unstable-crack-mechanismUnstable crack growthFatigue crack in thehub boreCracks become unstable, i.e. a forced fracture occurs,when more energy is released during crack growth thanis necessary for the crack to progress.Symbolic depiction of the elastic energy that is stored in the engine partunder the operating loads.Crack growth1,234During crack growth, the material opens elastically. This kinetic energyis then transferred into splitting the material (crack growth).