Temperature- and humidity-induced damage processes caused by multi-axial cyclic loading

在本研究提案中，来自材料科学（斯图加特大学材料测试研究所）和连续介质力学（应用力学研究所-连续介质力学主席）的科学家们将联合收割机力量结合起来，以更好地理解由于高性能混凝土的多轴循环压缩引起的热和水分引起的劣化过程。作为一个基本的工作假设，它是假定在混凝土结构中的多轴应力状态有一个显着的影响过程中的宏观裂缝的形成。另一方面，与疲劳相关的损伤过程受到样品调节的显著影响，即受水分含量和温度的影响。根据目前的研究状况，疲劳损伤发生在微观层面上的水泥石结构。本研究提案的重点是将第一个资助阶段对单轴应力条件下损伤过程的研究扩展到多轴应力条件下。在材料科学工作组内，正在开发一种高压三轴单元，用于对直径达60 mm、应力状态达100 MPa的圆柱形试样进行循环压力阈值试验。这使得温度和湿度引起的疲劳损伤过程的系统调查。在循环多轴载荷之后，复杂的载荷序列也将随后叠加。损伤过程的表征进行了BET，压汞法和XRCT的援助。此外，成像方法，如光学显微镜和REM是跨尺度使用。预计所获得的知识将有助于建立一个模型，根据全球损害指标描述损害现象。有了这个模型，就有可能首次对退化的发展作出定性预测。在工作组中，连续介质力学双试样的特点是在静态三轴预载荷下，在循环载荷下的切向空间中的宽频率范围内。为了测试温度的影响，在三轴室内的干、湿试样中通过围隔介质加载温控流体。与此同时，在开放式XRCT扫描仪中对受损样品进行高分辨率扫描。在此基础上，对损伤混凝土基体上宏观单裂纹的水-力相互作用进行了数值研究。使用高效的混合有限元法，孔隙解析模拟将在第一个应用阶段进行的数值分析的扩展。该项目的总体目标是更好地了解在第一个资助阶段获得的损伤过程的研究结果，并进一步推广损伤启动和裂纹发展的衍生模型。

In the present research proposal scientists from materials science (Materials Testing Institute University of Stuttgart) and continuum mechanics (Institute of Applied Mechanics - Chair of Continuum Mechanics) combine forces to contribute to a better understanding of the thermal and moisture induced deterioration processes due to multiaxial cyclic compression of high performance concrete. As a basic working hypothesis, it is assumed that multi-axial stress states in the concrete structure have a significant influence on processes of macroscopic crack formation. On the other hand, fatigue-related damage processes are significantly influenced by sample conditioning, i.e. by moisture content and temperature. According to the current state of research, fatigue damage occurs on a microscopic level in the cement paste structure. The emphasis of this research proposal is to expand the investigations done in the first funding phase on damage processes at uni-axial stress to multi-axial stress conditions. Within the working group materials science, a high- pressure triaxial cell is being developed to perform cyclic pressure threshold tests for cylindrical specimens with diameters up to 60 mm and stress states up to 100 MPa. This allows the systematic investigation of temperature- and moisture-induced fatigue damage processes. Following the cyclic multi-axial loads, complex load sequences will also be superimposed later. The characterization of the damage processes is carried out with the aid of BET, Hg porosimetry and XRCT. In addition, imaging methods such as light microscopy and REM are used across scales. It is expected that the knowledge gained will allow a model to be derived to describe damage phenomena on the basis of global damage indicators. With this model it should then be possible for the first time to make qualitative predictions about the development of degradation. In the working group continuum mechanics twin samples are characterized under static triaxial preloads for a wide frequency range in the tangential space under cyclic loading. In order to test the temperature influence, dry and wet samples in the triaxial cell are loaded with a temperature-controlled fluid via confining medium. In parallel, high- resolution scans of damaged samples are characterized in the open XRCT scanner. Based on these scans, the hydro-mechanical interaction of macroscopic single cracks on the damaged concrete matrix is numerically investigated. Using a highly efficient hybrid FEM, pore resolved simulations will be performed in extension of the numerical analyses undertaken in the first application phase. The general aim of the project is to better understand the findings of the damage processes gained in the first funding phase and to further generalise derived models for damage initiation and crack development.