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# Do Materials Get Tired? How Long Will a Paperclip Last? (MS)

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### Overview

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 to determine what uses the materials may have. One property that is tested is the amount of stress a material can handle before it breaks. You have probably tested the amount of stress a material can handle before by twisting or pushing on an object such as a toy until it breaks. The amount of stress a material can handle before it breaks measures how strong the material is. Also, as a material gets older, it can handle less stress which can cause it to fail at much lower stresses. For example, if a material is loaded over and over again and then fails it has undergone what is known as fatigue.

Fatigue is a very common mode of failure for materials and has been studied for centuries. Fatigue occurs every day in objects that you’re familiar with. For example, airplane wings fatigue thousands of cycles on every flight and bridges fatigue every time a car drives over them. However, just because a material is undergoing fatigue does not mean that it will always break. In fact, engineers run careful experiments so that they can be sure that things will not break due to fatigue while you are using them.

### Content Objectives

• Students will predict how paperclips will fatigue based on prior experiences.
• Students will be able to measure, compare and contrast the fatigue of the paperclip based on its angle and the material (whether metal or plastic).
• Students will compare and contrast how differing angles change the outcome of how a material will fatigue.
• Students will predict and measure how different sizes or materials will affect the failure of materials.

### Process Objectives

• Students will be able to determine the fatigue of paperclips using quantitative data.
• Students will make observations of paperclips fatigue while bending the paperclips in different angles.
• Students will be able to determine how the angle affects the fatigue of paperclips.
• Students will compare and contrast how different variables affect the fatigue such as materials, size and angle.

### Suggested Time

• 60-90 minutes (approximately 2-3 class periods)

### Assessment Strategies

• Completion of the hypothesis.
• Completion of the lab questions.
• Informal evaluation of participation in group discussion.

### Procedure

• Part 1: Strain of Materials
• Introduce lesson by asking students to view video clips Do Materials Get Tired? Intro QuickTime Video (2 minutes 21 seconds) and Do Materials Get Tired? Fatigue QuickTime Video (2 minutes 11 seconds).
• After viewing videos, ask the students to share their ideas about the fatigue of materials. What happens to an object when bent or twisted? Does it retain its shape? Does the object break? Use examples of different materials such as rubber, plastic, metal, etc.
• Ask the students how they could devise a plan to test how each of these objects break? Would they all break in the same way or would they break in different ways?
• Now, introduce other factors that may affect how an object breaks. Does the age of a material affect how easily the object breaks? Give some examples.
• Talk to the students about how useful it is for people to test how a material breaks. Engineers are scientists that construct objects, buildings, airplanes, etc. out of materials that have specific properties. How would it be useful to know how an object fatigues before you build with it?
• Complete the and when the lab is over, use data from the lab to reiterate parts 1-4 above. Draw on real-life experiences and objects used in everyday life.
• Part 2: Other Examples
• Teacher-led discussion about how an object fatigues is useful.
• What are the advantages and disadvantages of a material that does not fatigue easily? Are there any advantages of a material that would fatigue easily? Explain.
• Extension
• Try the experiment again and compare the results to the first time. Were they similar or different?
• Try adding different materials to this experiment, such as different quality plastic and metal paperclips
• Students may view video Fracture Surfaces of Paperclips QuickTime Video (10 seconds) as an introduction. After downloading a Virtual Microscope, students can view various paperclips and their fatigue surfaces, as well as data collected by Penn State researchers. Discussions as well as activity questions can be designed based around the images and the experiment.