The Stress of Strain
Stephanie Poyer
Stonewall Jackson Middle School, Hanover County
Developed with funding from the American Council of Engineering Companies
of Virginia and the MathScience Innovation Center
Question(s)
What materials make buildings the strongest?
Grade/Subject
Physical Science, Pre-Algebra
Virginia Standards of Learning: 2010 Physical Science (PS.1, PS.6); 2009 Math
8 (8.12, 8.13)
21st Century
Curriculum
Engineering: Nature of STEM (1.23); Design and Build (2.22); Civil
Engineering (4.43)
Background
Students will understand the process of how engineers conduct materials testing
to determine the ultimate tensile strength of a beam. Students must know and
understand how to solve ratios. Prior to this activity, the concepts of stress,
strain, and deformation will be introduced. Review the process of making ratios.
Materials
Supplies are for EACH pair of students participating in this activity:


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2 oz of polymer clay, such as “sculpey clay”- packaged in 1.75 lbs.
(For more information, visit http://sculpey.com/teacherslounge.htm)
String or rope about 2 feet
Weights to hang on strings (or textbooks can be used)
Scale to measure beam weight
Safety
The cured clay will be hot when it comes out of the oven. Do not bake the clay
in a microwave and follow all baking instructions on the package of the clay.
Procedure
1. Divide the 1.75 lbs. of clay into 2-oz. cubes, resulting in 14 equal cubes.
2. For the stress testing, make sure you have a place, such as between two
tables, desks or chairs, to rest the beams and add the weights.
3. If you want to cure the clay with students, preheat the oven to 275F.
4. Make two long, thin lengths of clay for demonstration. Cure one piece in the
oven but not the other piece.
5. Ask students to vote on the following question, “Do engineers construct
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buildings with solid beams or hallow beams?” Tally the responses on the
board. Tell them they will find out more about what engineers do in this
activity.
6. Use an uncured length of clay to demonstrate plastic deformation by putting
across a gap and showing that it bends but does not spring back to its
original shape after a weight is added to it and removed. Use the cured
length of clay to demonstrate elastic deformation by showing that the clay
returns to its original shape after the weight is removed. Challenge the
students to design a beam that is very strong but does not weigh very much.
7. Divide the class into groups of 2-4 students in each group.
8. Give each group a 2-oz cube of clay. Explain that using less clay may
increase their strength: beam weight ratio.
9. Have students design a 7-inch long beam to span a 6-inch gap. Ask students
to sketch their ideas for various designs before constructing the one they
predict will have the best strength: beam weight ratio. The beams can be
square, circular, l-shaped, triangular or any other shape they think will be
successful.
10. During beam construction, suggest students use a pencil point to help join
vertical clay slabs to any horizontal clay slabs (perpendicular surfaces) of a
beam design. This reduces any gaps between two surfaces which would
weaken the beam.
11. Follow the directions on the packaging to cure the clay beams by baking
them in the oven. This can be done at the end of the day or overnight.
12. To complete the curing process, let the beams cool to room temperature.
13. Weigh and record each group’s beam design.
14. To test the beam strength, straddle each beam across a six-inch gap.
15. Tie several loops of string or rope around the beam, which helps distribute
the weight and provide a place to attach the weights.
16. Add weight until the beam breaks. Record the maximum amount of weight
each beam held (=yield strength).
17. After analyzing the data, announce the winning team design as the beam
with the highest strength: beam weight ratio. Have the winning team present
their design concept to the rest of the class.
Data Analysis /
Results
The Stress of Strain
This graph can be completed by two groups or as a whole class.
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Design
Conclusion /
Questions
References
Amount of
clay used
Weight
added
Beam weight
ratio
Observations
Class discussion questions:

Back at their desks, have the students calculate strength: beam weight ratio
(ex: 120z/ 20z =6). Which beam had the highest strength: beam weight
ratio?
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Are they the same three beams that held the most weight?
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Which beams would be preferred for construction purposes?
Lesson was adapted from http://teachengineering.com
Resources for K-12, TE Activity: Breaking Beams
Math Science Innovation Center
Information on educational programs available to students, teachers and school
divisions and procedures for registering for programs.
http://msinnovation.info
The Stress of Strain
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©MathScience Innovation Center, 2009