Processes and Engineering in Mechanics and Materials Laboratory (PIMM, UMR CNRS 8006, Arts et Métiers ParisTech (ENSAM)), 151, Boulevard de l'Hôpital, Paris, F-75013, France.
The Structural Health Monitoring (SHM) process is classically decomposed into four steps: damage detection, localization, classification and quantification. Here the focus is put on aeronauticcompositestructures and specifically on the damage quantification step. For SHM purpose, such structures are equipped with piezoelectric elements that can be used both as sensors and actuators. To quantify a detected damage, measurements are first performed in a reference state. Then, during the life cycle of the structure several measurements at unknown states are performed. Several damage indexes are then extracted from the difference between the reference and unknown states. This damage indexes matrix is the basis of any algorithms dedicated to the quantification step but still contains many more dimensions that just a quantification of damage size. The question raised here is the efficiency of dimension reduction algorithms in the damage indexes space for quantification purposes. Performances of simple direct regression (SDR), principal component analysis (PCA), partial least squares (PLS), canonical correlation analysis (CCA) and autoencoders (AE) are investigated for this purpose. It is shown that PCA, PLS and CCA are all able to discover a low-dimensional space within the damage indexes space that is linearly related with the physical damage size, and that average prediction errors of the order of ≃ 1% can be achieved by projecting data through that low-dimensional space.
tips position [20, 21]. However, none of these methods addressed the case of a delamination type damage which is crucial in compositestructures.
In this paper, an alternate damage quantification strategy based on a post-processing step of the results of damage imaging method and focusing on delamination-type damage is presented. Such a method allows for damage size assessment of a delaminated area by post-processing the images produced by damage localization algorithms. Localization methods take raw signals from sensor as input and return a map of index representing the likelihood of presence of a damage over the surface of the structure under study. From this spatial probability map, region of high localization index is identified around the estimated damage location and the area of this region is computed. A data-driven model representing the mathematical relationship between the computed area and the actual size of the damage is then inferred. As more and more aeronauticstructures are made of composite , SHM processes have to be tested on this type of material. That is why the proposed method is validated on numerical simulation data carried out on CFRP plate samples. These samples are equipped with a stiffener and of a piezoelectric sensor-actuator network with several configurations of damage size. The proposed method is detailed in Sec. 2 and validated on numerical data coming from simulation of a composite plate equipped with a stiffener described in Sec. 3. Results are shown and discussed in Sec. 4.
THE 19 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS
The composite aerospace industry grows continuously and the needs for reliable non- destructive testing (NDT) technologies and techniques are high. The Centre Technologique en Aérospatiale (CTA) is unique in its kind, for it gathers three major inspection methods in one place: shearography, thermography and laser-ultrasonics. In order to challenge and demonstrate their capacities, those three cutting-edge technologies were used to inspect aeronauticcomposite parts provided by the Centre Spatial de Liège (CSL). 2 Inspection techniques
Fig. 8. Composite in A380 (courtesy of Airbus Group Innovation).
This trend has reached its maximum and now new evolutions are expected.
4. Some recent trends and perspectives
Research in aeronauticcompositestructures is still a growing field. If researches have mainly focused on sizing methods and damage tolerance, nowadays, advanced materials, structures and manufacturing methods are put forward. For materials, the introduction of thermoplastics is a challenge. This material has interesting capabilities in damage tolerance but its main interest is the ease of manufacturing especially with short cycles like in the automotive industry. A new kind of structures, already used by the Russians for spatial applications (Vasilliev et al, 2001, 2012), see Fig. 9 known as “geodetic structure” is studied by the aeronautic industry in Europ (www.wasis.eu). This technology is known to be very efficient (lighter than sandwich structures) but very complex and expensive. Another trend is to increase the thickness of the parts to sustain very high loads like in landing gears. Some prototype parts have reached 120 mm thick (EU program ALCAS). Finally, a strong on-going tendency is to insert out-of-plane strengths by stitching or pinning the composite (Toral et al.). The use of knitted or braided composite is also proposed. It allows designing fully 3D tailored structures. For example, the fan blade of the new Leap-X engine is based on an interlock composite (De Luycker et al, 2009). Many trials were also made to create multifunctional compositestructures and are reported in Duarte et al. For example, smart structures with grids of captor, sensors or piezo actuators is a way to improve the capability of aeronauticstructures by morphing the wings for example or to follow its ageing and health. It may also possible to make multifunctional parts. An example is an antenna integrated in the core of a sandwich structure which has both on the mechanical and radar functions (Smyers et al, 2008).
1.3 Continuité des séquences : blending
Enfin, on décrit ici le dernier type de contraintes qu’un problème d’optimisation de structures aéro- nautiques fait intervenir, à savoir la continuité des séquences entre éléments structuraux voisins. L’objec- tif est d’assurer une continuité entre les plis adjacents. Cette contrainte de continuité permet d’obtenir des performances mécaniques importantes. Cette continuité garantit de fait une bonne intégrité du panneau. Ces contraintes de continuité sont connues sous le nom de blending. Il existe différentes définitions du problème de blending. On peut par exemple souhaiter que des panneaux de même épaisseur aient les mêmes séquences d’empilement (cela conduit à la définition de tables de drapage) ou bien simplement souhaiter que des séquences voisines partagent les mêmes orientations sur leur épaisseur commune.
on an Aeronautical Carbon Composite Structure $SSO&RPSRV0DWHU SS
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Composite sway structures are prone to global instability phenomena and to second-order effects; the latter have to be predicted carefully because they may govern the design. These second order effects are amplified by an additional source of deformability with regards to steel sway structures: the concrete cracking. Indeed, this effect, which is specific to concrete and composite constructions, tends to increase the lateral deflection of the frame, amplifies consequently the second-order effects and reduces the ultimate resistance of the frames. In other words, for a same number of hinges formed at a given load level in a steel frame and in a composite frame respectively, larger sway displacements are reported in the composite one. The objectives of the present thesis is to investigate the effects of these phenomena on the behaviour of composite sway frames, to highlight the particularities of their behaviour and to propose simplified analytical procedures for the design of such frames; these objectives will be achieved by means of numerical and analytical studies on plane frames extracted from actual buildings. The research steps are the following:
With flat plates, elastic properties are generally determined by standardized experiments. When the sample is curved, their identification is more complex. Nowadays, there is a lack of representative samples capable both of predicting the mechanical behavior of a composite material from curved structures and taking into account the manufacturing process. Regarding filament winding, specimens extracted from the cylinder present two curved faces and two straight ones (respectively along the curvature and the generatrix of the cylinder). Moreover, it is well known that such a process induces residual manufacturing stresses [1, 2] which can be released during the cutting of the coupons. Some authors [3, 4] have found resulting strains after cutting a structure manufactured by filament winding. The cut sections exhibit a smaller radius of curvature than that of the cylinder before cutting and a significant warping of the assembly. The specimens extracted in this way are therefore not exactly representative of the initial material.
First A. Author, Second B. Author and Third C. Coauthor.
2 2 PROPOSED METHOD
The use of neural networks requires a large set of data in order to learn a process as well as to improve their capacity to generalize and predict the process they have been trained for. In the present case, ANNs are trained to predict the position of a single damage in a composite plate. In order to generate a significant dataset relative to different damage localization, a coupled-field finite-element model of the EMI technique is first developed (figure 1). The resulting electrical impedance at the terminal sensors is then utilized as a damage indicator. Finally, damage metrics derived from the impedance spectrum are used as inputs to train, validate and test the ANN.
In this paper, harvesters coupling magnetostrictive and piezoelectric materials are investigated. The energy conversion of quasi-static magnetic field variations into electricity is detailed. Experimental results are exposed for two macroscopic demonstrators based on the rotation of a permanent magnet. These composite/hybrid devices use both piezoelectric and
15. Allix, O., Lévêque, D. and Perret, L., "Identification and forcast of interlamination in composite laminates by an interlaminar interface model", Composite Sructures, Vol. 31, 1995, pp. 61-74.
16. Gornet, L,. "Simulation des endommagements et de la rupture dans les composites stratifiés", Thesis, Université Pierre et Marie Curie Paris 6/LMT/ENS-Cachan, ISBN 2-11- 088-9705, 1996.
Experimental composite joint tests in isolation
The test campaign realised at Stuttgart University was performed in strong collaboration with Liège University. The tested joint configuration (coming from the substructure designed and tested at Liège University) is presented in Figure 2. The tested joint configuration was designed so as to exhibit a ductile behaviour at collapse and with account of the M-N combined loading (Demonceau, 2008). The materials were ordered as follows: S355 steel for the profiles and the end-plates, ductile S450C steel for the rebars and C25/30 for the concrete.
 Jones RM. Mechanics of Composite Materials. 2nd Ed. London: Taylor & Fransis; 1999.
 H.K. Jeong, R.A. Shenoi. Probabilistic Strength Analysis of Rectangular FRP Plates Using Monte Carlo
Simulation. Comput Struct 76(3):219-35, 2000.
 S. Sriramula, M.K Chryssanthopoulos, Quantification of Uncertainty Modeling in Stochastic Analysis of
An helicopter blade is a characteristic example of complex sandwich structures : the leading edge is made of unidirectional glass-epoxy, reinforced with a thin layer of titanium. The core is made of a polyurethane foam, and covered with a hybrid glass-epoxy and carbon-epoxy skin. During impact on the leading edge, buckling of the skin or rupture of the foam can lead to a partial rupture of the structure. Therefore it is important to understand the mechanics of the rupture initiation.
5. Model applied to unidirectional plates
The approach described above to model damage accumulation has been used to predict the failure of unidirectional plate specimens loaded in the fibre direction. It has been shown that the model accurately predicts tensile failure to within ±2% and that by increasing the complexity of the model to include fibre-matrix debonding the scatter of composite properties can be simulated (17). The same reference explores the effects of the viscoelastic properties of the matrix on the long term behaviour of the composite under a steady load. It is shown that the model is capable of predicting the scatter of rates of fibre failures when compared to experimental curves obtained by acoustic emission. The present study has taken into account the variation of Weibull modulus which occurs when selecting just thirty two fibres from a very large fibre population. It has been shown that, even though the large population will have a determinant value of Weibull modulus, the random choice of thirty two fibres will lead to a wide distribution of the value ( 20 ). The results of including this effect can be seen in Figure 4 ( 21 ). These results confirm that damage increases with time and that the spread of the increase of damage, when different specimens are subjected to the same applied load, is increased over and above that obtained due to the scatter in fibre strengths because of the effects of considering a limited number of fibres in the RVE.
The Fig. 12 shows the optical micrographs of PP/flax com- posites for different holding times using the newly-developed manufacturing process. Once again, the interfacial debonding within the fibrous yarn is observed when no pressure holding time is considered (Fig. 12 a). A better quality of the interface cohesion is observed from 30 s holding time. It should also be mentioned that the optical micrographs emphasize the debonding interface at fiber bark and matrix, which is not observed with SEM analysis (Fig. 12 e). Some residual pieces of bark that were not removed during the preparation of the yarns may be observed between technical fibers (fiber bun- dles). The debondings observed at the interface between re- maining barks and the polymer matrix suggest that the prep- aration of flax fiber polymer composite should lead to bark free technical fibers. This point should be investigated into more details in future works so that to quantify the impact of residual barks (for different levels of bark quantities) on the quality of the fiber/ matrix interface and therefore, on the mechanical properties.
Theoretical evaluation of negative plastic moments of composite beams
Even when an effective width of slab is well defined, there are several definitions of the plastic moments of beams.
In a beam submitted to vertical loading and designed with allowance for some redistribution of moments, meaning some available ductility, Eurocode 4 prescribes that only those bars coming in addition to the welded mesh should be considered in the plastic resistance. The reason for this option is the ability of normal re-bars to yield, to be opposed to the low elongation at failure characterising welded bars of mesh. In the context of seismic design, the situation is more complex because it is not enough to define a reliable lower bound of the plastic moment : the capacity design of zones of the structure neighbour to the intended dissipative zones requires the neighbour zones to be more resistant than the real plastic resistance of the dissipative zones. This real resistance may mobilise all re-bars, welded mesh included, even if these fail when large displacements are applied to the structure. We can conclude that, for seismic checks of a structure, two plastic resistances of composite sections submitted negative moments should be considered :