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The present work introduces a different data processing strategy, proposed in order to improve sub-surface defect detection on industrial composites; in addition, a resume of thermal data processing with most common algorithms in literature is presented and applied with new data. A deep comparison between the common absolute contrast, DAC, PCT, TSR and derivative methods and a new proposed contrast mapping procedure is implemented. Thermographic inspection was done in reflection mode on a Glass Fiber Reinforced Plastic plate, with flat bottom hole defects. Thermal data computation method is found to be critical for simultaneous defect detection and automatic mapping, optimized to identify defect boundaries at specific depth, with help of accurate image processing, implemented in a Matlab GUI for a reliable and rapid characterization of internal damage. The new processing approach, the Local Boundary Contrast method, elaborates different contrast maps and facilitates recognition of damage extension. Tanimoto criterion and the signal-to-noise ratio method were applied as a criterion to assess defect detectability of various processing methods.

The use of mechanical fasteners is still main assembling method for CFRP sub-structures in aircrafts and helicopters. However, this type of joint introduces complex stress field in the hole surroundings producing failures risk. In this work, a progressive damage 3D model of the riveted joint has been implemented to predict the residual strength and compliance after first damage signs and reproduce the final failure of composite joints under tensile test. Two 3D FEM Models were used and results are compared to experimental tests. The junction stiffness under load was evaluated and preliminary analysis shows both the coefficient of friction and preload induce not significant alteration of the composite joint behaviour. On refined model, a 3D Hashin-type failure criterion was used to analyze damage of matrix and fibers, beginning in the hole where contact conditions with the bolt pin and head are more severe and is evenly distributed in various plies, leading to final rupture of second or third one. Successively, delaminations were introduced with cohesive model, since is considered to produce reliable results, because the initial compression collapse in critical layers in contact with rivet is proved to affect shear and compression load transfer to other layers, giving rise to localized internal delaminations, propagating successively in width.

The fatigue life prediction of components in CFRP subjected to random fatigue load was carried out considering different fatigue damage models. The selected component is a filled hole tension specimen, which is representative of the typical aeronautical riveted joint. The stress state of the riveted section was determined through a FEM model, created with CODE ASTER. Subsequently, the fatigue load history was introduced, calculating the stress field for the different load levels. Applying the constant life diagrams, fatigue cycles were counted and the damage was evaluated. Finally, the damage that occurs in the different ply of the specimen was calculated using different fatigue damage criteria present in literature. The creation of a parametric FEM model offers the possibility of optimizing the study and constitutes a useful tool in order to choose the most suitable damage criteria for the particular geometry and loading mode of the component. Moreover, different severity of the load history and different load sequence were introduced to show their effect in the fatigue failure.

This paper describes the application of active pulsed Thermography (PT) as a 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 plates with internal production defects. Several set-up tests allowed to identify optimal configurations for the defect detection, according to specimen geometry and defect location. A custom post-processing algorithm has been developed to improve thermographic data for more precise defect characterization, whilst a successive full-field contrast mapping allows to achieve a reliable defect distribution map and a better definition on larger areas. Detection of defects was studied with a specific thermal contrast evaluation, with a suitable choice of undamaged reference area during the transient cooling phase. The influence of heating time and experimental set-up on the thermal contrast results has also been studied; moreover, the ability of thermographic technique to detect real small production defects with accuracy and reliability is verified for CFRP aeronautical components.

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.

This paper deals with an overview of improved UT control procedures, related to defect investigation on advanced CFRP composite elements for aeronautical structures; the inspection procedures were evaluated with different frequency probes, employing direct contact, water stream and immersion methods. In this work, ultrasonic and thermographic controls of CFRP components are presented for detection of different types of defects (real and artificial) in order to evaluate sensitivity and detectability techniques. This work presents a preliminary ultrasonic study on CFRP components employing different probes and tools. For these aims, inspections of CFRP plates with artificial defects are optimized and compared to thermographic acquisitions with active pulsed technique, in order to calibrate Ultrasonic Technique tools and verify procedures for suitable results. In a second stage, improved ultrasonic scanning investigations are applied on real parts, containing real production defects in terms of small delamination and diffused porosity of T-shape stringer and resin/matrix excesses between layers, localized in the thin cap and web of structural civil aircraft stringers.

The identification of defects in real components made in composite material assumes considerable importance in the aeronautical field, as irregularities in the material can compromise its functionality. Also, the possibility (NDT) to verify the effectiveness of repairing on the component during exercise through non-destructive testing techniques has great importance. The goal of the present study is the application of the ultrasound and thermographic techniques for the identification of defects in a repaired component and the evaluation of the damage caused by the application of cyclical fatigue loads. All the important factors were studied, improving ultrasound scanning and allowing evaluating the effectiveness of the techniques used. In this paper, one case of study was proposed for non-destructive damage evaluation employing Phased Array ultrasonic and thermographic methods on aeronautical CFRP component. The structural element analysed is a spar with a double-T section with a hole and presents a repair by scarfing and hot bond process. The experimental results obtained shown the validity of the ultrasonic and thermographic technique for the sensitivity in detecting defects on full-scale aeronautical components with short execution and scan times.

This work investigates the possibility to apply the improved hybrid capacitor discharge welding (CDW) process, based on projection welding principles, to aluminium alloy Al 5754. The CDW process is an electrical resistance welding technology, realised with high-intensity current pulses discharged by large capacitors. The innovative aspect is the effective possibility to weld aluminium alloys with CDW process and improve the mechanical weld characteristics and the presence of defects as a function of the technological parameters; intrinsic CDW process characteristics need to be investigated on the basis of interaction between the technological and geometrical aspects and the related mechanical properties, in order to improve welding shape and reduce defect size. In order to optimise the process, visual and ultrasonic inspections of the most significant welded joints were performed, and residual stress values were checked; in addition, high-cycle fatigue tests after room temperature tensile tests were executed to optimise the weldments.

In this article, a combination of numerical and experimental methodology for delamination evolution analysis on unidirectional CFRP elements under fatigue is suggested. Fiber-reinforced composite structures exhibit continuous damage accumulation with degradation of effective mechanical properties during cyclic HCF loads. Since advanced composites applications are allowed after extensive experimental certification tests, proposed work is based on experimental procedures to better detect and predict damage initiation and growth, monitoring static displacements and strains under 4-points bending by digital image correlation and measuring compliance variation under fatigue. In addition, non-destructive investigation of composite plates is conducted during cycling tests by using infrared thermography and ultrasonic measurements to detect damage location and validate FEM predictions. Experimental results are analyzed and compared, employing also digital image correlation technique; in addition, thermographic and ultrasonic monitoring inspection with Matlab elaborated measurements are implemented to check results for fatigue damage analysis of same specimens.

In this work, the ultrasound technique was used to monitor the damage of material subjected to fatigue loads. Prediction of structural damage is critical for safe and reliable operation of engineered complex systems. In these measurements, conventional ultrasonic probes (transmitter and receiver) were stably fixed to the tested samples with steel brackets, in order to eliminate ever possible variability associated with the coupling of probes. The transmitted and received ultrasonic signals were recorded and analyzed using a digital oscilloscope. The data were converted into the frequency domain using an algorithm developed in Matlab based on Fast Fourier Transform (FFT) for received signal in dependence of the applied stress level and the accumulated fatigue damage was deeply studied in order to recognize quantitative effects, suitable for an experimental prediction of the integrity of the material. The acquired data were compared with the reference signal, at the beginning of the fatigue tests. Particular care has been paid to UT signal attenuation and to the study of the frequency spectrum as the number of load cycles varies. The applied experimental technique has proved efficient for detecting damage induced by mechanical stress.

In this work, a numerical analysis of the buckling behavior of a panel in CFRP with longitudinal and transverse stiffeners is reported. The focus was to evaluate the influence of damage at the center of the panel, simulated with a cut that affects the central stringer and the underlying skin. Two FEM models have been created: one with damage and the other intact. The numerical models reproduce the component to estimate the axial rigidity, the deformation field and the unstable behavior before buckling. Experimental tests were carried out using a test facility for medium-sized structural elements on a real damaged component with saw-cut. Two comparisons have been made. In a first comparison, the experimental campaign confirmed the results of the numerical model performed for a damaged component. Subsequently, a comparison between numerical cases was made. The influence of damage on the buckling load and on the axial stiffness was evaluated.

Questo lavoro presenta l'applicazione di metodi di controllo non distruttivi utilizzando ultrasuoni Phased Array e convenzionali per lo studio di componenti strutturali in composito CFRP per uso aeronautico. Le analisi sono state inizialmente eseguite e ottimizzate su piastre di carbonio laminato, contenenti difetti artificiali di forma simile ma con geometrie variate per consentire l'adattamento dei test di rilevazione UT su difetti grandi e piccoli a determinate profondità, con diverse condizioni di interfaccia sonda. Sebbene i risultati ottenuti abbiano una validità limitata nel caso delle piastre laminate, i metodi sperimentali e gli strumenti utilizzati sono invece di validità generale e sono quindi applicabili agli studi di componenti strutturali aeronautici con difetti reali, derivati da prove meccaniche, da processi di produzione o processi di riparazione. Sono stati studiati tutti i fattori importanti, migliorando la scansione ad ultrasuoni e consentendo di valutare l'efficacia delle tecniche e degli strumenti utilizzati sui diversi componenti.

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 behavior of welded material is characterized by the absence of the tertiary creep.

This work contains an overview of innovative procedures related to the optimization of non-destructive control ultrasonic techniques for defect investigation on composite plates. The inspection procedure improvement allows developing ideal ultrasonic setup and methods, giving the operator appropriate criteria and guidelines in terms of equipment, material and control procedures. Ultrasonic inspections are conducted on different GFRP laminates with artificial defects; tests are improved using special parts designed for probe positioning and contact conditions on inspected components. The data processing of UT procedures allows comparing detection sensitivity of different probe frequencies and plate material behavior. Contact ultrasonic method presents best results for GFRP plates using 1 MHz Olympus A103S probe, detecting small defects with maximum signal amplitudes. Finally, a statistical study is performed for repeatability demonstration of UT inspections.

In this work, numerical models and experimental tests are performed to validate analysis methods applied on aeronautical CFRP components. In particular, tests of a multi-stingers panel under static compression load and a repaired spar under dynamic bending load are reported. The FEM model are realized to reproduce the panel’s geometry and stacking sequence, the constraints and representative loads of the component's service. Numerical models are improved to reduce the differences between the calculated and experimental deformations, allowing the experimental test validation. In the multi-stringer panel, the experiences evaluate the effects of the out-plane deformations due to compression failure, the difficulties of alignment and distribution’s load along and finally the validation of the numerical model. While in the spar’s study, the experimental measurements confirmed the predictions of the numerical model.

In this work, active Pulsed Thermography (PT) was used to improve CFRP aeronautical parts investigation in order to detect sub-surface defects. Composite materials are prone to successful thermal/infrared NDT analyses, since defects are easily characterised by their surface ‘footprints’ under thermal pulses with reliable results. Better defect characterisation is achieved analysing data with better data processing and procedures, providing detailed contrasts maps where defects’ edges are distinguished. Thermal data are analysed for different specimens with artificial defects and experimental procedures are verified on real a failure in structural aeronautical component. Finally, improved data processing allowed better identifying the damaged areas on specimens.

The work presents a numerical and experimental analysis of a full-scale fatigue test on a CFRP rear spar of an aircraft solicited by a load history representative of variable load levels during the flight conditions. The structural element is essentially a double-T section with a hole and presents a repair by scarfing and hot bond process. The global stress field of the component was determined by a numerical model realized with a commercial FEM code. The numerical model is realized to reproduce the component geometry and stacking sequence, the representative constraints and service loads. The real component has been subjected to cyclic random fatigue stresses and subsequently a static bending load to evaluate the residual strength. Finally, the experimental results obtained, illustrated and discussed, were compared with the numerical ones.

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.

Aluminium foam sandwiches are subjected to four-point bending fatigue test considering the effect of geometric parameters of panels, such as core and plate thickness, and loading mode, such as 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 finite element method model. Despite, the notable fatigue data scatter, originated by foam intrinsic inhomogeneity, experimental fatigue curves are coherent and allow obtaining unified fatigue curves.

Two different full-field techniques are here used to investigate the heat sources and dissipation sources that compare onto the surface of a composite laminate under fatigue load. The material here studied is a glass fibre reinforced epoxy laminate employed in the field of wind turbine. During fatigue tests, 8-bit (256 grey levels) images and thermal images were acquired and elaborated to relate the hysteresis area and the heat sources, finding the energy of damage in three points of specimens. The data shows a clear intensification of dissipative phenomena just in that region where final failure is then registered.

In this paper an associative-parametric approach is proposed in order to model the mesh of an aeronautical concept starting from a set of high-level structural primitives. This approach allows the designer to carry out the geometric modelling and the automatic mesh generation within one software environment in a fast and interactive way. The structural optimisation process is then simplified, with a relevant man-hours saving. A lower number of data transfers between different software is, moreover, involved with less problems related to the data corruption. To assure orders of continuity higher than C0 between adjacent instances, a suitable mathematical description of the structural primitives has been proposed. This description assures the maintenance of the required continuity constraints when the mesh is modified. Appropriate schemes of dependences are identified to guarantee the automatic propagation of the modifications complying with the continuity constraints.

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 work, enhanced thermal data processing is developed with experimental procedures, improving visualization algorithm for sub-surface defect detection on industrial composites. These materials are prone to successful infrared nondestructive investigation analyses, since defects are easily characterized by temperature response under thermal pulses with reliable results. Better defect characterization is achieved analyzing data with refined processing and experimental procedures, providing detailed contrasts maps where defects are better distinguished. Thermal data are analyzed for different CFRP specimens with artificial defects and experimental procedures are verified on real structural aeronautical component with internal anomalies due to impact simulation. A better computation method is found to be useful for simultaneous defect detection by means of automatic mapping of absolute contrast, optimized to identify defect boundaries.

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.

Aluminium foam represents a new class of materials characterized by a large variability due to its porous structure. A simple and easy-to-use indicator of foam inhomogeneities is represented by apparent density, but this parameter does not give indications about differences existing in structural stiffness. To overcome this limit, the variability of natural frequency has been considered in this work in the case of commercial closed-cell aluminium foam. Moreover, fatigue and fracture properties of aluminium foam are not well studied. In this work standard three-point bend specimens SE(B) have been used to determine critical Crack-Tip Opening Displacement. The pre-crack phase, consisting in the application of appropriate fatigue load, has been monitored through modal analysis. Experimental results confirm the possibility of using this technique also in the initial step of fatigue failure process to quantify the amount of fatigue damage induced by repeated loads.

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.

Capacitor discharge welding (CDW) is analysed to repair aeronautical components made with Inconel 718 and Ti6Al4V alloys, currently used for compressor and turbine applications. The CDW process is a resistance welding technology; it makes use of high-intensity current pulses, discharged by large capacitors. The use of multipoint contact features allows us to reduce stress concentration effects at the weld toe, obtaining thin weld beads, and achieve good material continuity. Base material and as-welded specimens with the same heat treatment were welded, using different parameters, and mechanically tested; the weld bead extension and the microstructural aspect of the welds were finally analysed. The development of advanced technologies for aeronautical components repair is of particular interest and the structural performances at room and working temperatures for capacitor discharge welds have to be carefully studied.

In this work, we evaluate the residual stress field originated by a new welding process named Friction Stir Welding. We analysed aluminium alloy butt-welded joints. Both similar and dissimilar joints have been considered in 2024-T3 and 6082-T6 aluminium alloy 0.8 and 3 mm thick. For each joint, the longitudinal and transversal residual stress distribution have been obtained in a direction normal to the weld cord. In the thicker dissimilar joints, the longitudinal residual stress distribution is very similar to the distribution present in the traditional welded joints. It presents, in fact, a tensile region near the weld cord, which is balanced by compressive regions away. On the contrary, other joints present a low compressive stress at the weld toe, which began tensile away. Moreover, the effect of the shoulder geometry on the residual stress field was evaluated on 1.5 mm thick joints.

This paper deals with the application of a non-destructive technique based on natural frequencies and damping evaluation to predict fatigue failure of closed cell aluminium foams. Two different numerical methods have been used to analyse data, and a detailed analysis about frequency and damping evolution of metallic foam specimens subjected to fatigue load is shown. Moreover, significant correlations between modal data changes and the evolution due to the relevant fatigue loads in the high-cycle regime have been extracted; these correlations may be efficiently used to predict the residual life with good accuracy from the beginning of the fatigue tests.

This work concerns the mechanical behaviour analysis of Capacitor Discharge Welded joints made with Inconel 718 and TiAl6V4 superalloys in the form of cylindrical bars. The research activities have been conducted in the frame of AWFORS project, financed by the European Community. The Capacitor Discharge Welding (CDW) technique is of particular interest for repair processes; it allows the substitution of the damaged part after its removal and the replacement of the new component without consistent additional machining. Good mechanical properties and geometrical integrity are to be achieved; consequently, the multipoint contact profile for the CDW process has been conceived to enhance the microstructural weld characteristics and improve the fatigue and creep behaviour at high temperatures. Fatigue tests have been performed both at room and working temperature, comparing the results between base material and as-welded specimens with the same heat treatment. The microstructure has been deeply analyzed through optic and scanning electron microscope (SEM) observations; this way the microstructural modifications and the Heat Affected Zones have been studied in the welded section as well as the interaction between the weld layers material and the parent metal.

Residual stresses develop during most manufacturing processes involving material deformation, heat treatment, machining or processing operations that transform the shape or change the properties of a material. They have a not negligible effect on the material strength, especially on fatigue. For this reason, it is important that some knowledge of the internal stress state can be deduced either from measurements or from modelling predictions. The object of this paper is forecasting the modification and the evolution that a residual stress field, originated by welding, suffers after chip-forming machining, such as milling and cutting. Numerical results have been critically compared to experimental measurements and show the potentiality but also the limitations of numerical techniques.

This work presents a non-linear continuum damage mechanics model based on general thermodynamic framework developed by Lemaitre and Chaboche. The proposed model has been formulated to take into account the material damage evolution at different load levels and it allows the effect of the loading sequence to be included; by means of a recurrence formula derived for multilevel loading, complex load sequences can be considered. The fatigue test data for a hardened and tempered steel have been used to verify the model proposed in this paper and the results show a good agreement to predict fatigue life under complex load sequences.

The aim of this work is to evaluate by numerical simulation the real stress state of a welded component subjected to a sinusoidal external load. In particular, the relaxation process of the residual stress, due to the application of a cyclic external load, has been studied. Longitudinal welded joints, laser welded and 3 mm thick, are analysed in this work following the residual stress evolution during a fatigue test. A numerical procedure, implemented by the ABAQUS code, carries out the residual stress field originated by a welding process and it is taken as a pre-stress condition in the present ABAQUS simulations. In order to evaluate the influence of the amplitude level on the residual stress relaxation, eight different sinusoidal loads at the load ratio R = 0.1 are applied to the model. The results show a significant reduction in the initial residual stress level, even after the first load cycle. They offer a very precise explanation of the fatigue behaviour of this kind of welded joints, confirming the behaviour experimentally observed.

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 and 538°C) in both base and TIG welded material. Welded material showed an increase of the data scatter and lower fatigue strength, which was anyway not excessive in comparison with 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.

In this article, a methodology to analyze the shear behavior of aluminum foam with closed cells is proposed. A biaxial load device was expressly designed and the elaboration with digital image correlation technique of the data acquired during the test with a charge coupled device (CCD) camera allowed determining the displacement and strain fields. This procedure made it possible to evaluate the procedure as suitable or not for conducting a shear characterization for metallic foam. The τ-γ curves obtained showed an initial elastic outline followed by the yield plateau, a peak load, and a rapid load drop.

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.

In this work, a scheme of representation for aircraft structural concepts is identified. Based on this scheme, a parametric-associative geometrical modelling of the aeronautic structure, consisting in a quad-mapped mesh, is proposed. The mesh generation is based on a hierarchical scheme ensuring the one-to-one correspondence between mesh elements belonging to adjacent primitives. The automatic propagation of modifications is efficiently implemented according to well-defined schemes of dependence thanks to which the modifications involve only the concerned instances. This scheme is implemented in an original software, called MeshFEM and developed using C++, Matlab and the VTK library for 3D graphic visualisation.

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.

This work concerns single crystal Ni-based superalloy welded through the capacitor discharge welding (CDW) process. The static and fatigue behavior of CMSX-4 type single crystal alloy has been analyzed, performing mechanical tests on square specimens in order to establish in detail the superalloy behavior at the expected service stresses. Tests have been conducted both at room and working temperatures (800°C), comparing base material and capacitor discharge welded joints. The material microstructure has been observed with optical and scanning electron microscopy in order to characterize welding parameters effects on the weld characteristics, analyzing base material and as-welded specimens; this way, the CDW process has been studied to perform butt joint with the better characteristics ad achieving sound welds. Microstructural modifications and the heat affected zones have been observed in the welded section as well as the interaction between weld material and parent metal. The research activities have been conducted in the frame of AWFORS project, financed by the European community.

The aim of this study is the evaluation of mechanical behaviour of T-joints made in wood and reinforced with bidirectional long glass fiber and epoxy matrix, using numerical and experimental methods of investigation. Joints in composite / wood are not uniform and different interfaces are present in them, each of which responds differently over time when requested. These composites are also anisotropics and many important mechanical quantities, such as stiffness, vary independently of each other. Due to the characteristic heterogeneity of constituent materials, the diversity of the thickness [1,2,3], the characteristic loading mode, the study of these composites is very complex to be analyzed. The difficulty in developing analytical models able to interpret and to predict their behaviour over time, is complicated by the different phenomena interacting with each other. Obtaining coherent results from numerical-experimental methods of investigation, therefore, means having an ideal instrument to study and monitor the evolution of damage [4,5]. In this work, a numerical FEM model of a simple T-joint has been developed in order to evaluate stress field in the critical point. Finally, experimental static tests are carried out on the same kind of joints.

In the present work, the capacitor discharge welding process (CDW) applied on AISI 304 circular bars was studied. The CDW process is essentially an electrical resistance welding technology, realized through current pulses of high intensity and discharged by large capacitors; the process allows to reduce stress concentration effects at the weld toe, obtaining thin welds and achieve good material integrity. CDW process characteristics lead to conceive the idea to investigate on the interaction between the weld technological aspects and the related mechanical properties. In this research activity, 150 cylindrical specimens with bore diameter 6 mm and different igniter dimensions were machined in AISI 304 austenitic stainless steel; a special equipment was designed to clamp specimens and to assure perfect electrical continuity. The main CDW welding parameters (energy input P, applied forces and igniter dimensions) were studied in order to optimise the welding process. The static and fatigue properties were finally analysed for the welded bars and the results were correlated to process parameters; mechanical tests give good results with respect to base metal if the proper welding parameters are used, despite the fact a brittle character was observed for the welded joints.

Le tendenze attuali sulle possibili procedure produttive e di repair di componenti aeronautici in superlega vertono sull’utilizzo di sistemi di laser cladding e processi di saldatura avanzata. Tra le tecniche di saldatura, quella capacitiva denominata Capacitor Discharge Welding (CDW) si rivela particolarmente adeguata in quanto consente, dopo la rimozione della parte danneggiata, l’applicazione di una nuova pala senza la sostituzione dell’intero blocco, evitando lavorazioni aggiuntive e con l’ottenimento di ottime proprietà meccaniche. Il processo CDW consente di ottenere cordoni di saldatura di piccolissimo spessore, effetti di intaglio minimi ed una buona caratteristica strutturale e di forma del materiale saldato. Nel presente lavoro è stato analizzato il comportamento meccanico statico, a fatica ed a creep delle superleghe Inconel 718 e TiAl6V4 prima e dopo la giunzione CDW su particolari provini cilindrici saldati di testa, soggetti anche a specifici trattamenti termici. Le prove sono state eseguite sia a temperatura ambiente che ad alta temperatura sul materiale base e sul materiale sottoposto a tecnica di saldatura CDW, confrontando i risultati ottenuti. E’ stata inoltre analizzata la microstruttura delle leghe mediante osservazione al microscopio ottico e al microscopio elettronico a scansione (SEM). La tecnica è stata infine impiegata anche per la saldatura di provini complessi quali mock-up di turbina aeronautica, con risultati al momento non del tutto soddisfacenti

During the high cycle fatigue of aluminium alloys, an energy dissipation occurs. This dissipation is hard to be estimated because of the high diffusivity of such alloys and the importance of the thermoelasticity effects in comparison with others standard metallic materials (e.g., steels). Nevertheless the study of the energy balance gives valuable information about the nature of deformation mechanisms facilitating the construction of constitutive models associated with the microplasticity and damage of the aluminium alloy. In this work, the different energies involved in the energy balance were deduced from two complementary imaging techniques. The dissipation and thermoelastic sources were derived from an infrared thermography system, while the deformation energy was estimated from a digital image correlation system. Three tests with various loading blocks were carried out and a comparison between deformation and dissipation energies was systematically performed. (C) 2009 Elsevier Ltd. All rights reserved.