DISEG

After a brief discussion on crack models, it is demonstrated that cracking is often accompanied by snaps and jumps in the load-displacement response which complicate the analysis. This paper provides a solution by simplifying non-linear crack models into sequentially linear saw-tooth models, either saw-tooth tensionsoftening for unreinforced material or saw-tooth tension-stiffening for reinforced material. A linear analysis is performed, the most critical element is traced, the stiffness and strength of that element are reduced according to the saw-tooth curve, and the process is repeated. This approach circumvents the use of incremental-iterative procedures and negative stiffness moduli and is inherently stable. The main part of the paper is devoted to a regularization procedure that provides mesh-size objectivity of the saw-tooth model. The procedure is similar to the one commonly used in the smeared crack framework but, in addition, both the initial tensile strength and the ultimate strain are rescaled. In this way, the dissipated fracture energy is invariant with respect not only to the mesh size, but also to the number of saw-teeth adopted to discretize the softening branch.

A combined compression and AE test is proposed, in order to study the scaling of fracture and Acoustic Emission in concrete-like specimens with different size and slenderness. The cylindrical concrete specimens were drilled from two pilasters sustaining a viaduct along an Italian highway built in the 1950s. A two-dimensional FEM numerical model of the pure compression tests is also presented, which is able to describe both the discrete cracking at the matrix-aggregate interface and the smeared cracking of the matrix. The adopted mesoscale modeling directly accounts for the aggregate dimensional distribution, being each aggregate explicitly represented. The distribution of aggregate size and position was generated according to the Füller distribution. In this way, the crack patterns can be simulated correctly, as well as the load displacement curve. During microcrack propagation, acoustic emission events can be clearly detected experimentally. Therefore, the number of acoustic emissions can be put into relation with the number of Gauss points in the finite element model where cracking takes place. A good correlation is found between the amount of cracking simulated numerically and the experimental acoustic emission counting for different specimen sizes, and the two quantities show the same exponent with respect to the considered volume. This evidence reconfirms the assumption, provided by fragmentation theories, that the energy dissipation during microcrack propagation occurs in a fractal domain comprised between a surface and the specimen volume.

We examine an application of Acoustic Emission (AE) technique for a probabilistic analysis in time and space of earthquakes, in order to preserve the valuable Italian Renaissance Architectural Complex named "The Sacred Mountain of Varallo. " Among the forty-five chapels of the Renaissance Complex, the structure of the Chapel XVII is of particular concern due to its uncertain structural condition and due to the level of stress caused by the regional seismicity. Therefore, lifetime assessment, taking into account the evolution of damage phenomena, is necessary to preserve the reliability and safety of this masterpiece of cultural heritage. A continuous AE monitoring was performed to assess the structural behavior of the Chapel. During the monitoring period, a correlation between peaks of AE activity in the masonry of the "Sacred Mountain of Varallo" and regional seismicity was found. Although the two phenomena take place on very different scales, the AE in materials and the earthquakes in Earth's crust, belong to the same class of invariance. In addition, an accurate finite element model, performed with DIANA finite element code, is presented to describe the dynamic behavior of Chapel XVII structure, confirming visual and instrumental inspections of regional seismic effects.

We present some numerical simulations of AE due to damage propagation in quasi-brittle materials under compression. To this purpose, the AE cumulative number, the time frequency analysis and the statistical properties of AE time series will be numerically simulated adopting the so-called "particle method strategy". The method provides the velocity of particles in a set simulating the behavior of a granular system and, therefore, is suitable to model the compressive wave propagation and acoustic emission (corresponding to cracking) in a solid body. The numerical simulations correctly describe the compression test in terms of mean stress-strain and crack pattern. The size effects on the peak compressive strength and on the AE count are correctly reproduced. In addition, the amplitude distribution (b-value) and temporal evolution of AE events due to cracking, crucial for the evaluation of damage and remaining lifetime, was simulated according to the experimental evidences.

In the present paper a comparative numerical study of two XIII century masonry towers preserved in Alba (Italy) is described. The buildings have been recently analyzed and monitored because of an emerging damage pattern, also due to some seismic events during the last few years. Nondestructive evaluation techniques have been adopted to assess the present situation of the towers without perturbations. Such investigations revealed the presence of concentrated damage zones as well as the deviation of one of the towers from verticality. Both towers have been analyzed through a preliminary linear elastic analysis, and a good agreement has been found with in situ measurements of the stress level by means of flat jack tests. This model validation allowed proceeding with the nonlinear analysis in order to evaluate cracking and crushing in the masonry. In the case of "Torre Sineo", an increasing tilt mechanism has been simulated. Cracking and crushing of the masonry have been both ...

: The former clinker warehouse, also known as the "Paraboloide" of Casale is an architectural symbol of the concrete Italian industrial heritage. The constructive technology and the structural shape of the thin concrete vault are comparable to the outstanding realizations achievements of the beginning of the XXth century, as for examples the works of E. Torroja and of P.L. Nervi. The metric survey has been achieved using high resolution terrestrial laser scanning technology (TLS), in order to obtain an accurate 3D model with uniform level of detail and precision, in addition to a continuous metric information for each portions and elements of the structure. This procedure allowed a proper reading of the structural typology of the building and the recognition of possible design and construction principles, essential for the conservation project. The laser survey was carried on both intrados and extrados of the huge parabolic vault, performing a high number of scan positions...

The 2010 Maule earthquake produced brittle compression failure of reinforce concrete (RC) walls, due to high axial load, deficient detailing of boundary elements, and the use of thin and slender walls. To prevent new failures of existing buildings, there are several techniques to improve the confinement of boundary walls. The use of carbon fiber fabrics (CFRP), like transversal CFRP bow tie reinforcement, is a non-invasive and of fast application technique, but the effect of that confinement in thin walls has not been studied in detail. Six reinforced concrete specimens, with 100mmx400mm cross section, and 600mm tall, were tested under uniform compression. The specimens were not adequately confined with steel hoops to represent the actual unconfined boundaries of walls. Of the six specimens, two were reinforced with nine transversal CFRP bow-tie reinforcements anchored on both faces of the wall, two were reinforced with twelve transversal CFRP bow-ties, and two were left unconfined....

The Acoustic Emission (AE) technique was used to assess the structural stability of the Asinelli Tower, the tallest building in the city of Bologna, which, together with the nearby tower, named Garisenda, is the renowned symbol of the city. AE is a passive, non-destructive structural evaluation technique based on the spontaneous emission of pressure waves by evolving fracture processes. The monitoring program was carried out with the aid of a USAM tool, which is part of the equipment used at the Fracture Mechanics Laboratory of the Department of Structural Engineering at the Politecnico di Torino. This tool makes it possible to conduct a complete analysis of AE signals, acquire a huge quantity of data from on site monitoring, and identify the microcracks triggering the damage processes in a structure. In the second part of the paper, the results from a preliminary linear analysis are presented, in order to assess the structural behavior of the tower. The cracking and crushing strengths of the masonry have both been compared with the calculated stresses. The numerical analysis gives a valuable picture of the modal response of the tower, providing useful hints for the prosecution of structural monitoring.

In the past two decades, Fractal Geometry developed as a new and powerful mathematical tool able to model a wide class of complex natural systems. Particularly, when joined with renormalization techniques, Fractal Geometry represents the natural and synthetic way to characterize the so-called self-organized processes, emphasizing their universality and the scaling laws arising at the critical points. Nevertheless, the choice of the most relevant seismic parameter is often not trivial, as in the case of Italian seismicity, where magnitude must be replaced by the seismic energy release, in order to obtain unambiguous multifractal spectra. Moreover, to model the stick-slip phenomenon, it is useful to consider the contact mechanics of self-affine rough faults. A numerical model provides the exact distribution of normal and tangential forces at any rough surface. The Renormalization Group technique represents the theoretical framework able to explain the multifractal distribution of contact micro-forces obtained numerically. As a consequence, the experimentally detected multifractal release of seismic energy derives directly from the mechanical work of a multifractal distribution of forces during the slip event.

In the present paper a new method able to easily evaluate the critical height of rupture of liquid films on various substrate surfaces is described and commented. It is automatic and based on the different IR reflectance of solids and liquids. Its limits and advantages are analysed. The heights of rupture of water have been measured on different kinds of polymeric solid surfaces as well as on surfaces whose water repellency has been increased by different mixtures of substituted ammonium salts, used as autophobic rinse aids. In both cases the effect of experimental conditions and the value of contact angles measured by the Wilhelmy microbalance have been correlated with the height of rupture. The peculiar sawtooth trend of force versus immersion in Wilhelmy experiments, performed by using some of these autophobic substances solutions, is described and interpreted.

The process of indentation of brittle and quasi-brittle materials has been investigated both from the experimental and the theoretical point of view. As far as we know, only a few studies have tried to explain the mechanics of cutting due to an indenter which penetrates inside the material. In this paper, an attempt is made to find some general relations for the cutting process in brittle and quasi-brittle materials, under different hypotheses for the microscopic failure behaviour. Fracture patterns in homogeneous brittle solids are obtained by the finite element method in the framework of linear elastic fracture mechanics. Microstructural heterogeneities are taken into account by the lattice model simulation. Although the reality is often much more complex than the theoretical models applied, the study provides interesting indications for improving performance of cutting tools.

In this paper, an attempt is made to find some general relations for the microcutting process in brittle or quasi-brittle materials, under different hypotheses of microscopic failure behavior. Fracture beneath the indenters and sudden chip formation are the main dissipation mechanisms taken into consideration. Fracture patterns in more homogeneous brittle solids are obtained by the Finite Element Method in the framework of Linear Elastic Fracture Mechanics (LEFM). On the other hand, the quasi-brittle response due to microstructural heterogeneities is taken into account by Lattice Model simulations. The analysis is not limited to the more common study of a single indenter. When two indenters are acting in parallel, their mutual distance plays an important role. If the indenters are very close, they behave like a unique larger indenter, whereas if the distance is relatively large, their mechanical interaction vanishes. In addition, when the distance is approximately three to four times their dimension, the mechanism of chipping (with formation of secondary chip between the two parallel grooves) can take place, improving the ratio of removed volume to spent energy and then the demolition ability of the two indenters. Some comparisons are proposed between the presented approach and more sophisticated and computationally demanding models from the literature, as well as with experimental data. The analysis should provide useful hints for the optimal design of super-abrasive tools.

In a previous work [1] the authors showed how to acquire the meso-structural characteristics of undamaged concrete-like materials by a peculiar laser equipment [2]. In order to extend the analysis to damaged disordered materials, a new direct tension test equipment has been developed, that minimizes flexural effects by freely rotating boundary conditions. Increasing levels of damage are obtained, after reaching the peak load, by proceeding along the descending strain-softening curve. After the desired damage level is reached, the load is removed and the specimen is cut to permit the laser acquisition of the most damaged zone. The progressive rarefaction of the effective stress-carrying cross section is described by means offractal concepts. It is worth noting that both the fractal dimension and the measure of the stress carrying cross section decrease after the peak load, and vanish when the specimen is broken apart. Lots d'une pr&~dente ~tude [1], les auteurs ont demontr(la possibilit(d' appr&ier les caract&istiques m&o-structurelles des matMaux desordonn&, comme le b(ton, au moyen d' un dispositif laser [2]. Afin de pouvoir dtendre l'analyse a des mat&iaux desordonn& endommages, il a fallu mettre au point un equlpement d essal a traction directe, pour minimiser les effets deflexion tout en permettant la rotation des sections des extr~mit& de l'&hantillon. Des niveaux d'endommagement croissants sont obtenus par l'application d'une charge sup&'eure a la valeur de pic, suivant la courbe decroissante de radoucissement (strain-softening). Une fois le niveau d' endommagement souhait~ atteint, la charge est enlev(e et l'&hantillon est COUld. Le dispositif laser est ainsi capable d' analyser la section la plus endommage'e. La rardfaction progressive de la section effective est decrite en utilisant des concepts appartenant au domaine de la g&m(trie fractale.

The fracture process in concrete-like materials cannot be properly modelled in an Euclidean framework, due to its complex morphology at the micro-and meso-level. The inherent¯aws interact through a multi-scale process, leading to self-ane fracture surfaces. Moreover, the self-organized network of microcracks displays fractal properties prior to the formation of the ®nal fracture surface. At the same time, due to the presence of pores and voids, the stress-carrying cross section is a rare®ed fractal domain, even from the beginning of the loading process. A new experimental equipment has been developed which allows the entire fracture surface, or any plane cross section, to be digitised and analysed. This represents an important progress with respect to the study of one-dimensional pro®les. In this paper, the three-dimensional algorithms for evaluating the fractal dimension of invasive surfaces and lacunar sections are described. The invasive fractal character of the fracture surfaces is con®rmed. Moreover, the lacunar fractal character of the stress-carrying cross sections, a priori assumed by Carpinteri [A. Carpinteri, Mechanics of Materials 18 (1994) 259± 266], is now proven experimentally.

The preservation, maintenance and restoration of the wood and masonry architectural heritage is traditionally taught in European Community Faculties of Architecture and Civil Engineering, in the context of MS BC, Master and PhD courses especially devoted to the issue. At the beginning will be described the sector of historic masonry construction and subsequently the one of wood.

In the present paper, we describe the results obtained from double flat-jack tests performed varying the size of the masonry prism involved in the test. In addition, not only the deformations have been acquired, but also the acoustic emissions (AE), in order to get information about local cracking during the test. We present a meso-scale numerical model of the test, where every brick of the masonry is modeled in the details. Discrete cracks can arise both in the mortar joints and in the brick units. A good correlation is found between the amount of cracking simulated numerically and the experimental acoustic emissions for different prism sizes. The model is also able to catch the decrease in the compressive strength with increasing size. It is not possible to obtain an easy direct relation between the acoustic emission and the amount of cracking; nevertheless, it is possible to state that the two quantities are proportional to each other when increasing sizes are considered.