Breukanalyse van brosse bouwmaterialen onder monotone en cyclische belasting met behulp van geavanceerde monitoring

Brittle construction materials such as concrete are prone to micro-cracking and sudden fracture. This is becoming even more important with the growing demand for a more economical and efficient design, which results in thinner structures with increased exposure to external stresses. This is especially the case for structures that are exposed to fatigue loading at stresses well below their yield strength, such as bridges, sea walls, and wind turbines. These structures experience progressively increasing micro-damage build-up. Hence, dedicated and robust monitoring techniques that would lead to an improved understanding of the fracture mechanisms are important. This research work envisioned an in-depth understanding of these fracture mechanisms by incorporating advanced non-destructive testing methods (NDT) and developing an efficient test protocol for fatigue behaviour of brittle cementitious materials. Accordingly, the overall research work focused on monotonic and fatigue fracture analysis of quasi-brittle materials such as cementitious mortar using NDTs. First, the sensitivity of the fracture behavior to the sample size is investigated. In monotonically loaded cementitious mortars under the Brazilian splitting test, failure became more brittle as the sample size increased. The physical reasoning behind this phenomenon was identified using the acoustic emission technique (AET). As fracture in quasi-brittle materials is mainly due to tensile stresses, there is a research interest in identifying the modes of fracture (tensile or shear). Accordingly, the next research objective was set to identify and investigate AET-based fracture mode analysis methodologies. This was accomplished by applying various AE-based methods to different types of tests: direct tensile test, Brazilian splitting test, and friction test. In addition, for an even more robust analysis, a novel test setup that combines AET and 4D X-ray CT (X-ray computed tomography) on cementitious mortars was developed to investigate the complex fracture mechanism of the Brazilian splitting test. Furthermore, the combination of the two techniques was used to validate the capacity of AE-based fracture mode analysis. Besides validating the capacity of the AET, this research work was able to highlight how material heterogeneity and boundary conditions can induce shear cracking in Brazilian splitting tests. Fatigue fracture, size effect and overall fracture development are closely related to the development of a fracture process zone (FPZ). Hence, another research objective was set to investigate the evolution of FPZ in cementitious mortars under monotonic and fatigue Brazilian splitting tests. The research used AET and digital image correlation (DIC) to quantify the size of the FPZ. In that, novel modifications were proposed to both techniques so that similar quantitative results were obtained. It was observed that, in fatigue loading, the cause for the progressive and cumulative failure is the existence of a well-defined micro-cracking zone (FPZ). This FPZ causes energy dissipation that allows for the slow and progressive failure in fatigue loading. The main research interest in fatigue damage is commonly to predict fatigue life. Hence, the final research question aimed at developing short-time fatigue test protocols that would allow for fast and efficient fatigue life characterization of brittle materials. This test protocol was designed for cementitious mortar and concrete samples subjected to fatigue Brazilian splitting tests. The damage in these tests was measured using AE, ultrasonic, and LVDT (deformation) measurements. The developed test protocol enabled a prediciton of fatigue life at various load levels from a single stepwise fatigue test. Hereby, the relation between Phase II fatigue damage rates and fatigue life was applied. Overall, new insights and further understanding were highlighted in the fracture mechanisms of quasi-brittle materials. In addition, the work proposed NDT-based fracture analysis methodologies that can readily be applied to other materials and testing conditions.