Fatigue properties and fatigue crack growth in puddled steel with consideration of microstructural degradation processes after 100-years operating time

摘要

A subject of the authors' research were the fragments of steel structures (of the puddled steel) coming from the turn of the 19th and 20th centuries. The material gained for the research comes from monumental (and still operated) Wroclaw bridges and the hall of the Main Wroclaw Railway Station. The works were focused on the phenomenon of microstructural degradation of those steels and its potential influence on their strength properties. Analysis of the achieved results indicated that those long operated steels are a subject for 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. The microstructure degradation processes negatively influence fatigue properties. The results of low cycle fatigue test for the puddled steel (from the Main Railway Station) in the after-operation state indicate for worse plastic properties of that steel in comparison to the normalised state. The results of fatigue crack growth test indicate for worsening of fatigue crack propagation resistance of the material depending on the degree of degradation processes intensity in the puddled steels. A new kinetic equation has been proposed for description of the fatigue of crack growth rate due to the Dimensional Analysis approach. From the utility point of view, the achieved results acquire fundamental importance because of the fact of continuous operation of a high number of the bridges aged more than 50 years (about 68% for Europe). It is worth noticing that almost 28% of them are objects aged over 100 years.