Stop the Stretching
Students learn about composite materials, tension as a force and how they act on structural components through the design and testing of strips of plastic chair webbing. Engineers use their knowledge of material properties, such as the ability to withstand tension and compression, to choose the most suitable materials in their design of structures. Composites are designed to obtain a combination of properties exhibited by two or more components. From foundations for buildings to artifical tissues used to transplant organs, composites are extensively used by engineers to obtain the perfect material properties they need for specific designs. composite material design load structure tension Students learn about tension as a force and how it acts on structural components through a hands-on group design problem. They also learn about composite materials and how they can withstand higher forces. To build 1 test station: link chain, at least a 14' section (make sure the links fit around a 3/8" bolt) 8" section of link chain 2 3/8"x6" round head bolts threaded entire length 6 3/8" hex nuts duct tape 5-gallon pail with strong handle (such as a school floor wax bucket) small pea stones (uniform size) or sand, enough to fill the 5-gallon pail (sand is messier) ruler small coffee can (16 oz.) scale to weigh sand or stones For One Sample (each team need 4 sample sets): 1 strip of plastic sheet, 4mm x 3" x 18" long (1 mil = 1/1000 in.) (Do not use trash bags; purchase a roll of plastic sheeting at Home Depot or hardware stores) 5' of masking tape 5' of string Tools for Sample Construction: rulers scissors marker The Need: Because of the increasing cost of making plastic chair webbing (plastic strips), your company needs to find a new way to make lawn chairs. An idea was introduced to make strips out of plastic sheets, and develop a new product line. However the plastic alone is not strong enough in tension, and it stretches way too much to be used to make lawn chairs. The Challenge: Your engineering team has been assigned to design and test a new composite material to use for chair webbing. A composite material is one that is made from two or more materials bonded together. Your goal is to design a 3" wide strip of chair webbing made from thin plastic, masking tape, string, and hot glue. Your chair webbing must be designed to hold the greatest load possible in tension, with the smallest amount of elongation (stretch). Set up only two test stations. This focuses all students' attention on the testing so they learn how to improve their own designs by seeing the results of other teams' tests. Before teams test their own chair webbing designs, demonstrate to the class the process of running a test. Run the first test on a single 4 mil thick plastic strip (3" x 18") and have a student record the data on the board. Have the entire class graph these results on the grid provided in their packets. By doing the plain plastic test first, students are able to see the improved stiffness and strength of their composite material designs. Teacher Background Notes: Structural elements subjected to tension (pulling forces) 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 stretches 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; observe that the middle of the material gets skinnier and thinner. All materials in tension, even steel, stretches and necks down, before failure (break). Wen 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 strung between poles; wires used to hang or support signs and traffic lights, 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 deforms (compresses, stretches, bends) 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 (see Graph 1). 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. 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 (such as less thermal conductivity, higher electrical resistance, etc.). Considering having students to do an Internet search to find different composite materials and learn how they are made, what they are used for and 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 research include: glass fiber-reinforced resins (fiberglass), carbon-graphite composites, ceramic composites, plastic laminates and plastic-metal laminates, and many others. Gather materials. Copy the attached files: Constructing the Tensile Test Stations (instructions on how to construct the tensile test stations, also see Figure 1) and Problem Statement Worksheet (student procedures). Follow the Problem Statement Worksheet. Fundamental Facts about Forces and Structures (doc) Fundamental Facts about Forces and Structures (pdf) Constructing the Tensile Test Stations (doc) Constructing the Tensile Test Stations (pdf) Problem Statement Worksheet (doc) Problem Statement Worksheet (pdf) What is tension? What effect does tension have on structures and structural elements? What are some real-life examples of tension? What are some actual examples of structural elements that are loaded in tension? What are composite materials and how are they made? What are some real-life examples of composite materials? What are their special properties? Example Rubric Parameters: Maximum load carried Minimum amount of stretching (elongation) Minimum amount of material used