The first step in designing anything with composites is to perform materials tests in order to validate the manufacturing processes. Until we validate all of the mechanical properties of the materials we work with, we have no idea what appropriate loading parameters are, and there is no way to proceed safely with the design.

Criteria to determine the core thickness

Ideally, the chassis should be as stiff as possible to ensure that the suspension can function. However, the chassis must also be lightweight, so the core cannot be excessively thick.

Tests to complete

• Bending strength of the panels Used to verify the manufacturer's specs.
• Maximum tensile force a glue joint can withstand per unit length
• Maximum bending moment a glue joint can withstand per unit length
• The in-plane pullout strength for bracket inserts in the composite
• The normal plane pullout strength for the bracket inserts in the composite

Sandwich panels used:

The sandwich panels that we will use are up for discussion.

Materials:

Procedure:

1. Cut the Nomex core into a rectangular sheet 
2. Layer the prepreg on the core as follows
   1. 1 layer of fiberglass scrim cloth (to ensure that it bonds with the core)
   2. 1 woven carbon sheet running at 0 degrees and 90 degrees
   3. 1 unidirectional carbon running at 45 degrees
   4. 1 unidirectional carbon running at -45 degrees
   5. 1 woven carbon sheet running at 0 degrees and 90 degrees
   6. 1 unidirectional carbon running at 45 degrees
   7. 1 unidirectional carbon running at -45 degrees
   8. repeat for the other side
3. Cure the prepreg by putting it in an autoclave or an oven

Youtube Reference: Carbon Fiber Prepreg With Nomex Honeycomb Core (Spacegrade)

Bending strength of the panels Used to verify the manufacturer's specs

Purpose: Performing a bend test will enable us to validate the safety of applying loads onto the car. The main mounting points on the car will be near the front and rear suspension, the seats and the battery box. This will give a hard value of the amount of force that the composite and be loaded with before failure.

Reference: Hexcel's mechanical testing of Sandwich PanelsMechanical Testing of Sandwich Panels.pdf

Long beam flexture test. Procedure

1.Manufacture the custom prepreg sandwich panel using the materials and procedure above. use a rectangular plate with a length of 25" by 10" or so.

2.Ask for permission to use the beam testing machines in E3.

3. Set up deflection gauges and force gauges and add increasing forces to the material, say 100N each time. Record the deflection and force at each interval.

4 Continue until failure. At failure record observations about the failure mode, such as tension/compression failure of the skin, localized face wrinkling failure, localized face dimpling failure4. 

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Loading of composite p