Fatigue of prestressed tendons

具有预应力筋的预应力混凝土构件在非静力荷载作用下需要进行抗疲劳破坏设计。在实践中，这些构件用于起重机跑道梁、工业建筑板、风力涡轮机塔、电线杆或桥梁。这些构件的疲劳破坏基本上可以发生在混凝土内部、混凝土与预应力钢之间的粘结层内部以及预应力钢本身内部。在这里，预应力钢的疲劳往往是决定性的破坏模式。此外，在预应力混凝土构件中预埋预应力筋的疲劳试验与单一预应力钢的疲劳试验之间存在显著差异。在申请人开展的研究项目和进一步的前期工作中，建立了疲劳荷载下预应力混凝土梁的数据库，并对预应力钢样品进行了各种调查分析。与EC2+NA和MC10的SN曲线相比，大部分预应力混凝土梁的测试结果——尤其是预应力钢的应力范围小、荷载循环次数多的预应力混凝土梁——表明其疲劳强度较低。申请人在布伦瑞克工业大学iBMB进行的预应力混凝土梁测试证实了这一点。结果表明，预埋预应力筋的疲劳性能受结构参数、系统参数和损伤机制的影响，其疲劳强度较预应力钢试件有所降低。这种偏差在普通模型中没有得到充分的认识。由于对这些参数和损伤机制的识别、定义和建模缺乏基本的认识，需要进行系统的研究以弥补现有的知识空白。因此，本研究项目将对预应力筋的疲劳强度进行实验、理论和数值研究。通过对预应力钢试件和预应力混凝土梁的疲劳试验，探讨了预应力筋在复杂疲劳过程和固有条件应力作用下的不同参数和损伤机制。结合理论模型和数值研究，将确定、量化、建模和描述单个参数和机制。这个项目的目的是建立一个模型-基于这些调查-描述预应力钢的疲劳行为，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.