Strain Hardening of Aluminum

Location:

Concepts Shown:

strain hardening, dislocations, mechanical work

Equipment:

One 1100 aluminum rod approximately 15 inches long and 5/16 of an inch in diameter (can be purchased from Alcoa 800-622-5262).

Procedure:

Ask one of the weaker students in class to bend the rod. At first this should be easy so ask the student if the rod was easy to bend. Well, if it was easy to bend, it should be easy to straighten. Instruct the student to continue bending and straightening the rod until he/she no longer can. Pass the rod to another student who appears to be stronger than the first. Tell him/her to continue where the first student left off. Once he/she can no longer bend the rod, explain that it was strain hardened.

Science:

Dislocations are inherent to the crystalline structure of the rod. Each time the rod was cold worked, bent or straightened, slip occurred causing the dislocations to move. However, the dislocations eventually become impeded by each other. For further movement, these dislocations have to bend around each other, and this causes less slip for a given stress. The result is an increase in yield strength. Graphically, the yield strength and ultimate strength increase as the percentage of cold work increases. Remarks: It is imperative that the rod is straightened completely. If not, the rod will fatigue and break. Although the rods may be annealed (345� C for 35 minutes) and used again, they will eventually fatigue and may not strain harden as expected. This experiment should be practiced beforehand.

References:

Author:
Dave Goodman
Credits:


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