Multiscale investigation of fatigue crack growth in high performance concrete based on computer tomography and phase-field fracture modeling

高性能混凝土（HPC）如今广泛用于桥梁、塔架、海上钻井平台、起重机跑道、高层建筑、风力涡轮机和道路的混凝土构件的构造，所有结构都特别容易受到循环载荷的影响。目前，由于过于保守的设计标准，使用HPC带来的机会无法充分利用。这主要是由于缺乏关于疲劳载荷下材料退化的知识。这个项目的目的是填补这些知识空白。现在人们普遍认识到，通过显式建模其异质结构可以获得对混凝土行为的新见解。在这个项目中，HPC的多尺度计算模型将被建立，以阐明疲劳性能的基本原理，特别是损伤和裂纹模式和裂纹扩展速率如何受到材料特性和非均匀几何形状的影响。计算模型将集中在所谓的中尺度，其中HPC将被认为是由散装水泥砂浆，粗骨料，空隙和骨料和散装砂浆之间的界面砂浆。该模型的多尺度性质将出现从需要适当分配的散装和界面砂浆，这反过来又是由水泥浆，细骨料和孔隙在较低的尺度（微尺度）的材料特性。细观尺度的转变将基于砂浆的断裂特性主要取决于局部孔隙率的假设。将用相场模拟方法描述损伤和开裂，扩展到包括疲劳效应。将进行全面的实验计划。的散装和界面砂浆的断裂性能将其特征在于作为局部孔隙率的函数，从而证实了微观-介观尺度的过渡实验。中尺度模型的几何形状将完全基于计算机断层扫描（CT）的几何形状采集。为此，将开发创新技术，以增强骨料和砂浆之间的对比度，并实现自动图像分割。将通过试验观察裂纹萌生和扩展现象，包括在CT室内进行试验，并在单调和循环载荷条件下对载荷下的应变和位移进行全场三维测量。实验将允许一个彻底的验证的计算模型，都将揭示在HPC疲劳破坏的基本机制。

High-performance concrete (HPC) is nowadays widely used for the construction of concrete components for bridges, towers, offshore rigs, crane runways, tall buildings, wind turbines and roads, all structures that are particularly susceptible to cyclic loading. Currently, the opportunities that arise from the use of HPC cannot be fully exploited because of overly conservative design standards. This mainly results from the lack of knowledge regarding the material degradation under fatigue loading. The aim of this project is to fill these knowledge gaps.It is nowadays widely recognized that new insight on the behavior of concrete can be gained through an explicit modeling of its heterogeneous structure. In this project, a multiscale computational model of HPC will be set up to shed light on the fundamentals of the fatigue performance, and in particular on how the damage and crack pattern and crack growth rate are influenced by the material characteristics and by the heterogeneous geometry. The computational model will focus on the so-called mesoscale, where HPC will be considered constituted by bulk cement mortar, coarse aggregates, air voids and interfacial mortar between aggregates and bulk mortar. The multiscale nature of the model will arise from the need to properly assign material properties to the bulk and interfacial mortars, which in turn are constituted by cement paste, fine aggregates and pores at the lower scale (microscale). The micro-mesoscale transition will be based on the assumption that the fracture properties of mortar mainly depend on the local porosity. Damage and cracking will be described with a phase-field modeling approach extended to incorporate fatigue effects.A comprehensive experimental program will be carried out. The fracture properties of the bulk and interfacial mortars will be characterized as functions of the local porosity, thus substantiating the micro-mesoscale transition experimentally. The geometry of the mesoscale model will be entirely based on geometry acquisition from Computer Tomography (CT). For this purpose, innovative techniques will be developed to enhance the contrast between aggregates and mortar and enable automatic image segmentation. Crack initiation and propagation phenomena will be observed through testing, including testing within the CT chamber with full-field three-dimensional measurement of strain and displacement under load, under monotonic and cyclic loading conditions. The experiments will allow for a thorough validation of the computational model and both will shed light on the fundamental mechanisms of fatigue failure in HPC.