This work considers the possibility to apply the improved hybrid Capacitor Discharge Welding process (CDW), based on projection Welding principles, to aluminium alloy Al 5754. The innovative aspects are the effective possibility to weld aluminium alloys with CD Welding process and establish the mechanical weld characteristics and defects presence as function of the technological parameters. Intrinsic CDW process characteristics need to be investigated on the base of interaction between the technological aspects and the related mechanical properties. The geometrical and technological factors are combined to improve the welding efficiency and reduce defects size. In order to assess the technological parameters, visual and ultrasonic inspection of the most significant welded joints were performed and room temperature tensile and fatigue tests were executed to verify the welding results.

In case of composites, fatigue damage can be analysed by measurements based on compliance variation and dissipation energy, during the fatigue cycling load. Final results are interesting and show the possibility to monitor the damage state with these procedures

This work studies the improved hybrid Capacitor Discharge Welding process (CDW), based on projection welding principles applied to stainless steel AISI 304. The innovative idea is to modify the igniting points geometry on the section to be welded and optimize the weld characteristics in order to guide the local fusion processes more uniformly on the whole area and enhance the weld properties. Eight different profile geometries for the contact surfaces have been realized in order to evaluate the geometry influence on joints quality and according to process parameters influence. The mechanical behavior of the welds has been verified with static characterization at room temperature and fatigue tests for welded samples with the better observed microstructure.

Lo scopo di questo lavoro è quello di identificare le potenzialità e i limiti della tecnica termografica pulsata nell’individuazione e caratterizzazione delle difettosità su GFRP, confrontando i risultati sperimentali con quelli di analisi numeriche agli elementi finiti che riproducono le stesse indagini. A partire dai risultati sperimentali ottenuti, che permettono di determinare caratteristiche calorimetriche del difetto in base alla risposta termica in superficie, viene simulato con modelli numerici appositi agli elementi finiti il comportamento termico del difetto in relazione ai parametri di conducibilità, allo spessore del composito e alla profondità ed estensione del difetto. Il metodo consente di implementare in maniera rapida differenti modelli numerici e caratterizzare numerose tipologie di difetti, stabilendo delle correlazioni iniziali fra posizione dello stesso e risposta termica del modello di partenza.

Quest 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 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 dei bulloni.

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.

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.

Complex time dependent damage phenomena are characteristic of Composites under fatigue; this is due to material structural heterogeneity and fibre/matrix interactions. Fatigue life is mainly determined by the progressive damage evolution, difficult to be modeled with numerical tools or mathematical frames. Experimental methods are to be applied to achieve correct information about the evolving fatigue damage and the authors propose a methodology based on Digital and Thermal Image analyses, to measure on notched GFRC specimens the dissipative sources in terms of temperature and deformation based data; starting from previous works, the measurements procedures were improved for these techniques, in order to obtain useful results with notched composites and capable to detect different damage states on specimen surface.

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.

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.

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.

Purpose of the job is to perform measurements on two specimens with ND control procedures on composite material with carbon fibers and different thickness, containing artificial defects similar in shape, but with different typology, in order to allow the detection tests to be adapted on large and small defects at certain depths. The analysis was performed using the Thermography Pulsed technique and Phased Array ultrasonic inspection, comparing the detection of artificial defects of different sizes and different typology, identified at different depths as with the real ones. All the important factors that enable the success of a scanning method with ND controls were studied, allowing to evaluate the efficacy of the used techniques and tools in the studied cases.

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.

This work explores the possibility to apply thermographic technique as non destructive test to control glass fiber reinforced polymers having artificial defects. Three symmetrical composite panels reinforced with fiberglass (unidirectional E type fiberglass and epoxy resin) were realized through hand lay‐up and artificial inter‐laminar delaminations of various geometry were introduced at different depths. Several thermal stimulation procedures with different ways to consider the defects in the specimens enable to optimize the thermographic technique, by solving some initial problems related to the experimental set -up and issues. Above all, the minimum requirements for defect which can be revealed trough pulsed thermography have been established, according to present experiences. The importance of geometric and thermo-physic characteristics of the defect related to the material in which it is inserted has been observed, as well as the surface conditions of the observed laminate faces

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.

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.