Olivier Barge

Diaa Albukaai

Manfred Boelke

Kévin Guadagnini

Emmanuelle Régagnon

Rémy Crassard

New Arabian desert kites and potential proto-kites extend the global distribution of hunting mega-traps

Keywords: Desert kites, Saudi Arabia, typology, geographical distribution, mega-traps, hunting techniques

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

In recent years, several field expeditions have contributed to documenting and excavating desert kites, these gigantic archaeological constructions that have long remained mysterious or subject to controversy. These teams, which include the Globalkites Project (see www.globalkites.fr), have proposed chronological markers showing that kites were used over a very long period of time, ranging from the late Neolithic to recent periods (Abu-Azizeh et al., in press, al Khasawneh et al., 2019[START_REF] Barge | The 'desert kites' of the Ustyurt Plateau[END_REF][START_REF] Brochier | Kites on the margins: the Aragats kites in Armenia[END_REF]. Furthermore, their function as hunting "mega-traps" for herds of wild ungulates has been clearly demonstrated [START_REF] Abu-Azizeh | Discovery and excavation of desert kites in the southeastern badia of Jordan[END_REF]. The geographical range of kites is for its part documented by the observation of high-resolution satellite images. A total of 6,023 kites were counted in a recent study [START_REF] Barge | Desert-kites et constructions apparentées: découvertes récentes et mise à jour de l'extension géographique[END_REF], extending from the Aralo-Caspian zone, the steppes and desert east of the Levant to western Arabia. However, new discoveries have been made in the past two years (2020 and 2021) in Saudi Arabia. They concern desert kites in sectors where they had never been identified before, and constructions with an original morphology which do not comply with the strict definition of a kite -a combination of long walls converging to an enclosure surrounded by peripheral pit-traps -because they are not closed structures in the same way as desert kites stricto sensu. However, they display many kite attributes and are undoubtedly related to them. We thus refer to these constructions as Arabian "open kites". They were previously mentioned as "pseudo-kites" (see [START_REF] Barge | Desert-kites et constructions apparentées: découvertes récentes et mise à jour de l'extension géographique[END_REF], and their number has since increased from 320 to 835. The new and more complete corpus obtained from satellite imagery enables us to characterise their morphological features as well as their location in the landscape. Furthermore, the observation of several of these open kites during a field campaign in spring 2021 in Khaybar, western Saudi Arabia, and the implementation of remote sensing resulted in an enhanced description of this new type of construction. The observations made in Khaybar in 2021 confirm some first analyses on the satellite images. Here, we propose a preliminary assessment of the archaeological structures from the ground, and their patterns.

Desert kites stricto sensu comprise varied morphological characteristics [START_REF] Barge | Morphological diversity and regionalization of kites in the Middle East and Central Asia[END_REF] across the entire geographical area of extension, and intra-regional similarities have been distinguished. Yet, recent discoveries in Arabia do not only concern Arabian open kites, but also other desert kite types with sizes, shapes and layout details that can be linked to families of kites identified elsewhere. For this reason, these new desert kites are of great interest even if they are less numerous (n=87) than Arabian open kites.

A regionalization of kite families is now taking shape in Arabia, to the south of the geographical area of global kite distribution and is filling some previous gaps in regions between the Negev and Khaybar. On account of the scarcity of available absolute dating and the difficulties involved in obtaining dates [START_REF] Crassard | Addressing the desert kites phenomenon and its global range through a multi-proxy approach[END_REF], this improved regionalization provides crucial data on the phenomenon of the diffusion of kites and associated hunting techniques.

Methods

The methods used here are based primarily on the identification and observation of kites on highresolution satellite images. These images allow us to cover large areas and to analyse the spatial distribution of kites on a wide scale. With these images, it is also possible to obtain fine-scale information and thus to characterise kite morphology. It is even sometimes feasible to define a relative chronology in the case of reuse or overlapping with other archaeological structures. However, data from field investigations remain essential; they are complementary in terms of the scale and type of data collected and allow for a more detailed analysis of architectural traditions. Fieldwork is also crucial in order to better define the chronology of constructions. This is identifiable in particular by the search for dating elements or the observation of the state of preservation of the structure.

All this information must be based on a systematized methodology, in order to be perfectly applicable and comparable. This methodology has already been presented on several occasions, but requires restating here to avoid misunderstandings [START_REF] Groucutt | Mass-kill hunting and late quaternary ecology: new insights into the 'desert kite' phenomenon in Arabia[END_REF]. The Globalkites Project research team (e.g. [START_REF] Brochier | Kites on the margins: the Aragats kites in Armenia[END_REF][START_REF] Crassard | Addressing the desert kites phenomenon and its global range through a multi-proxy approach[END_REF][START_REF] Barge | Morphological diversity and regionalization of kites in the Middle East and Central Asia[END_REF] proposed the following definition of a kite: it is composed of long walls or stone alignments (driving lines, or antennae) which converge towards an enclosure equipped with pits along the external periphery (pittraps, or cells). It should be noted here that one of the objectives of the Globalkites Project was to examine the function of kites and that a functionally neutral terminology was therefore adopted at the outset of the project. As the function of kite components has now been established [START_REF] Abu-Azizeh | Discovery and excavation of desert kites in the southeastern badia of Jordan[END_REF], here we will use the terms "driving lines" and "pit-traps". Based on the combined presence of these three components (driving lines, enclosure, pit-traps), it is possible to distinguish kites from other mega-traps known in the world from the Palaeolithic to the modern era (e.g. [START_REF] Brink | Inukshuk: caribou drive lanes on southern Victoria Island, Nunavut, Canada[END_REF]Leondorf and Stone, 2006;[START_REF] Santiago | Guanaco hunting strategies in the northern plains of Tierra del Fuego, Argentina[END_REF][START_REF] O'shea | Nobody knows the way of the Caribou": Rangifer hunting at 45° North latitude[END_REF][START_REF] Lemke | Literal niche construction: Built environments of hunter-gatherers and hunting architecture[END_REF][START_REF] Imamura | Prehistoric Japan, New perspectives on insular East Asia[END_REF][START_REF] Sato | Late Pleistocene trap-pit hunting in the Japanese Archipelago[END_REF][START_REF] Blehr | The need for ethnographical analogies in the study of prehistoric caribou/wild reindeer hunting: A case study from Sumtangen, Norway[END_REF][START_REF] Jordhøy | Ancient wild reindeer pitfall trapping systems as indicators for former migration patterns and habitat use in the Dovre region, southern Norway[END_REF][START_REF] Olsen | Chasse, culte ou artisanat¬? Les fosses "à profil en Y-V-W" ; Structures énigmatiques et récurrentes du Néolithique aux âges des Métaux en France et alentour. Châlons-en-Champagne[END_REF]. This definition also backs up the hypothesis that kites are the material evidence of the diffusion of a hunting technique. An approximate definition would compromise the examination of this hypothesis by running the risk of integrating distinct devices and techniques. A strict definition does not, however, exclude exceptions, to be examined on a case-by-case basis. On the one hand, topographical configurations can make driving lines or enclosures superfluous, as is the case for some kites in Armenia [START_REF] Barge | Hunting or pastoralism ? Comments on 'seasonal use of corrals and game traps (desert kites) in Arménia[END_REF] or on the Ustyurt Plateau, Kazakhstan [START_REF] Barge | The 'desert kites' of the Ustyurt Plateau[END_REF]). On the other hand, from the point of view of the diffusion phenomenon, the kite-hunting technique may have been adapted on a local or temporal scale to structures lacking one of the three attributes. Conversely, kites can be the result of earlier hunting techniques using traps with less systematic architecture. However, the pit-trap is a compulsory attribute. Indeed, the pit-trap has been clearly identified by excavations as the final trapping feature [START_REF] Abu-Azizeh | Discovery and excavation of desert kites in the southeastern badia of Jordan[END_REF]. It cannot be defined as a small subsidiary enclosure (contra [START_REF] Groucutt | Mass-kill hunting and late quaternary ecology: new insights into the 'desert kite' phenomenon in Arabia[END_REF], but as a dug-out pit, which the inner side was carefully lined (or stone-faced) with rows of stones, and into which animals fell. It is therefore essential to the final function of the whole structure as a trap. The absence of the pit-trap from certain structures resembling kites, sometimes interpreted as trapping systems, therefore identifies constructions used in a very different manner to kites. The huge geographical distance separating structures or groups of structures comprising pit-traps from large clusters of desert kite concentrations may also be an additional parameter of distinction, notably in the case of constructions in the Atacama Desert in Chile [START_REF] Barge | Desert-kites et constructions apparentées: découvertes récentes et mise à jour de l'extension géographique[END_REF], or those in South Africa [START_REF] Lombard | A case for springbok hunting with kite-like structures in the northwest Nama Karoo bioregion of South Africa[END_REF], for example. In these cases, we can refer to technical convergence where the invention of the trapping concept using long walls-type system was reinvented in other regions. In this way, we propose the following classification of mega-traps identified on satellite images (Fig. 1):

-kites (stricto sensu), -kite-like (equipped with at least one pit-trap but lacking the presence of a well-defined enclosure, as is the case for kites stricto sensu), -other mega-traps.

Figure 1: classification of mega-traps identified on satellite images

Based on this definition of a desert kite, it is possible to define variables that describe the morphology of each of the three constituent elements. These variables can be recorded for each kite by field observation and/or from satellite images. The potential acquisition of this information using these two data sources has validated the approach in the past. Apart from cases where the image is not sufficiently legible (poor image quality or poor kite preservation), it is possible to draw up an identity sheet for each kite by means of quantitative data, particularly measurements (enclosure size, length of driving lines, width of entrances, number and size of pit-traps), or by identifying morphological features (entrance shape, presence or absence of particular layouts) [START_REF] Crassard | Addressing the desert kites phenomenon and its global range through a multi-proxy approach[END_REF]Supplementary data, 1). In five different regions of the area where kites are present, the field study of more than 150 kites confirmed the reliability of data obtained from satellite images. A few rare cases showed that measurements may be slightly underestimated, but these inaccuracies are negligible when working on hundreds of structures, which in any case remain comparable. In the case of open kites, where spatial organisation is less elaborate and systematic, the cumulative length of walls was measured, pit-traps were counted, contiguous pit-traps were identified and orientation was measured (Supplementary data, 1). These data, recorded in tables in a database, are analysed using univariate or multivariate statistical methods, allowing for comparisons between kites and between groups of kites.

Geographical coordinates are also associated with each kite. It is thus possible, using geomatics tools, to analyse the geographical context of constructions, and in particular their topographical context.

New discoveries in Arabia

Most of the desert kites stricto sensu, and kite-like structures identified over the last two years are located in north-western Arabia (Fig. 2). Some of these constructions have been presented elsewhere [START_REF] Groucutt | Mass-kill hunting and late quaternary ecology: new insights into the 'desert kite' phenomenon in Arabia[END_REF], but their initial description appears to be rather approximate and the comparisons poorly documented. Here, we present a much more complete inventory. Many of these new discoveries are open kites, and their numbers have increased considerably, especially in basaltic plateau areas (harrat is the Arabic word to describe these lava fields; [START_REF] Kennedy | Kites in Saudi Arabia[END_REF], such as in Harrat Nawasif, Harrat Kishb and Harrat Rahat. New open kites have also been identified in Harrat Khaybar and Al-Hait region, albeit in a more limited way. Large numbers of open kites are now known and their distribution is well defined, suggesting much more massive hunting activity with these traps than previously thought. The distribution of open kites south of the previously documented area of global desert kite distribution further extends the scope of the kite phenomenon. However, the temporality of these southernmost structures has yet to be more clearly defined.

In the Tabuk area, just south of the city, 37 constructions with all the characteristics of the Negev V-shaped kites [START_REF] Holzer | Desert kites in the Negev Desert and northeast Sinai: their function, chronology and ecology[END_REF][START_REF] Nadel | Ramparts instead of walls: building techniques of large game traps (kites) in the Negev highland[END_REF] extend over an area of about 300 km in length from northwest to southeast. They consist of two driving lines converging directly into a single pit-trap (Fig. 3). These driving lines are between 100 m and 400 m long. The topographies and orientations of these V-shaped kites are varied, but the pit-trap is frequently hidden by a slope break.

Meanwhile, the numbers of kites stricto sensu has remained almost stable in northern Saudi Arabia, in Harrat-al-Shaam south extension and in the northern confines of the Nefud. Similarly, little change is observed in Harrat Khaybar and the Al-Hait regions. In contrast, other kites have recently been identified at a latitude between 27 and 28.5 degrees north, i.e., in an area between the northern Nefud and the Khaybar region, which was previously thought to be a hiatus. They are divided into two concentrations. The first is located to the west, south of the town of Tabuk, in the area with V-shaped kites, but over a smaller area of about 40 km by 40 km. They are clearly different from the latter. They are small in size, but consist of driving lines, an enclosure and pit-traps and are therefore kites stricto sensu. The second concentration, to the east, is divided into four sub-groups to the north and east of the town of Ha'il, over a 120-kilometre-long area. The numbers of these structures are quite low in both regions (30 in Tabuk region, 20 in Ha'il region) but these desert kites are of particular interest as their morphology resembles that of other known kites further north, notably in the Jordanian and Syrian parts of Harrat al-Shaam.

Open kites

Geographical distribution

The geographical range of the 835 recorded Arabian open kites extends for about 700 km from north to south, from Harrat Khaybar to Harrat Nawasif (Fig. 4: A). Their location is limited to basaltic areas, mostly on the periphery of lava fields. They are particularly abundant in Harrat Khaybar (n=320) and around Harrat Nawasif (n=368), and rather more scattered elsewhere. They are generally found grouped together, a few hundred metres apart, sometimes with gaps of several dozens of kilometres between the groups (Fig. 5 and supplementary data, 2). In Harrat Khaybar, open kites are located in the same areas as desert kites, and the latter are more numerous (n=693). The geographical extension of kites does not go beyond Harrat Khaybar to the south, whereas open kites extend over nearly 500 kilometres further south (Fig. 4). Harrat Khaybar is thus a unique region where kites stricto sensu (and especially very characteristic Khaybar-type kites) spatially coexist with open kites.

Figure 4: Distribution area of open kites (A) and kites (B) in Western Arabia. Circles are proportional to the number of constructions counted in 10-km-square grids. Location of the study area in the geographical range of kites (C) (supplementary data, 3).

Morphology

Open kites do not comprise all three attributes of desert kites (driving lines, enclosure, pit-traps): they do not delimit a closed area and therefore do not encompass an enclosure. They consist of long walls or stone alignments that resemble kite driving lines, but which are usually shorter and do not always converge. Open kites are made up of sections of walls of varying length that connect variable numbers of pit-traps (Fig.

6). A distinction can be made between different types of open kites:

-those with only one pit-trap (or with a single group of contiguous pit-traps) at the end of two converging walls forming a point (Fig. 6: A; n=203 of this type). They are comparable to the Negev "V-shaped" kites [START_REF] Holzer | Desert kites in the Negev Desert and northeast Sinai: their function, chronology and ecology[END_REF][START_REF] Nadel | Ramparts instead of walls: building techniques of large game traps (kites) in the Negev highland[END_REF], with however more variability in driving line length and the angle formed by them. The distinction with the latter is also based on their spatial distribution, which suggests making them a subtype of open kites, -those made up of the juxtaposition of several of these pointed shapes (Fig. 6: B; n=92 of this type), outlining a W shape, -those that combine walls and pit-traps in a complex manner without any discernible recurring shape (Fig. 6: C, D, E, F, G andH). The latter are by far the most frequent (n=540).

The specificities and size of open kites can be assessed by the number of pit-traps and by the total length of the walls forming them (Fig. 7). The number of pit-traps varies greatly and can be very high in complex and unstandardized open kites: about 100 of these structures comprise at least 15 pit-traps and this number can reach 91. The total cumulative length of walls exceeds 500 m for the largest but there are also smaller structures. The length of simple V-or W-shaped walls is sometimes less than 100 metres. We observed that simple V-shaped kites often have more than one pit-trap, showing that contiguous pit-traps are very frequent. No distinct geographical distribution of these sub-types could be detected based on size or type (Fig. 5 and supplementary data, 2)

Figure 6: Morphological variability of Arabian open kites (A to H); Comparison of contiguous pit-traps (I and J) and pit-traps with micro-points (K and L) between Arabian open kites and desert kites stricto sensu.

Figure 7: Histograms of the number of pit-traps and the total cumulative length of open kite walls, according to shape.

Pit-traps are a feature of kites and open kites. In the case of desert kites stricto sensu, our excavations in many parts of their global distribution area indicate that pit-traps are pits with a depth of several metres. The whole inner side of pits is stone-faced with rows of stones. They form the trapping element of the kite into which hunted animals fell [START_REF] Abu-Azizeh | Discovery and excavation of desert kites in the southeastern badia of Jordan[END_REF]. Satellite images show characteristics shared by the pit-traps of the Harrat Khaybar kites and open kites. Outside Harrat Khaybar, the presence of desert kites with contiguous pit-traps is uncommon (15%). In Harrat Khaybar, this frequency increases to 72%. In particular, series of contiguous pit-traps (from three to more than a dozen, Fig. 6: J) are frequently observed. Open kites often share this characteristic too (Fig. 6: E, F, H andI): 41% of them comprise at least one series of more than three contiguous pittraps. When they are not contiguous, it is also common in kites and open kites, for pit-traps to be connected, over a short distance, by a curvilinear wall delimiting a small indentation towards the outside, a micro-point (Fig. 6: K and L). In Harrat Khaybar, kite pit-traps are circular (which is not necessarily the case elsewhere), which is also the case for open kites. They are quite similar in size: 3.95 m in diameter on average for kites, 3.39 m for open kites. They are therefore slightly larger in kites, with more standardised dimensions (the standard deviation is 0.28 m for kites, 0.54 m for open kites). We also observe that the size of open kite pittraps tends to increase from south to north and is more similar to that of kites in the north (Fig. 8). Still unknown chrono-cultural factors may explain this north-south size gradient, with the gradual evolution of trap architecture as the hunting technique developed throughout the region.

Topographical implantation

The Harrat Khaybar kites clearly show that entrances predominantly open towards the west (animals coming from the west and going towards the east, Fig. 9: A). Open kites do not follow the same pattern as no particular orientation is discernible (Fig. 9: A). Our results contradict [START_REF] Groucutt | Mass-kill hunting and late quaternary ecology: new insights into the 'desert kite' phenomenon in Arabia[END_REF], but this can be explained by the fact that we used continuous azimuth for this analysis and not simply the four single cardinal directions (N, E, S, W). When the orientation of each open kite

Figure 9: Orientation of kites and open kites (A); Histogram of the angular differences between the orientation of open kites and slope orientation (B ), see supplementary data, 4

is compared with the orientation of the slope on which it is built, there is a clear tendency for open kites to conform to the local slope since 68% of them show an angular deviation from the slope of less than 60° (57% have an angular deviation of less than 45°). This trend can be clearly observed (Fig. 9: B) even if the method used for this comparison can be improved as the orientation of the slope was only determined by a single pixel (located approximately at the centre of the open kite) of the SRTM DTM with 30-metre resolution (Supplementary data, 4). This measurement from the open kite centre point is then less representative of the whole open kite surface area, while the measurement of an average slope surface area would yield more accurate results. In addition to these quantitative data, we also observed the relief on which open kites are built. Direct observations confirm that open kite orientations are adjusted to local slopes, as open kites are mainly positioned on a slope with a steeper gradient than the areas below, with the pit-traps located in the upper part of the structure. This can be the slope of a hillock overlooking a sub-horizontal area such as a mudflat (Fig. 10: A) or slopes delimiting the embankment of a wadi. In the latter case, we frequently observe that open kites are built on the opposite banks of the same wadi, with opposite orientations (Fig. 10: B). The fact that open kite pit-traps are predominantly located at the top of the structure may well be related to the fact that gazelles tend to flee upwards when frightened [START_REF] Blank | Behavioral responses of goitered gazelles to potential threats[END_REF]. In any case, open kites show opportunistic adaptation by builders to the natural topography, with for example the construction of pit-traps behind a convex break of slope.

Study case in Harrat Khaybar: fieldwork in the Khaybar oasis area

The modern oasis of Khaybar, in western Saudi Arabia, is located in the central western side of Harrat Khaybar (Fig. 11). The region comprises a dense concentration of desert kites stricto sensu, characterised by a very specific type. Enclosures are often triangular, sometimes with a succession of several generally very regular and symmetrical giant triangles, and the driving lines leading to the enclosure are very straight and can be very long, up to more than several hundreds of metres (Fig. 12). In October 2020, a new archaeological research project was set up in the oasis area of Khaybar, with a comprehensive survey of a 56 sq. km area. The Khaybar Longue Durée Archaeological Project (KLDAP) documented a number of open kites in this area, the characteristics of some of which are presented here. Sixteen open kites have been identified so far in the research area (Table 1). Most of them are isolated structures in the landscape, and are rarely associated with other open kites. Indeed, they certainly do not operate in chains, as desert kites stricto sensu sometimes can. However, open kites can be positioned alongside a wadi, but at a considerable distance from it, and the two are never connected. The overall size of open kites in Khaybar is always smaller than desert kites. We can differentiate two main groups of open kites in the Khaybar oasis: simple forms, such as V-or W-shapes, and more complex forms. There are at least nine examples of simple forms with a simple plan consisting of two driving lines leading to a single pit-trap (Fig. 13: A), rarely to two coalescent pit-traps. When two of these V-shaped open kites are connected and conjoined at the extremity of one of their driving lines, they form a W-shaped open kite (Fig. 13: B). The topographical contexts of these simple forms are varied. Sometimes, the pit-trap is located on top of a plateau, with driving lines leading to it from a wadi bed. Driving lines can also be located on a flatter plateau surface sloping down slightly to a single pit-trap located at the edge of the same plateau. The pattern can also combine several of these features, with driving lines running upwards from a valley, to a hill or a narrow plateau area, and leading to a pit-trap located on the opposite slope.

The other examples of open kites in Khaybar are complex forms with very diversified sizes and layouts, as observed on satellite pictures elsewhere in western Arabia (Fig. 14). The Khaybar examples show rather small enclosure-like spaces, and short driving lines, when they exist. The latter are short and never straight, generally following natural topographical features such as cliff edges, and natural landforms.

The pit-traps in Khaybar open kites are generally located on a slope break at the edge of a natural escarpment. This configuration keeps the pit out of the animals' sight when they are following the driving lines, which generally run uphill. Open kite pit-traps show diversified sizes, ranging from a rather similar size to desert kite pit-traps of around 4 m in diameter (although always much more collapsed in open kites), to smaller modules less than 2 m in diameter (Fig. 15). Construction techniques are similar in both cases, with a rather organized piling of uncut basalt blocks available in the direct vicinity of structures. The remaining number of stone courses above present-day ground level is between one to five rows for open kites, while desert kite pit-traps are particularly well preserved in Khaybar with preserved elevations of sometimes up to 2 m above present-day ground levels.

Figure 13: Aerial pictures of Khaybar simple open kites. A: V-shaped open kites (O-AB831 is approximately 130 m in length), driving line extremities were destroyed by bulldozer activities; B: W-shaped open kites (O-AB821 is approximately 22 m in length and 25 m in width). Photo credits: KLDAP. Figure 14: Aerial pictures of Khaybar complex open kites. A: vertical view of O-AB834 open kite with a difficult to follow layout, it may reflect consecutive uses of different structures; B: map of O-AB834; C: Aerial pictures of O-AB829 open kite, the funnel entrance and driving lines follow the edge of the thalweg escarpment, and the main trapping zone with at least six pit-traps is out of the axis alignment and follows the left driving line; D: map of O-AB829. Photo credits: KLDAP. Figure 15: Pit-traps from Khaybar. A: set of six agglomerated and conjoined pit-traps at O-AB829 open kite, with different states of erosion and collapse; B: three pit-traps from AB40 desert kite strictly speaking, the preserved elevation of pit-trap walls is about 1.5 m the present-day ground level; C: single pit-trap from O-AB831 V-shaped open kite, the wall running across the picture is modern; D: single pit-trap from O-AB820, heavily eroded and collapsed; E-F: pit-traps from O-AB834 open kite with remains of clear inner wall delimitations. Photo credits: KLDAP.

In terms of understanding how these open kites functioned, and the possibility that they may have functioned simultaneously, it is interesting to note that in the Khaybar study zone (Fig. 16), there are two to four V-and W-shaped open kites along the western bank of the wadi, and a more complex open kite along the eastern bank. The rest of the landscape along this same bank is covered by five gigantic Khaybar-type desert kites. Two additional open kites, that may have functioned together, and that bear witness to two chronological phases of use, complete the inventory. The whole landscape is quite difficult to interpret as no dating elements are yet available. However, the general pattern of open kites on the opposite banks of the same wadi, with opposite orientations, has also been regularly observed in other regions south of Khaybar (e.g.: Fig. 10: B).

Relative chronology of open kites and desert kites: erosion and superimposition

On satellite imagery, open kites are more difficult to detect than desert kites, as they were regularly adapted to the natural topography. When driving lines, for example, strictly follow the edge of a cliff, it is difficult to recognize an anthropogenic construction, unless a detailed observation can be made on the ground. Also, open kites are often made of smaller blocks, and they are systematically more collapsed and eroded than nearby desert kites. This is clearly observable on the ground too, and the surface state of the rocks used in the construction of open kites seems to have endured more weathering. At first sight, these observations may have chronological implications for the interpretation of the two types of structures. It seems obvious that open kites might be an older form of desert kites, possibly a more primitive or simpler construction. Future excavations and dating of kitelike and kite structures will confirm whether or not they were synchronous.

One example of superimposition is directly observable in the field in the Khaybar oasis area, confirmed by the high-resolution (2 pixels per cm) drone pictures made on site. In the window of study (Fig. 16), a series of desert kites are clearly observable. One of them (AB13) shows a southern driving line overlapping an open kite driving line (O-AB822). This relative chronological evidence is clear on the ground (Fig. 17), as well as on the aerial picture (Fig. 18).

Figure 17: Relative chronology: the driving line from the AB13 kite (in dark colour) overlaps the driving line of the O-AB822 V-shaped open kite (in light colour). A: view from the southwest; B: view from the southeast. Figure 18: Orthophotograph of the superimposition of AB13 kite on O-AB822 open kite. A: raw image; B: interpreted image with AB13 kite (in dark colour) overlapping the driving line of the O-AB822 V-shaped open kite (in light colour). Photo credits: KLDAP.

This example of the relative chronology of both types of structure tends to reinforce the hypothesis that open kites are older forms of hunting-traps than desert kites, the latter being much larger, more regular in shape, and comprising bigger pit-traps. The different erosive states, and preservation states, of open kites and desert kites seems to be a reliable chronological indicator, that will need to be confirmed by future excavations of both kite-like structures and kites with absolute dating elements.

Other indications of relative chronology, such as superimpositions, or reuse of kite stones in later, and better dated, constructions, will be given particular attention in forthcoming fieldwork seasons in Khaybar.

Modifications of the distribution area of kites in Saudi Arabia

Morphological characterisation

Among the recent discoveries, apart from the numerous open kites, the number of kites stricto sensu in the Tabuk region and the Ha'il region is relatively low (respectively 30 and 20 kites). They nonetheless present interesting morphological characteristics as they are comparable to kites located further north.

The kites in the Tabuk area consist of two or three driving lines of 70 to 440 metres in length. They end in a very small enclosure (between 0.1 and 0.5 ha), usually with a single pit-trap, many of which are point-shaped. Kites in the Ha'il region have two to four driving lines. Their length is generally limited to a few hundred metres, but it is not easy to observe them on the images and some of them may have been much longer (there is one observed case of a driving line of almost 3 km). Enclosure size is moderate, but nevertheless larger than kites in the Tabuk area. Their shape is characterised by a star shape made up of points ending in five to seven pit-traps.

In order to establish a comparison between these two groups of kites and other kites from the southern part of the distribution range, we first calculated the central values of morphological variables observed or measured on satellite images for these two groups, as well as for four other groups made up of morphologically homogeneous kites distributed to the north and south of these two groups (Table 2, supplementary data, 1). Group 1 includes kites found west of Harrat al-Shamm and as far as the Damascus region. Group 2 corresponds to kites found mainly east of Harrat al-Shaam. Group 3 consists of relatively scattered kites north of the Nefud and west of Harrat al-Shaam. Groups 4 and 5 correspond to the stereotypical kites of Harrat Khaybar, the former consisting of a single triangular base while the latter combine several.

Table 2: Central values (mean, median or frequency) of morphological variables used to compare kites in the regions of Ha'il et Tabuk with kites in clusters to the south of the general distribution sone of kites: A-Number of driving lines (mean); B-Mean length of driving lines (median); C-Surface of enclosures (median); D-Frequency of proximal indentations; E-Width of entrance (median); F-Number of pit-traps (mean); G-% of straight pit-traps; H-% of angle pit-traps; I-% of pointed pit-traps; J-% of incipient pointed pit-traps

The values of the small kites in the Tabuk area are close to those of group 1. They have no funnel entrance, no proximal indentations, reduced entry width and few pit-traps. In fact, size values (length of driving lines, enclosure area, number of pit traps) are always smaller than those of group 1, which has the lowest values of all the comparison groups. In summary, the kites of group 1 and those of the Tabuk region have very similar morphological characteristics, the former can be described as small, the latter as very small.

Kites in the Ha'il region are morphologically close to those of group 3. Like them, they do not have funnel-shaped entrances, almost systematically have proximal indentations, the same number of pittraps (with a large majority of pointed pit-traps), and equivalent enclosure size. Only the length of driving lines is shorter, but we know that this may have been underestimated for the Ha'il kites.

This comparison can be represented graphically using the central values of the variables proposed in figure 19. Enclosure size is represented by a square with an opening at the top proportional to the width of the kite entrance. This opening can be accompanied by graphic signs corresponding to specific features (funnel entrance and proximal indentation) when these are significantly present. The size and number of driving lines are represented by a red rectangle at the top and the number of cells of each type by a circle in the centre. Similarities between the profiles of group 1 and the kites from the Tabuk area are clearly visible: the latter are merely smaller and more frequently feature pointed pit-traps. The similarity is even more obvious for group 3 kites and those from the Ha'il area.

Conversely, the kites from these two new regions do not show any similarities with kites from the eastern Harrat al-Shaam (group 2) or the Khaybar region (groups 4 and 5).

Figure 19: Graphic representation of the central values of morphological variables of the kite groups; for each group, the red rectangle represents driving lines, the black figure enclosure size and the characteristics of the entrances and the central circle the number and proportions of each type of pit-traps (see legend below).

In a second stage, the comparison of enclosure shapes confirms these similarities (Fig. 20). The comparison of enclosure shapes entails the comparison of specific cases of kites (with the question of representativeness). However, statistical variables only indirectly take into account enclosure shape. The comparison of shapes, which is qualitative and less abstract than the comparison of statistical variables, therefore provides additional information. In the case of the Tabuk region and Group 1 kites, the circular or geometrically simple shape, the connection to the driving lines and the distribution of a small number of pit-traps are obvious common features. In the case of the kites from the Ha'il region and group 3, the layout of pit-traps in a point determines in each case a different star shape, but the eight presented cases clearly belong to the same set. The difference with kites from Harrat al-Shaam (group 2), to which the kites from the Ha'il region could have been hastily associated, is clear. In Harrat al-Shaam, the star shape is less regular because the points are only really proximally marked, and the pit-traps in the distal position are actually incipient pointed pit-traps. Pit-traps are also more numerous and enclosures are larger. Kites from the Khaybar region (groups 4 and 5) stand out from all the others, with very pronounced proximal indentations, frequent contiguous pit-traps and a very high proportion of straight enclosure walls. The two methods of comparison thus yield identical results. After comparison with the five groups, kites in the Tabuk and Ha'il regions can be associated with two groups of kites, while they are clearly distinguished from the other three groups.

General cartography of the southern part of the global kite distribution area

Once these morphological relationships have been established, we can map out the different kite families (Fig. 21). These consist of V-shaped kites, Arabian open kites and kites stricto sensu from the Tabuk and Ha'il regions discussed in this study. In addition, a sample of other kites north of the 28th parallel and in Harrat Khaybar is represented based on their affiliation to the groups used for comparison (groups 1 to 5). The sample represents about 10% of kites, the others are considered as indeterminate (Fig. 21: grey circles). We observe a regionalisation of kite families on the basis of morphological similarities, particularly in the southern part of the overall distribution zone. The numerous open kites significantly extend the scope of kite distribution southwards, along the lava fields of western Saudi Arabia. They only share the north of the area where they are present with the large concentration of kites in the Khaybar and Al-Hait region. Further north and up to Harrat al-Shaam, kite distribution is more diffuse but the morphological families are nevertheless regionalised. The well-known V-shaped kites of the Negev extend to the south and occupy an elongated area about 500 km long running north-northwest (Negev) south-southeast (Tabuk region). In this latter area, a small group of small kites can be related to the numerous kites found west of Harrat al-Shaam to Damascus (Group 1). Groups of star-shaped kites (including those in the vicinity of Ha'il), scattered to the east between 27 and 30 degrees latitude, fill the space between the two large concentrations of Harrat al-Shaam and Harrat Khaybar, but are nonetheless morphologically distinct from them. Examples of this group of kites (Group 3) are also found in the western half of Harrat al-Shaam.

Discussion

The need for hunting and the reasoning behind it shifted during the Neolithic from a survival practice to a more complex tradition infused with various meanings. The economic and social values of hunting activities can be defined by food and raw material procurement, territorial demarcation of areas with different functions, ritual or social use (offering, sacrifice, feasting), prestige, elite or cultural practice (trophy, royal hunts). The degree of dependency on these values, the selection of specific prey, the knowledge of animal ethology (where they occur and how they can be captured) are the main factors that might have led to the adoption of a specific hunting technique and mobility patterns by Holocene hunters. Furthermore, the outcome of a hunt has two facets: single prey or masskilling of herds, involving choices in the pre-selection of the outcome, or how to use, process and transport animal products. The economic costs and benefits associated with communal hunting are plausibly related to meat provisioning and ensuring a tradable resource. The costs of a hunting technique encompass effort during hunting, the making and maintenance of tools and structures, efficiency and reliability of capture (productivity) [START_REF] Smith | Anthropological applications of optimal foraging theory: A critical review[END_REF]. The choice of a meat surplus could be a reason behind the choice of communal mega-trap hunting, while communal hunting is also considered as a large-scale socio-cultural event [START_REF] Kornfeld | Prehistoric hunter-gatherers of the High Plains and Rockies[END_REF]).

Almost nothing is known about mega-traps users in the kite distribution area. More dating is required as well as the excavation of related sites in order to clearly associate them with a cultural facies. Were hunting excursions organized by specialized hunters from a local authority? Were tribes specialised in hunting activities and trade? Most ethnographic accounts consider hunting systems of driving and trapping animals as low-rank activities in societies (e.g., Solubba, Arabian Bedouin societies; Betts 1989), where closed groups exchange and trade their skills and the products of the hunt. In addition, the development of such large structures would involve some notion of property rights, since a community would not invest its time in building mega-traps if the captured prey could be claimed by outsiders.

Hunting activities vary depending on capture techniques (trapping, pinning down, etc.), community investment (planning, time, number of hunters), the outcome and purpose of the hunt (number of prey, selected animals, a single or several captures) and the tools used to kill (trap, arrow, spear, etc.). How did they operate and how were they organized? In the case of mega-traps, and according to most ethnographical data, the hunt was more active and even cooperative [START_REF] Legge | The exploitation of animals[END_REF] than regular 'passive' traps (e.g., wheel traps, snares, nets), most probably requiring the involvement and presence of hunters throughout the process. But exactly how these prehistoric mega-traps worked remains unknown.

Kites and open kites are two distinct types of traps, but with a number of shared characteristics, which establishes a certain kinship between them. This raises the question of the similarities and differences between these two types of traps from the point of view of how the hunt operated. It is difficult to grasp how hunting was carried out with kites and open kites. The active participation of hunters seems to be vital for these traps to work, but data are lacking to establish hunting methods (number, location, and behaviour of hunters/trappers, possible use of dogs) and temporality (duration of the hunting action, succession of the different trapping phases, seasonal or continuous use). However, it is worth noting that the layout of both types of traps opportunely takes advantage of local topography, making use of slopes and counter-slopes probably to limit the animals' visual range. We know above all that pit-traps mark the final phase of the hunt. In both cases, animals were trapped and were probably killed as they fell into these pits. However, the morphology and orientation of constructions show that the phase(s) prior to trapping as such, during which animals were forced to move towards the pits, were different. The overwhelming westerly orientation of kites stricto sensu suggests that kite builders had excellent knowledge of the temporality, rhythms and routes of seasonal migrations and other wild animal movements. The systematic layout of driving lines opening onto a vast space and converging on an enclosure with a narrow entrance shows sophisticated overall architecture, suggesting a standardised hunting technique and a search for efficiency. The morphology of open kites appears less elaborate. Some of them attain considerable sizes, but they are more often smaller constructions than their counterparts. Their layouts are very varied and not systematised in the same way as kites. The orientation of these structures is independent of cardinal orientation and is directed more towards the top of the slopes, suggest a more localized driving area. It is thus possible to affirm that the hunting techniques used with these two types of traps were related, but different and probably more formalised in the case of kites. The on-site observation of the superimposition of a kite driving line on the terminal part of an open kite at Khaybar establishes the anteriority of open kites (Fig. 18 and Fig. 19). This precedence can also be observed in the field at Khaybar by the better state of preservation of kites, both in terms of their preserved elevation and the state of the weathered surface of the basaltic blocks used in their construction. The construction technique of open kites also appears to be more rustic. Thus, it would seem that an early hunting technique was developed using open kites, from Harrat Khaybar to Harrat Nawasif. Later, another more sophisticated hunting technique was implemented in the north of this region using only kites. We therefore suggest that open kites should be considered as 'proto-kites'. Further north, in Harrat al-Shaam, long undulating walls, called meander walls, have also been recognized as hunting constructions prior to kites [START_REF] Kempe | Desert kites in Jordan and Saudi Arabia: Structure, statistics and function, a Google Earth study[END_REF]. Some include pit-traps in their layout and it seems that these constructions evolved over time (bag-shaped kites) before the appearance of kites with a clearly closed enclosure [START_REF] Betts | Game drives in the Black Desert, eastern Jordan[END_REF]. Other 'proto-kites' therefore probably exist, but, in the absence of absolute dating, it is still impossible to design a chronology of these different constructions.

The number of known kites and kite-like structures, including V-shaped and open kites, currently amounts to 7151. In this southern part of the overall kite distribution area, the clear identification and mapping of morphologically different groups offers a spatialized image of the different types of kites (Fig. 22). This image probably carries chronological connotations [START_REF] Barge | Desert-kites et constructions apparentées: découvertes récentes et mise à jour de l'extension géographique[END_REF]. Each group of kites can be considered as material evidence of a singular kite hunting technique in a given region and period. These data provide a basis for recording the spatial and temporal spread of the kite-hunting technique, once absolute chronological data become available. Within this complex web, the identification of a proto-kite form to the south of the extension area is an important piece of information. However, it would be premature to deem that open kites are the only early form of kites from which Khaybar kites originated, a model from which the hunting technique spread further north to Kazakhstan. On the one hand, there are other potential forms of proto-kites; the meandering walls of the Harrat al-Shaam, which largely overlap kite structures, also have pit-traps, at least in some of their extensions. On the other hand, if open kites are primitive kites, the technical innovations that led to kites may have been developed in another region further north, before spreading southwards again to give rise to the Khaybar kites.

Conclusion

In western Arabia, kites and open kites show many morphological similarities: the presence of pittraps, with the same layout, shows that these constructions are related. Hunting techniques, at least for some hunting phases, on the other hand, must have been different. The examination of topographical layouts shows that kites and open kites do not follow the same logic and that their general architecture is different. Kites appear to be more sophisticated, with the recurrent layout of standardised elements (driving lines, enclosures, pit-traps), whereas open kites show very diversified layouts. However, the partial overlapping, in Harrat Khaybar, of the geographical distribution areas of the two types of constructions suggests that they derive from each other. Kites and open kites may have been in operation at the same time, defining related but different hunting techniques. They may also have followed each other in time, with one technique leading to the emergence of the other, forms of 'protokites' predeveloping the sophisticated and standardized forms of desert kites. The more general question of the phenomenon of the diffusion of the kite hunting technique, and its documentation, is a current issue in research on these constructions. The discovery and characterisation of open kites through the examination of satellite images and the overall mapping of the region undoubtedly provide new elements in this respect. However, the understanding of open kites (age, coveted species, groups behind their constructions), like that of the Harrat Khaybar kites, is totally dependent on fieldwork.

Forthcoming research in the field, involving excavations and dating, will undoubtedly enhance our knowledge of these structures, their evolution, diffusion, and operational methods.

Figure 3 :

 3 Figure 3: Examples of V-shaped kites in the region south of Tabuk

Figure 5 :

 5 Figure 5: Open kites types in Harrat Nawasif.

Figure 8 :

 8 Figure 8: Diameter of open kite pit-traps (red) and desert kite pit-traps (blue) according to latitude (supplementary data, 3)

Figure 10 :

 10 Figure 10: Two examples of the topographical layout of open kites.

Figure 11 :

 11 Figure 11: Overall map of the Khaybar Longue Durée Archaeological Project research area around the Khaybar oasis, and location of identified desert kites strictly speaking and open kites. The window in red (B) locates the study zone, enlarged in Fig. 16.

Figure 12 :

 12 Figure 12: Oblique drone pictures of typical Khaybar desert kites with their generally triangular, regular and symmetrical shapes. Naming of the kites according to the Globalkites Project numbering system are: A: AB41; B: AB8; C: AB7; D: AB19. Photo credits: KLDAP.

Figure 16 :

 16 Figure 16: Eastern case study zone; the rectangle illustrates where kite AB13 covers the open kite O-AB822.

Figure 20 :

 20 Figure 20: Comparison of enclosure shapes of kites from the regions of Ha'il et Tabuk with kites from clusters to the south of the overall kite distribution area.

Figure 21 :

 21 Figure 21: Distribution of families of kites and kite-like structures in the south of the overall distribution area.

Table 1 : List and preliminary description of open kites observed in the field in Khaybar; the description in the field sometimes requires the subdivision of constructions, which explains why the KLDAP digitization can include several numbers for the same open kite.

 1

	Structure	Structure	Observations		Type	Pit-trap	Pit-trap	Cumulative
	No	No					number	mean	wall length
	Globalkites	KLDAP						(m) diameter	(m)
	O-AB822	KH05000	Very eroded, cut by a	V-shape	1	4.3	226
			more recent kite
	O-AB828	KH05001	Triangular shape with	Complex	4	3.3	161
		KH05002	one disconnected pit-trap	shape
	O-AB821	KH05003	Close to wadi cliff		W-shape	2	4.3	114
	O-AB819	KH05004	Close to wadi cliff		Complex	10	3.9	313
						shape
	O-AB820	KH05007	Close to wadi cliff		V-shape	1	5	106
	O-AB824	KH05008	Above natural gully with	Complex	2	3.5	130
		KH05009	a slight slope, heavily	shape
			collapsed, driving lines
			hard to see on the
			ground, good potential
			for excavation
	O-AB823	KH05010	Open kite using natural	V-shape	1	3.9	141
			round	shape	small
			depression
	O-AB46	KH05011	Unclear pattern, Difficult	Complex	16	4.9	283
		KH05012	to read in the field and	shape
		KH05013	heavily collapsed.
		KH05014
		KH05015
		KH05016

Acknowledgements

Fieldwork in Khaybar was carried out as part of the Khaybar Longue Durée Archaeological Project (codirection: Guillaume Charloux, Rémy Crassard, Munirah AlMushawh). We thank the French Agency for the development of AlUla (AFALULA) and the Royal Commission for AlUla (RCU) for their support and fieldwork permit. This research is also part of the Globalkites Project, supported by the French National Research Agency (2013-2018) and the CNRS team Archéorient.

Supplementary data

Open kites and kites feature classes (1), Regional mapping of open kites (2), Use of regular grid of 10 km x 10 km (3), Angular differences between the orientation, of open kites and slope orientation (4).


Luc Averous

Jean Christophe Quantin

Alain Crespy

Dominique Lafon

Evolution of the Three-Dimensional Orientation Dist『ibution of Glass Fibers in Injected lsotactic Polypropylene

The mechanical properties of fiber reinforced thermoplastics depend on the characteristics of the fibers, such as their three-dimensional orientation distribu tion, which is the result of the processing conditions and of the initial products before mixing. Different composites were obtained by injection of reinforced isotac tic polypropylene containing 30 wt% fibers. Different size distributions of fibers were investigated. Their average lengths range from 60 to 4 70 µm. Specific new tools were developed by combining microscopy and image analysis to determine the microtextural characteristics of the fibers, such as their size distribution or their orientation in the matriX. Our technique allows the following of the size distribution evolution of the fibers as the composite is processed. The three-dimensional orien tation of the fiber is established using the in-plane angle and the angle of inclina tion. The evolution of the three-dimensional orientation distribution is studied in the sample, by differentiating central and border behavior and also as a function of fiber size. It is concluded that the coupling of scanning electronic microscopy with image analysis is an interesting technique that allows the 3-D orientation of the fibers to be determined. This technique is validated by sho叨ng that the results obtained agree with those that other authors obtained on comparable materials.

INTRODUCTION F

iber reinforced thermoplastics are increasingly used in applications where it is essential to accurately predict their composite properties. The perfor mance of such materials is particularly dictated by the microtextural characteristics given to the matrix by the fibers. We can define microtextural characteristics as (1): the parameters of the fibers, such as their size, their shape (ratio length/diameter), and their spatial organization, determined by fiber orientation and fiber dispersion. The microtexture of a composite of given composition depends on the processing conditions and on the characteristics of the different initial prod ucts (fibers and polymer), before mixing. All these in teractions can be described by the system illustrated in Fig. 1. It consists of the initial products, the pro cessing conditions, the microtextural parameters, and the material properties. In order to predict material properties from initial products and processing con ditions, it is necessary to analyze the different rela tions between each point in this system.

The particular field in which we use this approach is the analysis of the evolution of glass fiber orientation distribution in injected reinforced polypropylene. Pro cessing conditions are fixed and controlled. The char acteristics of the fibers before extrusion are varied by incorporating fibers with various size distributions into the polymer.

To determine the size distribution and three-dimen sional orientation distribution of the fibers, specific procedures are developed that combine microscopy and image analysis (1�4). We define the shape factor (Fs) as the ratio of the length to the diameter. A fiber is defined by its shape factor, Fs > 1. A second pneumatic selection is carried out on SFgl; the fraction containing the longest fibers is eliminated and the resulting glass fiber called SFg2. Thus, three different populations of selected glass fi bers are obtained with different average lengths.

Micro textural characteristics

Spec.imen Preparation

The composites were prepared with a polypropylene matrix (Appryl 3030 MNl-Atochem-France). Its mo lecular weight is 325,000 Daltons. The fiber content was 30 wt%.

Processing conditions were identical for all injected specimens. A co-rotating twin-screw extruder (Clex tral BC 45, Firminy, France) was used to prepare glass fiber-polypropylene pellets. An injection molding ma chine (Billion, Bellignat-France) with a clamping force of 90 tons was used to mold standard dumbbells. Dumbbell specimens produced have an active portion 10 mm wide and 4 mm thick (according to French standard NFf 51-034 1981). The various samples were taken from the central part of the dumbbell.

METHODS

Deten血ation of the Size Distribution of Glass Fibers (1, 2, 4)

Most of the usual size distribution determination techniques (scattering, gravitational measurements, etc.) are not appropriate to determining the length of acicular particles such as fibers, because these meth ods are based on a spherical shape model. The most usual technique is therefore measuring the length of the fiber using microscopy. Semi-automatic image an alyzer processes are increasingly used because of the tediousness of manual measurements (5,[START_REF] Allen | Particle Size Measurement[END_REF].

The most recent development has been automatic image analysis, which works more quickly and mini mizes human intervention [START_REF] Talbot | [END_REF]. We therefore developed a specific automatic technique by coupling optical mi croscopy and image analysis (1, 4). It is based on an individual analysis to obtain the size distribution of the fibers. The size factor for these cylindrical particles which is used in the analysis is the exodiameter, which is estimated from the maximum Feret diameter. The Feret diameter is the length of the particle projec tion in a determined direction on the plane.

For each analysis, a population of at least 800 fibers is treated and analyzed. Before image acquisition, fi bers are carefully dispersed between two glass slides, in an appropriate dispersing solution, see Fig. -The size distribution in weight (each fiber is weighted by its length) is represented by the average weight length, L w , as follows:

L1d; x n1 Lw = Lidi x n1 (2)
The size distribution range is given by a dispersity factor: the variation coefficient (CV n l, the ratio between the standard deviation and the average length in num ber.

Determination of the Three-Dimensional

Orientation Distribution of Fibers in the菹atri:a:

(1, 3)

To determine the orientation angles of fibers, most of the more advanced methods use their elliptical marks on a sectioned polished surface (Fig. 3). The three-dimensional orientation of the fiber is deter mined by two angles (Fig. 4): the inclination (0) and the in-plane angle(中) ,in a spherical coordinate system.

In the last decade, some authors (8-13) have fol lowed similar approaches, but without entirely resolv ing the determination of both angles. Their ap proaches do not fully deal with the problem of 180 ° ambiguity on the determination of the in-plane angle. This ambiguity is due to the fact that two possible plunge senses correspond to the same elliptical mark (Fig. 5). Only, Clarke et al. ( 14), using a confocal scan ning laser microscope, have proposed a process to determine three-dimensional orientation from two parallel optical sections of the material. Unfortu nately, until now this type of microscopy has been uncommon and rather expensive.

We therefore developed an original technique by coupling scanning electron microscopy and image analysis. This technique is described in a previous paper (3). This method requires two different polished sections of the material to obtain the three-dimen sional orientation distribution of the fibers in the ma trix (Fig. 6). Grey level frames of the polished sections are taken by scanning electronic microscopy. Each image is then treated to prepare it for the quantitative analysis phase. Each frame is cleaned and a threshold is operated in order to simplify it. It is then necessary to separate touching fibers, in order to individualize and label each one. During the quantitative analysis the elliptical fiber marks are interpreted and trans lated into orientation results. We must be careful that fibers with irregular shaped cross section are not taken into account. 1 show, this operation re suits in a notable reduction in the average length and a narrowing In the length distribution. The effects of grinding and processing on fiber length distribution (after extrusion and Injection operations) are shown in 凡g. 9. For short glass fibers (SFgl and SFg2) under the chosen processing conditions, we note that fiber breakage is quite low. As noted by other authors [START_REF] Arroyo | [END_REF]19), this length reduction concerns principally the longest fibers, producing shorter fibers. , """"""""'""', 才 i 4 亿 S F g 1 ] i J 0,2 f-'"""""""",志µ.""升乒.....f o Initial fibers (before processings). • Fibers, after processings. In border zones, in-plane orientation tends to be parallel to the principal border. We can note that the 0,08 0,09 0,08 0,30 0,33 0,03 0,12 0,25 0,27 0,30 0,16 0,09 0,40 0,57 0,59 0,03 0,08 0,77 0,85 0,01 0,07 0,80 0,84 0,01 0,02 0,83 0,89 0,00 0,03 0,83 0,68 0,00 0,01 0,67 0,65

SD Orientation Results

Sampling Mode

Most sampling planes for orientation investigations on the injected fiber composites are established taking both core and skin behavior into account (20-22). In some cases, however, it is not possible to differentiate and identify clearly the two types of layers by

CFg results.

0,07 0,10 0,89 0,87 0,03 0,12 0,90 0,76 0,02 0,06 0,88 0,69 0,02 0,05 0,89 0,67 0,03 0,04 0,82 0,66

SFgl results.

0,04 0,08 0,78 0,76 0,03 0,21 0,84 0,70 0,06 0,11 0,78 0,50 0,03 0,05 0,61 0,39 0,01 0,02 0,54 0,38

SFg2 results. The in-plane orientation中and the symmetric angle 中+180 °do not have the same probability. In our case, the in-plane angle中close to 0 ° is more frequent than the 180 ° angle. This is due to the curvature of the advancing front during mold filling (Fig. 1 7), which favors a particular plunge sense.

We observe that, for the same fiber concentration, on a polished section, the shorter the fibers are, the

  presentation of the system. The purpose of this work is to show and analyze the results obtained by the application of these image analysis methods in order to show the value of such techniques, which have been until now, rarely used in the polymer composite field. The results obtained for the evolution of the 3-D orientation distribution of the fibers can illustrate the use of such a method. MATERIALS Fibergla霪11 Generation All the fibers we used have the same diameter equal to 14 micrometers. Three different length distribu tions of glass fibers are obtained using different gen eration processes: • a first batch of fiberglass was obtained from 4.5 mm cut thread (CFg). • two batches of shorter glass fibers were obtained by a fragmentation process of continuous fibers. After fragmentation by pneumatic selection (Alpine ATPSO Rotoflex, Augsburg, Germany), the powder (ground glass) is eliminated leaving only glass fi bers, coded SFgl.

Fig. 2 .

 2 Fig. 2. Captured image of dispersed Ji山ers between two g/nss slides.

Fig. 3 .Fig. 4 .Fig. 5 .Fig. 6 .

 3456 Fig. 3. Image of the elliptical mark of a fiber on a polished suiface.

  micro scopic observation. So, applying stereological laws, the section studied is sampled in zones of equal area. After verification of the presence of two principal sym metry axis on the plane formed by the section through vas. Each point Is translated Into a unitary segment with the same plane orientation. The addition of the different segments gives the direction rose, which pro vides a synthetic illustration of the main fiber orlen tations on the plane. It shows the in-plane angle dis tributions, and the stereographic density plot, which gives the probability distribution of the three-dimen sional orientations.

Fig. 9 .

 9 Fig. 9. Stze distributions tn weight. Cumulative presentation.

Fig. 10 .Fig. 11 .

 1011 Fig. 10. Prtncipal samplings zones and symmetry axis.

value of a 11 is higher than a 22 .

 22 Towards the center, along the X axis, the a 11 tensor component increases. The further from the secondary wall, the more the fibers are orientated parallel to the principal wall along the X axis. In-plane orientations in the two zones close to the secondary wall are confused due to the influence of both borders.For central zones, we note that the further away from the secondary border, the more the fibers are orientated progressively parallel to the X axis (Figs.15 and 16). The influence of the principal wall is predom inant. This is shown by an increase in a 11 and a de crease in a 22 values. The shorter the fibers are, i.e. mobile in the flow, the greater is the effect. By analyz ing stereographic density plots, a dissymmetry in the 3D orientation distribution is detected. We can notice that the density extrema are shifted toward the center, along the X axis. This is due to the rheology of the system and this is in agreement with mold filling mod els, given e.g. in the simulations of Advani and Tucker (26) or Baraldi et al.[START_REF] Vincent | Predicting Fiber Orlen tation In Injection Molding[END_REF].

Fig. 12 .Fig. 13 .

 1213 Fig. 12. Stereographic density plots of CFg. Fig. 13. Stereographic density plots ofSFgl.

Fig. 14 .砌. 17 .

 1417 Fig. 14. Stereographic density plots of SFg2.

Table 1 . Size Distribution Results (Average Lengths and Variation Coefficients).

 1


J C Lav亡rous

D Quantin

A Lafon

Crespy

Granulometric Characterization of Short Fiberglass in Reinforced Polypropylene. Relation to Processing Conditions and Mechanical Properties

Keywords: Fiber length, granulometry, image analysis, mechanical properties, reinforced polypropylene, short fiberglass

In most cases, for reinforced thennoplastics, a knowledge of average filler length is not enough infonnation. The entire granulometric filler distribution needs to be known, if structure is to be correlated with properties. We have developed a technique of granulometric detennination based on image analysis to analyse rapidly the evolution of the granulometry of short fiberglass. Once images in grey levels from polarized light microscopy have been acquired, each selected fiber on the frame is individualized and labelled before measurement and data treatment. For the fiber, the size factor chosen is the length, or the maximum Feret diameter. With this tool, we have been able to study more precisely the evolution of the fiber length distribution during processing with a polypropy lene matrix. Finally, we elucidate the dependence of mechanical properties such as impact and tensile strengths on glass filler size.

I. INTRODUCTION

Reinforced thermoplastics are produced by incorporating reinforcing fillers such as fiber glass into polypropylene. They are being used increasingly in applications where a precise prediction of the material characteristics is essential. The properties and performances of such materials are conditio ned by the microtextural characteristics brought by the fillers to the matrix. We can define microtextural characteristics by [ l]: the filler parameters, such as granulometry (size) and granulomorphy (shape); and the filler-matrix parameters, such as spatial organization determined by the orientation and dispersion.

In this paper, the effects of processing on the mechanical properties of reinforced thermoplastics are reported as a function of a microtextural characteristic. In particular, we report the effect of varying filler granulometry (length distributions).

Granulometry of Fiberglass

During the two last decades, there has been considerable interest in granulometry of fibers. Different techniques, such as light scattering [START_REF] Jones | [END_REF]3] or sieving [4], have been devel oped to determine fiberglass granulometry. However, most granulometry determinations scattering, gravitational measurement,...) [START_REF] Allen | Particle S比eMeasure,砒nt[END_REF] are based on a spherical shape model, inadequate for investigating fiber-like particles. Therefore, the most usual technique is length measurement of fibers, using microscopy. For each selected fiber, the diameter of the circle circumscribed to the particle is determined. However, manual measurements are very tedious because the basic sampling is made up of hundreds of fibers.

To bypass methods like this, a semi-automatic image analyser process was developed. For poorly dispersed fibers, it is also possible to treat images and to separate connect ed fibers with recently developed routines [START_REF] Coster | Precis d'analyse d'images[END_REF], although with current personal computers the treatment time is rather long.

It is based on a digital table, where each extremity of the fiber representation is noted [6]. A computer program calculates the fiber length from the table coordinates of

Influence of the Processing on Fiberglass Granulometry

According to Turkowitch and Erwin [START_REF] Turkowitch | [END_REF], the principal causes of fiber damage are fiber fiber interaction, fiber contact with processor and fiber interaction with polymer. These and other authors have shown that considerable fiber bre吐age occurs during processing. The brealcage of fibers depends on their initial lengths (10] and is affected by processing conditions (4,6,10-14). The most significant important cause of fiber bre吐age is shear ing. For example, Vax.man et al [15]. observed extensive fiber damage at low shear. The conditions under which the fibers are mixed into the polymer (11,12,14,16] bring about significant reductions in fiber lengths.

Granulometry一Mechanical Properties Relationship

The relationship between fiberglass length and macroscopic mechanical properties is complex [START_REF] Tancrez | Contribution a l'itude de la fragiliti de composites polypropyll叱 -fi bres de verre courtes moulis par i nj ection[END_REF][START_REF] Denault | and 8.氏ster[END_REF][19][START_REF] Chou | Microstructural design o f fi ber composites[END_REF]. It is well known that the tensile behaviour is dependent on the fiber length. Indeed, elastic modulus can be described by Cox's rule of mixture [START_REF] Cox | [END_REF], involving a fiber length factor.

Fracture mechanisms have been proposed by different authors [START_REF] Tancrez | Contribution a l'itude de la fragiliti de composites polypropyll叱 -fi bres de verre courtes moulis par i nj ection[END_REF]. They point out rela tionships between fracture energy and fiber length. In the same way, a critical length is introduced in the Kelly and Tyson's model [22] (load transfer matrix-fiber by sliding) which determines the kind of fracture mechanism. For granulometric analysis of the fibers in the molded samples and in the pellets (extruded product), the matrix was pyrolized in a muffle furnace for two hours at 500 °C to separate the fibers.

Tenslle Testing

An electro-mechanical tensile tester (Adamel Lhomargy DY26, lvry sur Seine-France) , coupled with a servo-controlled, optical, elongation-measuring system, was used to study the Ioad-elongational behaviour of the specimens. A computer controls the testing machine and facilitates data processing. The testing conditions for elastic modulus deter-mination was room temperature (20 °C) and a speed of 1 mm/mn for the dumbbell cali brated length of 50 mm. For each determination, 10 specimens were tested.

Impact Strength Studies

A mechanical impact tester (Zwick Sl02, Ulm-Germany) with a 4 Joules pendulum was used for Charpy tests. The testing conditions for impact strength measurements were room temperature (20 °C) and 10 specimens tested for each determination. The distance between supports was 40 mm. Impact strength is usually quoted as energy per unit of area.

Short Flberglass Granulometry Determination by Image Analysis

The granulometric approach chosen is individual analysis, where each particle must be sepa rated and individually measured [START_REF] Coster | Precis d'analyse d'images[END_REF]. The process was comprised of three principal steps [l]: 一Image acquisition: a system composed of a sensor (CCD camera) coupled with microscopy allowed images to be captured. Particles were dispersed between two glass microscope slides, in a solution of diethylene glycol (dispersing). The fiber concentration was very low to minimize fiber contacts. For a representative sampling, more than 800 fibers were analysed [24], requiring more than 25 different images. Observations were made with a polarizing microscope (Leitz, Wetzlar-Germany) in transmission mode.

一Image treatment: once the image had been filtered of noise, each selected fiber was individualized and labelled. Captured images were digitalized with a video card. The image treatments improved contrast and corrected light intensity variations. This was done using morphological tools [START_REF] Serra | [END_REF]. Initial images (512*512 pixels) were in grey levels (256 levels). By thresholding, they were transformed into binary images (2 levels). Fibers were coded to 1 and the background to 0, each fiber thus being individualized. All the image manipulations were done with a computer and dedicated software: Visilog (Noesis, V elizy-France).

-Quantitative analysis: for each labelled fiber, we determined the size factor. The data treatment allowed us to determine length distributions. The retained size factor for this cylindrical particle is the exodiameter or the maximum Feret diameter [23].

Each analysed particle had to be totally included in the frame of measurement. Feret diameter data were corrected for the probability of inclusion, longer fibers having a lower probability of inclu sion. The Miles and Lantuejoul method [7] allowed us to correct this bias. Usually, gran ulometric determination offers two kinds of representation:

-Granulometry in number, where each particle is weighted by its own number. An average number length could be represented by: Ld; x n; Ln and a variation coefficient (VC), the ratio between standard deviation and average length.

Ill. RESULTS AND DISCUSSION

Granulometrlc Distribution of SFG1 and SFG2

As with the majority of powders obtained by a breaking process, granulometric dis tributions have a log-normal shape [START_REF] Allen | Particle S比eMeasure,砒nt[END_REF] (see Figs. 1 and2

). The fitting of the granu lometric distributions to a log-normal law was done with acceptable correlation co efficients (see Table II). Using the equation coefficients resulting from the fitting to a log-normal law, it was possible to determine other estimators of the average lengths (in number and weight) and dispersity coefficients (see Table II). Average lengths determined by this approach are lower, due to the length data display (histogram pre sentation).

In Table I, results of short fiberglass before processing show a sharp difference between both average lengths of SFG 1 and SFG2. These quantitative results were confirmed by the granulometric distribution presentation in Figures 1 and2

. The graphs show clearly the positive result of the separation of two short fiberglass populations (SFG 1 and SFG2),

Frequency in number 0,3 S F G 1 ["/ F『 equency in weight 0,3

••••-.p.."二.-•• •• --• -• •1 一一• •• -•• • • ••-••口• • •-•• ••• • • 一一 ` ', ; , 口, d, 0 , 2 仁 •-• •••一• 气.. , .. 一•• …• •••••`-• • 一一 ; "•• • • • • • • • , 0 -4, ./ F G.~2 _ 0,1 o i 口
一••••上"••••••••• •• • sm -1 .七.......... • •--•--• ••-------• •--•--•• 0,2仁 ,o-、 • •----•• •--• • •••• • 一••• • • • • • 一•一睿~. 一一•--一旦. 一-• --.. . 一一一• .一•.一• 一• 一• 一• • 凸 IJ \S FG 2 ,

Evolution of the Granulometry of SFG1 and SFG2 with the Processing

The results of the evolution of the granulornetry with extrusion and injection processes are presented in Table III. We can note a weak but progressive decrease in average length, for both granulornetry by number and by weight, as the processing continues.

The variations in weight-average fiber lengths (L w ) are more important than those in number averages (L n ). In fact, the average L w is more sensitive to fiber length. As con firmed by the representation in Figures 3 and4, we show that it is mainly the longest fibers that deteriorate by breakage during the processing.

lliV'

The dispersity coefficient (D) and variation coefficient (VC) follow the same trend, and can be interpreted in the same way. For SFG2, dispersity decreases with the processing, due to the disappearance of some of the longest fibers by fracture and, consequently, a nar rowing of the granulometric distribution. This effect is less important for SFG 1, because its initial granulometric distribution is narrower.

However, with processing conditions chosen, the size of short fiberglass is little affect ed by processing.

Relatlonshlp Between Granulometry and Mechanical Properties

The results of impact (Charpy) and tensile tests are presented in Table IV. A statistic equality Student's test is carried out on the averages so that each result appears signifi cantly different from the other with a confidence threshold greater than 99,9%. A syn thetic representation is presented in Figure 5. Mechanical properties variations with filler length are important. The evolution of the impact strength as a function of the filler size is inversely proportional to the elastic modulus evolution. For such material, a compro mise in filler size is necessary to optimize the mechanical properties.

IV. CONCLUSIONS

We have developed and described a granulometric tool based on automatic image analy sis. With this tool, we can analyse more precisely the evolution of short fiberglass length distribution during processing. For our processing conditions, fiber mechanical degrada tion is low and affects mainly the longest fibers, with little effect on the number average length.

Different filler sizes were chosen in order to test the mechanical behaviour of reinforced thermoplastics. It is interesting to note different responses even for two such similar prod ucts as SFG 1 and SFG2, which present principally a different granulometric distribution but are in the same granulometric range. The main trend is that the evolution of the resilience as a function of the filler size is inversely proportional to the elastic modulus evolution.

  the end points. The length is expressed in the form of length distribution and calculated averages. The last evolution is, with a minimization of human interventions and faster character ization, automatic images analyser (7-9]. The principal problem encountered particularly for long fiberglass, is the dispersion of fibers for image acquisition. Contacts between fibers must be minimized, in order to individualize each one easily, and to facilitate image treatment. To improve the dispersion, Sawyer [9] treated the fiber surface. The treatment is based on a silane-glass reaction with methyl trimethoxy silane catalysed by acetic acid and a cationic lubricant. However, there are a great diversity of other solutions (1,4,9] like ly to improve, with different efficiencies, the dispersion of fibers.

  of fiberglass filler (Vetrotex, Cbambery-France) were used with dif ferent shape factors (f.):• Long fiber glass (LFG) with f 1 > 100, obtained from 4.5 mm cut thread.• Short fiber glass (SFG1 and SFG2), 100 > f 1 > l.• Fiberglass powder (FGP); 1 > f, 矿 Shape factor is the ratio of length to diameter. Since filler populations have a constant diameter (13 micrometers), shape factor is characterized here by the length. The compos ites were prepared with a polypropylene matrix (Appryl 3030 MNl-Atochem). The filler weight content was constant at 30%.11.2. Short Fiberglass and Flbergl霾sa Powder GenerationShort fiberglass and fiberglass powder were obtained by fragmentation of the longest fibers. After fragmentation, short fiberglass and powder were separated by pneumatic selection (Alpine ATPSO Rotoplex, Augsburg-Germany). During another step, short fibers were divided by the pneumatic process into two populations (SFG 1 and SFG2) character ized by two different average lengths, with two different granulometric distributions. 11.3. Sample Preparation Processing conditions were identical for all specimens injected. A co-rotating twin screw extruder (Clextral BC45, Firminy-France) was used to prepare fiberglass-polypropylene pellets. The diameter of the screw was 50 mm. The extrusion temperature was 230 °C and the screw speed was 100 rpm. An injection moulding machine (Billon, Bellignat-France) with a clamping force of 90 tons was used to mold standard dumbbells. The screw speed was 100 rpm and the injection temperature was 2500C. The holding pressure was 20 bars. The mold temperatllre was 20 °C and the total cycle time was 1 minute. Dumbbell speci mens produced had an active portion IO mm wide and 4 mm thick (according to French standard NFr 51-034 1981). The Charpy samples were cut from the central part of the dumbbells to get samples without notches whose dimensions were 10*4*60 mm (accord ing to French standard NFr 51-035 1983).

LFollowing Tancrez [ 17 ]

 17 n =..1 工ni n; is the number of fibers with a Feret diameter:d;. -Granulometry in weight, where each particle is weighted by its own length (fiber diameters and glass density are constant). A representative average weight length could be: , we used a dispersity factor (D), representing the granulomet nc range: D= 匕 -L,.

  FIGU邸 I Fiber length distributions in number of SFG I (initial) and SFG2 (initial) with log-normal fittings.

FIGURE 2

 2 FIGURE 2 Fiber length distributions in weight of SFG 1 (initial) and SFG2 (initial) with log-normal fittings.

FIGURE 3 FIGURE 4

 34 FIGURE 3 Evolution of SFGl granulometry (in weight) with the processing.

10 FIGURE 5

 105 FIGURE 5 Results of impact and tensile test on powder , short and long fiberglass.

TABLE Ill Granulometric data and processing Average length Average length in number in weight (Ln in microns) (Lw i w in microns)

 Ill

							Variation	Dispersity
							coefficient	coefficient
							(VCn)	(D)
	SFG I (initial) SFG I (after extrusion) SFG I (after injection) SFG2 (initial) SFG2 (after extrusion) SFG2 (after injection)	l l 1	66 62	56 27	19 07	130 124 99 297 234 209	0,99 1,00 0,88 1,16 0,98 0,97	0,97 1,00 0,77 1,34 0,97 0,95

TABLE IV Stiffness and impact strength data

 IV

		PP Matrix	LFG	FGP	SFGI	SFG2
	Elastic modulus (MPa) E composite/E matrix Impact strength	1529 (2,5%)	6847 (3,3%) 4,48 16,9	2855 (2,6%) 1,87 27,6	3088 (1.9%) 2,02 22,9	4280 (2.6%) 2,80 19,9

(kJ/rn勺 (6,0%) (8.8%) (3,1%) (6,4%)