cjf2019 t1_j99548b wrote

I think you might be confusing fatigue failure and strain hardening which are different concepts. Strain hardening occurs when plastically deforming a metal and generating dislocations which are a type of defect in the crystal structure. Dislocations require the atomic bonds joining the atoms around the defect to stretch in different ways, creating areas of compression and tensile strain. The strain fields between different dislocations tend to repel each other, so as more dislocations are introduced to the material as it is deformed, they have a harder and harder time moving around due to the repulsions which in effect strengthens the material.