Research Goal/Thesis

The purpose of this research project is to investigate the effectiveness and viability of composite chassis components through the design and implementation of structural “sandwich” panels. These will be pursued for an increased strength-to-weight ratio as well as higher stiffness as compared to a conventional tubular frame.

Design Process

Sandwich Panel

To start, the design of the structural “sandwich” panel is achieved by utilizing two outer carbon fiber layers and inserting a core structure between them, typically with aluminum honeycomb, foam, or Nomex.

Each carbon fiber layer is created by using multiple carbon fiber lamina at different orientations to create a laminate, bonded together with epoxy resin. The orientation stacking of the carbon fiber lamina is to maximize material properties in multiple directions, in order to retain high stiffness and low deformation properties in a multitude of loading situations.

Additionally, the core material of the panel is utilized to effectively transfer loads across the entire of area of the carbon fiber, as well as resist shear loads and bending stresses across the carbon fiber. The material of the core structure is determined based upon the application.

Honeycomb Properties and Dimensions

Honeycomb is a core material used as a structural stiffening medium between plies of Carbon Fiber. A honeycomb structure is the ideal material for energy absorption because ----- . Because of this, the use of honeycomb materials creates a great strength-to-weight ratio, when comparing the capabilities of sandwich panels to that of conventional tubing.

Honeycomb Mechanical Properties

When dealing with honeycomb, there are a few different cell configurations, which contribute to the loading and material capabilities of the honeycomb and by extension the sandwich panel. These configurations include:

• Hexagonal Core/Reinforced Hexagonal Core

  • This is the most common core structure, available in metals and nonmetals

Open

• OX Core

  • A hexagonal type honeycomb that has been overexpanded in the W-direction, making more of a rectangular configuration

  • Because of this, this cell is better in shear in the W direction, but weaker in the L direction, as compared to Hexagonal

Open

• Flex Core

  • This unique shape makes this cell configuration ideal for panels of curvature, because their shape makes the cells curve able and resistant to buckling

• Double Flex

  • This has some of the same properties as Flex-Core, just on a larger scale, making this the most formable and compression-resistant cell

In addition, after honeycomb structures have exceeded their ultimate compressive strength, they continue to deform plastically and crush uniformly, which is deemed crush strength. This absorption capacity is predictable as it is at a constant stress level, making it ideal for absorbing energy. With respect to shear properties, the honeycomb is highest parallel to the length direction, while it is lowest in the width direction.

These mechanical properties and dimensions play an important part in determining how a core material can be utilized. Specifically, with length and width, larger paneling can take more loads due to increased stress capacity, but can be more susceptible to buckling if too large. Core thickness is directly related to bending resistance and flex rigidity, as well as shear strength. Cell size, as it relates to cell density determines stiffness and strength, as well as overall weight. Overall, it is important to take in each one of these parameters and understand how each one affects the properties of the honeycomb core.

Material Selection

When choosing honeycomb core material, there are two clear choices, being Nomex (aramid fibers) or Aluminum.

Firstly, aramid or Nomex honeycomb is a non-metallic core material made of phenolic resin and Aramid fiber paper, which offers low weight with a good compressive strength to weight ratio. Secondly, aluminum honeycomb is a metallic core material made of extruded aluminum, which offers high compressive and shear strengths. For this reason, aluminum honeycomb has been chosen to be utilized in crash structure and mounting areas with Nomex being utilized in all other body panels.

We utilized these data sheets to determine these configurations:

With these sources, we understand the differences in shear and compression strength between the two materials.

Carbon Fiber Properties and Dimensions

Look into the different types of carbon fiber available (uni-directional, twill weave, figure out their stiffness, use case, source ability, cost, ease of manufacturing when creating a laminate)

Unidirectional (UD) Carbon Fiber:

UD carbon fiber can be used in both prepreg and non-prepreg forms, meaning that a dry or wet layup manufacturing process can be utilized to manufacture parts with UD.

UD carbon fiber is a non-woven carbon fiber sheet, which features all fibers running in a single, parallel direction. This makes UD carbon fiber light weight compared to woven counterparts.

Stiffness:

• Provides a concentrated density of fibers to provide max longitudinal tensile potential, extremely stiff

• Resin ratio is usually low (prepreg) = high stiffness and strength to weight

• Dry form (non-prepreg) = requires binding fibers that run perpendicular to the direction of the carbon fiber

Use Case:

• Able to stack/overlap fabrics at varying angle orientations to achieve strength in multiple directions w/o sacrificing stiffness

• Ideal for applications where front-to-back strength is most important.

• Not suitable for draping, can reveal gaps, wrinkles or creases when draped over complex surfaces

  • Improved with resin infusion

• Great choice for maximizing compressive/tensile with low weight, but must have many layers of UD to create a quasi-isotropic laminate

Sourceability/Cost:

• Compared to woven, due to lower fiber content, and less intensive weaving process, save on production costs

• Straightforward production, involving less complexity

Ease of Manufacturing:

• Can be laid down in multiple directions, allowing us to achieve balanced properties

• Orientated with greater percentage along single axis, creating different stiffness and strength along different axes

• Spreading resin going against the grain will cause nonwoven fibers to break free from binder

• A vacuum bag manufacturing process produces the greatest strength-to-weight ratio

  • Vacuum bag manufacturing process utilizes a sealed bag in

• With vacuum infusion, you can get the ideal resin-to-fabric ratio

  • Compared to woven fabric, it is more difficult due to the slower resin infusion process

• The process is can be detailed due to the preciseness when generating directional strength

• Tends to fall apart during layup process due to lack of interlaced fibers

Twill Weave Carbon Fiber:

TW carbon fiber is characterized by a diagonal pattern, fibers are woven in a staggered manner, which causes a more

Used much more in an aesthetic with suitable strength properties

Stiffness:

• Good pliability and can form to complex contours

• Better at painting its fabric stability than satin weave but not as good as plain weave

Use Case:

• Used in many cosmetic and decorative applications while having moderate formability and moderate stability

• Can be utilized in combination with UD carbon fiber to create a high strength laminate with improved machinability

Sourceability/Cost:

• Higher production costs, due to intricate diagonal pattern which results, in more advanced weaving techniques and additional processing steps

Ease of Manufacturing:

• Must be handled more carefully than a plain weave fabric to avoid adding distortions to the weave

• More complex, a traditional vacuum bagging process and autoclaving prepreg will produce what appears to be a flattened or crushed surface

  • Added layer of deep epoxy surface allows the carbon fiber to develop full depth, and preserve a 3D appearance.

Pre-preg vs Non Pre-Preg

Pre-Preg Carbon Fiber

• Created by impregnating carbon fiber fabrics with a controlled amount of resin (epoxy or phenolic) in a factory setting

  • Cut into sheets/rolls and stored in a cooled area to prevent complete curing

• Requires an oven or autoclave to fully cure and form the final composite part

• Advantages:

  • Higher fiber-to-resin ratio

    • Better mechanical properties + lighter weight

  • Better control over resin content + distribution

    • Consistent quality + performance, as opposed to wet layup

  • Less waste due to excess resin removed during the impregnation process

  • Cleaner + safer = resin exposure minimized

• Disadvantages:

  • Higher cost = materials are more expensive than dry fabric + resin

  • Complex processing - Specialized equipment + skills to handle and cure

• Automotive Use Cases:

  • Body panels, Chassis,

Non Pre-Preg Carbon Fiber (Wet)

• Applying resin to dry carbon fiber by hand or machine in a mold

  • Placing resin-coated fabrics into a mold, and manually removing air bubbles with a roller or brush

• Resin can be epoxy, polyester, or vinyl ester

• Cured at room temperature or with heat

• Advantages:

  • Lower cost = process use cheaper materials + equipment

  • More flexibility = process can accommodate complex shapes + large parts

• Disadvantages:

  • Lower fiber-to-resin ratio = lower mechanical properties + heavier

  • Less control over resin content + distribution

    • Variable quality + performance

  • More waste = Excess resin discarded or cured in the mold

  • Messier + riskier = resin exposure is higher

• Automotive Use Cases:

  • Bumpers, Hoods, Fenders

Bonding Processes + Orientation

Unidirectional (UD) Carbon Fiber

• Bonding Process (Epoxy/resin properties)

  • Fibers are bound by trace amount of polyester binder (composed of less than 3% of overall construction)

  • Spread of fibers against the grain causes nonwoven fibers to break free from the binder

    • More of a risk with UD because carbon fiber is not woven together

  • Epoxy/Resin Properties

    • Align with high stiffness, strength, and specific orientation

    • High shear + tensile strength = support direction of fibers

    • Needs to have excellent bonding properties for layer stacking, in order to achieve the desired stiffness

    • Precise Resin to fiber ratio = ease of application

• Orientation

  • Binder is used on one side of material to provide clean, opposite face of the carbon

    • No twisting or crimping of fibers

  • Some brands have a slight weave in-and-out of binder (not true UD fabric)

    • Slight weave reduces single direction max strength

Twill Weave Carbon Fiber

• Bonding Process (Epoxy/resin properties)

  • Expoxies should offer high tensile + flexural strength

    • Require durability + strong adhesion properties due to woven layers with varying orientations

    • High modulus of elasticity = Enhance resistance to deformation + improve load-bearing capcity

• Orientation

  • 2x2 Twill Weave

    • Diagonals are synchronized

    • Braid is over-over-under-under

      • Provides an elastic pattern = used for complex chapes because weave is looser

  • 4x4 Twill Weave

    • Fancier pattern, looks like arrow heads/tractor traces

    • Not as popular, but will bend around curves better than 2x2 twill weave

Mounting Solutions

In terms of mounting components to the panels or even mounting the panels themselves to a tubular area of the car, there are various solutions to accommodate each specific application.

Thru Bolts

A commonly used solution is using thru-bolts, which can end up leading to shear and moment in the core material or compression and bearing stress if an axial load is placed on it. This means that selecting a core material and evaluating the loads of each mounting joint go hand in hand together, as they can both directly affect the effectiveness of the joint and panel as a whole.

A thru bolt solution is effective when the bolted joint is transferring loads to the panel that are in-plane, as opposed to out of plane. Additionally, a custom flange can be utilized with thru bolts, in order to mount any part to the paneling which could be effective for suspension mounting.

Potted Inserts

Potted inserts are another viable mounting solution, which utilizes an insert that is placed in a cutout section of the core material and adhered to the panel by a potting adhesive, which constrains the insert in all six degrees of freedom.

There can be 3 types of potted inserts, one being through-the-thickness inserts, which have inserts bonded through the the entire thickness of the sandwich panel with potting material around the insert. Next, fully potted inserts include an insert that sits in the core material of the laminate, that does not go through both sides of the thickness, where potting material sets it inside the sandwich panel, reaching the bottom layer of the laminate. Lastly, partially potted inserts are smaller inserts that only go through about half of the laminate, with potting material around the insert.

In addition, there are various potting methods that have their own benefits and drawbacks

Additionally, thru-bolts could be used in a different configuration, where a custom mounting bracket is created that bolts onto the panel, and any moving part/anything experiencing load is mounted to the bracket, and not directly onto the panel.

Mounting Solution Specifics

• need to examine each one of these mounting capabilities

  • Strengths, weaknesses, ease of manufacturing, cost, usable applicationsetc.

  • Decision matrix

Manufacturing Process

tbd → advait helping with this

Testing Methodology

In order to test the viability and capabilities of the structural composite panels, both empirical testing and FEA simulation shall be utilized.

To start, materials testing, which includes tensile, 3-point bend, shear, and compression testing, shall be employed to understand the material properties of the composite paneling, as well as ensure the panels are viable at least in the early stage to continue research, in pursuit of a fully composite panel chassis. Each material test will result in critical numerical data, such as yield and ultimate strength, elastic modulus, as well as fracture toughness and flexural modulus. Each one of these data points can then be used to characterize the composite material as a whole. Additionally, this data can be used in hand calculations or brought into a Finite Element Analysis program such as ANSYS, to then predict and understand how the material, based on its properties, will react in situations relevant to the ASC and FSGP competitions.

In terms of FEA testing, ANSYS, utilizing its ACP program, can accurately represent the composite structure in junction with the data collected from the empirical testing, which allows us to simulate the entire chassis structure as one piece, helping us account for the complex geometry of the chassis and load cases of the ASC regulations, which would be hard to replicate in an empirical testing setup.

Empirical Testing

• need to go into specifics on how each empirical test will be conducted to get accurate results

  • Also explain how each material property will be extracted/calculated given the test results

KETIV Technologies Video: Intro to Composite Analysis Using Ansys Mechanical

FEA Testing

To utilize FEA to simulate composite structures, shell elements are utilized

• Use a shell element (planar element). This creates a model based on the length and width dimensions, while using tabular data for thickness, making the simulation run more efficiently.

• Define material for each layer, and define stiffness for each layer. Ansys will then supposedly “automatically” recognize this and will make it so that you can simulate it.

To run the FEA simulation, we need orthotropic material properties, ply thickness and orientation, stacking sequence, the orthotropic strengths, and failure theory/theories.

We will utilize the Tsai Wu failure criterion in order to determine the factor of safety and tensile/compressive failure strengths

Delamination Analysis:

A common failure method of composites is the delamination of the laminate, in which the plies of the material start to separate, which disallows the composite to transfer load and stresses throughout the entire structure, leading to stress concentrations and failure of the structure.

There are a few ways you could approach analyzing this in Ansys

CZM (Cohesive Zone Modeling) - Calculates energy needed to pry/shear apart a composite panel structure (I think this would be useful for sandwich panels).

VCCT (Virtual Crack Closure Technique) - Another method to see how composite panels can delaminate.

Found two things over this:

Science Direct article

NASA Research

Basic Overview of how to Simulate a Panel: (17:40)

1. Insert a “Layered Section”

2. Specify what Geometry layered section applies to

3. Define a coordinate system (pretty much tell ansys what a “zero degree” orientation looks like)

4. Define layers - select worksheet option, and define each layer

1. If you go to engineering data on the ANSYS workbench → composites material, there are a lot of predefined ones to use.

2. Click coordinate sys → insert new coordinate sys → from there, can go to properties of coord plane and set geometry relative to plane.

AFTER SETTING COORDINATE PLANE:

3. Right click geometry → Insert layered section → select surface for lay sec geom → select coordinate system to use

4. Go to worksheet, from there can add different layers, different thickness, different angle.

5. From there can treat it as a normal ANSYS simulation (inserting various tests and defining certain wanted solutions)

   1. Can also define which specific layer you want.

Tsai-wu code below

Geometry Preparation of FSAE Composite & Monocoque Chassis in ANSYS SpaceClaim - Part 1

Basically just a demonstration of how to generate a good mesh.

Some notable tools:

• Stich - select this tool under repair to make your mesh better

• Split edges - merges all edges that ansys thinks are seperate

• Imprint - if you have suspension mounts you can do this so coincident edges are “imprinted” (red areas)

• Can also create “origins” at tire locations (unhide tire) if applicable in order to simulate torsion

Materials & Meshing Setup for FSAE Composite & Monocoque Chassis Using ANSYS Mechanical - Part 2

In ANSYS mechanical do the following…

1. Upload your model

2. Go to engineering data and remove the default materal, then go to engineering data sources → composite materials

   1. Should you want to use a material other than those already in ansys, go to DoD Composite Materials Handbook (has mat properties)

3. Adds epoxy carbon woven, honeycomb, and resin epoxy (last two are relevant to what we want)

4. Close engr data tab and go to model tab

   1. If there is a question mark then smt needs to be fixed before you proceed

5. In materials tab u see ur materials

6. Mesh it

Note that curves and suspension attachment areas are weirdly meshed (see part 1 image)

Supposedly if you change element size to 10mm, Capture Curvature to “No”, and select faces of high curvature then “insert → face meshing” your mesh would be a lot better.

7. From there you can go to “Named Selections” select various panel(s) and name them things like “Top Mold” “Bottom Mold” etc.

   1. Helps with defining materials.

Research Results and Findings

Resources

HexCell documentation/manuals

FSAE/ASC composite research

UMinn Solar Car Chassis Explanation

Effects of core cell size and panel width on stiffness