Tighten bolt beyond yield point - Preload the bolt in the plastic area 2

That's because it's something you're not supposed to do. So most articles will be about how to do what you're supposed to do....

If an event occurs where people suspect the bolts were over tightened or subject to high tensile or bending loads and have actually yielded or could have, they normally just replace the bolts as they are pretty cheap.

There is also a difference between "bolts" and stud bolts. Most high pressure equipment uses stud bolts with threaded studs and separate nuts.

what are you looking at?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

In my case, I'm specifically looking for articles on the plastic tightening of bolted joints and the potential benefits or otherwise that this could have. Some studies show that pre-stressing or applying tightening beyond the elastic limit is beneficial, as can be seen from Nagata's study: DOI:10.1115/PVP2009-77689 or Chapman's study ( which states "The experimental results show substantial benefits in
tightening a joint to the bolt torque-tension yield point.

(a) The joint can withstand higher working loads
before opening.

(b) Fatigue strength of the joint is increased to its max-
imum value because fatigue failure mainly occurs
when the joint opens."

Another interesting article is

In modern times lots of car manufacturers use "torque to yield" fasteners in their engines.
I think an "angle torque" spec //can// be a clue, but not a guarantee.
"Do not re-use" can be driven by other details, like thread locking goop applied to the screw by the manufacturer.

My suspicion is the result is not vey deep into the yielded zone.

Not sure if i understand your question correctly. You should not tighten the bolts beyond its elastic limit( as LI stated). However ,if the question is , to make sure that the bolt tightened beyond its elastic limit, the answer is not complicated. You may calculate the force corresponding to yield stress and ultimate stress and you may calculate the necessary strain level .
Then you may apply one of the following methods;

-Turn-of-Nut Method,
-Direct Tension Indicator Method,
-Calibrated Wrench Method.





According to the grace of God which is given
unto me, as a wise masterbuilder, I have laid the foundation, and another buildeth thereon. . . .
I Corinthians 3:10

Hi

You might find this article interesting










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

I'm really not sure I agree with what is written there myself.

Going into the plastic Zone generally results in necking and thinning of the stud at some point and therefore must result in loss of strength as there is less square area of the bolt to apply surely?

Just doesn't make sense to me.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

LittleInch,

The concept is that you want to tighten the bolt down as hard as possible. Tightening into the plastic region of the bolts makes them a bit tighter and safer. It also means that the bolts can be used only once, and they must be discarded if removed. You need to work in a professioal QA environment.

Has the OP read anything by John H. Bickford? He gets recommended here fairly regularly. He is the gold standard for fastener engineering.

--
JHG

It's not as if entering the plastic range eliminates elastic behavior, it just means that the removal of the load doesn't return the fastener to the original length. As long as the design of the joint is such that there can be no load that forces the fastener further into the plastic range, this will generally produce the maximum performance. Typically the necking is insignificant compared to the gain in clamping strength, but it does depend on knowing what is happening and using the correct monitoring tools to get there.

I suspect the lack of significant information is that there isn't a plug and chug formula to get the values correct and depend on the geometry of the mating parts.

Keep in mind that under elastic loading the diameter of the fastener also decreases and under moderate excursion into the plastic area the difference between the elastic reduction in area and the plastic reduction are very small. It's when someone just won't lay off the wrench and keeps on going that visible and notable necking occurs.

---

In one example I investigated we had weld-studs that were reported as shearing off from the structure. The expectation was the production line was twisting them off. The stress group calculated everything was sufficient as designed. I asked for a sample plate of a half-dozen studs to see what happened during over-tightening. Made a spacer block same thickness as the actual mating part, put on the nuts and -it sure feels good to twist the daylights out of a fastener on purpose instead of the usual breakage during a repair.

None of them failed at or near the weld. They all failed under that test the same way - elongating so much the thread on the stud no longer matched the thread on the nut and the mismatch acted as a stop preventing the stud from being pulled up into the nut causing the threads of the stud in the nut to shear.

My conclusion - they had originally overtightened the nuts, but only enough for plastic deformation of the stud threads and not enough to strip them. Then the part was removed and the replacement installed, allowing the nuts during reinstallation to give a false torque against the deformed threads and not clamping the load; the load was then free (several hundred pounds static, so no one was giving it a shake test to see if it was held tightly) to rattle on the rough road operations and fatiguing the studs in bending. AFAIK after that the problem did not reoccur which I assume meant the QA people were ensuring the use of torque wrenches and not the biggest cheater bar the workers could carry.

Hi Littleinch

I have to say in all my working life I have never designed a bolted joint to go into the yield reason for all the reasons that you yourself abide by. I suspect vast 3DDave alludes to, that it’s exceptabke provided you know exactly what you are doing and the joint is monitored correctly with equipment, I doubt it truly applies with the average fitter taking a wrench to the joint.

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

Another good article on this subject:-


The interesting thing to bear in mind is that when the bolt yields during tightening it is usually only at the outer surface of the bolt, so most of the bolt shank is still within its elastic limit.

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

Nearly every bolt of importance in automotive applications has been torque to yield for 40 years now. It is a superior to fastening systems it has replaced. It does produce strong, lighter weight connections in less space. It is likely not worth the cost of development in other applications, or the other applications are a victim of, "this is the way I've always done it".

I think this is normal for fully tensioned high strength structural bolts, so there should be information out there.

Ooo eee ooo ah ah ting tang walla walla bing bang
Ooo eee ooo ah ah ting tang walla walla bing bang

I think Carroll Smith's "Engineer to Win" has a few paragraphs about torque to yield.

Bickford too probably.

Maybe a productive conversation would be to discuss situations where TTY is not superior.

For example, systems that are subject to stress corrosion cracking or creep may want to operate at lower stress levels.

Very brittle fasteners that have limited elongation before failure.

Clamps materials that cannot support the bearing loads.

Any other ideas?

Hi,

I made a few theoritically checks on excel for this case with hex.h bolt M16, grade 8.8 and 10.9, layout as below:



Test 1 with 10.9 and pretension 100% yield. Seperation is higher than ultimate, so bolt will break before seperation occurred, at an external force 85kN:



Test 2 with 10.9 preloaded at 90% yield, external force can be applied up to 155kN:



If reduce external force to 85 kN, gain some clamp force compared to fully yielded of 10.9 above:



Test 3 with 10.9 yielded 70%, then 120kN external force can be applied:



If we reduce external force to 85kN, clamp force remains:



Test 4 with grade 8.8 and preloaded 100% yield. Seperation is close to ultimate while 120kN external force can be applied before seperated:



And clamp force when applied 85 kN external force:





Engineering is a journey of thousand miles begins with a single step

The concept of structural connection does not apply to pressure vessels (PV).
In PV, studs with a diameter of 50, 100, 200 mm and larger are common. And no torque is used (the worst method). Hydraulic tensioner is used.

Regards

Hi cherish

Perhaps I am reading the graphs incorrectly but I can’t get my head round how the preload can be less than the external load and yet the joint doesn’t separate until the force gets close to the ultimate load (test 4) for example

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

The preload is always in excess of the external load until the external load matches the preload. At that point the load in the fastener exceeds the preload.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

Hi dik

I agree but look at the graphs above and they don’t reflect that.

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