Olivier De Almeida
email: olivier.dealmeida@mines-albi.fr

Aurélien Mazzoni

Assessment of induction heated mould technologies for high-temperature thermoplastic composite manufacturing

Keywords: Thermoplastic composites, thermo-compression moulding, induction heating, hot press

The performances of Cage System ® and 3iTech ® moulds designed by Roctool for the thermocompression moulding of high temperature composites have been investigated and compared to a conventional hot press equipped with cartridge heaters. The Cage System ® technology demonstrated its capacity to achieve rapid temperature cycles with heating and cooling rates higher than 200°C.min -1 , while allowing significant electricity energy saving. However, this heating technology induces critical thermal deformation of the moulding surfaces at high temperature. Advanced design is thus required for obtaining high thermal homogeneity and for satisfying dimensional accuracy of structural composites. As for the 3iTech ® technology, the studied mould exhibited higher thermal performances than a conventional press with a heating rate up to 40°C.min -1 and a temperature homogeneity on the moulding surface lower than 10°C. Moreover, the carbon/PEEK laminate fabricated with this mould satisfied the dimensional tolerance for aerospace applications and therefore represent an interesting alternative to conventional heating systems.

Introduction

Hot press compression moulding has been used for many decades for the fabrication of thermoset and thermoplastic composites parts, in particular for Bulk Moulding Compound (BMC) or Sheet Moulding Compound (SMC) [START_REF] Park | Compression molding in polymer matrix composites[END_REF][START_REF] Rosato | Reinforced Plastics Handbook[END_REF]. This composite manufacturing route is actually an attractive solution for highvolume production, as it allows obtaining good surface finish, gives flexibility in part design, reduces raw material wastage, and satisfies dimensional accuracy. Moreover, although the initial capital investment is relatively high, thermo-compression moulding presses have interesting cost/performance characteristics, as they require low maintenance.

Hot press compression moulding is also the most common way of manufacturing long fibre reinforced thermoplastic composites. Indeed, due to the high viscosity of thermoplastic resins, bulk consolidation of thermoplastic composites requires the application of both high temperature and high pressure and hot presses are efficient for enabling the resin to flow into and around fibre bundles [START_REF] Hoa | Principles of the manufacturing of composite materials[END_REF][START_REF] Kim | Effect of pressure on the impregnation of thermoplastic resin into a unidirectional fibre bundle[END_REF]. And although many efforts have been devoted to the development of complex semi-finished products to enhance the availability of the resin to the fibre surface [START_REF] Ye | Processing of thermoplastic composites from powder/sheath-fibre bundles[END_REF][START_REF] Bourban | Material phenomena controlling rapid processing of thermoplastic composites[END_REF], autoclave and fibre tape placement processes are still limited to the consolidation of prepreg tapes: tapes are already well impregnated and consolidation then mainly consists in enhancing the intimate contact between the plies [START_REF] Hoa | Principles of the manufacturing of composite materials[END_REF][START_REF] Phillips | Prediction of the consolidation of woven fibre-reinforced thermoplastic composites. Part I. Isothermal case[END_REF][START_REF] Li | Modeling the consolidation process of thermoplastic composites[END_REF].

Whether a hot press is used as a stamping unit or for the direct consolidation of thermoplastic semifinished products, heat of press platens is generally supplied by integrated heating systems working on the principle of resistive heating according to Joule's first law. Steam, oil or hot water systems are actually limited to applications where severe heating and cooling cycles are not experienced [START_REF] Knights | Rapid mold heating, cooling technology on cutting edge[END_REF], and resistive cartridge elements regularly distributed in the platens are thus preferred for thermoplastic composites manufacturing.

The main advantage of electric cartridges among the various heating solutions is the possibility to control the temperature of the platens by a multi-zones regulation. It consists in dividing the platens into different areas containing one or several heating devices. The heaters of each zone are individually switched on or off by a temperature controller after comparing the pre-set target temperature with the actual temperature recorded by a temperature sensor in the zone. Resistive heating is thus an efficient technology when temperature uniformity is critical. However, this technology does not allow achieving a higher heating rate than 10°C per minute, which represents a strong restriction of the production rate of long fibrereinforced thermoplastic composites for which large temperature amplitude is required during the manufacturing cycle.

This drawback of conventional hot presses is one of the main factors that explain the slow growth of thermoplastic composites applications in the aerospace or automotive industry. And consequently, the development of several innovative out-of-autoclave processes has been encouraged in the last decade with aim of increasing heating and cooling rates [START_REF] Knights | Rapid mold heating, cooling technology on cutting edge[END_REF][START_REF] Haro | Rapid Out-of-Autoclave Composite Manufacturing for Aerospace-Grade Prepregs[END_REF][START_REF] Staff | Rapid Temperature Cycling[END_REF]. Among them, the two inductive moulding systems developed by Roctool, namely the Cage System ® (CS) and 3iTech ® (3iT) technologies, have already proved their efficiency with engineering matrices (PP, PA) [START_REF] Feigenblum | Cost effective and advanced fiber reinforced thermoplastic processing for performance applications[END_REF][START_REF] Feigenblum | Device for transforming materials using induction heating[END_REF][START_REF] Guichard | Induction heating device and method for making parts using same[END_REF]. However, even if these technologies are attractive solutions for the reduction of the total processing time, their efficiencies for high performances thermoplastic composites manufacturing have never been demonstrated.

The aim of this study was therefore to analyse the capabilities and limitations of two induction-heated moulds dedicated to the consolidation of high-temperature thermoplastic laminates. The CS and 3iT moulds have been designed respectively in 2009 and 2012 for high temperature processing, and this study then represent an inventory of these technologies that must be balanced as regards to Roctool's know-how at this time.

For matter of comparison, the performances of a conventional hot press are first analysed by means of the process data recorded during the consolidation of carbon/PEEK prepregs. The CS and 3iT moulds used in this study are then described, with a particular focus on heating and cooling performances, temperature uniformity and dimensional accuracy. The performances of both systems for the fast consolidation of carbon/PEEK composites are finally discussed.

Conventional hot press

Technology

The conventional hot press used as reference of comparison in this study is a fully automatic hydraulic lab press specially designed by Pinette Emidecau Industries (PEI) for high temperature composite manufacturing. The press is equipped with cartridge-heated platens of 700x700mm 2 that allow applying a process temperature up to 450°C, and has a maximum clamping force of 750kN.

All electric cartridges represent a total heating power of 65kW which allows heating the platens with a maximum heating rate of 10°C.min -1 according to the technical data sheet. A rough estimation of electricity consumption was calculated considering that 80% of the power of all cartridges is required during a heating ramp at maximum rate. Reaching a platen temperature of 400°C, i.e. typical processing temperature for Carbon/PEEK composites, was estimated to 40kWh of electric power.

As for cooling, it is achieved by pumping pressurized air and water through parallel cooling channels drilled in press platens, and the temperature control is assured by the simultaneous regulation of electric cartridges. The configuration of the press platens enables controlled cooling rate of -15°C.min -1 above 300°C and -10°C.min -1 between 300°C and 180°C.

For thermal regulation purpose, each platen is divided into 9 zones individually controlled by a digital temperature regulator. Platens temperature is monitored by thermocouples located in the centre of each zone. As mentioned in the technical data sheet of the press, the temperature homogeneity has been measured with temperature sensors regularly distributed at the surface of the platens, 50mm far from the borders of the heating platens. The measurements showed that temperature uncertainty was +/-5°C at 400°C and +/-2.5°C at 200°C after 15min of stabilisation. In order to assess the actual hot press performances, the temperature recorded in each zone during the consolidation of a 400x400mm 2 carbon/PEEK laminate was analysed. The composite was made with 40 plies of Cytek AS4/APC-2 prepreg using a picture frame ([90/+45/0/-45] 5S stacking sequence). A conventional processing cycle was used for the fabrication of the laminate, i.e. a heating rate of 5°C.min -1 under a pressure of 1bar, 35min isotherm at 395°C under a pressure of 7bars and a cooling stage at 4°C.min -1 . The actual processing cycle applied during consolidation is shown in Figure 1. The consolidation pressure is calculated from the applied force and the displayed temperature corresponds to the average temperature of all zones of top and bottom platens.

Material and processing conditions

Thermal homogeneity

Figure 2 displays the temperature in each zone of the bottom press platen and the maximum temperature amplitude recorded during the entire processing cycle. The recorded data clearly demonstrate the capability of resistive cartridges for controlling temperature homogeneity when regulation only deals with heating: the ramp up is perfectly linear and the temperature gradient is less than 2°C until the end of isotherm. Inertia of resistive heating is however observable when changing from the heating step to the isotherm one, although it has no influence on temperature amplitude of the moulding surfaces. Contrary to heating, the cooling stage is highly irregular. The overall cooling stage actually complies with the set cooling rate, but the temperature decrease is characterized by regularly spaced temperature fluctuations that progressively disappear below 200°C. These fluctuations are the consequence of a too strong heat transfer at high temperature between the mould and the coolant fluid in cooling channels. When temperature is too low below the set temperature, the temperature controllers switch the heaters on in order to counterbalance the rapid decrease of temperature. These fluctuations appear periodically every 5 minutes during the cooling stage, which surely depend on the cooling rate setting, the design and the inertia of the cooling system as well as on the PID parameters of the temperature controllers.

The temperature oscillations during the cooling phase also induce a large temperature disparity in the press platens. The temperature amplitude is 50°C at the beginning of cooling (395°C) and slowly decreases to 10°C at demoulding temperature.

The fluctuations of temperature during cooling make difficult the control of solidification mechanisms in thermoplastic matrix composites, in particular when semi-crystalline polymers are involved. Prediction of process-induced residual stresses or degree of crystallinity is therefore complex and assessment of in-service performances of thermoplastic composites then requires composite properties to be experimentally identified after manufacturing. This is particularly true for polymers which crystallisation kinetic is highly sensitive to cooling conditions.

Dimensional stability

Likewise thermal homogeneity ensures a homogeneous molten state during consolidation, the dimensional stability of the mould is a critical parameter in hot press manufacturing as it governs the pressure homogeneity during impregnation and the dimensional accuracy of composite parts for later assembly operations.

Planarity and smoothness of hot platens usually amount to less than +/-0.1mm, and when settled in hot presses, the parallelism is generally ensured to be lower than +/-0.1mm. In the present case, because of the reasonable platen size, the press specifications are even better with a planarity and a parallelism lower than +/-0.05mm. The geometrical accuracy of press platens at room temperature is however not the key factor for high quality composite manufacturing.

The dimensional accuracy of composite parts actually strongly depends on the mould deformation during matrix solidification, i.e. during matrix crystallization and/or at glass transition temperature. This deformation results on one hand from the thermal expansion of hot parts and on the other hand from their deformation under the stroke force. Measurement of mould dimensions during a thermal cycle is anyway challenging at high temperature and moreover, it would not give direct indications about the quality of composite parts. For this reasons, it was decided to use the thickness of the processed carbon/PEEK composite as an indicator of performance of cartridge heating technology on dimensional tolerances.

Measurements performed with a micrometre in many locations of the flat laminate show that the thickness of the 400x400mm 2 carbon/PEEK plate is 5.1mm +/-0.1mm, with randomly distributed thickness variations. This dimensional stability complies with standards of aerospace industry for structure applications and ensures obtaining high mechanical performances when optimal impregnation of fibre bundles is achieved. The Figure 3, that shows a representative micrograph of the as manufactured laminate, also demonstrates the high accuracy of hot platens: after consolidation, the initial pre-impregnated tapes remained perfectly flat and inter-ply planes are parallel.

Induction heated moulds

The performances of Roctool's CS and 3iT technologies for the manufacturing of high-performance thermoplastic composites were assessed by analysing the in-service behaviour of two moulds dedicated to the fabrication of flat composite laminates. Both moulds are comparable as they both have a moulding surface of 400x400mm 2 and were used in the same environment (press, induction generator and cooling system). As shown in Figure 4, the CS mould used in this comparative study is composed of two 500x500x80mm 3 metallic blocks made of high permeability steel, and a 7-turn inductor coil positioned in two halves outside of the mould and shielded by the mould itself. When the mould is closed, both parts of the coil are electrically connected and form a solenoid. Then, when the induction generator supplies the highfrequency electrical power, the magnetic field induces alternative Eddy currents at the surface of the tool and generates the rapid heating of mould surface by Joule effect. In the present case, the inductive configuration was designed for moulding composite laminates of 400x400mm 2 at a temperature up to 450°C.

Cage System ® induction heated mould

Because of the induced electric currents, risk of short-circuit between the two moulding surfaces exists. A ceramic layer had been coated on the mould surfaces, that makes it possible to process carbonreinforced composites. However, this technology prevents the use of highly electrically conductive materials that would create a conductive contact between the two faces, like additional picture frames, inserts or stop blocks/shims made of metal. As the moulding parts are free from any heating device, two rows of 10mm diameter cooling channels could be drilled through the steel blocks (Figure 5). The channels close to the moulding surface (C in Figure 5) correspond to the cooling channels. These 2x10 channels are used when fast cooling is desired and must be purged before heating. To the contrary, on the opposite side of the mould blocks, 2x8 channels, called regulation channels (R in Figure 5), are used during heating to create a thermal barrier and protect the press from the heat of the mould.

The simultaneous water flow in regulation channels and induction heating at moulding surface create a large thermal gradient through mould thickness (Figure 5). Therefore, although the efficiency of induction heating is 80-90%, heat exchange in regulation channels decreases the energy efficiency of this technology. Nevertheless, the regulation channels can be used for monitoring relatively slow cooling rates and yet allow achieving complex temperature cycles for dynamic control of processing conditions (for instance heatingcooling-heating cycles).

Mould with 3iTech ® heating technology

Contrary to CS technology that directly heats the moulding surface, in 3iT systems, the induction heating is produced in inductive channels machined in the mould block, by means of a continuous inductor placed in this inductor network. The 3iT heating system then preferably heats the moulding surface by a conductive effect. With this technology, the risk of an electrical short-circuit between different parts of the tool is totally avoided.

The 3iT induction heated mould used in this study is shown in Figure 6. It features a moulding surface of 400x400mm 2 with an integrated picture frame. The inductor used is a copper braid of about 10mm diameter insulated by two insulating braids (inserted picture in Figure 6) that fit with the diameter of the inductive channels in order to optimize the inductive output. As shown in Figure 7, the inductive network (I in Figure 7) is formed of a single raw of 12 regularly spaced parallel inductive channels drilled at half height of each mould block. As for the cooling network, it is formed of two rows of 10mm diameter channels on either side of the inductive channels and machined perpendicular to the latter (C in Figure 7).

Thermal regulation of the mould bases is also required in 3iT technology to prevent damaging of the press. A row of 8 regulation channels machined in each mould base must be feed with a heat fluid during induction heating (R in Figure 7). The main difference with CS technology is that 3iT heats the entire moulding blocks (Figure 7), and heat loss is therefore a critical issue when designing 3iT moulds. Thermal bridges between the mould and other surrounding parts, in particular the mould bases, are thus reduced to a minimum and all non-moulding surfaces are covered with insulating material to limit convective heat loss. In return, the efficient insulation of the mould requires the use of the cooling system to decrease the temperature of the moulding surface, whatever the expected cooling rate. Consequently, as the cooling channels must be purged before heating to prevent steam pressure issues, it limits the possibilities of applying complex thermal cycles including heating or isotherm steps after cooling phases.

Pilot plant description

Contrary to conventional hot press, induction-heating technologies are build-in solutions in which the heating technology and the cooling system are included in the tool. The thermo-compression unit used in this study to assess the performances of CS and 3iT moulds was thus build around the moulds (Figure 8).

The moulds were installed in an instrumented vertical hydraulic compression press of a capacity of 1000kN. The press used is special in that the applied force is not controlled by the hydraulic pressure, but with a load sensor located in the nose of the press actuator like in tensile test machines. Moreover, press speed and position are monitored thanks to a LVDT sensor, which gives additional information to analyse the processing behaviour.

The heating system, regardless the inductor in the mould, consists of a 200kW medium-frequency induction generator and its capacity matching box. The power of the induction generator was selected according to the size of the moulding area (400x400mm 2 ) and the expected heating rate, although this latter also depends on the inductive technology used. Heating and temperature regulation are performed with a temperature controller that collect the mould temperature from a thermocouple sensor in the mould. In each mould, the sensor is located a few millimetres below the moulding surface in the middle of the bottom part of the mould.

A water-cooling unit was specifically designed for achieving fast cooling cycles whatever the processing temperature: the cooling unit can deliver regulated cold water at a flow rate of 150 l.min -1 at anytime of a cycle. This cooling capacity is obtained by using a 1m 3 buffer tank between the factory and the mould cooling systems and a heat exchanger of 200kW capacity. Moreover, for the purpose of cooling monitoring, distribution feeders with solenoid valves control the flow rate in the cooling channels and allow switching the water circulation from regulation channels to the cooling network.

Finally, all the equipment is controlled from an operating panel that coordinates the actions and centralizes the data from all plant, in particular mould temperatures, platen displacement and applied press load.

Capabilities and Limitations of induction technologies Heating and cooling performances

The maximum heating and cooling performances of CS mould are depicted in Figure 9. They were obtained by continuously supplying the 200kW electrical power to the inductor in the heating phase and by using the maximum water flow during cooling. As shown in Figure 9, a processing temperature of 400°C can be reached within 3 minutes with a heating rate decreasing progressively from 270°C.min -1 to 85°C.min -1 . The 2min30s at maximum power that are required to heat the mould up to 400°C represent an electricity consumption of about 9kWh, which is far less than the consumption of a conventional hot press.

The maximum water flow rate allows applying cooling rates high enough to quench a thermoplastic matrix. For instance, a PEEK matrix processed at 400°C can be cooled below the glass transition in less than two minutes. Moreover, in CS technology, the thermal heat is concentrated in the skin of the mould and the thermal inertia of CS moulds is therefore particularly low. This low inertia is clearly observable by the absence of overshoot at the end of heating and by the prompt drop of mould temperature at the end of isotherm.

The performances of the pilot plant with the 3iT mould are shown in Figure 10. During the heating phase, as this technology requires the entire moulding blocks to be heated, the temperature rate is drastically decreased compared to CS. The 200kW generator output only allows a heating rate of 40°C.min -1 , which is nevertheless four times faster than with a conventional hot press. Also, the 9min30s of heating here represent an electricity consumption of about 32kWh, which is comparable to the consumption of a conventional hot press.

The Figure 10 also shows that cooling of 3iT mould can be highly efficient with a rate of 80°C.min -1 despite the high quantity of energy that the hot moulding blocks represent. This performance is significantly higher than what can be applied with a conventional press, although it is lower than with CS technology. However, contrary to the CS mould, the cooling rate was not obtained with the total capacity of the cooling unit but with a water flow rate of 20L.min -1 , i.e. less than 15% of the unit capacity. Faster cooling cycles can be achieved but it was limited in this study to preserve the cooling system: the cooling performance results from the highly energetic transformation of liquid water into superheated steam, and thermal fatigue and pressure issues may be critical for higher flow rates.

Thermal homogeneity

As there is only one and unique heating system for the entire moulding surface, heating and cooling rate performances of inductive technologies must be analysed considering the thermal homogeneity of the moulding surface. In order to quantify the thermal homogeneity of the CS mould, 27 thermocouples were placed at the surface of the bottom part of the mould. They were positioned in three parallel lines of nine thermocouples perpendicular to the cooling channels direction so as to characterize a representative moulding area of 300x360mm 2 (width x depth). The temperature data were recorded using a Pico USB TC-08 Thermocouple Data Logger during a thermal cycle consisting first in a heating phase at 50°C.min -1 until 400°C interrupted by 2min isotherm every 100°C and then in a cooling phase at maximum rate.

The mean temperature and maximum temperature difference recorded during the test are displayed in Figure 11. The results show that the temperature heterogeneity is all the more large that the mould temperature is high: the temperature difference increases from 10 to 60°C when the mould temperature is heated from 100°C to 400°C. These temperature gradients however also depend on the heating conditions as switching from heating ramps to isotherms favours homogeneity.

The Figure 12 gives details of the temperature gradients in the moulding area during representative phases of the thermal cycle: at the end of heating when the generator supplies high electric power, and during the cooling phase. In both conditions, the temperature gradients exhibit a thermal imbalance between the centre of the moulding surface and the front and back edges of the mould. The cold area close to the edges is due to the systematic water circulation in either the cooling or regulation channels that are close to the surface in this zone (Figure 5). The temperature field at the end of heating (Figure 12a) also exhibits a thermal imbalance between the left and right parts of the mould caused by the solenoid dissymmetry. During cooling (Figure 12b), although the front and back gradient remains critical, the lateral dissymmetry is largely decreased. Black dots illustrate the thermocouples location during the test.

A similar study was achieved on the 3iT mould with 21 thermocouples homogeneously distributed on the moulding surface 15mm far from the edges. The temperature recorded during a conventional processing cycle at 250°C is shown in Figure 13.

During the heating phase, the temperature gradient in the mould is less than 10°C and remains lower than 10°C during the temperature stabilization. This thermal homogeneity is obtained thanks to preferentially conductive heating from inductive channels to the moulding surface. This gradient increases during the cooling phase especially over the first minutes with a maximum of 25°C. This heterogeneity is due to the temperature difference between cold inlets and hot outlets of the cooling system, but no specific temperature pattern could be identified.

Dimensional Stability

As for the hot press, the dimensional performances of the CS and 3iT moulds for the fabrication of high-temperature thermoplastic composites were assessed by means of composite panels fabricated with the moulds. Carbon/PEEK laminates were thus manufactured with both technologies using similar processing conditions. The laminates were made with 6 plies of 2-faces powdered carbon fabric provided by Porcher Composites (Pi-Preg®). The material used was a 3k carbon 5H Satin fabric of 285g.m -2 areal weight covered with 200g.m -2 of PEEK powder, which corresponds to a fibre volume ratio of 50% in the prepreg.

Figure 14 shows the processing conditions used for the consolidation of a 300x300mm 2 laminate in the CS mould. The consolidation was performed under a pressure of 10 bars during 10min at 395°C, and the heating and cooling rates were respectively 50°C.min -1 and 20°C.min -1 . Moreover, neither stop block nor picture frame was used for the fabrication of the laminate.

The press platen position was recorded during the test, which allows following the consolidation kinetic afterwards. As shown in Figure 14, the platen position increases during the heating step because of the thermal expansion of both the mould and the material: in order to keep a constant force during heating, the press platen moved up until the PEEK matrix is molten and becomes viscous. Then, the application of the consolidation pressure startes the impregnation process: the prepreg stack thickness decreases and stabilises when consolidation is globally achieved. After cooling, the final position of press platen is lower than the position at the beginning of the trial. The position difference corresponds to the total reduction of the stack thickness. After manufacturing, the laminate thickness was measured with a micrometre and the microstructure of the composite was investigated in many locations (Figure 15). The thickness measurements show that the mean laminate thickness is 1.8mm, which is in good agreement with the nominal thickness of the prepreg ply that is 0.3mm. However, the laminate exhibits a critical gradient with a minimum thickness in the middle of the laminate and a maximum thickness in the plate corners. The final variability is +/-0.3mm, which is far too much according to the aeronautical specifications for structural composite parts.

The investigation of the resulting composite microstructure also leads to the distinction of three different concentric areas. In the middle of the laminate (3), macro-porosities (0.5mm) can be observed between the carbon yarns while the fibre bundles are well impregnated. Between 50mm and 100mm from the laminate centre (2), the composite is fully impregnated and only a few micro-porosities remain visible around the carbon filaments. And finally, outside this central area of 200mm diameter (1), the microstructure is characterized by poor impregnation: the core of fibre bundles lacks of impregnation while all the space between the yarns is filled with PEEK matrix.

The apparition of these porosities and thickness gradients is the consequence of differential thermal deformation of the mould during the consolidation. Indeed, the large thermal gradient through the mould thickness causes a convex curvature of both moulding blocks, and therefore induces an increasing gap distance from the middle to the edges of the mould.

As a consequence, the pressure distribution during manufacturing is heterogeneous on the stack and the periphery lacks of pressure for achieving complete impregnation. The resulting microstructure close to the edges (3) is thus similar to what has already been observed in film stacking configurations when a low pressure is applied [START_REF] Loendersloot | Textile impregnation with thermoplastic resin -models and application[END_REF]. In the centre of the laminate, the apparition of large porosities may result from a different mechanism. At the end of isotherm, thanks to the high pressure applied in this area, consolidation is likely totally achieved and matrix even squeezed towards the periphery. The macro-porosities are thus probably created during the cooling step, as a result of the mould contraction: the loss of mould curvature during cooling may lead to the composite deconsolidation before the matrix solidification, which has been observed as a source of macro-porosities in rich matrix zones [START_REF] Ye | Thermal de-consolidation of thermoplastic matrix composites -I. Growth of voids[END_REF][START_REF] Lu | Thermal de-consolidation of thermoplastic matrix composites -II. "migration" of voids and "re-consolidation[END_REF]. As shown in Figure 16, similar processing conditions were applied in the 3iT mould for the consolidation of a 6 plies stack of 300x300mm 2 carbon/PEEK prepreg: 10min at 390°C under 12bars. Both the heating rate above the melting temperature and cooling rate until PEEK glass transition at 143°C were approximately 40°C.min -1 . After processing, the laminate was analysed in term of microstructure and thickness homogeneity, like the one fabricated with the CS technology. Like for the CS laminate, the composite microstructure was investigated in different locations in the panel. And contrary to the laminate made in the CS mould, the microstructure of the composite panel fabricated in the 3iT mould is identical whatever the sample (Figure 17): the fibre bundles are fully impregnated and only few micro-porosities can be observed around carbon filaments. Thickness measurements with a micrometre lead for the 3iT laminate to a constant thickness of 1.8mm, which agrees with the nominal ply thickness. The micrometre technique is however deficient for identifying an effect of the inductive technology and ultrasound time of flight method was thus used to characterise the thickness distribution over the entire surface of the 300x300mm 2 panel (Figure 18). This sensitive measurement technique confirms the mean laminate thickness, but it also highlights a specific pattern of thickness distribution in the laminate.

The laminate part consolidated in the rear side of the mould indeed exhibits a lower thickness than the rest of the laminate. The thickness variability is however only +/-0.1mm and confirms the consistency of impregnation in the laminate as well as it satisfies aeronautical specifications.

Like in the CS mould, the slight variation of laminate thickness is likely caused by a different thermal expansion in the rear part of the 3iT mould. The dissymmetry of the inductor network in this area is then probably the cause of a locally different thermal deformation in the mould at high temperature. Part of the inductor network indeed follows the back side of the mould until the outlet box protecting the electric connectors.

Discussion

Conventional hot press technology is a reference process for the fabrication of high temperature thermoplastic composite parts as it complies with quality standards of the aerospace industry in terms of dimensional accuracy and thermal homogeneity. The main drawback of this process is however the thermal inertia of the resistive heating platens. Indeed, although cartridge heaters allow achieving homogeneous heating thanks to individual zoning regulation, platens heating rates are limited to 10°C.min -1 . Moreover, the simultaneous use of the air-water cooling system with these heating devices induces imperfect control of cooling rates that may be critical for the prediction of final composite properties.

The comparison with inductive technologies for high temperature composite manufacturing shows that induction heating is an attractive solution for the reduction of the processing time. Both CS and 3iT technologies allow applying high heating rates, and as the heating phase can represent a large part of the total processing time, these processes offer new possibilities for the optimization of thermoplastic composite manufacturing as well as thermoset composite curing. Moreover, inductive technologies allow significant reduction of electricity consumption, in particular the CS system. The use of induction heating is therefore an interesting way for reducing the manufacturing cost of a composite part.

The use of heating rates higher than 200°C.min -1 nevertheless requires particular attention to be paid to the through-the-thickness thermal homogeneity of the processed material. While conventional processes allow considering the material as thermally homogeneous during manufacturing, this is no longer the case in a thick composite stack of low thermal conductivity.

Although inductive technologies demonstrated their efficiency for achieving fast heating, they exhibit some critical drawbacks inherent to the use of induction heating. Induction process generates localized heating and in the case of CS technology, non-uniform thermal expansion that may cause unacceptable defects in the processed composites is induced. Moreover, one unique heating system is used for the thermal regulation of the CS and 3iT moulds and the thermal behaviour then strongly depends on the initial design of the inductive network whether it is a solenoid or an inductive copper braid.

Because 3iT technology consists in heating the entire moulding blocks, this induction heating system provides an interesting compromise of heating rate and dimensional accuracy. Its performances make it the most versatile mould for composite manufacturing among the three studied technologies, and hence represent an interesting solution for reducing the processing time without decreasing the quality of composite parts.

The thermal inertia of 3iT system and the insulation of all non-moulding surfaces is the main limitation of this moulding technology. The possibility to achieve complex temperature cycles combining heating and cooling steps is limited, and complicates the control of low cooling rates. The simultaneous use of the heating and cooling systems should be avoided in 3iT technology as it would lead to temperature oscillations like in hot press, and would degrade the benefit of induction heating. In order to improve cooling control, one option consists in adjusting the cooling system to the desired processing condition. This can be achieved by modifying the cooling network of the 3iT mould: the fast coupling used for the connexion of the cooling system allow reducing the number of cooling channels or reorganising heat fluid circulation easily, which is usually impossible with the rigid cooling network of conventional hot platens.

Contrary to 3iT and hot press, only the extreme surface of the mould is heated with the CS technology and a fast cooling can therefore be promptly interrupted by an isotherm stage. The CS capability of applying complex cooling paths then offers new options for the optimization of processing cycles. For instance, during the fabrication of thermoplastic composites, the degree of crystallinity of a thermoplastic matrix may be improved by means of an isothermal crystallisation stage while reducing the overall cooling time using a high cooling rate.

Of course, these performances must be balanced as the studied CS and 3iT moulds were respectively designed in 2009 and 2012. Since then, Roctool's know-how has evolved and the better understanding of induction heating as well as composite behaviour (thermal expansion, shrinkage) may have led to more advanced mould design with better homogeneity. In particular, nowadays CS technology can be likely optimized for a specific application, i.e. a specific matrix and a specific part geometry, which would make it a cost effective solution for mass production where the cycle time is crucial. As for the 3iT technology, solutions exist for running multi zone heating. This option is likely to counterbalance moulds dissymmetry for obtaining a homogenous moulding surface temperature.

At last, although the optimisation of the induction network is crucial and may affect the cost of an inductive tool, the final cost of inductive moulds is substantially identical to the cost of a conventional tooling with cartridge heaters, the cost of the inductor being very lower than the cost of cartridge heaters. Therefore, the selection of induction heating technology must be made after analysing the cost-effectiveness of the technology, taking into account the cost of the inductive technology itself, its maintenance, the return on investment in the generator, and the benefit of the expected processing time.

Conclusions

The thermal characterization of the induction technologies designed by Roctool show that induction heating is a promising solution for saving time during the consolidation of high temperature composites, as they allow applying high heating and cooling rates up to 280°C.min -1 , while requiring less electrical energy for heating.

Among the two studied technologies, the studied Cage System ® mould however exhibits critical drawbacks for high-performance thermoplastic composites manufacturing, which main one is due to differential thermal expansion of the moulding surfaces that cause defects in the processed composite parts. Nevertheless, as Roctool's know-how has evolved since the Cage System ® mould was designed; part of its limitations may have been solved. This technology thus represents a particularly interesting solution for mass production.

On the other hand, although heating performances of the 3iTech ® technology are lower, this heating system allows obtaining high thermal homogeneity and dimensional accuracy while improving the heating capability compared to a conventional hot press with cartridge heaters. Therefore this is an interesting versatile technology for composite manufacturing.

Figure 1 -

 1 Figure 1 -Processing cycle used for the consolidation of AS4/APC2 prepreg in conventional hot press

Figure 2 -

 2 Figure 2 -Temperature recorded in each of the 9 zones of the bottom part of the moulding surface by the thermocouples sensors used for thermal regulation of press platens, and maximum temperature difference at the surface of the moulding surface of a hot press during consolidation of a C/PEEK laminate.

Figure 3 -

 3 Figure 3 -Microstructure of C/PEEK composite manufactured from prepreg tapes with a conventional hot press

Figure 4 -

 4 Figure 4 -Cage System ® induction heated mould

Figure 5 -

 5 Figure 5 -Simulated temperature distribution through mould thickness after 170s of induction heating at maximum generator power (200kW) and 60s of temperature stabilization. C and R refer respectively to cooling channels and regulation channels.

Figure 6 -Figure 7 -

 67 Figure 6 -3iTech ® induction heated die and copper braided wire inductor

Figure 8 -

 8 Figure 8 -Thermo-compression pilot plant with induction-heated mould

Figure 9 -Figure 10 -

 910 Figure 9 -Maximum heating and cooling rates allowed by the Cage System ® technology

Figure 11 -

 11 Figure 11 -Mean temperature and maximum temperature difference at the surface of the moulding surface of Cage System ® mould. Temperature recorded at the surface of the bottom part of the mould by 27 thermocouples regularly distributed on the tool's surface.

Figure 12 -

 12 Figure 12 -Interpolated temperature fields at the surface of CS mould at the end of heating (a) and during cooling (b).Black dots illustrate the thermocouples location during the test.

Figure 13 -

 13 Figure 13 -Mean temperature and maximum temperature difference at the surface of the moulding surface of 3iTech ® mould. Temperature recorded at the surface of the bottom part of the mould by 21 thermocouples regularly distributed on the tool's surface.

Figure 14 -

 14 Figure 14 -Processing cycle used for the consolidation of powdered carbon/PEEK prepregs with the Cage System ® technology

Figure 15 -

 15 Figure 15 -Thickness distribution and morphology of a C/PEEK laminate made from powdered prepregs after consolidation with Cage System ® technology.

Figure 16 -

 16 Figure 16 -Processing cycle used for the consolidation of powdered prepreg in the 3iTech ® mould

Figure 17 -

 17 Figure 17 -Microstructure of C/PEEK composite fabricated from powdered prepreg and manufactured with 3iTech ® technology

Figure 18 -

 18 Figure 18 -Thickness distribution of a 300x300mm 2 C/PEEK laminate manufactured with 3iTech ® technology from powdered prepreg


Substitution effect (MtCo2 avoided/yr) Material substitution

Energy substitution

Carbon sequestration and the reduction of greenhouse gas (GHG) emissions, as strategies to limit climate change, are objectives of the highest global importance, deserving close study by every nation. Forests and the forestry & wood sector are recognized as strategic elements in the question of climate change mitigation due to their large capacity for carbon storage and their potential to limit GHG emissions. The overall carbon impact of the forestry sector includes both the effects of carbon storage in forest ecosystems and forestry products, and a substitution effect that can result from the use of wood in place of other materials or energy sources that are greater emitters of GHG. In this context, the French Ministry of Agriculture and Food requested INRA and IGN to conduct a study of the GHG emissions mitigation potential of the forestry-wood sector in France through the year 2050. The scope of the study included all forests available for wood production in mainland France, all wood-processing industries, and all social actors contributing to the development of the bioeconomy via this primary production and processing supply chain.

Using simulations of the effects of three contrasting forest management scenarios through the year 2050, the study confirms the central role of the French forestry & wood sector in climate change mitigation. In addition to carbon storage in forest biomass, significant benefits could be gained through increased use of wood as an energy source and -more importantly -as a material. The latter strategy, which could be encouraged through proactive forestry management and tree-planting programs, is likely to become more important as climatic conditions deteriorate and as forests become increasingly subject to severe weather events and associated threats such as forest fires and pest outbreaks.

Four aspects of the forestry & wood sector enable action with respect to climate change: (i) carbon storage in forest ecosystems; (ii) carbon storage in wood products; (iii) substitution effect from the use of wood materials in the place of non-wood materials; and (iv) substitution effect from the use of wood energy in place of fossil energy. Forestrysector policies and practices influence all four of these aspects. It is worth considering which strategies favour carbon storage in forest ecosystems via reduced tree harvesting; and conversely which strategies favour substitution effects and carbon storage in wood products via stimulation of tree harvesting and the use of wood-sector outputs.

Carbon storage in the French forestry sector: The current situation

The first step in assessing the carbon impact of France's forestry sector with regard to these four action mechanisms was to conduct a review of the extensive international scientific literature on the subject. Our primary goals here were (i) to identify and refine the relevant assumptions and coefficients with respect to carbon storage and substitution effects, as applicable to the French national context; and (ii) to identify points of uncertainty and evaluate how these affect key parameters.

Carbon storage in forest ecosystems results from forests' capacity to absorb atmospheric CO2 and retain it in living biomass aboveground and belowground, in deadwood, and in forest soils. Forest dynamics -influenced by such factors as stand age, management methods, climate, weather events, and pest pressureare critical to this capacity.

Carbon storage in wood and wood-based products depends on forestry yields, the uses that are made of wood, and the lifespan of the products or materials in question.

Energy substitution relates to the quantity of CO2 emissions avoided by using wood energy in place of a reference energy source (oil, gas, coal, electricity from multiple sources, national grid, etc.)

Product substitution relates to the quantity of CO2 emissions avoided by using wood instead of other materials (concrete, steel, plaster, aluminium, etc.) in key sectors such as construction.

Accounting for substitution effects is central to understanding the role of forests and forestry products in climate change mitigation. Doing so is challenging, however, because it requires comparing supply chains for each product, from primary materials through to eventual disposal, according to the principles of Life Cycle Analysis. Substitution coefficients are thus specific to a given national industrial context.

By adapting carbon storage and substitution effect coefficients drawn from the international literature to the current situation in France, we can convert material flows between different stages in the French forestry & wood sector (expressed in Mm 3 /year) into fluxes of CO2 equivalent relative to the various mitigation mechanisms (expressed in MtCO2eq/yr) (Figure 1).

Currently, the sector's net carbon impact is dominated by carbon storage in forest ecosystems, estimated at 88 MtCO2eq/yr. The largest portion of this is in above-and belowground biomass of hardwood stands. Carbon storage in biomass of softwood species, in deadwood, and in soils also plays a role, but at much lower amounts. Annual storage via the use of wood products is considered to be nil, meaning that at present, the amount of carbon stored in wood materials produced in a given year is equal to the amount of carbon released at the end of materials' life or through the destruction of older wood products. The favourable impact of wood product use thus only comes into play through substitution effects. Substitution effects from the use of wood materials are significant (32.8 MtCO2eq/yr according to our assessment), while substitution effects from the use of wood energy have little impact on the sector's total carbon budget.

Forest management decisions and the future development of the timber industry -notably with respect to harvest intensities, replanting cycles, and product use -will directly influence the sector's carbon impact. Key factors include forest stand dynamics and their capacity to store or release carbon, as well as the sector's capacity to market products that can successfully compete with rival materials. Maintaining low harvest levels allows for additional carbon storage in forests as long as stands are healthy and growing, but limits further development of the bioeconomy and thus the potential substitution effects resulting from an increased use of wood products. Minimal harvesting may also leave forests more susceptible to weather events and at a potentially greater risk of losses to pests and other threats. More aggressive harvesting, on the other hand, accompanied by a more rapid turnover of tree populations and a policy of climatechange adaptation, would temporarily reduce carbon storage in forests, but this effect would be compensated for, at least in part, by a reinforcement of substitution effects further down in the supply chain. By strengthening the forestry-wood sector, the latter strategy would help develop the bioeconomy and offer associated advantages in terms of job creation and economic growth. NB: The variation in carbon levels in wood products is estimated at 0 and -0,1 MtCO2eq/yr for LW and PW respectively, and thus does not appear in CO2 fluxes for the sector in 2013

Three forest-management scenarios to enhance climate-change mitigation potential

To assess the effectiveness of different strategies in enhancing climate change mitigation in the forestry & wood sector, three contrasting management scenarios were simulated, using three complementary modelling tools (see Box, p. 5). The models allowed for the integrated analysis of: forest resource dynamics (variable according to the effects of climate change and other challenges to forest health); harvesting intensities; and the use of wood products. The three management scenarios were differentiated according to their relative emphasis on forests as carbon sinks, carbon storage in wood products, and substitution effects resulting from wood use.

Scenario 1: Extensification and reduced harvesting

In this scenario, societal preferences for more "natural" landscapes combined with weak prices and policy signals lead to trends of extensification, with large forested areas left unmanaged or only minimally managed. Forests in mountainous and Mediterranean regions receive minimal management, while forests in the Massif Central are under-utilized. Forest managers allow forest ecosystems to adapt to climate change through spontaneous evolution.

The total national tree harvest remains close to its current level (50 Mm 3 TAV/yr), amounting to a reduction in logging rates from 50% of net biological growth, to 42% in 2035 and 37% in 2050. There is a continued gradual loss of large-diameter hardwoods for woodenergy markets. A lack of specialised machinery for hardwood felling combined with a deterioration of hardwood timber quality and quantity lead to an increased focus on softwood species.

With the exception of the Landes forest, forestry practices in this scenario are based on natural regeneration. Given an absence of active management in the majority of mainland forests, continued rapid capitalisation (as observed over the past 30 years) and an increase in carbon sinks are expected. At some point, however, aging and density of stands may lead to a loss of available wood resources and of carbon storage capacity in forests.

Scenario 2: Territorial dynamics

This scenario is characterised by regional pressure and other developments that prompt sector actors to adopt strategies and policies specific to their local area, with significant decision-making taking place at the regional level rather than at the national level. A key driver in this scenario is a strong demand for biomass, especially for energy. Low prices nevertheless lead to a simplification of management practices and a specialisation of management objectives. Industrial wood usage is penalized by the strong demand for wood biomass for energy, especially heating systems. Management strategies remain extensive in some regions, however, notably in high mountain areas and around the Mediterranean. Forest managers and timber processors are aware of climate risks, but other constraints -economic, sylvicultural, and environmental -allow them little opportunity to modify their practices.

Harvest levels remain at current rates for the entire period (averaging 50% of net biological growth), with total harvest volume thus increasing to 75 Mm 3 TAV/yr by 2035. New strategies for making use of hardwoods are developed thanks to the investment capacity within the French timber industry that can be increased thanks to the involvement of foreign companies buying wood on contract. Investment in forests is uneven, however, with modest development of woodlot access and an increase in logging found only in those regions where downstream demand and local policy initiatives work together to bring about such changes.

Scenario 3: Intensification and active replanting

This scenario assumes an economic and political context favouring a rapid transition toward a new "bioeconomy." Use of hardwoods is facilitated by technological innovations, investments in forestry and timber industry infrastructure, changes in consumer behaviour, training and education programs, strong public incentives for property consolidation, increased use of contracts, and simplification of planning policies. This combination of forces supports greater investment in forests, with strong markets for wood products and a more favourable tax regime. This situation results in turn in more active forest management, including the implementation of diverse strategies for climate-change adaptation (strengthening and protecting ecosystem services, improving sector resilience to shocks from extreme events).

Forests in mountainous areas are partially brought back into production, while Mediterranean forests provide more energy-wood, pulpwood, softwood timber and potentially even small-diameter hardwoods. Forestry management methods shift in favour of harvesting of younger stands (allowing for reduced risk and the use of processing methods for smaller-diameter trees) in the Landes forest and other regions. Strategies foster diversification of these forests in terms of both tree species and age class.

A specific replanting plan is implemented in this scenario. It involves large areas that are already forested but are currently unproductive or not economically viable. The stated objective is to replant 50,000 ha/yr for the next 10 years, or a total of 500,000 ha. These plantings have a major impact on total forestry output in terms of both quality and quantity, thanks to the diffusion of new, highly productive varieties (softwoods, poplars) from breeding programs reoriented to address the impacts of climate change and the requirements of the bioeconomy.

The national harvest increases (more in some regions than in others), moving toward an average logging rate of 70% of net biological growth by 2035 and remaining stable thereafter. This gradual but significant increase requires a corresponding growth in capacity within the relevant timber-industry sub-sectors (nurseries, primary processing in France, secondary and tertiary processing).

Biodiversity and ecosystem services in the three scenarios

Tangible benefits with regard to biodiversity and ecosystem services can be expected in all of these scenarios, although the nature of these benefits will vary. Expanded biodiversity monitoring would be useful in all three scenarios to verify anticipated benefits and detect any signs of deterioration.

Strategies to expand the forestry sector's role in climate change mitigation

The impact of these three scenarios on the overall carbon footprint of the French forestry & wood sector were simulated through 2050 using the IGN resource-modelling tool known as MARGOT (see Box, p. 5).

The three different trajectories result in marked differences in the total volume of wood harvested annually from French forests, corresponding to different levels of intensification. The corollary to increased logging rates is a reduction in standing wood as one moves from the "extensification" scenario to the "intensification" scenario.

Projections for the carbon sequestration capacity of forest ecosystems (forest biomass, deadwood, and soils) in the three scenarios diverge accordingly (Figure 2). The "extensification" scenario results in a considerable increase in annual carbon storage in forest ecosystems, rising to more than 130 MtCO2 eq/yr by 2050. A more moderate increase in carbon storage is achieved in the "territorial dynamics" scenario, reaching less than 100 MtCO2 eq/yr by 2050. Annual carbon storage actually decreases in the "intensification" scenario, a result of increased logging rates through 2035 and the effects of the replanting program.

The replanting scheme leads to an initial drop in carbon storage as a result of concentrated clear-cutting between 2021 and 2030 (Figure 2). This loss is only partially regained in the following decades because the 2050 horizon is too close in time for the benefits of the new plantings to take effect. These benefits will only appear clearly after 2050 and will peak in 2070. The gain from these plantations is lower than might be hoped, moreover, due to the "realist" constraints imposed on the simulated replanting program, including limits of accessibility and limits of economic viability (with the least-productive areas assumed not to be subject to replanting). This result must be qualified, however, by noting one of the limitations of the MARGOT resource model. This model is based on observed data and simulates the growth of a given number of trees, but does not consider the effects that a significant capitalisation of French forests could have on forest productivity. This could increase sharply by 2050, given the unfavourable effects of stand density on stand growth. Including this constraint in the model ("dd" in Figure 2) produces a substantial alteration in the projected carbon storage capacity of French forests under the three scenarios, with the difference between scenarios being substantially reduced. Growth in annual carbon storage capacity between 2016 and 2050 is only 25% in the "extensification" scenario (compared to +60% without this constraint); annual carbon storage capacity remains stable in the "territorial dynamics" scenario; and the reduction is slightly more marked in the "intensification" scenario.

Carbon storage in wood products and GHG emissions avoided thanks to the use of wood materials and wood energy are also important components of the forestry sector's overall carbon impact.

The substitution effect from wood use is consistently positive: it remains steady over time in the "extensification" scenario; increases slightly in the "territorial dynamics" scenario; and increases substantially in the "intensification" scenario (Figure 3A).

Using the assumptions adopted here, total carbon storage attributable to the substitution effect for the period 2016-2050 is thus 33% higher in the "intensification" scenario than in the "territorial dynamics" scenario, and 60% higher than in the "extensification" scenario (Figure 3B).

Again, however, it should be noted that in the current state of research, estimates of avoided emissions are highly sensitive to the substitution coefficients used for wood materials and how these are assumed to change over time. Uncertainty in this regard exists at several levels. The value used here, for both pulpwood (PW) and lumber wood (LW), is 1.6 tCO2 avoided/m 3 of wood, or the midpoint within the range of values reported in the literature (0.59 to 3.47 tCO2/m 3) .1 Substitution coefficients for wood materials depend on the specific type of wood product in question as well as on any alternative technologies considered in establishing the reference value, making the result sensitive to the level of precision applied in disaggregated PW and LW products, among other factors.

Three complementary models for simulating resource and harvest dynamics for forestry systems

The centrepiece of our approach to simulating the effects of various scenarios through the year 2050 was the MARGOT model (Matrix model of forest Resource Growth and dynamics on the Territory scale). Developed by IGN, MARGOT allows for a detailed analysis of forest resource dynamics at the regional level. MARGOT is based on "study areas" defined using data collected as part of the National Forest Inventory (IFN), with comparable forest stands grouped together in terms of species, parcel characteristics, environmental conditions, and management features. It combines two demographic models to assess changes in the forest resource: the first describes a stand of trees (excluding poplar plantations) by diameter class (from 5 cm to 90 cm and above) and applies a growth rate for its average volume based on the actual observed growth rate of a comparable stand of the same diameter class. The other is specific to poplar plantations and employs an analogous approach but by age class. Logging rates can be determined from this in several ways. For the "territorial dynamics" scenario, harvest rates were held constant relative to the prior period. For the "intensification" scenario, they were modified to reflect increased logging rates. The "extensification" scenario was based on harvest levels obtained from simulations with the FFSM model. UMR LEF in Nancy developed FFSM (French Forest Sector Model), an economic modelling tool primarily designed for theoretical analyses but calibrated using actual data. It is a multi-modular model, one module of which describes the economic sector in partial equilibrium. Using a set of parameters describing the economic behaviour of different actors (landowners and forest managers, processors, consumers), FFSM creates an output of economic variables relating to supply and demand at different points in the supply chain for the time period requested (prices, volumes proposed and ordered, international trade, surpluses). It thus enables an economic analysis of different scenarios, anticipating, among other things, those public policy instruments that are needed to guide sector dynamics in specific directions. Finally, to account for the effects of climate change on forest dynamics, we used the GO+ model. UMR ISPA in Bordeaux developed this tool which depicts the primary biophysical and biogeochemical processes of a forest ecosystem for specific production forestry species (maritime pine, Douglas fir, and beech). It can be used to model growth, production, and forestry management while allowing for changes in environmental conditions, including climate effects, in the analysis of forest dynamics. Anomalies in growth dynamics highlighted by GO+ in the case of accentuated climate change effects (RCP 8.5) were then introduced into the MARGOT model as parameter modifications: results from the "Beech" model were used for all hardwood species; results for "Maritime pine" for all pines; and results for "Douglas fir" for all other softwoods.

5). (A) Annual carbon storage in MtCO2 eq/yr; (B) Cumulative carbon storage from 2016-2050 in MtCO2 eq

Changes in these substitution coefficients through 2050 are likewise difficult to predict since they will depend on the development and performance of technologies in competing sectors relative to future uses for wood products. An increase in the use of wood for construction and a decrease in pulpwood applications (a scenario not considered here), coupled with a small improvement in the environmental impact of rival sectors, for example, could lead to an improvement in these coefficients over time.

Notwithstanding these two important points of uncertainty (long-term changes in forest productivity; determination of substitution coefficients for wood products), the lessons that emerge from the three scenarios described here underscore the key role of France's forestry & wood sector in mitigating GHG emissions through 2050. This role is based on forest ecosystem capacity for carbon storage as well as on the substitution effects (avoided GHG emissions) available through a more widespread use of wood products and wood materials. Carbon storage effects appear to be more favourable in the "extensification" scenario, although they may also be limited by the impacts of stand aging on forest productivity. Substitution effects appear to be more favourable in the "intensification" scenario, and may be further improved if technological innovations can achieve improved substitution coefficients for the use of wood products over other materials.

Efforts needed to expand the use of forest resources

The analysis using the FFSM model (see Box, above) highlighted the economic barriers to increased forest harvesting as envisaged in the "intensification" scenario and (to a lesser extent) in the "territorial dynamics" scenario. If the structure of the forestry & wood sector remains unchanged, and if consumer habits with respect to the use of wood products continue at current levels, it will be both difficult and expensive to use public funding to increase logging activities to maintain current rates of harvesting (the central principle of the "territorial dynamics" scenario).

To address this lack of « spontaneous » response of the market, strong economic signals to both consumers and producers would be needed. On the demand side, consumers could be encouraged to see the value of wood and to make greater use of wood products in place of other materials. On the supply side, forest landowners and wood processors could be assisted in developing and marketing wood materials and products that appeal to consumers and that meet specific needs.

Whatever efforts are pursued to increase logging and replanting activities, the potential advantages for the sector as a whole are significant. The economic benefits to consumers, landowners, and timber processors of the "territorial dynamics" scenario are twice those of the "extensification" scenario, for example, not including any impacts in terms of jobs creation (which deserve to be studied in more detail).

Impacts of climate change on forestry-sector carbon dynamics

Another important variable to be considered is the impact of climate change on forest resources over time. The three forest-management scenarios we have outlined will have different impacts on the forestry sector's carbon footprint under different climate conditions. We used average climate data for the period 2003-2013 as the basis for what we refer to here as the "current climate." In 2050, this climate comes with a series of dry years similar to that experienced from 2003 to 2006. This "reference" climate can be compared to an alternative possibility in which climate change effects are intensified (corresponding to the IPCC's RCP 8.5). The impacts of this "degraded climate" on forest productivity were examined using the GO+ model and then integrated into the MARGOT model (see Box,p. 5). Estimates of the effects of drought on tree mortality relied on data from the national forest surveillance network. Under "degraded climate" conditions, a drought of equivalent or greater severity compared to the 2003 drought is foreseeable in the initial quadrennial of 2016-2020, with potential reoccurrence every several years. These conditions are predicted to trigger additional tree mortality, especially for softwoods, affecting single adult trees of medium or large size, without spatial or temporal distinction.

Under these circumstances, and regardless of which of the three forest-management scenarios is followed, the carbon storage capacity of forest ecosystems in France will be significantly reduced (Figure 4). Annual storage will increase by just 30% from 2016-2050 under the "extensification" scenario (vs. +60% in the current climate); will remain nearly level in the "territorial dynamics" scenario (vs. +20% in the current climate); and will fall by 40% in the "intensification" scenario (vs. a drop of 20% from 2016-2050 under current climate conditions). Impacts on substitution effects will be less dramatic. For the "extensification" scenario, avoided GHG emissions as a result of product substitution will be nearly identical under degraded climate conditions compared to current climate conditions; and they will be only slightly lower for the "territorial dynamics" and "intensification" scenarios (Figure 3A). The estimated total amount of avoided emissions over the 35-year period considered here is only slightly affected by an anticipated worsening of climate trends (Figure 3B).

The reduction in carbon storage in forest ecosystems is potentially overestimated in this model due to our use of beech as a stand-in for all hardwoods, a particularly drought-sensitive species. Impacts on carbon storage under aggravated climate change conditions may also be compensated for by the maintenance of substitution effects. In terms of the total forestry-sector carbon footprint, the impact of degraded climate conditions will be less damaging under scenarios that rely more heavily on the substitution effect ("intensification" and "territorial dynamics") and more damaging in scenarios of the "extensification" type.

Major events that reduce the gaps between different management scenarios

The frequency and intensity of pest outbreaks and weather events that can threaten French forests are likely to increase in the future.

Major events such as severe storms, forest fires, or large-scale pest outbreaks may dramatically reduce anticipated gains in carbon storage in forest ecosystems nationwide. To address this possibility, three types of unfavourable incidents were introduced into two of the scenarios described above ("extensification" and "territorial dynamics"): (i) widespread forest fires that overcome containment measures, which are more likely following periods of repeated drought and will therefore increase with the effects of climate change;

(ii) severe storms and their attendant risks, including forest fires and bark beetle outbreaks; and (iii) large-scale pest outbreaks causing severe damage to a range of hardwood and softwood tree species.

Drought followed by forest fires

Periods of drought considerably increase the risk of forest fires, as was the case in 2003, when nearly 75,000 hectares burned. To measure the impact of such events on the total carbon balance of the forestry sector, a "drought followed by forest fires" event was introduced at the beginning of the simulation quadrennial 2026-2030.

The location and scale of this event could vary according to a "fire risk index," linked to climate conditions. Taking the months of July and August 2003 as a reference point for "current climate," this index predicted a fire risk 2.4 times higher under degraded climate conditions (RCP 8.5). Thus if the total land area burned in a fire event was 75,000 hectares under current climate conditions, the burned area would be estimated to total 175,000 hectares under degraded climate conditions. Tree mortality was assumed to be 100 percent within the burned area.

Compared to the total national forested area, however, this is a relatively small area of impact, corresponding to a small volume of living biomass consumed. Viewed at the national level, the effect of this type of event on the total carbon footprint of the forestry sector remains low (Figures 5 and6). Total carbon storage in forests, as well as GHG emissions avoided via substitution effects, are unchanged relative to the "no-incident" simulation, even under degraded climate conditions. It should be noted, however, that a series of repeated droughts and forest fires is to be expected, rather than a single episode. The cumulative impact of all these incidents could ultimately make itself felt on the total sector carbon footprint, even if each incident alone has a relatively small effect.

Storms followed by forest fires and/or bark beetle outbreaks

This incident includes a severe winter storm affecting the entire country and causing widespread damage to trees, followed by an outbreak of bark beetles in pines and spruces, followed by forest fires the next summer. Storms Lothar and Martin in 1999, which destroyed 176 Mm 3 of wood in France, were used as reference incidents. The simulated storm would take place in the quadrennial 2026-2030. It would begin in the area of Charente and move across the country from the southwest to the northeast, impacting an area of 700,000 to 1 million hectares. In the direct path of the storm, damage to trees exceeds 40%. In addition, in an area totalling 685,000 hectares on the periphery of the storm, tree populations are impacted at levels below 40%.

Although direct damage from the storm is not affected by climate, degraded climate conditions (RCP 8.5) result in additional damage from bark beetle infestations. Tree mortality from bark beetles under current climate conditions reaches 6-12% depending on the tree species and the region; this rate is multipled by 1.7 in the case of a degraded climate (RCP 8.5).

Forest fires the following season are concentrated in the path of the storm. The scale predicted is the same as for forest fires following drought (75,000 ha in the current climate and 175,000 ha in a degraded climate, RCP 8.5). Mortality of remaining trees following the storm is assumed to be total.

Here the effects are much more extensive, at least episodically. Thus, for example, in the case of the "territorial dynamics" scenario, under current climate conditions, annual carbon storage in forests falls dramatically, dropping by 60% from the quadrennial 2021-2025 to the quadrennial 2026-2030, following the storms (Figure 7). The impact on forest biomass is moderated to some extent by the large quantity of deadwood, conserving a portion of the stored carbon in forests.

Assuming that large amounts of this wood would be cut and sold, moreover, carbon storage in wood products and the resulting substitution effects would increase in the wake of the storm: avoided GHG emissions would rise sharply, by 75%, and storage in wood products, although still low, would be temporarily multipled by 10.

Overall, the loss of cumulative carbon storage in forests from 2016-2050 due to the combined "storm, bark beetle, fire" incident is greater in the "extensification" scenario than in the "territorial dynamics" scenario (Figure 5). On the other hand, the increase in avoided emissions via substitution effects is slightly more favourable in the "territorial dynamics" scenario than in the "extensification" scenario (Figure 6).

Other pest and disease outbreaks

Finally, we introduced different types of severe pest and disease outbreaks affecting oak and pine species. Parasite spread, mortality dynamics, and loss of growth as determined by the duration of the outbreak were modelled on those seen with ash dieback caused by a fungus. The fungal outbreak results in heavy mortality among young trees, moderate mortality among adult trees, and significant loss of growth. This type of incident was considered independently of other unfavourable circumstances, including climate conditions. It was thus simulated only under "current climate" conditions. The species impacted (oaks or pines) and the severity of the attack determine the scope of the incident. The impact is potentially small if the outbreak is restricted to maritime pine (starting from a localized outbreak in south-western France): in this case, cumulative carbon storage in forests through 2050 would be reduced by less than 5% (Figure 5) and avoided emissions would remain unchanged (Figure 6). The impact would be more substantial in the case of an outbreak affecting pedunculate oaks (Quercus robur), given the importance of this species in French forests and an assumed genesis of multiple localized outbreaks in eastern France. A similar impact would be seen if all species of pines were affected.

If, on the other hand, all species of deciduous oaks were susceptible to infection, the impact of an outbreak on the overall carbon footprint of the forestry sector would be major. In this case, annual carbon storage in the biomass of deciduous species would fall significantly from the quadrennial 2021-2025 onward, becoming negative in the two subsequent quadrennials. The increase in stored carbon contained in deadwood would be insufficient to maintain annual carbon storage levels in forest ecosystems and, for the overall period of 2015-2050, total storage would be reduced by a third in the "extensification" scenario and by 42% in the "territorial dynamics" scenario (Figure 5). At the same time, carbon storage in wood products and avoided emissions via substitution effects would increase significantly as a result of the outbreak, but would remain low relative to the loss of carbon storage in forests: +9% for the substitution effect for the full period 2016-2050 in the "extensification" scenario; +12% in the "territorial dynamics" scenario. This is assuming that the timber industry would be able to process 70% of the dying oaks in the "territorial dynamics" scenario, or an annual average volume of 15 Mm 3 /yr.

Conclusions and future research needs

The findings of this study suggest that the role of French forestry sector in mitigating climate change is likely to become more significant between now and 2050. Although some components of the sector may show minimal or even negative carbon storage during some periods (for instance, in the wake of a severe storm or pest outbreak), the overall scale of the forest resource makes it a key factor in determining the national inventory of greenhouse gas storage and release. The various compartments and mechanisms considered by this study -i.e., storage in forest ecosystems and in wood products; emissions avoided through substitution effects -play complementary roles in determining the overall carbon balance of the forestry sector. Reductions in carbon storage in living biomass (for example, through intensified timber harvesting, use of short-rotation plantings, or the processing of windfall timber) can be counterbalanced by or made up for over time through avoided emissions resulting from substitution effects, through temporary carbon storage in deadwood, or through more long-term carbon storage in wood products. The benefits to be gained through substitution effects increase with more active forestry management and increased timber harvesting. Stepped up timber harvesting could also generate significant economic and social benefits, but such a shift would require considerable changes in consumer behaviour as well as a some degree of reorganisation of the forestry sector.

Carbon storage in forests is likely to decrease as global temperatures rise and as storms, drought, forest fires, and pest and disease outbreaks increasingly impact forests. Substitution effects, by contrast, are relatively unaffected by these factors. Future scenarios emphasizing more active forestry management thus offer substitution effects that can help compensate for reductions in carbon storage capacity in living biomass.

Given the uncertainty surrounding certain key parameters, it is extremely difficult to rank the three forestry management scenarios presented here in order of their anticipated carbon impacts through 2050. Just as important is the fact that this timeframe falls well short of the length of the typical forestry cycle for hardwood-dominant forests in mainland France. If projections had been attempted for a longer timeframe (through the year 2100 or beyond), the findings of this study might have been different. Modelling tools and methodologies as currently available do not allow for robust simulations over this length of time, however.

Figure 1 -

 1 Figure 1 -Flows of material and CO2 at different stages of the French forestry & wood sector in 2013 (TAV= Total aboveground volume, LW=Lumber, PW=Pulpwood, EW=Energy wood)

Figure 2 :

 2 Figure 2: Annual carbon sequestration in forest ecosystems under three management scenarios, with and without "density dependence" effect (dd), current climate, in MtCO2 eq/yr.

Figure 3 :

 3 Figure 3: Avoided GHG emissions from substitution effects, using wood products based on three management scenarios and two climate backgrounds, current climate and degraded climate (RCP 8.5). (A) Annual carbon storage in MtCO2 eq/yr; (B) Cumulative carbon storage from 2016-2050 in MtCO2 eq

Figure 4 :

 4 Figure 4: Annual carbon storage in forest ecosystems under three different management scenarios, and under a current climate vs. a degraded climate (RCP 8.5), in MtCO2eq/yr

Figure 5 :Figure 6 :Figure 7 :

 567 Figure 5: Cumulative C storage in forest ecosystems 2016-2050, two management scenarios with unfavourable events (MtCO2 eq)

Further

  research is needed to improve methods to measure and assess the carbon balance of the various compartments of the forestry sector, as well as to understand the mechanisms and drivers underlying changes in carbon storage and release as forestry sector strategies, relationships, and conditions change. This study highlights the need for additional data collection, methodological clarification, and multidisciplinary coordination, particularly with regard to forecasting the future impacts of different climate pathways and associated weatherdependent events. Another key research need is to improve the capacity for integration across different modelling tools: for example, a resource model based on demographic assumptions, a biophysical model with the ability to modify climate conditions, and an economic model allowing analysis of the market factors inhibiting increased timber harvesting. Finally, evaluating the effects of disease and pest outbreaks as linked to biotic and abiotic conditions requires better coordination of data sources, modeling tools, and research on these interactions.To learn more:Roux A., Dhôte J.-. (Editors), Achat D., Bastick C., Colin A., Bailly A., Bastien J.-C., Berthelot A., Bréda N., Caurla S., Carnus J.-M., Gardiner B., Jactel H., Leban J.-M., Lobianco A., Loustau D., Meredieu C., Marçais B., Moisy C., Pâques L., Rigolot E., Saint-André L., Schmitt B. (2017). Quel rôle pour les forêts et la filière forêt-bois françaises dans l'atténuation du changement climatique? Une étude des freins et leviers forestiers à l'horizon 2050. Study report for the Ministry of Agriculture and Food, INRA and IGN, 96 pp. + 226 pp. of annexes. 147, rue de l 75338 Paris Cedex 07 France T . : + 33(0) 1 42 75 90 00 www.inra.fr

Sathre, R., O' Connor, J.,

Meta-analysis of greenhouse gas displacement factors of wood product substitution. Environmental Science & Policy, 13 (2): 104-114.