Abstract

Have you ever broken a fishing rod? Or seen a treetop bend over and touch the ground (or even snap off) during an ice storm? These are examples of the effect of bending stresses on flexible rods. There are scientists who actually study this phenomenon and discover ways to prevent breakage, which leads to stronger fishing rods, building materials, car parts, and more. In this science project, you'll explore the bending stresses in flexible rods by testing asparagus stalks.

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.

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.

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 and Concepts

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

• Stress
• Structural failure
• 2-point bending test
• Stress riser
• Force
• Maximum bending stress
• Bending failure
• Stress concentration
• Stress accumulation

Questions

• Where does maximum bending stress occur in a flexible rod?
• Can you modify the point of maximum bending stress? How?

Materials and Equipment

For this science project you will need the following materials and equipment:

• Fat, fresh asparagus stalks, approximately 9 inches long (15)
• Chicken leg bands (12), available at farm animal supply stores. An alternative is any thin, strong, metal or plastic adjustable banding material that can be snugly wrapped around an asparagus stalk at any point along its length, such as a cable tie.
• Measuring tape
• Lab notebook
• Digital camera (optional)
• Graph paper

Experimental Procedure

Variations

Careers

If you like this project, you might enjoy exploring these related careers:

Related Links

• Science Fair Project Guide
• Other Ideas Like This
• Mechanical Engineering Project Ideas
• My Favorites
• Stress, Strain, & Strength

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