Notice that material "B" stretches 1.5-inch with only a 30 lbs. 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 "A" only stretches 1/4-inch when loaded to 50 lbs. 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. The steeper the slope of the linear (straight-line) part of their graph, the higher the material's stiffness (see graph in Figure 1). Their graphs tell them if they are successful. In this experiment, students are trying to develop not only a stronger material, but also one that has a much greater stiffness. 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. 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. All materials in tension, even steel, stretches and necks down, before failing (breaking). Have students watch for the necking on their plastic samples that are loaded in tension they should observe that the middle of the material gets skinnier and thinner. 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-inch initial length). Lesson Background and Concepts for Teachersīefore they break, structural elements subjected to tension (pulling forces) stretch and "neck down." The amount of elongation (stretching) depends on the load and the original length of the material. When creating new buildings, bridges and other structures-or even smaller products such as washing machines, bicycles, chairs and cell phones- engineers choose materials and come up with designs that can withstand the tensile forces that they are expected to encounter. The amount of force that a material can undergo before snapping is its tensile strength. What are some other materials that are easily stretched? What are some that are not so stretchy? What happens when you stretch a rubber band? What happens to the rubber band if you stretch it too far? (Listen to student responses.)
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