**5. Stresses in welds with cracks**

The fatigue behavior of metals is determined precisely in [20, 24–26]. Fatigue hypotheses, dislocation structures, fatigue cracking, and their fractures are given there. The issue of no crack growth in existing structures in service is also discussed there. Such cracks appear when the initiation crack stress is lower than the value of


#### **Table 1.**

*Effect of cover plates on the fatigue strength.*

#### **Figure 9.**

*Undamaged specimens after 3637<sup>10</sup><sup>3</sup> and 2827<sup>10</sup><sup>3</sup> load cycles (cracks after 1817.2<sup>10</sup><sup>3</sup> and 1990.2<sup>10</sup><sup>3</sup> load cycles).*

stress necessary for crack propagation. The undamaged service of these structures and the lack of propagation cracks is because no situation has arisen during service which would lead to their appearance. Many hypotheses resulting from laboratory tests have been put forward, including the oldest tests on wagon axle models (Ø 50 mm), carried out by T.V. Buchwalter as early as 1938 [27, 28]. Generally, there is no one solution for the three-dimensional problem of fatigue fractures. However, a material experiment review laid out the directions for further research to find a more precise solution to the problem. There is the optimistic fact that as early as 1965, Kudriawcew [29] stated "structural sections in which non-propagating cracks develop may be stronger than sections constructed with notches."

The prediction of eventual fatigue cracking in welded butt splices in the railway bridges tested was assessed on the basis of strength analysis for three selected bridges. These are plate girder bridges constructed in 1938, 1938, and 1947 on different railway lines. Their technical characteristics are given in **Table 2**. This is a compilation of requirements collated in two papers [7, 16] relating to the structures.

The analysis was undertaken on the three bridges. They were constructed either side of WWII. In total, there were six examinations of the welded butt splices, and three additional tests limited to the testing of splices with cracks. The additional examinations were executed in 5–8-year time intervals. Overall, 632 X-rays were taken and 49 technological hot cracks were discovered. The calculated stress values at
