Air Tank Material Selection
There are two general modes of failure in mechanical components: "brittle" or "ductile" fracture.

"Ductile" fracture occurs when the material stretches (yields) like taffy before breaking apart.

"Brittle" fracture occurs when a crack propagates through the material "ripping" the component into pieces before much stretching occurs.

Materials will yield (stretch) unless they have the necessary conditions to crack. The "Fracture Toughness" of a material as well as the size, shape and location of any cracks or other stress concentrators determine if a material will crack under a given loading condition. What makes this difficult is knowing the information about the cracks in the material, an impossible task without expensive equipment like x-ray units and the fact that cracks may grow over the service life of the component due to cyclic stresses or creep.


Pressure vessels can contain huge amounts of energy in the from of compressed gas. If a tank were to break due to brittle fracture shock-waves through the material may cause the tank to explode into hundreds of pieces which would travel at incredible speed causing damage to the equipment or personnel in the area.

There are two methods used for making safe pressure vessels: Yield Before Break, and Leak Before Break.

The yield before break design method ensures that the pressure tank will yield (stretch) before breaking apart. This means you must insure that there is no yielding by measuring or observing the tank. For example, when a 2-liter bottle is over-pressurized it is easily apparent as the bottle expands significantly (before it explodes due to brittle fracture).

The leak before break design method ensures that even a crack as large as the thickness of the wall of the pressure vessel will still not allow the material to fracture. This means the pressure vessel will leak before the tank breaks due to brittle fracture, lowering the internal pressure and avoiding further propagation of the crack.

Results
So far, the PVC pipe survived all but the 12 foot freezer temp drop. What does this mean? Hard to say anything conclusively, but the pipe is stronger than I expected - but looks to get dramatically less safe at lower temperatures. More testing is required!