Numerical modelling and experimental characterisation of mechanical performance of ceramic materials at multiple scales

Among countless engineering applications of ceramic materials, the design of reliable ceramic armour remains one of the most challenging. The complexity of deformation and failure mechanisms amplified by the rate-dependent behaviour, observable in service and corresponding experiments, still hinders the development of novel materials capable of demonstrating a significantly improved performance. The main aim of the research discussed in this thesis is to increase the understanding of the physical phenomena surrounding the response of ceramic materials to rapidly applied loading, such as that arising from explosion and collision, by combining experimental and numerical methodologies at different length scales. Multiple experimental techniques have been adopted to characterise the materials considered. At the micro-scale, the mechanical properties of single grains, single grain boundaries, and polycrystalline structures have been measured with micro-cantilever beam tests, and the results. At me macro-scale, instead, the quasi-static characterisation tests have been combined with the outcome of high strain-rate tests (i.e. impact) to provide an overview of the macroscopic behaviour of the ceramic materials considered at multiple rates, thus increasing the understating of the rate dependent deformation and failure processes in ceramic materials. The numerical approach also focused on two distinct scales, aiming to reproduce the geometrical features of the microstructures of polycrystalline ceramic materials, as well as the mechanical response at both the micro- and the macro-scales. At the micro-scale an innovative method to model the grain structure of polycrystalline materials has been developed. Experimentally obtained results were used both in the generation of the models to guarantee their representativeness, as well as to validate the developed numerical simulation methodologies. This provided the required confidence in the algorithms developed to simulate the response of realistic, statistically representative micro-structures which would be extremely difficult to investigate experimentally. This offered the unique opportunities to explore the physical phenomena which would otherwise be very difficult to generate, let alone observe and measure in reality (i.e. in-situ dynamic crack propagation and branching within three-dimensional domains). At the macro-scale a constitutive model for simulation of deformation and failure of ceramic materials has been developed, overtaking some limitations of the state-of-the-art material models. An analytical model of the crack growth has been implemented, based on the study of the behaviour of a single sharp crack randomly oriented within an elastic continuum medium, and embedded into an ad-hoc modified constitutive model to lay the fundaments for the future development of methodology for concurrent multi-scale modelling of ceramic materials subjected to impact loading.