Shaft Failure

[user71]

Hello,
Anyone care to look at this failed shaft? - I'd like to know if it is fatigue or something else that might cause it to break like this?

I'm a little baffled because I can't see any beach marks or similar looking breaks from any books I have. It is 1045 material 1" diamter. It's load history is not known but is subject to bending and torsion.

Failure occured at a step down to 1" with a fairly tight radius and pretty heavy tooling marks...

I'll upload another picture in a bit

Thanks

 

[user71]

Second pic

 

[user71]

Last pic.

Thanks

 

[metengr]

user71;
Based on the very limited background information you provided and the attached photos, it appears to me that in the first posted photo toward the left side of the shaft diameter there are two deep- looking corrosion pits that may be the suspect failure initiation site. This is pure speculation.
The lack of ductility along the fracture surface and the rather flat fracture appearance suggests either low cycle fatigue crack propagation or a notch sensitive material that failed under dynamic load conditions in the presence of a local stress riser (corrosion pits). Despite the rubbing damage between mating faces of the fracture surface there does not appear to have been any pre-existing crack because based on the extent of surface corrosion away from the fracture surfaces, this would have been apparent. That is all I can tell at this time.

 

[TVP]

I agree with metengr that LCF or brittle fracture could be possible failure modes. The flat, almost featureless fracture surface is very reminiscent of some torsional overload failures that I have seen, so I would add that as a third possibility.

 

[user71]

Thanks for the replies.
One thing I could add is that the shaft is rotating and failed in a matter of hours - about 200,000 cycles.
Just worked through possible stresses and looks like the possible bending stress is much higher than torsional stress.
There is a 3/8" weld about 3/8" away so perhaps it is brittle given that it is 1045. I would have thought it was out of the heat affected zone, but I'm not really sure.
Might be time to send it for analysis to get to the root cause.

Thanks for your time, much appreciated.

 

[stucathome]

1045 is medium tensile. From the pic it's too tight a rad for the grade. Chances are it's hot rolled as well, not normalised.Either increase radius to 1.5d + or use a different grade