G. Lesiuk et alii, Frattura ed Integrità Strutturale, 34 (2015) 290-299; DOI: 10.3221/IGF-ESIS.34.31 290 Focussed on Crack Paths Kinetics of fatigue crack growth and crack paths in the old puddled steel after 100-years operating time G. Lesiuk, M. Szata Wroclaw University of Technology, Faculty of Mechanical Engineering, Department of Mechanics, Materials Science and Engineering, Smoluchowskiego 25, PL-50-370 Wroclaw, Poland , A BSTRACT . The goal of the authors’ investigations was determination of the fatigue crack growth in fragments of steel structures (of the puddled steel) and its cyclic behavior. Tested steel elements coming from the turn of the 19 th and 20 th were gained from still operating ancient steel construction (a main hall of Railway Station, bridges etc.). This work is a part of investigations devoted to the phenomenon of microstructural degradation and its potential influence on their strength properties. The analysis of the obtained results indicated that those long operating steels subject to microstructure degradation processes consisting mainly in precipitation of carbides and nitrides inside ferrite grains, precipitation of carbides at ferrite grain boundaries and degeneration of pearlite areas [1, 2]. It is worth noticing that resistance of the puddled steel to fatigue crack propagation in the normalized state was higher. The authors proposed the new kinetic equation of fatigue crack growth rate in such a steel. Thus the relationship between the kinetics of degradation processes and the fatigue crack growth rate also have been shown. It is also confirmed by the materials research of the viaduct from 1885, which has not shown any significant changes in microstructure. The non-classical kinetic fatigue fracture diagrams (KFFD) based on deformation (  ) or energy (  W) approach was also considered. In conjunction with the results of low- and high-cycle fatigue and gradual loss of ductility as a consequence (due to the microstructural degradation processes) - it seems to be a promising construction of the new kinetics fatigue fracture diagrams with the energy approach. K EYWORDS . Puddled steel; Kinetic fatigue fracture diagram; Mechanical degradation. I NTRODUCTION he rapid development of metallurgy in the 19 th century resulted in intensive erection of numerous bridges. Many of them are still operating. Operation of the advanced in age technical objects becomes a common phenomenon in the current reality. The problem becomes particularly acute in bridge sector. It can be estimated from the works [3] and [4], that the age of 68% of railroad bridges exceeds 50 years, while 28% of them are over 100 years old objects. The maintenance of such aged objects in full usefulness and reliability requires adequate diagnostic methods and evaluation of their reliability. It should be prepared independently from the procedures adopted in normative regulations T