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ASPECTS OF STRUCTURAL DEGRADATION IN STEELS OF OLD BRIDGES BY MEANS OF FATIGUE CRACK PROPAGATION G. Lesiuk1, 2 and M. Szata1 We present conclusions drawn from the studies related to old steel structures, especially those erected on the turn of the 19th century. The objects of interest of the authors are the Wroclaw Pomorskie Bridges: the Central Pomorski Bridge and the North Pomorski Bridge (1885 and 1930, respectively), as well as the Sand Bridge (1861). The material used for their construction was puddled steel or cast steel. In the course of long-term operation, these steels (especially puddled steel) show susceptibility to degradation processes. We present the results of metallographic tests (light microscopy and SEM) and mechanicalproperties tests (hardness measurements and static tensile tests) describing the state of structural degradation. In addition, the results of initial study for the puddled steel coming from the Sand Bridge and concerning the development of a fatigue crack are presented. Basic quantities describing the kinetics of fatigue crack growth are determined. Keywords: fatigue crack growth rate, puddled steel, microstructural degradation.

The published data [1–7] and our own research of old steel structures pose the problem of worsening of the usefulness of these structures. The degradation theory initiated at the Wrocław University of Technology in the 1990s [1] was initially dealing with the problems related to surface mining machines. Its development led to the systematic analysis of machine (structure)–environment systems. In the course of works, several problems connected with the structural degradation of steels have been documented [2, 3]. The problem is becoming more serious when referred to bridge structures from the turn of the 19th century. In particular, this concerns the puddled steel which, as the studies show, is more susceptible to the processes of structural degradation than the cast steel. The material aspects of the degradation theory are discussed in [2, 6]. The essential issue in the studies of old structures is the evaluation of the degree of wear of steel parts. The lack of comparison material—the modern technologies of production of steels are far different from those in the past centuries—makes the evaluation of wear of structural parts quite difficult. The only solution elaborated within the degradation theory and its material aspects [2, 7] is to accept the normalized state as the level for comparison. The steels from the Wroclaw Pomorskie Bridges, i.e., the puddle steels of the Sand Pomorski Bridge (1861) and the Central Pomorski Bridge (1885) and the cast steel of the North Pomorski Bridge (1930), all in the afteroperation state, were the objects of our study. The goal of the present work is the investigation of the influence of the processes of structural degradation on the kinetics of fatigue cracking. According to the above, the basic mechanical properties: yield stress σ 0.2 , ultimate tensile stress σ u , Young modulus E , uniform elongation δ u , and reduction