×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Bolt shear capability independent of preload?
5

Bolt shear capability independent of preload?

Bolt shear capability independent of preload?

(OP)
I've recently heard of some testing that indicates that the shear capability of a bolt is effectively independent of the applied preload, for "normal" values of preload. The tests have been done with a bolted double shear joint, with the faying surfaces of the clamped members lubricated to minimize the clamping friction. They've found that varying the preload between 50% and 85% of yield has around a 1% influence on ultimate load capability.

I haven't thought it through yet, but on the surface, this doesn't make a lot of sense to me. Clearly, the body of the bolt is under combined tension and shear stress, and that should reduce the capability. Maybe it's the little bit of friction remaining between the plates, maybe there are some nonlinearities that are playing havoc with the linear lens through which I view the world.

Does anybody have any insight on this? I apologize, I don't have a reference for the testing.

--Tom

RE: Bolt shear capability independent of preload?

This is not a little friction. Even with lubricated joint the friction coefficient can be ~0.2. Recall that it is on both sides of the lug therefore, the total friction force is doubled. In case of 0.2 the friction force is 40% of the clamping force.

RE: Bolt shear capability independent of preload?

(OP)
Thanks for the link.  It seems like it's saying that the shear stress on the bolt near the ultimate failure elongates the bolt sufficiently to reduce preload.  This seems like it would be sensitive to the relative stiffness of the bolt and the joint - a stiff bolt and a compliant joint would get more benefit from this effect than the reverse.  It also seems like a short grip length would be more beneficial, because a short region near the joint winds up in shear, and whatever strain you get in this region, the actual change in length will be small; a long joint would require more actual change in length to alleviate preload.

I guess I need to look up the references cited there.

RE: Bolt shear capability independent of preload?

I think they way to envision this is that fastener interaction between axial and shear stress is important up to the yield point (e.g. von Mises equivalent stress), which is widely documented for fasteners (see same reference above and no-cost MIL-HDBK-5).  After yielding, the stress distribution in the entire joint has changed so much that the largest principal stress is the important factor.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources