Well we went out to our local Big5 and bought a 75cm workout ball that was rated for 1000 lbs pressure on it, we inflated it about 8 in over the "recommended" maximum and it popped (into 2 pieces...) , so we still don't have any balls for testing
Edit (Update): Sigh... so we actually went back to the store and managed to replace the ball, claiming it was "defective", and that it broke when we sat down on it (into 2 pieces?). Surprisingly, this worked and since they were out of stock on the one we bought (on sale for 15$), we got a slightly different one (30$ value), and actually got a dollar back
. Unfortunately, when we were testing our prototype lift mechanism, our new 75 cm ball hit an edge of the sheet metal we were using, and popped also. Hopefully, the people in the store wont recognize us when we go to return it again tomorrow...