Roller Latch Shaft
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Overall - 14X
Stack: 0.010 inch increments
Roller Latch Shaft at 14Xmagnification
Closeup of oldest cracked area - 50X
Stack: 0.001 inch increments.

Roller Latrch Shaft Fracture Detail at 50X magnification
Closeup of Central Area of Fracture - 50X
Stack: 0.0005 inch increments.
Roller Latch Shaft Fracture Detail at 50X magnification
I remember finding this thing - it's got a loosely fitting steel roller between two washer securely affixed to a splined shaft.  I forget what its exact function was.  It looks like a part of a door latch.  The roller is quite worn.

The shaft failed in two stages.  The first stage formed an open crack which became quite rusty as seen in the upper row of images.  The second stage was low cycle fatigue without ongoing corrosion.

It would be quite instructive to make a cross section of the shaft to see the microstructure of the steel and find out if there are other, unopened cracks.