Abstract

Objective

The goal of this project is to test the weight-bearing capacity of polystyrene structural beams with various cross-sectional geometries. Which is strongest? Which has the best strength-to-weight ratio?

Introduction

• How do you choose which material to use for a particular purpose?
• How do you know that manufactured materials meet the advertised specifications?
• How do you know that the finished products have been fabricated properly?
• How long will the finished product last?
• For research and development of new materials, how do you measure progress?

Materials scientists have specialized equipment for testing the strength and other properties of different materials in order to answer these types of questions. In this project you will use a simple test stand to measure the weight-bearing capacity of various structural shapes made from styrene plastic. Which structural shapes provide the greatest strength? Which shapes provide the greatest strength-to-weight ratio?

Terms, Concepts, and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• stress,
• strain,
• ductile,
• brittle,
• strength-to-weight ratio.

Questions

• What is the difference between compressive stress and tensile stress?
• When you hang a weight from the center of a beam which is supported at both ends, what stress(es) do you induce on the beam, and where?
• Which cross-sectional shape do you expect to be strongest? Weakest?
• Which cross-sectional shape do you expect to have the best strength-to-weight ratio?

Bibliography

• A good place to start is this Science Buddies resource written by Stanford Mechanical Engineering Professor Beth Pruitt and her students:
  Stress, Strain and Strength.
• This PBS website has great information on structural engineering, including online labs where you can learn about forces, materials, loads and structural shapes:
  WGBH, 2001. "Building Big: Bridges, Domes, Skyscrapers, Dams and Tunnels," PBS Online [accessed February 17, 2004] http://www.pbs.org/wgbh/buildingbig/index.html.

Materials and Equipment

To do this experiment you will need the following materials and equipment:

• safety glasses,
• at least 15 15-inch lengths of polystyrene (or ABS) plastic structural beams:
  • 3 samples each of 5 different cross-sectional shapes,
  • plastic beams are sold under the brand name "Plastruct,"
  • a web search for Plastruct will turn up the company site, where you can find a list of local retailers,
  • you can also locate many online suppliers;
• sturdy "S" hook (for hanging weight bucket),
• 5 gal plastic bucket,
• weights (water, sand, bricks),
• ruler,
• postal scale (for weighing beams),
• bathroom scale,
• test stand, consists of:
  • two supports at equal height, with a gap (about 12 in) between them,
  • 2 "C" clamps (one for each end of the beam).

Experimental Procedure

1. Safety note: Wear safety glasses when testing beam capacity. Keep hands and feet clear of the area underneath the weight bucket, which may fall at any time.
2. Do your background research and make sure that you understand the terms and concept and can answer the questions above.
3. Set up your test stand for supporting the plastic beams. You will want a gap of about one foot, and you will need to clamp each end of the beam firmly in place with a "C" clamp across this gap. You could use a space between two workbenches, or you could build a sturdy frame with pieces of 2×4 and cross-bracing. Your test stand will need to be tall enough to hang a 5-gallon bucket from the beam, plus about 25–30 cm.
4. For hanging weight from the beams, get a sturdy S-hook from the hardware store, and hang a 5 gallon plastic bucket from it by the handle.
5. For weight, try water (up to 18 kg/bucket), sand (up to 29 kg/bucket for dry sand, 35 kg/bucket for wet sand), or iron weights from a weight-lifting set (somewhere in the range of 70–140 kg/bucket, depending on air space).
6. Weigh each beam before testing.
7. Clamp each end of the beam down firmly with "C" clamps.
8. Test at least 5 different beam shapes.
9. Test at least 3 beams of each shape (5 or more is better).
10. Add weight bucket (in small, measured increments) until the beam breaks. Weigh the bucket on the bathroom scale to see how much weight was required to break the beam. Record the amount of weight needed to break each beam.
11. Watch carefully and record any observations in your lab notebook. Does breakage consistently start in a particular location on all of the beams of a particular type?
12. Calculate the strength/weight ratio for each beam, and the average for each cross-sectional shape of beam.
13. Graph your results.

Questions

• From your observations and measurements, is polystyrene ductile or brittle?
• From your observations of each beam failure, do the various beam shapes perform better, worse or about the same under compression vs. tension?

Variations

• For a more basic project, see: Strength in Numbers?.
• Orientation of the beam. For beams with asymmetric cross-sections, does the orientation of the beam affect its weight-bearing capacity?
• Hobby shops also sell similar structural parts made of other materials, including wood (various types), ABS plastic, brass and aluminum. Select structural parts with similar dimensions and cross-sections, but made of different materials. Test them with similar methods. Compare the strengths (and weaknesses) of the various materials.
• Measure the vertical deflection of each beam as you add more weight. Remove the weight and see if the material recovers its original shape (elastic deformation) or remains permanently deformed (plastic deformation). Use the additional information to create a stress-strain plot for each type of beam.
• Devise a method for applying and measuring a torque stress to a beam. Test different beam types for resistance to this type of stress.

• For more science project ideas in this area of science, see Materials Science Project Ideas.

Credits

Andrew Olson, Ph.D., Science Buddies

Last edit date: 2006-02-23 12:10:30