Fatigue crack propagation behavior of old puddle iron including crack closure effects

Abstract

In this paper the fatigue crack growth behavior in structural components from the old 19th century structures (e.g. bridges) has been investigated. The delivered material for investigation was extracted from a beam made of puddled iron, commonly used in 19th century. The obtained results from several ancient railway metallic bridges (located in Lower Silesia, Poland) have shown the presence of microstructural degradation processes in puddled iron. In all analyzed materials (low carbon puddled iron) microstructure degradation processes were related to: the presence of numerous precipitations of carbides and nitrides (or the carbides–nitrides) of iron inside the ferrite grains, the presence of continuous precipitations of cementite at ferrite grain boundaries. In order to restore the initial state of the microstructure, all tests were carried out in two stages of heat treatment; as-received state and after normalization (950°C, 2h, cooled in air) state. The kinetic fatigue fracture diagrams (KFFD) have been obtained. The problem of crack closure has been involved in fatigue crack growth process during the experiments and its understanding is fundamental for the analysis of stress ratio effects on KFFD. In the paper, a few experimental and numerical techniques for the evaluation of the crack closure/opening forces based on the experimental data have been compared. The implemented algorithm in the numerical environment gives promising results in description of the kinetics of fatigue crack growth of the old puddled iron with consideration of crack closure effect.