Mark said:
> Within the
> elastic limit you can flex something almost an infinite number of times
> without changing its strength or shape. Speed and temperature also play a
> big
> part of this too.
>
while Nolan said, earlier:
> Failure happens when you go into plastic
> deformation. Simply flexing below that level doesn't result in failure.
> That's why your cars springs sit there going "boing boing" without
> snapping
>
I believe you may be confusing elastic limit with fatigue limit. They are
not one and the same thing.
Reference to a Soderberg diagram (he did this work in 1935, and I believe
it is still valid):
http://www.efunda.com/DesignStandards/springs/images/soderberg.gif
shows that fatigue failure is dependent upon the relationship between the
alternating stresses and the mean stress in a component. If the mean stress
is greater than the yield strength, the component will most certainly fail.
If the variable stress is greater than the fatigue limit (mean stress = 0),
the component will also fail. BUT, the component can also fail at any point
whose coordinates are above the line joining these two limits, even when
neither the fatigue limit nor the yield strength is exceeded. This is why
fatigue failure is so insidious.
Michael Marr
1960 TR3A
Naperville, IL
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