Determination of Tensile Stress-Strain Common
Properties in Materials

Developers:

Dean O. Roberts
Wm. Tennent High School
Warminster, PA

Dr. Allen Marks
Rohm and Haas Company
Spring House, PA

Grade
Levels:

11 and 12

Discipline:

Physics (Statics, Elasticity, and Fracture)

Specific
Objectives:

The students, upon completion of this lab, will be able to:

a) demonstrate the relationship between stress and strain
b) graph stress-strain curves for various common materials
c) interpret graphed results and discuss differences in mechanical properties of materials
d) develop procedures for testing tensile strength in other common materials
e) calculate the modulus for the materials

Background:

There is a wide range of solid materials which have extreme variations in tensile strength and elastic properties. These properties can be tested in the high school lab with some fairly simple materials and methods. The materials and procedures listed below comprise a very small segment of possibilities. The materials and procedures you use will be limited only by the equipment available to you and safety procedures followed.

Materials:

The following are some of the common materials that could be tested:

a) curling ribbon (cut to less than .15cm in width)
b) rubber bands
c) thin strips of latex caulk
d) electrical tape (cut to less than 1cm in width)
e) crepe paper streamers
f) fishing line (2 pound test)
g) fresh pasta
h) polyester thread
i) thin strips of plastic wrap or plastic bags
j) other common elastic or non-elastic materials with ultimate strength no greater than 2-3kg.

Equipment:

The following equipment should be very helpful:

a) ring stands
b) large c-clamps (to secure ring stands)
c) metal bars
d) right angle clamps with v-grove
e) paper clips large and small
f) masking tape
g) large #3 clips (see diagram)
h) hooked masses and slotted masses
i) meter sticks and 30cm rulers
j) vernier calipers or micrometer
k) safety glasses
l) scissors or utility knife
m) foam rubber or shock absorbing material to catch weight

Procedures*:

1. Obtain at least 3 materials with different elastic properties (from flexible to nonflexible).

    

2. Measure and mark a distance of 5cm or 10cm in the middle of each piece of material (5cm for high elasticity and 10cm for low elasticity).

    

3. Measure the thickness and width of the material between the marks (thickness for very thin materials may be determined by measuring multiple layers of the material and dividing).

    

4. Secure one end of the material to the metal bar which is tightly clamped to a ring stand which is also weighted or secured to the lab table so it will not tip when weights are added. Be sure the material hangs vertically.

    

5. Secure to the other end of the material a hooked mass or some type of mass that is no more than 50 grams.

    

6. Measure the distance between the marks to determine if any elongation has occurred with this small amount of mass.

    

7. Place some type of cushion below the material so that when the break point is reached the weights added will not damage the floor or table. It would be a good idea to keep your feet clear of this area and also to wear safety glasses in case material rebounds when it breaks.

    

8. Now add more weights carefully and slowly and measure the elongation after each mass has been added.

    

9. Be sure to record accurately all measurements.

    

10. Continue to add masses until material breaks.

     

11. Note the position of the break in relationship to the original marks you made.

     

12. For some materials, creeping will occur&emdash;the material will continue to stretch without additional weights being added. For this type of material a time limit will need to be determined after each mass is added. Once the time limit is reached, one member of the group will need to support the masses and then add the next mass. (Thirty seconds or less is a good time unit, but this does depend on the material).

     

13. Be careful that your material does not slip through the clamps at each end. Tying some materials will help prevent this to some degree.

     

14. After all data are recorded, have students graph the stress (force) vs. strain (elongation) for each material. Place strain on the x-axis.

* Note: An interesting alternative to the above list of procedures is to simply give the students the materials to be tested and make available the equipment listed. Then allow the students to determine the methods necessary to meet the objectives. Students will need to decide on what variables to control and what equipment to use much like a research scientist must do.

Questions:

1. Compare the curves for each sample.

   a) Which sample had the greatest elongation?
   b) Which sample had the least elongation?
   c) Which sample had the highest load before breaking?
   d) Which sample had the least load before breaking?

    

2. Does the sample with the highest load mean that that material has the greatest ultimate or tensile strength? Why or why not? (Hint: Tensile strength is force/area).

    

3. Compare your graphs with those on the next page taken from
   Reference 4.

    

4. Do the graphs agree with the relationship that is found in Hooke's' "law"? If so, to what extent? If not, why not?

    

5. Calculate the elastic modulus (Young's modulus) for each curve. This should be done by finding the slope of the curve before the elastic limit is reached.

    

6. What is the relationship between stress and strain?

    

7. If time permits, repeat the experiment to see if the results are similar. Another option would be to compare your results with other groups who used the same materials.

    

8. What sources of error exist in this experiment? Discuss the methods you could use to avoid them.

Further
Activity:

If enough data are collected on one or more of the materials used, a statistical analysis could be done to determine the reliability, precision, and accuracy of the data and the experiment.

References:

1. Giancoli, Douglas, Physics Principles with Applications, 2nd Edition, Prentice Hall, NJ. 1985.
2. Billmeyer, Fred W. Jr., Textbook of Polymer Science, 2nd Edition. John Wiley and Sons, N.Y. 1971.
3. ASTM Designation D 412, Annual Book of ASTM Standards, Philadelphia, PA.
4. Williams, J.G., Stress Analysis of Polymers, 2nd Edition, John Wiley & Sons, NY, 1950.

Small Strain Linear Elasticity       a) Rubbery behavior of polymers abovethe glass transition temperature.
(b) Behaviour below the glass transition temperature.
(c) A linear-elastic-plastic material.
(d) An elastic perfectly plastic material.