The investigation is just beginning 3

[Tmoose]

The investigation is just beginning. I only have 2 pictures for now, and almost no details.

One brand new M36 "bolt" out of a litter of 26 snapped at installation.

The thread runout detail may not be the best. That will be part of the investigation.

The material nominally is hot rolled 1045, about Grade 8.8, but with yield strength more like 50% of UTS, not 80%.

The full diameter body bolts I've broken by tightening to maximum Moosage generally show some some necking as I recall.

 

[desertfox]

hi T.Moose

I did a quick calculation based on the 1900lbf-ft and the 55100 psi figure from the specification and it appears the tension in the bolts after tightening should be around 95% of the yield, now given that torqueing of bolts is subject to huge errors, I just wonder whether that particular bolt got overloaded because of some change in friction value or thread geometry etc.
I also note that the bolt failed during tightening and that is when the bolt see's the most stress because its subject to both tensile and shear stress until the wrench is removed.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[Tmoose]

thanks all.

The attached image shows XRF test results for the broken bolt, and an old bolt.
It includes the material certs for the new bolts.

One thing that has me scratching my head is the different results for two locations on the broken new bolt.
Maybe the surface prep needed to be more severe.

The fact XRF does not measure carbon content makes me (an XRF ignoramus) wonder how useful it really can be.


More news at 11.

thanks

Dan T

 

[EdStainless]

You are right, you need C to know much. That will take some OE work.
The other thing is that most people are not careful enough with surface prep, and they don't know how to turn on the 'high precision' mode.
Is your cert chem from the melt or a product check?
I will assume melt, so given the variation in a portable XRF and the allowances for check chem, it looks like a match to me.
I usually re-test the same spot 5 times and record each time. That gives a good idea of how precise the readings are.

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P.E. Metallurgy, Plymouth Tube

 

[Tmoose]

" it looks like a match to me. "
Should I take that to mean the new, broken bolt //could be// 1045?

thanks

Dan T

 

[EdStainless]

Nothing there clearly tells me that it isn't 1045.
I would be more concerned with mechanical properties and microstructure.
Though at some point a detailed analysis would be good to have.
Mix-ups have been known to happen.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[chicopee]

Let us know about your findings from your investigation particularly as alluded above about cold temperature condition and overtightening.

 

[tbuelna]

The pictures in the OP show a failure right at the thread run-out. The thread also appears to be cut rather than rolled. And the transition from the cut thread root run-out to the bolt body looks like it creates a significant stress concentration. The corrective action would be to "waist" the bolt body profile to provide better stress distribution, and roll form the threads to provide better fatigue performance.

 

[mfgenggear]

where's the SEM test, charpy and tensile. what is the actual hardness of the bolt and what heat treat process was used?

 

[mfgenggear]

http://www.enme.umd.edu/~ccfu/ref/Bolt&Weld.pdf