Students will conduct several simple lab activities to learn about the five fundamental load types that can act on structures: tension, compression, shear, bending, and torsion. In Part One, students will play the role of molecules in a beam subject to various loading schemes. In Part Two, students break foam insulation blocks by applying these five fundamental load types (tension, compression, shear, bending and torsion). Students will study carefully each type of fracture pattern (break in the material) and make drawings of the fracture patterns in their notes in order to learn the telltale marks of failure due to each fundamental load type.
LEVEL OF DIFFICULTY [1 = Least Difficult : 5 = Most Difficult]
100 minutes (2 class periods)
$10 per class
WHAT WILL THE STUDENTS LEARN?
Mass. Science & Technology / Engineering Grades 6-8 / Construction Technologies & Engineering Design
5.3 Explain how the forces of tension, compression, torsion, bending and shear affect the performance of bridges.
2.2 Demonstrate methods of representing solutions to a design problem, e.g., sketches, orthographic projections, multiview drawings.
2.3 Describe and explain the purpose of a given prototype.
2.5 Explain how such design features as size, shape, weight, function and cost limitations (i.e., ergonomics) would affect the construction of a given prototype.
The students will learn to identify the five fundamental loads: compression, tension, shear, bending and torsion.BACKGROUND INFORMATION:
What is meant by something being elastic and non-elastic Molecules + bonds
A lot of information is available in the directions
Each type of load has its own telltale marks which engineers use to identify the mode of failure (type of load causing failure) of a structure or its component parts. This is exactly what National Transportation Safety Board (NTSB) engineers do when you see them on the evening news talking about a crash investigation; they recover and analyze all the parts of the aircraft or train to determine what part(s) failed, how they failed, and why, in an effort to determine the cause of the crash.
"Fairly Fundamental Facts about Forces and Structures" Attached
Fracture- A break, split, or crack in an object or a material.
Elastic- The ability of an object to return quickly to its original shape and size after being bent, stretched, or squashed.
Inelastic- The inability of an object to return quickly to its original shape and size after being bent, stretched, or squashed.
The five fundamental loads with animated drawing and real life examples.
Simulations and explanations about forces, momentum, etc
Tension and compression description and pictures
extruded foam insulation, 1" X 4' x 8' (should be enough for 6 groups out of one piece)PREPARATION:
black sharpie marker
magnifying glass (optional)
"Fairly Fundamental Facts about Forces and Structures", should be reproduced for each student and discussed as a class before or during the activity. Discussing each load in "Fairly Fundamental Facts about Forces and Structures", before performing the experiment, which analyzes that load, may be very helpful.DIRECTIONS:
Cut the extruded foam insulation into strips 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 will also need one-piece 1"x1"x2" for part 2b.
PART ONE: Modeling Loads on Structures Using “Human Molecules”INVESTIGATING QUESTIONS:
Each person will represent a molecule of steel inside a steel bar and their arms will represent the internal bonding forces, which hold molecules together (a molecule is the smallest piece of steel that can exist with the chemical and physical properties of steel - billions of molecules link together in lines to make a piece of steel).
1. Form two lines of ten people each, lining up side by side, facing each other (see diagram). These two lines represent a structural element. Each person must use his/her left hand to hold hands with the person whom they are facing in the other line. Each person should then lock his/her right arm around the arm of the person on his/her right. See Figure 1 (To see figures download the Word Docuement or PDF)
2. Four other students will act as an applied load. Position one student at each end of both lines, and have them pull with equal force (if possible). Have the students pay attention to what they are feeling while the molecules are being pushed and pulled. Next, form the same lines again, but have the four people applying the loads push equally on each line end. The job of the molecules is to try to maintain their original formation, like a solid non-elastic object.
What type of loads did you model? What did it feel like to be a molecule inside the material (note which arm took most of the load - left or right?)?
3. Now have the “applied load” students pull one line of molecules to the left, and the other line of molecules to the right (as shown in the Figure 2 (To see figures download the Word Docuement or PDF)).
What type of load did you model this time? What did it feel like to be a molecule inside the material?
PART TWO: Looking at Loads: Studying the Five Fundamental Loads and Their Effect on Materials.
1. From the pieces supplied to each group have them cut (10) 1” X 1” X 6” blocks of extruded foam insulation.
2. Instruct the students to do the tests included below; make drawings of each fracture, and record observations on appropriate data sheets:
Have the students measure the length of the block, before breaking it. Two students should 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. A slightly dished in fracture (break) that is characteristic of a tensile failure should be noted.
A student should use a 2” high block of insulation foam and stand on the block or place a weight on it; try to keep the load perfectly vertical and stable. You should 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. Next students should try the compression test again using a 6” long block. Because of its slender (long and thin) shape, it will fail by buckling.
Two students should use two textbooks each, as is shown in figure three, to demonstrate shear. When blocks are sheared apart as shown in Figure 3 (To see figures download the Word Docuement or PDF), a rough angular fracture will be observed (an uneven fracture with several planes of the material angled in different directions).
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).
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 will actually fail 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.
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, will 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.
Describe the fundamental loads and the effect each load (of force) has on a structure or structural member (or component). Give real life examples of tension, compression, shear, bending, and torsion.REFERENCES:
Tufts University, Center for Engineering Educational Outreach
(see links)SAMPLE RUBRIC:
"Fairly Fundamental Facts about Forces"
Figures 1-3 for Loads Acting on Structures
See Associated Download.
This rubric is not in the same format as all of the others. Class participation, List and define the five fundamental loads (possible quiz)