Bolt snapped earlier than expected

[52 Ford]

Once I torque to failure, how much should I back off for my actual torque value?

HA! I see what ya did there.

Very funny, Strostovsky

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[cyanidekid]

All good points but it makes the most sense for these sorts of projects to design it to use the reasonably priced, reasonably strong bolts I can get from Fastenal (I get a good discount since the business I work for buys a $20k in bolts per year).

I think I'll look into the standards they comply with to get the ideal tensile strength, and do some testing and/or calculations to determine what gets to that.

Honestly half the stuff I make could be held together with a threaded potato, but hey, I'm learneding.

No, absolutely not. Buy by going with a reputable manufacturer, DO NOT trust the retailer to maintain quality control. Period. They have every motivation to Sorce from low price vendors, while claiming some QC standard, a manufacturer has their name on the box and reputation on the line.
P. S., the fact you are seeing “mis-shapened” fasteners with some regularity is proof you are getting junk.

 

[52 Ford]

No, absolutely not. Buy by going with a reputable manufacturer, DO NOT trust the retailer to maintain quality control. Period. They have every motivation to Sorce from low price vendors, while claiming some QC standard, a manufacturer has their name on the box and reputation on the line.
P. S., the fact you are seeing “mis-shapened” fasteners with some regularity is proof you are getting junk.

About 10 years ago my dad got into a discussion with a guy who turned out to be a representative for a large fastener distributor (not sure which one - one of the BIG names, though). They did a lot of quality checks on bolts.

The guy explained how Chinese or other low cost fasteners might perform WAY under their rated specifications, while some very reputable mfgs may have bolts that out perform the advertised specs by a sizable amount. Basically, the point the guy was making was that high quality cost money.

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[neilho]

Qualifying the torque wrench might be helpful, too.

 

[WizardOfBoz]

https://www.fastenal.com/content/feds/pdf/Article - Bolted Joint Design.pdf

Super useful reference, eKretz, thanks for that. Also useful is Carroll Smith's "Nuts, Bolts, Fasteners and Plumbing Handbook". Dated, but some good info.

 

[steve45]

After looking at the title of this thread, 'Bolt snapped earlier than expected', don't they ALWAYS break earlier than you expect?

 

[DDoug]

Huck Bolts if they can work in your application.

 

[dian]

bolts/screws are poor creatures, because they get twisted. if in undirectional tensile state the 10.9 bolt has a min. elongation (total) of 9%.

 

[BT Fabrication]

question, how and where on the handle were you holding the torque wrench? that can effect bolt torque values easily by 25% OVER or under depending where you have it held. other thing is cheaper torque wrenches only work in the middle 50% of the rated range, if at the top or bottom 25% it might actually be over torqued when it clicks off, especially on the lower very small values. That is where digital wrenches shine but they are $$$$$$

 

[dian]

very true. if you use a 200 nm wrench on a m6 screw you have no chance.

 

[dian]

yes, it will snap. m6/10.9 torque is 13.2-16.5 nm (vdi 2230-2003). this is based on 90% ys. so 14.9 (lightly lubed)/0.9 = 16.5. uts/ys is 1000/940 mpa, corresponding to 17.5 nm. even starting with the dry value it should snap.

today i had to use some 20 mm m6 10.9 screws. i experimentally threaded a piece of 12 mm 6060/AlMgSi0.5 and put a steel washer under the screw. it was fine up to 18 nm, started yielding at 20 nm and immediately snapped at 22 nm. (would have snapped at 21 nm probably). i losened the screw inbetween and the thread had a bit of wd-40 on it.