Custom Fastener Design

[cyph]

Background:
I am designing a custom 10-32 threaded fastener for an underwater vehicle. The fastener head will be exposed on the surface of the vehicle and thus needs to be hydrodynamic (better than button or pan head), hence the need to custom design.

I am trying to determine the strength of the fastener/modify the design for strength in compromise with the hydrodynamics guys. I have done some very basic (I am no FEA guru) analysis in Solidworks Simulation and have been trying to compare it to hand calculations.

The problem:
The calculations for standard fasteners seem to have the max bolt loads derived from the "tensile stress area" but seem to not take into account any stress concentrations. Isn't there a stress concentration at the fillet on the bolt head?

My FEA results seem to match the standard calculation with stress concentration factor in a round shaft with shoulder fillet, which for my geometry I am getting a factor of 2. Is this round shaft shoulder fillet geometry an appropriate approximation for the fastener head/shank geometry?

In general, what is the best way to determine how the head geometry affects bolt strength?

We were also thinking of boring out the fasteners to reduce weight. How will this affect the stress in the bolt?

Thanks!

 

[unclesyd]

Checkout the last section of this course paper, there is some explanation of the reason for considering the head/shank radius.

https://engineering.purdue.edu/ME452/Homework/HW 12 Bolt Stresses/hw12sol.Fall.2010x.pdf

 

[TVP]

cyph,

Fastener standards have accounted for the underhead stress concentration and its effect on nominal tensile strength by requiring a minimum radius that is based on the thread size. For example, standard M12 fasteners with a variety of heads (pan, hex, hex flange, socket, etc.) have a requirement for the underhead radius to be 0.6 mm minimum. High strength fasteners for the structural (construction), automotive (con rod & other critical engine apps), and aircraft/aerospace markets specify the radius to be 1.2 mm minimum for size M12 (0.047 inches or 1.19 mm for 0.5 inch US aero). As long as you use these standard dimensions, you should not have any problems with underhead stress concentration.

 

[hydtools]

cyph,
instead of drilling a hole to reduce fastener weight, consider reducing the shank diameter to equal the minor thread diameter. This will allow you to create a larger head/shank fillet. Assuming the fastener length is sufficient allows doing this and retain sufficient thread length.

Ted