**2. Fatigue-induced structural changes in steels**

It is assumed that the basic factor describing the properties of a material is the changes in internal material structure resulting from the structural degradation processes described among other things in [3, 9–11]. This is mainly concerned with the decreasing value of the impact strength, sometimes even by several times. The comparison level for such a phenomenon is the difference in material properties of actual steel versus normalized, as the other has material properties from the time of the structure's construction. The simulation of these properties is carried out by thermal annealing. For this purpose, specimens are annealed at a temperature of 930°C (steels of C ≤ 0.26%) for an hour and then cooled in air. This way the minimal possible grain size in the steel is achieved.

This process increases yield strength and at the same time lowers the ductile-brittle transition temperature, i.e. significantly increases mechanical properties (**Figure 1**). Sometimes, astonishing results are obtained. For example, from the railway bridge over the Warta River in Gorzów Wielkopolski (western Poland), two types of steel specimens were tested for Charpy impact energy.

The bridge was constructed for the German Railways in 1938, using German normalized mild steel St37-12 (**Figure 1**). The tests refer to specimens which were


A significant aging effect was found in the structural steel after 77 years in service. At �20°C, the impact energy was 19.2 times higher.

*Quality and Fatigue Assessment of Welded Railway Bridge Components by Testing DOI: http://dx.doi.org/10.5772/intechopen.104439*

#### **Figure 1.**

*Impact energy KV(T) for naturally aged (S) and normalized (N) specimens from a plate girder railway bridge constructed in 1938.*

#### **Figure 2.**

*Notch toughness of tested bridge steels at temperature* �*20°C for naturally aged (S) and normalized (N) specimens.*

The results of impact energy tests at temperature �20°C for nine steel grades from eight bridges constructed in the years 1887–1938 are shown in **Figure 2**. Two types of specimens were tested: naturally aged and normalized. The steel in post-service conditions showed a very small KV impact energy value.

The actual ascertained KV values are only from 4 to 12 J. This dependence concerns all the steels tested independently of carbon content from 0.016% to 0.258%.

Such a condition shows brittleness in the material; this is a particular danger when it is located in areas of stress concentrations, for example around welding imperfections (WIs) in a weld – **Figure 3**. Welding imperfections (WIs) are crack initiators when the loads reach a prescribed critical value. The largest concentration of normal stresses σ<sup>x</sup> is caused by ellipsoidal welding imperfections and longitudinal ones with elliptical cross sections. For these two groups of welding imperfections, the maximum stress gradient increases as the curvature radius value of the sharpest shape of welding imperfections lowers.

For example, for welding imperfections with shape characteristic t/ρ = 100, the shape coefficient values u = y/t = 1 are 13.63 and 21.00 **Figure 3**. In the case of globular welding imperfections of a small stress concentration—class III with a sharp shape—it
