Skateboard Cap Screw (3/8-24 size):
The microscope is an Olympus SZH stereo; the camera is an Olympus e-Volt E-500.

This is an example of reversed low-cycle bending fatigue.  If you right-click on a selected image, you can then view it at full resolution in a new browser window.  The 8X image shows the arrest lines when so viewed.  The 64X images don't, because they were mashed against the mating faces before the screw broke.  That's why we can tell that the loading was in reversed bending.
Overall view at 8X magnification.
Stack: 0.010 inch per step.
Detail views (turned 90 degrees CCW) at 64X.
Stacks: 0.0005 inch per step.
Skateboard screw at 8XAnimation
Top edge detail at 64X magnificationAnimation
Bottom edge at 64X magnificationAnimation