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When
I’m under a lot of stress I sometimes have the nervous habit of grabbing a
paper clip, straightening out the bends, then repetitively bending it back
and forth. Eventually the wire reaches a point where it just
breaks apart.
My
paper clip broke due to metal fatigue. Metal parts are said to
become fatigued when they’re subjected to forces of a repetitive nature such
as occur due to twisting and bending. The metal cracks, then
eventually breaks due to the stress.
So
what’s happening when metal becomes fatigued? Figure 1 shows the
simplified atomic structure of a sample metal.
Figure 1
When
the metal is deformed, such as during bending, its rows of atoms are forced
to move with respect to each other as shown in Figure 2.
Figure 2
The
movement of rows of atoms leads to an alteration in structure, breaking bonds
between atoms. This results in small cracks forming along the
metal’s surface, cracks which eventually migrate deeper inside the metal with
each subsequent bend. With time the metal will become so
compromised by the cracks that breakage occurs.
Metal
fatigue can occur in centrifugal clutch mechanisms as well. Power
tools such as grass trimmers typically operate between idle and working
speeds many times during a day’s usage. As we learned in previous
articles, when the engine runs at idle speed, the springs in the centrifugal
clutch mechanism stay retracted. As the engine speeds up, the
centrifugal force acting on the clutch shoes extends the
springs. Successive extensions and retractions cause the metal in
the springs to bend, and over time they, like my paper clip, will become
fatigued and metal springs will break.
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