Composites Research

Goal: MSXIV requires a detailed overview of the different possibilities for incorporating composites as structural components into the chassis.

Context: In MSXII, the majority of the composite use was for the aerobody. The aerobody was mounted to the chassis and was not designed to be a structurally component of the car. In essence, the aerobody and chassis systems were isolated from each other. The main advantage of this was to avoid the mistakes made with the monocoque design used in MSXI, which are detailed in other confluence pages. By having a strong, overbuilt chassis, the car had a reliable structure which is something that MSXII did very well.

The push for using composites as structural members was inspired by a few factors. At ASC 2018, the team had the opportunity to see the use of composites in other cars. There are many advantages to using composites including weight reduction, better use of space and higher strength. However, the use of these composites requires lots of knowledge and experience and a well defined schedule. In order for this to be successful, the team needs to research the various possibilities and perform materials tests to validate the reliability of the material.

University of Toronto Composite Sandwich Panel Forming:

Goal: Determine a method for manufacturing composite sandwich panels.

Background: The bottom section of the chassis will use carbon fiber sandwich panels due to their high torsional stiffness. There is a broad base of knowledge from various student design teams that can be utilized to help us determine the best solution that optimizes the key criteria:

• High torsional stiffness
• Reliable, will not fail catastrophically
• Manufacturable with the team’s resources
• Cost effective
• A large safety factor to account for manufacturing defects

https://www.mie.utoronto.ca/mie/undergrad/thesis-catalog/319.pdf

According to this thesis, the UofT FSAE team optimized the design of their sandwich panels by using a software called Altair OptiStruct. They also used Pro/Engineer and Pro/Mechanical. Their layering consisted of carbon laminate plies, a thin layer of fiberglass cloth to ensure an adequate bond to the core material, then the nomex core material. To ensure a firm bond of the carbon laminate to the core, stiff plates were placed above and below the layup and the entire assembly was sealed with vacuum pressure

Bends were created in the panels by removing the carbon laminate layer on the inner side of the bend. Some of the Nomex core was removed to allow the panel freedom to bend. The open cut was then filled with a lightweight epoxy, bent to the appropriate geometry and held to cure. Several layers of plainweave carbon fiber cloth were added to inner side of the bend to add strength.

Goal: Discuss sesign and analysis methodology used by UofT

The univeristy of Toronto decided to use composites for the following reasons:

1.  Using composites in addition to steel tubing allows for a lighter chassis in comparison to a full steel frame chassis.
2. Increased torsional stiffness

During their design process, they believed it was important to note that their chassis was designed for optimal stiffness, not maximal. It's important to note, seeing as the closer you get to a "perfectly stiff chassis" results in more dead weight (i.e. the weight to stiffness ratio begins decrease exponentially).

Composite Design and Optimization

For their composite design they focused on using 2 laminate layers with one core layer. Examples of their panel design can be seen above.

During their optimization process they focused optimizing the following three parameters:

1. Laminate thickness
2. Core thickness
3. Ply directions

By optimizing the above mentioned parameters, their goal was to improve the stiffness to weight ratio. From their optimization, they found that ply direction affected their design and results the most out of the three as it greatly impacted force loading results and projected performance of the car. UofT used Altair OptiStruct in order to determine the optimal direction of plies.

UofT's process for simulating a side impact:

• simulated with dynamic loading
• to show how the chassis was being loaded, they decided to look at the amount of energy absorbed at different points in the chassis, as well as how the car maintained and distributed the load.
• to estimate minimum laminate and core thickness allowable, they used generalized panel calculations, in order to find the safest structure 

For examples of equivalency calculations used please, see appendix B in the above linked document.

Chassis Design and Optimization

To begin with, UofT defined what their optimal torsional stiffness range was. They decided on this to ensure their design was optimized and not maximized. First they determined what the difference between the expected front and rear stiffness values and then multiplied them by 6-8. For example, their front stiffness value was 663 ft-lb/deg and their rear stiffness value was 343 ft-lb/deg; as a result their optimal stiffness range was 1950 ft-lb/deg to 2800 ft-lb/deg. In their simulation software, they represented all of the tubes as beam elements, and applied a linear analysis. While performing their baseline analysis, they represented their composite panels as shell elements with isotropic properties (which is not the case in reality) they did this in order to simplify the analysis greatly, and focus mainly on optimizing the design of the steel frame. 

The main advantages of this are:

• faster simulation times and simplified analysis set up
• faster design iterations
• focus on improving relative stiffness of steel geometries.

After they had roughly optimized the design of the steel geometries, they began improving the representations of the composite panels. They switch the composites to be represented as "advanced shells" specifying directional properties (representing the panels as orthotropic materials) as well as the ply direction and thickness.

Model constrains

Constraints in the model included limiting the movement of the left, rear, lower suspension point from translating in the x, y, and z directions. The right, rear, lower suspension point was restricted in the x (longitudinal), z (vertical), but allowed to translate in the y (lateral direction). Allowing this lateral movement serves to avoid over-constraining the chassis, which would produce stiffness values much higher than is realistic.

A torque of 2000 ft-lb was applied at a center point of the forward axle of the chassis. Because all analyses carried out in these studies were linear, the actual value of the torque applied is arbitrary, though the relative angular deflection values of each iteration of the chassis under this load are of interest in optimizing the design.

Composite Strucuture Design and Optimization

In this section, the article was very lacking, no new information was discussed and only mentions, they just specified number of initial layers, material properties of each layer and its orientation, and then boundaries of how much each layer can rotate. Afterwards, their software just gave them an answer for what the optimal design would be.

Lastly, it is important to note this paper focuses solely on optimizing stiffness, and does not focus on or necessarily discuss how to design and simulate and ensure their chassis is safe for impact and rollover cases.

For further information about their materials and manufacturing process, please refer to chapter 3.

For further information about how they validated their design and performed physical testing please refer to chapter 4.

Stanford's composites dd

Stanford's team had an extensive use of sandwich panels in their monocoque design. They were able to mount their leading arm, front shock, upper control arm, rear shock brace, trailing arm, steering bell crank, steering column, topshell, canopy linkages. To begin, they created an extensive report on the Tensile Testing of Threaded Bushings in Sandwich Panels in  2017 in order to better understand design choices for load chassis inserts. They determined the maximum loading configurations to guide the design of chassis connections. 

They also performed FEA on the key mounting points to determine the loads in the x, y and z directions.

List of non structural materials used with composites:

Foaming Adhesive

Econotac

Cutting Carbon:

Scribe lines should be built into the molds from the mold manufacturer

3-5 extra layers of carbon should be added to places that will be cut to prevent rattling while cutting and make stronger edges.

Vacuum Bagging Tips:

• Vacuum bagging on large molds: put stressreleaving dog ears every 2­3 ft along the mold in
  addition to large dog ears near areas where
  more bag will be needed (fairings)
• While assembling large vacuum bags, DO NOT STEP ON THE BAG OR LET IT DRAG ON THE FLOOR. This will cause micro­tears and holes which are really hard to find.
• When joining vacuum bags, do not use an overlap joint ­ use a butt joint so that the vacuum tape can be inspected at each point.
• If you are pulling vacuum on four fairings at a time and you plan to use one pump place the pump so that it is near the center of the string of the four fairings. I.e. don't daisy chain four
  fairings and put the pump at the end of the line. Place the pump in the middle. Place the vacuum pump at one end of your large vacuum bag and place the vacuum gauge at the
  other end. Sometime vacuum failure willnot propagate to the pump on the vacuum. Hardened carbon can easily pierce AirTech Stretchlon stretchy vacuum bag
  Place a fan to blow on the vacuum pump so it does not overheat.
• When you have vacuum failures spend a few minutes with the entire team searching for the failure, but then you should send everyone out of the room except for one careful person with
  good hearing. Turn off all noise sources in the room so it is silent. Have them pull vacuum on the mold, but then turn off the pump. Have that person search alone for the hissing noise of
  vacuum failures. Using a stethoscope may help, but sometimes it is a hinderance. Don't forget that you can have punctures in the vacuum bag in the center of your mold in addition to gaps in
  the vacuum tape around the perimeter of your mold. You could even have cracks in your mold itself, but if you didn't realize you had cracks in 10/17/2018 Composites Fundamentals - SSCP
  https://sites.google.com/a/stanfordsolarcar.com/sscp/home/sscp-2012-2013/composites/composites-fundamentals-and-planning 8/12 your mold before you laid down carbon you
  should just go cry or something.

Mold preparation and Release

• Apply Chemlease 15 in two coats to remove
  porosity
• Apply 3­5 coats of Frekote 700­NC
  Inspect the mold for cracks. If you have already
  pulled parts and you noticie cracking have
  someone vacuum bag the entire mold to see
  how bad the cracks are. If it looks like the cracks
  will cause vacuum failure you need to seal the
  cracks now before you start laying carbon down.
  Also check to see if the cracks get worse when
  you heat or cool the molds.

Additional Resources

https://www.fpl.fs.fed.us/documnts/fplr/fplr1574.pdf

Composites simulation resources: