Impact Load Lab Exercise
Problem
Eggs break on impact. The goal of this lab is to build a structure capable of absorbing repeated impact load to protect a chicken egg.
Design Constraints
- design must allow egg to be installed, removed, inspected, and reinstalled before each brick drop without making any modifications
- construction may not exceed a height of 12", and the footprint must be between 10 square inches and 100 square inches
- only the provided materials may be used: 15 toothpicks, 12 cubic inches of floral foam brick, 5 sheets of newspaper, 2ft masking tape
- fabrication must be completed within a period of 25 minutes
- must accomodate for natural variation in egg size
Brainstorming
Our first idea involved making the foam block housing into a triangle shape to deflect the brick (figure 1). We started with a right triangle, and upon further analysis, reasoned that an equilateral triangle would be stronger as its uniformity would distribute the force more evenly (figure 2).
Figure 1. Right triangle shaped structure.
Figure 2. Equilateral triangle shaped structure.
We then had the idea to build a tall structure around the egg so that the brick would have less time to accelerate and therefore lessen the impact (figure 3). We reasoned that the brick’s force would be mostly absorbed by the cushioning upon reaching the structure closest to the egg.
Figure 3. Tower-shaped structure.
As a general theme, we agreed that we would need a hard shell around the egg, with cushioning on top to slow the brick. In addition, our remaining toothpicks would be most useful placed inside the foam block to increase the tensile strength, using the analogy of rebar in concrete.
Evaluation of Ideas
When we received the materials, we realized that the physical properties of the floral foam block were different from our initial understanding, so we did not cut it up to build the triangular structure. Instead, we carved a hole for the egg on the bottom of the block. We also put toothpicks in a cross-hatch pattern at the top of the block (tensile strength), and toothpicks perpendicular to the ground, from the bottom, to prevent the block from cracking the egg by means of compression. We used our leftover newspaper taped to the top of the foam block to act as a cushion.
We ultimately ruled out the tower design reasoning that the tall structure would be less stable, so we decided to go with a shorter design.
Our final design looked like this.
Figure 4. Floral foam block with hole for egg on bottom and toothpicks to increase the tensile strength of the brick.
Figure 5. Final design for testing.
Figure 6. Drawing of final design. The bottom view (left) shows the hole for the egg and toothpicks surrounding the hole. The side view diagram shows the orientations in which all the toothpicks were placed inside the foam block, as well as the newspaper cushion and tape.
Post-testing Analysis
Figure 7. Floral foam block after testing.
Maximum height survived = 24in = 0.61m Height failed = 30in = 0.762m E_absorbed = mg (h1 + h2 + h3) ⇒ Eabsorbed = (1.58kg) (9.8m/s2) (.3m + .46m + .61m) Using a mass of 1.58kg for the brick, we calculated that our design cumulatively absorbed 21.2J of energy prior to failure.
Conclusion
After the first drop, the newspaper padding fell off the top because the tape didn’t stick well to the foam block. In addition to securing the newspaper with tape, attaching the tape to the block with toothpicks would make it more durable. With the impact of the brick from 18”, the structure showed signs of cracking from the bottom. To improve this for next time, more toothpicks could be added closer to the bottom, perpendicular to the direction of the force, to increase the tensile strength of the brick. When the brick was dropped from 24”, the crack split the foam block in half, and the impact of the brick cracked the eggshell. In future designs, more toothpicks could be used to increase the tensile strength of the block. In addition, the foam block could be wrapped with newspaper or tape to further support the block from splitting.