Massimo Viscardi

Damage identification and location on a typical aeronautical structure

A technique for detecting and locating structural damages is presented in this paper. It is based on the analysis of experimentally evaluated frequency response functions (FRFs) and consists of a comparison of the FRFs of the healthy structure which are assumed as reference and the FRFs collected at different times. A damage detection index interprets the differences between the FRFs. The results obtained by this technique when tested on a partial frame of a commercial aircraft were very interesting. It is possible to detect and locate all damages which were simulated/induced and also gave an indication of the extent of the damage. Moreover, the technique has the basic features required of a new NDE technique such as being non- model related and having the possibility of performing real- time monitoring.

Piezoresistive strain sensing of carbon nanotubes-based composite skin for aeronautical morphing structures

Nowadays, smart composites based on different nano-scale carbon fillers, such as carbon nanotubes (CNTs), are increasingly being thought of as a more possible alternative solution to conventional smart materials, mainly for their improved electrical properties. Great attention is being given by the research community in designing highly sensitive strain sensors for more and more ambitious challenges: in such context, interest fields related to carbon nanotubes have seen extraordinary development in recent years. The authors aim to provide the most contemporary overview possible of carbon nanotube-based strain sensors for aeronautical application. A smart structure as a morphing wing needs an embedded sensing system in order to measure the actual deformation state as well as to “monitor” the structural conditions. Looking at more innovative health monitoring tools for the next generation of composite structures, a resin strain sensor has been realized. The epoxy resin was first analysed by means of a micro-tension test, estimating the electrical resistance variations as function of the load, in order to demonstrate the feasibility of the sensor. The epoxy dogbone specimen has been equipped with a standard strain gauge to quantify its strain sensitivity. The voltamperometric tests highlight a good linearity of the electrical resistance value as the load increases at least in the region of elastic deformation of the material. Such intrinsic piezoresistive performance is essentially attributable to the re-arrangement of conductive percolating network formed by MWCNT, induced by the deformation of the material due to the applied loads. The specimen has been prepared within this investigation, to demonstrate its performance for a future composite laminate typical of aerospace structures. The future carbon-fiber sensor can replace conventional metal foil strain gauges in aerospace applications. Furthermore, dynamic tests will be carried out to detect any non-reversible changes to the sensing response.

MULTIFUNCTIONAL SYSTEM FOR ACTIVE NOISE CONTROL AND DAMAGE DETECTION ON A TYPICAL AERONAUTICAL STRUCTURE

The present work relates to the assessment and testing of a multifunctional intelligent system, based upon the use of piezoelectric devices, devoted both to the active noise and vibration control and to damage detection f the structure. In the control application, the piezoelectric devices (in form of patches) play the role of actuators; their induced secondary vibration field has the target to reduce the primary one through a specific control algorithm and system. In the health monitoring application, the piezo devices play both the roles of actuators and sensors. In fact the developed technique is primarily based upon the evaluation and comparison of the structure Frequency Response Functions (FRF) that intrinsically contains all the information regarding the structural properties whose change may be correlated with incipient damages. The aforementioned application were investigated and experimentally assessed with good results with reference to a typical partial fuselage structure (three frames, eight stringers and the skin panels: 1.2 m x 1.7 m). On the noise control application side, a height sensors/height actuators control architecture was then assessed and experimentally tested whose results may be synthesized in a 30 dB vibration level reduction at sensors locations and more than 20 dB of reduction of the associated noise field. In the optic of a multifunctional intelligent system, the aforementioned set of piezos was decided to be used also for health monitoring application. As a preliminary activity, an extensive monitoring was performed on the integer structure to verify the sensibility of the system and the stability of the defined Damage Index (DI) in respect to environmental factor not related to structural real modification. To verify the sensibility of the technique to reveal and locate a typical shear clip damage, a set of rivets were successively cut in the area surrounding the frame shear clip, and relative FRFs were acquired and relative DI calculated. The analysis of the data showed a good sensibility of the system to identify the presence of a damage with maximum values of the DI in the sensor closest to the damage location and with an absolute value of the index growing up with damage extension.

Active noise control experiences on a GT high‐speed car

The presentation will show some results of an extensive experimental work aimed at the reduction of the engine noise inside the binnacle of a GT high‐speed car. The car is completely made by carbon fiber and the engine is rigidly attached to the body and for this reason the vibrations of the engine are directly transmitted inside the binnacle, thus producing very high noise levels in all the oeprating conditions. Preliminary laboratory experiments were made by simulating the noise in the cavity by means of different loudspeakers and a first configuration of the ANC system was optimized. This last included some speaking panels driven by several piezoceramic patches elastically suspended from the roof. The laboratory results were encouraging, and the measured noise reductions were sensible even if the simulated primary field was scaled down due to the nonlinear behavior of the speaking panels at high driving voltages. The second step of the work consisted in optimizing a second configuration of the ANC syst...

Experimental vibro-acoustic analysis of the gear rattle induced by multi-harmonic excitation

The paper reports a wide vibro-acoustic experimental investigation of the gear rattle phenomenon induced by multi-harmonic excitation. The analysis is performed by using different measurement techniques which allow some of the significant parameters in this type of investigation to be acquired on a specific test rig: the angular rotations of the gears by using encoders; the accelerations obtained from a triaxial accelerometer; the sound pressure level determined by employing both acoustic microphones; the correct evaluation of the acoustic sources by utilizing a p–v sound intensity probe. Performance indices were adopted to compare the dynamic behaviours of the system with respect to some parameters, such as the speed of the pinion, the fluctuations in the speed of the pinion and the lubrication conditions. The results of the comparative analysis show very good agreement between the vibro-acoustic measurements and the results from the encoder-based method; this has helped us to interpret the physical behaviour of the gear pair with respect to the impacts occurring between the teeth during the different phases of the phenomenon. Moreover, the study indicates interesting aspects of the effects of multi-harmonic excitation on the rattle phenomenon, with particular attention to the influence of lubrication on the reduction in the rattle noise.

Acoustic performance assessment of innovative blankets for aeronautical applications

Polyurethane blankets are increasingly used for many aeronautical NVH applications. These foams, generally available in various thickness and density, are great sound absorber, therefore suitable in the aircraft interior. These foams are used as replacement to traditional combination of mineral wools / rock wool along with perforated panels, which require labor and also health hazardous. Polyurethane foams are generally available in various densities and thickness. The acoustic performance of sound absorbing poroelastic materials is characterized by intrinsic physical parameters like flow resistivity, and absorption coefficient. This paper presents a detailed discussion on measurement of flow resistivity as well as acoustic absorption coefficient of PU foam samples. Such numerical database of examined samples has been then validated through other laboratories activities, which shows the good accuracy of the methodology implemented within.Polyurethane blankets are increasingly used for many aeronautical NVH applications. These foams, generally available in various thickness and density, are great sound absorber, therefore suitable in the aircraft interior. These foams are used as replacement to traditional combination of mineral wools / rock wool along with perforated panels, which require labor and also health hazardous. Polyurethane foams are generally available in various densities and thickness. The acoustic performance of sound absorbing poroelastic materials is characterized by intrinsic physical parameters like flow resistivity, and absorption coefficient. This paper presents a detailed discussion on measurement of flow resistivity as well as acoustic absorption coefficient of PU foam samples. Such numerical database of examined samples has been then validated through other laboratories activities, which shows the good accuracy of the methodology implemented within.

Experimental technologies comparison for strain measurement of a composite main landing gear bay specimen

The development of advanced monitoring system for strain measurements on aeronautical components remain an important target both when related to the optimization of the lead-time and cost for part validation, allowing earlier entry into service, and when related to the implementation of advanced health monitoring systems dedicated to the in-service parameters verification and early stage detection of structural problems. The paper deals with the experimental testing of a composite samples set of the main landing gear bay for a CS-25 category aircraft, realized through an innovative design and production process. The test have represented a good opportunity for direct comparison of different strain measurement techniques: Strain Gauges (SG) and Fibers Bragg Grating (FBG) have been used as well as non-contact techniques, specifically the Digital Image Correlation (DIC) and Infrared (IR) thermography applied where possible in order to highlight possible hot-spot during the tests. The crucial points identification on the specimens has been supported by means of advanced finite element simulations, aimed to assessment of the structural strength and deformation as well as to ensure the best performance and the global safety of the whole experimental campaign.

Impact detection method for composite winglets based on neural network implementation

Maintenance tasks and safety aspects represent a strategic role in the managing of the modern aircraft fleets. The demand for reliable techniques for structural health monitoring represent so a key aspect looking forward to new generation aircraft. In particular, the use of more technologically complex materials and manufacturing methods requires anyway more efficient as well as rapid application processes to improve the design strength and service life. Actually, it is necessary to rely on survey instruments, which allow for safeguarding the structural integrity of the aircraft, especially after the wide use of composite structures highly susceptible to non-detected damages as delamination of the ply. In this paper, the authors have investigated the feasibility to implement a neural network-based algorithm to predict the impact event at low frequency, typically due to the bird collision. Relying upon a numerical model, representative of a composite flat panel, the approach has been also experimentally validated. The purpose of the work is therefore the presentation of an innovative application within the Non Destructive Testing field based upon vibration measurements. The aim of the research has been the development of a Non Destructive Test which meets most of the mandatory requirements for effective health monitoring systems while, at the same time, reducing as much as possible the complexity of the data analysis algorithm and the experimental acquisition instrumentation. Future activities will be addressed to test such technique on a more complex aeronautical system.

A modified Shunted Switch Architecture (SSSA) for active vibration control

During the last years, the research interest in assessing noise and vibration optimization has been addressed on different control typologies, based both on active and passive architectures. Within the paper, some preliminary activities aimed at the realization of a structurally simple, cheap and easily replaceable active control systems is discussed. Under these premises, the paper deals with the assessment of an Enhanced Synchronized Shunted Switch Architecture (SSSA) control architecture, based upon the use of piezoelectric devices, specifically optimized for a cantilver beam structure. Main activities regarded the control system set up and optimization, both under the electronic than the piezo location points of view, and control results under deterministic and stochastic forcing actions. Experimental results have been compared with the numerical one as well as a comparison between the SSSA approach and other active control architectures has been also presented and discussed. Results have shown a good performances of the proposed approach that present also a relative easy implementation if compared with already assessed control technologies.

Acoustic characterization and noise reduction strategies for a general aviation aircraft

This work concerns with the vibro‐acoustic characterization of a general aviation aircraft with the objective of designing solutions to reduce the inside noise level during the flight; first of all a new passive insulation, and then verifying the possibility of an active noise control approach. The acoustic characterization was performed into two different step; in‐flight acoustic and vibration measurements during a typical mission of the vehicle were initially measured, to characterize the acoustic levels and spectra elated to the different flight operations. Ground measurement were also performed to characterize the sound insulation properties of the aircraft structure. The next item was the partial re‐design of the sound insulation pack for the vehicle by the use of innovative material; the choice of the best technical solution has been performed by the use of numerical prediction models and through experimental tests on modified samples of the aircraft structure. The in‐flight acoustic and vibration measurement showed also the presence of dominant low frequencies tonal components related to the BPFs and upper harmonics. This situation suggested the possibility to implement an active noise control approach. All these activities will be herein presented and discussion of the main results will be performed within the paper.