The aim of this work is the development of microstructural numerical models of metallic foams. In particular, attention is focused on closed cell foam made of aluminium alloy. By means of a finite elements code, the material cellular structure was shaped in different ways: firstly the Kelvin cell, with both plane and curved walls; finally an ellipsoidal cell defined by random dimensions, position and orientation has been adopted as base unit. In order to validate the foam numerical models, static tests were performed to obtain the typical stress–strain curves and then compared with the numerical analysis results.

DIC (Digital Image Correlation) based methodology gives full field measure of the displacement using a well defined algorithm for matching the images of loaded and load free component, so that displacement in a plane can be evaluated for a certain number of grid reference points on the analysed surface. In this work, the authors present an application of DIC technique to analyse fatigue damage phenomena in two notched GFRC laminates under tensile load. Damage analysis based on optical DIC technique has been performed to detect the damaged areas on the specimen surface. The damage evolution and failure mechanism has been followed monitoring two parameters: the local hysteresis area of stress-strain cycles, the local stiffness variation.

Nel presente lavoro viene presentato lo studio del comportamento a fatica di provini in laminato composito in fibra di carbonio e resina epossidica uniassiali realizzati con tecnologia RTM. L’obiettivo è quello di valutarne il danneggiamento all’aumentare del numero di cicli di carico. A tale scopo, il processo di danneggiamento è stato studiato sia in termini globali, valutando il decadimento della rigidezza del composito, sia in termini locali, valutando le variazioni delle mappe di temperatura in funzione del numero di cicli.

The residual stresses introduced into a component as a result of the mechanical processes necessary for its manufacture significantly influence of fatigue behaviour. The application of loads themselves can, however, alter the initial residual stress distribution, so it is feasible to believe that the residual stress field of a fatigue-stressed structure develops over its entire lifespan. The aim of this article is to analyse the development that the residual stresses initially present in a welded butt joint undergo following the application of the external cyclic load. Comparisons between two residual stress measurements, conducted on the same joint before and after the application of fatigue cycles, have made it possible to obtain interesting information regarding the development of residua stresses in components subjected to fatigue. In particular, it has been found that unlike what is commonly reported, there are particular conditions where the application of a cyclic load results in an increase in the residual stresses initially present, rather than their relaxation. This phenomenon should be taken into consideration in order to avoid unexpected failure in components subjected to fatigue.

In this paper the possibility to employ the Digital Image Correlation (DIC) technique for the mechanical behaviour analysis of metallic foam was investigated. Image Correlation allowed to measure displacement and strain fields on closed and open cells aluminium foam specimens under two different loading conditions (compression and shear) and to characterise, with opportune calculation, information on the mechanical behaviour of foams. The adopted technique is suitable to conduct a deep analysis and also to appreciate the local heterogeneities that appear in a specimen during test. The parameters obtained with DIC analysis are confirmed by the global data furnished by testing machine proving the adopted methodology represents a valid tool for the study of these new materials.

In the present study a new dissipation device for seismic protection of structures is proposed. This device is designed to dissipate the energy entering a structure during an earthquake through the activation of hysteretic loops of an aluminum plate located in the middle of the device itself. In order to maximize the amount of dissipated energy, the design of the device has been performed requiring that the aluminum plate is stressed in an almost uniform way. In particular, the device is designed to concentrate energy dissipation in the aluminum core, while the external steel plates are dimensioned in order to give an adequate stiffness to the device and to limit instability phenomena. Characterization tests have been performed on two typologies of device designed for different levels of the maximum shear force (20 kN and 40 kN, respectively). Moreover, in order to verify the behavior of the aluminum-steel device, characterization tests have been performed on the aforementioned devices realized without the aluminum plate. The results show that the steel plates behave elastically in the range of forces expected in the device during an earthquake, confirming that the aluminum plate is the main element for the hysteretic energy dissipation.

In this paper a discussion about the determination of the stress state corresponding to the application of four-points bending load on a sandwich panel having a core made of closed cell aluminium foam is reported. An analytical model based on laminated plate classical theory is compared to a more complex FEM model, considering the effect of geometric parameters of panels, like core and plate thickness, and of loading mode, like span length. The results show the difficulties to define a reliable model to calculate stress state in this kind of composite material.

Aluminium foam sandwiches are subjected to four point bending fatigue test considering the effect of geometric parameters of panels, like core and plate thickness, and loading mode, like arm distance. Fatigue strength curves are expressed in terms of different stress amplitude parameters calculated using an analytical model based on laminated plate classical theory and a solid FEM model. Nevertheless the relevant fatigue data scatter, originated by foam intrinsic in-homogeneity, experimental fatigue curves are coherent and allow obtaining unified fatigue curves.

Several studies on Digital Image Correlation technique allowed this methodology to be recognised in the last decade as a welloptimised tool for mechanical experimental measures, but much work is to be done to apply DIC based analyses on advanced applications, such as fatigue damage studies and composite materials failure investigations. DIC based methodology gives direct measure of the displacement field using a well defined formulation of matching the images of loaded and load free component, so that displacement vectors can be evaluated for a certain number of grid reference points on the analysed surface. In this work, the authors present the latest research with this method, and analyse fatigue damage phenomena in GFRC laminates under tensile fatigue load employing notched and notch-free specimens. Damage analysis based on optical DIC technique has been performed to detect the damaged areas on the specimen surface and to follow failure mechanisms during lifetime; the damage evolution in composites was performed monitoring two parameters: the local hysteresis area of stress strain cycles, the local stiffness variation. These parameters are evaluated on a grid by means of DIC strain measurements during the entire fatigue life showing encouraging results.

A CDM (continuum damage mechanics) model for damage valuation is here considered and applied to the study of two different typologies of notched and cylindrical specimens. The model presents some parameter and authors determined them in a previous work testing cylindrical and smooth specimens. Firstly fatigue characterization was conducted and the SN curves found. In order to evaluate the capability of CDM model to predict the sequence effect and to simulate a more realistic loading condition, tests with various loading blocks were carried on and in particular high-low, low-high and random blocks were applied to the three specimens considered. Model previsions showed good agreement with results for each geometry considered.

Fatigue behaviour of fastened joints represents a critical issue for aeronautical structure, considering also that a notable amount of data has been collected for static behaviour. In this work, fatigue test of riveted single lap joint made of carbon/epoxy laminates were carried out at different load levels and test frequency. Experimental results showed the importance of monitoring the temperature field in the region between fasteners. Moreover, the evaluation of the residual strength of specimen previously subjected to fatigue load showed a notable improvement of all the mechanical properties.

Purpose - The aim of this work is the development of a procedure able to model the highly irregular cellular structure of metallic foams on the basis of information obtained by X-ray tomographic analysis. Design/methodology/approach - The geometric modelling is based on the feature "pore" characterized by an ellipsoidal shape. The data for the geometric parameters of the instances are obtained with a methodology which is driven by the pore volume distribution curve. This curve shows how much the cells, whose diameter belongs to a given dimensional range, contribute to the reduction of the total volume. Findings - The presented methodology has been implemented into a CAD tool consisting of a Matlab routine identifying the instances of the feature "pore" and a CATIA's macro modelling the closed cells foam. Originality/value - The presented methodology allows to obtain in an automatic way the CAD model of the complex structure of closed cell aluminium foam approximating by considerable accuracy both the density and the volume distribution of the real foams.

Il presente lavoro ha come obiettivo principale lo studio del comportamento a creep della superlega per applicazioni aeronautiche Udimet 720Li. E’ stato seguito un piano di prove di creep a due diverse temperature di prova (650 e 700°C), confrontando i risultati ottenuti su provini lisci e su provini con intaglio. E’ stato inoltre implementato un modello numerico per l’analisi agli elementi finiti dell’influenza dell’intaglio sul comportamento a creep del materiale. Questo piano di prove è inserito in un progetto più ampio avviato in collaborazione con Avio S.p.A. di Torino che prevede, tra le altre cose, l’esecuzione di prove di fatica a basso numero di cicli. Il materiale è stato fornito sotto forma di manufatto dalla Avio S.p.A. di Torino.

Superalloys are the most diffused material for aeronautical and aerospace applications, mainly for turbines and compressors production, because of their excellent resistance at high temperature. Among them, polycrystalline superalloys Udimet 720Li and Inconel 718Plus are interesting candidates for turbine disc application, due to their high temperature strength, good corrosion resistance and excellent workability. The aim of the present work is to study the low cycle fatigue (LCF) behaviour of these two nickel-base superalloys for aeronautical applications. The experimental tests plan has been predisposed with two different working temperatures (650 and 700°C). Materials were provided from Avio s.p.a. Torino in the form of manufactured article. This project includes tensile and creep tests, too.

In this work the mechanical behaviour of CFRP laminates having an artificial wrinkle introduced in the critical section has been studied in the Open Hole Tension and Open Hole Compression configuration. The experimental test allowed determining the failure mechanism and the knockdown of the ultimate strength in five different configurations.

In this work, the possibility using two different technologies to repair industrial flat components made of Waspaloy superalloy was investigated. Specimens present a V notch in the central zone of the gage length, which has been refilled by MicroPlasma and Electro Spark Deposition in order to recover the original thickness of the material. These specimens have been used for a complete mechanical characterization, carrying out static, low-cycle fatigue and creep tests. Finally, results have been compared to base material. Static and low-cycle fatigue test have been carried out at Room Temperature and 538°C, while creep test considered the temperature of 704°C. Results of uniaxial tensile tests showed that the two repair process have a different effect on the mechanical properties. While MicroPlasma produced a reduced yield stress and tensile strength but a good performance whit respect to the elongation to failure, Electro Spark Deposition assured a better mechanical strength but a reduced elongation to failure. Low-cycle fatigue properties have been determined carrying out tests at different temperature (Room Temperature and 538°C). Repaired material showed lower fatigue strength and an increase of the data scatter, especially ESD at 538°C. Finally, creep test carried out on a limited number of specimens allowed establishing some changes about the creep rate and time to failure. MP behaviour was more similar to base material, while ESD show the presence of a marked tertiary creep.

Obiettivo del presente lavoro è la caratterizzazione a taglio di schiume di alluminio tramite un sistema di carico biassiale appositamente progettato e lo sviluppo di modelli numerici microstrutturali di schiume soggette a taglio. Sono state considerate due differenti morfologie di cella. Per la loro rilevanza industriale, le tipologie di schiume studiate sono state la schiuma metallica a celle chiuse Alulight e quella a celle aperte M-Pore. Il modello della schiuma a celle chiuse consiste in un cubo costituito da celle ellissoidali le cui dimensioni e posizione sono variabili in maniera casuale uniforme nel volume di riferimento ed entro un range di valori. Il modello per la schiuma a celle aperte è basato sulla tessellatura casuale di Laguerre, che riproduce sia la regolarità macroscopica che la casualità microscopica delle schiume M-Pore. L’adeguatezza dei due modelli è stata valutata mediante il confronto delle curve sperimentali con quelle numeriche.

In this work, the mechanical properties of Waspaloy superalloy have been evaluated in case of welded repaired material and compared to base material. Test program considered flat specimens on base and TIG welded material subjected to static, low-cycle fatigue and creep test at different temperatures. Results of uniaxial tensile tests showed that the presence of welded material in the gage length specimen does not have a relevant influence on yield strength and UTS. However, elongation at failure of TIG material was reduced with respect to the base material. Moreover, low-cycle fatigue properties have been determined carrying out tests at different temperature (room temperature RT, 538°C and 760°C) in both base and TIG welded material. Welded material showed an increase of the data scatter. Fatigue strength was reduced , with respect to the base material. During test, all the hysteresis cycles were recorded in order to evaluate the trend of elastic modulus and hysteresis area against the number of cycles. A clear correlation between hysteresis and fatigue life was found. Finally, creep test carried out on a limited number of specimens allowed establishing some changes about the creep rate and time to failure of base and welded material. TIG welded specimen showed a lower time to reach a fixed strain or failure when a low stress level is applied. In all cases, creep behaviour of welded material is characterized by the absence of the tertiary creep.

This paper describes the application of active pulsed Thermography (PT) as Non-Destructive Test (NDT) method for the investigation of CFRP aeronautical components. The analyzed specimens include T-shaped stringers, previously monitored by ultrasonic analysis, and laminated flat panels with internal production defects. Several set-up tests allowed to identify optimal configurations for the defects detection according to specimen geometry and defect location. A custom post-processing algorithm has been developed for more precise defects characterization, elaborating thermographic data; whilst a successive full-field contrast mapping construction allowed to propose a reliable defect distribution map and better definition on larger areas. Detection of defects was studied through proper thermal contrast evaluation, with respect to suitable choice of a sound reference area during the transient cooling phase. Influence of Heating time on the thermal contrast has also been studied and the Thermography ability to detect with accuracy and reliable results real small production defects is verified in this work on typical CFRP aeronautical components.

In recent years many techniques for the seismic control of structures have been developed. Among these, the metallic hysteretic devices are able to dissipate a great amount of the energy entering the building during a seismic event, thanks to a stable behavior under cyclic loads that produces a wide hysteretic loop. Steel shear panels are examples of elasto-plastic elements, which dissipate energy under a shear behavior. Generally such dampers are known to possess large energy-dissipation capacity relative to their size; they are cost-effective and are able to protect non-structural elements too. Moreover, the shear panels may be easily installed and substituted in the structure by mean of diagonals on which may be mounted and then connected to the frame. As disadvantage, this kind of energy dissipating devices can dissipate energy only after they sustain inelastic excursions. As a consequence they are ineffective for vibrations that produce interstory drifts smaller than the yielding drift of the device. To overcome this constraint, Rai and Wallace and Foti and Diaferio proposed shear panels made in aluminium alloys. In fact, these alloys are very ductile with a yielding limit lower than ordinary steel. Numerical and experimental researches have been developed on aluminium shear links. Foti and Nobile performed characterization and shaking-table tests on some aluminium shear panels showing instability phenomena and problems of the connections of the devices to the structure. The aim of the present note is to find out the optimum geometrical configuration of an aluminium-steel shear panel in order to dissipate a large amount of the seismic energy.

In this chapter, a new seismic protection device is proposed. It is designed to dissipate the energy entering a structure subject to seismic action through the activation of hysteresis loops of the material that composes it. These devices are characterized by a high capacity to absorb the seismic energy and the ability to concentrate the damage on it and, consequently, to keep the structure and the structural parts undamaged. Moreover, after a seismic event they can be easily replaced. In particular, this chapter proposes a new shear device that shows the plasticity of some areas of the device at low load levels. In order to maximize the amount of dissipated energy, the design of the device was performed by requiring that the material be stressed in an almost uniform way. In particular, the device is designed to concentrate energy dissipation for plasticity in the aluminum core while the steel parts are responsible to make stiffer the device, limiting out-of-plane instability phenomena. The geometric configuration that maximizes the energy dissipation has been determined using a structural optimization routine of finite element software.

In questo studio è stato esaminato l’effetto di trattamenti termici post-saldatura sulle proprietà meccaniche di giunti doppi sovrapposti in lega 2024T3 e 7075T6 ottenuti per FSW. I trattamenti termici sono stati condotti a temperature comprese tra 200° e 450°C e tempi tra 0,5 e 6h. I giunti sono stati caratterizzati con misure di microdurezza Vickers lungo tutto il profilo e prove di trazione ed i risultati analizzati in funzione della temperatura e del tempo di trattamento. I campioni con omogeneità nei profili HV giungono a rottura nel nugget. Il comportamento a trazione nei giunti doppi dopo trattamento a medie temperature (200° e 300°C) è simile a quello del giunto originale e la rottura avviene nel materiale base 7075T6. I giunti trattati ad alta T esibiscono una duttilità ridotta del 50% rispetto ai precedenti ma con la frattura che avviene nel nugget. E’ stata prodotta una correlazione tra durezza e dimensione dei grani nel nugget dei vari campioni.

Residual stresses, introduced into a component by manufacturing processes, significantly affect the fatigue behaviour of the component. External load application produces an alteration in the initial residual stress distribution, so it is reasonable to suppose that residual stress field into a component subject to a cyclic load presents an evolution during the total life. In this work, the authors analysed the evolution that the residual stress field, pre-existing in a butt-welded joint, suffers following the application of cyclic load. The comparison between two residual stress measurements, carried out on the same joint before and after the cyclic load application, allowed to obtain interesting information about the residual stress evolution. It was found that in particular condition, unlike the general opinion, a cyclic load application produces an increasing in the residual stress level rather then a relaxation. This phenomenon is to take well in account in order to avoid unexpected failure in components subjected to a fatigue load.

In this paper a methodology to analyse the shear behaviour of aluminium foam with closed cells is proposed. A biaxial load device expressly designed and the elaboration with Digital Image Correlation technique of the data acquired during the test with a CCD camera allowed determining the displacement and strain fields. This procedure made possible to evaluate the procedure as suitable or not for conducting a shear characterization for metallic foam. The curves obtained showed an initial elastic outline followed by the yield plateau, a peak load and a rapid load drop.

Many factors are involved in determining the fatigue strength of welded joints. It is, however, very difficult to consider their relative importance. The aim of this paper is to isolate the effect of residual stress from other factors, establishing a relation between the amount of residual stress and fatigue life. A geometrical notch due to the weld bead is removed by milling the upper surface of the welded plates. Moreover, specimens are subjected to four-point bend loading. Before conducting the fatigue test, the magnitude of residual stress for each specimen is experimentally evaluated, and then linked to the number of cycles to failure. This relation is analyzed for three different plate thicknesses and for different stress amplitude levels in the high cycle regime. The results clearly show the significant influence of residual stress on fatigue behaviours when the load level is near the fatigue limit.

The structural behaviour of bolted joints of composite laminates for aerospace applications was modelled comparing the shape, amplitude and phase of stress–strain cycles. This study proposes a model for the bolted joints resulting in a typical load–displacement curve, under cyclic loading, significantly affected by hysteretic effects. From the data gathered through the experimental activities, a constitutive relationship between strain and stress was proposed, starting from simple physical models. The assumption of a rigid shift between the laminates was used to correlate load and displacement curves in the different phases of the load cycle. The hysteretic behaviour was attributed to friction phenomena and interpreted using damping coefficients characterizing the global dynamic response of the structural joint.

Si riportano i dati delle tensioni residue introdotte dalle lavorazioni meccaniche per asportazioni di truciolo su provini a sezione cilindrica realizzati nella superlega Inconel718Plus e se ne studia l’evoluzione a seguito dell’applicazione di cicli ad ampiezza di deformazione costante. Le prove di fatica in controllo di deformazione sono state eseguite a temperatura ambiente e con ampiezze di deformazione comprese tra 0.76% e 1.33%. Le prove sono state interrotte dopo 10 e 100 cicli per consentire la misura delle tensioni residue assiali e circonferenziali mediante diffrattometria a raggi X. I risultati ottenuti hanno permesso di stabilire che l’instaurarsi di fenomeni di rilassamento è fortemente condizionato dall’ampiezza di deformazione applicata. La riduzione di tensioni residue, inoltre, è concentrata nei primissimi cicli, più che essere graduale, a differenza di alcuni modelli proposti in letteratura.

Questo lavoro descrive lo studio sperimentale svolto per valutare la resistenza a trazione di alcuni giunti in schiuma di alluminio ottenuti mediante diverse tipologie di collegamento, sia mediante bulloni che adesivi a base di resina epossidica. Sono state considerate diverse tipologie di disposizione dei bulloni e per ogni prova è stato ricavato il valore della forza massima necessaria per la rottura del collegamento. Le migliori prestazioni meccaniche si ottengono con il collegamento realizzato mediante due piastre coprigiunto e resina epossidica. Nel caso di collegamenti mediante bulloni, il carico trasmesso aumenta al crescere del numero di bulloni.

In questo lavoro sono state valutate le proprietà meccaniche di una superlega di Nickel tipo Waspaloy qualora risulti riparato per saldatura con riferimento al metallo base. Sono state eseguite delle prove sperimentali di trazione a diversi valori di temperatura, su provini in materiale base e, a temperatura ambiente, su provini sottoposti ad una saldatura trasversale tipo TIG. La presenza di materiale saldato risulta quasi ininfluente nel caso delle proprietà statiche, ad eccezione dell’allungamento percentuale a rottura che si riduce, anche se non in maniera drastica. E’ stata effettuata, inoltre, una caratterizzazione a fatica oligociclica della lega a temperatura ambiente, sia in condizioni base che su provini saldati TIG. Il materiale saldato presenta una resistenza a fatica inferiore ma comunque ottima rispetto al metallo base. I dati a fatica sono stati esaminati anche alla luce dell’evoluzione di alcuni parametri di danneggiamento quali il modulo elastico e l’area di isteresi.

In this paper the behaviour of a new hysteretic dissipater for seismic passive protection of structures is analysed. The device is made in aluminium and steel and the results of a series of shaking table tests on a 3D two-floor steel frame equipped with these dissipaters are described. The device is subjected to shear forces during a seismic event and it has been designed on the basis of an optimization procedure having the objective of maximizing the energy dissipation. The experimental analysis gives a better characterization of the devices and the real quantity of the dissipated energy. The results show the values of the maximum acceleration and maximum displacements at each floor, together with the maximum interstory drifts measured on the 3D frame subjected to some earthquake records. The same measurements realized on the 3D model without dissipaters subjected to the same earthquakes give the level of the efficacy of the proposed device in reducing the seismic effects on structures.