Lab Experiment 4: Torsion

Lab Experiment 4: Torsion

Due to prior to the start of lab

Objectives:

The objective of this lab is to study the torque-twist relationship of various materials, and to compare experimental and theoretical torques that will cause shear yielding of a prismatic bar subjected to torsion.

Procedure:

Three different torsion members will be tested to failure.  The three different members are made of A36 Steel, Gray 20 Cast Iron, and 6061-T6 Aluminum.  Each torsion member will be placed in the Tinius-Olsen torsion testing machine (TOTTM) and a torque will be applied to the member.  The twist of the member will be recorded in two different ways:

  1. Using the scale on the TOTTM, and
  2. Using lasers attached to each end of the member.

In both cases, students will record by hand the torque and the data required to determine the relative twist angle between the member ends.  Testing and data collection will continue until yielding and failure of each torsion member.

Analysis:

  1. Plot the experimental torque-twist relationship along with (i.e. on the same plot) the theoretical torque-twist relationship for each member. Create two plots: one for steel and one for cast iron.  For steel, you should also print out a zoomed-in version of your figure, focusing on the elastic region.
  2. How does changing the material alter the torsion response of member? Which material could hold the largest torque?  Which had the largest yield torque?  Which could twist the most?
  3. Compare the results from the TOTTM and from the lasers. Comment on the method you think is more accurate for small twist angles.
  4. Identify the experimental torque that will cause the section to yield in shear on your experimental torque-twist plot.
  5. Calculate the theoretical yield torque that will cause shear yielding of the extreme fibers. Assume shear yielding will occur at 60% of the tensile yield stress (i.e. use 60% of the yield stress obtained in Lab #2 or from a source of your choice.  Be sure to clearly state your source).  Comment on any differences between the theoretical and experimental values.
  6. Comment on possible sources of error/inaccuracies in this experiment. Comment on anything that looks strange or unexpected.

Note:  See the table on the inside back cover of the textbook for tabulated values of the shear modulus (G) for each material.

Format:

  • This lab assignment will be a formal laboratory report, following the format and rubric distributed in lab and posted on Blackboard.
  • You may work alone or in groups of 2-3 students; however, your group should consist of different students than those you have previously worked with on prior labs this semester.
  • Because the write-ups for this lab report will be more substantial than prior labs, your grade for this lab assignment will count 5 times more than an ordinary lab assignment, i.e. your grade will be out of 15 possible points instead of 10 possible points.
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