Objective
In this science project, you will modify the point of bending failure and the maximum bending stress in a model of a flexible rod.
Introduction
So what do dipsticks, bow and arrows, car antennas, fishing poles, pole vaults, kites, tents, dryer vent brushes, surgical probes, and fly swatters have in common? They're all so different, it's hard to imagine, right? Well, to work, they all use a flexible rod—a long, slender, bendable cylinder. Flexible rods are found throughout the automotive and construction industries. Some historians even think they were used to help build the Great Pyramids of Egypt. Flexible rods are also important in building instruments for medical testing and treatment, since the human body is made up of many curved vessels and tubes, and flexibility is essential to maneuver through those curves.
Whether it's hollow or solid, when a flexible rod is bent, almost all the stress occurs at the surface of the rod, as shown in Figure 1. In designing the rod, mechanical engineers and materials scientists think about how much bending the rod will undergo during normal use, and how many times it will be bent. They want to avoid structural failure of the rod, so they choose the right material, limit bending motion, and avoid designs that concentrate stress along the length of the rod as it bends.
Figure 1. Shown here is a rod undergoing a 2-point bending test. Dashed lines indicate the location of stress at the surface of the rod.
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Have you ever tried to open a cellophane bag of chips or candy? It's usually very hard to rip open the bag unless you find the special notch in the packaging, or unless you make a notch yourself with your teeth or scissors. This notch is an example of a stress riser, a place where stress concentrates, and cracks can start and grow. A flexible rod can withstand greater bending forces if the stresses are evenly spread out along the length of the rod. However, if there is a concentration of stress along the rod—for example, at a joint, drill hole, or notch—as shown in Figure 2, then a crack may form and grow when the rod is bent, and the rod will break, even under normal bending forces.
Figure 2. Shown here is a rod with a notch undergoing 2-point bending, which shows stress concentration around the notch. |
In this science project, you'll find the place where a flexible rod model tends to break naturally when it is tested in bending to the point of failure, meaning you'll identify the location of the maximum bending stress. Then you'll see if you can modify where the flexible rod model experiences bending failure by introducing stress risers, or points of stress concentration or stress accumulation.
For the experimental procedure, you'll be using asparagus stalks as models of a flexible rod. You'll bend the asparagus stalks until they break, both with and without bands. The bands are stress risers and introduce a point of stress concentration. Where do you think the asparagus stalks will break when they are not banded? How about when they are banded?
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:
Bibliography
Materials and Equipment
For this science project you will need the following materials and equipment:
Experimental Procedure
| Group | Banding, in Inches from the Cut End |
| A (control) | Unbanded |
| B | 2 |
| C | 4 |
| D | 5 |
| E | 6 |
| Group A (control) | Asparagus 1 | Asparagus 2 | Asparagus 3 |
| Total length | |||
| Break length=length from cut end to break | |||
| Break ratio=Break length/Total length |
Figure 3. Asparagus stalk from Group A (control group) being tested until bending failure. |
Figure 4. Asparagus stalk from Group B (banded 2 inches from cut end) being tested until bending failure. |
| Group Description | Average Break Ratio |
| Group A, control group (no bands) | |
| Group B (banded 2 inches up from cut end) | |
| Group C (banded 4 inches up from cut end) | |
| Group D (banded 5 inches up from cut end) | |
| Group E (banded 6 inches up from cut end) |
Variations
Credits
Kristin Strong, Science Buddies
Last edit date: 2008-04-07 11:00:00
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