Experimental and Theoretical Characterization of Fretting of CFRP Prestressing Strands and Effect on Fatigue Life of Partially Post-Tensioned Girders with Unbonded Tendons

由于其高强度重量比，碳纤维增强聚合物（CFRP）部件正在成为各种土木工程应用中钢的可能替代品。最常见的应用是在大跨度混凝土桥梁中使用CFRP束作为预应力筋。在其他几种选择中，最受欢迎和最有效的变体是无粘结钢筋束的部分后张法。这可以归因于与部分后张相关的设计灵活性、加速施工、更好地利用所有加固元件的极限承载力、更高的极限挠度等。在这种桥梁中，要考虑的关键设计标准是车辆力矩引起的预应力元件疲劳。当这种预应力元件与施加法向力的其它元件之间存在相对运动时，这种预应力元件的疲劳寿命会显著降低。这种现象被称为微动磨损。在PPTGUT的寿命期间，管道和肌腱之间发生连续的往复相对运动。这可能导致CFRP筋的微动磨损和疲劳寿命的降低。当相对运动的位置与高筋曲率和弯曲裂缝的位置重叠时，这种影响可能更有害。相对较新的CFRP材料很少针对这种微动疲劳现象和CFRP PPTGUT内发生的其他复杂情况进行测试。在这种情况下，所提出的项目被调用，其目的是从理论和实验上捕捉CFRP预应力束中微动的微观现象，并研究其对CFRP疲劳寿命的宏观影响。柏林工业大学参与了钢绞线微动疲劳的研究和CFRP作为受拉元件的应用。在此基础上，提出了本课题的研究思路，即通过试验研究碳纤维增强复合材料单根筋的微动疲劳特性。在空气中的疲劳试验与横向压力和疲劳试验的碳纤维增强塑料绞线弯曲的鞍。在接下来的部分中，将通过实验测试缩小的CFRP PPTGUT的微动疲劳研究。接下来，实验结果将用于建立数值有限元（FE）模型，以预测20米长的CFRP PPTGUT的疲劳寿命，即演示器（在主席的实验室建造和提供）。此外，将为相同的简化分析模型开发。验证机疲劳寿命理论预测完成后，将进行疲劳失效（或250万次循环）试验。在这些练习之后，所有研究的结果将被均匀化，并再次系统地进行比较，为CFRP PPTGUT的设计和应用提出具体的指导方针。

Owing to its high strength to weight ratio, carbon fibre reinforced polymer (CFRP) components are emerging as a possible replacement for steel in various civil engineering applications. The most common application is the usage of CFRP strands as prestressing tendon in long-span concrete bridges. Among several other options, the most popular and efficient variant is partial post-tensioning with unbonded tendons. This can be attributed to the design flexibility, accelerated construction, better usage of the ultimate capacity of all reinforcing elements, higher ultimate deflection etc associated with partial post-tensioning. In such bridges, the critical design criterion to be considered is the fatigue of prestressing element occurring due to the vehicular moment. The fatigue life of such prestressing elements can be considerably reduced, when there is a relative motion between the element and other element applying normal force. This phenomenon is termed as fretting. During the life of the PPTGUT, continuous to and fro relative motion occurs between the duct and the tendon. This may lead to fretting of CFRP tendon and reduction in fatigue life. When the location of relative motion overlaps with the location of high tendon curvature and the flexural cracks, the effect might be more detrimental. The relatively new material of CFRP is seldom tested against this phenomenon of fretting fatigue and the other complex situations occurring inside a CFRP PPTGUT. In this context, the proposed project is invoked, that aims to theoretically and experimentally capture the microscopic phenomenon of fretting in CFRP prestressing strands and study its macroscopic effect on the fatigue life of CFRP.For more than a decade the scientists at the Chair of Conceptual and Structural Design-Concrete Structures, TU Berlin have been involved in investigating the fretting fatigue of steel strands and the application of CFRP as tension elements. With this previously conducted research, the present project is proposed, wherein, the fretting fatigue characteristics of individual CFRP strand will be tested via. in-air fatigue test with lateral press and fatigue tests on CFRP strands bent over the saddle. In the next part, the fretting fatigue will be studied by experimentally testing the reduced scale CFRP PPTGUT. Next up, the experimental results will be utilized to setup numerical finite element (FE) models, in order to predict the fatigue life of the 20 m long CFRP PPTGUT viz. the demonstrator (built and available at the Chair’s laboratory). Also, few simplified analytical models will be developed for the same. After the theoretical prediction of fatigue life of the demonstrator, it will be tested till it fails in fatigue (or 2.5 million cycles). After these exercises, the results from all the studies will be homogenized and once again systematically compared to suggest concrete guidelines for the design and application of CFRP PPTGUT.