The reason that a pre-crack is necessary is that fatigue cracks grow very slowly and may not be detectable without a starting point. Using a pre-crack allows the crack growth to be monitored over a period of time and allows the fatigue life of the material to be determined.

It is important to note that the pre-crack is not the same as the cyclic loading that is applied during the test. The cyclic loading is applied to the material after the pre-crack has been introduced, and it is the combination of the cyclic loading and the pre-crack that causes the crack to grow. The cyclic loading is necessary to induce fatigue in the material and cause the crack to grow, but it is not sufficient on its own to initiate the crack growth.

La prefisuración contribuye al proceso de crecimiento de la grieta y con ello al seguimiento de la misma dentro de la prueba de laboratorio.

If you want to measure fatigue crack growth (not fatigue crack initiation), the pre-crack must be obtained by submitting a sharp notch (as sharp as possible) to a cyclic load initiating a small fatigue crack extension. The cyclic load must not be too high in order not to modify too much the stresses at the crack tip.

I agree with Salaman Shahid. Additionally, precracking of a specimen prior to testing will help in conducting adequate cyclic loads. These loads will produce stress intensities sufficient to cause a crack to initiate from the starter notch and propagate to a length that will eliminate the effect of the notch.

The straightforward answer is to create a natural crack, which eliminates the notch effect.

Fatigue crack growth testing is used to determine the rate at which a crack will grow under cyclic loading conditions, and is typically performed on pre-cracked specimens. The pre-crack is necessary because it provides a starting point for the crack to grow from, and allows the crack growth rate to be measured over a range of crack lengths.

Using a cyclic load for fatigue crack growth testing is beneficial because it allows the crack growth behavior of the material to be studied under realistic loading conditions. The cyclic load can be varied in terms of the magnitude, frequency, and number of cycles, and the effect of these parameters on the crack growth rate can be investigated.

It is important to note that the presence of a pre-crack does not necessarily mean that the material is already damaged or weak. The pre-crack can be introduced into the specimen using a variety of methods, such as machining, drilling, or laser cutting, and is typically only a small fraction of the total thickness of the specimen. The pre-crack is simply used as a starting point for the fatigue crack growth test, and does not significantly affect the mechanical properties of the material.

Thank you everyone for your answers to this question.

I think the best explanation is provided by Naresh Reddy Kolanu

We do not need stress concentration with a notch (notch effect), what we need is to have a stress intensity factor from the natural crack. And this should be there at the very beginning of the FCGR test. This is the reason for having the "pre-crack".