Fatigue of prestressed tendons

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Prestressed concrete members with pre-tensioned tendons under predominantly non-static loads need to be designed against fatigue failure. In practice, such members are used e. g. in crane runway beams, slabs in industrial constructions, towers of wind turbines, poles or bridges. The fatigue failure of these members can basically occur within the concrete, within the bond between concrete and prestressing steel as well as within the prestressing steel itself. Here, the fatigue of the prestressing steel is often the decisive failure mode. Furthermore, a significant difference between fatigue tests with singular prestressing steel and tests with embedded pre-tensioned tendons in prestressed concrete members can be noticed.In research projects and further preliminary work, carried out by the applicant, a database of prestressed concrete beams under fatigue loading was established and various investigations on prestressing steel samples were analyzed. A large part of the test results of prestressed concrete beams - especially with small stress ranges in the prestressing steel and high numbers of load cycles - indicate a lower fatigue strength compared to the SN curves according to EC2+NA and MC10. This was confirmed by a test of a prestressed concrete beam conducted by the applicant at the iBMB, TU Braunschweig.It can be concluded that the fatigue behavior of embedded pre-tensioned tendons is influenced by structural and systematical parameters and damage mechanisms, leading to a reduction in the fatigue strength compared to fatigue tests on prestressing steel samples. This deviation is not sufficiently recognized in common models. Due to the lack of basic understanding regarding the identification, the definition and the modelling of these parameters and damage mechanisms, systematic investigations are required to close existing gaps in knowledge. Therefore, the fundamentals of the fatigue strength of pre-tensioned tendons will be investigated in this research project experimentally, theoretically and numerically. Based on fatigue tests on singular prestressing steel samples and prestressed concrete beams the different parameters and damage mechanisms which are caused by the complex fatigue processes and stresses due to built-in conditions of pre-tensioned tendons are to be examined. In combination with theoretical models as well as numerical investigations the individual parameters and mechanisms will be identified, quantified, modeled and described.The aim of this project is to develop a model - based on these investigations - which describes the fatigue behavior of prestressing steel including the effects of the various relevant parameters and damage mechanisms.