In this work is shown the design of a fatigue crack generator. This machine is used for generating a crack, which is sufficiently sharp to provide a valid result from the fracture toughness test for metallic materials. This precracking for fatigue make cracks in flexion specimens type Bend and stress compact type C(T) in agreement to the norm ASTM E399 - 90, E813 - 89 and E1290 - 89. This is necessary in the determination of the KIC, CTOD and JIC factors, for the characterization of the analyzed material. The functional principle of this equipment is based in charging cyclically the schemed specimens between the levels of maximum stress and minimum tension loads, the applied load is the result of the radial movement of a rigid bearing of balls, which is mounted in a double gear excentricity device. This excentric is designed in steel AISI 4340, with a maximum excentricity of 0.6 mm, which can be changed by the relative movement of the gear. The device consists of an engine with driving frequency, which transmits power to a shaft and that to the excentric. The structure was made in 1020 and 4340 AISI steel. Another of the advantages of the present design it's low cost and easy manufacture, allowing the implementation and use in research processes of materials characterization.