Luca Collini

Oscillations Control of Rocking-Block-Type Buildings by the Addition of a Tuned Pendulum

This study deals with the dynamical evolutions exhibited by a simple mechanical model of building, comprising a parallelepiped standing on a horizontal plane. The main goal is the introduction of a pendulum in order to reduce oscillations. The theoretical part of the work consists of a Lagrange formulation and Galerkin approximation method, and dry friction has also been considered. From the analytical/numerical simulations, we derive some important conclusions, providing us with the tools suitable for the design of absorbers in practical cases.

Strain Heterogeneity and Damage Localization in Nodular Cast Iron Microstructures

Fracture mechanisms of ferritic/pearlitic nodular cast iron are influenced by the heterogeneous matrix microstructure. This contribution is aimed at understanding the role of a heterogeneous matrix on the localization of damage due to mechanical loading. Therefore, bend specimens made of different nodular cast iron were plastically deformed and the degree of strain heterogeneity determined by comparison with a homogenized response. To address these observations elastic-plastic finite element models of local microstructures were developed. Computed strain maps are examined to understand the link between microstructural features and conditions for damage localization.

A micromechanical model of the evolution of stress and strain fields in ultrafine-grained metal structures under tension

In this study, the authors present a micromechanical finite element modelling approach to the analysis of a set of ultrafine-grained polycrystalline copper structures. The models take into account both realistic grain distribution and the number of refinement passes of the microstructure. The grain size effect is successfully predicted, with non-uniform deformation and shear localization reproduced during global plastic flow in agreement with experimental observations. The models provide a useful tool with which to interpret damage and fracture mechanisms occurring in ultrafine-grained structures.

Oscillations of a rocking block with an added pendulum

Interactions between buildings and earthquakes have been studied for a long time, and there is much literature about them. This research deals with the dynamical evolutions exhibited by a simple mechanical model of building, comprising a parallelepiped standing on a horizontal plane. The main goal is the introduction of a pendulum in order to control the vibrations; dry friction has also been considered. The theoretical part of the work consists of a Lagrange formulation and Galerkin approximation method. From this theoretical standpoint, we derive many important analytical or numerical results, providing us the tools suitable for the design of absorbers in practical cases.

Modal tests on buildings: correlating large amounts of acquisitions with different space–time collocations

This article deals with a method for the time and space superposition of data acquired in the dynamical testing of large structures. The method permits two procedures comprising the integration of sets of measurements taken at different times and the integration of sets of measurements taken at different places. The latter is a very useful feature when dealing with huge structures, such as big buildings comprising a number of different architectural details. The validation of this method is a case study consisting of many dynamical tests performed on an ancient castle.

Mechanical properties of copper processed by Equal Channel Angular Pressing – a review

The Equal Channel Angular Pressing is a hardening treatment with which ductile metals can beprocessed to refine their grain and sub-grain structure. This process enhances the mechanical strength of metalsin terms of tensile strength, stress-controlled fatigue strength, and fatigue crack growth resistance. In this paperthe authors draw a review of the major results of a wide research activity they carried out on a coppermicrostructure processed by Equal Channel Angular Pressing. The essential results are that tensile and fatiguestrengths of the so obtained refined structure are improved by a factor of two with respect to the originalcoarse-grained metal. The fatigue crack initiation mechanism and the stability of the refined microstructureunder cyclic loading are topics also discussed, evidencing the essential role of the process and of the materialparameter, as the content of impurities in the microstructure. In this review, the authors also underline somecritical aspects that have to be more investigated.

La modellazione microstrutturale di materiali a struttura eterogenea: princìpi ed applicazioni

Molti problemi della Meccanica e Fisica dei Solidi e della Scienza dei Materiali, non sono facilmente risolvibili con gli approcci tradizionali. Oltre allo studio delle proprieta effettive dei solidi eterogenei, vi e la crescente necessita di incorporare un maggiore numero di informazioni sui meccanismi di deformazione e danneggiamento generati alla microscala, anche per i materiali abitualmente considerati omogenei. Micromeccanismi di cavitazione e concentrazioni locali di tensione e deformazione, sono indispensabili per spiegare fenomeni non-lineari come la rottura di fatica o il cedimento duttile, altrimenti non inquadrabili con approcci classici di tensioni e deformazioni medie. La micromeccanica si occupa della determinazione precisa, o di una stima accurata, di grandezze di campo microstrutturali locali. In questo lavoro sono illustrati i principi che sono alla base dell’approccio micromeccanico, come i concetti di multiscala, di distribuzione statistica delle fasi, di descrizione mediante volumi di riferimento e di omogeneizzazione e localizzazione, e, attraverso alcune applicazioni pratiche delle principali tecniche di modellazione, sono illustrati e discussi criticamente i risultati della ricerca effettuata su varie strutture di ghisa nodulare.

Vibration Analysis for Monitoring of Ancient Tie-Rods

This paper presents an application of vibration analysis to the monitoring of tie-rods. An algorithm for the axial load estimation based on experimentally measured natural frequencies is introduced and its application to a case study is reported. The proposed model of a tie-rod incorporates elastic bed-type boundary conditions that represent the contact between stonework and the tie-rod. The weighed differences between experimentally and numerically determined frequencies are minimized with respect to the parameters of the model, the main being the axial load and the stiffness at the tie-rod/wall interface. Thus, the multidimensional optimization problem is solved. Results are analysed in comparison to a model with simple fixed-end boundary conditions. In addition, the analytical formulation of the problem is delivered.

NUMERICAL METHOD FOR ESTIMATION OF TENSILE LOAD IN TIE-RODS

The work introduces a two-phase method for determination of axial loads in tie-rods. The method described here consists of an experimental activity and an automated numerical calculation. The influence of considering an elastic Winkler-type bed to model the tie-rod constraint inside the wall has been investigated. The algorithm used for calculation involves a solution of a functional minimization problem, where the tensile load and the stiffness of elastic foundation at the edges are used as optimization parameters and the error function, which describes the deviation between the frequencies measured and those calculated using finite element method, is minimized. Qualitative analysis of the results showed a significant reduction of the error compared to models with different boundary conditions. The method showed to be conservative for the strength evaluation of the rods, because the optimal values of tensile loads appeared to be higher than the load in perfect encastre conditions.

Fatigue Crack Propagation in Friction Stir Welded Joints of Particulate Metal-Matrix Composite

In this work the fatigue crack growth properties of friction stir welded butt joints are evaluated. Fatigue Crack Growth (FCG) tests have been carried out on two particle-reinforced aluminium alloys (AA6061/Al2O3/20p and AA7005/Al2O3/10p). FCG properties have been evaluated at the centre and at the side of the weld, respectively. The results are compared with FCG properties of base materials. The role of alumina in particles inside the matrix is evident in the threshold region: this can be explained in terms of Roughness Induced Crack Closure.