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Thread length not a variable in determining tensile strength? 2

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rollingcloud

rollingcloud

Aerospace
Aug 9, 2022
172
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.
 
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The nut threads stripped out? I'd say length of engagement is really important.

Look at shear stress of threads.
 
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While a double post - a typical thread develops the full strength in 3-4 full turns with 30-50% of the load being taken in the first turn.

This is 100ksi tensile limit material so 50ksi shear limit approximately; the shear area at the root is roughly 80% of 4 turns times pi times the root diameter 0.25 inches.

50,000 psi shear * .8 * 4/28 * pi *.25 = 4490 pounds. Since this was shear failure, it likely plastically involved more threads, but it would have damaged the ones that take all the load earlier.

Tensile yield strength is only 60,000, so,

30,000 psi shear * .8 * 4/28 * pi *.25 = 2700 pounds when it first starts to go.
 
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3DDave,
It takes 3-4 turns for full strength, but the more thread engagement the better for the nut to withstand the load?
Are you saying that such nut without defects will typically pass the tensile test up to 4490 lbs of tensile load, theoretically?
And the thread of the nut would experience strain hardening when the load is 2700 lbs?

Looking at the machining handbook, the stress area for UNJ threads is =.25*pi * pitch diameter^2, is this just a stress area per thread for tensile stress? Hence why it's different from your shear stress area. What are the top factors leading to premature shear stress failure of a nut in terms of design features such as the root diameter?
 
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The load is transferred by stretch of the bolt and compression of the nut. The load cannot be uniform because each turn of engagement removes load, so the next thread doesn't have as much to generate elongation. It doesn't take many turns before all the load to be removed from the bolt and so no load is available to be transferred to the nut. 3-4 turns is about it; sometimes bolts are given a tapered internal relief to better spread the load, but typical solid-section bolts won't.

That formula is for a screw failing by pulling the screw apart by tensile failure, not stripping the threads out of the nut by shear failure.
 
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Thanks for the clarification. Is the compression of the nut due to the radial force exerted by the bolt? Which is perpendicular to the tension force.
 
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2 diameters of thread engagement, even in pretty crappy materials, permit a US Socket head cap screw to be fully tightened with little likelihood of failure.

Or so says Holokrome .
 
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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? but what happens when the nut has less than 4 fully formed thread? such as 3.5
 
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Does the 4590 lbs of tensile load come from NASM1312-8 ?

1/4" - 28 Grade 8 bolt is rated for about 3300 lbs clamp load
1/4" - 28 Grade 5 bolt is rated for about 2300 lbs clamp load
Probably less than proof load, that would be a little less than ultimate yield.

What is the installation torque and predicted loading ?
 
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4590 was based on old testing results. I am hoping to find a standard that I can use. NASM1312-8 is just a generic tensile test procedure for fasteners. I know its for low torque application.
 
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ISO has bolt and nut joint failure philosophies that sometimes seem to vary from US philosophy.

ISO 898-2
7 Failure loads for nuts with nominal height > 0,50 but < 0,8D
The values of failure Loads given in table 7 for guidance apply to different bolt classes.
Bolt stripping is the expected failure mode for lower strength bolts, while nut stripping
can be expected for’ bolts of higher property classes​

Recent research has, in addition, shown that a nut tested with a hardened
mandrel is capable of sustaining a higher load before stripping than when tested with a bolt of the
appropriate property class.​

Failure of threaded fasteners due to over-tightening can occur by bolt shank fracture or by stripping of
the threads of the nut and/or bolt. Shank fracture is sudden and therefore easily noticed. (NUT) Stripping is
gradual and therefore difficult to detect and this introduces the danger of partly failed fasteners being
left in assemblies.
It would therefore be desirable to design threaded connections so that their mode of failure would always
be by shank fracture but, unfortunately, because of the many variables which govern stripping
strength (nut and bolt material strengths, thread clearances, across-flats dimensions, etc.), nuts
would have to be objectionably thick to guarantee this mode in all cases.
A bolt or screw of thread M5 to M39 assembled with a nut of the appropriate property class, in accordance
with table 2, is intended to provide an assembly capable of being tightened to the holt proof
load without thread stripping occurring.
However, should tightening beyond bolt proof load take place, the nut design is intended to ensure at
least IO % of the over-tightened assemblies fail through bolt breakage in order to warn the user that
the installation practice is not appropriate.​

 
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