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

由短纤维(应变硬化水泥基复合材料)制成的高延性混凝土是一种很有前途的新型纤维混凝土，它在单调拉伸荷载下具有很高的变形能力。它的塑性是渐进的多重开裂的结果，并伴随着明显的应变硬化。为确保小骨料混凝土构件在循环荷载作用下的安全性和有效性，深入了解这种材料的疲劳性能和理论-数值模型是必不可少的。对于实际应用和新型纤维混凝土的有目的的改进来说，主要的问题是：由于疲劳，处于开裂状态的材料在多大程度上失去了其固有的延性和抗拉强度，以及如何通过面向目标的材料设计来减少这些损失？在第一个项目阶段，已经从微观和细观层面研究了小体积混凝土在循环荷载下的复杂行为。重点研究了聚合物微纤维的降解机理，如纤维疲劳、去纤化、挤压等，以及纤维与基质的粘结性能。在二期工程中，将主要通过施加不同的加载方案和温度，在宏观水平上对不同成分的小尺寸混凝土的疲劳性能进行试验。为了更好地了解损伤过程，实验应伴随着详细的形态调查。本项目阶段的目的是将高循环载荷下小细胞肺癌的宏观行为，包括预期的各向异性材料退化，与在微观和细观层面上检测和描述的损伤机制结合起来。为了实现中观尺度和宏观尺度数值模式的尺度耦合，需要为应力和应变测量以及内部材料变量建立热力学一致的均匀化。为了获得高的计算效率，在高性能计算机上使用了时间均化和自适应离散化方法。项目结束时，将全面了解不同观测水平下的损伤机理，一致的材料表征方法，以及预测和优化循环荷载下小混凝土力学行为的数字计算工具。除了SCC材料设计之外，该虚拟工具还应能够针对不同的加载场景有针对性地开发其他类型的纤维混凝土(数字化材料设计)。

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).