**2.1 Fracture control concepts**

A few different fracture control concepts are applied in railway systems, and one of them is damage tolerance concept (Zerbst et al., 2005). Within the frame of this concept, the possibility of fatigue crack growth is basically accepted. The aim is to prevent the crack to grow to its critical size during the lifetime of the component, i.e. to estimate number of cycles to critical crack size. In fatigue, crack extension is expressed as a function of stress intensity range *∆K* and the crack extension rate, *da/dN*, whereby *da* denotes an infinitesimal crack extension due to an infinitesimal number of loading cycles *dN*. The basic idea is that the largest crack that could escape detection is presupposed as existent. After that, the initial crack can extend due to various mechanisms such as fatigue, stress corrosion cracking, high temperature creep, or combinations of these mechanisms. Such a failure process is visible, and catastrophic rail failure can be prevented by regular examination of the top surface of the railhead. Maintenance methods (lubrication and grinding) help combat the wear and rolling contact fatigue phenomena referred to in local parameters. By applying these methods appropriately, maintenance costs can be reduced (Vitez et al.,2005). Rail grinding prolongs rail service life by preventing the emergence of defects or by delaying their development, preventive grinding to improve the quality of the running surface of newlylaid rails and corrective grinding to remove rail defects that have already developed by reprofiling the rail to optimize wheel/rail contact.
