Asimov wrote:
"There is a thing called metal fatigue of which I was strongly reminded
this spring."

Yes. It usually begins with a microscopic crack. The greater the stress,
the shorter the life. Damage is cumulative. It is affected by
temperature and surface finish.

Some metals such as steel and titanium have lower stress limits below
which they are mostlly im,mune to failure from repeated flexing. Other
metals such as aluminum and its alloys have no such immunity and
eventually fail under repeated flexing. In these, designs are kept below
the fatigue threshold for the number of stress cycles expected within
their lives. Or, a lifetime is specified and replacement is required.
Another technique is non-destructive testing to find cracks and the
piece is retired for cause.

Best regards, Richard Harrison, KB5WZI