**4. Fatigue tests on butt welds**

All the cracks in the butt splices of the bridge structures are internal hot cracks. They arise in the weld metal and HAZ of a joint during the crystallization process in the liquidus–solidus temperature range (**Figure 3**).

Service fatigue cracks appear in stress concentration zones caused by structural details and increase their value by a concentration factor caused by nonmetallic inclusions. Therefore, the development of a fatigue crack depends on many factors: the shape and dimensions of a structural component as well as the way and magnitude of loading. The larger the structure, the smaller the critical crack length necessary to cause the final brittle fracture [3, 18]. Development of fatigue cracking appears in stages under fluctuating loads, and their increase is caused by the weakening of a structure's strength. This is why, in the literature, the stress is put down to the influence of the imperfection's geometry and its location in the weld.

The literature gives different models for the initiation and proliferation of fatigue cracks in a nondeterministic approach. Despite many attempts to describe the fatigue mechanism, given in 64 hypotheses [19], no general hypothesis has yet to be formulated [18, 20, 21]. We are still stuck in the phenological description, despite having more and better research tools and numerical calculations. It appears that when assessing the endurance of bridges with cracks in their welded joints, it is highly useful to analyze their service behavior and the results of laboratory tests.

In Poland, the pioneer of the in situ field testing of welded butt splices on railway bridges was Professor Andrzej Fabiszewski from the Technical University of Szczecin. The procedure understood the principle that a weld is the weakest point in welded structures. The results of these tests were a great surprise to the organizers. In 34 bridges, internal technological cracks and hot cracks were ascertained in 437 welded butt splices (**Figure 6**).

To answer the question, "What do we do with theses bridges?", laboratory fatigue strength tests were carried out on three typical structural solutions which reflected the details in the early welded bridges. Specimens U, C, and P and the test results are given in **Figure 7**. The tests were carried out on 60 specimens, each time loaded at 5 loading levels. The tests are presented more precisely in [3]. They allowed, using the least-square method, fatigue class values according to EN ISO 5817: 2014 to be estimated [22, 23]. The following fatigue classes (ΔσC) were obtained for individual specimens from different constructions:


#### **Figure 7.**

*Fatigue strength test results for U – Sound welds (uncracked), C – Welds with internal cracks, and P – Welds covered by one-sided rhomboid cover plates.*

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

#### **Figure 8.** *Fatigue test results on 16 specimens and regression analysis.*

The tests were carried out using a pulsator of frequency 5 Hz and stress ratio R = 0.1. Of note is the low fatigue class Δσ<sup>C</sup> = 79 MPa for specimens "strengthened" with rhombic cover plates. The rhombic cover plates had been intended to secure welded butt splices in early welded bridges, but the fatigue effects appeared to be quite the opposite. The results of the tests (**Figure 7**) clearly show that for the number of load cycles Ni larger than 1.1106 , the fatigue strength of the specimens with cover plates is lower than the fatigue strength of the specimens with cracked butt welds (type C).

The results of fatigue tests on 16 specimens with rhomboid cover plates give cause for reflection (**Figure 8**). Specimens with dimensions 18012720 mm were manufactured from Polish mild steel St3M for bridges (C = 0.19%, Mn = 0.66%) of fy = 312 MPa and fu = 452 MPa. The tests were carried out according to the Polish standard on fatigue tests on metals using five stress levels: 75, 80, 100, 120, and 140 MPa. The tests were performed on a pulsator with 5-Hz frequency. The first cracks appeared near the welded end of the cover plate and spread toward the specimen edges. In three specimens with stress levels 80, 100, and 140 MPa, the cracks appeared at <sup>99</sup>103 , 168.9103 , and 20103 cycles before total fracture. However, two specimens at stress level 80 MPa were not damaged, despite being loaded by 1819.8103 and 836.8103 cycles after the first cracks appeared (**Figure 9**). The test results for 13 damaged specimens allow us to work out the logarithmic regression equation.

The tests show stress concentrations by rhomboid cover plates mainly at their ends [14]. The fatigue strength value resulting from using cover plates depends on their shapes, as well as their length (**Table 1**). The lowest value is reached when the additional element is shortened up to 300 mm.
