I’ll present this below without the answer, in case you want to enjoy a little brain teaser. It is a solid and experimental mechanics problem that, while not terribly practical, I found very interesting:
A part fractures cleanly in two by brittle fracture (no plasticity) under the action of residual and applied stresses. You only have the broken part in front of you, no prior information.
What were the original residual stresses on the fracture plane?
Should this problem be solvable? The original stresses were of course relaxed by the fracture. There are no longer stresses to measure.
I won’t spoil the answer for you in case you want to think about it yourself. The solution is available at http://www.lanl.gov/contour/fracture.html, which proves the method on a fractured aluminum forging, with the results validated by neutron diffraction measurements. Just published in EFM.