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

[Eric Phillips]

Thanks for the info.  I do agree that it's possible to design the
bracket to stand up to a half billion cycles, with aluminum, and that
would be an acceptable life.  Maybe even an order of magnitude more
than acceptable.  :)

I have stated previously that I believe designing one is possible.
Whether they are, or not, is in question.  Or whether the designs
available are compromised by having DIYers install them (introduction
of stress risers, etc.).

While I do see your point (and the points made by others), I still
have concerns about aluminum in that particular application.  I think
we *can* agree that having the choice is a good thing for all of us.
And that it's nice to have a civil discussion about the whole issue.
Thanks for that.  :)

Eric

On 10/25/06, lebakken1 at netzero.net <lebakken1 at netzero.net> wrote:
> Eric writes,
>
> > >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
>
> This is pretty conservative Eric. For steel, the endurance strength
> for infinite life, is generally considered to be .5Sut. The above
> statement you reference is kind of correct in that aluminum and
> copper, for fatigue considerations, can fail at any stress or number
> of cycles, since they do not test as consistently as steel. It is way
> too broad a stroke however. .3Sut is a good starting point for any
> aluminum design that will be cycled a half a billion times. That's
> 5xE08 - not infinite, but a pretty large number. I did give a quick
> look at my ASM Metals Handbook from 1944. I love this book because
> most metals testing that is referenced is derived from WWII munitions
> and aircraft testing and use. Here they describe endurance strength
> for a few aluminum alloys at around 6500 psi, which is about .18 Sut,
> again at 5xE08 cycles.
>
> So there is some theory and some practice.
>
> Alluminium is some pretty good chit really.