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之一。为了全面描述和解释控制疲劳损伤扩展的基本机制，并识别与正常强度混凝土的现象学差异，需要先进的数值模型与适当的实验方法相结合。最近对正常强度混凝土提出的主要由应变累积测量驱动的疲劳损伤假设，显示出捕捉和解释胶凝材料疲劳现象的巨大潜力。因此，本研究选择将其作为高强混凝土的基础理论概念，并对其进行进一步的发展和验证。协调的数值和实验研究将跨越三个层次的观察，包括局部破坏的非均匀损伤演化（1级），均匀损伤演化（2级）和聚集间相互作用（3级）。在第1级，将使用骨料骨架的详细3D镶嵌来研究晶格离散模型（LDM）中由于骨料间滑动和微裂纹演化而导致的损伤局部化。在此基础上，设计了一种新的试验方法，旨在分离扭转和轴向复合载荷作用下管状试件疲劳损伤演化的关键机理。所开发的试验方法的一个显著特点是能够在测试区域内诱导稳定的、宏观均匀的疲劳损伤演变。在第2级，损伤过程的均匀性将被用于模型的尺寸减小，这允许在多达数百万次循环的疲劳加载期间有效地模拟三轴应力重新分布。微平面模型（MM）将用于在测试过程区域内再现均匀演化的、损伤引起的各向异性。在第3级，骨料间的相互作用将通过协调的数值模拟和测试来解决，以表征在水泥膏体层内引起剪切或压缩的边缘骨料配置。基于对局部机制的系统表征所描述的三个观察水平的覆盖范围，假设的假设将使用非均匀疲劳损伤演变的标准测试方法（压缩测试）进行验证。这些结果将作为高强度混凝土结构暴露在一般荷载条件下的疲劳行为的现实和有效预测的基础，可以纳入SPP2020优先计划中建立的实验-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.