Hands-on Activity Testing Fundamental Loads

Learning Objectives

In this activity, students learn:

• To identify the five fundamental loads: compression, tension, shear, bending and torsion.
• What is meant by something being elastic and non-elastic.
• About molecules and bonds.

Educational Standards

Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).

In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.

International Technology and Engineering Educators Association - Technology

State Standards

Materials List

• extruded foam insulation, 1" X 4' x 8' (should be enough for 6 groups out of one piece)
• Xacto knife
• black sharpie marker
• ruler
• (optional) magnifying glass

Pre-Req Knowledge

An basic understanding of compression, tension, shear, bending and torsion as provided in the Fairly Fundamental Facts about Forces and Structures lesson.

Introduction/Motivation

What if a highway overpass bridge collapsed? What if a car and its air bags didn't protect people in a crash? Materials and structures can sometimes fail when subjected to large enough loads. Each different type of load can cause its own mode of failure.

Similarly, each type of failure leaves clues for the engineers who investigate to find the causes. For example, people with the National Transportation Safety Board (NTSB) look for these clues after a transportation accident. Finding a cause for transportation failures allows the NTSB to prevent accidents and keep passengers safe.

The goal of engineers who design structures and vehicles (and everything else, too!) is to consider all the possible loads and stresses and forces on the materials and the design, and plan accordingly to minimize the chances of failure.

Now let's get going to feel and see these forces.

Procedure

Before the Activity

• Gather materials. Make sure students have paper and pencils, too.
• Cut the extruded foam insulation into strips of 1"x 1" x 4'. Each team needs at least one full piece (1"x1"x4') and one 1/4 piece (1"x1"x1'). They also need one-piece of 1"x1"x2" for part 2b.
• Divide the class into teams of two students each.

With the Students

Loooking at Loads: Studying the Five Fundamental Loads and their Effect on Materials

1. From the pieces supplied to each group, direct students to cut (10) 1" X 1" X 6" blocks of extruded foam insulation.

2. Instruct students to do the tests included below; make drawings of each fracture, and record observations on appropriate data sheets.

2a. Tension: Measure the length of the block, before breaking it. Two students work together to pull on the block as straight as possible (from the ends) until it breaks. Put the two pieces back together and measure the change in length of the block. Note a slightly dished-in fracture (break) that is characteristic of a tensile failure.

2b. Compression: Have a student stand on (or place a weight on) a 2" high block of insulation foam; try to keep the load perfectly vertical and stable. Observe wrinkles on the outside of the material, as well as the bulging of the material, both of which are signs of a compressive failure. Then, try the compression test again using a 6" long block. Because of its slender (long and thin) shape, it will fail by buckling.

2c. Shear: Have two students use two textbooks each, as is shown in Figure 3, to demonstrate shear. When blocks are sheared apart as shown in Figure 1, a rough angular fracture is observed (an uneven fracture with several planes of the material angled in different directions).

Each student clamps foam blocks of 1" X 1" X 6" between 2 books and the two students slide their books in opposite directions on the table.

2d. Bending: On each side of a block of insulation draw a 5" line down the middle and then slowly bend it until it breaks. As load is applied, notice that the lines form a bowed shape on the two sides of the beam (like a smile).

Group Discussion: What happened to the lines on the top and bottom of the block? A bending moment makes a beam "smile," causing one side of the beam to be pulled apart (tension) and the opposite side to be pushed together (compression). While the load is applied, this should be observable. (A beam subjected to bending fails in tension because materials have a lower tensile strength and a higher compressive strength.) On the side of the beam that experiences tension, the same flat or slightly dished-in fracture seen in the tension test will be visible on the opposite side of the beam small wrinkles indicating compression and a bump in the fracture plane may be visible. Because of the combination of tensile, compressive, and shear stresses (internal forces) in the beam, the same clean break seen in pure tension is not likely to occur.

2e. Torsion: On each side of a block of insulation draw a 5" line down the middle and slowly twist the block about its center until it breaks. A twisting (torsional) moment causes angular rotation in a beam. This means that each slice of insulation (within a single plane of molecules) actually rotates slightly and the molecules are being sheared or slid apart. Beams or any structural member loaded solely in torsion, experience a shear failure because torsion forces produce high internal shear stresses (sliding and ripping) between molecules and layers inside the material. You can tell it's a shear failure because of the rough, angular ripped-apart quality of the fracture.

3. Conclude by having teams discuss and summarize in writing their results and findings, as described in the Assessment section.

Vocabulary/Definitions

elastic: The ability of an object to return quickly to its original shape and size after being bent, stretched, or squashed.

fracture: A break, split, or crack in an object or material.

inelastic: The inability of an object to return quickly to its original shape and size after being bent, stretched, or squashed.

Assessment

Team Discussions: Ask students to hold group discussions and write down key points. Review their write-ups to gauge their comprehension.

References

Copyright

Supporting Program