In this paper an overview is given on some relevant activities performed by CIRA within the field of noise and vibration, guidance, monitoring and control. Conventional microphones, piezoelectric materials and fiber optics are the sensing strategies used for the proposed applications. High level of integrability, wide operational frequency range are just some features that make them particularly suited for noise and vibration, structural monitoring and sensing, micro actuation and harvesting. The first study regards an active headrest system developed for aircraft cabin seats based on the filtered- X LMS algorithm. Experiments carried out on a mock-up of active headrest are described along with noise attenuation results in a representative acoustic field. After that, a structural monitoring application is presented via strain gauges, piezoceramic sensors, and fiber optics. A series of drop tests are executed on a representative model of a flexible fuel tank, that is dropped from a certain height on an instrumented steel plate and equipped with such deformation sensors. Finally, guided elastic waves generated by piezoelectric transducers are investigated for de-icing purposes. The development of such a specific technology has required the assessment of a dedicated approach that is addressed in this paper, along with its level of maturation and possible future developments.
In the present work, an overview is provided on the activities performed by CIRA, DLR and Univ of Bristol to develop and test a morphing system aimed at altering the twist of a blade to enhance the performance of the main rotor. The activities were performed within the research Project of “Shape Adaptive Blades for Rotorcraft Efficiency” (SABRE, H2020, 2017-2021), a Consortium constituted by six Partners (Univ. of Bristol – leader, Univs. of Munich, Delft and Swansea and the research centers of DLR and CIRA). Moving from the original features of the blade and the requirements of the reference rotorcraft, a layout of the architecture was sketched. A refined numerical model was then implemented to accurately predict the functionality of the concept and verify its safety compliance with the test facilities it was conceived for. Laboratory tests were thus performed on a dedicated prototype. Finally, on this basis, two other demonstrators were built and finally tested in the just mentioned wind tunnel and whirl tower plants.
This work resumes the results achieved by a no-model based approach for SHM. The proposed methodology provides a signal to noise ratio improvement by cross-correlation function applied to a derived signal of the strain vector. The bonding line integrity of skin-stringer interface is the committed target, and the capability of debonding detection after low energy impact is estimated. The methodology is tested on aeronautical stiffened CFRP panels under different loading conditions after impact: residual strain, static load and quasi static load are considered. The encouraging results drove a full scale applications.
We report on the use of Fiber Bragg Grating (FBG) sensors integrated onto an aircraft landing gear for remote and realtime load monitoring. Several FBGs strain sensors, both in a linear and tri-axial configuration, have been integrated on different locations of true landing gears (both Main and Nose gears) based on their load condition derived from FEM numerical analysis and exposed to numerous qualification lab tests where the load applied to the gears was varied in the range 0-20kN. To this aim, the gears were mounted on a 25kN hydraulic press, that changed the shock absorber route from 0 mm up to 200 mm (corresponding to the maximum take-off weight,~4600 kg). Obtained results are in good agreement with those provided by reference electrical strain gauges located very close to their optical counterparts, and demonstrate the great potentialities of FBG sensors technology to be employed for remote and real time load measurements on aircraft landing gears.
Composite structures need structural monitoring systems to improve maintenance and design processes. Maintenance may be supported by prompt detection of damage insurgence, moving towards condition-based rather than scheduled approaches. Design can attain adequate safety levels with lighter structures, in force of a continuous knowledge of their status. Required practices and systems are dependent on damage type, each with its own particularity; therefore, complex systems are necessary to respond to such a necessity. Among the many, bonding defects are particularly important. They can be classified as bonding deficiency, as adhesive misses in some parts, or de-bonding, as attachment collapses. Moving from activities performed within OPTICOMS, a project funded within the European flagship Clean Sky 2 JTI, the present work focuses on the preliminary characterization of bonding imperfections effect on selected composite aircraft components. In detail, how local adhesive absence influences static structural response and how this flaw type can be detected through a proprietary algorithm is investigated. A multi-element beam is referred, representing a main spar of the primary structure. A large numerical campaign is conducted on a tuned FE model, implementing different defect layouts, for size and location. Numerical structural response is computed through a representation of a distributed strain sensing system. Supported by a basic theoretical discussion, results are processed and commented, to individuate specific parameters that can describe applied failures. Finally, an in-house code verifies preliminarily its capabilities in exposing presence and size of the applied imperfections, correlating numerical outcomes with performed estimations.
Traditionally, metallic vessels have to comply with specific regulation rules (Pressure Equipment Directive from the European Parliament) involving periodic re-qualifications. However, for high pressure composite vessels, standards, and particularly non-destructive techniques, have to be developed, tested and validated. In this new frame, a research project has been set up for VECEP program (VEga launcher Consolidation and Evolution Preparation Programme) aiming at funding developments to set-up new control/monitoring techniques applicable to composite vessels, for both future regulatory and inspection needs. To reach the market, these techniques have also to be cost-effective in comparison with traditional ones. To monitor such vessels behavior and finalize the sensor system to the detection of structural defects, CIRA and AVIO have conducted R and D activities based on high resolution distributed strain profiles sensitivity analysis along single mode optical fibers. To elaborate this method, preliminary tests were carried out for testing different bonding agents and different surface finishing, on a set of representative coupon from composite overwrapped pressure vessels under bending incremental solicitation until reaching ultimate load. Additionally, their sensitivity, analyzed during the test, provided additional valuable data about structure integrity. A mechanical criterion based on OFDR (Optical Frequency Domain Reflectometry) differential strain profiles analysis was preliminarily implemented in order to evidence structural anomalies during the test.
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.
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.
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.
The object of this work is a numerical model aimed at predicting the electrical conductivity of polymeric matrices filled with Carbon Nano-Tubes (CNT). The model, developed within the GRAPSS, ”Graphene-Polymeric Spray Sensor for Shape Recognition of Super-Deformable Structures” a National Project entirely funded by CIRA, will be used to address the design of a sprayable sensor aimed at measuring large deformations. The phenomenon of the tunneling, at the basis of the electrical and thermal conductivity of CNT filled polymeric matrices, was modeled through the finite element, FE, approach. Each particle was schematized as a cluster of nodes connected by highly conductive elements, in compliance with the large conductivity of the CNTs. When the tunneling condition was verified between two particles, a link was realized; the specific electrical resistance was computed on the basis of parameters like the mutual distance and the tunnel cross section area. The resulting system, a truss-structure network contained within a reference cubic volume, was then solved through a thermal analogy. The inward and outward currents, passing through two opposite faces of the cube, were simulated by applying thermal fluxes of opposite sign; the voltage drop caused by the global resistance was then estimated through a steady heat transfer analysis, giving the temperature gradient between the opposite faces. The ratio between the voltage drop and the inward-upward current (respectively, the temperature and the heat flux) represented then the global resistance of the cube. A parametric investigation was finally performed, finding out the dependence of the gage factor (strain vs resistance variation) on CNT concentration and aspect ratio parameters (curvature, diameter-length ratio) and the electrical conductivity.
The manufacturing and the preliminary numerical and experimental testing results of a fiber optic based sensor, able to recognize different load paths, are herein presented. This device is conceived to identify load directions by strain detection along a circumferential geometry. A demonstrator is realized by manufacturing a circular shaped, flexible glass/epoxy laminate hosting the sensible elements. Three loops of optical fiber, laying at different quotes along its thickness, are there integrated. The sensor system is supposed to be bonded on the structural element and then able to follow its deformations under load. The working principle is based on the comparison of the strain paths detected at each fiber optic loop at homologous positions. Rayleigh backscattering optical technology is implemented to measure high spatial resolution strains. A finite element model is used to simulate the sensor behavior and assess its optimal configuration. A preliminary experimental campaign and a numerical correlation are performed to evaluate sensor performance considering in-plane and bending loads.
It is the aim of this paper to present the design of a sensor system based on fiber Bragg gratings (FBG) for the strain monitoring of an adaptive trailing edge (ATE) device. Some of the activities herein showed comes from developments inside the project SARISTU (EU-FP7), funded by the European Union inside the VII Framework Programme and focused on smart aircraft structures. Because the TE is immerged into 3D structural and aerodynamic fields, the sensor system network should have chord- and span-wise features. The ATE device will be equipped with a shape monitoring system using a widely distributed sensors based on fiber optic (FO) elements herein referred to, mainly with the aim of reducing the number of channels (then expense, complexity, etc.). In what follows, the mathematical modelling of a sensor system concept based on FBG is applied to evaluate the chord-wise strain of a trailing edge device. A hinge rotation detection capabilities based on strain measurements is presented. The detection and process of data concerning the in-flight ATE local deformation are necessary to reconstruct the shape produced by the action of a dedicated actuation system.
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