From airplane wings to overhead power lines, through large blades of wind turbines, a buildup of ice can cause problems ranging from low performance to catastrophic failure. Therefore, it is of the utmost importance to control or prevent ice formation, especially on the critical areas of the structures. However, de-icing and anti-icing countermeasures can result energetically expensive and harmful to the environment. In addition, excessive use thereof will reduce the life of an ice protection system (IPS) and introduce fatigue to the controlled structures. Therefore, in order to manage properly the available resources, it is desirable to have an IPS that can both detect ice formation and monitor the ice thickness on critical surfaces. This would allow the IPS to operate when it is necessary. Ultrasonic guided-wave-based techniques have proved to be reliable for ice detection but approaches to assess ice state over time have not been reported yet. The present work investigates the interaction of ultrasonic waves, propagating in a composite plate, with an ice mass changing state, as it melts. The use of a metric is discussed as indicator of ice condition variation.
This work aims to provide a quick solution for impressing a well-defined and repeatable speckle pattern on the surface of a material sample. The proposed technique is based on a water-soluble stabilizer on which the speckle pattern generated and optimized via computer is printed. To verify the application, a two-dimensional (2D) digital image correlation (DIC) system is employed to measure the full-range strain distribution at the macroscopic level during a tensile test on open-hole aluminum specimen. The experimental setup consists mainly of an action camera, a macro lens, and an open-source 2D DIC software. The measured data obtained from the DIC are compared to the other ones provided both from a traditional measurement method based on strain gauge and by a numerical simulation. The results indicate that the approach is both accurate and reliable to obtain stress-strain curves especially in the presence of plastic deformations.
Carbon fiber reinforced composites are widely used in the aerospace industry, but barely visible impact damage can lead to delamination and compromise the structural integrity. The scattering of the fundamental anti-symmetric guided wave mode (A0 Lamb mode) at an artificial circular delamination in a quasi-isotropic laminate was investigated experimentally. A 5 cycle Hanning windowed wave pulse was used as the excitation signal for the experiments. Fast Fourier Transform was employed to identify the guided wave amplitude of the scattered field along various directions. The experimental wavefield was captured using a laser Doppler vibrometer. Experimental results are presented for the scattering pattern and scattering amplitude as a function of distance from the damage. The results of this study can help to improve delamination detection techniques using guided waves and to gain physical insights into the scattering of guided waves at a delamination.
The paper presents a preliminary study about a de-icing system using ultrasonic waves. The activity has been developed within the project “SMart On-Board Systems” (SMOS), which is part of Italian Aerospace National Research Program, funded by the Italian Ministry of Education and Research and coordinated by CIRA. Conceived for an aircraft wing leading edge, the system shall be extended to other aircraft components, once its efficiency and reliability will be demonstrated. Herein, the results of a preliminary numerical work on a NACA 0012 profile are presented. Guided waves are generated by a piezoelectric transducer bonded on the structure and they cause shear stresses that induce ice delamination and fracture. The investigation is focused on the selection of most suitable excitation frequency for the actuator. Finite element analyses are performed to demonstrate the effectiveness of this approach.
Structural Health Monitoring deals mainly with structures instrumented by secondary bonded or embedded sensors. Sensors, acting passively or actively as both signal generators and receivers, are able to “listen” to any event happening in the structure (passive SHM) and to “interrogate” the structure to check its “health status” (active SHM). Structures embedded with sensors appear promising for reducing the maintenance costs and the weight of aerospace composite structures, without any reduction of the safety level required. Among many actuators/sensors technologies under investigation for active SHM systems, the combination of piezoelectric patches employed as guided wave exciters or impact sensors and optical fiber Bragg gratings (FBG) as stress wave detectors look promising for their distributed sensing capability as well as weight reduction compromise in a so-called “hybrid structural component”. FBGs have been employed only recently as stress ultrasonic wave sensors due to the reduced number of high-frequency optical interrogators available. One such device, a multi-channel fiber optic acoustic emission (FAESense™) system developed by Redondo Optics, has been employed by the authors for this purpose. Hybrid SHM systems employing FBGs as sensor arrays could provide more distributed data about the local integrity of the structure with less weight addition compared to other sensor types. Typical diameter of fiber optics could allow the embedding of sensor arrays within the composite laminate. Finally, FBGS can provide simultaneously high frequency data characterizing guided wave propagation as well as low frequency local deformations permitting an SHM approach combining global and local impact and damage detection. Intent of this paper is to summarize the first experience gained by the authors in developing SHM systems for composite plate-like hybrid structures for impact detection.
Lamb waves propagating in thin plates and shells are being widely studied for their potential applications in nondestructive inspection of large-scale structures. These structures are generally characterized by the presence of geometrical discontinuities such as stiffeners, mechanical joints or variable thicknesses that affect the propagation characteristics of Lamb waves that can be very similar to those from defects occurring in service (delamination, disbond, etc.). Therefore, the knowledge of the effects of such discontinuities on the propagation of guided waves is essential to avoid their false identification as defects. In this work Lamb waves propagating in a metal plate with a downward step are studied through laboratory experiments. A single 10 mm piezoceramic disk (PZT) bonded to the host structure using cyanoacrylate gage adhesive is utilized for Lamb waves generation and the responses are measured at multiple locations, along a line crossing the step, using a scanning laser Doppler vibrometer (LDV). The interaction of the fundamental Lamb mode A0 with the geometrical discontinuity in the isotropic plate is investigated and discussed.
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.
KEYWORDS: Structural health monitoring, Waveguides, Composites, Nondestructive evaluation, Damage detection, Signal analysis, Signal detection, Wave propagation, Statistical analysis, Algorithm development
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 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 health monitoring (SHM) based on guided waves allows assessing the health of a structure due to the sensitivity to the occurrence of delamination. However, wave propagation presents several complexities for effective damage identification in composite structures. An efficient implementation of a guided wave-based SHM system requires an accurate analysis of collected data to obtain a useful detection. This paper is concerned with the identification of small emerging delaminations in composite structural components using a sparse array of surface ultrasonic transducers. An ultrasonic-guided wave tomography technique focused on impact damage detection in composite plate-like structures is presented. A statistical damage index approach is adopted to interpret the recorded signals, and a subsequent graphic interpolation is implemented to reconstruct the damage appearance. Experimental tests carried out on a typical composite structure demonstrated the effectiveness of the developed technique with the aim to investigate the presence and location of damage using simple imaging reports and a limited number of measurements. A traditional ultrasonic inspection (C-scan) is used to assess the methodology.
Maintenance approaches based on sensorised structures and Structural Health Monitoring systems could represent one of the most promising innovations in the fields of aerostructures since many years, mostly when composites materials (fibers reinforced resins) are considered. Layered materials still suffer today of drastic reductions of maximum allowable stress values during the design phase as well as of costly and recurrent inspections during the life cycle phase that don't permit of completely exploit their structural and economic potentialities in today aircrafts. Those penalizing measures are necessary mainly to consider the presence of undetected hidden flaws within the layered sequence (delaminations) or in bonded areas (partial disbonding); in order to relax design and maintenance constraints a system based on sensors permanently installed on the structure to detect and locate eventual flaws can be considered (SHM system) once its effectiveness and reliability will be statistically demonstrated via a rigorous Probability Of Detection function definition and evaluation. This paper presents an experimental approach with a statistical procedure for the evaluation of detection threshold of a guided waves based SHM system oriented to delaminations detection on a typical wing composite layered panel. The experimental tests are mostly oriented to characterize the statistical distribution of measurements and damage metrics as well as to characterize the system detection capability using this approach. Numerically it is not possible to substitute part of the experimental tests aimed at POD where the noise in the system response is crucial. Results of experiments are presented in the paper and analyzed.
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