Chapter 3:  Examples of Using the SEM

Metal Fatigue Observed in Mead Paperclips

 Theory:

One interesting property of metals is that they can undergo fatigue. Metal fatigue occurs when a metal is exposed to repeated mechanical stresses and eventually enough chemical bonds in the metal are broken by the mechanical stress so that the metal fractures and breaks. One notable example of metal fatigue occurred several years ago to a commercial passenger jet flight to Hawaii. Unbeknownst to the passengers and crew, the cabin of the airplane had started to exhibit microscopic stress fractures due to the relative changes in outside air pressure relative to inside air pressure that routinely occurred as the plane made repeated flights over the years. These fractures caused the roof of the passenger cabin to tear away from the fuselage while the plane was in flight, causing several deaths.

Procedure:

I obtained a container of 150 Mead silver paperclips from the Ames in Frostburg. After opening the container, I carefully removed 30 paperclips from the container using a pencil. I used a pencil to do this so I would not get oils from my skin on the remaining paperclips and then have to clean them before mounting them on specimen stubs. I then proceeded to straighten out the paperclips. Next, I began bending each paperclip in 180 degree increments at a rate of one bend per second until the paperclip broke. I then recorded the number of bends that it took for each paper clip to break. I recorded these results in Table-2, which is located at the end of this document.

Using this table, I determined the minimum number of bends that it took to break the paperclips (11 bends). I decided to examine three paperclips in the SEM to see if it would be possible to observe metal fatigue with SEM. One paperclip was unbent. This acted as a control sample in the experiment. Using the value for the minimum number of bends until metal fatigue occurred, I determined that it would be possible to bend one of paperclips 6 times without it breaking (11/2 is approximately 6) and it should begin to exhibit some signs of metal fatigue at this point. Then, while wearing rubber gloves, I bent a paperclip 6 times and used this paperclip for my second sample. For my last sample, I bent a paperclip until it broke (still while wearing rubber gloves). I then mounted each specimen to a stub following the previously outlined mounting procedure. The images I obtained appear after the data table for this experiment.

To:
Table of Contents
Previous Page(Imaging with the SEM)
Final Comments and Future Experiments