Structural as well as material deformation and failure cannot be understood at a single scale alone: they require the investigation of multiple scales to capture the progression of the fundamental physical mechanisms. The breakdown of the basic constituents of any material ultimately leads to the failure of its overall structure and intended function. Materials failure, ranging from the earth crust in earthquakes, to the collapse of buildings, to the swelling and fracturing in energy storage materials, to bio-chemo-mechnical effects in cells and living tissues, impacts on millions of lives and is a major issue in the health and wealth strategic plan of UNIBS. The group of Mechanics of Solids ans Structures, with its Applied Seismology and Structural Dynamics Research Center frames in this vibrant context.

Centers

CeSiA - Applied Seismology and Structural Dynamics Research Center

Interests

• Constitutive modeling of geomaterials. Geomaterials are a wide class of materials, usually identified as brittle, or quasi brittle. For example, rocks, soils, concrete and masonries are geomaterials. Also advanced materials such as ceramics belong to this class. We develop constitutive models for this class of materials, under the classical Theory of Plasticity. Such constitutive models may be applied in nonlinear Finite Element analysis of advanced engineering problems, as geotechnical applications or simulations of the mechanical behavior of old masonries. From a numerical point of view, the constitutive modeling of these materials is particularly critical, due to the difficulties to achieve convergence in Finite Element analyses of boundary value problems. For this reason, differently from the typical literature approaches, the numerical efficiency and the stability of the integration algorithm is a key point in order to assure the applicability of the developed models to a wide class of practical engineering problems.

• Constitutive modeling of metals subject to large strains. This research theme concerns the development of advanced constitutive models for metals subject to large deformations and their integration and implementation in the Abaqus FE code. An accurate modeling of the material mechanical behavior is fundamental to simulate nonlinear mechanical processes involving metals using the Finite Element Method. These simulations are very useful in the engineering practice, since they can be adopted to design metal forming processes. In this way, firstly, using parametric analyses one can optimize productive processes in order to minimize the costs. Secondly, especially for cold processes, one can simulate (and optimize) the mechanical properties of the produced pieces. Especially for this scope it is fundamental the correct reproduction of residual stress profiles, that requires an accurate modeling of the material mechanical behavior, usually subject to severe loading-unloading cycles at large plastic strains during the process. Finally, in case of wrong combination of the design parameters, an advanced constitutive model should reproduce the formation of defects (such as chevron cracks or surface defects) in the workpieces during the simulations.

• Crack propagation modeling in brittle materials: numerical simulations, multiscale analysis, plasticity analogies, SIF evaluation algorithms, real-life applications. This research frames the problem of three-dimensional quasi-static crack propagation in brittle materials into the theory of standard dissipative processes.Variational formulations are stated. They characterize the three dimensional crack front quasi-static velocity as minimizer of constrained quadratic functionals. An implicit in time crack tracking algorithm that computationally handles the constraint via the penalty method algorithm is introduced. Novel outcomes arising from a visco-plastic regularization allows 3D crack tracking and solution of real-life applications.

• Development of analytical solutions for drawing processes. This research project is focused on the development of analytical tools for the estimation of the force to cold draw wires or rectangular plates. These analytical models must take into account the different combination of die geometries, area reduction, and the friction conditions. Such models are very important to the design of drawing metal forming processes. In fact, even if the numerical analyses are probably the most powerful tool today available to optimize metal forming processes, the design of a real industrial process involves parametric analyses, which require a single numerical simulation for each combination of the process parameters. For this reason, analytical models, allowing (at least) an initial design of the process, are very important. In fact, it should be noted that the most adopted design procedures of metal forming processes in the engineering practice are still based on the limit analysis technique. The developed analytical solutions are based on the limit analysis techniques.

• Effective mechanical properties of polycrystalline metals, with particular focus on the size effects. We develop strain gradient plasticity models accounting for dislocation mechanics with the purpose of describing the size effects observed in metal components in the size range spanning from a few tens of nanometers to a few tens of micrometers. At this scale, the mechanical properties of a metallic specimen subject to nonuniform strain fields strongly depend on the specimen size and the observed mechanical response can be quite different with respect to that observed at the macroscopic scale: diminishing the specimen size, one observes an improvement of the mechanical properties (strengthening and variation of strain hardening). The classical Plasticity Theory cannot take into account such phenomena, since it is not explicitly referred on any intrinsic material scale. Other models have been proposed in the literature, called "Gradient Plasticity models". They are based on nonlinear partial differential equations in which the spatial gradient of the plastic strains explicitly appears. The numerical integration of such differential equations using the Finite Element Method is particularly complex, due to their strong nonlinearity. The research topic is then based both on the development of this class of constitutive models, and on the numerical techniques allowing their integration for the simulation of engineering boundary value problems.

• Effective mechanical behavior of syntactic foams. We develop micromechanical models to predict the effective properties of syntactic foams. They are particulate composites in which a thermoset polymer matrix, usually made of vinyl ester or epoxy resin is filled with of hollow spheres, also called balloons. They are usually made by glass, ceramic, or metal. These composites find applications in aerospace and marine systems for their closed-cell microstructure. The mechanical behavior of the syntactic foams is reproduced by means of numerical homogenization techniques, based on Finite Element micromechanical simulations of a representative volume (RVE) of composite material. To perform these analyses, it is fundamental the modeling of the mechanical behavior both of polymeric materials constituting the matrix, and of the failure of the fillers, typically of brittle type.

• Electro-chemo-mechanical behavior of Ionic Polymer Metal Composites. Ionic polymer-metal composites (IPMCs) are synthetic composite nanomaterials that can be developed as artificial muscle under an applied voltage or electric field. IPMCs are composed of an ionic polymer (typically Nafion) whose surfaces are chemically plated or physically coated with conductors such as platinum or gold. Under an applied voltage, ion migration and redistribution due to the imposed voltage across a strip of IPMCs result in a bending deformation. IPMCs are candidated to be used as smart devices for medical applications. For instanace they could be adopted to guide wires and endovascular steeres and stirrers for navigation in human blood vasculature, artificial muscles to correct paralysis or for assisting a weak heart. We study the electrochemomechanics of Ionic Polymer Metal Composites (IPMCs) with the purpose of developing predictive models for IPMCs' sensing, actuation, and energy harvesting.

• Mechanical behavior of composite structures. This research focuses on developing structural models for the accurate estimate of the stress field in sandwich panels and other composite structures subject to relevant boundary layer effects.

International collaborations- in alphabetical order

• B. Lecampion, EPFL Lausanne, Switzerland;

• M. Gei, Cardiff University, United Kingdom;

• N. Gupta, Tandon School of Engineering, New York University, USA;

• J. Llorca, Universidad Politécnica de Madrid, Spain;

• E. Martínez-Pañeda, Imperial College, London, United Kingdom;

• C.F. Niordson, Technical University of Denmark;

• M. Porfiri, Tandon School of Engineering, New York University, USA;

• A.M. Puzrin, ETH Zurich, Switzerland;

• J. Segurado, Universidad Politécnica de Madrid, Spain;

• J. Reinoso, Universidad de Sevilla, Spain;

• P. Wawrzynek, Cornell University, Ithaca, USA;

• J.R. Willis, University of Cambridge, United Kingdom.