Fatigue crack propagation in cold drawn steel

[EN]This paper deals with the fatigue crack growth in pearlitic steel wires in the form of hot rolled bar and cold drawn wire. The progress of crack front was analysed by means of the evolution of the aspect ratio with the relative crack depth, and this latter was evaluated during the tests by a compliance method. Results show that cold drawing is beneficial from the point of view of crack growth rate, i.e., cracking is slower in the cold drawn wire (final commercial product) than in the hot rolled bar (base material). In spite of the oriented pearlitic microstructure of the cold drawn steel, fatigue crack propagation develops in mode I, i.e., cracking takes place by maintaining its original plane. A materials science reasoning is proposed to explain this behaviour on the basis of the pearlitic microstructure of the steel and the large geometry changes in the vicinity of the crack tip

Effect of sudden load decrease on the fatigue crack growth in cold drawn prestressing steel

This paper analyzes the overload retardation effect (ORE) on the fatigue crack growth (FCG) of cold drawn prestressing steel when different loading sequences are used. The ORE is more intense for elevated load decrease or for low initial stress intensity factor (SIF) range ΔK0. A transient stage can be observed in the Paris curve (da/dN–ΔK) when the KmaxΔK value suddenly decreases, associated with the ORE and with the evolution of the plastic zone and compressive residual stresses near the crack tip. In tests with Kmax decrease, a small zone appears related to FCG initiation, with a fatigue fractography resembling the tearing topography surface (TTS) mode, and associated with a decrease of crack tip opening displacement (CTOD)

Compliance evolution in round cracked bars under tensile fatigue

[EN]This paper studies the dimensionless compliance in cylindrical geometries with transverse surface cracks subjected to axial tensile loading. Compliance evolution is analyzed when round bars are subjected to fatigue with free and constrained sample ends, initial crack geometries of straight or circular fronts and several materials characterized through of the Paris parameter m. With this aim, a computer application that calculates the crack front’s geometric evolution and the dimensionless compliance was made by discretizing the crack front and assuming that every point advance perpendicular to the crack front according to the Paris law. The results show that dimensionless compliance grows with the increase of the relative crack depth and the decrease of the aspect ratio, showing greater values for free sample ends than for constrained sample ends. Furthermore, during fatigue crack growth, materials with higher values of the Paris parameter m produce slightly greater dimensionless compliance and a better convergence between the results for straight or circular initial crack

Diseño y elaboración de material docente para el aprendizaje de fractura de materiales

Memoria ID11-229. Ayudas de la Universidad de Salamanca para la innovación docente, curso 2011-2012