[DML] Re: TA Bolt questions

[tobyp@xxxx]

Hello List -- This is a reply to a request for technical comparisons 
of different types of bolts. It will require some technical details. 
Those that are not interested can click the "next" button. As defined 
earlier, I will recap some terms that need to be discussed. All are 
expressed in "KSI", which is 'thousands of pounds per square inch'. 
"Ftu" is the allowable ultimate tensile stress (tensile strength). 
"Fty" is the allowable tensile yield stress (when it starts to 
permanently stretch). "Fcy" is the allowable compressive yield stress 
(when it starts to permanently buckle or crumple). "e" is the 
elongation, expressed as a percentage. Basically, the lower the 
number, the more brittle the material at failure. "E" is the modulus 
of elasticity, which is a measure of the 'stiffness' of the material 
when subjected to a load (this number is expressed as KSI X 1,000). 
The lower the number, the more bending or deflection you will get for 
a given load. Okay ... are you ready?

The "8.8" bolt has Ftu = 120, Fty & Fcy = 96, E = 29, e = 12%
The "10.9" bolt has Ftu = 151, Fty & Fcy = 136, E = 29, e = 9%
The "12.9" bolt has Ftu = 177, Fty & Fcy = 160, E = 29, e = 8%

The above came from the ASTM Materials Handbook. The 8.8 is about 
equivalent to the SAE grade 5, the 10.9 is about the same as SAE grade 
8, and there is no SAE equivalent for the 12.9. The above bolts are 
alloy steel, and must be plated for corrosion protection. I should 
note that my fractured 10.9 checked out at Ftu = 136 using Rockwell 
hardness methods. I had said in post #21114 that my guess at the 
material for the stock bolt was 4130 steel with a Ftu range of 125 - 
145 KSI. This was based on the opinion of the metallurgist who did 
the hardness test. I didn't ask for a spectroscopic analysis of the 
material to verify the chemistry. For that material, the Fty is 103 
KSI, and the Fcy is 113 KSI.

For the custom bolts, Ftu = 220, Fty & Fcy = 200, E = 29.4, e = 12%.
These bolts are corrosion-proof, and require no plating or other 
protection.

Just for fun, I also looked up titanium material to see how it might 
stack up. The usual alloy for bolts is called Ti-6AL-4V. It has an 
Ftu = 160, Fty = 150, Fcy = 155, E = 16.0, e = 10%. It falls in 
between "10.9" and "12.9", except for the "E" of 16.0, which shows 
that it is about half as stiff in terms of bending at a given load. I 
won't even discuss the 316 CRES bolt option. With an Fcy = 83, it 
doesn't even belong here.

John H. admitted in post #21251 that his 12.9 bolt was bent. Others 
have reported bent 10.9 bolts all over the place. This (to me!) gives 
an idea of the stresses (and loads) involved in this critical joint. 
Without doing an instrumented strain survey (like I do in jet 
aircraft), this is the best way to guess at what the bolts and other 
components need to stand up to. BTW - For all of you that are 
concerned about brittle fractures of hard bolts, please note that my 
special bolts are more ductile (higher "e" value) than either the 10.9 
or 12.9. Just thought that I'd throw that in for free. One other 
thought - how and when the threads are formed in the bolt. If the 
threads are cut, or ground in, the grain of the material is disturbed, 
and this is an unacceptable situation for any bolt that you are 
concerned about (cut threads are CRAP). The only acceptable method is 
"rolled threads", which preserves the grain structure. But ... if the 
threads are rolled in before heat treatment, the fatigue strength 
(resistance to cracking, basically) is as low as 1/3 that of bolts 
with the threads rolled after heat treatment. The special bolts are 
rolled after ... Does anybody happen to know about this for the stock 
bolts? I personally wouldn't want to guess. Well, that is all for 
now. As always, feedback will be appreciated.

Toby Peterson VIN 2248
Winged1

--- In dmcnews@xxxx, "Walter" <Whalt@xxxx> wrote:

> It would be interesting to know (if you can tell us) what the 
differences are between an 8.8, 10.9 & 12.9 hardness bolt.