Enhancement of fatigue resistance of strain-hardening cement-based composites by means of experimental-virtual multiscale material design

应变硬化水泥基复合材料（应变硬化水泥基复合材料，SHCC）是一种新型的纤维增强混凝土，在单调拉伸载荷下具有很高的变形能力。其延性是由渐进的多重裂纹形成的，并伴有明显的应变硬化。为了确保在实际应用中常见的SHCC建筑构件在循环荷载作用下的安全性和有效性，对这种材料的疲劳行为和理论数值模型的深入了解是必不可少的。关于新型纤维增强混凝土的实际应用和有目的的改进的主要问题是：由于疲劳，开裂状态下的材料在多大程度上失去了其固有的延性和抗拉强度，以及如何通过目标导向的材料设计来减轻这些损失？在项目第一阶段，从细观和细观两个层面对循环荷载作用下SHCC的复杂行为进行了研究。特别是对聚合物微纤维的疲劳、除颤和纤维挤压等降解机理以及纤维-基质结合性能进行了全面的研究。所有微观组织成分及其行为用数值模型描述。在二期工程中，不同成分SHCC的疲劳性能试验将主要在宏观层面进行，采用不同的加载场景和温度。为了更好地了解损伤过程，实验应伴随着详细的形态学研究。本项目阶段的目的是将SHCC在高循环荷载下的宏观行为（包括预期的各向异性材料降解）与在微观和中观水平上检测和描述的损伤机制结合起来。中观和宏观尺度数值模型的尺度耦合要求应力和应变测量以及内部材料变量的热力学一致的均质化发展。为了达到较高的计算效率，在高性能计算机上采用了时间均匀化和自适应离散化方法。在项目结束时，将全面了解不同观察水平下的损伤机制，连贯的材料表征方法和数字计算工具，以预测和优化循环加载下SHCC的力学行为。除了SHCC材料设计之外，虚拟工具还应该能够针对不同的加载场景（数字化材料设计）有针对性地开发其他类型的纤维增强混凝土。

A new promising type of fiber-reinforced concrete – highly ductile concrete made by short fibers (Strain-Hardening Cement-based Composite, SHCC) – exhibits high deformation capacity at monotonic tensile loading. Its ductility results from progressive multiple cracking, which is accompanied by distinct strain hardening. To ensure safety and efficiency of building components of SHCC subject to cyclic loads, which are common in practical applications, profound knowledge of the fatigue behavior of this material and theoretical-numerical models are indispensable. The main question with respect to practical applications and to the purposeful improvement of the new fiber-reinforced concrete is: To what extent does the material in the cracked state lose its inherent ductility and tensile strength due to fatigue and how can these losses be mitigated by a goal-oriented material design?The complex behavior of SHCC under cyclic loading has been investigated in the first project phase at micro- and meso-levels. In particular, the degradation mechanisms of polymer microfibers, such as fiber fatigue, defibrillation and fiber squeezing, as well as the fiber-matrix bond properties were comprehensively studied. All microstructural constituents and their behaviors were described using numerical models.In the second phase of the project, experiments on the fatigue behavior of SHCC of different composition shall be performed predominantly on the macroscopic level by applying various loading scenarios and temperature. For a better understanding of the damage processes, the experiments shall be accompanied by detailed morphological investigations. The aim of this project phase is to combine the macroscopic behavior of SHCC under highly cyclic loading including the expected anisotropic material degradation with the damage mechanisms detected and described at the micro- and meso-level. The scale coupling of the numerical models for the meso- and macro-scale requires the development of a thermo-dynamically consistent homogenization for the stress and strain measures, as well as the internal material variables. To achieve a high compu-tational efficiency, time homogenization and adaptive discretization methods at high-performance computers are used.At the end of the project, there will be a comprehensive understanding of the damage mechanisms at different levels of observation, a coherent material characterization methodology, and a digital computational tool to predict and optimize the mechanical behavior of SHCC under cyclic loading. In addition to the SHCC material design, the virtual tool should also enable the targeted development of other types of fiber reinforced concrete for different loading scenarios (digitally enabled material design).