What is elastic fatigue

Under fatigue we understand changes in material properties that are caused by constantly changing loads (e.g. vibrations and oscillations) and that, in extreme cases, cause a sudden and often catastrophic break.It is crucial (and initially incomprehensible) that fatigue can also occur if the amplitudes of the vibrations, and thus also the stresses occurring in the material, remain far from the yield point, so that actually only deformation should be present.When plastic deformation takes place, it is clear that we must expect a dependence of the fatigue phenomenon on the frequency and the amplitude of the alternating load. Even in the simplest case of permanent load with constant frequency and amplitude, complex behavior is to be expected, which of course will also depend on the structure of the given material.If the frequency and amplitude fluctuate over time, the matter becomes completely confusing, as the effects cannot simply be added up.If a material that has lasted for years and of which we are certain that it originally did not contain any microcracks suddenly breaks through fatigue without warning and without major plastic deformation, we must assume that microcracks have formed over time, which slowly have grown until one of them has reached the critical size defined for the present load.So we have to look primarily for mechanisms that contribute to the formation and slow growth of microcracks Alternating load (and with alternating loads).Unfortunately there is no answer. What is observed when a specimen is cyclically deformed in fatigue experiments is a maximum tensile strength or breaking stress that decreases continuously with the number of cycles R.M. (that was the value of the maximum in the stress-strain curve). If one carries for a given amplitude of the vibration load R.M. against the number of cycles, one obtains the so-called - Curve. It typically looks like this:
The mechanism of fatigue is extremely complex and far from being understood in detail. However, two fundamental mechanisms can be distinguished in principle: 1. Processes in the forward and backward movement of dislocations which lead to fundamental changes in the arrangement of the dislocations in the course of load changes.2. This special offset arrangement creates microcracks, mostly on the surface of the material, which grow until the critical crack length is reached at one point and the material breaks.Let's take a quick look at these two mechanisms in an extremely simplified and idealized model. Let us first consider how the dislocations are present in the first stress cycle at the maximum of the amplitude. Since we are working below the yield point, they are anchored to obstacles and do not move. If the load changes from tension to compression, the dislocation wants to run back, and it can. Because the excretions to which it clings when subjected to tensile load block the backward movement. It is now running back until it is stuck on other excretions again.In the case of a single dislocation, it looks like this, for example, when anchored to excretions:

© H. Föll (MaWi 1 script)