Damage Processes in Ultra-High Performance Fiber-Reinforced Concrete Under Cyclic Tensile Loading

超高性能混凝土(UHPC)的应用为建造承载能力高、耐久性好的质量优化结构构件提供了极大的优势。为了实现这种结构构件的资源节约型设计，应通过在混合料中添加大量高强度钢纤维来避免超高性能混凝土的脆性破坏。超高性能纤维混凝土(UHPFRC)在弯曲、剪切和扭转作用下的结构构件中纤维的承载性能已有模型，但在循环荷载作用下纤维的承载性能还没有得到充分的评价。在SPP2020的第一个资助期内，对超高性能混凝土拉伸试件在循环拉伸加载下的研究表明，即使在经历了大量恒幅加载循环(单级试验)后，纤维仍表现出相当大的抗疲劳性能。然而，对于超高性能混凝土在复杂加载条件下的循环退化问题(多阶段试验)还没有得到充分的研究和了解，因此，本研究在第一个资助期的调查和研究成果的基础上，对超高性能混凝土在复杂循环拉伸荷载作用下的损伤机理和退化过程进行了基础性的试验和数值研究。一方面，计划进行连续和系统的多阶段循环试验，以评估不同几何形状的试件(包括两个演示器)的顺序效应和损伤累积。损伤将使用新的机械和光学测量技术进行测量和记录，如数字显微镜(DM)、扫描电子显微镜(SEM)、计算机断层扫描(CT)和摄影测量(GOM)，以及声发射分析(SEA)。另一方面，从第一个资金期开始，将开发和实施一种基于介观债券模型的有效均化策略，以便在连续介质-力学模型中描述复合材料UHPFRC在循环拉伸载荷下的退化行为。第一个和第二个资助期的实验结果将被用来验证数值模型。其目的是建立一个模型来预测结构构件在单调和循环荷载作用下的宏观反应。研究的最高目标是在实验-虚拟实验室中结合实验和数值获得的结果和模型，以便能够预测超高性能混凝土在循环拉伸载荷下的退化预测。为了能够对现代超高性能混凝土结构受拉受弯构件进行预测，演示人员将在实验虚拟实验室中进行模拟，以验证其预测的准确性。

Application of ultra-high performance concrete (UHPC) offers great advantages for the construction of mass-optimized structural members with high load-bearing capacity and outstanding durability. To attain a resource-efficient design of such structural members, the brittle failure of UHPC should be avoided by adding considerable amounts of high-strength steel microfibers to the mixture. While models exist for the evaluation of load-bearing behaviour of fibres in structural members made of ultra-high performance fibre reinforced concrete (UHPFRC) subjected bending, shear and torsion, the load-bearing behaviour of fibres under cyclic loads is not evaluated adequately. Investigations conducted on UHPFRC tensile specimens under cyclic tensile loading within the first funding period of the SPP2020 indicate that even after a high number load cycles with constant amplitude (single-stage tests), the fibres show still a considerable fatigue resistance. However, the cycle degradation of UHPFRC under complex loading scenarios (multi-stage tests) has not been sufficiently investigated and understood yet.Therefore, the present research project aims to conduct fundamental experimental and numerical investigations on the damage mechanism and degradation process of UHPFRC under complex cyclic tensile loads, which are based on the investigations and findings of the first funding period. On the one hand, continuous and systematic multi-stage cyclic tests are planned for the evaluation of sequence effects and damage accumulation on specimens with various geometries, including two demonstrators. The damage will be measured and documented using novel mechanical and optical measurement techniques like digital microscopy (DM), scanning electron microscopy (SEM), computer tomography (CT) and photogrammetry (GOM), as well as acoustic emission analysis (SEA). On the other hand, an efficient homogenization strategy will be developed and implemented based on the mesoscale bond model from the first funding period in order to describe the degradation behaviour of the composite material UHPFRC under cyclic tensile loads in a continuum-mechanical model. The experimental results of the first and second funding period will be implemented for the validation of the numerical model. The aim is developing a model to predict the macroscopic response of structural members under monotonic and cyclic loading. The superior goal of the investigations is the combination of experimentally and numerically obtained results and models in an experimental-virtual-lab to enable a degradation prognosis of UHPFRC under cyclic tensile loading. To enable a prognosis for modern UHPFRC structural members subjected to tension and bending, the demonstrators will be simulated in the experimental-virtual-lab to validate its prognosis accuracy.