Final Design

The final sandwich panel has a total thickness of 1.125" and is comprised of:

• 8 face plies of prepreg carbon fiber with a balanced 0/45/0/45/45/0/45/0 layup schedule

• 1" 3000 aluminum 1/8" cell 7.7 pcf honeycomb core

• 2x 1" round critical G-10 inserts

• 6x square non-critical G-10 inserts

The full front impact structure, including the honeycomb impact attenuator, the composite anti-intrusion plate, and the first 50 mm of the chassis is shown below.

Skin

In a sandwich panel, the skin is designed to withstand in-plane tensile shear forces. Additionally, as required by the rules of the FSAE competition, the anti-intrusion plate must be quasi-isotropic such that the measured properties are the same regardless of direction.

Selection of the carbon fiber weave was based on availability of materials from General Atomics. The twill weave 584 3K-8HS/TC250 prepreg weave was chosen for its greater properties. Each face skin, top and bottom, consists of 8 plies of the carbon fiber weave and follows a 0/45/0/45/45/0/45/0 layup schedule to ensure quasi-isotropic material properties.

Core

The core of the anti-intrusion plate will carry much of the compressive load upon impact of the crush structure. To ensure the safety of the driver, the core was selected to be a 1" thick 3000 series aluminum honeycomb core with 1/8" cell size and 7.7 pcf density. This configuration ensures that the core shall not fail during impact.

The honeycomb core was machined according to Figure 1. There are eight insert locations that were waterjet cut from the core. The insert locations were machined with a 0.05" tolerance to account for the thickness of the foaming adhesive used to bond the G-10 inserts to the core.

Inserts

The inserts are necessary to support the bolts that are used to attach the anti-intrusion plate to the front of the vehicle. The inserts must be able to withstand the shear and bending stresses present upon impact. G-10, a composite laminate material made of layers of woven glass fiber binded in epoxy, was selected, and machined into 1" diameter round inserts and 1.142" square inserts.

For the 1" diameter round inserts, the relevant results of the FEA are shown below. The max stress concentration induced by these inserts is 301.35 MPa, and the max stress seen by the inserts is 150.32 MPa.

Expected Performance

The FEA results for the final plate design are shown above. Based on these results, the final plate design has a:

• Max deflection of 1.1 mm

• Factor of safety of 1.8

Additionally, the plate has been designed to prevent core failure, as can be seen in Figure 6. Based on these results, the anti-intrusion plate meets the deflection requirements of the competition which prescribe a maximum deflection of less than 25 mm.

Real Performance

Three tests were performed to determine the performance of the design: perimeter shear, 3 point bend, and quasi-static crush. The material properties for the sandwich panel were also determined from the obtained data using a MATLAB script.

Perimeter Shear Test

The perimeter shear test was performed to determine the shear strength of the skin of the plate. The force vs. displacement graph of the test is shown in Figure 7 below. The highest peak is used to determine the skin shear strength, which is 30.737 kN. This easily passes the minimum requirement of 4 kN.

3 Point Bend Test

The 3 point bend test was performed to determine the energy absorption of the panel and compare it to the energy absorption of rules-compliant 1.0" diameter, 0.065" wall thickness 1020 steel chassis tubing.

For 19 mm of displacement, steel tubing absorbs 62.99 J of energy. Our CFRP plate absorbed 232.05 J.

This test was also performed to determine the bending stiffness and yield strength of the plate, which were found to be 3633.4 kN/m and 32.13 kN, respectively.

Figure 8a and 8b. Force and Energy Absorption vs. Displacement for CFRP coupon and rules-compliant steel chassis tubes.

Quasi-static Crush Test

A quasi-static crush test of a representative front chassis bulkhead was conducted to verify energy absorption and maximum anti-intrusion plate deflection. Total energy absorbed was 8205 J, exceeding the requirement of 7350 J. The maximum recorded deflection was less than 1 mm, which compares well with our FEA results.