[s-cars] Cracked Big Brake Brackets - ECS Stage 2

[Eric Phillips]

On 10/24/06, lebakken1 at netzero.net <lebakken1 at netzero.net> wrote:
> Eric wrote:
>
> >2.)  No, comparing alloys of aluminum as "apples and oranges" doesn't
> >address fatigue limit.  No alloy of aluminum has a fatigue limit.
> >Alloying does affect fatigue strength.  I would imagine those main
> >wing spars in the 40+ year old B-52 Stratofortresses have some darn
> >good fatigue strength.  747s, too (And built by the same company, of
> >course.  :)  )
>
> Ummm. Yes it does sort of. Fatigue limit (Endurance Strength, Es) is
> a direct function of ultimate tensile strength, Stu. Stu varies
> widely among the different Al alloys. Subsequently, so does Es.

This is directly contradicted by the website reference I gave in my
first e-mail on this subject - here is the quote from that site:

"The significance of the fatigue limit is that if the material is
loaded below this stress, then it will not fail, regardless of the
number of times it is loaded. Material such as aluminum, copper and
magnesium do not show a fatigue limit, therefor they will fail at any
stress and number of cycles. Other important terms are fatigue
strength and fatigue life. The stress at which failure occurs for a
given number of cycles is the fatigue strength. The number of cycles
required for a material to fail at a certain stress is fatigue life."

Is the discrepancy one of terminology, or one of "theory versus practice?"

Eric