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Determining the Stress Range for Wood

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Source:
Author: Bob Bonnet and Dan Keen

Purpose or Problem

Determining the safety stress range of a 2×6 piece of balsa wood, while making it lighter in weight.

Overview

Many times a material needs to be very strong, because it will undergo a lot of stress or pressure. Sometimes, a material must be made from a strong substance, like steel, but it may also have a requirement of being as light as possible. Some bones in birds are strong, yet they are hollow to make them light. The struts used in aircraft often have large holes in them to make them lighter, yet they must still be strong enough for the job they are required to do.

Design engineers must know how much force a material can withstand before breaking, and whether or not that material can be made lighter by cutting holes in it, yet still being able to support the weight needed.

While engineers may need a certain material for its strength, the material may be able to withstand much more stress than required. Therefore, they can reduce the mass (weight) of the material by cutting holes in it. A margin of safety must also be included to ensure a safe design. For example, if 2 pounds of stress is to be exerted on a material, you may want that material to be able to withstand 6 pounds before it breaks, giving you a two-thirds margin of safety.

Hypothesis

Hypothesize that you can lighten a piece of balsa wood by cutting holes in it, while still keeping much of its structural strength.

Materials' List

• Plastic gallon jug
• Two-foot-long piece of strong string
• Wooden sawhorse
• Two pieces of balsa wood, 2 inches wide by 1 foot long
• Piece of 2×4 lumber about 12 inches long
• Two pieces of plywood about 1 foot by 6 inches
• Wood screws
• Screwdriver (or electric screwdriver)
• Hand saw (or a power saw used under adult supervision)
• Gram weight scale
• Bathroom scale or scale to measure pounds
• Utility knife
• Pitcher of water
• Ruler

Procedure

The wooden box device that holds the balsa wood in place is held constant, as is the water jug device for adding weight to stress the balsa wood. The mass of the piece of balsa wood is the variable.

Cut two pieces of thin balsa wood, 2 inches wide by 6 inches long, and set these strut-like pieces aside.

Cut two pieces of 2×4 lumber in 6-inch lengths. Cut two rectangular pieces of plywood into pieces 1 foot wide by 6 inches deep.

Using screws, attach one piece of plywood to the top of a wooden sawhorse. Using that as the bottom piece, make a rectangular box by using the 2×4 pieces for sides and the remaining piece of plywood for the top. Leave two sides of the box open.

Using a utility knife, cut a small V-shaped notch on one of the long sides of the balsa wood near the end of the wood.

Stand the balsa wood vertically and insert one end into the opening of the box to a depth of 1 inch. Position the balsa wood so the end with the notch in it is outside the box and facing upward.

Tie both ends of a 2-foot-long piece of strong string to the handle of an empty plastic gallon jug, making a loop. Hang the jug from the balsa wood by placing the loop of string in the notch.

Slowly add water to the jug until the balsa wood breaks. Weigh the water in the jug and record this weight. (If the weight of a full jug is not enough to break the wood, tie a second jug to the first one and begin adding water to it.)

Take the second piece of balsa wood and cut a series of holes in the wood, each spaced at equal distances. Each hole should be 1 inch in diameter.

Perform the same weight test. Record the amount of weight needed to break the "swiss cheese" balsa wood strut.

Engineers need to know weights, percentage relationships, strengths, and other important factors about a material. Use the data you have determined by experimentation to compile stress data on the 2×6 pieces of balsa wood. The fact sheet you compile should include these figures:

• Weight of the solid balsa wood piece (use a gram weight scale):_____
• Weight of the lightened "swiss cheese" balsa wood piece:_____
• Breaking weight of the solid balsa wood piece: _____
• Breaking weight of the lightened "swiss cheese" balsa wood piece: _____
• Percent lighter the lightened strut is to the solid strut (the weight of the lightened piece divided by the weight of the solid piece, times 100 for percent):_____
• The percentage of weight the lightened piece breaks at compared to the solid piece (breaking weight of the lightened piece divided by the breaking weight of the solid piece, times 100 for percent):_____
• The maximum weight the lightened piece can safely support, the safety margin being two-thirds beyond what is required (breaking weight of the lightened piece divided by one-third):_____

Results

Write down the results of your experiment. Document all observations and data collected.

Conclusion

Come to a conclusion as to whether or not your hypothesis was correct.

Something More

If the balsa wood was twice as thick, would it be able to hold twice the weight? Laminate two pieces of balsa wood together with wood glue. Let dry and test.