Ratings Grade 5/8:
Proof Load (testing) 85/120 kpsi
Tensile Strength 120/150 kpsi
Yield Strength 92/130 kpsi
Rockwell Hardness 54/58.6
Composition: Medium carbon steel (Grade 5), medium carbon alloy steel
(Grade 8), both are quenched and tempered. Fine-grain steel is
specified for grade 8. Carbon content range is the same, slightly more
sulfur allowed for Grade 5.
---from Society of Automotive Engineers J429, for nominal bolt
diameters < 1 in.
The Grade 5 will begin to yield ("permanent stretch", if you will)
well before the Grade 8; in fact, it will have failed in tension
before the Grade 8 has even begun to yield. This may or may not be
desirable, depending on the application. You might actually WANT a
bolt to fail or stretch in some circumstances, where damage to
surrounding structure is more critical than the importance of the bolt
holding. Try to use what the original designers did where possible
(presumably they did the calculations or had their standard policy to
justify their choices), unless you observe some compelling evidence to
make you switch.....for example, bolts that regularly snap under
normal loads or the like.
M. Dietsche
---Ulix Goettsch <ulix@u.washington.edu> wrote:
>
> 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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