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. The stress at which a material breaks is a measure of its strength. During use a material may degrade, which may 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.
- Students will conduct an experiment mimicking a torsion test.
- Students will determine the shear stress on a paperclip.
- Students will infer reasons for the quantitative shear stress for various paperclips.
- Students will calculate the shear stress on various paperclips.
- Students will compare the shear stress on various paper clips.
- Computer with Internet access
- Video clips (online)
- 4 different paper clips, for example:
- 1 Small metal paperclip
- 1 Large metal paper clip
- 1 Small plastic paperclip
- 1 Large plastic paperclip
- Metric ruler
- Part 1
- Students should view Do Materials Get Tired? Intro QuickTime Video (2 minutes, 21 seconds) and Do Materials Get Tired? Fatigue QuickTime Video (2 minutes, 11 seconds).
- Teacher should lead a discussion on how to test the fatigue of various objects. What are some products students would want to make sure were tested for fatigue? Have any students ever had a product just suddenly break due to fatigue?
- Teacher should inform the students that they will test the fatigue of a variety of paperclips.
- Students should create the three hypotheses listed in the lab.
- Students should collect data for the laboratory experiment. This can be done with groups of students testing just one variable (two groups can test the same paper clips, but each group would change the rotation angle) and then share the data between the groups, or each group can test all of the variables including type of paperclip and rotation angle.
- Part 2
- Students should graph the number of cycles vs the angle of rotation. Multiple graphs may need to be completed, one for each type of paper clip.
- Students should complete the analysis questions.
- Students should watch Fracture Surfaces of Paperclips QuickTime Video (10 seconds).
- Teacher should readdress the discussion around various objects that may have failed under stress forces. Teacher should also discuss the various careers in which the experiment pertains and the importance of such careers to society and today’s lifestyles.
- Using the 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.
- Groups of students can test only one type of paper clip. After the tests have been conducted, the various data can be recorded and averaged.
- Some of the paper clips can be frozen or heated and same experiment conducted. This can show how changes in temperature can affect the strength of metals.