Possibility of fatigue damage detection by non-destructive measurement of the surface hardness

The cyclic material properties of metals are reflection of their internal structure and chemical composition. The fatigue process inside material is the consequence of the cyclic plastic deformation localized in the points of stress concentration. The sleep processes inside material induced by cyclic loading gradually lead to creating slip curves, slip bands and then generating and spreading of microcracks. Those processes change the state and also properties at the surface of material. This fact led to the idea of fatigue damage level detection by measuring the hardness at the surface layers. The presented contribution will present results of surface hardness measurements of material specimens during cyclic loading. There will be also introduced the methodology of surface hardness measurement by Brinell´s method and hardness measurements of constitutive phases by Vickers method on steel specimens cyclically loaded. The results will be presented graphically in the form of dependency between that surface hardness respectively constitutive phases and the number of cycles to fracture. Measurements were made for several levels of loading stress amplitude and for two kinds of steel material. Obtained experimental results are analyzed and discussed in relation of other experimental measurement in detail.