Activity: Stop The Stretching

GRADE LEVELS:  6-8

SUMMARY:

Students will learn about composite materials, tension as a force and how they act on structural components through the design and testing a strip of plastic chair webbing.

LEVEL OF DIFFICULTY [1 = Least Difficult : 5 = Most Difficult]

5-most difficult

TIME REQUIRED

(30 min) class period demo/initial computer graphing
1 (30 min) class period design
½ - 1 (30 min) class testing and follow-up

COST

Materials to build one tensile test station: $6
Student materials: $3 per class


STANDARDS:

5.3 Explain how the forces of tension, compression, shear, bending, and torsion affect the performance of bridges and towers.
1.1 Identify materials used to accomplish a design task based on a specific property (i.e. weight, strength, hardness, flexibility).
2.2 Demonstrate methods of representing solutions to a design problem (ex. sketched, prototypes).
2.3 Describe and explain the purpose of a given prototype

WHAT WILL THE STUDENTS LEARN?
Students will learn about tension as a force and how it acts on structural components through a hands-on group design problem. They will also learn about composite materials and how they can be made for increased strength.
BACKGROUND INFORMATION:

• Set up only two test stations. This will focus all students attention on the testing, and they will learn how to improve their designs after watching the results of other teams’ tests

• Before the class tests their own chair webbing designs, you should demonstrate to students the process of running a test. Run the first test on a single 4 mil thick plastic strip (2” X 18”) and have a student record the data on the board. Have the whole class graph these results on the grid provided in their packets. By doing the plain plastic test first, students will be able to really see the improved stiffness and strength of their composite material designs.


Teacher / Background Notes:

Structural elements subjected to tension (pulling forces) will stretch and “neck down” before they break. The actual amount of elongation (stretching) depends on the load, but it also depends on the original length of the material; the longer the piece of material, the more it will stretch when subjected to a given load (so it is important for all students to mark off the 5” initial length). Have students watch for the necking on their plastic samples that are loaded in tension; they will observe that the middle of the material gets skinnier and thinner. All materials in tension, even steel, will stretch and neck down, before they fail (break). When a high enough load is placed on a structural member in tension, the ultimate tensile strength of the material is exceeded and it fails.

Direct students to find these real-life examples of structural elements in tension: cables (wire ropes) used to hold up bridges, antennas and small towers, and also used in hoists and cranes; telephone lines hanging between poles; wires used to hang or support signs, and hold up sailboat masts; ropes used with pulleys to lift heavy loads (block and tackle), or used in rope ladders, playground equipment and boat rigging.

The stiffness of a material is a measure of its rigidity or flexibility; the greater a material’s stiffness, the less it will deform (compress, stretch, bend) when a certain load is placed on it. In this lab, students are trying to develop not only a stronger material, but also one that has a much greater stiffness. Their graphs will tell them if they are successful. The steeper the slope of the linear (straight-line) part of their graph, the higher the material’s stiffness (For graph see download). A steep slope indicates a very rigid material - the amount of stretch increases slowly as the load increases - this is the goal for designing the chair webbing. Notice that material “A” only stretches 1/4” when loaded to 50 lb. A curve with a less steep, flatter slope (graph B) indicates a more flexible or stretchy material - the amount of stretch increases quite a lot as the load is applied. Notice that material “B” stretches 1 1/2” with only a 30 lb. load applied (*For graph see download*). Composite materials are quite common today. A composite material is one that is created by bonding two or more materials together to create a material that is stiffer, stronger, lighter or has some other improved property (less thermal conductivity, higher electrical resistance, etc.). Maybe it would be interesting for your students to do a net search to find different composite materials, find how they’re made, what they’re used for and what are their improved properties. Students might investigate the following: reinforced concrete, insulation and other building materials; materials used to make skis, snowboards, racing bicycles, tennis rackets, fishing poles and golf clubs; materials used to make spacecraft, airplane and automotive bodies. Specific materials that they might look into include: glass fiber-reinforced resins (fiberglass), carbon-graphite composites, ceramic composites, plastic laminates and plastic-metal laminates, and there are many others.

MATERIALS:
To build 1 test station:
At least a 14’ section of link chain (make sure the links fit around a 3/8" bolt)
An approximately 8” section of link chain
3/8” x 6” round head bolt threaded enter length
3/8” hex nuts
Duct tape
5-gallon pail with strong handle (school floor wax buckets)
Small, pea stones (uniform size) or sand - enough to fill both the 5-gallon pails (sand is messier)
Ruler
Small coffee can (16 oz.)

For One Sample (each team will need 4 Sample sets):
1 strip of plastic sheet, 4 mils thick X 3 in wide X 18 in long
(1 mil = 1/1000 in.) - A roll of plastic sheeting can be purchased from Home Depot - don’t use trash bags
5' of masking tape
5' of thread
Tools for Sample Construction:
rulers
scissors
marker

PREPARATION:
Obtain Materials & Photocopy Worksheets
Constructing the Tensile Test Stations and problem statement

DIRECTIONS:

See Problem Statement Worksheet in Download

INVESTIGATING QUESTIONS:
What is tension?
What effect does it have on structures and structural elements?
Give real life examples of tension and actual examples of structural elements that are loaded in tension.
What are composite materials and how are they made?
Find real life examples of composite materials and identify the special properties possessed by them.
REFERENCES:
(none)

WORKSHEETS:

See Associated Download.

SAMPLE RUBRIC:

See Associated Download.