Abstract
This paper analyzes the effect produced by sudden load changes on the fatigue of cold drawn prestressing steel wires to be used in prestressed concrete. Fatigue tests were performed on cylindrical samples under load control, using fatigue steps with different stress, and observing the appearance of a transient state in the fatigue crack propagation curve (da/dN-ΔK) when the product KmaxΔK suddenly decreases, a sort of overload retardation effect that allowed the calculation of the plastic zone size. In addition, the fatigue crack growth rate in the Paris regime, da/dN, obeys mainly to the parameter ΔK and it is hardly ever affected by Kmax. The study of the fatigue fracture surface shows: (i) that the micro-roughness of ductile micro-tearing patterns is a function of Kmax; (ii) the existence, in some tests, of a small zone with fatigue initiation fractography (with a tearing topography surface), related to the decrease of crack tip opening displacement, a phenomenon similar to that of crack initiation and growth from a micro-notch.
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