Mounting Solutions
- Mike Lin
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 fiber body. This in turn would make our composites more predictable and reliable to use.
Conclusions/Insights: While this method was tried in tested in MSXII, they were not intended to be a structural interface between the chassis and the aerobody, unlike what we hope to achieve.
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.
Conclusions/insights: This method does not require pre preg carbon fiber (which may or may not be available to the team due to sponsorships). The thesis in which this information was gathered from offers a detailed resource on manufacturing and analysis. It is a very promising solution but will need to be vetted out with material testing before we can safely apply it to the rest of the car.
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
6. Click bond bushings
https://www.clickbond.com/products/bushings/
The Stanford solar car team has used these bushings within their sandwich panels for many generations of solar cars to ensure strong structural bonds between mounting points and composite.
They have extensive experience and resources for building a composite chassis. In their 2014-2015 cycle they experienced numerous mechanical failures, so in 2016 they wrote a report called "Tensile Testing of Threaded Bushings in Sandwich Panels" to validate the structural bonds for these:
https://www.clickbond.com/products/bushings/
This is primarily meant for components of the car that exert a stress at a hard mounting point. For our purposes we want to simply have a structural bond between the chassis and the composite, which means that there will likely be a continuous surface area over which the tubes can be in contact with the composite.
Conclusions/insights: These fastening methods are great because they are widely used by the aerospace industry and other solar car teams, however testing must be done to understand its loading/ failure characteristics.
Goal: Determine the best mounting solution between the steel tube members of a chassis and the proposed structural bottom panel of the chassis.
Factors to consider
- The fastening method must not protrude through the aerobody
- Fastening system should be elegant
- Off the shelf solutions are preferable because they are cheaper, save time and are more reliable
In order to accomodate for potential manufacturing defects int he bottom panel, should we use hard mounting points at specific areas of the chassis, or should we assume that a long interface will exist where the tubes can lie flat on the composite panel?
A solution comprising of clickbond threaded bushings, welded sheet metal plates and mechanical fasteners is displayed below. The solution was inspired by the Stanford Solar car team's design. There are many advantages to this solution.
- It's an off the shelf solution used for many applications in many industries, likely by lots of people.
- It will provide an elegant, lightweight solution in a clean form factor
- It comes with jigging tools making it relatively easy and straightforward to install
- It uses mechanical fasteners, which is a physical link between the two components. This will give the team more peace of mind as opposed to relying purely on the shear strength of epoxy and hoping for the best.
- It is cost effective
- It is predesigned, so integrating this solution will not take as much time
- Because the solution mounts at specific hard points, we can adjust it and add some room/ tolerancing to make sure that the components will fit should there be manufacturing defects
We will have to perform some materials tests before using it.
Here is a description from the click bond website:
Click Bond adhesive-bonded bushings enable construction using lightweight structures manufactured from composite sandwich structures with crush-sensitive or low strength core materials such as honeycomb or foam. They provide localized reinforcement and strongpoints for threaded attachment.
The accompanying disposable installation fixtures offer rapid, consistent installation as well as continuous clamping of the joint during cure, ensuring a strong bond. Internal elastic fixtures are available with select bushings for reduced footprint applications.
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