A sub-micron fracture is a minuscule fracture that can be caused by neutron fatigue.
A sub-micron fracture in the metal-casing of a dilithium chamber hatch replaced at Earth Station McKinley in early 2367 was the cause for an explosion in the USS Enterprise-D's dilithium chamber, and not sabotage by the Klingon spy J'Dan, as was originally suspected. The fractures were detected by means of microtomographic analysis. (TNG: "The Drumhead")