Pressure Test #2

Just a quick update today.  Yesterday I replaced both 3mm thick windows with 6mm thick ones and let the sealant dry over night.  This evening after work I resumed pressure testing.  At about five meters down (approx seven psi) the front window cracked and the hull lost its vacuum.

I made a crucial mistake with this window, and that was to use the old 3mm window's holes as a template to drill into this one.  I didn't think about the fact it would curve slightly different to the 3mm one, being a lot thicker, and of course some of the holes didn't match up toward the end of the window.  I then decided just to clamp it down, seal it up and test it anyway.

I concluded that it cracked due to a few reasons;

1. it didn't have any bolts in the last few holes so the forces may not have been equal across the window, also this section may have flexed more against the seal under pressure.
2. the forces involved in the straps holding the window to a curve would have not been there in the final build, and...
3. the main reason of using poly-carbonate is that it can be flexed quite dramatically and not break, so even under normal atmosphere pressures the whole window is under a lot of stress just simply being held in a curve.  That, coupled with the pressure difference would have added to why it cracked.

For next time, I plan to get the piece remade, and if possible have it heated and curved to the right shape.  I will make sure to drill the holes by holding the window in place, and I will put every single bolt in!

At least this will give it a good chance for next time.

Fig. #1.  The crack occurred when the window had approx 780kg across it at roughly five meters down.

Fig. #1
04/01/2017