Fastener thread shear failure 1

[mathlete7]

Hi, I have a question regarding fastener thread shear failure. I know that often when analyzing fastener threads its common to assume that all of the load reacts on the first two or three threads. I was wondering if its appropriate, when dealing with ultimate loads, to just claim however many threads as you need since as the first threads yield subsequent threads will pick up load (basically treat the threads the same as you would fasteners in a metallic joint for ultimate loads). The limiting factor at some point would obviously be the tensile allowable of the fastener itself.

Thanks for you help,
Dan

 

[rb1957]

how many threads in the nut ?

 

[mathlete7]

I don't have a specific application - I'm just trying to establish some guidelines. I'm just wondering in general if its appropriate to claim up to all of the threads at ultimate loads (similar to what you'd do with a joint with a bunch of fasteners) since the threads will be yielding and spreading load to subsequency threads.

 

[SWComposites]

Is this for a tension load on a fastener (along the axis)? If so, allowables for fastener tension are developed from tests, not analysis.

If you are concerned with a shear load on the joint, then the fastener length is selected such that the threads are not in bearing on the plates, and since the threads are not at the shear plane of the joint, the fastener (generally) will not fail in the threads (except for some cases where there is a bending moment on the fastener)

 

[mathlete7]

Correct, this would be for a tensile load on a fastener. Tensile failure modes of concern would be fastener tensile failure (Ftu exceedance) and fastener pullout due to thread shear.
I've found that in the aircraft industry, where they use lots of standard fasteners, there are allowable tensile ratings that are readily available that cover all modes of tensile failure.
It seems that programs I've worked on with space hardware, however, they often don't have good tensile allowables that cover any aspect of tensile failure for the fastener and people separately calculate the tensile allowable of the fastener and the thread shear allowable for the fastener.
Granted I've only been in aerospace for ~8 years so can't claim to have seen it all by a long shot.

 

[CoryPad]

Thread shear capacity is calculated. Here is one type:

thread725-35222