An electromagnetic method allowing to localize and characterize main damages inside composites such as CFRP and GFRP, has been developed. This method is based on the local measurement of the induced electric field, by electric excitation (GFRP) or magnetic induction (CFRP). All damages inducing a local variation of electric conductivity and/or dielectric permittivity, one can detect and characterize damages inside composites materials by analysis of these two parameters. On the other hand, this technique allows detecting some damages having non mechanical origin such as liquid ingress (e.g. water oil, fuel...) or small burns generated by electric sparks (e.g. short circuits, lightning impacts), these kinds of damages being quasi non detectable by more classical methods such as ultrasonic analysis.
Magneto-Optical Imagers (MOI) appear to be good alternatives to conventional eddy current sensors for defect detection in large metallic structures. Indeed, they allow short time inspection of large structures such as airplanes fuselage or wings, thanks to the visualization of "real time" images relative to the presence of defects [1]. The basic principle of the MOI is to combine a magnetic inductor, used to induce the circulation of eddy currents into the structure under test, with an optical set-up used to image the resultant magnetic field, thanks to the Faraday effect occurring in a magneto-optical garnet. The MOI designed by G. L. Fitzpatrick and Physical Research Instrumentation provides two-level images relative to the presence of defects, with an adjustable detection threshold. These so-called qualitative images, although highly contrasted, are rather poor and limited in terms of defect characterization possibilities. In, this paper, the authors present a new kind of MOI, called Quantitative Magneto-Optical Imager (Q-MOI), based on the use of a dedicated "linear" magneto-optical garnet associated with a specific instrumentation. The Q-MOI should considerably reduce the inspection time and allow to fully characterize the encountered defects. First images obtained with a demonstration prototype are shown for surface and buried flaws and further enhancements of the device are proposed.
Recently, a new SHM system has been developed at ONERA. This system based on the interaction between an electromagnetic field and a composite structure, allows detecting all types of damages having an influence on the electrical characteristics of the structure such as carbon fiber breaking, organic matrix pyrolysis, liquid ingress... The basic principle of this SHM system, called HELP-Layer (Hybrid ELectromagnetic Performing Layer), consists to induce eddy currents inside the structure and to perform a local measurement of the resulting electric field. All damages inducing a significant variation of the two main electrical parameters (i.e. electrical conductivity and dielectric permittivity), induce a significant variation of the resulting electric field. The sensitivity of this technique depends of the type of damage; as regard to “mechanical” damages (e.g. delaminations, fiber breaking, crack), the sensitivity is comparable to techniques using ultrasonic methods, as regard to other kind of damages (e.g. matrix pyrolysis, liquid ingress...) the electromagnetic method is more sensitive than ultrasonic methods. The analysis of the two resulting components of electric field (i.e. Ex and Ey, the conductivity direction being y) allows to determine the kind of damage (mechanical or other), by resolution of a part of the inverse problem, the direct problem being solved by the DPSM (Distributed Point Source Method) concept.
A new concept of Structural Health Monitoring System for composite materials, (CFRP and GFRP) using electromagnetic properties of the material, has been developed at ONERA (Office National d’Etudes et de Recherches Aerospatiales, France). This concept, based on the detection of local electric conductivity variations and/or local
dielectric permittivity variations, has allowed to design a demonstrator having a great sensitivity to detect main defects
such as delaminations, fiber breaking, burning and liquid ingress. However, this technique at the present state, does not presently allow to perform quantitative measurements of electric conductivity and dielectric permittivity. In order to remedy to this disadvantage, a numerical simulation with an original method developed at ENS-Cachan (Ecole Normale Superieure de Cachan, France) by D. Placko and N. Liebeaux, has been performed. Various results obtained by
simulation are presented discussed and compared with experimental measurements.
Two health monitoring systems for damage detection in composite structures have been developed. First, a magnetic probe measures the magnetic field reflection during a frequency exploration. The local electrical conductivity of the structure is deduced from the cut-off frequency of the transfer function. Secondly, a probe with piezoelectric elements analyzes the local visco-elastic response of the structure produced at the moment of touching and stressing it. The same piezoceramic elements induces the mechanical solicitation and senses the response signal. The probes are compared in the analysis of damages induced in a coupon of quasi isotropic Carbon-epoxy material by various aggressions: 4 J- and 2 J-impacts inducing delaminations, a local burning by contact with a hot body and a simulated lightning impact by electric spark. Both probes show sensitivities to these various types of damage. For the piezoelectric probe, a "real-time" strategy, based on the processing of spectral power densities of the sensor signals, leads to an automatic measuring system classifying the damages. A software, based on fuzzy logic and implemented on a dedicated micro controller, elaborates the input data in order to realize the material damages classification. Combination the two techniques in a hybrid probe and use of the fuzzy logic procedure to the full signals to classify the structure response in a more subtle and extended way is the starting point of the design of a multisensor tactile probe able to recognize damages for a given material (NDE) or several classes of materials (robotized examinations in hostile environments).
A model of electromagnetic behavior of composite materials such as carbon epoxy or glass epoxy structures has been developed. Based on this model, an electromagnetic method to evaluate the electric conductivity and the electric polarization of this type of material, by measurement of magnetic and electric components of an incident electromagnetic field crossing through the material, has been also developed. A Health Monitoring System (HMS) derived from this technique is presented, which allows to detect and characterize a wide variety of defects inside composite structures. The HMS is constituted of emitting and receiving networks sensitive to magnetic or electric fields, integrated into the composite structure. This system delivers electric images in which the damages can be detected and localized. Various electric images obtained and related to structures including various damages such as impact delamination, fiber breaking, local burning and liquid ingress, are presented and compared to images resulting from classical ultrasonic NDE. The complementary of the present technique with acousto-ultrasonic technique based on Lamb waves propagation, and the interest of combining them in a unique integrated system is evoked.
This work aims at presenting two electromagnetic methods for non-destructive evaluation of delaminations in graphite- epoxy composites: the eddy current holography and the measurement of field reflected by delamination. The obtained results permit to get delamination images that are compared to the results obtained by ultrasonic C-scan procedure which is considered as reference method.
In composite materials, delaminations are discontinuities producing mode conversion processes generating various out-going modes. The Discrete Wavelet Transform allows isolating various propagation modes and extracting them in order to measure the time delay between the arrivals of the main burst and a specific out-going mode, for various propagation paths. This process permits, with a good accuracy, to localize a damage and to estimate its extension. An active health monitoring system composed of integrated disc-shaped, 100 (mu) m-thick and 5 mm-dia PZT transducers working sequentially as actuators and receives is presented. The diagnostic is based on multiresolution process by wavelet transform applied on recorded Lamb wave signals obtained before and after damage. The robustness and portability of this technique is demonstrated by the fact that, after validation in our laboratory it was successfully applied to data coming from an experiment conducted in an other Laboratory using its own Health Monitoring system.
The feasibility of an integrated system for permanent detection and estimation of damaging impacts on composite plates has been evaluated. This system is based on the existence, during a damaging impact, of an intense acoustic emission in the high frequency range. This acoustic emission is registered by a network of piezoelectric sensors and is used to obtain estimations of the location of damaging impacts and estimations of damage areas. Experiments have been carried out with carbon-epoxy plates equipped with four small and thin disc-shaped piezoelectric sensors. Each plate has been impacted using a weight drop machine equipped with a Boeing window. The impact energy varies from 2 J to 6 J for 16 plies coupons and from 4 J to 8 J for 32 plies coupons. The procedure used to identify the damage as a noise source from the signals received by the sensors allows to localize them with an accuracy of a few centimeters. It is also possible to obtain a nearly linear relation between the amplitude of the identified noise source and the projected area of the damage. It is then possible to estimate the damage area in a plate from the amplitude of the noise source after a calibration of the sensors of a reference plate.
The feasibility of an integrated system for permanent detection of damaging impacts on composite structures has been evaluated. This system is based on the existence, during the damaging impact, of an intense acoustic emission in the high frequency range. This acoustic emission is registered by a network of piezoelectric sensors. Experiments are carried out with carbon-epoxy plates equipped with four miniaturized disc- shaped piezoelectric sensors. Both fully embedded and surface positioned sensors are used. Each plate is impacted using a weight drop machine equipped with a Boeing window. The impact energy varies from 2 J to 6 J for 16 ply coupons and from 4 J to 8 J for 32 ply coupons. The procedure used to identify the damaged area as a noise source from the signals received by the sensors allows localizing them with an accuracy of a few centimeters. It also allows to correlate the <<amplitude>> of the identified noise source to the area of the damage. It is then possible to estimate the damaged area from the amplitude of the noise source.
We present shearographic imaging of the interaction of AO waves with defects in plates. The images are good when the plates are composite plates, due to some attenuation of the waves which prevents reflections. Those images contain information on the interaction between waves and defects. Nevertheless, this information is very difficult to extract and this extraction need numerical models. Here we show some examples of interaction. It is expected that this images will help to better understand the wave-defect interaction and will help to define and optimize health monitoring systems for carbon-epoxy plates and sandwich plates.
When an imperfectly conductive structure is illuminated by an electromagnetic wave, its response, on the magnetic component, is comparable to a first order integrator, if the skin effect is negligible at the wave frequency. Using this model, we have developed a method and a magnetic prove allowing to build 'electric images' of the defects present in carbon epoxy composite materials, by measurement of magnetic field transmission in these materials. After a presentation of the theory of electromagnetic scattering in a non perfectly conductive materials from M.A. Bethe and K.F. Casey, we demonstrate that it is not necessary to perform the magnetic field attenuation measurement by transmission; we can proceed on one face of the sample, by reflection, obtaining the same result. We present the experimental set up and the magnetic probe developed, we show several images achieved on samples having induced defects and we compare these image with the images achieved by a classical ultrasonic method on the same specimens. Furthermore we present images produced by integration in a structure of a 'sensor-network' based on the principle used by the probe.
The amplitude and phase spacing distributions of electromagnetic fields are imaged and measured using microwaves interferometry revealed by photothermal films and lock-in infrared thermography. Such EM fields imaging is a powerful tool for NDE of dielectric and radar absorbing materials.
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