×
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

Fastener Failure Modes

Fastener Failure Modes

Fastener Failure Modes

(OP)
I have a question regarding fastener failure modes.  I was reading through my "Spacecraft Structures and Mechanisms" textbook the other day and read the following:

"In tension, a threaded fastener with a suitable head will fail first in the threaded section, usually at the thread nearest the head."

They then go on to show how the ultimate tensile stress for the fastener is established based on this failure load as a function of the tensile area of the fastener.

I've never heard this before.  I always thought that the Ftu for a fastener was that of the material and you checked the thread shear strength separately using Fsu for the material (and check capability of first couple threads).

Has anyone heard of this before?  That is, that the Ftu for a fastener is some contrived value that actually reflects the threads failing rather than the tensile ultimate strength of the fastener material itself?

RE: Fastener Failure Modes

It is not the thread that fails, but the net section resulting from the cutting of threads. It fails as the first couple of threads because of the transition stress concentration at the interface between the shank and the threaded section.

RE: Fastener Failure Modes

(OP)
Thanks for your response Ron.  Would this basically mean that to establish the tensile allowable for a fastener you would need an Ftu specific to a given fastener (rather than just looking up the Ftu for the parent material)?  That is since the tensile allowable would be a function of not only the parent material, but also the thread geometry?

RE: Fastener Failure Modes

Isn't it just the material properties and the reduced section at the threads?

RE: Fastener Failure Modes

As OCI says, it is just the material and the reduced cross section.  Ftu is a material function, not a geometry function.  In order to apply Ftu, you only need to know the material, then compute the net cross section.

RE: Fastener Failure Modes

(OP)
Thanks for you input everyone!

RE: Fastener Failure Modes

No - the fastener tensile strength comes from test data where the fastener system is loaded in tension.  This is not a contrived value.  The allowable strength is typically given in terms of load for each fastener diameter (not as a stress) - at least in the aerospace industry; don't know about the civil field.  And no, its not just the reduced section at the thread times the material Ftu as that does not account for the stress concentrations.
 

RE: Fastener Failure Modes

With high strength steel fasteners (such as ASTM A325), the allowable tensile stress on the fastener is modified so the gross (nominal) area can be used for design.  This way, you don't need to know the area at the root of the threads.

DaveAtkins

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