Benoit Rosa
email: benoit.rosa@irccyn.ec-nantes.fr

Pascal Mognol

Jean Yves Hascoet

LASER POLISHING OF ADDITIVE LASER MANUFACTURING SURFACES

Keywords: laser polishing, ALM process, experimental investigation

The Additive Laser Manufacturing (ALM) technique is an additive manufacturing process which enables the rapid manufacturing of complex metallic parts and the creation of thin parts so as, for example, to decrease parts weight for biomechanical or aeronautic applications. Furthermore, compared with Selective Laser Sintering (SLS) technology, the ALM process allows creating more huge parts and material gradient. However, for aesthetic or tribological functions, the ALM surfaces need an additional finishing operation, such as the polishing operation. Polishing processes are usually based on abrasive or chemical techniques. These conventional processes are composed by many drawbacks such as, accessibility of complex shape, environmental impact, high time consumption and cost, health risks for operators etc... In order to solve these problems and to improve surface quality, the laser polishing (LP) process is investigated. Based on melting material by laser, laser polishing process enables the smoothing of initial topography. However, the ALM process and the laser polishing processes are based on laser technology. In this context, the laser ALM process is used directly on the same machine for the polishing operation. Moreover, an alternation between both processes can be established during the manufacturing operation in order to treat non accessible surfaces. Currently, few studies focus on laser polishing of Additive Laser Manufacturing surfaces, and it tends to limit the industrial use of additive manufacturing technology.

The proposed study describes an experimental investigation of laser polishing surfaces obtained by Additive Laser Manufacturing process. The investigation results in the improvement of complete final surface quality, according to laser polishing parameters. This experimental study introduces the laser polishing of thin section parts, in order to develop laser polishing applications.

According to a manufacturing chain context, the final objective is to create a multi-process mastery in order to optimize the final topography and productivity time.

Introduction

Based on laser melting of powders projection, the Additive Laser Manufacturing (ALM) technique is an additive manufacturing process which enables the direct manufacturing of complex metallic parts. Compared with SLS technology, the ALM process enables the creation of bigger parts and material gradient [START_REF] Muller | Modeling and control of a direct laser powder deposition process for Functionally Graded Materials (FGM) parts manufacturing[END_REF]. ALM process also enables the creation of thin parts so as, for example, to decrease parts weight for biomechanical or aeronautic applications. Integrated on a 5 axis milling machine or on an anthropomorphic robot, the laser melts the powder in order to create the desired part, layer by layer or in a continue deposition (Fig. 1). Based on CNC machine, it is possible to use hybrid manufacturing technology such as ALM, laser polishing and milling operations. Fig. 1: principle of ALM process However, the final quality of ALM manufactured surfaces is a real problematic [START_REF] Gharbi | Influence of a pulsed laser regime on surface finish induced by the direct metal deposition process on a Ti64 alloy[END_REF][START_REF] Gharbi | Influence of various process conditions on surface finishes induced by the direct metal deposition laser technique on a Ti-6Al-4V alloy[END_REF], that tends to limit the industrial application of the process. For aesthetic or tribological function, the manufactured surface needs a finishing operation, such as a polishing operation. Based on abrasive or electrochemical technics, these conventional polishing operations hold many drawbacks. For automated abrasive process, the drawbacks relate to the accessibility of complex surfaces caused by the diameter of the abrasive tool. Actually, the polishing operation is effectuated manually and it remains a slowly task. The efforts applied to the surface are not mastered, and can create nonfunctional parts. Finally, the repetitivity of task and the presence of residual powder of metal can cause a health risk for operators. Regarding the electrochemical polishing process, it holds an environmental drawback, due to the use of a chemical flow and because it requires an identic cathode part to polish the surface. In order to solve these problems, and to improve ALM surface quality, the laser polishing process is investigated. Used a few years ago to polish diamonds [START_REF] Erdemir | Durability and tribological performance of smooth diamond films produced by Ar-C60 microwave plasmas and by laser polishing[END_REF][START_REF] Gloor | 30 5.2 mm 2.9 mm Thickness : 0.8 mm Before LP After LP "UV laser polishing of thick diamond films for IR windows[END_REF] or optical lenses [START_REF] Bol | laser polishing of glass articles[END_REF], the laser polishing technique is now chosen more frequently, especially to polish metals. Thereby decreasing processing time and reaching 10 to 200 s/cm² according to the initial topography. The energy of the laser beam is applied to the surface and the topography peaks are melted. With the surfaces tensions, the molten material flow is reallocated into cavities in order to smooth the initial topography (Fig. 2). The final topography depends on the operating parameters of the laser on the material, and is also linked to the initial topography and the strategy of laser polishing [START_REF] Willenborg | Polishing with Laser Radiation[END_REF].

Problematic

Some studies focus on SLS polished surfaces optimization [START_REF] Lamikiz | Laser polishing of parts built up by selective laser sintering[END_REF][START_REF] Ramos-Grez | Reducing surface roughness of metallic freeform-fabricated parts using non-tactile finishing methods[END_REF][START_REF] Kruth | Experimental investigation of laser surface remelting for the improvement of selective laser melting process[END_REF] and less are concentrated on ALM laser polished surfaces [START_REF] Rombouts | Surface finish after laser metal deposition[END_REF]. Laser polishing optimizations [12; 13] of surfaces of thick section parts are proposed and it tends to limit aeronautic and biomechanical applications of laser polishing process.

The proposed study focuses on the laser polishing of ALM topographies obtained according to a complex and thin part context. Both processes are based on the laser technology. The investigation enables to apply ALM process and laser polishing process on the same 5 axis CNC machine, and it helps to improve productivity. Furthermore, the laser polishing operation can be applied between each ALM deposition layer. Based on experimental investigation regarding laser parameters and scan strategy, the proposed study analyzes final topography optimization according surface roughness and surface integrity. Finally, this study takes into account the final surface roughness and, in a first approach, the geometrical deviation of a real complex geometry, in order to define the process parameters.

Experimentations

ALM Process

The initial topography is obtained according to a complex parts' manufacturing context. Indeed, many strategies are workable in order to create functional parts. The ALM strategy can be based on vertical or flat strategy. The initial topography is function of the primary process strategy. According to a complex part manufacturing context, the vertical strategy is used (Fig. 3) to create 316L thin section sample. The argon gas is used during both processes in order to protect the oxidation of surfaces. The gas is projected through the powder projection nozzle of the additive manufacturing machine. The operating parameters used for the manufacturing sample are laser power (P) [W], feed rate (V f ) [mm/min], powder flow rate (Q m ) [g/s] and superposition step (h) [mm]. The 316L powder diameters used for experiments are situated within the range of 45-90 µm and the composition is conform to the standard ISO (Table 1). The initial and final topographies are characterized by surface roughness (S a ) [µm] (Eq.1) [14] and calculated according to the standard ISO 4288. The topography optimization is characterized by the surface roughness reduction [%] (Eq.2).

∬ | | (1)
(2)

Material Measurement

The measurements were carried-out thanks to a focal variation microscope ALICONA Infinite focus. This equipment enables to obtain a three dimensional topography of polished surfaces. The roughness parameters were extracted after a 3Dreconstruction, established with the software used. In order to control the surface integrity, a Scanning Electron Microscopy was used. This measure device enables to characterize micro-cracks and helps to measure the composition of the material.

Results

Initial Topography Analysis

The final surface topography depends on laser polishing parameters, strategy, material and initial topography. Unlike milled surfaces, which can be considered as an homogeneous laser polished topography [START_REF] Benoit | Topography modeling of laser polishing on AISI 316L milled surfaces[END_REF], ALM surface is more chaotic, heterogeneous and composed of two different textures (Fig. 4) correlated with the process principle. The first one is directional and caused by the layer-by-layer deposition direction. The second one is chaotic and caused by the partially melted powder. The analysis of the initial topography shows an oxidation of the surface. This phenomenon is highlighted by the color of the surface. According to the vertical manufacturing strategy, the argon protection is not sufficient. In other words, the efficacy of protection gas is depending on the argon flow direction.

Feasibility Study

The first step of the experimental campaign considered the feasibility of laser polishing process according to two objective functions.

These objective functions are the final topography smoothing and the form of the polished surface. In this study, the form analysis focuses on the collapse of the surface. Finally, the best determined parameters are a laser energy density E = 525 J/cm² obtained with P = 210 W, V f = 3000 mm/min, O f = 0 mm and an overlap (O v ) of 60 %.

After laser polishing, the surface is not collapsed and the topography is smoothed. However, the final topography is not perfectly smoothed and not optimized (Fig. 5). The final topography still includes some defects such as material drops and micro-cracks. These material drops tend to increase the surface roughness and impact on the tribological behavior. The micro-cracks reduce the fatigue resistance of the functional surface. Moreover, laser polishing creates a form distortion, caused by the heat transfer onto the thin surface.

Metallography Analysis Of Material Drops Defect

In order to perceive the composition of surface defect, the scanning electron microscopy device was used. The analysis showed a concentration of silicon oxide into the material drops (Fig. 6). The silicon component comes from the AISI 316L powder and becomes a silicon oxide drop during laser polishing process. The oxide component is linked to the ALM surface environment, which is composed of a non-mastered protection gas flow.

Laser polishing strategy

In order to improve the surface smoothing, a multipass strategy was chosen. The multi-scan strategy consists in applying N times the same laser parameters on the same surface, according to the same laser path. Keeping the same laser parameters operated for each pass, this strategy simplifies the mastering of laser polishing process. The number of passes varied from 1 up to 5 according to the same laser parameters. As a result, increasing the number of passes improves the topography smoothing (Fig. 7). After five laser polishing scans, the initial topography is greatly smoothed (Fig. 8). This laser polishing strategy enables to obtain a final surface roughness of 0.79 µm for an initial S a of 21 µm, and a surface roughness reduction of 96 %. The cut-off filter used for the polished topography is 800 µm and 2500 µm for the initial topography, according to ISO 4288. x 450 Oxygen Silicon 100 µm Fig. 7: evolution of topography according to the number of passes Fig. 8: profiles roughness before and after laser polishing according to a 5 passes strategy Fig. 9: evolution of silicon oxide phenomenon according to the increase of passes number Additionally, increasing the number of passes tends to limit the silicon oxide drops on the surface (Fig. 9). The increase of passes number creates a directional texturing of silicon oxide phenomenon, following the laser direction. After five passes, the silicon oxide phenomenon decreased. With a one-pass strategy, some micro-cracks are perceptible after laser polishing. After a multi-pass strategy, the micro-cracks are eliminated. A step structure phenomenon is visible after multi-scan strategy (Fig. 10) and there are no micro-cracks.

Decrease Of Silicon Oxide Drops

In order to eliminate silicon oxide defect from laser polished surfaces, a containment cell was used during ALM and LP processes. Mastering the environment enables to create a homogenous gas flow around the melt pool of ALM process and limits the oxidation (Fig. 11). Indeed, the silicon concentration decreased with the mastery of the argon flow. As a result, the decrease of silicon concentration on ALM surface reduces the silicon oxide drops phenomenon on laser polished surfaces.

Laser Polishing Of Real ALM Complex And Thin Part

Conforming to an industrial application, a real thin and complex ALM part was polished by laser.

According to a two dimensional laser path (Fig. 12), the offset parameter varied during the laser polishing process. This type of path enables to simplify the laser polishing programming.

In order to eliminate form deviation after laser polishing, the energy density was decreased regarding previous tests. A 100 w power 3000 mm/min feed rate for a five pass strategy was used. As a result, the ALM surface is smoothed after laser polishing process (Fig. 13) and no geometrical deviation is highlighted. A 62% surface roughness reduction is obtained for an initial S a of 14 µm and a final S a of 5.39 µm. Moreover, the final surface roughness is less optimized than with a 210 w laser power. The decrease of energy density enables to eliminate the geometrical deviation but increases final surface roughness.

As a conclusion, the laser polishing process must be optimized according to two objective functions, which are surface roughness and geometric deviation.

However, the protection gas is heterogeneous and the final topography is not optimized. The protection gas defect is highlighted by a surface color gradient. As a conclusion, the offset parameter variation enables to smooth a complex surface according to a simple laser path. An investigation of five axis and normal strategy to the surface can improve the final polished topography.

Conclusions And Further Research

This study focuses on the laser polishing of ALM surfaces according to a thin and complex manufacturing part context. Based on experimental investigation, this study enables to optimize ALM surfaces according to laser polishing parameters, and laser polishing strategy. As a result, the initial topography is smoothed, and the integrity of the surface is improved. Finally a real laser polishing of ALM part is presented. After investigation more conclusions can be established:

 The laser polishing process improves considerably the surface quality of ALM parts.  The increase of number of passes improves the smoothing and decrease the silicon oxide drop phenomenon.  The master of argon flow during ALM process improves the surface integrity after laser polishing.  The multi-pass strategy enables to eliminate the micro-cracks created on the first laser polishing scan.  Laser parameters settings must be correlated, with initial topography, material and topology of parts.  The multi-pass strategy operated according the same laser parameters, enables to simplify the laser polishing optimization.  Laser polishing process enables to smooth thin section parts obtained by ALM process.  A simple laser polishing strategy enables to improve a complex ALM surface quality.  The laser polishing process must be optimized according to surface roughness and geometric deviation Further research will be focusing on a five axis laser polishing strategy, in order to treat an entire complex ALM surface. Due to a re-melting process, more metallurgical analysis will be taken into consideration such as hardness, microstructure fatigue behavior, and corrosion resistance.

The final objective will focus on the final topography prediction according to an experimental methodology.

Fig. 2 :

 2 Fig.2: principle of laser polishing process

Fig. 4 :Fig. 5 :Fig. 6 :

 456 Fig.4: topography of ALM surface

Fig. 10 :Fig. 11 :

 1011 Fig.10: step structure phenomenon after a multipass strategy

Fig. 12 :Fig. 13 :

 1213 Fig.12: laser polishing strategy of real ALM complex and thin part

Table 1 :

 1

		composition of 316L powder used for
		manufacturing ALM surfaces
	Component	Si	C	Mo Ni	Mn	Cr
	%	0.8 0.02 2.5 12.2 1.4 17.3
	3.2. Laser Polishing Process

LP and ALM processes are based on the same 5 axis machine, and use the same laser fiber. The maximum laser power is 800 watts, for an 1070 nm wavelength and a 0,8 mm top-hat spot diameter at offset (O f ) equal to 0 mm. Each ALM sample is divided into several 7 mm x 30 mm laser polished areas. The laser polishing path is perpendicular to the initial ALM topography. The tested laser polishing parameters are:

 Laser power (P) [W]  Feed rate (V f ) [mm/min]  Overlap (O v ) [%]

which means the distance a line overlaps with the last one  Strategy of laser polishing

Acknowledgments

This work is co-funded by the European Union. Europe invests in Brittany with Regional Development European Funds (FEDER). The authors would like to thank the Ecole Normale Supérieure de Rennes, and Gilles Carabin from IRCCyN.


S Lo Feudo

A Allani

G Cumunel

P Argoul

D Bruno
email: d.bruno@unical.it

8-22 Mai 2015, Presqu'île de Giens (Var) Experimental Tuned Mass Damper Based on Eddy Currents Damping Effect and Adjustable Stiffness

Keywords: TMD, Eddy currents, vibration control, wavelet transform

An experimental Tuned Mass Damper (TMD) is proposed in order to damp vibrations induced by external excitations. This TMD is based on the Eddy Currents damping effect and is designed in such a way as to allow a manually adjustment of its own stiffness and inherent damping. The TMD's modal parameters estimation is therefore carried out by applying the Continuous Wavelet Transform to the signals obtained experimentally. The influence of the manual adjustment of the TMD's components on its dynamical properties is then studied.

Introduction

In the framework of vibration control of civil engineering structures, an important role is covered by the adoption of devices called Tuned Mass Damper (TMD). The basic idea consists in attach a passive control device to a primary structure, for example a tall building or a bridge, which is tuned with the main structure's natural frequency and is able to reduce vibrations induced by harmonic excitation, wind loads and/or earthquake ground motions. Part of the vibration energy can be in fact transferred from the principal structure to the TMD, so that the capability to dissipate energy is improved, with obvious advantages in terms of durability and structural safety.

In this research, an innovative experimental TMD is proposed and designed to be placed on a top floor of building scale model. The proposed TMD is manufactured in such a way as to allow a change in the dynamical properties of the device itself, namely the stiffness and the damping coefficient. In fact, since the mass of the TMD is fixed for practical issues, TMD's stiffness and damping represent the design parameters, so that it is necessary to easily set them in order to test some of the optimization criteria proposed in the literature and reviewed by Allani et al. [START_REF] Allani | Optimization of one or multiple tuned mass dampers[END_REF].

The damping of the TMD is obtained with eddy currents damping effect. In particular, when a conductive material is subjected to a time-varying magnetic field, due to the electromagnetic induction an electromotive force (emf) is produced in the conductor, and eddy currents are generated. According to the Faraday's law of induction, this electromotive force opposes the rate of change with respect to time of the magnetic flux. Moreover, for the Lenz's law, the consequent currents circulate in such a way as to generate a magnetic field that opposes the original change in magnetic flux. For this reason, when the flux rises the emf is negative and the eddy currents contrast this flux increase. The currents circulation causes therefore a magnetic field which flux is opposed to the other. Conversely, when the flux rate drops, the emf is positive and eddy currents generate a magnetic field which flux is concordant with the other. In addition, a Lorentz force appears contrasting the motion but owing to the resistance of the conductor, energy is dissipated into heat, so that a vibration reduction is achieved. According to Graves et al., [START_REF] Graves | Theoretical comparison of motional and transformer EMF device damping efficiency[END_REF], electromagnetic dampers can be based on motional or transformer electromotive force depending on the mechanism that generates the emf. The transformer emf devices generate an emf in presence of a stationary conducting circuit and of a time-varying magnetic field linking the circuit, whereas the motional emf devices generate an emf within a conductor moving through a stationary magnetic field. The authors remarked that, although the transformer emf devices are more efficient, the motional emf are more suitable to realistic situations. Some researches related to the use of the eddy currents damping mechanism were then reviewed by Sodano & Bae in [START_REF] Sodano | Eddy Current Damping in Structures, The Shock and Vibration Digest[END_REF]. As advantages of adopting this damping strategy, the authors highlighted the lack of contact between the eddy current damper and the system to damp, so that not only the dynamical response and the material properties are unaffected by its addition, but also the damper performance does not degrade over the time. As far as the vibration control is concerned, Larose et al. adopted in [START_REF] Larose | Modelling of tuned mass dampers for wind-tunnels tests on a fullbridge aeroelastic model[END_REF] a TMD to reduce the oscillations of a full-bridge aeroelastic model, providing the adjustable inherent damping of the added mass by an eddy currents mechanism. An eddy currents damper was also proposed by Sodano et al. in [START_REF] Sodano | Concept and model of eddy current damper for vibration suppression of a beam[END_REF] with the aim to suppress vibration of a beam in case of permanent magnet placed on the top of the beam and sets perpendicular to the beam's vibration direction. In this case, only the radial component of the magnet flux generates the damping force since the magnetic flux density component parallel to the velocity of the conducting sheet does not produce any contribution. In order to measure the damping of the beam as a function of the gap between the used copper plate and the surface of the magnet, Sodano et al. had calculated the damping ratio either by determining the logarithmic decrements of the initial condition response and by applying the unified matrix polynomial approach to the frequency response. According to them, eddy currents increase greatly the damping and the eddy current damping force is highly non-linear with respect to the gap between the magnet and the beam. Ebrahimi et al. had studied in [START_REF] Ebrahimi | Design and modeling of a magnetic shock absorber based on eddy current damping effect[END_REF] the problem of a magnetic shock absorber based on the motional emf, and Bae et al. also proposed in [START_REF] Bae | Vibration suppression of a cantilever beam using eddy current damper[END_REF] a theoretical model able to predict the eddy current damping effect on the suppression of a cantilever beam vibrations. A magnetically TMD was also recently tested in case of a large beam by Bae et al. [START_REF] Bae | Vibration Suppression of a Large Beam Structure Using Tuned Mass Damper and Eddy Current Damping, Shock and Vibration[END_REF]. Finally, a large-scale horizontal TMD based on eddy current damping was designed and tested by Wang et al. in [START_REF] Wang | Feasibility study of a large-scale tuned mass damper with eddy current damping mechanism[END_REF]. The damping ratio of the TMD was evaluated by using the logarithmic decrements obtained from free vibrations test data, and it was showed to radically drop as the gap between magnet and conductive mass increases. A feasibility analysis was also carried out by considering durability and economic viability of such a TMD.

The present work is arranged as follows: Section 2 provides a general theoretical background on eddy currents and on the methods adopted for the TMD's parameter estimation; Section 3 illustrates the proposed experimental TMD model together with the achieved results and finally, Section 4 gives some concluding remarks.

Eddy currents Tuned Mass Damper

When a conductive material moves along a stationary magnetic field, eddy currents are induced, (Figure 1.a). By neglecting the surface charge in the conductive plate, the eddy currents density can be written as:

()   J v B ( 1
)
where  is the electrical conductivity of the conductive material,

() T x y z v v v    v
is the relative velocity between the permanent magnet and the conductor,

() T x y z B B B    B
is the magnetic flux density due to the magnet and  vB represents the electromotive force driving the eddy currents. Considering that velocity has component only in the x direction, the currents density is given by: ()

x z y v B B     J j k (2)
The magnetic flux density components due to a permanent circular magnet can be evaluated by applying the Biot-Savart law to a circular loop as follows:

0 1 3 1 4 Mdz d d      R B R s (3)
where 0

 is the permeability, M the magnetization per unit length and dz a differential length in the magnet thickness direction.

              R R r r i j R j k i j s (4)
where 1 R is the distance between an infinitesimal element ds on the strip and the point P on the () yz -plane, R the distance of P from the center of the circular magnet and b its radius. By substituting Eqs. (4) into Eq. ( 3) and integrating through the magnet thickness, the magnetic flux density due to the permanent circular magnet can be expressed as [START_REF] Bae | Vibration suppression of a cantilever beam using magnetically tuned-mass-damper[END_REF][START_REF] Sodano | Concept and model of eddy current damper for vibration suppression of a beam[END_REF] /

                                             
The eddy currents J produce in turn a magnetic field with opposite polarity with respect to those generated by the permanent magnet, and a repulsive force appears. This force can be evaluated by adopting the Lorentz's Law and can be expressed as follows:

()

V dV   F J B ( 5
)
where V is the conductor volume. Substituting Eq. ( 2) into Eq. ( 5) and by doing the cross product, the electromagnetic force becomes 22 [( ) ]

x z y x y x z V v B B B B B B dV        F i j k
Hence, according to [START_REF] Wang | Feasibility study of a large-scale tuned mass damper with eddy current damping mechanism[END_REF] and by virtue of Eq. ( 2), the

x component of the electromagnetic force does not depend on the component of the magnetic field parallel to the velocity of the conductor,

x B : 22 () x x y z V F v B B dV     
The damping force in the

x direction is thus proportional to the velocity vector with opposite direction. It is therefore possible to define the linear damping coefficient and the damping ratio of the TMD [START_REF] Wang | Feasibility study of a large-scale tuned mass damper with eddy current damping mechanism[END_REF] as follows:

Parameter estimation

The numerical evaluation of the frequency and of the damping ratio is carried out by applying both a classical method and the Continuous Wavelet Transform (CWT) on experimental data. The first method consists in evaluating the natural period of the system from a free vibration test by measuring the distance between two peaks in the temporal domain, and the damping ratio by applying the logarithmic decrement technique. In addition, the modal parameters identification from experimental data is also performed by applying the CWT to the achieved signals. The CWT allows to study a signal in a time-frequency plane, allowing the time evolution estimation of natural frequencies, damping ratios and mode shapes, which is useful for noised and non-stationary or non-linear systems. Generally speaking, the CWT of a signal consists in the decomposition of the signal itself in a family of function localized in time and in frequency, called wavelet. A wavelet family is built by scaling and shifting in time a base wavelet, called mother wavelet. The principal characteristic of the mother wavelet is to have zero mean and finite energy. In this way, the signal can be analyzed at different scales, with large windows for low frequencies and narrow windows for high frequencies. In particular, the CWT of an asymptotic signal is concentrated along some curves in the time-frequency domain called ridges, and the restriction of the CWT to each ridge is called skeleton of the wavelet. The skeleton gives the natural frequency of the signal, whereas the time evolution of the logarithm of the Wavelet transform amplitude gives information about damping, for further explanation please see Le & Argoul [START_REF] Le | Continuous wavelet transform for modal identification using free decay response[END_REF].

Experimental TMD model

The proposed TMD is made of a 150 g copper mass supported by two thin aluminum sheets. The device was designed in order to permit a manual adjustment of the natural frequency and of the inherent damping, and to be placed on the top floor of a building scale model (Figure 2). One of the cable's support position is variable, so that the amount of stretch in the slender cable can be manually changed. As long as the cable tensile stress is increased, the TMD stiffness rises, as well as the natural frequency. In this study, by considering the frequency bandwidth of interest, the support is supposed to move over a range of 0.4 mm and the step size here considered is of 0.04 mm . This range will be denoted hereafter as 1 D . In particular, when 1 D is sets equal to 0 mm , the TMD frequency is the higher among those considered. The damping of the TMD is conversely given by a magnet located up to the mass, which distance is adjustable. Since the conductive material, that is the copper mass, is subjected to a time-varying magnetic field, eddy currents are induced in the conductor. The time-varying magnetic field is in fact generated by the oscillation of the conductor in the stationary field and the normal of the magnet surface is perpendicular to the velocity. The magnet is therefore moved over a range of 2 mm , and the step size adopted here is of 0.2 mm . This range will be denoted hereafter as 2 D . The initial chosen value of 2 D corresponds to a distance between the magnet and the mass of 3.35 mm.

Parameters estimation

In order to set the TMD with optimal parameters, it is necessary to characterize the TMD properties behavior when the magnet distance and the stretch in the cable are changed. To this end, the response of the TMD alone was recorded in several free vibration test when an impulsive shock is applied on it. For each fixed value of 1 D the response of the TMD in the temporal domain was measured by varying 2 D and each test was carried out three times. The signals recorded were then cut at the two ends to eliminate superfluous recordings, and a high-pass filter was adopted in order to remove a low frequency component deriving probably from external factors. The obtained smoothed and filtered data were therefore adopted in order to evaluate the TMD's damping ratio and frequency by using the CWT. The CWT of a signal allows not only to evaluate the modal parameters of the system, but also to appreciate their evolution in time. This represents an important issue in this context since TMD performance is very sensitive to a change in the parameters characteristics. The Cauchy wavelet was chosen as mother wavelet and the Q factor, which represents the ratio of the center-frequency to the frequency bandwidth, was set equal to 10 (for more details please see [START_REF] Le | Continuous wavelet transform for modal identification using free decay response[END_REF]).

In Figures 3 and4, results are presented for two cases where the obtained damped frequency is in the neighborhood of the fundamental frequency of the structure to be controlled, which is 10.77 Hz .

By looking at the ridge of the CWT, it emerges that the frequency keeps almost constant with time and the damping ratio, which can evaluated from the slope of the logarithm of the Wavelet transform modulus exhibits in the first instants a non-linear behavior due probably to geometric non-linearity. However, the non-linear effect keeps out of the zone of interest since the considered domain was chosen in such a way to neglect the so-called edge effect. By looking at the logarithmic of the amplitude of the Wavelet transform, it appears that the damping can be correctly modeled as viscous since the logarithmic of the amplitude has a linear behavior within the considered domain. In Figure 5, the TMD acceleration, the damping ratio and the natural frequency of damped vibration are shown within the considered domain and neglecting the edge effect. In Figure 5.a, the signal's peaks are detected, and in Figure 5.b, the damping ratio obtained from the CWT and from the logarithmic decrement are compared. Finally in Figure 5.c, the damped frequency achieved by applying the CWT and that obtained from the peak distance are presented. From Figure 5.b it emerges that the damping ratio obtained from the logarithmic decrement technique has an oscillatory behavior around the value obtained from the Wavelet transform. Also the damped frequency achieved from the peaks distance has an oscillatory behavior, but values are very close. In Figure 6, for a fixed 1 D , the damped frequency and the damping ratio, by varying the magnet distance, are shown when both methods are used. Results obtained from the classical methods slightly underestimate those obtained by the Wavelet transform, but the global trend appears to be quite similar. In Figure 7, results obtained in terms of modal parameters for each 1 D and 2 D value considered are summarized. The damped frequency and the damping ratio reported are the average values obtained from each of the three tests, as in Figure 6. In

Conclusions

In this study, an experimental prototype of Tuned Mass Damper based on the damping effect of Eddy Currents is proposed. The eddy currents are induced by the movement of a conductor, which is the TMD's mass, within a stationary magnetic field. As a consequence, a magnetic field opposite to the flux change arises, together with a Lorentz force, which is opposite to the conductor movement. Several vibration tests were performed and the TMD's parameter estimation was carried out with the aid of the Wavelet transform. The research objective was to evaluate the influence of the cable level stress and of distance between the magnet and the conductive mass on the modal parameters. In this framework, not only the eddy currents were shown to be effective in order to furnish the damping to the TMD device, but also the damping achieved was shown to be viscous. In particular, the damping ratio obtained depends on the distance of the circular magnet from the damper mass and decreases non-linearly with the considered gap. Conversely, the TMD's stiffness given by the slender cable connected to the mass increases linearly when the cable tensile stress is augmented.

Figure 1 -

 1 Figure 1a) Eddy currents mechanism and b) circular magnetized strip

  and the natural frequency of the TMD, respectively.

Figure 2 -

 2 Figure 2 -Eddy Currents Tuned Mass Damper

Figure 7 . 1 D 1 D

 711 a it appears how, for a fixed tension in the cable , the frequency remains nearly constant by varying the distance of the magnet. The damped frequency also varies linearly by changing and it does not depend on the magnet distance. Conversely, Figure7.b shows how the damping ratio varies in a non-linear way with respect to 1 D and 2 D . In particular, when the magnet is close to the conductor, the damping ratio tends to assume higher values.

Figure 3 -Figure 4

 34 Figure 3a) TMD's acceleration, b) Fourier transform, c) CWT, d) Logarithmic plot of the skeleton amplitudes and e) damping ratio of the TMD under an impulsive shock for 1 0.04 D mm  and 2 3.55 D mm 

Figure 5 -Figure 6 - 2 DFigure 7

 5627 Figure 5a) TMD's acceleration, b) damping ratio and c) damped frequency under an impulsive shock for 1 0.24 D mm  and 2 5.15 D mm 


Benoit Rosa
email: abenoit.rosa@irccyn.ec-nantes.fr

Jean-Yves Hascoet
email: bjean-yves.hascoet@irccyn.ec-nantes.fr

Pascal Mognol
email: cpascal.mognol@irccyn.ec-nantes.fr

Modeling and optimization of laser polishing process

Keywords: laser polishing, initial topography, experimental design, statistical model

Laser polishing is a finishing process based on melting material, with the objective of improving surface topography. Some operating parameters must be taken into consideration, such as laser power, feed rate, offset, and overlapping. Moreover, because of its dependence on the primary process, the initial topography has also an impact on the final result. This study describes a quadratic model, conceived to optimize final topography according to the primary process and laser polishing. Based on an experimental matrix, the model takes into account both laser operating parameters and the initial topography, in order to predict polished surfaces and to determine optimal set of parameters. After the phase of experimentation and the creation of the quadratic model, an optimal final topography is introduced, taking into account the initial surface and the laser parameters.

Introduction

Finishing operations on mechanical parts is a decisive step in the production chain, regarding the functionality of parts. In the molding field, the finishing process is often practiced manually, which represents up to 30 % of production cost [START_REF] Ukar | Laser Polishing Operation for Die and Moulds Finishing[END_REF]. These high costs are the result of high time processing operations that represent 10 to 30 min/cm² and require highly qualified operators [START_REF] Ukar | Laser Polishing Operation for Die and Moulds Finishing[END_REF][START_REF] Nüsser | Influence of Intensity Distribution and Pulse Duration on Laser Micro Polishing[END_REF]. Moreover, the geometrical shape of parts can be modified by un-mastered trajectories and efforts applied to the surface. The accessibility of a complex shape is also restricting [START_REF] Nüsser | Influence of Intensity Distribution and Pulse Duration on Laser Micro Polishing[END_REF]. The economical context forces the industrial companies to reduce production costs by automating the processes. In order to improve surface quality and to reduce processing time, some finishing processes were developed like six axis robot mechanical polishing. However, the accessibility of complex surfaces is limited by the abrasive diameter of tools. To improve the accessibility of complex surfaces laser polishing technology appears as notably promising. Used a few years ago to polish diamonds [START_REF] Erdemir | Durability and tribological performance of smooth diamond films produced by Ar-C60 microwave plasmas and by laser polishing[END_REF][START_REF] Gloor | UV laser polishing of thick diamond films for IR windows[END_REF] or optical lenses [START_REF] Bol | laser polishing of glass articles[END_REF], the laser polishing technique is now chosen more frequently, especially to polish metals. Thereby decreasing processing time and reaching 10 to 200 s/cm² according to the initial topography. The energy of the laser beam is applied to the surface and the topography peaks are melted. With the surfaces tensions, the molten material flow is reallocated into cavities in order to smooth the initial topography (Fig. 1). The final topography depends on the operating parameters of the laser on the material, and is also linked to the initial topography and the strategy of laser polishing [START_REF] Willenborg | Polishing with Laser Radiation[END_REF][START_REF] Hafiz | Influence of overlap between the laser beam tracks on surface quality in laser polishing of AISI H13 tool steel[END_REF].

Problematic

Few studies are focused on the optimization of laser polishing surfaces according to the primary process in context of large and complexes shapes, such as mold surfaces [START_REF] Gisario | Surface transformation of AISI 304 stainless steel by high power diode laser[END_REF][START_REF] Lamikiz | Laser polishing of parts built up by selective laser sintering[END_REF][START_REF] Ukar | Laser polishing of tool steel with CO2 laser and high-power diode laser[END_REF]. In this context, and to create a multi-processes optimization [START_REF] Rauch | An advanced STEP-NC controller for intelligent machining processes[END_REF], this study introduces a statistical model of prediction.

The surface quality optimization is realized according to primary process and laser polishing in order to decrease the processing time of the whole manufacturing chain. As a result, this study introduces a coupled optimization between the primary process and the laser polishing. The model includes the initial topography, realized with a hemispherical tool, operating parameters of laser and the overlap parameter. The following parameters are used to control the laser:  Laser power (P) [W]  Feed rate of the laser (Vf) [mm/min]  Offset (Of) [mm] which is the shift into laser's focal point and surface  Overlap (Ov) [%] which means the distance a line overlaps with the last one. This parameter is specific for the treatment of a complete surface. The milled surfaces obtained with a hemispherical tool are characterized by scallop height (Hc), which is a function of the radius of hemispheric milling tool R and the milling step (ae) Eq. ( 1). Hc = ae² / 8R.

(

) 1
The final surface is characterized by two parameters:  Surface roughness parameter Sa, which is the arithmetic average heights of the surface [12].  Percent reduction of surface roughness which gives information about surface roughness reduction, using the initial surface roughness [START_REF] Hafiz | Influence of overlap between the laser beam tracks on surface quality in laser polishing of AISI H13 tool steel[END_REF].

Different initial topographies were studied through the parameter Hc, in order to decrease the time of the primary process, and optimizing at the same time the reduction percentage of the initial surface roughness according to both processes. The present paper focuses on the modeling and coupled optimization of laser polishing on AISI 316L milled surfaces. The modeling is taking into account the initial topography and the laser operating parameters.

Experimental study 3.1. Surfaces polishing tests

A continuous fiber laser was used during the experimentation. The maximum laser power is 12 kW for a 600 µm fiber's diameter. A welding head High Yag RLSK SWS was positioned on a six axes robot for the experiments. The welding head allows the motion of the laser beam independently of the robot. To protect surfaces from oxidation, argon gas was employed during laser polishing process. This step was performed according to an experimental design based on L16 (45) Taguchi method. This design experiment contains 5 factors and 4 levels. The strategy of surface polishing was perpendicular to the initial topography and the input factors were: the scallop height, the laser power, the feed rate, the offset and the overlap parameter (Table 1). Indeed, to study the homogeneity of surfaces, 21 mm x 28 mm surfaces were employed. Two samples AISI 316L were used, and were divided in 8 areas. Each area is corresponding to one test of the design experiment. Choosing this type of design matrix was justified by the number of factors and by the number of levels. In this matrix, it is possible to integrate every factor of the problematic, such as: the characterization factor of the initial topography (Hc), the laser power, the feed rate, the offset and the overlap. Fewer tests are necessary, and that could have an important effect on industrial application. A number of levels superior to two enables to approach nonlinear systems. The optimal results of this experimental study are a surface roughness reduction of 85 % and a final surface roughness of 2.08 ± 0.1 µm for an initial surface roughness of 13.73 µm. The average surface roughness reduction of the overall trials is 65 %. The efficiency of laser polishing has been highlighted by the smoothing of profile before polishing, and the decrease of the amplitude (Fig. 2).

Fig. 2: Optimal topography profile of the experimental design (85 % of surface roughness reduction)

Optimization of the final surface

The surface response method was employed in order to optimize the final surface topography according to the initial topography and the laser parameters. Based on quadratic regression between the experimental data and the input parameters, it is possible to determinate the optimum position of the reduction of the surface roughness. The surface responses showed that the optimal domain of feed rate and Hc is in the experimental domain (Fig. 3a). For the laser power and Hc the optimal domain is also in the experimental domain (Fig. 3b). Finally, regarding the overlap parameter the optimal domain is not in the experimental domain (Fig. 3c). This observation shows a superior value of 50 % improved surface roughness reduction. Moreover, the surfaces responses showed that a scallop height of 60 µm is an optimal value according to the laser parameters. The next step consisted in studying the increase of the overlap parameters for 60 and 90 % values. These overlap values were applied with an optimal laser parameters determined from surfaces responses (Hc = 60 µm, P = 750 W, Vf = 1000 mm/min, Of = 30 mm). According to these values of input parameters, the maximum surface reduction was 93 %. According to the standard ISO 4288 a filter with a cut off of 800 µm was used. The final Sa was 0.94 µm ± 0.1 µm for an initial Sa of 13.6 µm. The augmentation of overlap considerably increased the final result. After this optimization, the initial topography was eliminated. This optimal surface smoothing was highlighted by the decrease of the profile amplitudes and the decrease of frequency (Fig. 4).

Modeling

Based on Taguchi design, the statistical model is available to the experimental matrix. As well as a nonlinear system [START_REF] Ukar | Laser polishing of tool steel with CO2 laser and high-power diode laser[END_REF], and a number of levels superior to 2, a quadratic model was used Eq. ( 2).

ymod = b0 + b1.x1 + b2.x2 + … + bk.xk + b11.x1² + b22.x2² + … + bkk.xk². (2)
The input parameters are represented by x1…x²k and the coefficients (monomials) by b0…bkk. The quadratic regression equation is divided into two parts: a linear part and a quadratic part expressed by squared factors. The number of monomials is a function of the numbers of factors and the number of degrees of regression. After having calculated the coefficients, the quadratic regression model was obtained Eq. ( 3).

Roughness reduction of Sa (%) = -166.77768 + 5.44880583*Hc -0.04559046*Hc² + 0.221260766*Power -0.1514.10 -3 *Power² + 0.026068521*Feed rate -0.12484.10 -4 *Feed rate² + 0.643237507*Offset -0.00642640*Offset² -0.9232599*Overlap + 0.012452807*Overlap².

Discussion of results

In order to test the predictive capacity of the model, the "leave one out" cross validation method was used. The aim of this methodology is to predict the extracted points of architecture of the model. In other words, this method informs about the comportment of the model between the points used for statistical regression. Thus, a model is calculated for each point. Each model is based on N-1 points and allows the prediction of the extracted point. After calculation of the 16 points, the average error of the model is 11.5 % (Fig 5). However, three predicted point has an excessive error. The maximum predicted error is 40 % for a minimal error of 0.02 %. In order to test the real predictive capacity of statistical model, a validation test was realized during the experimental campaign (Hc = 60 µm; P = 750 W; Vf = 1000 mm/min; Of = 30 mm and Ov = 30 %). The predicted value of the validation test is 87.3 % and the measured value is 84.5 %. The predicting error is 2.8 %.

Conclusions

This study focuses on the topography modeling of laser polishing of AISI 316L milled surfaces.

The experimental campaign takes into consideration initial topography, laser parameters, overlap parameter and allows the establishment of statistical model. According to these results, more conclusions can be established:  The laser polishing process is effective with regard to surface smoothing.  Thanks to the leave one out cross validation method the predictive capacity of the statistical model is acceptable. The average error is 11.5 %.  The surface responses are a good indicator with regard to the position of the optimum.  The used of experimental matrix enables to obtain an average percentage of roughness reduction of 65 % for a maximum of 85 %.  After optimization, the percentage of surface roughness reduction is 93 %, for a final surface roughness of 0.94 µm ± 0,1 µm and an initial surface roughness of 13.6 µm.

Fig. 1 :

 1 Fig. 1: Principle of laser polishing

Fig. 3 :

 3 Fig. 3: Quadratic response surfaces between input parameters and the surface roughness reduction

Fig. 4 :

 4 Fig. 4: Topography profiles for different overlap parameters on Y axis (Overlap 30, 60 and 90 %)

Fig. 5 :

 5 Fig. 5: Measured and predicted values by cross validation methodology

Table 1

 1

			: factors and levels of design experiment
	Levels	Hc	Laser Power	Feed rate	Offset	Overlap
		[µm]	[W]	[mm/min]	[mm]	[%]
	1	40	750	500	0	20
	2	50	850	1000	10	30
	3	60	950	1500	20	40
	4	70	1050	2000	30	50


F Balança

M Lemagnen

S Pompidou

N Perry

DEVELOPMENT OF A DESIGN FOR END-OF-LIFE APPROACH IN A STRONGLY GUIDED DESIGN PROCESS. APPLICATION TO HIGH-TECH PRODUCTS

In response to a growing concern for environmental problems and to waste management from mass production products, several regulations have appeared to tackle end-of-life (EoL) issues. They address for instance end-of-life vehicles or waste electrical and electronic equipment. EoL management mainly lays on both EoL industry and product design. Thus, new methods of design have already been implemented since the past decades to answer the regulation requirements, notably through material choices and product architecture. However, some high-tech products remain out of the scope of these legislations. But for some years, initiatives have emerged for these products, coming from governments, international programs or customers' requirements which become increasingly strict. This paper focuses on a new design approach that would allow taking into account EoL considerations for such type of products, based on EoL strategies and adapted to aeronautic and defence products.

INTRODUCTION

"Cradle to grave design" [START_REF] Mdonough | Applying the principal of green engineering to cradle to cradle design[END_REF] is the actual common way for designing products. A product is made of first extraction raw materials and is rarely designed for a second life. In other words, reusing the whole product, remanufacturing it, or recycling its components are not yet promoted options. Indeed, end-of-life (EoL) considerations are not enough taken into account during the design process to ease its EoL treatments due to conflicting economic and environmental policies in a company. However, some European directives tackle EoL issues of mass production products. One of the most advanced sectors in terms of waste management is the automotive one. End-of-life vehicles (ELVs) directive [START_REF]Council of the European Union[END_REF] aims at preventing the waste production from vehicles, by supporting recycling and other recovery processes, in order to reduce the waste disposal. According to this legislation, 85% of ELVs have been recovered in 2006, of which 80% recycled or reused. By 2015, these ratios will respectively rise to 95% and 85%. Tools and design methods have been developed to comply with this regulation. Besides, other directives also deal with EoL: WEEE (Waste of Electrical and Electronic Equipment) [START_REF][END_REF], Disposal of spent batteries and accumulators [4] and Packaging and packaging waste [START_REF]EC of 20 December 1994 on packaging and packaging waste[END_REF]. Despite that, some high-tech products remain out of their scope. But for some years, initiatives have emerged for these products, coming from governments (e.g. French Department of Defence [START_REF]Ministère de la Défense et des anciens Combattants[END_REF]) or international programs (e.g. European program Clean Sky [START_REF] Cleansky | Mission and objectives[END_REF]). Customers' requirements also become increasingly strict; designing more ecofriendly products is an important selling point, even in aeronautic, space and defence industries. Taking into account the product's end-of-life during its own design process is not an easy task, especially for complex and high-tech ones. In this study, we will focus on products that require long development stages, undergo high levels of constraints, with long lifecycles and low amount of produced units. Our purpose is to propose and define a new design method that would better take into account EoL issues in the design process, and based on EoL strategies adapted to aeronautic and defence products. To do this, three topics have been investigated and will be developed in this paper: the design process, the standard profile and the EoL industry. The first section will introduce the problem regarding high-tech products specificities. The second one will focus on our proposal: a method based on an EoL strategy adapted to defence and aeronautic products. Lastly, some aspects of the method will be detailed throughout a case study.

IDENTIFICATION OF THE PROBLEM

Context

The products

This article focuses on aeronautic and defence products from several classes of technologies: optics, electronics, mechanics, software, etc. Firstly, these products require long development stages, undergo high levels of constraints, are developed for long lifecycles, but count a low number of produced units. In this way, the design characteristics result from their complexity, their lifecycle duration (from the beginning of the development process to the end-of-life strictly speaking), and the strongly guided process used for their development. Moreover, these kinds of products are, most of the time, submitted to high confidential restrictions. As a consequence, companies have to track them in order to save these classified technologies. This leads to other EoL issues which differ from mass production products ( [START_REF]Commission de la Défense Nationale et des Forces armées sur la fin de vie des équipements militaires[END_REF], [START_REF] Feldhusen | End of Life strategies in the aviation industry[END_REF]). In these conditions, it is difficult to implement other design constraints without a major impact on the product design: time, cost and spent resources.

Regulations

Regulations dealing with environmental issues in defence and aeronautic industries can be classified in two categories: the ones affecting product design (REACh [START_REF]Parliament and of the Council of 18 December 2006 concerning the Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH)[END_REF], RoHS [START_REF][END_REF]) and those affecting EoL management (WEEE, Packaging and Packaging waste, Disposal of spent batteries and accumulators). These legislations introduce new design constraints to manage during the design stages in an already strongly guided process.

Specificities of the Department of Defence and Department of Aeronautics

Most of the time, defence and aerospace products are out of the regulations scope, and are not affected by the requirements previously listed, except for REACh and packaging. However, managing the product's end-of-life is a common problem which will impact further generations. In the coming years, some products may become useless or obsolete due to regulation restrictions evolution [START_REF] Lemagnen | Assessment of chemical risk during product life cycle: a new method to be used during product design[END_REF] and materials scarcity [START_REF] Dg Entr | Report on critical raw materials for the EU, Report of the Adhoc Working Group on defining critical raw materials[END_REF]. Then, even if some industries are not directly affected by these situations, they may suffer from an indirect impact. Moreover, legislations may evolve rapidly. It is not useless to be proactive on this matter. Indeed, it may be a strategic choice for a company to outpace the competition.

Highlighting the gap between requirements and aero-defence industries know-how

End-of-life is not a part of aero-defence culture. For this sector, it is a new subject because of the absence of regulations and the very few consumers' requirements in the past years. That leads to a lack of skills and knowledge to treat that kind of purpose. However, many challenges can be faced by tackling EoL issues, as material rarefaction or protection of classified technologies, but designers are not enough aware of these challenges and the complexity of these new tasks [START_REF] Lee | A framework for assessing product End-Of-Life performance: reviewing the state of the art and proposing an innovative approach using an End-of-Life Index[END_REF].

Aero-defence industries actual practices

End-of-life form

End-of-life form is a recrudescent type of deliverable demand in aerospace and defence industry. This document is created to gather all the information required for the management of the product's end-oflife. It contains data about materials, hazardous substances, painting and surface treatment, etc. It also introduces the best end of life treatment associated to each part of a product, reminding the applicable EoL regulations and the dismantling procedures. The detail level of this document is discussed during the expression of the consumer's need. For designers not trained to such purposes, it is not an easy task to find, sort and adapt relevant information coming from the definition of the product and manage such amount of information. Indeed, there is no existing tool or method to ease the edition of that kind of documents.

Guidelines

The Aircraft Fleet Recycling Association (AFRA) published in 2009 a collection of the best practices for management of used aircraft parts and assemblies [START_REF]Best Management Practice for Recycling of aircraft materials[END_REF]. But these guidelines are generic and theoretical; those make it hard to be directly applied on a product.

New issue: how to integrate end of life in design process?

State of the art: design for end of life and dismantling

According to the UNEP [START_REF] Unep | Recycling Rates of Metals: A Status Report[END_REF], the product or material end of life can be divided into two main phases. The first step is the collection (including dismantling, decontamination and cleaning), and the second is the material recycling. Several tools have already been developed in the literature [START_REF] Ziout | A holistic approach for decision on selection of end-of-life products recovery options[END_REF]; they can be classified in three categories:

-Design for dismantling or design for recycling guidelines ( [START_REF] Chiodo | Design for Disassembly Guidelines[END_REF], [START_REF]Conception en vue du démantèlement[END_REF]): mostly dealing with materials, product structure, fasteners and connections; -Indicators [START_REF] Diagon | Intégration de critères de fin de vie en conception, Cas de conception d'une éolienneé[END_REF]: it may be every quantitative or qualitative data, useful for the EoL treatment of a product; -Assessment methods ([14], [START_REF] Francia | DFD evaluation for not automated products[END_REF]): they allow the designer to measure the EoL performances of a product, as for instance cost evaluation or assessment of the disassembly and dismantability.

In order to identify the ways to assess and propose relevant paths to integrate EoL into product's design process, tools which have been identified above can be linked together. Assessment methods address particular sets of indicators to model end-of life characteristics. According to different results, combinations and / or comparisons, general guidelines can be described in order to direct as easily as possible designers choices.

Limits of the existing tools and assessment methods: basis for the proposal of a new method

These tools allow the designer to take into account EoL during the design process, and assess it according to some criteria: all these tools have to be used together to correctly treat EoL, each one at the right moment during the design of the new product. But no proper link has been described between all these tools types in order to use them the best way as possible to integrate EoL design process.

V-model in design is currently one of the most used design process type in aerospace and defence industry. This model corresponds to a succession of more and more detailed definitions of a product, all along the development progress. As represented in figure 1, the product definition at a given level is correlated to a set of validation and justification actions, like in a mirror, regarding reference documents [START_REF]RG 00041Management de Programmes-Recommandations pour la mise en oeuvre de la logique de déroulement[END_REF].

BoS Bill of Specifications TSN Technical Specification Need DF Definition File DJF Definition Justification File

Figure 1. V-Model design (translated from [20])

Our proposal, in order to match the design V-model, is to reverse the link we observed in bibliography between the three tools categories, as illustrated in figure 2.

Figure 2. EoL tools links: identification vs integration

Identify EoL indicators

Determine rules and principles Select the relevant indicators

Select the relevant assessment methods

How to identify ? How to integrate ?

We propose to start with an end-of-life strategy according to one product (or a product family). This strategy lays on specific guidelines. In order to implement the chosen guidelines into design process, specific set of relevant indicators is defined. Assessment methods are then employed in order to evaluate indicators all along the design process. Indicators are used in order to monitor EoL integration during the process in the design choices. These indicators allow checking if the final "End-of-Life" performances comply with initial strategy objectives.

We develop this proposal in the next chapter.

PROPOSAL OF A METHOD BASED ON AN END-OF-LIFE STRATEGY ADAPTED TO DEFENCE AND AERONAUTICS PRODUCTS

In this section, we will propose a new method based on an EoL strategy adapted to defence and aeronautic products in order to integrate EoL issues during the design stages.

Introduction to the method

This method must be applied upstream of the product design, during the expression of the customers' needs and the drawing up of the technical specifications. This method is composed of five steps (see also Figure 3):

1-An end-of-life strategy is defined for the product by both consumer and manufacturer. They have to choose the EoL requirements that their product must comply with (e.g. x% of recyclability, reuse of some parts, no hazardous substances, etc.).

2-The designer will refer a pair (standard profile/EoL industry). The standard profile is defined as the reference product in a product family, in order to gather all information regarding EoL, as materials or hazardous substances. Based on the standard profile and the knowledge of the end-of-life industries, the end-of-life performances of the new product can be estimated.

3-The designer checks if the EoL requirements previously defined are technically feasible, according to the end-of-life performances estimated at the previous step, and relevant regarding the needs and the evolution of the EoL treatment during the product life. This stage can be considered as a tollgate in the whole process, allowing continuing on step 4.

4-

The product design can be launched: according to the EoL strategy chosen, relevant guidelines and associated indicators will be applied during the whole product development. The following sections will investigate three topics: (i) the standard profile, (ii) the end-of-life industry and (iii) the design processes. End-of-life strategies may be correctly defined combining these three axes: it ensures that rules and indicators used for a given product are relevant for the chosen recovery options [START_REF]RG 00041Management de Programmes-Recommandations pour la mise en oeuvre de la logique de déroulement[END_REF] keeping in mind that EoL treatments will obviously evolve throughout the product's life.

Pair (Standard profile / EoL industry)

Definition of a standard profile

As most of new designed products are actually new generations of older ones, a product profile can be defined for a same product family (same functionality products composed of same classes of technologies); it will be called standard profile. This profile is the basic reference to assess EoL recoveries solutions. Based on this reference, a new generation of product can be compared, only, theoretically, to the oldest one, from the EoL point of view. The data families which constitute such standard profile have been chosen regarding EoL industry Defining a standard profile for each family of products in a company, regarding EoL applicable topics (design and EoL treatment processes) should be useful to capitalize the work done on the EoL issues, and helpful to estimate the gain linked to the design choices (comparing a new product profile to the older one).

EoL industry

Having a good visibility over EoL treatment processes can allow us to rightly answer to the needs and requirements of dismantling companies, without giving them useless information, and keeping in mind the evolutions of the dismantling processes during the product's lifecycle. In this paper, we will base on the assumption that no specific EoL industry has been developed for a product, but that companies rely on existing EoL industries, less effective than an EoL network specifically developed for a product. Some of them already exist mostly set up under the pressure of regulations:

-WEEE; -Packaging wastes; -Batteries and accumulators wastes. Moreover, some specialized EoL industries also begin to emerge due to the high value of materials or the scarcity of others. Such EoL industries can be taken into account in the study (for example composite recycling network). However, the readiness level of the existing industries can hardly be assessed for a company due to the difficulty to anticipate the short and long term evolution of the EoL treatment processes.

The pair

When matching the knowledge of the EoL industry and the standard profile data, we can ascertain what EoL strategy is really compatible with a new product development, what improvement has to be done to enhance the product's EoL performances to reach the objectives defined by the consumer. To do this, each element contained in the standard profile has to be checked regarding the existing recovery options and relevance must be evaluated.

Guidelines, indicators and assessment methods associated to an EoL strategy

According to the EoL strategy chosen, different guidelines and set of indicators will be used to design and assess the EoL performances of a product. Currently, a product must respect one EoL strategy but it may happen that the strategy can be different from one part to another, in a same product. Various design guidelines already exist for industrial products for dismantling, disassembly, or recycling. Most of them are compatible with aeronautic and defence products. The next list introduces relevant guidelines for design for recycling, taken from "Guidelines for designing for disassembly and recycling" [START_REF] Dowie | Guidelines for designing for disassembly and recycling[END_REF]:

Indicators

End-of-life strategy

Reuse

1. Materials 1.1. Minimize the number of different types of material; 1.2. Make subassemblies and inseparably connected parts from the same or a compatible material; 1.3. Mark all plastic and similar parts for ease of identification; 1.4. Use materials which can be recycled; 1.5. Hazardous parts should be clearly marked and easily removed. Moreover, most suitable design stages to implement both rules and indicators must be identified in order to maximize the efficiency of the process. Sooner the end-of-life aspects will be taken into account, better the EoL performances will be [START_REF] Sousa Ribeiro | A framework to integrate the End-of-Life Aircraft in Preliminary Design[END_REF]. But, designers must keep in mind that uncertainties will be high in early design. Then, after the application of guidelines and the measurement of indicators, the end-of-life performances of a product will be assessed through methods based on the indicators and product data. These methods are directly related to the recovery options. For instance, a company can assess technical performances (recyblability, dismantability) or economic performances as EoL costs. Meißner [START_REF] Meißner | In ElektronikschrottZerlegebetrieben können Informationssysteme den Werkern Angaben zur Steigerung der Prozesswirtschaftlichkeit machen[END_REF] proposed a model to assess these cost according the following equation:

Fasteners and connections

EoL costs = Dismantling costs + Disposal costs -Reuse and Recycling Profits

But, due to variability of the material courtyard and difficulty to quantify workload, it will be most relevant to assess these costs for example in number of hours, which present less fluctuation and can be multiplied by an updated cost factor.

CASE STUDY: BINOCULARS PRODUCT FAMILY

Product presentation

The chosen product for case study is a medium-range infrared multifunction military binocular. These compact and light binoculars are based on uncooled infrared technology, with all the functions essential to combat operations in a single housing: thermal vision, telemeter and magnetic compass. This product has been chosen to test the proposed methodology because of its complexity and the diversity of technologies it owns: optical, electronic, and mechanical; and also because of the high number of produced units.

Figure 4. Binocular

We can assume that the binocular is representative of the binocular family in term of:

performances (medium range); technologies; materials.

Standard profile: required data

Establishing the standard profile of a product requires some knowledge about the type of technologies, the product mass and materials repartition, hazardous substances and strategic materials presence and traceability.

As explained in the section 3.2.1, the standard profile presents an inventory of structural materials, components and fasteners of the product. This profile has been established Table 2, mainly regarding data taken from definition and maintenance tree-views.

Pair Construction

Table 2 introduces the pair (standard profile, EoL industry). Most suitable options for the whole product or for each of its components are determined when .

Table 2. Pair (Standard profile, EoL industry)

matching the existing recovery option with each data of the standard profile Thus, the result will be extended for the entire binocular family.

Proposal of an EoL strategy compatible with the binoculars family

For every elements of the standard profile, designers must check what the possible EoL strategies are, regarding the existing treatments and recovery options. First it is preferred to select a strategy highly valuing the product or parts of it, then the materials and finally the energy.

In this example, according to pair examination, recycling the whole product or reusing some parts seem to be the best EoL strategies to choose. In a theoretical calcul, assuming that first the recyclability of each part of the product compatible with operations is about 100%, and secondly the efficiency of the full recycling process is at 100%, the recyclability rate reaches a theoretical level of 86% according to the pair. This result represents the amount of product recyclable (sum of the mass percentage of the parts compatible with this end-of-life strategy). A more realistic evaluation could reach an objective of 50%, due to the lack of compatibility of the product with the existing recycling process or the emerging recycling solution for the very specific material of such products.At the components scale, it may be more interesting to reuse or remanufacture some high-value components, like lens. Moreover, that kind of components is rarely subject to wear or soiling, thus reuse seems to be the best strategy.

As described on table 1, specific set of indicators could be chosen by the designers in order to follow the good integration of the chosen strategy. According to the table, two indicators should be monitored in both ways: disassembility and dismantability. This approach enables to describe, between manufacturer and customer, some kind of relevant objectives for binoculars EoL, and determine which metrics, linked to indicator monitoring, would be relevant to assess product EoL performance. to be created between companies and end-of-life actors in order to (i) enhance the recovery processes and (ii) harmonize the work done earlier by designers and the recovery requirements given by the EoL actors.

CONCLUSION AND DISCUSSION

Ideally, in order to maximize the product recovery rate, a product and its own EoL industry have to be designed together.

In this paper, we focus on the product development.

However, the method should also be adapted and applied to the technology development, based on the Technology Readiness Level (TRL). Further investigations are planned in order to improve the whole cycle, from strategy definition to indicators identification and integration in design process. TRL links will also be an important field of inquiries.

Figure 3 .

 3 Figure 3. Method principle

Table 1 .

 1 EoL strategies and associated indicatorsTable 1 lists pertinent EoL indicators regarding the main EoL strategies.

	Remanu-	facturing	Recycling	Heat	recovery	Disposal
	Disassembility
	Dismantability
	Adaptability
	Reliability
	Traceability
	Fatigue
	Deterioration
	Structure
	Separability
	Composition
	Contamination
	Economic viability
	Risk
	Efficiency
	Material evolution

  Our proposal method is a new way to integrate endof-life in the design process, taking into account what the company is able to do and what the requirements of the EoL industries are, in order to propose a relevant products's EoL management. However, having a good visibility on the EoL industries is the hardest part, due to the lack of communication between these two actors and the permanent changes in the EoL industries. Hence, a company must be an actor of the EoL treatment of its products. Even if a company integrates design for end-of-life and manufactures wholly recyclable products, this does not mean that products will effectively be recycled. A synergy has

	Product composition	Quantity (Mass %)	Hazardous Substances (e.g. annex XIV REACh regulation)	Strategic materials [13]	Reuse	Remanufac	turing	Recycling	Energy	recover	Disposal
	Structural Materials:	20%
	-	Magnesium	16%	-	Mg				x		x
	-	Aluminum	3%	-	-				x		x
	-	Plastic	1%	-	-				x	x	x
	Functional composants:	76%
	1. Electronic Components
		-	DEEE RoHS	50%	-	-				x		x
		-	DEEE not RoHS	5%	Cr VI, Pb	-				x		x
		-	Wires	10%	-	-				x		x
	2. Optical components
		-	Lens	1%	-	Ge	x	x		x		x
		-	Restitution (LCD)	3%	-	-				x		x
	3. Others
		-	Strap	1%	-	-				x	x	x
		-	Battery	10%	-	-						x
	Fasteners and connections :	4%
		-	Screw	3%	-	-						x
		-	Glue	/	-	-						x
		-	Clips	0.5%	-	-						x
		-	Seal	0.5%	-	-						x