Bistable energy harvesting has become a major field of research due to some unique features for converting mechanical energy into electrical power. When properly loaded, bistable structures snap-through from one stable configuration to another, causing large strains and consequently power generation. Moreover, bistable structures can harvest energy across a broad-frequency bandwidth due to their nonlinear characteristics. Despite the fact that snap-through may be triggered regardless of the form or frequency of exciting vibration, the external force must reach a specific snap-through activation threshold value to trigger the transition from one stable state to another. This aspect is a limiting factor for realistic vibration energy harvesting application with bistable devices. This paper presents a novel power harvesting concept for bistable composites based on a 'lever effect' aimed at minimising the activation force to cause the snap through by choosing properly the bistable structures' constraints. The concept was demonstrated with the help of numerical simulation and experimental testing. The results showed that the actuation force is one order of magnitude smaller (3%–6%) than the activation force of conventionally constrained bistable devices. In addition, it was shown that the output voltage was higher than the conventional configuration, leading to a significant increase in power generation. This novel concept could lead to a new generation of more efficient bistable energy harvesters for realistic vibration environments.

This paper is aimed at developing a theoretical model able to predict the generation of nonlinear elastic effects associated to the interaction of ultrasonic waves with the steady-state nonlinear response of local defect resonance (LDR). The LDR effect is used in nonlinear elastic wave spectroscopy to enhance the excitation of the material damage at its local resonance, thus to dramatically increase the vibrational amplitude of material nonlinear phenomena. The main result of this work is to prove both analytically and experimentally the generation of novel nonlinear elastic wave effects, here named as nonlinear damage resonance intermodulation, which correspond to a nonlinear intermodulation between the driving frequency and the LDR one. Beside this intermodulation effect, other nonlinear elastic wave phenomena such as higher harmonics of the input frequency and superharmonics of LDR frequency were found. The analytical model relies on solving the nonlinear equation of motion governing bending displacement under the assumption of both quadratic and cubic nonlinear defect approximation. Experimental tests on a damaged composite laminate confirmed and validated these predictions and showed that using continuous periodic excitation, the nonlinear structural phenomena associated to LDR could also be featured at locations different from the damage resonance. These findings will provide new opportunities for material damage detection using nonlinear ultrasounds.

Adhesion is attracting increasing interest in the aerospace field since composite materials have become, together with aluminium alloys, the main structural materials for aircraft primary structures. Nano-graphite was demonstrated to improve the mechanical performance of several polymers used as composite matrices. In this work Single Lap Joints (SLJs) of unidirectional composite laminates were manufactured, tested and simulated: two families of specimens were investigated and compared, one joined using conventional epoxy resin, the other joined with an adhesive obtained mixing the same epoxy resin with nano-graphite particles. The dispersion of expanded and sonicated graphene stacks (EGS, 3% wt) in the epoxy matrix was obtained by the swelling method, dispersing first the filler in acetone and then mixing it with the epoxy oligomers. Finally the solvent was evaporated and the filler-epoxy mixture was degassed under vacuum before adding an amine curing agent in a stoichiometric quantity. The research demonstrates the superior mechanical properties of the adhesive with the addition of nano-graphite through experimental characterization of its behaviour in terms of strength and energy absorption. Finite element numerical simulations have been carried out using the Cohesive Zone Model (CZM) element, obtaining as parameters the maximum shear stress and the critical fracture energy for the two adhesives. A good correlation between numerical and experimental results has been achieved and the criteria for developing reliable and accurate non-linear models of the adhesive failure have been established.

The use of pulsed thermography as a nondestructive evaluation tool for damage monitoring of composite materials has dramatically increased in the past decade. Typically, optical flashes are used as external heating sources, which may cause poor defect definition especially for thicker materials or multiple delaminations. SMArt thermography is a new alternative to standard pulsed thermography as it overcomes the limitations on the use of external thermal sources. Such a novel technology enables a built-in, fast and in-depth assessment of both surface and internal material defects by embedding shape memory alloy wires in traditional carbon fibre reinforced composite laminates. However, a theoretical model of thermal wave propagation for SMArt thermography, especially in the presence of internal structural defects, is needed to better interpret the observations/data measured during the experiments. The objective of this paper was to develop an analytical model for SMArt thermography to predict the depth of flaws/damage within composite materials based on experimental data. This model can also be used to predict the temperature contrast on the surface of the laminate, accounting for defect depth, size and opening, thermal properties of material and defect filler, thickness of the component, and intensity of the excitation energy. The results showed that the analytical model gives good predictions compared to experimental data. This paper is one of the first pioneering work showing the use thermography as a quantitative non-destructive tool where defect size and depth could be assessed with good accuracy.

In this study the amplitude and the phase of the structural response of samples of Single Lap Joint (SLJ) subjected to ultrasonic harmonic excitation was evaluated to identify and characterize the defects within the bonded region. Different parameters such as frequency, shape, and amplitude of the response signal coming back from the adhesive joint are key criteria for understanding the quality of the adhesion. Different metallic samples with the same geometry were experimentally tested: the defects were artificially introduced bonding partially two plates and changing the extension of the debonded region: two piezoelectric sensors (one exciting, one receiver) were attached on each of the two bonded plates. In this way, different experimental tests were carried out in order to study the influence of debonded regions on SLJ structural behavior. The structural dynamic response of the debonded samples was investigated and compared with the predictions of numerical models, for each SLJ, introducing viscoelastic properties for the adhesive layer, and applying the harmonic excitation. Moreover the numerical modal analysis was used to understand the experimental results by a proper description of viscoelastic tape behavior. The numerical simulations were used to find correlation between the content of the acquired signals and the defects of adhesion.

The coupling between structural support and protection makes biological systems an important source of inspiration for the development of advanced smart composite structures. In particular, some particular material configurations can be implemented into traditional composites in order to improve their impact resistance and the out-of-plane properties, which represents one of the major weakness of commercial carbon fibres reinforced polymers (CFRP) structures. Based on this premise, a three-dimensional twisted arrangement shown in a vast multitude of biological systems (such as the armoured cuticles of Scarabei, the scales of Arapaima Gigas and the smashing club of Odontodactylus Scyllarus) has been replicated to develop an improved structural material characterised by a high level of in-plane isotropy and a higher interfacial strength generated by the smooth stiffness transition between each layer of fibrils. Indeed, due to their intrinsic layered nature, interlaminar stresses are one of the major causes of failure of traditional CFRP and are generated by the mismatch of the elastic properties between plies in a traditional laminate. Since the energy required to open a crack or a delamination between two adjacent plies is due to the difference between their orientations, the gradual angle variation obtained by mimicking the Bouligand Structures could improve energy absorption and the residual properties of carbon laminates when they are subjected to low velocity impact event. Two different bioinspired laminates were manufactured following a double helicoidal approach and a rotational one and were subjected to a complete test campaign including low velocity impact loading and compared to a traditional quasi-isotropic panel. Fractography analysis via X-Ray tomography was used to understand the mechanical behaviour of the different laminates and the residual properties were evaluated via Compression After Impact (CAI) tests. Results confirmed that the biological twisted structures can be replicated into traditional layered composites and are able to enhance the out-of-plane properties without a dangerous degradation of the in-plane properties.

Thermoplastic materials are getting increasing attention in the aerospace field for a wide range of applications not only related to primary structures. In this work two types of thermoplastic structures are manufactured and investigated: a L-shaped stringer used as stiffener of flat plates; a stiffened panel for aerospace applications. The panel has been reinforced using four L-shaped stringers joined to it by induction welding, an innovative technique alternative to mechanical fastening and adhesive bonding. The main focus of this work is experimentally demonstrating the capability of the welding to keep properly the loads arising from a compression test until the failure of the stiffened panel. Interesting results came out concerning the post-buckling behaviour of the adopted thermoplastic materials that exhibited outstanding load bearing capabilities.

The present paper is focused on the corrosion of austenitic (AISI304) and a Duplex (2205) stainless steel grades in H2O/KOH 50% at 120°C. Linear sweep voltammetry and electrochemical impedance spectrometry measurements were carried out with an AMEL modified potentiostat equipped with a digital FRA in an home-made cell for high-temperature work with gas control. The impedance spectra have been fitted with a novel numerical model. The experiments were complemented by metallographic (in-plane and cross-sectional SEM micrography), structural (X-ray diffractometry) and compositional (EDX line-profiles) characterization of the materials attacked under electrochemically controlled conditions. Electrochemical measurements have shown that AISI304 exhibits a passivating behaviour, with a secondary peak in the passive range and eventual transpassivity. Duplex electrochemical behaviour is characterized by a mixed kinetic control. AISI304 was found to fail by intergranular corrosion and to be covered in passive conditions by acicular compounds and in transpassive conditions by a compact layer of corrosion products. Duplex samples, instead, exhibits a more uniform surface morphology and a compact layer of corrosion products both in passive and in transpassive conditions.

The present paper focuses on the corrosion of austenitic (AISI304) and a duplex (2205) stainless steel grades in molten KOH/NaOH 50 w/o eutectic at 250°C. Experimental activities have been performed consisting in electrochemical measurements (linear sweep voltammetry and electrochemical impedance spectrometry) complemented by metallographic (in-plane and cross-sectional SEM micrography), structural (X-ray diffractometry) and compositional (EDX line-profiles) characterization of the materials attacked under electrochemically controlled conditions. Electrochemical measurements have shown that AISI304 exhibits a passivating behaviour, characterised by two passivation peaks and a transpassive threshold, while Duplex, does not yield a clear indication of passivation. AISI304 was found to fail by intergranular corrosion and to be covered in both passive and transpassive conditions, by an incoherent scale, containing electrolyte species. Duplex samples, instead tends to fail by homogeneous attack and exhibit a range of scale structures, depending on the applied potential.

Fabrication and testing of fuel-cells based on nanofilm electrodes and interconnects is a hot technological challenge for three key reasons: (i) miniaturisation and integration into electronic devices as well as implementation of on-chip logics, (ii) testing of the performance of nano-materials on their real scale and (iii) use of cuttingedge material characterisation techniques. The principal interest of a nanotechnological approach to material problems in electrochemical energetics is particularly related to long-term durability issues of critically degradable components. Among the degradation modes, mechanical failure by cracking of the functional thin films is being recognised as a crucial one, impairing the implementation of laboratory systems into real-life devices. In this paper we report on corrosion-induced local thinning and correlated cracking of electrode components in a RTIL-based Proton Exchange Membrane Fuel Cell with Pt micro-particles as catalyst, Au feeder electrodes and Fe interconnects. In situ imaging of the multi-material system in electrochemical environment, based on X-ray scanning and optical microscopies, has disclosed the formation of complex cracking patterns, including spiral cracks. A simple mechanical explanation of the peculiar cracking pattern is proposed.

Exposure to noise constitutes a health risk. The present paper is related to activities performed to test emerging technologies aimed to characterise noise sources. In particular, acoustic acquisitions have been achieved on a complex laboratory equipment composed by different machines including a rotor serving as blower. Advanced noise measurement techniques based on acoustic holography and beamforming have been employed to realise a deep characterisation of the noise sources. This analysis could fix, in effective way, the possible mitigation strategies and, if extended to the other noise sources, a comprehensive management of the workplace noise exposure.

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.

This paper presents a new approach to evaluating the mistuning effects on turbomachinery blades that is classified as neither deterministic nor statistical — it is based on the employment of genetic algorithms. A genetic algorithm has been exploited to find the structurally mistuned configuration that leads to the maximum value of blade vibration amplitude for an assigned domain of variations. A test case has been fixed and subjected to an assigned forcing field; the target of the test case was to perform a smart search of the worst mistuned configuration. The test case was a twenty-blade disc on which one thousand forced frequency response analyses have been performed. A comparison with the results, based on the Monte Carlo methods, proved the suitability and the relevance of the proposed approach. The investigation has demonstrated the applicability of this new possible engineering approach to the study of systems with uncertain properties.

SMArt Thermography exploits the electrothermal properties of multifunctional smart structures, which are created by embedding shape memory alloy (SMA) wires in traditional carbon fibre reinforced composite laminates (known as SMArt composites), in order to detect the structural flaws using an embedded source. Such a system enables a built-in, fast, cost-effective and in-depth assessment of the structural damage as it overcomes the limitations of standard thermography techniques. However, a theoretical background of the thermal wave propagation behaviour, especially in the presence of internal structural defects, is needed to better interpret the observations/data acquired during the experiments and to optimise those critical parameters such as the mechanical and thermal properties of the composite laminate, the depth of the SMA wires and the intensity of the excitation energy. This information is essential to enhance the sensitivity of the system, thus to evaluate the integrity of the medium with different types of damage. For this purpose, this paper aims at developing an analytical model for SMArt composites, which is able to predict the temperature contrast on the surface of the laminate in the presence of in-plane internal damage (delamination-like) using pulsed thermography. Such a model, based on the Green's function formalism for one-dimensional heat equation, takes into account the thermal lateral diffusion around the defect and it can be used to compute the defect depth within the laminate. The results showed good agreement between the analytical model and the measured thermal waves using an infrared (IR) camera. Particularly, the contrast temperature curves were found to change significantly depending on the defect opening.

Nonlinear ultrasonic non-destructive evaluation (NDE) methods can be used for the identification of defects within adhesive bonds as they rely on the detection of nonlinear elastic features for the evaluation of the bond strength. In this paper the nonlinear content of the structural response of a single lap joint subjected to ultrasonic harmonic excitation is both numerically and experimentally evaluated to identify and characterize the defects within the bonded region. Different metallic samples with the same geometry were experimentally tested in order to characterize the debonding between two plates by using two surface bonded piezoelectric transducers in pitch-catch mode. The dynamic response of the damaged samples acquired by the single receiver sensor showed the presence of higher harmonics (2nd and 3rd) and subharmonics of the fundamental frequencies. These nonlinear elastic phenomena are clearly due to nonlinear effects induced by the poor adhesion between the two plates. A new constitutive model aimed at representing the nonlinear material response generated by the interaction of the ultrasonic waves with the adhesive joint is also presented. Such a model is implemented in an explicit FE software and uses a nonlinear user defined traction-displacement relationship implemented by means of a cohesive material user model interface. The developed model is verified for the different geometrical and material configurations. Good agreement between the experimental and numerical nonlinear response showed that this model can be used as a simple and useful tool for understanding the quality of the adhesive joint.

Adhesive bonded lap joints are widely used in the aerospace field and non-destructive testing (NDT) techniques are critical in evaluating the quality of adhesion before and during use. Two types of bonded samples have been experimentally investigated in order to verify the reliability of non-linear elastic wave spectroscopy (NEWS) based on the use of ultrasound. Piezoelectric sensors have been attached to the samples and used as generators and receivers. Both the samples have shown non-linearities in their dynamic behaviour. Non-linear metrics have been applied to their structural responses over an assigned range of excitation frequencies based on higher order harmonic analysis in order to evaluate the degree of non-linearity of the samples. Possible interpretations of the experimental behaviour are provided in the paper based also on tomographic testing of the adhesive layer that showed the presence of microbubbles in the bond due to manufacturing process.

This paper presents a nonlinear imaging method based on nonlinear elastic guided waves, for the damage detection and localisation in a composite laminate. The proposed technique relies on the study of the structural nonlinear responses by means of a combination of second order phase symmetry analysis (PSA) with chirp excitation and inverse filtering (IF) method. PSA was used to exploit the invariant properties of the propagating elastic waves with the phase angle of the pulse compressed chirp signals, in order to characterise the second order nonlinear behaviour of the medium. Then, the IF approach was applied to a library of second order nonlinear responses to obtain a two-dimensional image of the damage. The experimental tests carried out on an impact damage composite sample were compared to standard Cscan. The results showed that the present technique allowed achieving the optimal focalisation of the nonlinear source in the spatial and time domain, by taking advantage of multiple scattering and a small number of receiver sensors.

This paper presents a nonlinear elastic wave tomography method, based on ultrasonic guided waves, for the image of nonlinear signatures in the dynamic response of a damaged isotropic structure. The proposed technique relies on a combination of high order statistics and a radial basis function approach. The bicoherence of ultrasonic waveforms originated by a harmonic excitation was used to characterise the second order nonlinear signature contained in the measured signals due to the presence of surface corrosion. Then, a radial basis function interpolation was employed to achieve an effective visualisation of the damage over the panel using only a limited number of receiver sensors. The robustness of the proposed nonlinear imaging method was experimentally demonstrated on a damaged 2024 aluminium panel, and the nonlinear source location was detected with a high level of accuracy, even with few receiving elements. Compared to five standard ultrasonic imaging methods, this nonlinear tomography technique does not require any baseline with the undamaged structure for the evaluation of the corrosion damage, nor a priori knowledge of the mechanical properties of the specimen.

Literature offers a quantitative number of diagnostic methods that can continuously provide detailed information of the material defects and damages in aerospace and civil engineering applications. Indeed, low velocity impact damages can considerably degrade the integrity of structural components and, if not detected, they can result in catastrophic failure conditions. This paper presents a nonlinear Structural Health Monitoring (SHM) method, based on ultrasonic guided waves (GW), for the detection of the nonlinear signature in a damaged composite structure. The proposed technique, based on a bispectral analysis of ultrasonic input waveforms, allows for the evaluation of the nonlinear response due to the presence of cracks and delaminations. Indeed, such a methodology was used to characterize the nonlinear behaviour of the structure, by exploiting the frequency mixing of the original waveform acquired from a sparse array of sensors. The robustness of bispectral analysis was experimentally demonstrated on a damaged carbon fibre reinforce plastic (CFRP) composite panel, and the nonlinear source was retrieved with a high level of accuracy. Unlike other linear and nonlinear ultrasonic methods for damage detection, this methodology does not require any baseline with the undamaged structure for the evaluation of the nonlinear source, nor a priori knowledge of the mechanical properties of the specimen. Moreover, bispectral analysis can be considered as a nonlinear elastic wave spectroscopy (NEWS) technique for materials showing either classical or non-classical nonlinear behaviour.

Literature offers a quantitative number of diagnostic imaging methods that can continuously provide a detailed image of the material defects in aerospace and civil applications. This paper presents a nonlinear Structural Health Monitoring (SHM) imaging method, based on nonlinear elastic wave tomography (NEWT), for the detection of the nonlinear signature in damaged isotropic structures. The proposed technique, based on a combination of higher order statistics (HOS) and radial basis function (RBF) interpolation, is applied to a number of waveforms containing the nonlinear responses of the medium. HOS such as bispectral analysis and bicoherence was used to characterize the second order nonlinearity of the structure due to corrosion, whilst RBF interpolation was applied to a number of signals acquired from a sparse array of sensors, in order to obtain an image of the defect. Compared to standard linear ultrasonic imaging techniques, the robustness of this nonlinear tomography sensing system was experimentally demonstrated. Moreover, this methodology does not require any baseline with the undamaged structure for the detection of the nonlinear source as well as a priori knowledge of the mechanical properties of the medium. Finally, the use of HOS makes NEWT a valid alternative to traditional nonlinear elastic wave spectroscopy (NEWS) methods for materials showing either classical or non-classical nonlinear behaviour.

In different engineering fields, there is a strong demand for diagnostic methods able to provide detailed information on material defects. Low velocity impact damage can considerably degrade the integrity of structural components and, if not detected, can result in catastrophic failures. This paper presents a nonlinear structural health monitoring imaging method, based on nonlinear elastic wave spectroscopy, for the detection and localisation of nonlinear signatures on a damaged composite structure. The proposed technique relies on the bispectral analysis of ultrasonic waveforms originated by a harmonic excitation and it allows for the evaluation of second order material nonlinearities due to the presence of cracks and delaminations. This nonlinear imaging technique was combined with a radial basis function approach in order to achieve an effective visualisation of the damage over the panel using only a limited number of acquisition points. The robustness of bispectral analysis was experimentally demonstrated on a damaged carbon fibre reinforced plastic (CFRP) composite panel, and the nonlinear source’s location was obtained with a high level of accuracy. Unlike other ultrasonic imaging methods for damage detection, this methodology does not require any baseline with the undamaged structure for the evaluation of the defect, nor a priori knowledge of the mechanical properties of the specimen.

The transmission of noise generated by fuel pumps inside the car is treated. At first typical fuel pumps have been tested in an acoustically qualified room with the aim of characterizing their noise emissions. The acoustical measurements led to the evaluation of typical noise spectra associated to the fuel pumps investigated and, therefore, of the acoustical power of these sources. Then the modal properties of the tanks in which the pumps work have been evaluated: modal tests have been performed to evaluate their dynamic properties in terms of natural frequencies and normal modes with special care for the modal damping that is crucial for the transmission of the noise generated through the walls of the tanks. A numerical modal characterization has been performed developing Finite Elements models of the tanks under investigation: a numerical – experimental correlation has been performed updating the Finite Elements models on the results provided by the experimental tests.

This study is concerned with the activation energy threshold of bistable composite plates in order to tailor a bistable system for specific aeronautical applications. The aim is to explore potential configurations of the bistable plates and their dynamic behavior for designing novel morphing structure suitable for aerodynamic surfaces and, as a possible further application, for power harvesters. Bistable laminates have two stable mechanical shapes that can withstand aerodynamic loads without additional constraint forces or locking mechanisms. This kind of structures, when properly loaded, snap-through from one stable configuration to another, causing large strains that can also be used for power harvesting scopes. The transition between the stable states of the composite laminate can be triggered, in principle, simply by aerodynamic loads (pilot, disturbance or passive inputs) without the need of servo-activated control systems. Both numerical simulations based on Finite Element models and experimental testing based on different activating forcing spectra are used to validate this concept. The results show that dynamic activation of bistable plates depend on different parameters that need to be carefully managed for their use as aircraft passive wing flaps.

Recent nonlinear elastic wave spectroscopy experiments have shown that the nonlinear ultrasonic response of damaged composite materials can be enhanced by higher vibrations at the local damage resonance. In this paper, the mathematical formulation for the generation of nonlinear wave effects associated with continuous periodic excitation and the concept of local defect resonance is provided. Under the assumption of both quadratic and cubic approximation, the existence of higher harmonics of the excitation frequency, superharmonics of the damage resonance frequency and nonlinear wave effects, here named as nonlinear damage resonance intermodulation, which correspond to the nonlinear intermodulation between the driving and the damage resonance frequencies, is proved. All these nonlinear elastic effects are caused by the interaction of propagating ultrasonic waves with the local damage resonance and can be measured at locations different from the material defect one. The proposed analytical model is confirmed and validated through experimental transducer-based measurements of the steady-state nonlinear resonance response on a damaged composite sample. These results will provide opportunities for early detection and imaging of material flaws.

The present job is focused on a full composite ultra-light aircraft, and has the main goal of evaluating the existing margins for the improvement of the vehicle performance through a dedicated optimization of the aerodynamic and structural properties. For what concerns the aerodynamic properties of the entire aircraft, at first the attention has been focused on the airfoil chosen for the wings. Then the analysis of the complete wing and aircraft has been carried out. The preliminary investigation on the structural properties of the aircraft has been concerning an accurate evaluation of the operational external loads in order to obtain a confident estimation of the structural solicitation parameters and the identification of the most stressed aircraft structural parts.

In this paper a simple approach based on the Carlson's method will be presented to define a supersonic conguration optimized in terms of sonic boom properties. The Carlson's method provides a simplified procedure for the calculation of sonic boom characteristics for supersonic airplane congurations and spacecrafts. The information required for the signature predictions are: aircraft shape factor KS, aircraft operating conditions and atmospheric data. Unfortunately, there is not an analytic expression of the shape factor. Nevertheless, a graphic representing the relationship between KS and parameters related to the aircraft geometry, can be used. In this paper KS have been approximated through a linear and a quadratic interpolation as a function of other parameters and its minimization problem has been formulated. Computational results show the optimal values to be assigned to the aircraft geometry parameters in order to obtain the minimal value of the shape factor and in consequence of the sonic boom overpressure.

In this work a simple model has been used that relates sonic boom effects to the main geometric and operative parameters of the civil supersonic aircraft. In particular, a relation between the maximum overpressure and the aircraft shape factor is employed to define an optimal preliminary design point for the supersonic civil aircraft. Carlson’s method has been widely adopted for preliminary numerical investigations of sonic boom signatures associated with different categories of supersonic aircrafts. Correlations of these numerical predictions with flight-test data have shown a reasonable agreement and confirmed the validity of the method in spite of its ease of use. Modern supersonic aircrafts are conceived to have minimum effects on people and structures through properly designed sonic boom signatures (“sine wave”- like signature is one option) and adjusting their geometry through sophisticated Computational Fluid Dynamics methods. The N-wave approach is based on simplified assumptions that do not take into account the rise time, which is one of the major factors influencing the human ear response to sonic boom and cannot be used for estimating advanced sound metrics like the perceived loudness decibel. Nevertheless, the N-wave model can be used in an early stage of the design, because it generally provides conservative estimations (upper limits) of the overpressures due to an assigned supersonic aircraft geometry, giving a rough figure of its effect on the community. Different geometries can be, therefore, easily optimized through this approach to better fix starting points for minimizing the effects of boom signatures, which are shaped subsequently through higher-order methods.

A reduced-order short-period model is derived, which allows for representing the effects of structural flexibility on aircraft response to pilot and disturbance inputs on the basis of a minimum set of relevant information on aerodynamic and structural configuration. The model is derived from first principles by means of a Lagrangian approach, featuring vertical (heave) and pitch degrees of freedom, together with deformation variables for flexural deformation of wing and after portion of the fuselage. As a byproduct, an explicit formulation for stability derivatives with respect to deformation variables is derived. Numerical results are reported for a configuration representative of a modern commercial jet aircraft.

Aircraft noise is associated to two main groups of acoustic sources: those which produce airframe noise and the other ones which produce engine noise. Airframe noise can be numerically simulated once defined aircraft geometry and its intensity depends on aerodynamic configuration of aircraft. Engine noise is due to contribution of several sources according to engine’s type: in several cases these sources are not all well theoretically defined for the difficulty of performing experimental campaigns, and this does not allow a general validation of numerical simulation tools. Sound pressure levels associated to fans, compressors, jets, propellers can be numerically evaluated through commercial codes that require a big amount of input parameters concerning the geometric description of the different engine parts and the engine operational conditions that are well known from a physical standpoint but can be difficult to manage in a preliminary assessment of the noise emission due to a typical civil aircraft engine. The numerical definition of all these parameters requires a deep knowledge of the mechanism of the engine part subject of investigation and, often, also having a clear view of how this part works, the values of operational parameters, during the aircraft engine working, can be not available. For this reason an investigation of the sensitivity of the overall sound pressure levels associated to the different noise sources to the engine operational parameters can be useful to understand how much the noise can change if the working conditions change and to rank the variables from an acoustic standpoint fixing the most influential parameters. The present paper focuses on some of the acoustic sources acting on a typical civil aircraft engine and is based on the use of commercial codes able to simulate numerically the sound pressure levels associated to these sources.

The increased usage of adhesive bonding as a joining method in modern aerospace components has led to developing reliable ultrasonic health monitoring systems for detection of regions of poor adhesion. Nonlinear acousto-ultrasonic techniques based on higher harmonics and subharmonic frequencies have shown to be sensitive to the detection of micro-voids and disbonds. Nonlinear resonance properties of disbonds generate various nonlinear phenomena such as self-modulation, subharmonics, hysteresis and so on. By exploiting the local natures of these phenomena, this paper demonstrates the use of subharmonics for detection and imaging of flaws in bonded structures. To optimise the experimental testing a two-dimensional analytical model and a three-dimensional finite element analysis simulation were developed for understanding the generation of nonlinear elastic effects with emphasis on subharmonic frequency components. The proposed analytical model qualitatively described the generation of subharmonics but also higher harmonics due to the nonlinear intermodulation of the driving and resonance frequencies associated with the disbonded region. The numerical model was developed by modifying the user defined cohesive element formulation with a bi-linear traction-displacement relationship in order to simulate the interaction of elastic waves with the structural disbond. Whilst the analytical model supported the selection of the driving frequency, the numerical one successfully predicted the generation of subharmonic frequencies originating in the disbonded area. Experimental tests were conducted on a disbonded single lap joint structure using surface-bonded piezoelectric transducers and a laser-Doppler vibrometer, and allowed to validate the analytical and numerical results. It was clearly demonstrated that the nonlinear resonance effects in the form of subharmonics could be used to discriminate reliably regions of poor adhesion in bonded structures. This work can lead to new in situ nonlinear acoustic based health monitoring system for locating and imaging defects in critical aerospace components.

In this paper a simple approach aimed to the definition of a supersonic configuration optimized in terms of sonic boom signature will be presented. This approach is based on the application of Carlson’s method that is a simplified method for the calculation of sonic-boom characteristics for a wide variety of supersonic airplane configurations and spacecraft. Sonic-boom overpressures and signature duration may be predicted for the entire affected ground area for vehicles in level flight or in moderate climbing or descending flight paths. This prediction technique is based on simplification of the purely theoretical methods, which are able to provide quite acceptable estimates of sonic-boom phenomena for a wide range of flight conditions for conventional airplane configurations. Experimental measurements have shown that this approach describes properly sonic-boom properties for extremely blunt bodies at high supersonic speeds, providing reliable numerical information at distances large relative to body dimensions. The effects of flight-path curvature and aircraft acceleration, however, are not considered, and the method is further restricted to a standard atmosphere without winds. Furthermore, it is assumed that the pressure signal generated by the aircraft is of the far-field type, the classical N-wave. The information required, for the calculations and the pressure-signature predictions provided by the simplified method, are: aircraft shape-factor; aircraft operating conditions; atmospheric data. The signature data provided by the method include: N-wave bow-shock overpressure; the signature duration; the location of the ground impact point relative to the aircraft position at the time the boom was generated. This approach has been adopted to evaluate the sonic boom properties of a supersonic reference configuration comparing the numerical results with a more refined CFD approach. Then an optimization procedure has been applied to minimize the maximum value of overpressure: the geometry of the reference configuration has been modified iteratively following the criteria suggested by the Carlson’s method and keeping in mind several operational constraints: this iterative method has permitted the definition, in a preliminary stage of design, of an optimized supersonic configuration.

The Shell Eco-marathon every year challenges high school and college student teams coming from around the world to design, build and test energy efficient vehicles. With annual events in the Americas, Europe and Asia, the winners are the teams that go the farthest distance using the least amount of energy. In the present paper the development of a structural frame for a Prototype car attending the competition is presented. The team is composed by students, researchers and professors from different scientific sectors of University of Salento and is called “Salento Eco Team”: the main objective of the team work is to identify the best solutions to achieve the target of the competition through a multidisciplinary investigation of all the technical aspects concerning the development of a ground vehicle with high energetic performance. The competition is based on general official rules provided by the Shell Ecomarathon organizers fixing the criteria of participation for the different teams. Participating teams can enter as Prototypes (three- or four-wheel vehicles) or Urban Concept (four-wheel vehicles similar in appearance to regular cars and which are fit for on-road use). Starting from the lesson learned at the first participation an optimization process has been implemented aimed to improve the global efficiency of the vehicle. The first aspect taken in account has concerned the choice of the structural materials for the car frame. In the first version the choice of the steel has led to a solution with weight properties not permitting high performance during the race: therefore, the use of aluminium alloys and structural concepts taken from the aerospace experiences has allowed the elaboration of structural solutions with high ratios of robustness to weight. The investigation has led to the identification of a solution optimized in terms of structural concept and the effect of the weight saving on the energetic performance has been evaluated. The next step of this study is the Prototype construction and test.

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.

Non-Destructive Testing of aircraft structures is of paramount relevance leading to key information regarding the structural characteristics and the residual life of a component. This paper is focused on the experimental and modeling activities related to vibrational analysis carried out on a typical aeronautical composite sample. A flap section of a regional aircraft has been studied applying the conventional tools of the modal analysis. The aeronautical component has been at first characterized in terms of natural frequencies and normal modes. Then it has been damaged using a drop tower that induced a controlled impact in the structural component. The vibrational analyses have been repeated and the normal modes in the two conditions have been compared. Then other approaches based on vibrational properties of the structures have been investigated to detect defect and damage.