Macroscale Mechanistic Modelling of Structured Materials

Research focus

The Peer review has evaluated this group as Average

The focus of this group is on the theoretical formulation and the numerical implementation of specific mechanistic models for describing the dynamic response of complex structured materials. In particular, the present research is aimed at develop computational models capable of describing the macroscopic mechanical behaviour of large structures made of heterogeneous solid materials. Since the execution of effective time-history analyses necessitates the use of discrete models with reduced number of degrees of freedom, the trick lies in achieving a balance between accuracy in the geometrical description and the need for a realistic damage material model. Thus macroscopic material models capable of accounting for the peculiar behaviour of structured materials are particularly required when engineering applications call for a realistic description of the global response of large structures. By means of a full discrete point of view the constitutive laws of structures and heterogeneous solid materials are described as discrete mechanisms made up of the assembly of masses connected by simple elastic-plastic springs in the spirit of the rigid body spring model (RBSM) or with a fibre column-beam approach. It is worth noting that these elastic-plastic devices can be defined by a direct discrete formulation that does not necessarily require a differential formulation of the field equations, as is the case for the continuum solid material. Focusing on the global dynamic response at a level of detail that was larger than the size of the minimum periodic cell, this approach proved successful in transferring the essential mechanical characteristics of the internal texture, at the micro-scale, to the macroscopic element scale, even when adopting very few degrees of freedom. An innovative rigid element model and a fibre beam-column element have been specifically developed and implemented in this frame, with applications to the dynamics of complex materials for which it is essential to model the combined axial and shear damages, namely masonry-like materials and reinforced concrete column-beams. The rigid element approach is proposed for application to the in-plane dynamic analysis of masonry-like composite materials whose response is strongly related to mechanical deterioration and hysteretic energy dissipation. The conceptual core of this model is the macroscopic unit cell defined by four quadrilateral rigid elements connected to each other by two normal springs and one shear spring at each side. The cell size should be equal or larger than the minimum representative volume (RVE) of the heterogeneous solid material whose macroscopic behaviour can be related an orthotropic Cosserat continuum. In particular, the orthotropy of the shear response and the local mean rotation of the blocks, which depend on the different geometric arrangement of the vertical and horizontal material joints as well as the shape and size of the original blocks, are features that can accounted at the macro-scale. Beyond the linear elastic field, and specifically for earthquake engineering applications, the description of the material response under cyclic loading is paramount. The main macroscopic constitutive aspects are: very low tensile strength; significant post-elastic orthotropy combined with texture effects; the dependence of the shear strength on vertical compression stress; progressive mechanical degradation during repeated loading; and the energy dissipation capability. To this end a simplified heuristic approach is proposed, based on the phenomenological consideration of the main in-plane damage mechanisms that can be described at the meso-scale by adopting specific separate hysteretic laws for the axial and shear deformation between the elements. This separation lead to a large reduction in computational effort, though a Coulomb-like law is adopted in order to relate the strength of the shear springs to the vertical axial loading. The classical fibre models are able to describe accurately and economically (computationally-wise) flexural behaviour and its interaction with axial force: therefore, they are widely applied in struc150 tural analysis of slender RC beam-columns, while they fail in the determination elements prone to shear failure. The complexity of this problem arises from the different interacting phenomena that need to be considered, namely, crack propagation, the transfer of compression stresses from the point of load application to the supports, aggregate interlock across open cracks, dowel action, and the steel-concrete bond. Coupling a rotating smeared-crack constitutive model with a fibre model, a beam-column element has been developed with the aim of predicting the nonlinear behaviour in shear of RC structures throughout a significant range of design parameters, load conditions, and reinforcement arrangements.

Dipartimento di afferenza

Dipartimento di Ingegneria Strutturale (DIS)

Docenti afferenti

Full Professors
Vincenzo Petrini
Associate Professors
Siro Casolo
Assistant Professors
Lorenza Petrini
Giacomo Boffi