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Shell Optimization

Shell Optimization

Parameters to Optimize

For a composite top shell, these are the main design variables:

  1. Ply count: The number of layers in the composite. More plies generally increase strength and stiffness but add weight and cost.

  1. Ply orientation: The angle of each ply relative to the reference direction. Orientation affects the shell's ability to withstand loads from various directions.

  1. Fiber type: Different fibers (carbon) have different properties and costs.

  1. Type of weave: The weave pattern impacts stiffness, strength, and flexibility.

  1. Core material

    1. Aramid honeycomb cores are typically made from aramid fiber paper impregnated with phenolic resin, forming a lightweight, waxy, honeycomb structure with high strength-to-weight ratio and exceptional stiffness

    2. Core shapes

      1. Hexagonal Core [Fig. 1]: The standard hexagonal honeycomb is the basic and most common cellular honeycomb configuration, and is currently available in all metallic and nonmetallic materials.

      2. OX-CoreTM [Fig. 2] The OX configuration is a hexagonal honeycomb that has been overexpanded in the W direction, providing a rectangular cell configuration that facilitates curving or forming in the L direction.The OX process increases W shear properties and decreases L shear properties when compared to hexagonal honeycomb.

      3. Reinforced Hexagonal Core [Fig. 4 blurry picture]: Reinforced honeycomb has a sheet of substrate material placed along the nodes in the ribbon direction to increase the mechanical properties.

      4. Tube-Core® [Fig. 3]: Tube-Core configuration provides a uniquely designed energy absorption system when the space envelope requires a column or small diameter cylinder. The design eliminates the loss of crush strength that occurs at the unsupported edges of conventional honeycomb.Tube-Core is constructed of alternate sheets of flat aluminum foil and corrugated aluminum foil wrapped around a mandrel and adhesively bonded. Outside diameters can range from 1/2 inch to 30 inches and lengths from 1/2 inch to 36 inches.

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    3. Find which specific Aramid honey comb we use, maybe try to find if we can change to a different type (check pg. 9 pdf linked above to see material types of aramid honeycombs)

    4. Types of Honeycomb material:

      1. HFT®-G: is a bias weave carbon fabric reinforced honeycomb dipped in either a heat-resistant phenolic resin or a polyimide resin to achieve the final density. This product was developed for use at service temperatures up to 500°F. However, it is well suited for short exposures at higher temperatures. HFT-G has a very low coefficient of thermal expansion and a high shear modulus value.

      2. KOREX®: Honeycomb is made from KOREX aramid paper dipped in a heat-resistant phenolic resin to achieve the final density. KOREX honeycomb offers improved strength-to-weight ratios and/or lower moisture absorption than Nomex honeycomb of a similar configuration.

      3. HFT: is a fiberglass fabric reinforced honeycomb that incorporates a ±45° Fibertruss® bias weave dipped in a heat-resistant phenolic resin to achieve the final density. This material is recommended for use at service temperatures up to 350°F but is well suited for short exposures at higher temperatures. The Fibertruss configuration greatly enhances the shear properties. HFT has a much higher shear modulus than HRP or HRH®-10.

      4. 5056 Alloy: Specification grade honeycomb in the 5056 H39 aluminum alloy offers superior strength over 5052 alloy honeycomb. It is also available in a broad range of cell size/density combinations in the hexagonal and Flex-Core configurations. The strength properties of 5056 alloy honeycomb are approximately 20% greater that the comparable properties of 5052 alloy honeycomb of similar cell size, foil gauge, and density.

      5. ACG®: Aluminum Commercial Grade (ACG) honeycomb provides a low-cost aluminum honeycomb product for industrial applications. All ACG materials are provided with CR III coating. Hexcel produces ACG from 3000 series aluminum alloys. 3003 aluminum alloy is used for energy absorption applications where previous qualification studies specified this particular alloy. Hexcel also uses 3104 alloy for the manufacture of honeycomb with the flexibility to provide either 3104 or 3003 ACG, whichever is more appropriate for the application.

Limiting Factors (ordered greatest to least priority)

  1. Money/Budget: High-performance fibers and resins (e.g., carbon prepreg) are more expensive.

  1. Material properties: Limits on maximum ply count due to available materials or manufacturing capabilities.

  1. Space/Geometry: The physical dimensions of the car and requirements for space inside for occupants or systems.

  1. Cost vs. value/performance

  1. Strength

    1. Compressive

    2. Impact

    3. Shear

    4. Fatigue

    5. Flatwise tensile

  2. Density

  1. Cell wall thickness

  1. Facings

    1. Material

    2. Bonding process, adhesive, conditions

    3. Thickness

  2. Moisture

  3. Processing and operating temperature range

 

Forces Applied (FBD - Free Body Diagram)

Forces Applied to the Top shell

  1. Out-of-Plane (Transverse/Through-Thickness) Loads:

    1. Weight of the shell and any components attached to or supported by it (downward due to gravity and potentially any load from mounted systems).

    2. Localized compressive loads at mounting points, connections to bulkheads, or areas of application-specific reinforcement (e.g., window or access hatch cutouts).

    3. Impact and point loads, including debris strikes or low-velocity impacts (e.g., handling, maintenance, crash events).

  2. In-Plane (Panel Plane) Forces:

    1. Shear transfer between upper and lower shell skins and through the honeycomb core when the shell is loaded in bending (e.g., aerodynamic forces, occupant or cargo weight, and emergency loads).

    2. Tensile and compressive in-plane forces due to bending moments—especially important around curved regions typical in an aeroshell.

  3. Aerodynamic Loads:

    1. Distributed pressure from airflow during motion (lift, drag), which can lead to significant forces depending on vehicle speed and shell geometry.

    2. Induced vibrations and dynamic pressures from turbulent flow or vortex shedding.

  4. Thermal and Environmental Loads:

    1. Expansion/contraction and associated stresses due to exposure to solar heating and external temperature cycling.

    2. Moisture absorption/desorption, potentially affecting both the carbon fiber facings and the aramid honeycomb core (especially relevant for materials with open cells or exposed edges).

  5. Fatigue Loads:

    1. Repeated cyclic loading due to aerodynamic pressure variation, vibration, everyday manipulations, and thermal cycling, all contributing to long-term material degradation.

  6. Bonding and Interface Loads:

    1. Shear, peel, and local compressive stresses at the adhesive or mechanical bonds between the facings and the honeycomb core (these can be the weakest zones if not properly designed).

  7. Typical Free-Body Diagram Force Locations

    1. Gravity: Uniformly distributed across shell.

    2. Aerodynamic Pressure: Over the exterior top surface, varying with curvature and velocity.

    3. Compression/Tensile/Bending: At attachment points, panel edges, and through-shell sections spanning between supports.

    4. Shear: Throughout the core material, maximized where the shell is curved or between spaced supports.

    5. Impact/Fatigue Loads: At points of expected contact or typical damage sites (hatches, mounts).

 

Forces on the Bulkheads

  1. Structural Support and Load Transfer:

    1. Bulkheads transfer the loads from the topshell to other structural elements, experiencing in-plane shear and normal stresses, especially at the joints and attachment points.

    2. They resist bending and local deformation caused by aerodynamic forces, impact loads, and internal pressure variations.

  2. Impact and Crash Loads:

    1. The bulkheads are critical in absorbing impact energy during collision or debris strike, experiencing localized high-stress zones.

  3. Aerodynamic and External Loads:

    1. They encounter pressure differences during motion, especially if exposed areas extend inward or are near aerodynamic surfaces.

    2. Turbulent flow and vortex-induced vibrations can induce dynamic stresses and fatigue cycles.

  4. Vibration and Dynamic Effects:

    1. Oscillations caused by vehicle motion and external airflow can produce cyclic loading, influencing fatigue life and stiffness requirements.

  5. Thermal and Environmental Stresses:

    1. Variations in external temperature and internal heating affect the stiffness and bonding conditions, leading to thermal expansion stresses.

  6. Bonding and Connection Stresses:

    1. Attachment points with the topshell or internal fixtures generate peel, shear, and tensile stresses, which are critical for joint durability and overall stiffness.

  7. Summary of Force Application Points:

    1. At attachment points to the topshell and other structural components.

    2. Distributed aerodynamic pressure acting along exposed surfaces.

    3. Impact zones during crash or debris contact.

    4. Localized shear and tensile zones at reinforcements or openings (hatches, windows).

    5. Vibrational and cyclic loading from operational and environmental conditions.

 

Order of Parameter Optimization

  1. New Order of Parameter Optimization

    1. Ply Orientation

      1. Prioritized first as optimizing fiber orientation aligns reinforcement with principal stress directions, maximizing strength and stiffness with potentially fewer plies required. This leads to an optimized balance between performance and weight.

  2. Ply Count

    1. Determined after orientation to ensure sufficient strength, stiffness, and safety margins. Minimizing ply count reduces weight and cost while meeting design requirements.

  3. Core Material Selection

    1. Choice of core (e.g., aramid honeycomb type or alternate core materials) impacts stiffness, strength, weight, and environmental tolerance. Core characteristics such as cell shape (hexagonal, OX-Core, reinforced) and material properties are optimized here to suit mechanical and thermal loads.

  4. Fiber Type

    1. Selection of fiber (carbon variants or others) considers cost-performance tradeoffs and is chosen after fundamental structural parameters are set.

  5. Weave Pattern

    1. Weave affects flexibility, stiffness, and damage tolerance, optimized alongside or after fiber type selection for manufacturability and performance tuning.

  6. Facings

    1. Material, bonding process, adhesive types, and facing thickness influence structural integrity and interface strength, optimized to support the core and ply structure effectively.

  7. Cell Wall Thickness and Core Density

    1. Adjusted to fine-tune strength, weight, and mechanical behavior especially in honeycomb cores, following core material selection.

  8. Moisture and Environmental Factors

    1. Address moisture absorption, processing limits, and operating temperature ranges to ensure long-term durability and dimensional stability.

  9. Cost vs. Value/Performance Evaluation

    1. At all stages, budgetary constraints guide trade-offs between high-performance materials and affordability.

  10. Space and Geometric Constraints

    1. Final checks to ensure the assembly fits within design envelopes for the car’s interior, occupant space, and systems.

 

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