Stefano de Miranda

Drying of salt contaminated porous media: Effect of primary and secondary nucleation

The drying of porous media is of major importance for civil engineering, geophysics, petrophysics, and the conservation of stone artworks and buildings. More often than not, stones contain salts that can be mobilized by water (e.g., rain) and crystallize during drying. The drying speed is strongly influenced by the crystallization of the salts, but its dynamics remains incompletely understood. Here, we report that the mechanisms of salt precipitation, specifically the primary or secondary nucleation, and the crystal growth are the key factors that determine the drying behaviour of salt contaminated porous materials and the physical weathering generated by salt crystallization. When the same amount of water is used to dissolve the salt present in a stone, depending on whether this is done by a rapid saturation with liquid water or by a slow saturation using water vapor, different evaporation kinetics and salt weathering due to different crystallization pathways are observed.

A Mixed 4-Node 3D Plate Element Based on Self-Equilibrated Isostatic Stresses

A new mixed stress 4-node flat shell finite element, designed for the linear and nonlinear analysis of folded plate structures, is presented. The kinematics of the element is defined by 24 dofs with in- and out-of-plane displacements assumed to be quadratic, controlled by displacement and rotation parameters through an Allman like interpolation, and flexural rotations assumed to be bilinear. The assumed stress approximation, described within a local Cartesian frame aligned with the element orientation, is self-equilibrated and ruled by the minimum number of parameters. The element does not suffer from kinematical locking and rank defectiveness. Many numerical tests show the very good performance of the element.

Numerical insights on the seismic behavior of a non-isolated historical masonry tower

In this paper, numerical insights on the seismic behavior of a non-isolated historical masonry tower are presented and discussed. The tower under study is the main tower of the fortress of San Felice sul Panaro, a town located near the city of Modena (Italy). Such a tower is surrounded by adjacent structural elements and, therefore, is not isolated. This historical monument has been hit by the devastating seismic sequence occurred in May 2012 in the Northern part of the Emilia region (the so-called “Emilia earthquake”), showing a huge and widespread damage. Here, in order to understand the behavior of the structure, its interaction with the adjacent buildings and the reasons of the occurred damage, advanced numerical analyses (both nonlinear static and dynamic) are performed on a 3D finite element model with different levels of constraint supplied by the adjacent structural elements and a detailed comparison between the simulated damage and the actual one is carried out. The results of the conducted numerical campaign show a good agreement with the actual crack pattern, particularly for the model of the tower that considers the adjacent structural elements.

On the consistency of finite element models in thermoelastic analysis

In thermoelastic analysis, compatible finite elements may predict oscillating stresses if the temperature distribution is not coherent with the element strain coming from the assumed displacement approximation. Modelling the stress field as an independent variable could be a viable alternative to avoid these spurious outcomes. To this purpose, the behavior of mixed, hybrid stress and equilibrium finite element models in the presence of initial strains due to temperature is investigated and compared with the usually employed displacement approach. The consistency requirement for each model is established and the spurious outcomes due to inconsistent temperature fields are discussed, examining also the effects of element geometry distortions. The analysis, illustrated through some numerical tests, shows that a direct approximation for the stress field can eliminate, or at least reduce, the generation of spurious stress modes, although this is not a priori guaranteed by a mixed approach.

Recovery of consistent stresses for compatible finite elements

Abstract In displacement based finite element models, stresses deduced directly from the constitutive relationship can show local erratic behaviour. This occurs in problems involving initial stresses or strains, or varying rigidities over the element domain, when local stresses do not meet a specific consistency requirement. In this context, an integrated procedure for recovering consistent stresses, that is stresses ridded of spurious outcomes, is proposed. The procedure is developed within a general weighted residual approach, suitably specialized for the purpose. The relationship between the proposed procedure and those based on the Hu–Washizu formulation is also elucidated. For illustration purpose, some numerical tests are included.

Direct imaging of defect formation in strained organic flexible electronics by Scanning Kelvin Probe Microscopy

The development of new materials and devices for flexible electronics depends crucially on the understanding of how strain affects electronic material properties at the nano-scale. Scanning Kelvin-Probe Microscopy (SKPM) is a unique technique for nanoelectronic investigations as it combines non-invasive measurement of surface topography and surface electrical potential. Here we show that SKPM in non-contact mode is feasible on deformed flexible samples and allows to identify strain induced electronic defects. As an example we apply the technique to investigate the strain response of organic thin film transistors containing TIPS-pentacene patterned on polymer foils. Controlled surface strain is induced in the semiconducting layer by bending the transistor substrate. The amount of local strain is quantified by a mathematical model describing the bending mechanics. We find that the step-wise reduction of device performance at critical bending radii is caused by the formation of nano-cracks in the microcrystal morphology of the TIPS-pentacene film. The cracks are easily identified due to the abrupt variation in SKPM surface potential caused by a local increase in resistance. Importantly, the strong surface adhesion of microcrystals to the elastic dielectric allows to maintain a conductive path also after fracture thus providing the opportunity to attenuate strain effects.

Consistency and recovery in electroelasticity. Part I: Standard finite elements

Developing finite elements for electroelastic analysis requires a special care as the coupling in the discrete equations depends on the matching between the approximations assumed for mechanical and electrical variables. To provide a formal basis to this intuitive remark, a notion of consistency is established and a rigorous consistency analysis is presented for the standard finite element model based on assumed displacement and electric potential. In this way, specific analytical requirements are obtained which serve as a guide to select the interpolation functions for primary variables. Moreover, the analysis shows that violating consistency can be reflected as spurious outcomes upsetting the local distributions of secondary variables. Indeed, this undesirable effect is shown to be typical of the standard approach if stress and electric flux density are computed via the constitutive equations. To cure the trouble, an alternative recovery procedure is devised based on the consistency analysis. The procedure is variationally correct and reconstitutes in a consistent manner the distributions of stress and electric flux density.

A MESH GENERATION METHOD FOR HISTORICAL MONUMENTAL BUILDINGS: AN INNOVATIVE APPROACH

The numerical modeling of historical monumental buildings is a challenging task for contemporary civil engineers. One of the main reasons for this is that the use of traditional simplified structural schemes is inadequate due to the complex geometry of such historical structures. Therefore, it is necessary to resort to a fully 3D modeling technique that is performed using Computer Aided Design (CAD). In general, CAD-based modeling is an expensive and complex process and it is often performed manually by the user, which inevitably leads to the introduction of geometric simplifications or interpretations. In this study, an innovative mesh generation approach for the structural analysis of historical monumental buildings is presented. The method consists in a peculiar breakdown of the geometry starting from laser scanner or photogrammetric surveys. Moreover, this new approach involves a structural discretization that guarantees the generation of 3D finite element meshes as well as their mechanical characterization. The most relevant feature of the proposed method is the possibility to directly exploit 3D and detailed point clouds surveyed on historical buildings for structural purposes. As a result, a large reduction in the required time in comparison to CAD-based modeling procedures is achieved. A geometrical and structural validation of the method is carried out on a masonry tower application. The findings show good reliability and effectiveness of the mesh generation approach.

Simple Beam Model to Estimate Leakage in Longitudinally Cracked Pressurized Pipes

Losses from water distribution systems are reaching alarming levels in many cities throughout the world. Leakage is often the principal cause of water loss because of aging and deterioration of these systems, and pressure has been verified to assume a key-role in water-loss management. This paper presents a simple analytical model based on a beam with elastic constraints to estimate the leak area (from which the leakage is then estimated) in longitudinally cracked pressurized pipes and to evaluate the effect of pressure on the opening area of the crack. The model is calibrated on the results of a three-dimensional finite-element analysis and then validated by experimental results. The validation has been carried out for a wide range of pipes made of different materials (PVC, cast iron, asbestos-cement, and steel) with radii ranging from 27.5 to 110 mm, thicknesses from 1.5 to 12 mm, and crack lengths from 50 to 200 mm. The beam model, notwithstanding its simplicity, provides reliable leakage evaluations in longitudinally cracked pressurized pipes.

On the seismic behavior of the main tower of the San Felice sul Panaro (Italy) fortress

The medieval fortresses are a very common and distinctive type among the Emilian historical constructions and the earthquakes of May 20th and 29th, 2012 underlined their high vulnerability. Among those heavily damaged, there is the fortress of San Felice sul Panaro located between the two epicenters. This study presents some FE results regarding the behavior under seismic actions of the main tower (Mastio tower). The Mastio has peculiar geometric features and represents a typical example of non-isolated tower. In fact, it is constrained in very different ways by the surrounding parts of the fortress along two of its sides: on the north side it is constrained by the perimeter wall until one third of his high, while a stiffer building constrains it on the west side. In order to remodel the entire fortress, a multidisciplinary project involving the Municipality of San Felice sul Panaro and four Universities of the Emilia- Romagna (Bologna, Ferrara, Parma and Modena) together with the University of Genoa is g...