However, there was a problem with this particular piece of polycarbonate in this particular door. When it was installed, one of the bolts was overtightened slightly, causing a small, too small to notice, crack that radiated barely a millimeter out from the bolt hole. This crack was hidden by the backing plate. Over time, what started as a millimeter lengthened little by little with each pressure cycle of the airlock. Once the crack extended past the silicon seal, it had begun to vent air to the Martian atmosphere, ever so slightly.

Air: Generation Mars, Book One

Coming in October

(image: https://doi.org/10.1016/j.polymer.2013.04.036)