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INSTRUCTOR MATERIALS
INSTRUCTOR MATERIALS
OVERVIEW OF THIS PROJECT AND WHAT TO EXPECT
In this project students design and make small cement composites. Students decide how to use reinforcement materials such as cotton, thread, and paper, in a cement matrix. During testing, the students' composites are placed, like a beam, across two blocks of wood that are about 7cm high and 10cm apart. A fragile body (a raw egg) is placed underneath the composite, between the blocks, and a golf ball is dropped from increasingly higher distances until the composite is completely penetrated and the egg is smashed.
SPECIFIC PURPOSE OF THIS PROJECT
The most important thing students learn from this project is that composite materials can be designed to perform the way you want them to. They see how adding only a few reinforcement materials greatly increases the strength of the matrix material. Finally, this project offers students the chance to participate in some of the same design and fabrication activities that materials science engineers are involved in.
THE REAL-WORLD PROBLEM RELATED TO THE PROJECT
The most significant aspect of this project is that designing composite materials for specific functions is relatively new in many areas. In applications where metal, ceramic, and polymer materials have been used exclusively by themselves, composites offer lighter, stronger more customized devices. The practical value of ceramic composite materials has been recognized for centuries. In fact, today it is almost impossible to find anything made out of concrete that hasn't been reinforced. Many cement and steel composites involve pre-stressing the steel, which greatly increases the bending strength.
SPACE AND TIME REQUIRED
This project requires about four or five hours to complete. Student teams should have about nine square feet of area to work on. They will be building forms, laying out materials, and mixing cement so give them plenty of space. Make sure there is plenty of newspaper on every thing that might get spilled on. It is a good idea to structure the project so that the cement composites have at least 30min. to dry before testing.
MATERIALS AND EQUIPMENT REQUIRED
There are two sets of construction materials for this project. One set is for building a form. The other set is for the composite. Make sure your students understand that one design requirement is that no FORM materials are used in their composite and no COMPOSITE materials are used in their form. Tell your students to begin gathering the listed materials in the weeks before doing the project. Supply the cement to make sure each design team gets 200g of ACEr Hardware Quick Plug Hydraulic Cement. It is important that you supply this kind of cement to assure good results.
Your students may use the following materials to construct their composite FORM.
Cardboard
Trash bag liner
Glue
Tape
2 liter bottle
Plastic milk jug
Each team may use the following materials in their cement COMPOSITE.
200g of hydraulic cement
Water
4 cotton balls
1 sheet of notebook paper
1 foot of cotton string
6 feet of cotton thread
Make sure students have the following equipment for their construction.
Mixing containers
Mixing sticks
Scissors, pencils, rulers
Before testing the composites you will need to find a good test site and make the dropping device. For testing the composites and making the dropping device you will need the following materials and equipment.
10ft length of 1.5in, thin wall (Schld 26), PVC pipe
A drill motor and 1.75in hole saw drill bit.
A 10ft tape measure
golf balls
SUGGESTIONS REGARDING STUDENT TEAMS
This project works best with student teams of two or three.
PREPARING FOR THIS PROJECT- WHAT TO DO IN ADVANCE
There are three things you need to do to prepare for this project besides making sure the students have enough of all the listed construction materials. First, read the Student Project Description for this project so that you will know what information they have been provided with and exactly what the design requirements are. Next, make two cement composites (described below), one with reinforcement fibers and one with no reinforcement. Finally, obtain the materials for and set up the testing device (also described below).
Make Two Cement Composites
Use the two composites that you make for your presentation of this project to the students. This effective demonstration shows how weak the cement is without the reinforcement materials. While you are talking about how fibers improve the strength of the matrix material, drop a golf ball on the non-reinforced composite as it rests on the test blocks. Explain how the non-reinforced cement is very weak in tension and easily cracks. Then, drop the golf ball on the reinforced cement over and over until it begins to crumble. You and your students will be amazed at the difference between the reinforced and non-reinforced cement.
First, make a form to cast the two demonstration cement composites in. A simple way to do this is to cut out one large side of a Milk Duds' box and line it with a piece of trash can liner. Use cardboard or any of the other listed FORM materials if you would rather make your own. It is important that the shape of the two cement composites you use in the presentation be simple, rectangular, wide beams; part of the design task for the students is to design the shape of their form to optimize the composites impact strength. Next, shred up four cotton balls, and cut up the string and thread into pieces 6cm to 8cm long. Mix 200g of hydraulic cement with just enough water to make it the consistency of pancake batter. Add the water slowly so you don't make it too runny. Next, mix in well the thread, string, and cotton. Quickly pour the mixture into the form, spread out the fibers as evenly as possible, and give it about 20min to dry.
Remove the composite from the form and set it aside. Mix 200g of hydraulic cement with about the same amount of water you used for the first cement composite. This time what you will be casting is not a composite so do not add any reinforcement fibers. Pour the cement in the form and let it dry as before.
Make the Dropping Device
Obtain a 10ft length of 1.5in, thin wall (Schld 26), PVC pipe. Drill 1.75in holes in one side as shown in figure 1. Mount this perpendicular to the ground with the first drop site about 1m from the ground (Fig 2).
GUIDELINES FOR TESTING THE SYSTEMS CREATE
Set up the dropping device against a wall, on a tall ladder, or to the railing of an open stair well. If you choose to mount the dropping device against a wall you will still need a ladder to reach the higher drop sites. The testing procedure involves dropping the golf ball from increasing heights until the composite fails and the egg is broken. Let students adjust their composites on the test blocks after each drop. Record the results and calculate each team's performance with the Basic Performance Index (BPI) and the Extra Performance Index (EPI).
The Basic Performance Index (BPI) is satisfied by protecting the raw egg from the falling golf ball dropped from the first height of 1m.
Measure each team's Extra Performance with the EPI.
EPI = # of subsequent drops that the composite successfully protects the egg
IMPORTANT STEPS IN MANAGING THE PROJECT
Divide the class into design teams.
Distribute the Student Project Description.
Present the project to the students (15-30 minutes).
Let the students read the materials, discuss it among themselves, ask questions, and generate design ideas (15-30 Minutes).
Have students present their ideas or comment on their proposed scheme for using the reinforcement materials.
Let the design teams construct their cement composite (60 Minutes).
Test the composites as described above. Discuss the possible reasons for successes or failures during testing (30-60 Minutes).
Let the students improve their design and reconstruct (60 Minutes).
Test the final designs (30 Minutes).
Summarize the results of the project and tie the concepts together in a positive closure (15-30 Minutes).
IMPORTANT PRINCIPLES INVOLVED
Materials science engineers study materials very carefully to determine their properties. A material's response to loading or impact is very important to designers. The behavior and strength of a material can be represented in a simple graph of stress vs. strain or force vs. deformation. The elastic modulus (slope of the stress vs. strain graph), elastic strength (the stress above which the deformed material will not return to its original shape), and ultimate strength (stress at which the material fails) are properties of a material while compression (material squeezed together), tension (material stretched out), and shear (material with transverse forces) are used to describe its state.
Encourage your students to visualize what will happen when the ball strikes and where the maximum compression, tension, and shear will be in the composite. Because the bullet proof composite must endure greater and greater impacts, design teams will have to optimize their design with the materials they have.
Composites
Composite materials involve some combination of two or more components from the fundamental material types such as metals, ceramics or polymers. Composites are called non-homogeneous materials (not the same throughout) and can be anisotropic (properties are different in all directions). There are three common fiber reinforcing configurations. These are continuous, chopped, and woven fibers. How the reinforcing materials behave in the matrix is very important, but not well understood by experts. Adding reinforcing fibers to a matrix will generally increase its strength in the direction parallel to the fibers (Fig. 3). Chopped fibers improve strength in all directions while woven fibers generally improve strength in several directions.
Figure 3. Continuous aligned fiber reinforcement generally improves strength in the direction parallel with the fibers. The chunk of material shown here would support a greater F than F.
PROVIDING SUCCESSFUL CLOSURE
The most important thing you can emphasize to the students is that the activity they have just completed is very much like the activities that engineer are involved in. Engineers build models and test them, redesign and change things, and work towards a good final design.
SPECIAL SAFETY CONSIDERATIONS
Make sure no one breathes or ingests the hydraulic cement. Also, make sure that each time the golf ball is dropped every one is out of way and not in danger of being struck by the ball or pieces of broken cement. Express to students that they should avoid getting the cement on their skin.
Notes
Teams of no more than three students are best for getting all team members involved.
Most students try to force as much material into the mold as possible, in spite of the warning to adhere to the specified dimensions. Explaining up front that any composite that does not fit into the holder will be trimmed with a pocket knife until it does fit helps keep those instances down to a minimum.
Be sure to point out that although it doesn't matter whether they pick foil or plastic wrap to line the mold, it is very important that the liner not have any tears in it. For example, if a team tears the foil while poking it into the mold, they will need to reline the mold or the composite will stick.
Most students have difficulty understanding the necessity of cohesion in the composite. Unless it is explained that the strength of the composite derives from how the materials are combined together, almost all of the teams construct a composite with separate layers - the key word here being separate. The usual tactic is to fold the paper to the exact length and width dimensions of the mold and place it in the middle of the composite between two layers of cement. With the cement on either side unable to bond with the other, the composite tends to pull apart like an oreo cookie.
Most students have never worked with hydraulic cement before and need to be told the following:
a) The desired consistency is somewhat like Ragu (not the extra chunky kind).
b) The cement needs to be thoroughly mixed. Be sure not to leave dry powder at the bottom of the container.
c) Once mixed, the team must move QIJICKLY, and one team member must be stirring the cement at all times. Letting the mixed cement sit will cause it to harden much more quickly.
d) The designated stirrer may need to add water at intermittent intervals to keep the cement from hardening if the team isn't moving quite fast enough.
e) At some point, whether to break up clumps within the mixture or to spread the cement, hands will most probably be used. Don't worry, the cement washes out of just about every thing.
It needs to be explained that the teams will have time to construct the composite in layers if they so desire. Just be sure to remember cohesion and have the layers ready to go BEFORE mixing the cement. Additionally, materials may also be stirred in with the cement.
This project was developed by Eddie Richert.
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