Ultra high strength bolt experience?

[WKTaylor]

Folks...

Recently encountered a unique part:

1.2500-Dia Threaded stud. It has precision ground shank ~1.5-long with 1.25-long threads on both ends [~4.0-L ttl]... made from 4340M [4340VAR] steel HT270--300-KSI FTu [~170-KSI FSu] with Cd-Ti alloy plating. The aerospace quality threads were specified as ROLLED.

The stud is used in a critical shear joint on a unique acft.

Not surprisingly a few of these studs have failed thru the threads due to hydrogen embrittlement.

Now I have seen/used 260-KSI steel, 300M, MP35 and MP159 bolts [or parts with threads]... but 4340M 270+-KSI fasteners were beyond my threshold od sanity.

Anyone else know-of steel alloy 270+-KSI fasteners in use.... or for that matter "any-alloy" 270+-KSI HT fastener-items [bolts, pins, studs, etc] in service?

This part is very scary to me...

Regards, Wil Taylor

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[israelkk]

Under what specifications those studs are made (NAS, MIL, MS)? Were they tested for hydrogen embrittlement removal after manufacturing as the Ni-Cd spec requires? Do you have the C.O.T and batch number of those studs?

 

[MikeHalloran]

If it were used as a shear pin, with the threads just for retention, I might be okay with it.

If it's intended to be highly stressed in tension, e.g. to maintain a clamp load, then those threads scare me too.


Mike Halloran
Pembroke Pines, FL, USA

 

[WKTaylor]

MH...

This is a "pure shear" application.

Torque on the nuts is a lowly 4,000-in#. Most tension rated nuts this diameter on a high strength pin would have been torqued 11,500--14,000-in#.

As far as the incident photos indicated, the pins with the failed threads barely moved... which is a good thing.

Note: I am concerned RE the "health" of the other stud-pins in each joint [LBL, RBL].

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

 

[CoryPad]

Wil,

You are wise to be scared of this part. While 4340 steel has reasonable fracture toughness, even with low-temperature tempering like yours, the electroplating will add hydrogen into the part and make it susceptible to environment assisted cracking (EAC).

Under the most favorable circumstances, the material at that strength level would have a plane strain fracture toughness of ~ 40 ksi[·]in^0.5, but the K_I,EAC would be closer to 7 ksi[·]in^0.5. That would translate into an allowable flaw size of around 0.0002 in (5 micrometers).

Those parts should be inspected routinely and replaced with a more suitable material as soon as possible.

 

[WKTaylor]

CoryPad

I suggested 'they' consider redesign using AirMet 100 [900F HT] steel or Ferrium S53 CRES alloys; made per an honest UHS fastener fabrication detail spec such as NASM8907.

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.