**Figure 3.** *Stress distribution σ<sup>x</sup> near welding imperfections: longitudinal – I, ellipsoidal – II, and globular – III [3, 12].*

is independent from the imperfection size and is σ<sup>x</sup> = 2.04σ [3, 12], where σ is the design stress.

Tensile stresses near a notch may be the effect of external loading, residual stresses, or both simultaneously [13]. It is not possible to exclude brittle fracture even if there is no external loading.

According to the literature [14], the fatigue strength at 105 cycles to strength at <sup>2</sup><sup>10</sup><sup>6</sup> cycles ratio is in the range 1.44–2.45, with the average value 1.85. Similar values of 1.75 and 1.73 were obtained for Polish mild steel (St3SX: C = 0.160%, Mn = 0.498%) [15] – **Figure 4**. Tested were as follows:


Specimens NN were cold-deformed up to 10% relative elongation and then heated up to temperature 250°C, kept at this temperature for an hour, and then cooled in air.

The value of the infinitive fatigue strength Zrj = 145.3 MPa of naturally aged steel for 35 years is 55.9% of its yield strength fy = 260 MPa and for the overaged steel this value is Zrj = 157.3 MPa which constitutes only 31.8% of its yield strength. As a result of additional aging (specimens NN), there was an unexpected very large increase in steel yield strength fy of 90.4%, and ultimate strength fu increased by 27.1%.

The specific character of bridge loadings and the structural changes in steels due to aging are the main reasons why a general hypothesis for their fatigue estimation has yet *Quality and Fatigue Assessment of Welded Railway Bridge Components by Testing DOI: http://dx.doi.org/10.5772/intechopen.104439*

#### **Figure 4.**

*Regression straights obtained from fatigue tests on: naturally aged specimens (S) and additionally aged specimens (NN) [15].*

to be proposed [16]. Phenomenological models are still used despite the significant number of studies undertaken, especially for riveted structures, and more excellent research tools for testing have appeared as well as the possibility of numerical analysis. A problem has arisen as to how to adapt the information from laboratory fatigue tests to the design for the durability of structures in service. Knowledge of both the loading spectrum which a structure will be carrying and problems with butt splices have become necessary.

Welded structures under high stresses are damaged mainly by fatigue crack growth or brittleness. Hence, fracture mechanics has recognized the most important issues:


Over the years 1970–1980, the International Welding Institute introduced the "fitness for purpose" criterion, which relies on the formulation of fracture mechanics calculations for determining the permissible size of welding imperfections, thereby confirming the required quality and durability of structures according to standards and technical requirements. The determination of specific quality levels and example calculations as well as the determination of safety coefficients are given in [17, 18].
