High resolution electron microscopy of fatigue behavior in high performance concrete and multiscale modelling using a bonded particle model

本项目主要集中在两个方面：第一，利用高分辨率分析电子显微镜分析水泥基高性能材料疲劳引起的结构损伤。该项目的第二个目标是使用粘结颗粒模型(BPM)对高性能混凝土中的疲劳进行多尺度模拟。该项目的重点是超高性能混凝土。在第一阶段，制作了S参考超高性能混凝土试件和粘结剂试件，并进行了压缩载荷下的疲劳试验。然后对超高性能混凝土和粘结剂试件在不同疲劳状态下的结构进行了分析。为了表征过渡区，制作了具有石英岩-粘结剂界面的试件并进行了剪切试验。采用石英岩作为超高性能混凝土石英砂的模型物质。借助于透射电子显微镜，首次在纳米尺度上观察到了超高性能混凝土和粘结剂试件在疲劳过程中产生的结构特征变化，而在微观尺度上，超高性能混凝土没有表现出任何特征变化。为了用BPM模拟力学行为，建立了不同的流变粘结模型，并通过粘结剂、砂和石英岩-粘结剂试件的静态力学试验和微观研究对其参数进行了标定。目前的模型能够很好地预测变形和断裂行为。在第二阶段，我们计划用透射电子显微镜更详细地分析结构变化，并对损害进行量化或至少分类。为此，将大量使用瞬变电磁层析成像，因为FIB层析成像被发现对系统的研究不是很有帮助。此外，还计划在瞬变电磁法中进行现场拉伸试验。在第二阶段，该项目还将侧重于使用纤维和粗集料的超高性能混凝土。用高分辨率的透射电子显微镜分析了两组分与基体的接触情况。这些研究将为建模提供重要信息。第二阶段的建模将集中在用于在不同尺度之间传输数据的均匀化方法以及基于循环跳跃的方法来模拟完整的疲劳行为。提出了一种新的基于BPM的结构载荷外推与再生方法。这项工作的另一个重点在于进一步发展流变模型，以描述不同组件的力学行为，并考虑由于循环加载而发生的变化。除了在第一个项目期间调查的成分外，还将开发纤维和粗毫米级骨料的结构和功能模型。该项目与优先方案的其他项目密切相关。它使用他们的实验数据，并在优先方案内分享自己的结果、开发的方法和模拟框架。该项目还将参与计划的基准4和5。

The project focuses on two objectives: Firstly, the analysis of structural damage caused by fatigue in cementitious high performance materials using high resolution analytical electron microscopy. The second aim of the project is the multiscale modelling of fatigue in high performance concrete using a bonded particle model (BPM). The project focuses on UHPC. In the first period, specimens of the priority program´s reference UHPC and of the binder were produced and fatigue experiments under compressive load carried out. Afterwards the structures of UHPC and binder specimens in different fatigue states were examined. To characterize the transition zone, specimens with quartzite-binder interface were produced and shear tests performed. Quartzite was used as model substance for the quartz sand of the UHPC. By means of transmission electron microscopy (TEM) characteristic structural changes of UHPC and binder specimens caused by the fatigue process could be observed on the nanoscale for the first time, whereas the UHPC does not show any characteristic changes on the microscale. To perform modeling of mechanical behavior with BPM, different rheological bond models were developed and their parameters were calibrated by means of static mechanical tests with the binder, the sand and the quartzite-binder specimens and microscopic investigations. The current model is able to predict well both the deformation and the fracture behavior. In the second period we plan to analyze the structural changes with TEM in greater detail and to quantify or at least to categorize the damage. For this purpose TEM tomography will be used intensively, as FIB-tomography was found to be not very helpful for the investigation of the system. Furthermore, in-situ tensile tests in the TEM are planned. During the second period the project will focus also on UHPC with fibers and coarse aggregate. The contact of both components with the matrix will be analyzed with high resolution TEM. These investigations will provide important information for the modeling. The modelling in the second period will be focused on homogenization methods to transfer data between different scales as well as on cyclic-jump based methods to simulate the complete fatigue behavior. A novel BPM-based approach for load extrapolation and regeneration of structure is proposed. Other focus of the work lies on further development of rheological models to describe mechanical behavior of different components and to consider changes occurring due to cyclic loading. In addition to the components investigated in the first project period, structural and functional models of fibers and coarse millimeter-sized aggregate will be developed. The project is strongly interconnected with other projects of the priority program. It uses their experimental data and shares own results, developed methods and simulation framework within the priority program. The project will furthermore participate in the planned benchmarks 4 and 5.