7.

Strength of Materials

 

Objective:

To understand how forces and moments create stresses and strains in materials and structures.

 

Science Standards

 

Physical Science

Motions and Forces

Science as Inquiry

Unifying Concepts and Processes

 

Process Skills

Observing

Collecting Data

Communicating

Inferring

Testing

Interpreting Data

 

Description:

 

Students will perform simple experiments in the UAH Mechanics of Materials Laboratory to demonstrate how forces and moments are created, and balanced to achieve equilibrium. 

 

They will visualize how these quantities act to produce stresses in materials using the method of photoelasticity.

 

Metal samples will be tested to failure in torsion and tension to demonstrate the consequence of overloading a structure.  Group members will take digital photographs to document their work.

 

 

 

 

 

Background:

Forces and moments create stresses and strains in materials and structures.  Stiffness, strength, and geometry are important components in engineering design.

 

A force represents the action of one body on another.  It may be exerted by actual contact or at a distance, as in the case of gravitational and magnetic forces.  A moment is created when a force tends to rotate a body about a point or an axis.

 

Forces and moments must be balanced to achieve equilibrium; i.e., for a body to remain at rest or move with a constant velocity.

 

Stress is a measure of the force intensity in a material or structure while strain represents the shape changes that occur as forces and moments are applied.

 

Instructions

7a. Force & Moment Considerations

 

1.     Weigh each group member and record this data on the report sheet.

2.     Set up balance beam with the fulcrum at the center.

3.     Have two group members stand on opposite ends of the beam at equal distances from the center.  What happens?  Why?

4.     Adjust the positions of the team members until equilibrium is achieved.  Measure the distance from the center of the beam to one group member and multiply by his/her weight.  Do this for the other group member.  What can you conclude?

5.     Assume that one member will stand at a fixed distance from the center of the beam and that another member will stand in a position to achieve equilibrium.  Record the weight and position of the first group member on the report sheet.  Record the weight of the second member and compute the position required to achieve equilibrium.  Test this configuration and list factors that may lead to errors on the report sheet.

6.     Photograph two team members standing on opposite ends of the balanced beam while it is in equilibrium.

 

7b. Photoelasticity Experiments

 

1.     Place a specimen with a hole in the center into a polariscope.

2.     Apply a tensile load and observe how fringes form in different portions of the specimen.

3.     Assuming that the number of fringes represents the stress level, what does the hole do?

4.     Observe the fringes in different hand-held specimens to confirm this observation.  Discuss how changes in geometry affect stress on the report sheet.

5.     Photograph group members while they are looking at the photoelastic specimens.

 

7c. Torsion Test

 

1.     Obtain a metal specimen from the instructor (brass, steel or aluminum).

2.     Record the type of material on the report sheet.

3.     Draw a straight line down the length of your sample.

4.     Have the instructor help put the sample into the torsion fixture.

5.     Have one group member turn the wheel and count the number of revolutions (watch the wheel).

6.     Have another group member take a photograph of the others while they are doing the test.

7.     Continue loading until your sample breaks.

8.     Record the number of revolutions and the maximum torque on the report sheet. 

9.     Discuss and compare the results obtained for two different materials.

 

 

7d. Tension Test

 

1.     Obtain a metal sample from the instructor (1020 steel or aluminum).

2.     Record the type of material and cross sectional area on the report sheet.

3.     Have the instructor put the sample into the tension test machine.

4.     Take a photograph of the experimental set-up.

5.     Load the sample until it breaks.

6.     Obtain a computer printout and discuss the results.

7.     Record the maximum stress on the report sheet.

8.     Look at the fracture surface and record what you see on the report sheet.

 

 

7e. Documentation

 

1.     Download the camera using the computer.

2.     Compile a photo album and print it out for distribution.

 

Discussion

 

1.     What is a force?

2.     How does a force create a moment?

3.     How can the moment created by a given force be increased?

 

More Information

 

 

http://www.aku.ac.ir/faculty1/aliniamm/Structural%20Slides/cables/Cable-stayed%20bridges.htm

 

http://www.waves.brantacan.co.uk/susptwo.htm .

http://www.matweb.com/