Thread length not affecting tensile strength?

[rollingcloud]

I have a .25 - 28 UNJF nut made of G87400 that was tested per NASM1312-8, the hardness measured 22 HRC, it failed the tensile test (requires 4590 lbs of tensile load, nut threads stripped. Is it correct to say that the fixture setup used in NASM1312-8 tensile tests focuses on the tensile strength of the fastener itself, making the length of thread engagement not a factor in the tensile strength value determined by the test, and therefore, the result is a measure of the inherent strength of the fastener material and the stress area, not the joint (nut + bolt) strength?

I mean it makes sense when testing a bolt, because the location of failure would most likely be outside of the thread engagement area. But when testing a nut, the location of failure is inside of the thread engagement area.
I was assuming the length thread engagement would be a factor in the stress area of the fastener during an axial tensile test, before reading into the NAS spec. But I am still not sure how a fixture could make the length of thread engagement irreverent in determining the tensile strength. When the nut and the bolt mates properly, it would be a full engagement.

 

[TugboatEng]

A general rule of thumb is that the first 3 threads of the nut carry almost all of the load. Beyond that, thread engagement isn't much of a factor. 22HRC is very soft for a steel nut.

 

[rollingcloud]

Thanks TuboatEng. Then for castellated nut with no threads on the slots, there is no strength reduction given that it has at least 4 turns of threads, correct?

 

[EdStainless]

We had both nut and bolt test fixtures, made from high alloy steels with UTS about 250ksi.
We could fail any nut or bolt on demand.
And since we had custom fasteners made for us this was important.
We did the testing just to show people.
No reliably measured difference in strength between four turns and twenty.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[rollingcloud]

EdStainless - Thanks for the confirmation!
If the castellated nut has slots that do not have threads, the slots will not contribute to stress concentration in the threaded region that is engaged with the bolt because they are not part of the load path. And therefore, no strength reduction in terms of tensile testing, correct?

 

[TugboatEng]

Are you including the first and last threads in your count of 4? I would not include them as they are not fully formed.

 

[rollingcloud]

The thread length is .158'', deducting the first and last threads, then it only has 2 fully formed threads?

 

[EdStainless]

We used to count first and last as 1/2 threads.
So if a nut was 5x thread pitch we considered it as having 4 threads.
You have to really look at how the threads were formed to make this decision though.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[rollingcloud]

You mean how the threads transition at the ends?
Which standard can I use as a reference for the subject nut? I am looking at MIL-HDBK-5, its showing .0333 nominal minor area and ultimate tensile strength of 4170 lbs, is this assuming at least 3-4 threads are engaged?

 

[3DDave]

MIL-HDBK-5 was cancelled almost 20 years ago. What material and fastener is that strength for? I suspect it is for a high strength bolt, and does not apply to a lower strength nut.

 

[rollingcloud]

Oh wow, good catch. That strength is for .250 - 28 fastener (2A & 3A), 125 ksi material. Which standard do you recommend to use?

 

[3DDave]

"it failed the tensile test"
By how much?

G87400 is a 60ksi tensile yield material.

The material book rights were transferred to the FAA as MMPDS and then some proprietary arrangement with Battelle Memorial Institute, for the low-low price of $1000 to get what the DoD provided US industry for free.

 

[rollingcloud]

The samples failed under 3000 - 3600 lbf ultimate load. It's strange because several parts a couple years ago reported conforming results (above 4590 lbs). We also tested a few parts from another lot from a different manufacturer, they all conformed. The exact material is per AMS6322, the composition is similar to G87400, it limits bars under .5 dia to have less than 120 KSI tensile strength or equivalent hardness. Is there a NAS or ASTM standard that I can use?

 

[EdStainless]

I am sure that there is a NAS or SAE std that covers these, but I haven't kept up with that side of things, sorry.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[LewisII]

I've always seen castellated nuts with lower tensile requirements than the standard nut in that same size. You aren't finding a procurement spec referenced in the nut spec?

For instance, MS14145-4 is a 1/4-28 unjf locknut. It has a lower tensile than you cite for your nut, at 2,290lbs min. It uses NASM25027 typically but the standard tensile in NASM25027 is 4580lbs for a regular steel nut, double the castellated requirements.

 

[rollingcloud]

FF-N-836 stated that the castellated nuts should be 67% of the regular nuts. However, since my castellated nut has no slots on the threads (as opposed to the normal castellated nuts), I just don't see how that reduction in tensile strength would apply. Unless the 67% reduction is due to other factors that I am not aware of. If the 67% reduction applies to my nut, it would be about 3100 lbs of tensile strength for a 125 ksi material, but parts have met 4590 lbs of tensile strength previously.