SCREW FAILURE

[CESSNA1]

HELLO ALL: Attached are two images of a 10-32 zinc plated steel screw that failed under a sidewise load. The load was applied under the head and the screw broke at the top of the threads in a rivnut. The part that bothers me is the hole in the middle of the screw that is seen in the endview. Your help is solicited. The screw material is only known as steel.

 

[Ron]

Could be an anomaly in the drawn wire, but more likely a fracture occurred during the "heading" process of screw manufacturing.

Is this a repetitive failure? Has it occurred more than once and with multiple screws?

 

[CESSNA1]

RON: Thank you for your reply. There was a failure of six of these screws simultaneously. There is no information about other screw failures in similar systems. It is believed that in service these screws loosened and ultimately failed. The failures occured approximately 0.5 inches below the underside of the head, This is the only screw I have for analysis. This is the first time that I have seen this. What about the microstructure of the metal? Itr seems odd to me, almost like a casting.

I have attached a magnified side view of the same screw. I believe the flattened threads occurred during the failure

Thank you
Dave

 

[unclesyd]

I have to go along with Ron concerning the possibility of the problem stating with the header wire as this is quite common. Another very good possibility is Hydrogen Embroilment from the plating process.

Were these screws bought under any specification?

Do you have the plating procedure?

What is the environment the screw sees?

The problem from wire defects stems from someone not trimming the ends of the bulk wire.

 

[Ron]

CESSNA1...even though the failure surface looks like a casting, I doubt that it is, since machine screws are not made that way, particularly small ones.

The failure is a bit too far from the head to be a head forming failure or crack induced by that process. I go back to a wire anomaly.

Failure looks like "single event" overload, as there is no indication of fatigue. Also, in the second photo, it looks like a tension failure with the final separation being a tearing at a thread root.

A polished section (longitudinal) would show the wire anomaly.

 

[TXMEEN]

I am not seeing much necking in the threaded portion next to the fracture surface. I do see thread edges that are flattened in the second photo you posted. Do you have a shear load applied to the threaded portion or is it possible that the flattening of the thread edges could have occured after the failure?

Do you have any photos of the pre-failure installation?

 

[CESSNA1]

THANK YOU ALL FOR YOUR RESPONSES.

UNCLESYD: I do not have the specification or the plating procedure. The environment is just ambient conditions.

TXMEEN: The applied load is a shear load. A .125: X .5" aluminum bar with six .190" holes is positioned under the head and the screw provides a clampinmg force. I believe the flattening of the threads occurred during the failure. The screw failed and as it was pulled out it hit the side of the rivnut, flattening the threads. There are no pre-failure photos.

Dave

 

[tbuelna]

CESSNA1,

I can't say what caused your fastener failure. But in the future, you might want to remember that it's poor design practice to put the threaded portion of a fastener body in shear.

The thread flank surface in that picture also appears to be somewhat abused. Was it possible that your fastener's thread length was insufficient, and the thread bottomed out in the rivnut before the parts were clamped up? Such a condition would result in the screws taking the applied load in bending, rather than the load being taken by friction of the clamped interface. This situation would be made worse by the stress concentration created where the threads gouged each other at the point of the screw's thread runout. Which also would likely be the point of max stress for a screw in bending.

Hope that helps.
Terry

 

[dhengr]

Cessna1:
You say you have a shear force on the screw, sidewise loading; plus tension, torsional shear and more than likely some bending if you have a shear force. The thread root is a reentrant corner, a stress raiser. That kind of screw failure isn’t that uncommon. You didn’t say, but I’ll bet the screw broke a thread or two into the rivnut. Change your design to get the threads out of the high stress area. Or, figure out how to take a few of the forces by a means other than through the screw cantilevering .5" out of the rivnut.