Threaded Hole Stress Concentration - Bending Stress

[ManicalEngineer]

Good Afternoon,

My co-worker and I are working on an analysis that includes a steel tube in bending & axial loading. This tube has two threaded holes through one side (to fasten a small piece of rubber). The section of interest for this particular analysis is through the center line of the holes. We've assumed that we can treat that particular area as a plate in tension for determining the stress concentration factor.

My co-worker believes we can take credit for the bolt that will be threaded into the threaded hole, I disagree, my engineering intuition tells me that the addition of a bolt in the threaded hole does not negate the hole being there. Does anyone have any insight in to this? Also can someone confirm that we are barking up right tree?

Thanks,

Sean Bicknell

 

[WARose]

I agree with you. The bolt being there doesn't negate the fact you will have a concentration factor.

But does it matter? Is this thing being subjected to a high-cycle loading?

 

[ManicalEngineer]

WARose,

Thanks for your response! It matters because we have a specific Strength Factor (3) we need to hit for certification through our trade association/product certification organization. They require calculations for all components in the load path.

Thanks again

 

[dauwerda]

For tensile strength, typically you would check yield strength using the gross area and tensile rupture strength using the net section area. I'm not sure where you are at or if you need to follow a code, but AISC 360, available here,

https://www.aisc.org/globalassets/aisc/publications/standards/a360-16-spec-and-commentary.pdf

adresses this in chapter D2.

 

[wannabeSE]

When the element is in compression, the bolt in the hole can help. In tension, the bolt’s help is negligible.

 

[BridgeSmith]

Under the AASHTO spec, that's only true until the hole(s) take up 15% or more of the area in tension, then you're using a reduced effective area. Not sure about other specs.

 

[CARunderscore]

So, you have two things going on here:

1. A change in the nominal stress due to the change in net section.
2. Stress concentrations (maximum stress > average stress within the cross section itself) for calculating resistance to fatigue and/or fracture.

In the structural world, our codes generally don't care about small, localized changes in net section for the purpose of strength checks, as HotRod10 alludes to.

I'm not sure what your application is, but if you're worried about stress concentrations on the sides of the hole, a *pre-tensioned* bolt in a hole on a flat plate will improve fatigue resistance (and, I assume, fracture resistance) under tension, but it will still not be as good as there being no hole at all. Plus, I don't know how that translates to a hole in a curved surface, so I would just ignore this effect and consider the hole.

 

[DrZoidberWoop]

This is the kind of fun problem that can be solved in 30 minutes with a good FEA program. Intuition and standard practice are great, but sometimes a pinpoint analysis can lay all arguments to rest.

 

[ManicalEngineer]

Thanks everyone for your input, it was extremely valuable!