Fatigue of prestressed tendons

预应力混凝土构件的预应力筋主要在非静力荷载作用下，需要针对疲劳破坏进行设计。在实践中，这样的构件被用于例如起重机跑道梁、工业建筑中的板、风力涡轮机塔架、杆柱或桥梁。这些构件的疲劳破坏基本上可以发生在混凝土内部、混凝土与预应力钢筋之间的粘结内以及预应力钢筋本身内部。在这里，预应力钢材的疲劳往往是决定性的破坏模式。在研究项目和申请人开展的前期工作中，建立了预应力混凝土梁在疲劳荷载作用下的数据库，并对各种预应力钢材试件的研究成果进行了分析。大部分预应力混凝土梁的试验结果表明，与EC2+NA和MC10的SN曲线相比，疲劳强度较低，特别是在预应力钢筋应力范围较小且荷载循环次数较多的情况下。申请人TU BraunschweigTU在IBMB进行的预应力混凝土梁试验证实了这一点，可以得出结论：埋入式预应力筋的疲劳行为受到结构和系统参数以及损伤机理的影响，导致疲劳强度比预应力钢试件的疲劳试验有所降低。在常见的模型中，这种偏差没有得到充分认识。由于对这些参数和破坏机制的确定、定义和建模缺乏基本了解，需要进行系统的调查，以弥补现有的知识差距。因此，本课题将从试验、理论和数值三个方面对预应力筋疲劳强度的基本原理进行研究。通过对单根预应力钢试件和预应力混凝土梁的疲劳试验，分析了预应力筋内置条件下复杂的疲劳过程和应力所引起的不同参数和损伤机理。结合理论模型和数值研究，将对各个参数和机理进行识别、量化、建模和描述。本项目的目的是在这些研究的基础上开发一个描述预应力钢疲劳行为的模型，包括各种相关参数和损伤机制的影响。

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.