Reply To: turn heat into electricity

[Lerner]

We need to be able to accurately calculate how to avoid the breakage of our alumina hat insulator as we move up to higher currents. So we’d like help from all you mechanical engineers out there to get us some data about the impact fracture of alumina under pre-existing stresses.
In the last breakage, we had inadvertently left a stress of about 1 MPA on the insulator. This is of course small compared with the static tensile strength of 267 MPA. During the pulse, the magnetic field imparts and impulse to the upper anode assembly which rises slightly and then comes back down. This then conveys an impulse (cushioned by rubber) to the insulator. Even ignoring the energy absorbed in compressing the rubber the total energy available from the motion of the upp0er assembly is only 0.02 mJ/cm^2, far less than the fracture energy of alumina which is 2.5 mJ/cm^2.
So the question is: is it possible for such a small impact, combined with a small pre-existing stress, enough to cause fracture? Are there tables showing fracture energy as a function of pre-existing stress for alumina? Or am I leaving something out of this calculation?