One aircraft structural engineer chiming in here.
When a car, or aircraft, or bridge is being designed, one criterion for
selection of fasteners is the maximum (limit) load that they are ever
expected to see. Add to this some margin of safety to account for unknowns
and you arrive at the ultimate load. The fastener material and size is
then selected to ensure that the fastener can accept its ultimate load
without fracture (with environmental factors considered also). These are
static, as opposed to fatigue, considerations.
It is true that for some fastener materials, heat treating to a higher
strength also results in brittleness, The material becomes more sensitive
to shock loads, particularly if there are nicks or scratches on the surface
of the fastener that can serve as "stress risers". Under shock or
alternating (fatigue) loads, cracks may develop that could lead to sudden
failure.
Obviously, you do not want to use a very high-strength, fatigue-sensitive
fastener in that application. The fastener salesman is correct in general.
A higher strength fastener than what was originally installed is not
necessarily a good thing.
Steve Byers
Havelock, NC USA
'73 Midget GAN5UD126009G "OO NINE"
"It is better to remain silent, and be thought a fool
than to speak, and remove all doubt" -- Mark Twain
----------
> From: Lancer7676@aol.com
> To: ckotting@iwaynet.net; carlson@navtech.com; spridgets@Autox.Team.Net
> Subject: Re: Sway bar mounting bolts
> Date: Thursday, September 24, 1998 10:58 PM
>
> Chris and all:
>
> I would like to hear from some of the engineers on this one. I always
thought
> that when you needed strength you should always go to a #8 bolt. When I
went
> to our local fastener supplier, Knoxville Bolt and Screw, the salesman
> explained that #8 bolts are more brittle than a #5, and that a #5 bolt
had
> some "give" or elasticity that would absorb sudden force. His bottom
line
> was that #8 is not always the answer where you did not want a bolt to
break
> under impact. How about it, some of you people who know far more than I
about
> this topic?
>
> ----David
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