Ulix -
Both Grade 5 and Grade 8 fasteners are made of carbon steel, with Grade 5
being specified as "carbon steel", and Grade 8 as "medium carbon steel" (
I don't have the specific composition). Grade 8's are heat treated to a
higher strength than Grade 5's, and the increase in strength is accompanied
by a decrease in ductility, which is generally true for heat-treatable
materials.
If you load-tested a Grade 5 bolt and a Grade 8 bolt of equal sizes, you
would see both of them perform the same up to the yield strength of the
Grade 5. Then the Grade 5 would fail plastically (i.e., permanent stretch)
and fracture at a higher load. The Grade 8 bolt would continue up to a
higher yield load than the Grade 5, of course (it's stronger), but when it
failed it would do so suddenly.
I think for most of our automotive applications, Grade 5 is completely
adequate, and Grade 8's aren't going to fracture even though they are less
ductile (because they are never going to see their ultimate load anyway).
Grade 8's are more expensive, so take your choice.
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
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> From: Ulix Goettsch <ulix@u.washington.edu>
> To: Steve Byers <byers@cconnect.net>
> Cc: Lancer7676@aol.com; ckotting@iwaynet.net; carlson@navtech.com;
spridgets@autox.team.net
> Subject: Re: Sway bar mounting bolts
> Date: Friday, September 25, 1998 2:20 PM
>
> 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
>
>
>
>
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