Bolt Fatigue Strength

[Skyhawk989]

Hello,
I have two questions:

I have a bolt made out of CRES material (type 302), can anyone direct me to where I can find fatigue data for such a bolt material. MMPDS does not have and the aerospace materials handbook isnt much help.


thanks in advance
SH

 

[TVP]

This is a very complex subject. The following thread discusses some of the issues involved:

thread725-178349

In order to understand your specific fasteners, you will need to know how the bolts will be manufactured: cold formed vs. hot headed vs. machined from bar; rolled threads vs. cut threads; strain aged vs. no post threading thermal treatment. Type 302 stainless steel strain hardens considerably during cold deformation, so it has a huge range of static/fatigue properties. Annealed 302 has a yield strength of only several hundred MPa while a heavily cold rolled spring temper 302 has a yield strength in excess of 1800 MPa.

 

[Skyhawk989]

thanks, the bolt is cold headed, threads are rolled. However, this relates to the threaded portion of the bolt for fatigue strength. I am more interested in the bolt head fillet.

SH

 

[israelkk]

What is the spec of the bolt (NAS, MS....)?

 

[Skyhawk989]

bolt spec is NAS1351C

 

[israelkk]

The procurement specification for NAS1351 is Federal spec FF-S-86. You can download it at

http://assist.daps.dla.mil/quicksearch/.

The spec doesn't give any info about fatigue but only the strength properties of the steels that can be used to make the bolts. Earlier versions specifically called for low alloy steels such as 4130, 4140, 4340, 8640 etc. However. the current version doesn't even specify the alloys just that these alloys when heat treated per MIL-H-6875 will meet the strength properties. To calculate fatigue properties you should look at the properties of the alloy the bolt is made off at the specified strength level.

 

[Skyhawk989]

Thanks, israelkk. However, thats the first thing I did. I looked at FF-S-86, which references 300 series as material callout with Ftu_min = 80 ksi. I could alternatively generate a fatigue curve based on existing literature but I wondered if there actual test data available.

 

[israelkk]

I don't believe you will find any. Every torqued bolt will behave differently depends on the clamp rigidity and type of loading. There is no point to give bolt fatigue data because a bolt is never loaded by itself. It is generally used to clamp parts at a specified load (bolt tension). The overall clamp rigidity and the load (bolt tension) induced due to clamping torque in the bolt will dictate the fatigue life.

 

[Skyhawk989]

israelkk,

can you advise me as to how these NAS 1351 (CRES) bolts are manufactured? would they be annealed, then cold worked?
spec doesnt call out any heat treatmeant also...

SK

 

[israelkk]

FF-S-86 on paragraph 3.1.2 specify the NAS 1351 CRES bolts alloys according to ASTM A 493. On paragraph 3.2.3 it gives the mechanical properties. From this you can find if it is an annealed, 1/4 hard, 1/2 hard, etc. From the properties it seams to be annealed.

Those alloys cannot be heat treated or cold rolled after the manufacturing of the bolt. Usually the thread is rolled and head is cold upset so there will be some cold work on the thread and head but rest of the bolt will have the properties of the base material.

 

[dimjim]

Try the manufacturer. They should have some data
on fatigue data.

 

[Skyhawk989]

I dont have ASTM A 493, is there a copy I can get online?

dimjim, I already spoke to manufacturer, they had nothing.

 

[dimjim]

Is there a QPL for this spec so that you can
check other vendors to see if they have this
information?

 

[CoryPad]

You can obtain ASTM A 493 online for $30 from the ASTM Internet site:

http://www.astm.org/cgi-bin/SoftCar...astm.org/DATABASE.CART/REDLINE_PAGES/A493.htm

Regards,

Cory

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