# Notch sensitivity in a threaded fastener

• posted

I'm doing a fatigue analysis on a threaded fastener. It's 1-14UNS-3B threaded, and experiencing an axial force and a bending moment. I'm using 2.8 for the stress concentration (it's a higher grade fastener with rolled threads). However, I'm having trouble with estimating the notch sensitivity. I'm probably being way too concervative with .8 as the radius of the thread is probably much smaller than .01". Any insights or advice?

Thanks in advance, Dave

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• posted

First, I have to wonder how much bending there actually is across the threads. If it's a bolt in single shear, treat it like a fixed guided beam and find the moments at the ends. Using distributed loads instead of concentrated loads will yield lower moments.

I believe the radius for root radiused UNR fasteners is p/8, where p is the pitch (1/14 in here). That gives a radius of 0.0089 in. If it's a regular UN fastener the radius will be smaller.

The book Mechanical Engineering Design by Shigley has a figure for notch sensitivities for steels under reversing axial/bending loads. Here's what I get for a radius of ~0.009 in.

Ultimate Strength q

200 ksi 0.86 150 ksi 0.73 100 ksi 0.61 60 ksi 0.47

Most likely the q for your fastener would be 0.73 or less.

Also, if you considered bolt preload, remember that does not reverse with the other loads. There's a non-zero mean stress there. An equivalent reversing stress can be determined for that if needed.

Good luck.

Jeff F.

• posted

Hello. You get this stuff or what?

• posted

Yeah, I did... thanks.

I used the numbers you gave me to get in the ballpark, but ended up approximating notch sensitivity by the "peterson model", where:

q=1/(1+(a/r)) and a=.001*(300/Sut)^1.8.

The final result was a 'q' of about 0.63 or so. There is some bending across the threads, as it's an adapter that connects an actuator to a rod end.

I appreciate the > Jeff F> >

• posted

Good. Just wanted to make sure my post made it to news servers other than the one I'm on.

I ran across that formula once and had forgetten it. Good reminder thanks..

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