Define Mechanism of Rotary Instrument Fracture - Cyclic Fatigue

Occurs unexpectedly and without any visible signs of plastic deformation o An instrument rotating freely in a curved canal is subjected to compression and elongation (tension) simultaneously, resulting in work hardening and metal fatigue lead to fracture

NOTE: During manufacture the machine of grinding can cause minute crack that is not visible to the eye... but I can see it under the microscope. This crack when insert it into curved canal but not tight canal .. with rotation movement there is repeated compression and tension eventually well lead to fracture of the file .. even if the file is new ( 1^st use)