Ernesto Monaco

Experimental and Numerical Activities on Damage Detection Using Magnetostrictive Actuators and Statistical Analysis

The purpose of this work is to present the formulation of a new experimental procedure to employ in problems of damage analysis of structural elements. The proposed method is based on the acquisition and comparison of Frequency Response Functions (FRFs) of the monitored structure before and after damage occurred. Structural damages modify the dynamic behaviour of the structure and consequently its FRFs making possible calculation of a representative “Damage Index.” The experimental activity was carried on using two prototypes of magnetostrictive actuators developed within the European Commission funded project named MADAVIC (Magnetostrictive Actuators for Damage Analysis and VIbration Control). Three kinds of damages have been simulated on two beam-like structures: little mass disturbances, partial cuts of the beam sections and constraint yielding. Two expressions of damage indices were calculated and analysed. The main target of the work was to assess the reliability of the damage identification by using a “repetitiveness index.” This index is related to the lowest measurable damage extension and the statistical t-test, in order to verify if each calculated index was really representative of a structural damage, rather than of unforeseen differences between the FRFs. A simple Finite Element Model (F.E.M.) of the second test-article has also been developed for numerically simulating the experimental tests and comparing the corresponding results.

Guided waves in a stiffened composite laminate with a delamination

The integrity of safety-critical structural composites can be enhanced by the use of innovative ultrasonic nondestructive evaluation techniques. Among the various existing techniques, guided wave methods provide a good promise in terms of sensitivity to a variety of damage types or defects and the extent of the area that can be monitored, given the ability of these waves to travel relatively long distances within the structure under investigation. In comparison with isotropic metallic structures, wave propagation in composite structures presents additional complexity for effective damage identification. The material inhomogeneity, anisotropy, and the multilayered construction of composite materials lead to significant dependence of wave modes on laminate layup configurations, direction of propagation, frequency, and interface conditions. In this article, a specific structure will be analyzed with different levels of complexities in an effort to determine the propagation characteristics of the waves. The investigated structure is a stiffened composite plate of finite thickness and infinite lateral dimensions. The work is carried out using theoretical analysis, numerical modeling, and laboratory experiments. Numerical (finite element) simulations are used for more realistic models, where the geometric and material complexities present practical difficulties in direct analysis using theoretical constructs only.

Model assisted probability of detection for a guided waves based SHM technique

Guided wave (GW) Structural Health Monitoring (SHM) allows to assess the health of aerostructures thanks to the great sensitivity to delamination and/or debondings appearance. Due to the several complexities affecting wave propagation in composites, an efficient GW SHM system requires its effective quantification associated to a rigorous statistical evaluation procedure. Probability of Detection (POD) approach is a commonly accepted measurement method to quantify NDI results and it can be effectively extended to an SHM context. However, it requires a very complex setup arrangement and many coupons. When a rigorous correlation with measurements is adopted, Model Assisted POD (MAPOD) is an efficient alternative to classic methods. This paper is concerned with the identification of small emerging delaminations in composite structural components. An ultrasonic GW tomography focused to impact damage detection in composite plate-like structures recently developed by authors is investigated, getting the bases for a more complex MAPOD analysis. Experimental tests carried out on a typical wing composite structure demonstrated the effectiveness of modeling approach in order to detect damages with the tomographic algorithm. Environmental disturbances, which affect signal waveforms and consequently damage detection, are considered simulating a mathematical noise in the modeling stage. A statistical method is used for an effective making decision procedure. A Damage Index approach is implemented as metric to interpret the signals collected from a distributed sensor network and a subsequent graphic interpolation is carried out to reconstruct the damage appearance. A model validation and first reliability assessment results are provided, in view of performance system quantification and its optimization as well.

Structural damage identification using magnetostrictive actuators

In this work an approach to Structural Damage Analysis based on the utilization of actuators made by magnetostrictive materials is presented. New developed magnetostrictive actuators used to vibrate very stiff structures and able to acquire Frequency Response Functions (FRFs) in many points using piezoelectric sensors are the constitutive components of this approach. The numerical elaboration of the FRFs permits to analyze correlation between damaged and integer structures. In this way it is possible to create one or more Damage Indices in order to identify, localize and quantify different kinds of damages simulated on the structure. In particular it has been possible to perform many analyses during experimental tests in many frequency ranges and for different kind of damages. These experiences have led to a Damage Index whose values: (1) show a dependence with the distance between the damage and the sensor by which FRFs have been acquired, (2) increase reducing the distance between the sensor position and the damage location, (3) become higher if the damage extension increases. Furthermore, aim of this paper is to demonstrate and discuss the suitability of magnetostrictive devices for the development of an integrated system devoted to the health monitoring of the structures.

Influence of experimental testing set-up and geometric parameters on damping measurements

The paper presents the activities performed by the authors in order to develop and validate an experimental set-up for measurements of damping characteristics of typical materials employed within aeronautical and industrial field for passive vibrations reduction. These activities have been carried out within the research program funded by the European Commission named “F.A.C.E.” (Friendly Aircraft Cabin Environment). The set-up has been designed to operate through a PC-based acquisition system developed in LABVIEW programming environment. The development of the experimental damping measurement set-up is based on the principle of the “Oberst beam”, and it has been improved to allow the implementation of different approaches with “contacting” sensors and actuators. The influence of the damping on the stability and reliability of the results will be investigated, by evaluating the effect of the beam thickness to the applied damping thickness ratio. The results will be presented for some damping treatments like “constrained layers” as far as for rubbery materials commonly employed within the aeronautical field. These activities are aimed to the implementation of a better damping modelisation of a typical finite element model of light structures.

Investigation on fundamental modes of guided waves propagating in symmetric and nonsymmetric composite laminates

A quasi-isotropic composite laminate is constructed in an attempt to create a structure that behaves like an isotropic plate. Its membrane behavior is similar to that of the isotropic plate while the bending behavior is quite different from the latter. Moreover, the laminae may or may not be arranged symmetrically with respect to the midplane thereby resulting in a different mechanical response. In this work, guided wave propagation along multiple directions in symmetric and not symmetric quasi-isotropic plates is evaluated. Experimental and numerical results for the fundamental modes A0 and S0 are analyzed for the symmetric and nonsymmetric layups. An eight-node brick type element based on the three-dimensional theory is used in modeling to predict numerically the velocity of wave modes propagating in the graphite/epoxy composite plates. Agreement between experimental and numerical approaches is found and interesting dependencies between velocity of propagating modes and laminate stacking sequence are discussed. A final comparison with analytical dispersion curves obtained by the implementation of the global matrix method is discussed.

Detecting delaminations and disbondings on full-scale wing composite panel by guided waves based SHM system

A full-scale lower wing panel made of composite material has been designed, manufactured and sensorised within the European Funded research project named SARISTU. The authors contributed to the whole development of the system, from design to implementation as well as to the impacts campaign phase where Barely Visible and Visible Damages (BVID and VID) are to be artificially induced on the panel by a pneumatic impact machine. This work summarise part of the experimental results related to damages production, their assessment by C-SCAN as reference NDT method as well as damage detection of delimitations by a guided waves based SHM. The SHM system is made by customized piezoelectric patches secondary bonded on the wing plate acting both as guided waves sources and receivers. The paper will deal mostly with the experimental impact campaign and the signal analyses carried out to extract the metrics more sensitive to damages induced. Image reconstruction of the damages dimensions and shapes will be also described based mostly on the combination of metrics maps over the plate partial surfaces. Finally a comparison of damages maps obtained by the SHM approach and those obtained by “classic” C-SCAN will be presented analyzing briefly pros and cons of the two different approached as a combination to the most effective structural maintenance scenario of a commercial aircraft.

Guided waves based SHM systems: parameters selection for better identification and localisation of damages in composites stiffened plates

Structural Health Monitoring deals mainly with structures instrumented by secondary bonded or embedded sensors that, acting as both signal generators and receivers, are able to “interrogate” the structure about its “health status”. This innovative approach to the damage analysis is particularly promising for reducing the maintenance costs and eventually the weight of aerospace composite structures, without any reduction of the safety level required. These structures are currently designed and employed with significant reduction of the pristine material allowables to account certain failure mechanisms frequently inducing relatively small hidden damages called Barely Visible Damages, consisting among others in delaminations and/or debondings and being detectable only by specific instruments operated by trained personnel. It has been proved that the propagation of guided waves is affected by the presence of such type of damages, but their effective identification and localization depends on the accurate “tuning” of the wave characteristic (frequency, amplitude, velocity, mode) as well as on the proper selection of the best parameter of the specific wave mode selected and data analysis algorithm. The intent of this paper is to summarize the experiences gained by the authors in selecting the most sensitive parameters according to the type of damage to be investigated in several typology composite plate-like structures.

Methodologies for Guided Wave-Based SHM System Implementation on Composite Wing Panels: Results and Perspectives from SARISTU Scenario 5

Within the SARISTU project, the Application Scenario 5 (AS05) was devoted primarily to the development of methodologies based on ultrasonic guided waves for Structural Health Monitoring (SHM) implementation on wing structural elements made of composite materials for detecting BVID or hidden flaws. These methodologies have been mainly developed by the authors of this paper, technologically integrated, and applied on small-scale structural elements within Scenario 5 (unstiffened and stiffened plates) focusing, at the end of the work, also on statistical assessment of damage thresholds levels for each methodology propaedeutic to a probability of detection (POD) evaluation of each approach. The paper will shortly present the methodologies developed and implemented, the main experimental and numerical results in terms of damage detection, and the statistical assessment of threshold damage detection levels. Finally, a short comparison about pros and cons of the methodologies as well as the migration strategy of the methodologies to the Integration Scenario 12 for full-scale wing implementation will be presented.

Interior Active Noise Control in Turbofan Aircraft: Optimal Actuators Positioning Using Dedicated Genetic Algorithms

This paper presents the activities developed by the authors within the first year of the research project named M.E.S.E.M.A. (Magnetoelastic Energy Systems for Even More Electric Aircraft) funded by the European Commission within the 6th Framework Program and coordinated by the “Dipartimento di Ingegneria Aerospaziale” of the University of Naples “Federico II” (DPA). One of the main targets for the MESEMA Consortium consists in reducing the level of disturbance noise in turbofan aircraft; a noise & vibration control system using magnetostrictive actuators has been designed and produced, with the goal of controlling noise & vibrations in a wide band frequency range between 150 – 500 Hz. The environmental noise & vibration excitations was representative of a small/medium turbofan aircraft case. Final results of the task will be represented by a system made up of 30 actuation/sensing devices connected to a system performing control of external disturbances as well as of the devices’ intrinsic non linearity. As experimental test article a fuselage mockup of the ATR42/72 aircrafts family has been chosen available at the acoustic laboratory in the Alenia plant; due to its geometry and overall dimensions it well represents a fuselage section of an hypothetic regional jet. Within this paper the authors present most of the numerical activities developed in the project in order to design the actuation system as well as to optimize the location of the control devices. The numerical (finite element) model of the mock-up has been developed, correlated with experimental modal analysis results and updated in order to match the best way possible the experimental reality. This model has then been employed to carry out a deep simulation activity aimed at the evaluation of the required control actuators performances in terms of force spectra as far as their optimal placements for control purposes. In order to select among the many possible set of control actuator configurations an optimisation activity was required. The used optimisation method is based on “genetic” algorithms. For this analysis 126 actuators potential locations were selected on frames or stiffeners of the two middle bays of the mock-up. The authors developed the genetic algorithm code in MATLAB framework. The paper presents in the end the experimental characterization of the control actuators demonstrating how a good correlation between numerical and experimental activities has been achieved.