Fatigue is sort of a mix of plastic and elastic deformation. It can happen when a material is only being elastically stressed - but the mechanism of operation is still dislocation motion (like plastic deformation).

The dislocations that move in this case require much lower stress to move - ie. not all dislocations move at exactly the yield stress of the material (that's sort of an average). But in this case, dislocations move back and forth along the same path (the path of low resistance), and eventually form a slip band which can eventually lead to failure of the material.

This is referred to generally as high-cycle fatigue, ie. it takes a lt of cycles to cause failure, because the stresses are low, and SN (stress vs #cycles) curves are used to assess damage and predict time to failure (compared with a stress/strain curve used to predict failure from overstress).

Vibration is what they are referring to, so +1.5g followed shortly after by an (eg.) -1.5g, and no net position change.

When a metal is molten and cooling, it's atoms start forming bonds and binding together to form the lattice structure. But this cooling process happens simultaneously all over the structure. So all over, new lattices are forming in all different directions! The places where these differently-oriented lattices meet are called "grains boundaries" and they are the weakest points in the metals, where failure eventually happens.

But there are a few examples of perfect crystal lattices through a metal structure - the most well known (the only one I know of) is in Jet Engines. Jet turbines undergo so much stress yet need to be so reliable that they have developed manufacturing processes to make a whole turbine blade be a single crystal lattice. How they do it is a pretty closely held trade secret. But the advantages are huge:

• NO plastic deformation - since there are no dislocations/imperfections
• ~10x the material strength compared to the same metal cast normally

There are probably more but this isn't really my field of expertise, maybe someone else can add more..

There is a difference though, between bending a paperclip back and forth a few times so it plastically deforms and breaks - and fatigue.

The former is exceeding it's ultimate strength and breaking the paperclip. Fatigue is a different phenomenon caused by cyclic loading and not related to the ultimate yield strength - slip bands form and eventually cause a crack.

Seems like a technicality but they are two different phenomenon, and material health is asses completely differently when looking at overstress vs. fatigue!