The determination of the fatigue crack propagation threshold offers a range of materials utilization in engineering, in application to projects that are tolerant to damage. [delta]Kth is a tool that allows the establishment of a loading criterion under which the cracks will not grow significantly during the service operations of the component. For its determination to be widely applied, it was necessary to establish and study the factors that could influence it positively or negatively. Among them, the microstructure was of great importance because it had to be controlled from the material manufacturing to the placing putting into service of a finished component. The aim of the component and the stresses to which it is subjected when in service determine the material selection. Thus, basic characteristics and minimum requirements were listed which have attend the safety and has reduced cost-benefit ratio. Materials (SAE 4340 and SAE 4140) were selected whose chemical composition and mechanical properties that were similar to DIN 42CrMo4 so that they could serve as an alternative in the use of crankshafts to propose a solution in the practical problem which they were breaking well below the limit fatigue life that they proposed. The results obtained were satisfactory because in the fracture toughness tests of one of the chosen materials (SAE 4140) [delta]Kth of 12.48 MPa[square root]m was obtained and in the reference material, [delta]Kth(average) of 6.1 MPa[square root]m. The observation of the microstructure of the reference material may indicate, with restrictions, as a corroboration of the result presented in the [delta]Kth, because occurrences of dispersed inclusions in the steel matrix.