Bolt Fatigue Life

[AJohnson]

Juvinall and Marshek (1991 text) and Shigley and Mischke (1989 text) use a similar approach for estimating S-N curves of notched metals, based on tests conducted in smooth rotating members. The test result numbers are then adjusted for certain various factors such as: load type, member size, surface finish, notch type, etc... The texts include surface finish factors for notches (threads) that are machined, ground, hot-rolled or polished, but neither include a surface finish factor for a notch (thread) that is cold rolled. Anyone have a good reference to recommend?

Andy

 

[unclesyd]

I have never seen an actual number on the surface finish in the root area. I think it would have to a visual inspection, not a machine measured value.

The general description is that the root will have a flaw free and smooth surface and the root should blend in smoothly at the point of tangency on the flank. It is understood that any defect below the pitch line is detrimental to the fatigue life.

I have seen a call out of a 32 micro inch finish on the shank of high strength > 200,000 PSI fasteners.

On our H11 fasteners we use a binocular microscope (40X) to examine the roots.

You might check out SPS as they have done considerable work on fastener fatigue.

http://www.spstech.com

 

[CoryPad]

I haven't seen a factor like the one you are requesting.

In ASTM STP 1236 Structural Integrity of Fasteners, a comparison was made between machined threads and rolled threads in the low cycle fatigue range. At 10⁵ cycles, rolled threads could withstand 1.56 times the stress that machined threads could.

I recommend both ASTM STP 1236 & STP 1391, available at:

http://www.astm.org

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[aviat]

MIL-S-8879 (superceded by AS8879) lists a surface roughness no greater than 63 microinches Ra for external threads and 100 microinches Ra for internal threads in accordance with ANSI/ASSME B46.1. This a Mil Spec for a J thread or reduced radius type thread.

 

[yates]

Whilst surface finish is a significant factor in machined threads, it tends to be less so in cold-rolled threads (also extremely difficult to actually measure). Our experience in bolt fatigue testing has shown a greater influence of defects (which are usually laps) upon fatigue life than surface finish - in particular defects below the pitch line as pointed out by unclesyd. However, I would be wary of quantifying the influence of either of these two variables upon fatigue life - I cannot speak for surface finish, but in the case of laps, the determining factor has been shown to be the level of residual compressive stress in the thread root area and this depends largely upon the material and its work hardening characteristics. Example : in some titanium alloys, the residual compressive stress is also a function of the load application rate and profile during the cold threadroll operation.