Fastener Failure Modes

[mathlete7]

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?

 

[Ron]

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.

 

[mathlete7]

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?

 

[OCI]

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

 

[Ron]

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.

 

[mathlete7]

Thanks for you input everyone!

 

[SWComposites]

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.

 

[DaveAtkins]

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