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Breaking Point: Testing Tensile Strength

  • Teacher Resource
  • Posted 01.12.12
  • NOVA

In this video excerpt from NOVA's "Making Stuff: Stronger" , host and New York Times technology columnist David Pogue visits DuPont™, where Kevlar® was invented. After learning how this bulletproof material is made, David puts its strength to the test with an ice pick. A related demonstration tests and compares the tensile strength and elasticity of Kevlar®, nylon, steel, and cotton thread by using them to lift weighted buckets. Students learn that materials can be strong in different ways—some have high tensile strength, others are more elastic—and that materials scientists test the strength of materials by stressing them to their breaking point.

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Supplemental Media Available: Making Stuff Stronger Demonstration (Document)

NOVA Breaking Point: Testing Tensile Strength
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  • Media Type: Video
  • Running Time: 2m 26s
  • Size: 8.6 MB
  • Level: Grades 6-12

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This media asset was adapted from NOVA's "Making Stuff: Stronger" .

Background

“Strong as steel” is a familiar saying, and it’s an apt one. Steel, an iron alloy, is one of the strongest and most versatile materials around. But, as host David Pogue discovers in NOVA’s "Making Stuff: Stronger" , scientists are creating new materials that push the idea of strong to extraordinary new limits.

The word strong actually refers to a range of properties, each defined by the ability to stand up to a different type of force. Strength is a measure of how well a material can resist a force (or load) before failing. The load is distributed over an area and is more accurately defined as stress (force per unit area). There are different kinds of stresses, including tension (pulling), compression (squeezing), impact (a sharp blow), torsion (twisting), and shearing (surfaces sliding past one another). We apply these stresses in our daily lives when we pull open a door, push a cart, or twist the cap off a bottle.

Materials scientists test the strength of materials by stressing them to the breaking point, called failure, at which point the material ruptures and cannot rebound to its original condition or shape.

Tensile strength is how much stress a material can withstand while being pulled in opposite directions. This stress causes the material to temporarily lengthen. If the stress is low enough, when you release the force, the material will return to its original length – this ability for the thread to stretch and rebound is a property called elasticity. The amount the material lengthens is called the elongation. (If you divide the elongation by the original length you get what is called strain.)

If you continue to pull, the bonds between atoms in the material will start to break, eventually reaching the point at which the material will not rebound when the stress is released; the deformation is permanent, or plastic. When enough bonds break, the material snaps apart. The amount of stress the material can endure at the time of failure is the strength of a material.

Resource Produced by:


					WGBH Educational Foundation

Collection Developed by:


						WGBH Educational Foundation

Collection Funded by:


						National Science Foundation

						U.S. Department of Energy



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