Tension allowable NAS1204

[dcascap]

Hi,
I'm using the NAS1204 bolt in a design and I encounter the following:
The tensile stress allowable vs the tension force allowable on this spec significantly differs (for around a factor of 1.3) if we compared it by converting the stress allowable to equivalent tension allowable, as F=sigma*A with A the tensile stress area.
I think it can be due to the countersunk head, which will lead to a different type of failure (stresses in countersunk are quite difficult to assess), and which it reflects in the tension allowable, assuming that the tension allowables are tested values.

Do you know if my reasoning is okay? or do you have a better explanation? Thanks in advance!

Attached a screenshot of the spec .

 

[SWComposites]

its either the head failing before the shank, or the bolt threads failing before the shank, or the nut failing before the shank.

 

[btrueblood]

Yes, the stress concentration from the countersunk head (and the internal cavity for the phillips drive) lowers the tensile strength of the fastener. Similar to the lower stress capacity of button-head socket cap screws.

 

[dcascap]

Thanks for your answer, this clear my question

 

[WKTaylor]

Ok... I had to look this bolt series up...

NAS1202-to-1210 is at Rev11, procurement spec is NAS498 [Rev14]... except for tabulated ultimate tension values, down the far-right column of the table shown.

This bolt has a flush-tension-head [FTH] mated with short threads... so working tension rating is likely reduced since a reduced height nut is unable to supporting the tension rating of the head. By definition this bolt should be restricted to shear loading... and NOT be purposefully loaded in tension... other than by specified [shear] nut-torque-tension. Although threads are per AS8879... it appears that rolling is prescribed... may be accompanied by machining and grinding for finishing [fillet/runout]. ALSO it appears alloys/metallurgy/heat-treatment is are older generation. AND the head-to-shank fillet does NOT require 'rolling for tension-fatigue improvement'.

This is a very old generation bolt... guessing mid-1960s... intended for structural Assy [in-place of lock-bolts with SL-shear-nuts]... or 'mechanical' when the threads are drilled for a cotter-pin/lockwire and low-height castellated nuts. Also plated alloy steel is generally undesirable to due long-terms corrosion [rusting] issues and cadmium/chromated plating toxicity.

Fab/forming, over-all, is vague in some aspects... unlike newer generation bolts made-per procurement spec NAS4002.

 

[dcascap]

Thanks! Yeah it was a question asked by a colleague to me, I'll check with them why and the purpose of the bolt if they ended up using it