Ducruet César

Joly Olivier
email: olivier.joly@univ-lehavre.fr

Europe in global maritime flows: Gateways, forelands, and subnetworks

Introduction

Whilst processes of globalization raise new challenges for research to quantify and map the geographies of spaces of invisible financial and trade flows, increasing virtual connectivity between places has in fact been accompanied by an intensification of physical movement, of people and goods, endorsing the ongoing importance of location and distance [START_REF] Hall | Cities, Regions, and Flows[END_REF]. This point is illustrated by the fact that in the new millennium, ninety per cent of both global trade and Europe's external trade volumes are transported by sea. Ports and maritime transport are thus key elements of globalization processes [START_REF] Shaw | Making links: On (re)engaging with transport and transport geography[END_REF], making it crucial for our research to examine the position of Europe's port gateways in global shipping flows.

As argued by [START_REF] Lewis | A maritime response to the crisis in area studies[END_REF], depictions of the metageography of the world have traditionally been dependent on a continental, land-based analytical perspective. Hence important questions have for too long been overlooked in spatial analysis. How do the specific patterns of maritime flows compare with those of other important global flows such as finance or air transport? How do they influence wider territorial dynamics within and outside Europe? The main purpose of this chapter is thus to verify how patterns of inter-port links can contribute to our understanding of the potential emergence of a European macro-region, and to our understanding of the position of Europe as a whole in global sea traffic.

The position of Europe in maritime flows is demonstrated by the connections of its port gateways with the rest of the world. Such connections vary from one port to another in terms of traffic concentration and geographic coverage, dependent on multiple factors including origin-destination time and cost factors, shipper and ocean carrier decisions, and the size, quality, and specialization of local port infrastructures. A key research challenge is therefore to unravel the relationships between internal and external dynamics; in other words, how do the emergence and position of European maritime flows interplay with the distribution of its port system and how have such patterns been influenced by the evolution of other regions, such as Asia, where port dynamics are profound and rapid.

Analysis of the geographic coverage and hierarchy of maritime flows is reliant upon the two major concepts and research fields in transport geography: port systems, and maritime forelands. Research on port systems, or systems of ports, often look at changing traffic concentration levels among neighbouring ports (see a review by [START_REF] Ducruet | Revisiting inter-port relationships under the New Economic Geography research framework[END_REF]. On the other hand, studies of maritime forelands focus more on the overseas connections of ports and their evolution, classically conceptualized as the maritime side of port dynamics and complementary to the port itself and its hinterland [START_REF] Weigend | Ports: Their hinterlands and 'forelands[END_REF][START_REF] Robinson | The hinterland-foreland continuum: Concept and methodology[END_REF][START_REF] Vigarié | Ports de commerce et vie littorale[END_REF]. There has been a recent revival of interest in the study of forelands, which were the focus of early port geography research (see [START_REF] Ng | Port geography bibliometrics (1950-2011): Community structure, background, and diffusion[END_REF], with notable recent case studies of ports in North Korea (Ducruet, 2008), Maghreb (Mohamed-Chérif andDucruet, 2011), China [START_REF] Wang | The geographical connectedness of Chinese seaports with foreland markets: A new trend?[END_REF], and France [START_REF] Guerrero | French deep-sea hinterlands: Some empirical evidence of the spatial impact of containerisation[END_REF]. More global approaches to the analysis of port systems and maritime forelands have yet to emerge due to longstanding difficulties in accessing relevant information on shipping flow s [START_REF] Ullmann | Mapping the world's ocean trade: A research proposal[END_REF][START_REF] Marti | Trade route 11: Methods to assess port exchanges of maritime containerized cargo flows[END_REF], notwithstanding some recent research contributions adopting a network approach to analysis, putting the overall structure of the network at centre stage [START_REF] Joly | La structuration des réseaux de circulation maritime[END_REF]Kaluza et al., 2010;[START_REF] Ducruet | The worldwide maritime network of container shipping: Spatial structure and regional dynamics[END_REF]. A further limitation to global maritime research is that studies are often embedded within broader research projects primarily concerned with urban and multimodal accessibility (European Union, 2010) and focusing on specific dimensions such as individual ocean carrier strategies and geographic coverage [START_REF] Frémont | Global maritime networks: The case of Maersk[END_REF], port and maritime network vulnerability in relation to their dependence upon transoceanic canals [START_REF] Ducruet | The polarization of global container flows by interoceanic canals, Paper presented at the International Colloquium on Interoceanic Canals and World Seaborne Trade: Past, Present and Future[END_REF], or specific historic time periods1 . This chapter tackles these lacunae by analyzing the global pattern of maritime flows using previously untapped data on merchant vessel movements in 2004 and 2011. This period has specific relevance because it allows investigation of the impacts of the 2008 global financial crisis which had deep repercussions for maritime traffic, amongst other dynamics [START_REF] De Monie | Economic cycles in maritime shipping and ports: The path to the crisis of 2008[END_REF]. Since recent research has mostly focused on container flows, the present analysis also includes raw materials flows (i.e. bulks) because different commodities may have different spatial patterns as containers, the most valued shipments, are carried through regular (liner) services and bulks are transported via nonregular, on-demand (also called tramping) services.

The chapter is organized as follows: First we explain in more detail, the methodological issues and choices which have confronted and structured our analytical approach. Second, we present out findings by mapping and interpreting Europe's position, influence and coherence in global maritime flows using cartographical and network analytical tools. Third we complement these results by classifying the maritime forelands of European ports based on the specialization of their geographic coverage. Finally, we present our conclusions about the importance of ports for European development and for the world economy as a whole.

Data and methodology

Our data were obtained from the Lloyd's List, the world's main maritime insurance company, covering about 80% of the world fleet. The list provides information on the daily movements of merchant vessels, including the sequence of port calls, and information about vessels' carrying capacity, type of commodity, etc. Due to the volume and cost of this information, our analysis has been restricted to specific key months of circulation: October 2004 and May 2011. Due to the absence of information on vessel types and capacities in 2004 in the paper version of Lloyd's Voyage Records (LVR), missing data were retrieved from additional vessel databases such as Fairplay World Shipping Encyclopedia 2 , MIRAMAR Ship Index 3 , DNV Exchange 4 , and the World Shipping Register 5 . This required considerable efforts due to the absence of IMO (International Maritime Organization) numbers of vessels. Since many vessels regularly change their names and flags, the risk of confounding them across databases was avoided by taking into account their type, subtype, year of build, and former names.

Another methodological issue was the choice of tonnage capacity. Although deadweight tonnage (DWT) provides a more accurate picture of a vessel's commercial capacity (excluding reservoirs, decks, rooms, etc.), the Gross Registered Tonnage (GRT), which corresponds to the volume of the entire vessel, was selected for analysis due to its wider availability in ship registers. Measuring the weight of maritime flows based on GRT figures, however, may undermine the importance of some commodities as there is no proportionality between DWT and GRT. In any case, the occupancy ratio of vessels (i.e. the number of tons actually carried on each trip) as well as the volume of freight handled at each port, is not specified by the data sources. Thus, it was assumed that the overall capacity (GRT) of vessels is a good indicator of the importance of flows. Vessel capacities were summed by port and by interport link during each period of movement. This results in the elaboration of an origin-destination (OD) matrix to be considered as a weighted, undirected graph as in other studies of accessibility in networks [START_REF] Rodrigue | The Geography of Transport Systems[END_REF]. Traffic flows are thus calculated by taking into account the volume and the frequency of vessel trips between locations.

Lastly, vessel types were aggregated in different categories, such as liquid bulk (i.e. asphalt, crude oil, oil products, chemicals, liquefied natural gas, liquefied petroleum gas, water, wine, edible oil, and unspecified tankers), solid bulk (i.e. aggregates, cement, ores, and unspecified bulks), and containers.

General cargo vessels were excluded due to the high cost of purchasing their movements in digital format for 20116 , as it is the most numerous fleet. Flows of roll-on/roll-off (ro-ro) vessels, which typically carry trucks and vehicles, are not analyzed separately since they are often restricted to shortsea shipping services and remain mostly intra-regional.

Macroscopic patterns

Three geographical levels are considered: continents, maritime ranges, and ports. Here and in the following section on nodal regions we make use of single linkage analysis methods to simplify the pattern of flows and identify key structures. For each node in the network (i.e. continent, region, and/or port), we keep only the largest flow link with another node, which is believed to extract hidden patterns such as node hierarchies and the geographic coverage of their tributary areas. In the case of continents and maritime ranges, the second largest link has been considered in order to avoid losing too much information, as in the multiple linkage analysis method [START_REF] Puebla | Spatial structures of network flows: A graph theoretical approach[END_REF].

At the level of continents (figure 1), the first results underline a rapid and paramount increase of Asia-Pacific in global shipping, from 36.9 per cent in 2004 to 49.1 per cent in 2011 of world total traffic. Europe has witnessed the highest drop in traffic shares from 24.4 to 20.8 per cent, followed by West Asia, North America, Africa, and Latin America. The overall pattern of flows has remained rather stable, with a polarization upon Asia-Pacific and Europe, the latter being a subordinate of the first because Europe's largest link connects Asia. While Latin America and Africa have their largest traffic links with Europe, North America, West Asia and Europe itself are dominantly connected with Asia-Pacific for both years. The relative importance of Europe in other regions' traffic has also remained stable, except for a noticeable drop in Latin America. The share of Europe has actually declined for all regions except Africa (+2.2%), the largest drops being Latin America (-5.1%) and Asia-Pacific (-3.4%). Conversely, intra-European flows have increased their share of total traffic from 36.3 per cent to 40.8 per cent. Such dynamics underline the shrinkage of Europe's global position and the reinforced polarization of world flows by Asia-Pacific.

[Insert Figure 1 about here] Regarding the level of maritime ranges (figure 2) that correspond to the definition of regions given by Lloyd's List, a number of disconnected components emerge and reveal macro-regional patterns. A large Asia-Pacific pole centered upon China and Korea, ranges from the U.S. West Coast to the Red Sea and East Africa. North and South Europe remain separate entities for both years. In 2004, the North American East Coast is included in a vast Atlantic pole comprising the rest of the Americas, Northwest Africa, and Europe; in 2011, it is included in another pole comprising the U.S. Gulf, Central America, and Southwest America. All of these poles strongly connect neighbouring regions. Europe and the Americas. The reinforced internal connectivity of Europe as well as its decreasing global importance is highlighted for numerous maritime ranges. Outside Europe itself, only Northeast America, Northwest Africa, and the Red Sea maintain significant shares of Europe-related traffic in their total traffic. The highest drops occurred in Southwest America (-6.5%), Southeast America (-5.4%), Central America and Caribbean (-4.5%), and Southeast Africa (-4.1%). Such trends directly illustrate macro changes in trade patterns such as the shift of African traffics towards Asia [START_REF] Chaponnière | Le basculement de l'Afrique vers l'Asie : enjeux pour les ports africains[END_REF] and the extension of Asia's influence towards Latin America, notably in the case of solid bulks.

[Insert Figure 2 about here] At the port level (figure 3), specialization in European traffic appears more evident. North and West African ports have a very significant share of their traffic polarized by European ports. This directly reflects Maghreb's specialization in bulk exports (e.g. liquefied gas and oil products, minerals) to Europe as well as its historical ties with France. Alexandria and Damietta (Egypt), Casablanca and Tangier (Morocco), as well as many Libyan, Algerian and Tunisian ports have the largest volume and share of Europe-related traffic. Outside of this Euro-Africa area, only Montreal (Canada) combines both high volume and high share; all other ports specialized in European traffics remain rather small, such as in the Antilles (e.g. Pointe a Pitre, Fort de France), South America's East Coast (e.g. Cayenne, Santarem, Natal), and the Indian Ocean (e.g. Mauritius). The rest of the world remains far less specialized in European traffic, especially the Asia-Pacific area. As a confirmation of previous results, the overall pattern did not change much in 2011, although the decline of European traffic is significant outside specialized ports. Europe's tributary area for trade shrunk spatially, whereas other poles extended their influence, notably across the Southern hemisphere.

[Insert Figure 3 about here]

Port hierarchies and nodal regions

A very useful method to capture both the hierarchical and geographical effects of flow patterns is the application of single linkage analysis to world shipping flows at port level (figure 4). Bisecting the world graph into subgroups, each with its own dominant port, allows for mapping distinct nodal maritime regions. They are interpreted here as basins of intense interaction, and their geographic dimension can be tested through cartography. The regionalization of shipping flows can highlight crucial aspects of a port's ability to dominate others as an effect of various factors such as port infrastructure size and quality, shippers and carriers' logistics decisions, etc. [START_REF] Robinson | Ports as elements in value-driven chain systems: the new paradigm[END_REF]. The way in which Europe as a single entity emerges from the flows beside other regional entities may complement studies of world regionalization.

In support of previous results, the largest emerging component is centered upon Asia (Singapore being the dominant node), followed by a portion of Europe mostly occupying the North with a small trans-Atlantic extension toward Canada (Montreal). This means that transatlantic ties have not only shrunk in size, but they have also become disconnected subparts compared with the more integrated Asian group, which extends in 2004 across the Pacific and the Mediterranean basins. The split between North and South Europe may be attributed to the exclusion of land-based networks and flows in the analysis, due to the lack of such data on a world level. However, such absence cannot explain the split of the Atlantic, especially the northern part, into small groups. Most groups outside the Asian one are thus reflections of spatial proximity of range effects among neighbouring ports. The fact that Asia as a whole is dominantly a maritime economy can explain such results, as a large proportion of its internal and external trade occurs by sea, and most of its population and activities are located along its coasts [START_REF] Lee | A tale of Asia's world ports: The spatial evolution in global hub port cities[END_REF]. Important discontinuities thus influence flow patterns, ranging from physical factors (e.g. coastlines, straits) to human factors of trade routes and carrier decisions. Further research should attempt to integrate road and railway networks, for instance, in order to maintain continental continuity among Europe's cities and regions. This would also provide drastically different results for Asia (weaker inland penetration of logistics chains) and North America (transcontinental land bridges). In addition, this would help to improve understanding of the intermodal importance of some European gateways as well as the relative importance of maritime flows for inland (non-port) cities and regions in Europe and other parts of the world. At present, based on these results, Europe is composed of different maritime subsystems, each having their own internal logic, despite the fact that, in reality, they are complementary and interconnected.

In Europe, Rotterdam stands out as the second largest port after Singapore in terms of its number of subordinates (i.e. ports connecting Rotterdam by their largest flow link). Independent ports can be seen as regional hubs exerting polarization within their maritime range. There is a tendency between 2004 and 2011 for most groups to have become smaller, except for the largest European group which extends further along the West African coast. This confirms results of the previous analysis of macroscopic patterns, revealing that Northwest Africa maintained its dependence on European traffics.

Valencia replaced Marseilles at the head of a Western Mediterranean group. The geographic coverage and size of the large Asian group centered upon Singapore is mostly explained by the latter's hub functions for both liquid bulk and container flows internally and externally. There are plans to develop a new liquid bulk hub port in Gwangyang, South Korea, to take over part of this role.

[Insert Figure 4 about here]

Geographic specialization of maritime forelands

Global analysis

The application of factor analysis to the regional distribution of world ports traffic reveals the dominant specialization of their maritime forelands. A factor analysis was applied to the each matrix in 2004 and 2011 based on the distribution of traffic between each port and each of the seventeen world regions or ranges. This method allows us to extract the main trends from the original dataset and group variables as well as ports. Four main factors comprise about 65% and 61% of the total variance in 2004 and 2011, which was considered sufficient to obtain a global snapshot of foreland specializations.

As expected, the first factor (F1) depicts a hierarchical trend for both years in which largest ports tend to connect primarily (and perhaps to locate in) Asian regions, followed by Southeast Africa, Northwest Europe (only in 2004) and the Middle East. This pattern remains nearly unchanged throughout the period. The second factor (F2) reveals a very interesting trend in which ports bordering the Atlantic and connected seas and basins are opposed to ports bordering the Pacific and connected seas and basins. This clear divide between the world's two main oceans confirms previous results and underscores the importance of proximity links and regional dynamics. In the global network of container shipping, for instance, about 80% of interport links occur along a distance of 500 kilometers or less [START_REF] Ducruet | The worldwide maritime network of container shipping: Spatial structure and regional dynamics[END_REF], but this can largely be attributed to the way that vessels circulate and connect ports. Unlike air transport, maritime transport connects many intermediate nodes on their voyage from origin to destination, thus greatly exaggerating the importance of local links among adjacent ports. This also stems from the application of hub-and-spokes systems in many regions where one dominant port multiplies connections toward secondary ports.

The third factor (F3) provides another dimension, that is, the opposition between the Americas and the Mediterranean basin (with a slight influence of the Middle East) for both years. This pattern may reveal the importance of North-South trades between North and South America as well as between South Europe and North Africa, where trade exchanges are intense. The fourth factor (F4) somewhat differs in its structure between 2004 and 2011. In 2004, there is an opposition between Northwest America and Southeast Africa, which can be understood when including additional variables as an opposition between two different historical legacies. On the one hand, the grouping of Southeast Africa, West Africa, and Southeast America (and, to a lesser extent, South Asia) with North Europe in general resembles Europe's former colonial world based on North-South linkages. On the other hand, the grouping of Northwest America with Northeast Asia, South Europe, and North Africa (i.e. the Mediterranean basin) constitutes an important segment of the round-the-world trunk line connecting major economic poles. The cartography of the position of individual ports along those four main factors should verify to what extent ports are specialized in regions other than their own, due to the importance of long-distance links. Each of the maps (figure 5) represents a cluster of ports defined by the crossing of the aforementioned factors. Overall, they confirm a certain stability of the spatial patterns over time since most trends are logically distributed across space in the form of continuous alignments of ports sharing identical profiles. This is a very satisfactory result given the changes that occurred during the period and the differences in data coverage and collection methods. One main change is the fact that all clusters contain European ports in 2004 whereas in 2011, three main clusters do not. In itself this result again confirms the shrink of European forelands on global and regional scales.

While the specialization on Mediterranean flows involves mostly Mediterranean ports, two other clusters including many European ports expand outside Europe: Europe/West Africa and Europe/South America. . They both illustrate the persistence of former colonial ties through maintained trade and cultural links. This pattern thus reflects the importance of longstanding North-South Atlantic links for Europe. In 2004, the Europe/West Africa cluster includes Rotterdam, Le Havre, Rouen, Southampton, Tees, Felixstowe, Tilbury, Milford Haven, and Gothenburg in northern Europe as well as Lisbon and Algeciras in southern Europe among the largest ports. In 2011, only Felixstowe and Southampton remain from the previous list. The presence of many French ports in this group (also with La Pallice, Donges, Brest, and Le Verdon) confirms the importance of post-colonial linkages. In 2004, the Europe/South America cluster comprises Amsterdam, Antwerp, Bremerhaven, Dunkirk, Hamburg, Immingham, Wilhelmshaven, and Zeebrugge while in 2011, Le Havre and Rotterdam are also included. This confirms the proximity created by specific links of various natures (e.g. Brazil-Germany, Netherlands-West Indies). Yet, the number of non-European ports in this cluster has reduced considerably, the common feature being the Brazilian pole around Santos and Rio de Janeiro. These two major clusters have thus both shrunk spatially and have the peculiarity to include mostly large northern European ports ensuring such long-distance connections. Another example of spatial reorientation is the fourth cluster including Bilbao, Vigo, and Thamesport in 2004 but including only American ports in 2011, partly due to the reinforced North-South integration within the Americas (see [START_REF] Guy | Shipping line networks and the integration of South America trades[END_REF]. In fact Bilbao has shifted to the Europe/ West Africa cluster in 2011. The results obtained from the application of factor analysis for each year are very significant, as the four main factors captured 83.7% and 82.7% of the total variance, respectively. The hierarchical trend observed for F1 is dominated by containers in both years. Although containers have less weight than bulks, their traffic is highly hierarchical due to fierce competition among European ports to develop transshipment and intermodal services [START_REF] Notteboom | Concentration and the formation of multi-port gateway regions in the European container port system: An update[END_REF]. The best represented forelands on F1 shifted somewhat between 2004 and 2011, from the dominance of Latin America to that of East Asia. This again confirms previous results emphasizing the increased polarization of global maritime flows by Asian ports in general and the parallel retreat of Europe from its traditional forelands. Other factors have similar structures between 2004 and 2011 but with noticeable differences making it difficult to establish direct comparisons.

The second factor (F2) opposes African forelands, liquid bulks and Asian forelands, containers. This is a classical pattern whereby Europe imports containers from Asia and raw materials from Africa, although the analysis does not take into account the directionality of flows. In 2011, the opposition is more between Middle East/Mediterranean/East Asia/Northwest America, containers and South Atlantic/East Africa, solid bulks. This trend seems to echo the 2004 pattern but with a more precise delineation, that is, an opposition between the eastward container trunk line and the North-South raw materials trades (e.g. agricultural products and minerals). The third factor (F3) in 2004, clearly opposes North Africa/Middle East, liquid bulks and Southeast Africa/South America, solid bulks. In 2011, North Africa and the Middle East are grouped with Oceania but without a specific commodity profile, in opposition to the Americas again, without a specific commodity orientation.

The distribution of such trends within Europe itself exhibits a certain spatial order among gateways according to their main orientations. In 2004, Rotterdam, Hamburg and Bremerhaven (together with Tilbury) were the largest ports being specialized in Asian forelands and containers, but also solid bulks and Southeast Africa/Latin America trades. This category is thus very much concentrated in the northern part of the megalopolis as it includes major ports of the so-called Northern Range. The parent category (Asia, containers) having more affinity with liquid bulks and North Africa/Middle East flows is mostly located across the Mediterranean (e.g. Barcelona, La Spezia, Gioia Tauro, Piraeus) and in Southeast England (e.g. Felixstowe). In contrast, numerous large ports are specialized on African trades and liquid bulks (e.g. Le Havre, Algeciras, Marseilles, Genoa), while the largest ports in the last category specialize in solid bulks. Latin American and Southeast African trades concentrate mostly along the Atlantic range. Thus, the global distribution of European forelands privileged a few ports having long-distance specializations, while most large ports remained focused on flows with adjacent and traditional partners.

In 2011, some similarities do appear, with Hamburg and several Mediterranean ports having dominant links with Asia and containers. Ports more specialized on East-West container flows are among the largest, except Rotterdam, Marseilles, and Algeciras which are turned more towards North-South flows and solid bulks. These patterns confirm the wider spread of Asian trades across European major gateways and the retreat of traditional Atlantic forelands. Among other factors, the 2008 global crisis might have fostered such dynamics by making European gateways even more specialized in containers and Asia. At the same time, Asian ports have tended to capture those traditional forelands that were once Europe's preferential partners (e.g. Africa and Latin America).

Conclusion

Using various methods and analyses, we have documented the position and evolution of Europe and its port gateways in global maritime flows based on original data on merchant vessel movements in 2004 and 2011. The main results demonstrate the accuracy of the data due to the fact that results are consistent from year to year and also the enormous potential for vessel movement data to study trade patterns and regionalization processes around the globe.

Overall, our analyses confirm that Europe is no longer a leader in global maritime flows, which are increasingly polarized by Asian ports. The non-inclusion of land-based flows and networks is, of course, partly responsible for such a picture, which is only based on shipping data. For instance, a previous study of combined shipping and air transport flows shows the persistent domination of world exchanges by Europe [START_REF] Ducruet | Cities in worldwide air and sea flows: A multiple networks analysis[END_REF]. Another important finding is the fragmentation of Europe The study of foreland specialization points out the different orientations of major gateways towards specific post-colonial and resilient forelands as well as the spatial shrinkage of such trading ties during the period studied. Asian ports have captured a significant share of Europe's forelands, reinforced by increasing south-south flows outside the main poles. This study thus confirms the influence of major trade shifts on the distribution and specialization of Europe's internal and external port traffics. The maritime perspective offered is specific in the sense that it complements former analyses of other global flows, such as airlines and multinational firms, which suggest a maintained dominance of Europe and North America over the global economy (see [START_REF] Cattan | Le monde au prisme des réseaux aériens[END_REF][START_REF] Bretagnolle | Métropoles et mondialisation[END_REF].

Although maritime flows mostly comprise raw materials and thus cannot be seen as the most strategic vector of globalization, they remain the backbone of international trade. Their current distribution thus expresses the potential of gateways and regions to organize the material pillars of the global economy.

  Many changes are highlighted, such as the stronger polarization of global shipping flows by the China-Korea region and the weakening of trans-Atlantic links, as already observed by Ducruet and Notteboom (2012) in the case of container flows between 1996 and 2006. The link between Southeast Asia and China-Korea remained the heaviest between 2004 and 2011. Such patterns may have been influenced by the combined effects of the 2008-2009 global financial crisis (i.e. the lowering of longdistance trades) and of pursued intra-regional trade cooperation in Asia and other regions such as

  All other European ports belong to clusters specialized in Asia-Pacific traffics. They are often much smaller than the European ports of Europe-Atlantic clusters and the trend is their absence in 2011 from the two last clusters of the figure. One interpretation of such patterns is the growing expansion of Asian trades across Europe and the Atlantic, which in turn depreciates Europe's influence by restricting the geographic coverage of its main gateways. [Insert Figure 5 about here] 5.2 Commodity specialization and foreland specialization in Europe Due to the very high influence of adjacency on the results at the global level, the specific foreland specialization of European ports appears somewhat blurred. For such reasons, only extra-European traffic flows were retained. This resulted in the deletion of 177 ports in 2004 and 159 ports in 2011 due to their absence of extra-European traffics. The choice was made to add three variables on traffic per major commodity group (solid bulk, liquid bulk, and containers) with the assumption that foreland specialization might correlate with commodity specialization.

  in different areas and orientations, resulting in difficult 'readability' of a single European region. Most of the time, Europe is torn between North and South, with Northern ports having more widely spread connections and Southern ports remaining bound to the Mediterranean basin, although Southern ports do also have long-distance worldwide connections. The different analyses (i.e. subnetworks, foreland specialization) thus highlight the challenge of considering Europe as a single and integrated entity.Northern ports, which are characterized by a wider diversity and coverage of maritime connections, appear more integrated than Southern ports. Beyond sole physical and logistics factors, such a divide within Europe may reflect wider phenomena such as Europe's difficulty in realizing internal integration, as discussed in other chapters of the book.

Figure 1 :

 1 Figure 1: Largest shipping flows among world regions in 2004 and 2011

Figure 2 :

 2 Figure 2: Largest shipping flows among world maritime ranges in 2004 and 2011

Figure 3 :

 3 Figure 3: Weight and share of Europe at world ports in 2004 and 2011

Figure 4 :

 4 Figure 4: Single linkage analysis of global flows in 2004 and 2011

Figure 5 :

 5 Figure 5: Foreland specialization clusters including main European ports in 2004 (left) and 2011 (right)

See the CLIWOC project homepage with maps of vessel circulations during the 1750-1854 period: http://www.knmi.nl/cliwoc/

http://www.ihs.com/products/maritime-information/ships/world-shipping-encyclopedia.aspx

http://www.miramarshipindex.org.nz

http://exchange.dnv.com/exchange/Main.aspx

http://e-ships.net/

Since 2009 onwards, Lloyd's List does not publish paper versions of its registers.

Acknowledgements

The authors would like to thank Liliane Lizzi for cartographical support and Olaf Merk for complementing the data.


Marc Bouchez

Fréderic Crampon

Bruno Le Naour

Christian Wilhelmi

Kai Bubenheim

Markus Kuhn

Bernd Mainzer

Jörg Riccius

Cecile Davoine

Jean-Francois F Justin

Jens Von Wolfersdorf

Meriam Abdelmoula

Victor F Villace

Johan Steelant

K Long

Combustor and Material Integration for

published or not. The documents may come

I. Introduction

For high-speed aircraft, material and cooling issues for both airframe and engine are the key elements which force the designer to limit the flight Mach number. The expected benefits of economical, high-performance and high-speed civil-aircraft designs that are being considered for the future will be realized only through the development of light-weight, high-temperature composite materials for structure and engine applications, to reduce weight, fuel consumption, and direct operating costs, as well as optimized structures and associated design methods.

The focus of the European Research program ATLLAS [START_REF] Steelant | Achievements obtained on Aero-Thermal Loaded Materials for High-Speed Atmospheric Vehicles within ATLLAS[END_REF]2 is on advanced light-weight, high-temperature material development strongly linked to a high-speed passenger aircraft design. ATLLAS stands for Aerodynamic and Thermal Load Interactions with Lightweight Advanced Materials for High Speed Flight.

The 4 years program ATLLAS-II is a logical continuation project built upon the experience and technology development gained within ATLLAS-I.

The previous ATLLAS study revealed in the end that the optimal cruise Mach number of such an high speed airliner could be around Mach 5 to 6. In ATLLAS2, a more detailed design and feasibility system study is performed here which aim to a globally optimized vehicle with respect to aerodynamic, propulsive, structural and thermal layout but nevertheless allowing restrictions imposed by emissions regulations and sonic boom mitigation. The validated tools developed previously along with the lessons learned allow the ATLLAS consortium (industry, research institutes and university laboratories) to further address and improve the multi-disciplinary design process of such future systems.

In parallel, a lot of effort is still foreseen to extend the precious built-up material database with durability characterization both for the aero-frame and combustor related structures. Also new materials and compositions are addressed to cope with limitations previously encountered.

II. Pin fin experiment

A ten times scaled Perspex channel is set at Stuttgart University to investigate in details pin fin channels with staggered configurations. The test rig is working under suction mode and is connected to a vacuum pump. An electrical mesh heater is used to generate a step change in the thermal boundary condition to achieve a significant and nearly constant fluid temperature jump. A schematic view of the test facility is shown in Figure 3. The main differences between these arrays are the relative span wise and stream wise spacing and the relative height. This parameter variation is achieved by changing the pin diameter. These test cases are summarized in table 1 and in Figure 4, showing preliminary CFD computations of the corresponding flows depending on flow pattern.

Tab. 1: Pin fin diameters and non-dimensional parameters of investigated cases

The heat transfer enhancement thanks to the pin fin arrays has as penalty a total pressure loss. The integral pressure drop is evaluated. Therefore the test section is instrumented with twenty two pressure tubes at the side wall as sketched in Figure 5. These pressure taps measure the static pressure, which corresponds to the total pressure at the wall. A differential pressure measuring system is used. acceleration in the pin fin wake. This is demonstrated with the echelon form of the pressure distribution between the pin fin rows. The higher the Reynolds number, the lower is the pressure level in the test section. The pressure loss can be quantified in terms of a dimensionless number, called friction factor and introduced with the following formula:

= Δ 2 2 (1)
The pressure loss Δ over rows is calculated from the linear regression between the measuring points 4 and 19. Consequently inlet and exit recovery effects are disregarded. The maximum averaged velocity is determined between two pin fins and is obtained according to the mass conservation law from the approach flow velocity and the specific configuration. The same velocity is used to determine the Reynolds number based on the pin diameter as characteristic length. The measurements are compared with the correlation of Jacob [START_REF] Jacob | Heat transfer and flow resistance in cross flow of gases over tube banks[END_REF] , that is valid for � ≥ 2 in the investigated Reynolds number range. This empirical relation is in good agreement with the experimental data as shown in Figure 7.

= ⎝ ⎜ ⎛ 0.25 + 0.1175 � -1� 1.08 ⎠ ⎟ ⎞ , -0.16
(2) The transient heat transfer measurement technique using Thermochromic Liquid Crystals (TLCs) on the Perspex walls is used to measure the heat transfer at the end wall. The lateral conduction is neglected due to the relatively short measurement time and the low thermal conductivity of the used Perspex material. The conduction process into the wall can then be considered one-dimensional and the wall analytical treated as being semi-infinite for the conduction process during the measurement period.

The transient measurement technique provides accurate two dimensional heat transfer distributions. The experiments start at the equilibrium state, in which the complete test section is initiated at a constant uniform temperature. The transient technique examines the response of the wall temperature to a sudden change in the fluid temperature. An abrupt temperature rise of the main stream is generated with the mesh heater so that temperature gradients between the fluid and the wall arise. The surface temperature is monitored using TLCs. The green color stands for the calibrated indication temperature and is tracked by a high resolution CCD camera.

An analytical 1D-solution of Fourier's heat conduction equation for the solid wall (index s) describes the time response of the surface temperature for a semi-infinite wall:

= 2 2
(3)

The material properties are assumed to be constant. The differential equation is then solved with the following initial and boundary conditions:

= 0: = 0 , �⎯� ∞: = 0 (4) = 0: -� � = ℎ( -) (5)
The surface is represented with y=0 and h refers to the local heat transfer coefficient.

By assuming a semi-infinite wall with a convective boundary condition, constant material properties, constant local heat transfer coefficient h , negligible lateral conduction effects and for an ideal fluid temperature step, the solution for the surface temperature ( ) at the end wall is obtained as:

() -0 -0 = 1 -� ℎ 2 � � ℎ√ � � (6)
The reference fluid temperature is chosen to be the fluid bulk temperature. Duhamel's superposition principle is applied to account for variations compared to an ideal temperature step especially at down-stream positions due to the upstream heat exchange. The local fluid bulk temperature history is therefore approximated by many small ideal temperature steps ( , 1) -∆ f jj T between two consecutive time steps 1 -j τ and j τ .

() -0 = ∑ [1 -exp ( 2 ) erfc(β)]∆ (,-1) =1 with = ℎ� - (7)
In equation (7) all variables are known or measured except for the heat transfer coefficient h. The USTUTT/ITLR in-house code "ProTeIn" solves the equation ( 7) iteratively and a detailed two dimensional heat transfer distribution is then obtained. The evaluation technique applied in this work is explained in detail in [START_REF] Poser | Transient liquid crystal thermography in complex internal cooling systems[END_REF] .

An example for the distribution of the local heat transfer coefficient on the end wall is illustrated in Figure 8. This contour plot provides evidence that the horseshoe vortex contributes significantly to the end wall heat transfer in pin fin arrays. In the first and second rows, the horseshoe vortices are less developed and their footprint can be barely distinguished. The overall local distribution of the heat transfer coefficient is quite symmetric and homogenous. The third to the fifth pin fin rows play a crucial role in the heat transfer enhancement. Remaining cases are analogically investigated for different Reynolds numbers and inlet conditions.

III. Flat sample long duration testing of CMC materials

Flat sample testing is performed at the ERBURIG K test facility of Airbus Group Innovations in Ottobrunn (Germany) [START_REF] Schmidt-Wimmer | Evaluation of Ultra High Temperature Ceramics and Coating-Systems for their Application in Orbital and Air-Breathing Propulsion[END_REF] . The test facility uses a combustion of kerosene with oxygen (O 2 ) to generate a hot gas comparable in temperature, composition and velocity to actual rocket combustion chamber atmospheres. The temperature range for the CMC flat samples is 1050°C -1700°C (measured in the hottest area using a pyrometer). The test duration varies between "short" term testing (1 hour) and "long" term testing (4 hours), split in 30 minute intervals for evaluation. During testing the samples undergo optical and weight evaluation after every interval (Figure 10). Once a major deterioration is visible (before the end of the planned test duration), the test is stopped for metallographic investigation.

Figure 10 : example of CMC (C/C-SiC with CVD-SiC coating) test sample tested at ERBURIG K test facility

Several ceramic matrix composites from DLR and Airbus Group Innovations have been considered for ERBURIG K testing to check their performance at high temperature in an oxidative environment. As DLR materials, C/C-SiC (with and without CVD-SiC coating), SiC/SiCN (in two fibre-coated variants) and WHIPOX (in two coated variants) have been investigated.

For SiC/SiCN composites, pyrolytic carbon (PyC) and monazite (LaPO 4 ) were chosen as interface material; they were "long" term tested at temperatures of 1300, 1500 and 1700°C. In general, the samples with PyC-coated fibres showed higher mass losses due to active oxidation of the coating. SEM pictures indicated that this behavior already occurred at 1300 °C. The thereby emerged gaps between fibres and SiCN-Matrix were filled with SiO 2 due to an oxidation of the SiC/SiCN system (Figure 11). This behaviour is strongly depending on exposed temperature levels and distances from the outer sample surface. At 1500°C and above, the coating was completely removed and filled with SiO 2 . In case of the LaPO 4 -coated fibre composites, the coating close to the surface already started to degrade at 1300°C and a SiO 2 scale could be observed between fibres and coating. As depicted in Figure 12, the entire LaPO 4 coating was degraded at 1700°C. At the center of the samples however, the coating stayed intact (Figure 13). X-ray diffraction showed a formation of La 2 Si 2 O 7 and SiO 2 at the sample surfaces at temperatures of 1500°C and above, forming a cracked, mixed scale (Figure 14). An example of preliminary computation of the ERBURIG K test on a generic flat sample by ESA/ESTEC is shown below. The commercial code CFD-ACE is used in these current calculations. The SST k-Ω turbulence model is considered here as the best tradeoff between solution accuracy and computational effort and therefore remains as the chosen model for the computations. A laminar simulation of the ERBURIG K test conditions has been performed so far, in Figure 15. This solution serves as baseline for comparison with the turbulent calculations and also to initialize the turbulent simulations. The achievement of a turbulent simulation of the flow field in the ERBURIG K facility is an ongoing work. The final goal will be to compare the available measurements with the corresponding computed data. Elevated temperature permeability measurements with GN2 and GH2

In addition to ambient temperature permeability measurements for C/C, OXIPOL and WHIPOX samples at DLR Lampoldshausen and other test facilities in the framework of the ATLLAS I project [START_REF] Bouchez | Multi-level Coupled Simulations of Cooled Structures in the ATLLAS European Programme[END_REF] , recently some C/C and OXIPOL samples have been tested at elevated temperatures in order to later characterize the permeability (Darcy and Forchheimer coefficients) of theses samples with different fluids in the framework of the ATLLAS II project. In comparison to the previously used ambient temperature permeability test set-up, the elevated temperature permeability test set-up additionally contains a test fluid heating system (based on copper tubes wound around cartridge heaters as shown on the left-hand side of Figure 16) as well as a flow probe heating system (based on flexible line heaters) as shown on the right-hand side of figure 8. Before the start of each elevated temperature permeability test, the (closed loop) control units of the fluid heater and the sample heater are set to the intended temperature levels. During the test, a variation of the mass flow rate trough the flow probe and the total pressure level in the flow probe is obtained by a wide variation of the opening levels of control valves situated upstream and downstream of the flow probe as indicated on the left-hand side of Figure 17. As the main test objective is to obtain a large number of stationary values for the mass flow rate, the total pressure level as well as the differential pressure between the inlet and the outlet of the flow probe, pre defined control valve opening setting sequences are used insead of a closed loop control of the valves. A photograph of the complete test set-up is shown on the right-hand side of Figure 17.

V. Cooled CMC micro combustion chamber long duration testing

To evaluated the geometric effect of actual combustion chamber in addition to the material behavior of the CMC in the combustion atmosphere, micro combustion chambers are manufactured using the PTAH-SOCAR technology 6, 8, 10 .

Using the ERBURIG K test facility of Airbus Group Innovations in Ottobrunn (Germany), the cooled CMC combustion chamber are tested for long duration (hours) instead of minutes [START_REF] Schmidt-Wimmer | Evaluation of Ultra High Temperature Ceramics and Coating-Systems for their Application in Orbital and Air-Breathing Propulsion[END_REF] . The test set-up is shown in Figure 18. The combustion chamber is encapsulated with a metal housing for gas tightness and cooled using gaseous nitrogen (GN 2 ). Further downstream the metal throat is cooled by water.

HSS cooled testing in scramjet environment

Based on the work performed in ATLLAS1 [START_REF] Steelant | Achievements obtained on Aero-Thermal Loaded Materials for High-Speed Atmospheric Vehicles within ATLLAS[END_REF][START_REF] Bouchez | Multi-level Coupled Simulations of Cooled Structures in the ATLLAS European Programme[END_REF] and in current WP3, a generic transpiration cooled metallic panel with hollow sphere core [START_REF] Bouchez | Multi-level Coupled Simulations of Cooled Structures in the ATLLAS European Programme[END_REF][START_REF] Davoine | Metallic hollow sphere structures manufacturing process[END_REF] will be tested in the METHYLE test facility [START_REF] Falempin | METHYLE -A new long endurance test facility fordual-mode ramjet technologies[END_REF] .

Many characterizations and computations are made to prepare this test series. The HSS panel is composed with a box with thermocouples (T/C) and coolant input and collector at the bottom part. The porous hot wall is in contact with the hot gases, with a low permeability. The hollow sphere core is welded to it and has a higher thickness and a higher permeability. The shell thickness of the hollow spheres (diameter = 2 mm) is around 50µm, and the resulting apparent density is 0,6 g/cm³. This multifunctional sandwich is then composed fo tow layers of different porous material. The permeabilities of the two porous materials are very dissimilar: around 5.10 -11 m2 for the thin porous layer in contact with hot gases, and around 100 times greater the for hollow spheres material, core of the sandwich material.

Before METHYLE hot test, it has been tested in cold and hot conditions with and without coolant. Cold GN2 calibration was realized at ONERA to check the pressure drop versus mass flow characteristic.

MBDA has realized hot test series without any flow (no coolant, no hot gaz) in unsteady manner under Infra-red lamps facility. This thermal facility is able to heat such a sample during a few seconds under a heat flux of 1MW/m². The results (Figure 21) allowed us to document the behaviour of the double layer porous panel and would especially allow to estimate the effective diffusivity with stagnant air inside.

VII. UHTC and CMC injection struts

Based on different projects and published results, and on the test facilities capacities, a generic diamond shape injection strut was defined. Different technologies of cooling (convective, transpiration), different materials (UHTC and CMC) will be applied to manufacture such samples. Concerning the UHTC materials which are studied at ONERA 11 , three compositions based on diborides compounds [START_REF] Fahrenholtz | Refractory Diborides of Zirconium and Hafnium[END_REF] have been investigated in the Workpackage 3 where new solutions are developed to deal with external or internal components of the ATLLAS vehicle. The Field Assisted Sintering Technology (or Spark Plasma Sintering) has been selected to manufacture these ceramics and a thorough characterization of basic properties has been carried out on the sintered discs.

An important work has been done on the design of the UHTC injector in order to determine several crucial aspects of this component : external shape, internal design (fuel pipe …), joining between the different parts (UHTC injector, CMC basis, metallic chamber). Furthermore, some modeling have been carried out to simulate realistic environment (scramjet functioning). Moreover, some machining trials have been done to demonstrate the feasibility of this type of component (WEDM, diamond machining, drilling).

VIII. Conclusion

The ATLLAS2 program gives the opportunity to investigate at academic level, both in basic and relevant environment, different solutions possibly usable to ensure the long range cruise of a high speed airliner.

Different materials (UTHC, CMC, metallic) and different cooling techniques (radiation, convective, transpiration) are studied. Available numerical or semi-empirical tools are used to prepare the test, to design the different architectures. Test facilities are used from laboratories to industrial environments.

First results in hot conditions are already available but test are going on up to the end of the Research Program, planned in 2015.

  duration test facility allow to characterize different ceramic matrix composite uncooled samples and will allow to realize, at small scale, a long duration (several hours) investigation of cooled ceramic structure in PTAH-SOCAR technology. A multifonctionnal metallic transpiration cooled HSS panel using Hollow Spheres Stacking as core material was designed and preliminary tested in cold conditions with GN2 and in hot conditions with infra red lamps under 1 MW/m² heat flux. CMC and UHTC materials are used to design, manufacture and test generic fin injectors usable in high speed combustors. Industrial hypersonic METHYLE test facility is used to test in relevant Mach 6 combustor environment HSS panel as well as advanced fin injectors. Hot and cold permeability of composites is documented with GN2 and GH2. Numerical models are used in accordance with the experiments, some examples are given in the present paper. Nomenclature C/C-SiC = carbon fibre reinforced silicon carbide CFD = computational fluid dynamics CMC = ceramic matrix composite ERBURIG K = Environmental Relevant Burner Rig -Kerosene HSS = Hollow Spheres Stacking (sandwich) GN 2 = gaseous nitrogen METHYLE= French acronym for long duration hypersonic technology test facility O 2 = oxygen SiC/SiCN = silicon carbide fibre reinforced silicon carbonitride TLC = Thermochromic Liquid Crystals UHTC = ultra high temperature ceramic USTUTT/ITLR = University of Stuttgart WHIPOX = wound highly porous oxide

A

  generic aircraft design for high-speed flight is then retained as a guideline in the Work Package 2 of the Program. Its cruise Mach number target is close to 6. The Work Package 4 of the current Program is entitled Combustor and Material Integration and is described in the present paper. Its objective is to study material and structures for combustors and advanced hot structures while WP1 covers with project management, WP3 deals with material development and characterization and WP5 with numerical research and boundary layer transition experiments. The work package WP4 is split up into 3 tasks dealing with: • Task 4.1. Durability for Combustion loaded Non-Metallic Liners 4.1.1 Flat and PTAH-SOCAR axisymetric samples in long duration test in ERBURIG K test facility 4.1.2 Permeability measurements in DLR Lampoldshausen at elevated temperatures, in addition to existing ambient temperature characterization of these porous structures 4.1.3 Pin fin channel well documented in an USTUTT experiment • Task 4.2. Durability for Combustion loaded Panels with Metallic Cores (hollow spheres) • Task 4.3. Integration and Testing of (Un)Cooled Injectors (CMC or UHTC material) WP4 work is connected with the other work packages of ATLLAS-2 : System studies from ATLLAS-1 and WP2 results will be used as guidelines to define the architecture and the test condition of WP4 samples.

Figure 1 :

 1 Figure1: system studies as a guideline for structures to be used in different sub-systems WP4 will use the materials enhanced and characterized in WP3. WP4 will provide a data base for numerical simulations in WP5.2 WP4 material and structures investigated in WP4 will be hot tested in laboratory and later in realistic environment : long duration test in the ERBURIG K test facility at Airbus Group Innovations[START_REF] Schmidt-Wimmer | Evaluation of Ultra High Temperature Ceramics and Coating-Systems for their Application in Orbital and Air-Breathing Propulsion[END_REF] in Ottobrunn (Germany) and industrial METHYLE dual-mode ramjet test facility[START_REF] Falempin | METHYLE -A new long endurance test facility fordual-mode ramjet technologies[END_REF] at MBDA Bourges (France).

Figure 2 :

 2 Figure 2 : Gantt chart and main test and interaction of WP4 in ATLLAS2 program

Figure 3 :

 3 Figure 3 : Schematic view of the test facility in Stuttgart University Three different staggered pin fin configurations are experimentally and numerically investigated.

Figure 4 :

 4 Figure 4 : different pin fin pattern studied (here CFD results) within warm air experiment

Figure 5 :

 5 Figure 5 : Staggered pin fin array pattern and photograph of the test section An example of the pressure distribution across the pin fin array is depicted in Figure 6 for different Reynolds numbers for Case 2. The pressure distribution is marked by likely constant values at the inlet (positions 1 to 3) and outlet (positions 20 to 22) sections. The flow is experiencing alternatively deceleration at the stagnation region and

Figure 6 :

 6 Figure 6: Pressure distribution at the side wall of the pin fin array, Case 2

Figure 7 :

 7 Figure 7: Validation of friction factor versus Jacob correlation [1] for Case 2

Figure 8 :

 8 Figure 8 : Two dimensional local distribution of the normalized measured heat transfer coefficient, Case2, Re D,max =25000

Figure 9 :

 9 Figure 9 : flat sample testing at ERBURIG K test facility at Airbus Group Innovations, Ottobrunn (Germany)

Figure 11 :Figure 12 :

 1112 Figure 11: PyC-coated fibre sample, tested at 1300°C, center of the sample; the PyC coating was removed due to active oxidation Figure 12: LaPO 4 -coated fibre sample, tested at 1700°C, close to surface

Figure 15 .

 15 Figure 15. Example of preliminary 3D CFD simulation of the ERBURIG K test by ESTEC

Figure 16 :

 16 Figure 16 : Additional devices neccessary for measuring the material permeability under elevated temperatues.

Figure 17 :

 17 Figure 17 : Flow scheme (left) and photograph (right) of the elevated temperature permeability measurement set-up.

Figure 18 :

 18 Figure 18: cooled CMC duct for long duration testing in ERBURIG K test facility at Airbus Group Innovations in Ottobrunn, Germany

Figure 19 :

 19 Figure 19 : panel testing at the exit of scramjet in METHYLE test facility A generic HSS panel was designed and manufactured for cold and hot characterization.

Figure 20 :

 20 Figure 20: HSS panel to be tested in METHYLEThe HSS panel is composed with a box with thermocouples (T/C) and coolant input and collector at the bottom part. The porous hot wall is in contact with the hot gases, with a low permeability. The hollow sphere core is welded to it and has a higher thickness and a higher permeability. The shell thickness of the hollow spheres (diameter = 2 mm) is around 50µm, and the resulting apparent density is 0,6 g/cm³.

Figure 21 .

 21 Figure 21. Infra red testing at MBDA of HSS panel METHYLE test series are planned with and without combustion under Mach 6 flight conditions in second part of 2014.

Figure 22 :Figure 23 :

 2223 Figure 22 : examples of injection struts taken into account to define the ATLLAS2 generic fin injector

Figure 24 :

 24 Figure 24 : Assessment of the thermal levels (in K) in the ATLLAS2 injector (during test)Manufacturing of injectors began with the machining of existing blocks of CMC from DLR or UHTC from ONERA. ONERA checked its capacity to manufacture them from 12 mm thick UHTC disks of 60 mm diameter, with the chosen compositions.

Figure 25 .

 25 Figure 25. UHTC materials for fin injector machiningHot tests of these different types of fin injectors are programmed at the end of 2014. They are planned to be fed with H2 or a mixture of H2 and CH4.

min

Acknowledgments

This work was performed within the 'Aero-Thermodynamic Loads on Lightweight Advanced Structures II' project investigating high-speed transport. ATLLAS II, coordinated by ESA-ESTEC, is supported by the European Union within the 7th Framework Programme Theme 7 Transport, Contract no.: ACP0-GA-2010-263913. Further info on ATLLAS II can be found on http://www..esa.int/techresources/atllas_II.


T Vandellos
email: thomas.vandellos@onera.fr

E Martin

D Leguillon

COMPARISON BETWEEN COHESIVE ZONE MODELS AND A COUPLED CRITERION FOR PREDICTION OF EDGE DEBONDING

Keywords: Failure analysis, Finite Fracture Mechanics, Cohesive Zone Model

The onset of edge debonding within a bonded specimen submitted to bending is modeled with two numerical approaches: the coupled criterion and the cohesive zone model. The comparison of the results obtained with the both approaches evidences that (i) the prediction of edge debonding strongly depends on the shape of the cohesive law and (ii) the trapezoidal cohesive law is the most relevant model to predict the edge debonding as compared with the coupled criterion.

Introduction

The prediction of the onset of interface debonding is generally performed using a stress criterion [START_REF] Brewer | Quadratic stress criterion for initiation of delamination[END_REF] or linear fracture mechanics [START_REF] Waddoups | Macroscopic fracture mechanics of advanced composite materials[END_REF]. In each case, a characteristic length is needed which has to be identified experimentally. A first alternative approach is the use of a cohesive zone model [START_REF] Needleman | A continuum model for void nucleation by inclusion debonding[END_REF] which simulates a progressive debonding build up in terms of continuum damage variables. It was shown by previous authors that the shape of the cohesive zone model (for a given value of the fracture toughness) does not have any influence on the steady state propagation of a rectilinear crack [START_REF] Jean | Non-smooth contact dynamics approach of cohesive materials[END_REF]. This result does not hold at crack initiation for which the shape of the cohesive law has a strong influence [START_REF] Jean | Non-smooth contact dynamics approach of cohesive materials[END_REF]. A second alternative approach is the coupled strength and energy criterion [START_REF] Leguillon | Strength or toughness? A criterion for crack onset at a notch[END_REF] which permits the prediction of the applied load involving the onset and the associated crack nucleation length. This approach has proved to be successful to analyze the onset of fracture mechanisms within composite materials [START_REF] Mantic | Interface crack onset at a circular cylindrical inclusion under a remote transverse tension. Application of a coupled stress and energy criterion[END_REF][START_REF] Martin | A twofold strength and toughness criterion for the onset of free-edge shear delamination in angle-ply laminates[END_REF][START_REF] Martin | A coupled strength and toughness criterion for the prediction of the open-hole tensile strength of a composite plate[END_REF] and bonded specimens [START_REF] Moradi | Strength Prediction of Bonded Assemblies Using a Coupled Criterion under Elastic Assumptions: Effect of Material and Geometrical Parameters[END_REF][START_REF] Nguyen | Bond failure of a SiC/SiC brazed assembly[END_REF]. An extension to the 3D geometry has been recently proposed [START_REF] Leguillon | An attempt to extend the 2D coupled criterion for crack nucleation in brittle materials to the 3D case[END_REF] but, at the moment, the use of cohesive zone models seems to be easier. The aim of this paper is to assess the capabilities of cohesive zone models to predict the failure onset of a bonded specimen submitted to bending as compared with the coupled criterion. In this study, it is important to note that the coupled criterion is just considered as a numerical reference solution; but this assumption should be experimentally verified. First, the both approaches are described. Second, several shapes of the cohesive law are compared to the coupled criterion in order to show their influence on the prediction. Finally, with the most relevant shape, the influence of the interfacial properties on the prediction of the edge debonding is presented to conclude about the use of the cohesive zone model.

ECCM16 -16

The coupled criterion

As proposed by a previous author [START_REF] Leguillon | Strength or toughness? A criterion for crack onset at a notch[END_REF], combining an energy and a stress condition allows to derive an initiation criterion in the vicinity of a stress concentration. First, an energy balance between an elastic state prior to any crack growth and after the onset of a crack extension of area δS leads to the following incremental energy condition:

      C inc G δS a W 0 W a G    (1)
where W(0) is the potential energy at the initial state (without crack), W(a) is the potential energy at the final state (with a crack of length a) and G inc is the incremental energy release rate in which the infinitesimal energy rates of the classical Griffith approach are replaced by finite energy increments. Second, a stress condition states that the normal out-of-plane stress σ along the anticipated path of crack nucleation is greater than the relevant strength σ C  

a x for σ x σ C   (2)
Finally, for a monotonic and increasing applied loading, the crack increment and the applied load at nucleation are obtained by combining the equations ( 1) and (2).

The cohesive zone models

Cohesive zone models are used to describe the behavior of interfaces. More precisely, the traction in mode I (i.e. the opening mode of fracture), T 1 (resp. T 2 in mode II (shearing mode)), between the top and bottom surfaces of the interface is related to the relative displacement in mode I δ 1 (resp. δ 2 in mode II). Several shapes of cohesive law (bilinear, trilinear and trapezoidal [START_REF] Vandellos | Proposition of a framework for the development of a cohesive zone model adapted to Carbon-Fiber Reinforced Plastic laminated composites[END_REF]), including an elastic part, can be obtained using the constitutive law written as

                  λ - 1 Kδ T Kδ α T 0 δ if λ - 1 Kδ T λ - 1 Kδ T 0 δ if 2 2 1 C 1 1 2 2 1 1 1 (3)
where K is the initial stiffness of the interface, α C is a penalization factor for out-of-plane compression, λ is the damage variable, related to the damage kinetics, and f(λ) represents the effect of damage. The evolution of the damage variable λ is defined by

0 λ and δ δ if ,1 ) δ δ(δ δ) (δ δ α ) δ δ(δ min λ δ δ δ if ) δ δ(δ δ α δ δ) (δ λ δ δ if 0 λ * f * f 0 σ f * * 0 * 0 0 σ * 0 0                                  ( 4
  2 2 2 1 δ δ δ    (5)
where < . > + are the classical Macaulay brackets defined by

  x 0, max x   (6)
It should be noted that, in order to avoid damage under pure out-of-plane compressive normal stress, the normal relative displacement  1 is only taken into account when positive. Parameters δ 0 and δ f are material constants corresponding respectively to the relative displacement associated with the damage threshold σ C and the interfacial stiffness K, and to the relative displacement attained when the energy release rate G is equal to the fracture toughness G C . The threshold δ * is the relative displacement associated with the maximal damageable stress σ * which represents the admissible stress on the interface at the end of the first part of the damage process. It is defined by σ * =  σ C where   is a shape parameter. The value of δ * depends on the shape parameter  δ . The couple ( δ,    defines where the negative slope of the cohesive law can changed. Finally, the relative displacements δ 0 , δ * and δ f , represented in Figure 1, are defined by The shape of the cohesive law only depends on the values of the both shape parameters ( δ ,  σ ) which are equal to (0,1) for the bilinear law and ( δ , for the trapezoidal one. The three laws are illustrated in Figure 2 for identical interfacial properties (σ C , G C ). In the next paragraphs, the value of the relative displacement δ 0 has been imposed to 1.10 -5 mm for all simulations. It is important to note that the parameter δ 0 is just considered as a numerical parameter. Moreover, two tri-linear shapes have been used, with ( δ ,  σ ) = (,) and

                     0 σ δ C 0 C C f σ 0 f C 0 C C δ 0 f δ 0 * C 0 δ ) α (α 0.5σ δ 0.5σ G δ α ) δ (δ 0.5σ δ 0.5σ G α with ) δ (δ α δ δ /K σ δ (7)

Prediction of edge debonding: coupled criterion versus cohesive zone models

In order to compare the two numerical approaches, simulations of a bonded specimen submitted to four-point flexure loading have been performed. Cohesive zone models and the coupled criterion are here applied to analyze the initiation of fracture mechanisms near the free edge between the bond and the The geometry of the specimen is schematized in Figure 3. It consists of two substrates with the same thickness h = 2 mm bonded with a thin interlayer which is here neglected. When the cohesive models are used, interface elements are inserted between the two bonded substrates. The elastic properties of the substrates are selected to be E S = 400 GPa (Young's modulus) and ν S = 0.2 (Poisson's ratio). Due to the symmetry of the loaded specimen, it is assumed that the onset occurs near each free edge. A bidimensional finite element procedure with strongly refined mesh is used to derive the results which are now presented. This result can be explained comparing the damage kinetics of the different cohesive laws. Indeed, contrary to the bilinear and the tri-linear laws where the evolution of the damage variable λ between 0 (unbroken state) and 1 (broken state) is continuous and relatively slow, the damage variable of the trapezoidal model can evolve very quickly when the relative displacement  exceeds  * (i.e. at the end of the plateau). This kinetics involves that the process zone (i.e. the area where the damage variable is positive but smaller than 1) with the trapezoidal law is smaller than the ones obtained with the bilinear and the tri-linear shapes, as shown in Figure 5. This observation could explain the better correlation obtained with the coupled criterion. Nevertheless, it is important to note that this correlation reduces when the fracture toughness increases (Figure 6 and Figure 7). This phenomenon results from the variation of the process zone length l cz . Indeed, for the same interfacial strength σ C , the larger the fracture toughness, the longer the length. Thus, the softening behavior becomes noticeable when the normalized crack length l cz /L is higher than 10%. Consequently, as indicated in Table 1 and Table 2, the percentages error of the fracture displacement dc and of the fracture load Fc exceed 5% as the consequence of softening (i.e. when G C > 15 J/m² for σ C = 1 MPa and when G C > 1000 J/m² for σ C = 10 MPa).

Conclusion

Simulations of a four-point bending test, for the prediction of edge debonding, with a coupled criterion and several cohesive zone models have been realized. First, the influence of the shape of the cohesive law on the prediction of the onset has been shown. The trapezoidal model appears the most adapted model to predict the initiation in a similar manner to the coupled criterion. Second, it has been demonstrated that the length of the process zone has to be small enough to verify a good correlation between the results obtained by the both ECCM16 -16 TH EUROPEAN CONFERENCE ON COMPOSITE MATERIALS, Seville, Spain, 22-26 June 2014 8 numerical approaches. Therefore, it seems possible to use the trapezoidal cohesive zone model, under a few material conditions, to predict precisely the edge debonding. A comparison between numerical and experimental results will be realized in future works in order to confirm the relevance of the trapezoidal cohesive zone model.

Figure 1 .

 1 Figure 1. Representation of the relative displacements of the constitutive law from a tri-linear cohesive model

4 (

 4  δ ,  σ ) = (,), and the shape parameter of the trapezoidal law has been imposed to  δ = .

Figure 2 .

 2 Figure 2. Illustration of the bilinear, tri-linear and trapezoidal shape of the cohesive zone model for the same interfacial properties

Figure 3 .

 3 Figure 3. The geometry of the specimen tested under four-point bending

Figure 4 .

 4 Figure 4. Comparison of load versus displacement curves between the coupled criterion (CC) and different cohesive zone models (CZM) with σ C = 1 MPa and (a.) G C = 1 J/m², (b.) G C = 5 J/m², (c.) G C = 15 J/m², (d.) G C = 60 J/m²

6 Figure 5 .

 65 Figure 5. Normalized length of the process zone l cz /L versus the fracture toughness G C (a.) with σ C = 1 MPa and (b.) with σ C = 10 MPa.

Figure 6 .Figure 7 .

 67 Figure 6. Comparison of load versus displacement curves between the coupled criterion (CC) and the trapezoidal cohesive zone model (CZM) for several fracture toughness with σ C = 1 MPa

TH EUROPEAN CONFERENCE ON COMPOSITE MATERIALS, Seville, Spain, 22-26 June 2014 2

) ECCM16 -16 TH EUROPEAN CONFERENCE ON COMPOSITE MATERIALS, Seville, Spain, 22-26 June 2014 3

The relative displacement δ is determined by

Table 1 .

 1 Percentages error of the fracture displacement d C and the fracture load F C obtained by the trapezoidal cohesive zone model with σ C = 1 MPa

	G C (J/m²)		100 500	1000	3000	8000
	    100	d	CC C	 CC C d d	CZM c	   	4.7	2.4	1	-1.8	-6.4
	    100	CC C F	 CC C F F c CZM	   	1.8	2.2	3.8	7.2	11.6

Table 2 .

 2 Percentages error of the fracture displacement d C and the fracture load F C obtained by the trapezoidal cohesive zone model with σ C = 10 MPa