Materials such as metals (aluminum, iron, copper, etc.), ceramics (silicon carbide, porcelain) or polymers (milk jugs made of polyethylene) are tested by scientists and engineers to reveal certain mechanical properties such as the maximum stress a material can withstand before it fails. Some materials will slowly deform when a constant force or displacement is applied to them. This time-dependent and permanent deformation is called creep.
If you have ever noticed that chewing gum gradually sags when it is stuck to something or watched a plastic grocery bag gradually tear apart when it is carrying too much weight, you have observed creep!
- Students will predict what happens to materials as they creep.
- Students will be able to measure, compare and contrast how weight and size of a rubber band will affect how creep changes with time.
- Students will describe the relationship between creep and time.
- Students will determine other factors that are involved to produce creep.
- Students will be able to determine the creep of rubber bands using quantitative data.
- Students will be able to determine how the creep of a rubber band changes with time.
- Students will compare and contrast how different variables affect the creep such as size of the rubber band, weight of the object and time the object is under strain.
Grade Level: 6-8
- 60-90 minutes (approximately 2-3 class periods)
- Students will complete the hypotheses and study the materials before completing the lab.
- Students will complete the lab and lab questions.
- Informal evaluation of participation in group discussion.
- Part 1: Creep of Materials
- Introduce this lesson by asking the students to share their ideas about the creep of materials. Ask them about materials that are new versus materials that are old. What types of materials seem to lose their durability with time?
- Now, introduce the factors that lead to creep: use of the object, forces on the object, heat, etc. Talk about each of these factors and their importance making a list on the classroom board. Also, ask the students for other ideas.
- Talk to the students about how useful it is for people to test the creep of a material. Engineers are scientists that construct objects, buildings, airplanes, etc. out of materials that have specific properties. How would it be useful to understand how a material will tire before you build with it?
- Complete the lab and when the lab is over, use data from the lab to reiterate parts 1-3 above. Draw on real-life experiences and objects used in everyday life.
- Part 2: Video
- Part 3: Other examples
- Teacher-led discussion about how an object tires.
- What are the disadvantages of objects that tire easily?
- Are there any advantages of a material that would fatigue easily? For example, have you ever heard about the environmental impact of 6-pack rings? 6-pack rings are commonly used to hold together soda cans and if 6-pack rings get into the ocean or other wild areas, animals can get stuck or choke on them. However, scientists have engineered 6-pack rings to break apart when they are exposed to sunlight (UV rays). Now, 6-pack rings are strong when you are carrying your soda home from the store but after being exposed to sunlight, they become brittle.
- Try the experiment again and compare the results to the first time. Were they similar or different?
- Try lengthening the experiment to a couple of weeks. Do you notice any difference in the creep of the rubber bands?
- Add heat to the rubber bands by using a blow dryer. Compare the effects of the rubber band exposed to hot air from a blow dryer to the rubber bands that are not.