Theoretically, if glass dropped and the stress incurred was just less than that required to shatter it, would the next drop, no matter how small, break the glass? If not can it continue to take infinitely more stress damage as long as they each individually do not surpass its stress limit, even though their sum does?Note: I am asking this with the real life application of the glass backs of iPhones in mind
Glass is made harder and stronger through heat tempering. (Both Kiln and Chemical processes are used) Repeated drop-ball tests do not minimize the integrity of the heat tempered glass. Chips, scratches, or virtually invisible hairline fractures weaken the integrity of the glass. If you drop it enough, from the right height, or against the wrong surface, It Will Break!
You should investigate what the amount of energy is required to cause a defect either in manufacturing or physical damage to initiate and propagate a crack. The defect may be improper heat treatment, a foreign inclusion, a void in the glass matrix, a nick or scratch in the casting process or in handling in the production line, for example. Each cycle of dropping could potentially have enough energy to further propagate the crack until full fracture is achieved, You may want to investigate fracture mechanics. .
