Bottom Panel Joint

Owned by Josh Dembicky
Last updated: Jul 18, 2021
4 min read

Joint/Details

  • Large, singular panel adhered to 21 separate chassis tubes

    • 16 on the bottom

    • 3 on the catamaran section

    • 2 on the trunk section

  • Other than the trunk section chassis connections, the adhered length is relatively short

  • The bottom is completely fixed

Materials & Form

  • Chassis - 4130 Chromoly Steel - hollow tubes

  • Panel - Carbon Fibre Composite Sandwich - ~1.5” thick molded plane

    • AX-5201XL carbon fibre, NOMEX honeycomb, and corresponding adhesive film for carbon fibre

  • Adhesive - LOCTITE EA E-120HP - pasted and compressed between the panel and chassis

    • Epoxy-based

Material Characteristics & Notes

  • 4130 Chromoly Steel

    • Excellent strength-to-weight ratio

    • High tensile strength, ductility, and toughness

    • Lightweight in tubing form

      • In non-tubing form:

        • Modulus of Elasticity - 205 GPa

        • Ultimate Tensile Strength - 670 MPa

        • Tensile Yield Strength - 435 MPa

        • Rockwell B Hardness - 92

        • Elongation at Break - 25.5%

  • AX-5201XL Carbon Fiber

    • High stiffness, tensile strength, chemical resistance, and temperature tolerance

    • Low thermal expansion and weight-to-strength ratio

    • Excellent in tension

      • In general:

        • Modulus of Elasticity - ~230 GPa

        • Ultimate Tensile Strength - ~3.5 GPa

        • Tensile Yield Strength - ~2.1 GPa

    • Horrible in compression, but epoxy’s advantage in compression usually cancels that out when they form a composite

      • However, since we are by no means experts who manufacture our parts in a professional setting with quality equipment, I think we should lower our expectations when it comes to the material quality of not only our composites made with the Prepreg method, but the epoxy component specifically. I mean to say that the compressive counteraction that we look for in the carbon fibre composite, due to the epoxy’s presence, may not be expressed as strongly as we need it to be. Although, the NOMEX component might fix this issue.

  • NOMEX

    • High compressive strength, shear strength, materials compatibility, and fatigue strength

    • Low volume density and weight-to-strength ratio

    • High structural integrity

      • In general:

        • Ultimate Compressive Strength - ~2 MPa

        • Most other qualities change drastically based on stress direction

  • LOCTITE EA E-120HP

    • Superior thermal shock resistance

    • Excellent mechanical and electrical properties

      • Specifically peel and impact forces

      • Suitable for low stress [I wonder if that will be a problem.]

    • Withstands exposure to a wide variety of solvents and chemicals

    • Bonds dissimilar materials including aluminum, steel, and other metals, as well as a variety of plastics and ceramics

      • No mention of metal to organic compounds however

        • From Technical Data Sheet (TDS):

          • Modulus of Elasticity (general epoxy resins) - ~4.5 GPa

          • Shear Modulus (general epoxy resins) - ~2.1 GPa

          • Average Tensile Strength - 41 GPa

          • Lap Shear Strength (Stainless Steel) - 23 GPa

[IT SEEMS THAT THE CHROMOLY STEEL WILL YIELD FIRST, SO THAT SHOULD BE TESTED, BUT ONLY THE ADHESIVE IS TESTABLE]

Bonding Characteristics & Notes

  • Typical “Apply, Fix, and Cure” adhesive

  • Cavities may have been unfilled by the adhesive

    • Cavities created by part geometry or air bubbles created by uneven adhesive application

  • Clamped and cured for at least 24 hours

Known Forces or Stresses

  • General compressive stresses to chassis (and by extension the bottom panel) due to car parts placed on top (eg. seats)

    • Plus the external weight of the driver

    • Overloading a specific adhered tube may cause malformation and strain to the tube and adhesive, respectively

  • General weight of the bottom panel

    • We can test this if we get the approximate mass of the bottom panel

Potential Forces or Stresses

  • Any random stresses due to car functions (eg. the wheels being bumped into the bottom panel while turning)

  • Any random stresses due to the environment (eg. driving over a large rock the presses against the bottom panel)

Quality and/or Physical Concerns

  • Neither yet

    • The chassis hasn’t been adhered to the bottom panel as of right now (3:35 PM EST on Friday, July 9th), so no quality concerns

    • Still need to study and determine the experience of the bottom panel during car usage

Test(s)

  1. Adhesive Yield Strength

    1. Determine how much the adhesive can withstand

    2. Tensile and shear stress must be tested

    3. Yield is different for application, so maybe just figure out when it crosses a certain elongation?

    4. Then find amount of force required

  2. General Weight

    1. Determine safety factor and actual elongation

    2. Needed: surface area, bottom panel mass

  3. Quality

    1. Determine air bubbles?

    2. Determine shrinkage?

Testing Methods

  • MODS/Pre-calculations

[MAX ELONGATION AND DISPLACEMENT ARE BASED ON WHEN THE ADHESIVE BOND WOULD BREAK]

  1. Constant elastic and shear modulus, max elongation of 3 mm, assumed height is 2 mm, max horizontal displacement of 1 mm, constant SA (in mm^2)

    1. σ=Eε, σ=F/A, and ε=∆h/h

    2. τ=Gγ, τ=F/A, and γ=∆x/h

    3. Find F for both.

  2. Constant SA (in mm^2), gravity is rounded to 9.8 m/s^2, mass unknown, constant elastic modulus, assumed height is 2 mm, average tensile strength of epoxy given

    1. F=mg, σ=F/A, σ=Eε, and ε=∆h/h

    2. SF=σ_avg/σ

  3. N/A

  • SW

  1. N/A

  2. FEA of weight

  3. N/A

  • Physical

  1. Test amount of force required for adhesive to yield directly

  2. Not needed.

  3. Physical examinations

Expectations

  • MODS/Pre-calculations

  1. I expect it to be a very large force needed in both cases

  2. I expect the change in height to be extremely minimal, and the safety factor to be incredibly high

  3. I don’t expect many air bubbles, but I expect a lot of missing edges

  • SW

  • Physical

Results

  • MODS/Pre-Calculations

  1. Approximate Yield Forces: 1.38 giganewtons and 0.284 giganewtons (tensile and shear respectively)

  2. Approximate Gravity-Induced Elongation: 0.241 nanometers

    1. Approximate Gravity-Based Safety Factor: 76 million

  3. N/A

  • SW

  • Physical

Analysis

  • MODS/Pre-Calculations

  1. The force required to dislodge the adhesive from the carbon fiber sandwich or the chassis bars is incredibly huge. This much was expected. Meaning that for this scenario to happen, something truly drastic would have to happen to the car.

  2. When sitting in place, the light bottom panel causes very little strain to the adhesive. The elongation seen isn’t even a nanometer. And in this scenario, the safety factor is almost 76 million!

  3. N/A

  • SW

  • Physical