Goal: The MSXIV chassis will require strong structural bonds between metal and carbon fiber materials.
Background: Due to a desire for a lighter and more space efficient chassis, a greater emphasis on use of composites will be necessary. To effectively take advantage of the various material properties, extensive research, design, analysis and testing of structural bonds between materials is required.
Permanent mounting solutions
1.Click bond studs
https://www.clickbond.com/products/studs-standoffs/
A flat disk is epoxied to the carbon fiber surface with a stud. This stud fits inside a hole on a tab that is welded to the chassis. This method was used for MSXII and was great because it was easy to install, had a short cure time of less than 5 minutes and offered some flexibility with tolerances to ensure that the holes lined up in order to account for manufacturing defects. It also eliminates the need for riveting, which would introduce stress concentrations within the carbon fibre body. This in turn would make our composites more predictable and reliable to use.
Conclusions/insights: This is a great tried and tested method of mounting aerobody panels and can be used for non structural aerobody panels. For structural aerobody panels, a different solution is necessary.
2. Epoxies
DP460 structural adhesive is the epoxy used in the University of Iowa’s solar car. Their car had an impressive use of composite materials and a structural aerobody. It is an alternative to riveting and welding, offers a structural bond, is impact resistant and vibration resistant. It has a 60 minute work life and a cure time of 24 hours.
Here is a summary of the evaluation of various epoxies from the UofT 2007 FSAE team. In the test a cyclic load of 170 pounds (compressive then tensile) was applied to a test fixture.
Hexion Paste MGS 235/K was chosen because of its thick consistency made it easy to work with.
Conclusions/insights: Our team will need to perform similar tests to validate the results. This will ensure the durability and reliability of our composite structures which is critical, because it will allow us to have peace of mind and avoid catastrophic failure.
3. Carbon Fiber Cloth Layup
The UofT FSAE 2007 chassis thesis has an extensive report that documents how they bonded their sandwich panels to the chassis. To summarize, they first epoxied the panels to the steel members, then did used carbon fiber cloth and a vacuum bag to directly do a lay up around the tube. This resulted in a bond stronger than the composite panel. If used, similar material testing will be necessary to validate these results.
This section outlines the process of bonding the pre-fabricated composite sandwich panels into the chassis: A) The bonding surfaces of the steel portion of the frame were prepared by abrading the surface, and cleaning thoroughly. Figure A13 shows a typical welded tube section. This view also shows a seat belt mount that has been welded together prior to bonding the panels in.
B) Panels were cut and the edges were shaped to fit the profile of the tubes. Figure A14 shows the panel bonded into place using the Hexion Paste epoxy, denoted as the blue areas in the diagram.
C) Two layers of plain-weave carbon cloth were soaked in epoxy and applied around all bonding areas. A vacuum-bag was placed around all edges to ensure a secure bond between the panel and tube. Figure A15 shows the application of the additional carbon, with Figure A16 representing an example of the final product.
To validate the strength of their bonds they used testing (sec4.2.1) because the finite element analysis did not take into account these bonds.
They created a test fixture which failed after a 471 lb impact was applied. The bonds to the steel surface did not break, and instead the sheets were delaminated from the core.
4. Flat plates
This method consists of welding flat sheets to the steel tubes and then adhering it with epoxy.
Conclusions/insights: An example of this would be helpful to visualize this solution. It is good to keep in mind in case any of our aerobody geometry requires creative mounting solutions.
5. Riveting
A rivet is a permanent mechanical fastener that is easy to install. Typically holes are drilled into a surface before a rivet gun is used to pop it in. It offers a strong structural bond.
Caution must be used when using rivets and composites because the holes will be local stress concentrations that will affect the structural integrity of the chassis, potentially causing delamination. Performing analysis on composites is difficult as it is, so introducing these unknowns will necessitate a large safety factor.
Conclusions/insights: An effective use of rivets with our composite components is a possibility. However, epoxies and carbon fiber cloth are preferable because they do not introduce unknown stresses.
Non Permanent Mounting Solutions
There are a wide array of non permanent fasteners that can be used including screws, bolts, washers, nuts. Another solution is to use an interference fit between two objects. There are entire books on fasteners that detail a breadth of information if you are interested. A few items to consider when choosing mounting solutions are:
- Mass: In order to keep our car competitive, the mass of our car should be kept to a minimum.
- Strength: Strong and secure fasteners should be used for all components to eliminate vibrating components within the car.
- Adjustability: Manufacturing defects are inevitable and the fasteners must have adjustable positioning with a good tolerance in order to ensure a tight fit.
- Ease of access: A mounting fixture should be designed such that there is sufficient clearance to work on the fastener