The following work carried out a theoretical study on the mechanics of the fracture to analyze the critical crack size that can have a certain material. After analyzing the critical crack size the work analyzed how such a defect propagated until it reaches the permissible size. In this work three different types of materials (SAE 1020, 1045 and 4340 steel) were analyzed in three different situations, one in which there is a central crack in the material, in the other there is a lateral crack and finally two lateral crackings. The work will verify how a certain axis of automotive transmission behaves.