Very nice writeup... I agree completely on every point. In my previous post I was simply exploring the possibility of fracture occouring where the polycarb top is affixed to the copper (this is usually done with four sockethead type bolts) Beacuse of their (polycarb and copper) unequal thermal expansion coefficients, stress will result from this... however, as you saw in my calculation, even assuming that the copper doesn't expant at all (which is untrue, thereby giving us a worst case scenario) inadequate stresses are produced to have any effect... somewhat off-topic, but also it is good to have eliminated this cause from our list.
You do however bring up a valid point. WE NEED TO FIND THE CRACK TIP ENERGY for polycarbs in order to continue this analysis on a scientific basis.
Also, we should focus on crack initiation. Crack tip energies are exponentially lower than crack initiation energies. If we develop a proper procedure to manufacture that top without initiating any sort of microcracks (or a method for reselaing them, with methylene chloride, for example), the whole crack propagation issue is out the window!
Note: Fiber composites and acrylics have much higher hardness on the Rockwell scale (or any other applicable hardness scale for that mater), resulting in a much steeper stress - strain curve. The polycarb curve is much longer and smoother because of the material's relatively high elasticity.
Again, we need more information on the material, namely we need numbers.
Hardness
Modulus of elasticity
"Toughness" (can be calculated from the above values, or using Rhiemann's sums to find the area under the stress - strain curve until failiure)
Stress Strain curves
Ductile to Brittle transition temperatures
Crack tip energies
Crack initiation energies
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