Mechanical Properties of Chocolate- How Strong is your Chocolate? Materials such as metals (aluminum, iron, copper, etc.), ceramics (silicon carbide, porcelain) or polymers (milk jugs made of polyethylene) are tested by scientists and engineers to reveal certain mechanical properties such as the maximum stress a material can withstand. The stress at which a material breaks is a measure of its strength. However, today you will be testing the strength of a delicious material you know as CHOCOLATE! One conventional method of mechanical testing is called a 3-point bend test, in which a load (weight) is applied to the center of a beam which has its edges restricted. An ideal 3- point beam bending test:

Experimental setup of a bar of chocolate:

Stress is defined as force divided by area. Because of the geometry of a beam bend test where the force is not applied uniformly to the bar, stress can be found using the following formula:

where σ is stress, w is the width of the bar (in meters), t is the thickness of the bar (in meters), l is the length of the bar (in meters), P is load (mass) applied (in Newtons).

You will be testing four types of chocolate bars: Hershey’s Milk chocolate Hershey’s Dark chocolate Nestle’s crunch bar Hershey’s Mr. Goodbar (peanuts) Hypothesis Do you think the milk chocolate will break before the dark chocolate? Why? Do you think the crunch bar will break before the Mr. Goodbar? Why?

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wt

P=!

Load

Load Load

Procedure a. Equipment needed:

i. 4 different Hershey’s chocolate bars, for example: 1. Regular milk chocolate (1.55 oz.) 2. Dark chocolate (1.45 oz.) 3. Hershey’s Mr. Goodbar (1.75 oz.) 4. Nestle Crunch bar (1.55 oz.)

ii. Plastic or Styrofoam cups (12 oz size) iii. Pennies (approximately 350 per group) iv. String/twine v. Scissors

vi. Ruler or tape measure. vii. Two desks that can be placed approximately 3 – 4 inches apart (approximately

the length of the chocolate bars) viii. Cylindrical Funnel (can be made from rolled-up piece paper)

ix. Suggested: mass balance x. 2 – 4 people per group

xi. Data sheet b. Work Steps:

Note: during the actual experiment procedures, be sure record all observations (ie – any bending noted in chocolate bar, if the cup is moving around, how hard the pennies are falling into the cup, etc.)

1. Using the scissors, punch two small holes the rim of the cup. The holes should be opposite each other.

2. Cut a piece of string that is approximately 1.5 ft long. The string needs to be

long enough to tie to both ends of the cup, and hang approximately 4 – 6 inches below the chocolate bar.

3. Tie one of end of the string to one of the holes in the cup. Tie the other end of

the string to the opposite hole.

4. Record the following dimensions (be sure to include units): • Type of chocolate bar (milk chocolate, dark, etc): • Length of chocolate bar • Width of chocolate bar • Thickness of chocolate bar

5. Place a mat on the floor to protect the chocolate when it falls. 6. Place the chocolate in between the two desks. Approximately ½ inch (or less) of

the chocolate bar should be touching each desk. Note which way the notches (or lettering) is facing and try to remain consistent throughout the experiment.

7. Place the string and cup assembly across the middle of the chocolate bar.

8. Using the funnel, start placing the pennies into the cup, one at a time. The pennies should be funneled in at a steady pace, ensuring that each penny lands in the cup before the next penny enters the cup. (2-3 pennies a second is a good rate.) Try funneling the pennies in a way that they do not fall a large distance when they enter the cup.

9. Continue placing pennies into the cup at the steady rate until the chocolate bar

breaks.

10. Record the number of pennies in the cup at the time of fracture.

11. Look at the fracture surface and write down any observations.

12. Find the mass of the cup, string, and the pennies in the cup at fracture using a mass balance. If your lab does not have the equipment to weigh these items, calculate the approximate mass of the pennies, using 1 penny = 2.35 grams = 0.00518 lb.

13. Repeat steps above for each chocolate bar to be tested.

Questions

1. How was each of the chocolate bars different from each other? Describe physical characteristics of each chocolate bar below:

Hershey’s Milk chocolate _________________________________________________________ ______________________________________________________________________________ Hershey’s Dark chocolate_________________________________________________________ ______________________________________________________________________________ Nestle’s crunch bar ______________________________________________________________ ______________________________________________________________________________ Hershey’s Mr. Goodbar (peanuts)___________________________________________________ ______________________________________________________________________________

2. Which candy bar broke first? Describe why you think that it broke first. How many pennies did it take to break the chocolate bar?

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3. Which candy bar broke last? Describe why you think that it broke last. How many pennies did it

take to break the chocolate bar? ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________

4. Using your data sheet (number of pennies needed to break the chocolate bar), determine the strength of the chocolate bars from strongest to weakest.

______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________

5. By observing the chocolate bar as you added pennies, were you able to predict when the candy bar was about to break? Describe below why or why not.

______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________

6. Describe below what you think would happen in an experiment that used a candy bar twice the thickness of the thickest candy bar used in this experiment.

______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________

7. What did the “breaks” in the candy bar look like? Do you think by examining the candy bar after it broke that you could put it back together?

______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ Extension Questions:

1. Calculate the load (P) at which each chocolate bar broke. Hint: use P (Newtons) = m (in Kg) * 9.81 m/s2

2. We determined the strength of the chocolate bars by finding each bar’s “breaking point.” Stress is the calculation of this breaking point and is defined as force divided by area. Calculate the stress (σ) at which each chocolate broke using the formula below. (σ is stress, w is the width of the bar (in meters), t is the thickness of the bar (in meters), l is the length of the bar (in meters), P is load (mass) applied (in Newtons).

3. Using the calculations you made in number 3, rank the different chocolates in order of their relative strength.

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4. Why do the same types of chocolate fail with different number of pennies in the cup?

5. Why do different types of chocolate fail with different number of pennies?

6. Would you expect the chocolate fail at a lower or higher load if the grooves were facing the other direction? Why?

7. What would you expect if the cross section was different (ie – the chocolate bar is thicker)? Would you expect it to take more or less pennies, and why?