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

超高性能混凝土（UHPC）的应用为质量优化构件的建设提供了巨大的优势，具有较高的承载能力和优异的耐久性。为了实现这种结构构件的资源高效设计，应通过在混合物中添加大量高强度钢微纤维来避免UHPC的脆性破坏。虽然现有模型用于评估由超高性能纤维增强混凝土（UHPFRC）制成的结构构件中纤维在弯曲、剪切和扭转下的承载性能，但纤维在循环荷载下的承载性能尚未得到充分评估。在SPP2020的第一个资助期内，对UHPFRC拉伸试样在循环拉伸载荷下进行的研究表明，即使在多次恒定振幅的载荷循环（单阶段试验）之后，纤维仍然表现出相当大的抗疲劳性。然而，UHPFRC在复杂加载情景（多阶段试验）下的循环降解尚未得到充分的研究和理解。因此，本项目拟在第一期资助期研究成果的基础上，对UHPFRC在复杂循环拉伸载荷作用下的损伤机理和降解过程进行基础实验和数值研究。一方面，规划了连续系统的多阶段循环试验，以评估不同几何形状试件的序列效应和损伤累积，包括两个演示体。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，从第一个资助期开始，将基于中尺度键模型开发和实施一种有效的均质化策略，以便在连续力学模型中描述复合材料UHPFRC在循环拉伸载荷下的降解行为。第一期和第二期资助期的实验结果将用于验证数值模型。目的是建立一个模型来预测结构构件在单调和循环荷载下的宏观响应。研究的主要目标是在实验虚拟实验室中结合实验和数值结果和模型，以实现UHPFRC在循环拉伸载荷下的降解预测。为了能够对现代UHPFRC结构构件进行拉伸和弯曲预测，将在实验虚拟实验室中对演示体进行模拟，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.