Warm-up

1. Socks Shoes lll lll l 12 - 7 = 5 people in bare feet! Warm-up There are 12 people in a room. 6 people are wearing socks and 4 people are wearing shoes, 3 of those people are wearing both. How many people are in bare feet?

2. Materials in Stress Objective Students will: • Understand the important distinction between failure stress and allowable stress • Experimentally determine the maximum force a material can take before breaking • Draw conclusions about the materials based on results • Take responsibility for a component of the task • Work to accomplish group goals • Discuss results and come to a joint conclusion

3. Materials in Stress - Vocabulary compressive stress failure tensile overload

4. Materials in Stress What does the word “stress” mean to you? Many of us use the word to express feeling overloaded or pressured. In fact, in engineering, stress has a different meaning. It is a measure of the force per unit area on the object.

5. Force = 20 pounds 20 pounds 10 square inches or 2 pounds per square inch Materials in Stress For example, if you push on a column with a force of 20 pounds and the area of column is 10 inches squared, the stress on the column will be 2 PSI or 2 pounds per square inch. Stress can be compressive (pushing) or tensile (pulling), depending on the force that is applied. So, using the same word to describe feeling pressured or pulled at in all directions is understandable.

6. Materials in Stress Can you really lie on a bed of nails without feeling pain or puncturing your skin? A good example of stress as a distribution of force is a bed of nails. We know from experience that stepping on a single nail can result in a painful wound. What do you think is different about a bed of nails?

7. Materials in Stress Suppose A.J. weighs 100 pounds. If A.J. stands on one nail, the force on the small area of skin in contact with the nail is 100 pounds. That force is more than the skin can withstand and it breaks, allowing the nail to puncture through – ouch!

8. Materials in Stress What if A.J. lies on 10 nails? How much force is on each nail? 100 pounds divided by 10 nails is 10 pounds per nail. Since the nails have a very small surface in contact with the skin, that may still be too much force: ouch again!

9. I did it!!! Materials in Stress But what if there were 1000 nails? The force per nail is 100 pounds/1000 nails or 0.1 pound per nail. That small a force may not hurt A.J. at all. Each nail is carrying only a small portion of his weight.

10. Materials in Stress Well, that’s basically what stress is all about. It is just a fancy word for how the force on an object is distributed over its area. For the same force, a larger area results in less stress than a smaller area. But why does stress matter? Where would knowing about stress come into play? Would you need to know how much stress the towers or cables or hangers could take in order to build a safe suspension bridge? How would you figure out how much stress a material can handle before it breaks? Is the answer different for different materials?

11. Materials in Stress - Challenge Today you will look at materials that could be used in tension. You’ll be given a selection of possible materials, and your challenge is to determine the failure stress of each. How will each material fail? Of the materials you have, which is the strongest? How much stress can the materials take before they break?