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

超高性能混凝土（UHPC）的应用为构造具有高承载能力和优异耐久性的质量优化结构构件提供了巨大优势。为了实现这种结构构件的资源有效设计，应通过向混合物中添加大量的高强度钢微纤维来避免UHPC的脆性破坏。虽然存在用于评估由超高性能纤维增强混凝土（UHPFRC）制成的结构构件中的纤维承受弯曲、剪切和扭转的承载行为的模型，但是没有充分评估纤维在循环载荷下的承载行为。在SPP 2020的第一个资助期内，在循环拉伸载荷下对UHPFRC拉伸试样进行的研究表明，即使在恒定振幅的高次数载荷循环（单阶段试验）后，纤维仍显示出相当大的抗疲劳性。然而，在复杂荷载情景（多阶段试验）下UHPFRC的循环退化还没有得到充分的研究和理解，因此，本研究项目旨在对UHPFRC在复杂循环拉伸荷载下的损伤机理和退化过程进行基础性的试验和数值研究，这是基于第一个资助期的调查和结果。一方面，连续和系统的多阶段循环试验计划的序列效应和损伤累积的各种几何形状的标本，包括两个示范评估。将使用新型机械和光学测量技术（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））测量和记录损坏。另一方面，一个有效的均质化策略将开发和实施的基础上，从第一个融资期的中尺度债券模型，以描述复合材料UHPFRC在循环拉伸载荷下的连续力学模型的退化行为。第一和第二个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单调和循环荷载下结构构件的宏观响应。研究的上级目标是在实验虚拟实验室中结合实验和数值计算获得的结果和模型，以实现UHPFRC在循环拉伸荷载下的退化预测。为了能够预测现代UHPFRC结构构件受到拉伸和弯曲，演示者将在实验虚拟实验室中进行模拟，以验证其预测准确性。

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.