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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
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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
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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
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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
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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.
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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:
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With these sources, we understand the differences in shear and compression strength between the two materials.
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
Carbon Fiber Properties and Dimensions (Abby)
Look into the different types of carbon fiber available (uni-directional, twill weave, figure out their stiffness, use case,
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source ability, cost, ease of manufacturing when creating a laminate)
Unidirectional (UD) Carbon Fiber:
It can be used in both prepreg and non-prepreg forms
Non-women, and features all fibers running in a single, parallel direction.
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
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 = to achieve balanced properties
Orientated with greater percentage along single axis = Different stiffness and strength along different axes
Spreading resin going against the grain will cause nonwoven fibers to break free from binder
Vacuum bag process = greatest strength-to-weight ratio
Vacuum infusion = 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:
Characterized by diagonal pattern, fibers are woven in a staggered manner
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
Sourceability/Cost:
Higher production costs, due to intricate diagonal pattern = result 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.
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Understand pre-preg vs non pre-preg
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
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
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Look into the bonding process for each type of carbon fiber (necessary epoxy/resin properties), as well as how orientation affects a composite structure
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.
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