Experimental and numerical framework for characterization of high-strength concrete fatigue accounting for local dissipative mechanisms at subcritical load levels

高强混凝土的疲劳响应是近十年来研究的热点。为了全面描述和解释疲劳损伤扩展的基本机制，并确定与正常强度混凝土的现象学差异，需要先进的数值模型伴随着适当的实验方法。疲劳损伤的假设，主要是由一个累积的应变测量最近假定为正常强度的混凝土驱动的捕获和解释水泥基材料的疲劳现象表现出很大的潜力。因此，它已被选定为基本的理论概念，以进一步发展和验证高强度混凝土在这项研究奋进。协调的数值和实验研究将跨越三个观测水平，包括非均匀损伤演化与局部失效（1级）、均匀损伤演化（2级）和骨料间相互作用（3级）。在1级，详细的三维网格的骨料骨架将被用来研究损伤定位，由于内部格点离散模型（LDM）中的骨料滑动和微裂纹演化。基于这些模拟，将设计一种新的实验方法，旨在隔离的关键机制的疲劳损伤演变的管状试样暴露于组合扭转和轴向载荷。开发的试验方法的一个显著特征是能够在试验区域内诱导稳定、宏观均匀的疲劳损伤演变。在2级，损伤过程的均匀性将被用于模型的降维，这允许在高达数百万次循环的疲劳加载期间有效地模拟三轴应力再分布。微平面模型（MM）将用于再现受试工艺区内均匀演变的损伤诱导各向异性。在3级，骨料间相互作用将通过协调的数值建模和测试来解决，以表征在水泥浆层内引起剪切或压缩的边界骨料配置。根据所述的三个观察水平的覆盖范围以及局部机制的系统表征，将使用具有非均匀疲劳损伤演变（压缩试验）的标准试验方法对假设的假设进行验证。结果将作为一个现实和有效的预测暴露于一般负载条件下的高强度混凝土结构的疲劳性能的基础，可以纳入优先程序SPP 2020内建立的实验虚拟实验室。

Fatigue response of high strength concrete has become a subject of intensive research during the recent decade. To comprehensively describe and interpret the fundamental mechanisms governing the fatigue damage propagation and to identify the phenomenological differences with respect to normal strength concrete, advanced numerical models accompanied with appropriate experimental methods are needed. A hypothesis on fatigue damage primarily driven by a cumulative measure of strain postulated recently for normal strength concrete has shown a great potential to capture and explain the fatigue phenomenology of cementitious materials. Therefore, it has been chosen as the basic theoretical concept to be further developed and validated for high strength concrete within this research endeavor. The coordinated numerical and experimental research will span over three levels of observation including non-uniform damage evolution with localized failure (Level 1), uniform damage evolution (Level 2), and inter-aggregate interaction (Level 3).At Level 1, detailed 3D tessellations of the aggregate skeleton will be used to study the damage localization due to inter-aggregate sliding and microcrack evolution within an lattice discrete model (LDM). Based on these simulations, a new experimental method will be devised aiming to isolate the key mechanism of fatigue damage evolution on tubular specimens exposed to combined torsional and axial load. A salient feature of the developed test method will be the ability to induce a stable, macroscopically uniform fatigue damage evolution within a tested zone. At Level 2, the uniformity of the damage process will be exploited for dimensional reduction of the model, that allows for efficient simulations accounting for the three-axial stress redistribution during fatigue loading with up to millions of cycles. The microplane model (MM) will be used to reproduce the uniformly evolving, damage-induced anisotropy within the tested process zone. At Level 3, the inter-aggregate interaction will be addressed by coordinated numerical modeling and testing to characterize the borderline aggregate configurations inducing either shear or compression within the cement paste layer. Based on the described coverage of the three levels of observation with the systematic characterization of local mechanisms, the postulated hypothesis will be validated using standard testing methods with non-uniform fatigue damage evolution (compressive tests). The results will serve as a basis for a realistic and efficient prediction of fatigue behavior of high strength concrete structures exposed to general loading conditions that can be incorporated into the Experimental-Virtual-Lab established within the priority program SPP2020.