Phd in STRUCTURAL AND GEOTECHNICAL ENGINEERING

Thesis title: Numerical modelling and mechanical characterisation of composite materials as micropolar continua

The present work focuses on numerical modelling and mechanical characterisation of composite materials. The mechanical response of composites is strongly influenced by the presence of heterogeneities and discontinuities in the internal microstructure. For this reason, the numerical modelling of composites, including ceramic and metal composites, polycrystals, as well as classical materials like masonry structures and granular materials, is a non-trivial task. A possible approach is to use a micromechanical description of these materials, which provides an accurate evaluation of the material mechanical response. However, the drawback is the high computational costs resulting from the numerous degrees of freedom of the model. Therefore, a possible solution is to assume the material as a continuum. Multiscale techniques are used to provide a reliable macroscale modelling of materials, taking into account the influence of their internal features. This latter approach proves to be computationally convenient. To properly account for scale effect, various nonclassical/ non-local formulations have been proposed in the literature. Among these, micropolar theory has been proved to be very effective in representing mechanical behaviour of anisotropic media, accounting for the particles arrangements as well as their size and orientation. The objective of the work is the modelling of composite materials made of rigid blocks and thin interfaces, characterised by different textures and block sizes, as micropolar continua. Discrete models implemented in a finite element commercial code are assumed as the benchmark of the problem. The constitutive parameters of the equivalent continuous models are identified through a homogenisation procedure. This technique is based on a energy equivalence criterion between the discrete micromechanical model of the material and the continuum model. In particular, modal analysis of composites is investigated in order to explore the advantages of the micropolar description compared to both the couple-stress and classical descriptions. Moreover, the homogenisation procedure is validated by exploiting a heuristic optimisation approach. Specifically, the Differential Evolution algorithm is utilised for the micropolar constitutive parameters identification. Two different numerical procedures are tested and validated in order to pave the way for potential experimental validation. The same approach is also applied in the nonlinear field for the micropolar yield criteria parameters identification.