Steve,
My question here is if a grade 8 fastener is stronger
because it has been heat treated and is thus more brittle, or could it be
that it is really made of a more expensive alloy that is stronger but has
a comparable ductility to a grade 5 fastener.
I have no basis for this theory, but I wouldn't be surprised if the story
of the brittle grade 8 bolt is an urban legend...
Ulix
On Thu, 24 Sep 1998, Steve Byers wrote:
> 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
>
Ulix __/__,__
.......................................................... (_o____o_)....
'67 Sprite
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