...
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
Look into the differences in each material (stiffness, use case, sourceability, cost, focus on nomex and aluminum)
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
...
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
...
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
Understand and label how each property of honeycomb affects the properties of the core material (length, width, thickness, cell size, cell shape, density, etc.)
- Decision matrix between aluminum honeycomb and nomex honeycomb
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:
| View file | ||
|---|---|---|
|
| View file | ||
|---|---|---|
|
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)
Understand pre-preg vs non pre-preg
Look into the bonding process for each type of carbon fiber (necessary epoxy/resin properties), as well as how orientation affects a composite structure= 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
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
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.
...
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
ANSYS Composite Simulation
need to explain a little bit more about how ANSYS/FEA testing works as it pertains to a composite structure
I (noah) can fill in this section but we need to essentially explain the features of pre-ACP and post-ACP on ANSYS, how they reflect the IRL sandwich panels in a simulation setting, and how we plan on using it to prove to ASC that our panels actually pass regs
KETIV Technologies Video: Intro to Composite Analysis Using Ansys Mechanical[Devansh]
Vocabulary
Ply - a bunch of fibers put together with some kind of adhesive.
Laminate - stack of plies.
FEA Testing
...
How to model? (9:40)
Shell Method 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.
There are two other methods - solid shell and layered solid - but I don’t really understand what the guy in the video is saying.
What data do you need? (11:23)
Orthotropic material properties, Ply To run the FEA simulation, we need orthotropic material properties, ply thickness and orientation, stacking sequence, the orthotropic strengths, and failure theory/theories.
Mentions that Tsai-Wu failure criteria is apparently a pretty popular one for composites (may want to look into this more):
Example from video:
...
I don’t really understand this, so I found a quick explanation online by SolidWorks that’s more understandable: https://help.solidworks.com/2021/English/SolidWorks/cworks/r_tsaiwu_failure_criterion.htm
After identifying composite material, you can pretty much do whatever you want with it.
Delamination Analysis:
Two things you can do with ansys: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). Guy in video does not go too much in-depth on this but I found a pretty good pdf over it online (will take notes later on it).
VCCT (Virtual Crack Closure Technique) - Another method to see how composite panels can delaminate.
...
Define layers - select worksheet option, and define each layer
...
...
If you go to engineering data on the ANSYS workbench → composites material, there are a lot of predefined ones to use.
Click coordinate sys → insert new coordinate sys → from there, can go to properties of coord plane and set geometry relative to plane.
...
From there can treat it as a normal ANSYS simulation (inserting various tests and defining certain wanted solutions)
Can also define which specific layer you want.
...
Note: Remember the tsaiTsai-wu stuff from above? apparently you can code ANSYS to do it:code below
...
Geometry Preparation of FSAE Composite & Monocoque Chassis in ANSYS SpaceClaim - Part 1
...