EG H167 Hands-on Lab

Lab 2: Material Joining

And Beam Bending




This week in the laboratory your team will be doing two tasks. First, you will perform a bending test on a series of materials of different shapes. This test will allow you to verify the Young’s modulus (E) of each material. Then, you will weld some plastic to evaluate the material and the process as a possible chassis material for the robot that you will build in the spring.

The objectives for the bending tests are as follows:

  • To make measurements for cantilever beam bending.
  • To compute the deflection and the Young’s modulus of each material and each shape.
  • To understand how engineers compare different materials.

·        To understand why cross-sectional shape is important in design.


Basic Principles

In this lab write-up, we will cover some basic principles behind:

1)            Bending Tests

2)            Moment of Inertia, and

3)            Plastic Welding.


Lab Experience

The lab experience will encompass:

1)            Material Bending Tests, and

2)            Plastic Welding.














Bending Tests

To do the bending tests, you will clamp the shapes to the bench top and measure deflection while you apply known loads.  By clamping the various shapes to the bench you are creating a cantilever beam.  You will use a gauge to measure the deflection.

The materials include a steel strip, and an aluminum strip all with the same cross section. An aluminum squared bar that is hollow, and a section of round PVC pipe will also be analyzed.





I = Area Moments of Inertia (depends on object shape) (See below and next page.)






   E = Young’s Modulus


Moment Of Inertia

1.     Rectangular:


The formula for the moment of inertia for a bar is


I = b1 h13/12    





2.     Box:


The formula for the moment of inertia for a bar is


I = b1 h13/12 – b2 h23/12










3.     Hollow Tube: 


The moment of inertia for a hollow tube with a circular cross section is 





Itube = Pi*( d24 -  d14)/64









Welding Plastics


For Spring quarter, the teams will be given the option of using steel Erector set parts which are assembled with nuts and bolts, of using PVC (Polyvinyl Chloride) pipe and adhesives, and of using welded PVC.  Your robot could also be constructed of a combination of any of the three material/assembly choices.

In this laboratory, your team will actually try welding PVC using a welding torch, a PVC welding rod and PVC parts.  The temperatures are high and you will have to exercise caution.  A set of instructions will be provided to your team on how to do the welding.





Make sketches of equipment used in class; include them in your lab write-up.


Material Bending Test

For each of the three beams (steel rectangular, aluminum rectangular, and aluminum box)

1.      Clamp beam and position the dial indicator such that L = 12.5 in. and s = 11.5 in.

2.      Load (by placing 2.5 lb. weights in the bucket) incrementally up to Fmax = 12.5 lbs.

3.      Record the deflection (d) for each load using given tables.

4.      Measure the beam’s cross-section dimensions and calculate moment of inertia I.

5.      Record all measurements and calculations using tables provided.


Plastic Welding
  1. Read the handout “Plastic Welding” before using the welding equipment.




  1. Weld the two PVC beams provided in order to create a “T “ bar as shown in the following figure:


  1. Sketch the test setup and write down your observations.


Note: All teammates will try to weld on the same structure.





·  Lab reports must be done in Teams.

·  Follow given lab report format.

·  Maximum 4-5 pages (including figures and tables)



General Guidelines

·  The report should have a cover with the title, date, class, instructor, and time along with the names and seat numbers of the team members.

·  Include a brief discussion of the lab test setup including a sketch of the test setup.

·  Calculate the theoretical deflection d based on the material properties and shape (use the equation provided in the Background section). Then compare the measured to the calculated values. Include your sample calculations.

·  Plot the theoretically expected deflection d vs. load F.

·  Overlay the five data points (experimental measured load and deflection).

·  Compare the theoretical prediction with the measurements. Can you explain discrepancies if any exist?

·  Include a brief discussion of how the plastic is welded and the critical factors for obtaining a good weld.

·  Include a sketch of your welded part or parts.

·  Discuss among your team members as to which materials and methods of assembly that you would choose for building a small (9 inch by 9 inch robot).