without filler first. We may test with silica or glass beads infused into the epoxy. This would complicate the process since we would have to determine the bead to epoxy ratio and injecting would be very difficult, so we would have to use slather-and-push method to connect the insert. The point of filler is to make sure the insert and tube are concentric creating equal stress on the epoxy bond.
Control Group
We will be testing carbon fiber and aluminum interface epoxy strength for our carbon fiber push rods. We want to find the optimal method for attaching these materials not only for push rods but for any future parts of the car. The goal of this is to test the optimal procedure to interface aluminum with CFRP using epoxy. Given the following dimensions below, we expect a maximum shear strength of 12kN or 2700 lbs.
For our control group we will start with the following:
46610 Tube - ID x OD | 0.375” x 0.435”
Bond gap - 0.008' or 0.203mm
Insert OD - 0.367” or 9.32mm (Determined by ID of tube and Bond gap)
Insert length - 1/2” or 12.7mm
Carbon Fiber Length - 4”
Insert material - aluminum
Filler material - none
Adhesive - Loctite EA E‐120HP Epoxy (Lap Shear Strength: 33 MPa for abraded aluminum)
Cured at room temp for 24 hours
Oven Curing - none
Surface Prep
Prep of carbon - the inner part will be sanded with 300 grit and degreased with acetone
Prep of aluminum - inserts will be sanded with 300 grit and degreased with acetone
Steps
Create Aluminum inserts
0.008 in gap
Have a length of 1/2” that will adhere to the inside of tube
Cut the Carbon Fiber tubes into about 4-inch sections
The tubes are 6 feet long, so we can get at least 18 tests from one tube
Prep the surfaces
The aluminum inserts will be sanded (~300 grit) and degreased (acetone)
The CF inner part will be sanded (~300 grit) and degreased (acetone)
Add Epoxy
Slather Loctite Hysol E‐120HP on the insert
Insertion
Slowly push insert into tube and let cure
Testing
After putting the insert into both sides of the tube, attach to Instron machine
Pull until failure
record yield strength
Repeat 2 more times
Second Test
We will add silica beads based on a volume fraction - 15-20% filler. For the bond gap and volume we measured, 0.036g of epoxy and 0.006g of filler will be needed for each insert. That is 0.216g and 0.066g respectively for six inserts.
Third Test
We will attempt epoxy injection by introducing tiny holes on the flange.
Fourth Test
We will heat cure the epoxy at 65 C or 150 F for 12 hours.
Data
Table 1 - Data from Pushrods Control Group
Test | Shear Strength [lbf / kN] | |
|---|---|---|
1 | 85 | 0.37 |
2 | 286 | 1.27 |
3 | 0 | 0 |
Table 2 - Data from Pushrods with filler
Test | shear Strength [lbf / kN] | |
|---|---|---|
1 | 810 | 3.60 |
2 | 870 | 3.87 |
3 | 872 | 3.88 |
Conclusions
We can see that the control group yielded poor results, with a breaking point at 0.547 +/- 0.653 kN. This only reached 4.5% of our expected yield strength. Some things that could have gone wrong were improper surface preparation as it seemed to have detached from the tube wall. The last test was pulled apart by hand so human error was a big factor here.
The filler group gave better results as it yielded at about 3.783 +/- 0.159 kN. This reached 31.5% of our expected shear strength. The same problems were seen where the adhesive seemed to have detached from the walls of the tube. There also looked like uneven coating of epoxy across the surface of the insert. In future iterations, we will try to try sanding with a lower grit sandpaper and try injecting epoxy through holes in the insert.
The failure point seems to be the bond on the carbon fiber. Poor surface prep could be the issue; Future testing may including switching from 300 to 120 grit surface roughing.