Frédéric Médail
email: frederic.medail@imbe.fr

Alex Baumel
email: alex.baumel@imbe.fr

Special issue of Biological Conservation: "Which rarity to conserve" Using phylogeography to define conservation priorities: the case of narrow endemic plants in the Mediterranean Basin hotspot

Keywords: Biogeography, Conservation genetics, Conservation units, Evolutionary legacy, Mediterranean hotspot, Narrow endemics, Phylogeography, Rare species

The preservation of rare and vulnerable narrow endemics is a key issue in biological conservation since they represents distinct evolutionary units emerging from various temporal processes. Phylogeography has proved to be a relevant tool for the distinction of evolutionary structures resulting from contrasted biogeographical events. However, the historical and evolutionary perspective provided by phylogeography is still curiously underused in plant conservation genetics. Here we perform a comprehensive review of almost all case studies related to the structure of genetic diversity of Mediterranean narrow endemic plants (MNEs) of the Mediterranean Basin hotspot. The use of genetic diversity structure for phylogeographical inference and for the definition of conservation units was examined for ninety-two studies corresponding to eightythree taxa. Most of these taxa are perennial herbs with a narrow ecological niche. Geographical coverage is heavily biased since 91.5% of the analyzed MNEs are located in the north-western part of the Mediterranean region. Half of the studied species have moderate to high genetic diversity, and genetic differentiation is geographically structured in 56% of the case studies indicating that MNEs are not "evolutionary deadends" but are sheltering a strong evolutionary legacy calling for conservation planning at populations' level. However, a minority of studies used these genetic structures to define conservation units. The main insight of this review is that phylogeography is generally overlooked in conservation genetics and that the design of conservation units has not been a priority issue, rather a way to enhance the scope of genetic diversity analyses. Nevertheless, the strong phylogeographic structure revealed in several studies of MNEs underlines the relevance of phylogeography and we argue that comparative phylogeography of several co-occurring taxa could greatly improve the proactive conservation planning of threatened endemic plants within biodiversity hotspots.

Introduction

Facing rapid environmental and biodiversity changes worldwide, it is necessary to go beyond the taxonomic diversity and to consider the putative large gains offered in conservation biology by the other facets of biodiversity, namely evolutionary diversity and functional diversity [START_REF] Pollock | Large conservation gains possible for global biodiversity facets[END_REF]. If there is an increasing interest to include these metrics in community ecology, biogeography and conservation [START_REF] Devictor | Spatial mismatch and congruence between taxonomic, phylogenetic and functional diversity: the need for integrative conservation strategies in a changing world[END_REF][START_REF] Mouquet | Ecophylogenetics: advances and perspectives[END_REF][START_REF] Jarzyna | Detecting the multiple facets of biodiversity[END_REF], there are still some gaps in this fruitful approach. First, the dimension of evolutionary diversity or evolutionary rarity is quasi exclusively considered by the phylogenetic attributes of species, because of the facilitated use of phylogenetic trees in conservation prioritization [START_REF] Tucker | Incorporating geographical and evolutionary rarity into conservation prioritization[END_REF][START_REF] Pollock | Phylogenetic diversity meets conservation policy: small areas are key to preserving eucalypt lineages[END_REF]. This is the domain of ecophylogenetics that merges ecology, biogeography and macroevolution [START_REF] Mouquet | Ecophylogenetics: advances and perspectives[END_REF]. But this macroevolutive perspective masks the need to consider also the evolutionary diversity at a finer regional or local spatial scales, and at the level of species or populations. To fill this gap between multispecies phylogenetic studies and intraspecific phylogeography will allow to better take into account the evolutionary continuum into conservation prioritization [START_REF] Lexer | Next generation' biogeography: towards understanding the drivers of species diversification and persistence[END_REF][START_REF] Carvalho | Spatial conservation prioritization of biodiversity spanning the evolutionary continuum[END_REF]) (Figure 1). Second, most of the studies of evolutionary conservation have focused on large datasets of species irrespective of the consideration of their rarity degree (but see [START_REF] Cadotte | Rarest of the rare: advances in combining evolutionary distinctiveness and scarcity to inform conservation at biogeographical scales[END_REF]. However, the contribution of rare species to predict the impacts of biodiversity loss is crucial, since rare species often being the first to go extinct [START_REF] Gaston | Rarity[END_REF][START_REF] Gaston | The importance of being rare[END_REF]. Furthermore, rare species that are often characterized by low functional redundancy, are likely to support the most vulnerable functions, even in highly diverse ecosystems [START_REF] Mouillot | Rare species support vulnerable functions in high diversity ecosystems[END_REF]). Third, very few studies examining biodiversity facets concerns plants [START_REF] Forest | Preserving the evolutionary potential of floras in biodiversity hotspots[END_REF][START_REF] Thuiller | Are different facets of plant diversity well protected against climate and land cover changes? A test study in the French Alps[END_REF]); however, this group is not governed by the same spatiotemporal processes as most of the vertebrates for example, and the key role of local persistencenotably in severe environmental conditionsmust be also considered [START_REF] García | Persistence, multiple demographic strategies and conservation in long-lived Mediterranean plants[END_REF]. Within the framework of the conservation genetics, the consideration of phylogeography to set Conservation Units (CUs) aims to conserve the evolutionary legacy within species mainly related to isolation process and historical biogeographical events [START_REF] Ryder | Species conservation and systematics: the dilemma of subspecies[END_REF][START_REF] Moritz | Defining ''Evolutionarily Significant Units'' for conservation[END_REF][START_REF] Moritz | The importance of an evolutionary perspective in conservation policy planning[END_REF]. But this approach has been addressed almost exclusively for vertebrate species and very few studies concern plants (e.g. [START_REF] Swarts | Defining conservation units in a complex of morphologically similar, sexually deceptive, highly endangered orchids[END_REF][START_REF] Pouget | Conservation unit allows assessing vulnerability and setting conservation priorities for a Mediterranean endemic plant within the context of extreme urbanization[END_REF]. This distortion between conservation genetics approaches and the numerous insights of phylogeographic studies for plants is quite surprising. Indeed, approaches of multiple facets of rarity for conservation need to consider both phylogenetic diversity but also evolutionary structures of the populations of rare and threatened taxa. This would undoubtedly lead to better conservation prioritization efforts of rare and endangered taxa according to their biogeographical structure, notably within biodiversity hotspots. Among plants that are naturally restricted in terms of global distribution range, narrow endemic species (see Materials and methods for definition) are the target of much conservation attention because of their often reduced, isolated and threatened populations (e.g. [START_REF] López-Pujol | The 'paradigm of extremes': extremely low genetic diversity in an extremely narrow endemic species, Coristospermum huteri (Umbelliferae)[END_REF][START_REF] Jiménez-Mejías | Narrow endemics in European mountains: high genetic diversity within the monospecific genus Pseudomisopates (Plantaginaceae) despite isolation since the late Pleistocene[END_REF]. But again, few studies have defined and taken into account CUs for an effective conservation biogeography (Ladle & Whittaker 2011) of these range-restricted plants and the centres of endemism they mainly define, which are however areas of high conservation concern. Distribution of abnormally high concentrations of narrow endemics can be linked to the local accumulation of palaeoendemic or neoendemic plant species, with the predominant role of long-term stable environments or high topographical heterogeneity respectively [START_REF] Molina-Venegas | Climatic and topographical correlates of plant palaeo-and neoendemism in a Mediterranean biodiversity hotspot[END_REF]. But often, centres of endemism combine these two environmental drivers and they are indicative of refuge areas that have experienced a long-term stability of climate and habitats on a reduced spatial scale (e.g. [START_REF] Jetz | The coincidence of rarity and richness and the potential signature of history in centres of endemism[END_REF][START_REF] Ohlemüller | The coincidence of climatic and species rarity: high risk to small-range species from climate change[END_REF][START_REF] Médail | Glacial refugia influence plant diversity patterns in the Mediterranean Basin[END_REF], Sandell et al. 2011[START_REF] Harrison | Endemism hotspots are linked to stable climatic refugia[END_REF]. In the present review, we examine the contributions of phylogeography to the conservation prioritization of narrow endemic plants within the Mediterranean Biogeographic Region (MBR) (see Materials and methods), one of the 36 biodiversity hotspots of the world [START_REF] Médail | Mediterranean Basin[END_REF]. This important plant diversity (ca. 30,000 species and subspecies = taxa) is mainly concentrated within the 10 regional biodiversity hotspots (Médail & Quézel 1997), in which we can distinguish smaller ones [START_REF] Cañadas | Hotspots within hotspots: endemic plant richness, environmental drivers, and implications for conservation[END_REF]. These areas are characterized by an exceptional concentration of endemic species, notably range-restricted taxa. As quoted by [START_REF] Thompson | Plant evolution in the Mediterranean[END_REF], narrow endemism represents 'the cornerstone of Mediterranean plant diversity': somewhere close to 60% of all native taxa occur only in the MBR and more than one-third (ca. 37%) of the native flora corresponds to narrow endemic species [START_REF] Greuter | Botanical diversity, endemism, rarity, and extinction in the Mediterranean area: an analysis based on the published volumes of Med-Checklist[END_REF], i.e. an huge number estimated of ca. 11,000 narrow endemic taxa. But, this Mediterranean hotspot faces a biome crisis due to strong disparities between habitat loss and protection levels, particularly in the southern and eastern Mediterranean [START_REF] Hoekstra | Confronting a biome crisis: global disparities of habitat loss and protection[END_REF]. Global change threatens the current plant biodiversity, and species distribution modelling suggests a high degree of range contraction for narrow endemics in the MBR [START_REF] Casazza | Climate change hastens the urgency of conservation for range-restricted plant species in the central-northern Mediterranean region[END_REF]. Because current genetic variation can play a major role in providing resilience and local adaptation to future change (e.g. [START_REF] Nielsen | Multispecies genetic objectives in spatial conservation planning[END_REF], it is important to examine if this genetic metric is properly considered into conservation planning of Mediterranean narrow endemic plants (MNEs). Previous studies have shown that despite their narrow geographical ranges MNEs can be characterized by several lineages and CUs concerned by different threats (e.g [START_REF] Pouget | Conservation unit allows assessing vulnerability and setting conservation priorities for a Mediterranean endemic plant within the context of extreme urbanization[END_REF].

We perform a comprehensive review of the studies dealing with genetic diversity structure of narrow endemic plants in the Mediterranean Basin (i) to examine the relevance of phylogeography for the conservation of this priority biological contingent, (ii) to point out the crucial necessity to better include phylogeography into the framework of conservation of the multiple facets of rarity.

Methods

Definition of narrow endemism

Narrow endemism, also known as micro-endemism, local endemism, range-restricted endemism or short-range endemism, is a common form of rarity used to describe very small areas of distribution of taxa (eg., [START_REF] Kruckeberg | Biological aspects of endemism in higher plants[END_REF][START_REF] Gaston | Rarity[END_REF][START_REF] Caesar | Outstanding micro-endemism in New Caledonia: more than one out of ten animal species have a very restricted distribution range[END_REF]. Nevertheless, the geographical quantification of narrow endemism is difficult because it depends on the biogeographical history of the region under consideration, on its geographic structure (notably isolation's degree), on the environmental determinants, on the genetic structure and on the dispersal capacities of each taxon, but also on more subjective parameters such as the taxonomic bias and the scaling effects of the chosen area of study [START_REF] Kruckeberg | Biological aspects of endemism in higher plants[END_REF][START_REF] Peterson | Problems with areal definitions of endemism: the effects of spatial scaling[END_REF]. All these factors explain why there are so many causes promoting endemism, and why there is no exclusive definition of narrow endemism. [START_REF] López-Pujol | The 'paradigm of extremes': extremely low genetic diversity in an extremely narrow endemic species, Coristospermum huteri (Umbelliferae)[END_REF] defined 'extremely narrow endemics' as 'those plant taxa that usually occur in one or very few populations (five or fewer) that total very small species' census sizes (500 or fewer individuals, including reproductive and vegetative)'. This corresponds to the definition of [START_REF] Kruckeberg | Biological aspects of endemism in higher plants[END_REF] which considers 'truly narrow endemics' as extremely local rarities. Since this is the extreme situation of a restricted distribution range for a taxon and it concerns relatively few taxa, we preferred to consider a broader definition of narrow endemism. In this study, a narrow endemic plant is defined as a taxon whose totality (or nearly all, i.e. at least 90% of its occurences) of its populations are distributed within a narrow biogeographical entity, i.e. at the province level or below within the MBR.

Definition of the Mediterranean Biogeographic Region (MBR)

We considered in this review the narrow endemics present in the Mediterranean Biogeographic Region (MBR) defined on the basis of bioclimatic, biogeographical and phytoecological criteria, following the proposal of [START_REF] Médail | Hot-spots analysis for conservation of plant biodiversity in the Mediterranean Basin[END_REF]. Its contours mainly follow, in the northern part, on the maps of vegetation of Europe and of the eastern Mediterranean [START_REF] Quézel | Carte de la végétation potentielle de la région méditerranéenne[END_REF]; the 100-mm isohyet of annual rainfalls remains the standard for defining the southern boundary between the Mediterranean region, versus the Sahara to the south and the Irano-Turanian region to the south-east. The defined MBR thus covers an area of around 2,300,000 km 2 . This practical biogeographical limit has since been used by many international organizations (UNEP/Blue Plan, Conservation International, Critical Ecosystem Partnership Fund). In biogeographical and ecological terms, it is more realistic than a simple approach based solely on climatic factors and not plant assemblages [START_REF] Klausmeyer | Climate change, habitat loss, protected areas and the climate adaptation potential of species in Mediterranean ecosystems worldwide[END_REF], that provides a too restrictive delimitation of the Mediterranean biome. Of course, biogeographical ecotones or transition zones must be considered more than strict boundaries between regions, since in many cases eco-biogeographical limits are gradual [START_REF] Blasi | Relation between climate and vegetation along a mediterranean-temperate boundary in central Italy[END_REF]. It is the reason why we have also considered the narrow endemics that can be called sub-Mediterranean and located at the interface between the MBR and (i) the Euro-Siberian region with the Alps (e.g. Moehringia lebrunii, Primula allionii), the Pyrenees (Glandora oleifolia) or the Balkans (Paramoltkia doerfleri), and (ii) the Irano-Turanian region in Anatolia (e.g. Barbarea integrifolia, Centaurea wiedemanniana, Verbascum alyssifolium).

Selection of narrow endemics of the MBR with their relevant attributes

The list of the narrow endemics of the MBR with their respective characteristics is included in the supplementary material (Table A1). For data compilation to identify studies reporting the use of genetic data for narrow endemic plants of the MBR, we have searched in various databases for the key-word combination 'narrow endemic plant', 'Mediterranean region', 'phylogeography', 'genetic structure', 'genetic diversity', 'genetic variation' in the title. We have also considered further studies cited in these references. We have only kept the ones dealing with the genetic/phylogeographic structures of populations strictly located within the MBR and we have excluded pure phylogenetic studies and those including obscure genetic structure for some complex species group or genera. Our final data set includes 84 studies (see References A1) encompassing 83 narrow endemic plants (see Table A1). The taxonomy refers, without exception, to that adopted by The Plant List (www.theplantlist.org). For each narrow endemic, we have assigned the biogeographical area in which it is distributed. Biogeographical regionalization is the prerequisite for a better understanding of species distribution [START_REF] Kreft | A framework for delineating biogeographical regions based on species distributions[END_REF], and thus for a robust conservation biogeography framework. But as there is no modern and comprehensive biogeographical subdivision of the Mediterranean region at the province (or domain) and sector levels, we have used a combination of various works to characterize the distribution of each narrow endemic: (i) the maps of [START_REF] Rivas-Martínez | Biogeographic map of Europe[END_REF]Moreno Saiz et al. (2013) for the western and central Mediterranean; (ii) the maps of [START_REF] Quézel | Carte de la végétation potentielle de la région méditerranéenne[END_REF] and of [START_REF] Parolly | The high mountain vegetation of Turkey -a state of the art report, including a first annotated conspectus of the major syntaxa[END_REF] for the eastern Mediterranean; (iii) the maps of [START_REF] Deil | Contribution à la connaissance de la phytosociologie et de la biogéographie des groupements rupicoles calcaires du Maroc[END_REF] and of [START_REF] Meddour | Bioclimatologie, phytogéographie et phytosociologie en Algérie[END_REF] for North Africa. Then, for each narrow endemic, we have considered the following items: (i) administrative geographical location; (ii) presence on mainland vs. island; (iii) presence or not within the 52 phylogeographic refugia of plants identified by [START_REF] Médail | Glacial refugia influence plant diversity patterns in the Mediterranean Basin[END_REF] in the MBR; (iv) presence or not within one of the 10 regional biodiversity hotspots of plants defined by Médail and Quézel (1997, completed); (v): ecology with edaphic preference and main habitats occupied, and niche breadth approximated by altitudinal range and ecological specialization, according to indications of national or regional floras or Red data books (see below); (vi) ecological distinctiveness (measures of floristic composition and/or habitat variables comparing populations in parallel to measures of genetic differentiation aiming to decipher environmental differences between CUs); (vii) growth-forms sensu [START_REF] Raunkiaer | The life form of plants and statistical plant geography[END_REF] (i.e. phanerophyte, nanophanerophyte, chamephyte, hemicryptophyte, geophyte, therophyte), which constitute a good proxi for a classification into broad functional groups; (viii) endemism type: we have taken into account, as much as possible, phylogenetic studies to get an accurate estimate of the tempo of divergence, and to distinguish palaeoendemics vs. neoendemics, i.e. respectively a differentiation before or after the Pliocene-Pleistocene transition (ca. 3 My); (ix) caryology, with indications of the chromosome number and the ploidy level according to the Chromosome Counts Database (CCDB, version 1.45: http://ccdb.tau.ac.il/home/), the Chrobase.it (Chromosome numbers for the Italian flora: http://bot.biologia.unipi.it/chrobase/index.php), and other sources notably Flora Iberica (Castroviejo, 1986(Castroviejo, -2015)); (x) genetic data: genetic markers used, mean population genetic diversity (He) and its standard deviation, plastid haplotype number, genetic differentiation (Fst, PhiST or equivalent); (xi) number of known populations, and putative extinct populations according to indications of Red data books and of national or regional floras; (xii) whole distribution area of the taxon considered as its extent of occurence sensu IUCN; (xiii) IUCN categories according to the IUCN Red list of threatened taxa (http://www.iucnredlist.org/), completed by indications in several national or regional Red data books (notably for Spain: Bañares et al. 2003Bañares et al. -2010;;France: UICN France et al. 2012, andItaly: Conti et al. 1997), or in particular papers related to the concerned taxa (see References A1).

Data analysis

The studies included into the data table (Table A1) were analyzed to summarize (i) the temporal trend of publications, (ii) molecular methods used to access genetic diversity, (iii) extent of mean population genetic diversity and genetic differentiation, (iv) reports by the authors of any geographical genetic structure that could lead to inference of phylogeographical structures, (v) explicit use of concepts based on phylogeography to set conservation priorities such as ESUs (Evolutionary Significant Units), CUs (Conservation Units), and MUs (Management Units) or equivalent. Comparison of diversity or differentiation measures across different studies based on different molecular markers, sampling efforts and methods is particularly risky and previously only the magnitude of values was compared [START_REF] Fernández-Mazuecos | Narrow endemics to Mediterranean islands: moderate genetic diversity but narrow climatic niche of the ancient, critically endangered Naufraga (Apiaceae)[END_REF][START_REF] Jiménez-Mejías | Narrow endemics in European mountains: high genetic diversity within the monospecific genus Pseudomisopates (Plantaginaceae) despite isolation since the late Pleistocene[END_REF]. Here, the extent of genetic diversity or differentiation were summarized according to quartile limits (<Q1, Q1-median, median-Q3, >Q3) to help analysis. We have looked only to the main trends: how frequent are studies reporting lack of or very low genetic diversity or genetic differentiation? How frequent are studies reporting high genetic diversity or differentiation? When data were provided by several methods of molecular markers, we used the values from nuclear markers. Results reported by authors about geographical structure of genetic diversity were summarized in Table A1, and then summarized according two categories: existence or not of any geographical genetic diversity structure. The same approach was used to assess conservation implications of the results provided by authors, i.e. if they use or not any concept associated to ESU, CU, MU or equivalent. Summary statistics of genetic diversity data from Table A1 are indicated in Table 1.

Results

Insights about the studied narrow endemics

Our critical review allowed us to retain 84 published studies between 1996 and 2017 (Figure 2; References A1), which correspond to 83 Mediterraneanean narrow endemic (MNE) plants (Table A1) since some taxa were analyzed several times, and on the other hand, some studies have included the analysis of different taxa. There was a strong geographical discrepancy because a large part (54%, n=45) of the studied endemics occur in the Iberian Peninsula. 25 MNEs (i.e. 30%) located in France and/or Italy (including the islands of Corsica, Sardinia and Sicily) were studied. The other Mediterranean territories are clearly under-sampled since only five taxa have been studied in North Africa (only in Morocco), and seven taxa in the eastern Mediterranean which includes countries (Turkey and Greece) characterized by very high level of endemic richness. These 83 MNEs are located at 60% on the continent and 40% on some large Mediterranean islands (in particular Balearic Islands: n=15; Sardinia: n=7; Sicily: n=6). We found that 75% of the MNEs are included in one (or more) of the 52 glacial refuge areas identified by phylogeographical studies, and that 65% of the MNEs occur in one of the 10 regional biodiversity hotspots of plants within the Mediterranean region. If we consider the ecological and functional attributes, we note the preponderance of narrow endemics on limestone substrates (41%, n=34), to the detriment of those present on siliceous outcrops (11%). Ten taxa linked to highly specific substrates (gypsum, ultramafic or volcanic rocks) are included, and only eight that are indifferent to the nature of the substrate. Most of these endemics (ca. 80%) are specialized taxa occuring in one (n=25) or two (n=41) major ecological habitat, whereas only five endemics possess a large ecological niche (number of habitats occupied> 4). Almost all the endemics studied are perennial taxa (n=79, i.e. 94%), since the main growth-forms sensu [START_REF] Raunkiaer | The life form of plants and statistical plant geography[END_REF] are represented by low shrubs (chamephytes: n=32) and perennial herbs (hemicryptophytes: n=26). The evolutionary legacy of these narrow endemics includes, according to phylogenetic studies, 55% of neoendemics and 40% of palaeoendemics (only 4 taxa cannot be properly assigned to one of this category). Most of them (70%, i.e. n=58) are diploid taxa, whereas polyploids are infrequent (n=18). Finally, in relation to the IUCN Red List categories, two thirds of the endemics represent threatened taxa (CR: n=16; EN: n=20; VU=19). Few taxa are therefore included into the categories of least threat (NT: n=3; LC: n=14), the others being not evaluated (n=11) or data deficient (n=1).

Molecular markers and methods

The makers used to assess genetic diversity of MNEs were, in decreasing frequency, AFLP (36%), ptDNA sequences or RFLP (31%), isozymes (30%), ISSR (21%), RAPD (14%), rDNA ITS (10%), SSR (7%) and nuclear gene sequence (1%). Sixty-two per cent of the studies were based on only one marker, 26.5% on two markers, 8% on three markers and 2% on four markers. When more than one molecular method was applied, the most common association was AFLP and plastid DNA markers. Interestingly, SSR markers, which were the most common markers for population genetics in the 1990s and the 2000s, were rarely used (7%) and they are based on a low number of markers (median of 5); this pattern was mainly due to transfer of previously designed SSR loci from a related species and not specific development of SSR markers.

Genetic diversity summary

Our review of data indicates that in general MNEs are not depleted in genetic diversity, since only 25% possess a low genetic diversity (inferior to 9.5%) and 50% of the studies reported a mean population genetic diversity superior to 17% (all markers and AFLP median Table 1); this value is in agreement with a previous review of some 17 narrow endemic case studies reporting a majority of "moderate to high genetic diversity" [START_REF] Jiménez-Mejías | Narrow endemics in European mountains: high genetic diversity within the monospecific genus Pseudomisopates (Plantaginaceae) despite isolation since the late Pleistocene[END_REF]. The range of genetic differentiation was rather large with a first quartile at 9% and a third quartile at 35% (Table1). Gene flow is most often limited in MNEs, with half of the studies reporting a moderate or high differentiation (i.e. Fst or Gst median = 0.16 for isozyme and 0.25 for AFLP; Table 1). However, genetic differentiation is not always geographically structured with only 56% of the studies reporting a geographical pattern, even weak. Bivariate or multivariate analyses (non shown) failed to reveal any significant statistical correlations between genetic indices (diversity and differentiation, computed for all markers or by separating isozyme markers from AFLP markers, with any of the variables used to describe the studies).

Design of conservation units

Only 27% of the case studies showing a geographical structure of populations used explicitly this information to set conservation priorities, and finally only 15 studies (18%) (for a total of 16 MNEs) inferred CUs, MUs, ESUs or Relevant genetic units for conservation (RGUCs) (Pérez-Collazos et al. 2008). Interestingly, 12 of these 15 studies were based on AFLP. Consequently, we observed a strong association between AFLP, geographical structure and the design of CUs.

Discussion

Phylogeographical and genetic structures of Mediterranean narrow endemics (MNEs)

4.1.1. Insights of the data review An exhaustive literature survey of within-species phylogeographical works in the Mediterranean Basin hotspot allows us to retain 83 MNEs, which are part of 84 published papers between 1996 and 2017 (References A1). We fail to detect a significant increase in the number of publications over the years, because of an idiosyncratic rhythm of publications (Figure 2). These 83 MNEs here represent only 0.75% of the ca. 11,000 MNEs (see Introduction), i.e. a tiny part of the Mediterranean plant endemism. This lack of intraspecific phylogeographical studies on plants is not peculiar to the Mediterranean Basin, and other global biodiversity hotspots such as the Cape Floristic Regionalthough of much smaller size (< 90,000 km 2 )include even fewer studies, only 5 carried out until 2013 (see [START_REF] Lexer | Next generation' biogeography: towards understanding the drivers of species diversification and persistence[END_REF]. Geographical coverage is also heavily biased since 91.5% of the analyzed MNEs are located in the north-western part of the Mediterranean Basin, in particular into the Iberian Peninsula, whereas 60% occur on the continent and 40% on Mediterranean islands. Concerning the ecological and functional traits of MNEs [START_REF] Lavergne | Do rock endemic and widespread plant species differ under the Leaf-Height-Seed plant ecology strategy scheme?[END_REF], the studied taxa exhibit a high habitat specialization (80% of the taxa), mainly on calcareous rocky outcrops or cliffs. Calcicolous taxa represent the major contingent (41%), but it remains less important than the similar one (60%) occuring within the endemic flora of a region like the south-east of France which is covered for about half limestone rocks (Médail & Verlaque 1998). The studied MNEs are almost totally characterized by perenial growth forms (94% of the taxa), and this is similar to the percentages obtained in various endemic flora of the MBR, where annual taxa are scarce and comprised between 6 and 15% (Médail & Verlaque 1998;[START_REF] Melendo | The endemic flora in the south of the Iberian Peninsula: taxonomic composition, biological spectrum, pollination, reproductive mode and dispersal[END_REF]. There is a balance between recent (55% of neoendemics) and ancient lineages (40% of palaeoendemics), but diploid MNEs predominate (70%). The evolutionary legacy of these narrow endemics is correctly taken into account even if this panel is slightly unbalanced: palaeoendemics are indeed less frequent in various Mediterranean floras (15-17% of the endemic floras in the East, 28-32% in the West: see [START_REF] Verlaque | Endémisme végétal et paléogéographie dans le bassin méditerranéen[END_REF]. Despite this strong eco-geographical discrepancy, the conservation biogeography framework (Ladle & Whittaker 2011) is rather relevant since 65% of these MNEs occur in one of the 10 regional biodiversity hotspots of plants (Médail & Quézel 1997), and 75% of the MNEs are included in one (or more) of the 52 identified glacial refuge areas [START_REF] Médail | Glacial refugia influence plant diversity patterns in the Mediterranean Basin[END_REF]. Nevertheless, 24 of these refugias (i.e. 46 %) do not include any of the the studied MNEs, these "orphan refugias" are, again, mainly localized in the southern and eastern Mediterranean Basin.

Main results from molecular ecology data

By examining molecular methods, we observed that studies dealing with MNEs were based almost on "low cost" markers such as isozymes, or dominant markers (72% RAPD, ISSR, AFLP). Only 31 % of the case studies investigated plastid molecular variation and very few found enough plastid sequence variation to build a sensu stricto phylogeography approach based on maternal lineages and migration routes inference (e.g. [START_REF] Pouget | Phylogeography sheds light on the central-marginal hypothesis in a Mediterranean narrow endemic plant[END_REF]. This limit could be due to the low mutation rate of plastid genomes and should be surpassed by whole plastid genome sequencing. However MNEs still do not benefit from Next generation sequencing (NGS) technology. We found only one study based on NGS [START_REF] Filatov | Demographic history of speciation in a Senecio altitudinal hybrid zone on Mt[END_REF]), but it was not incorporated in our review because of a focus on hybridization. Despite the importance of issues tackled by genomics, such as local adaptation and speciation, the least that can be said is that the cost of conservation genomics is seriously challenging for MNEs. We note also that phylogeography as intitially defined [START_REF] Avise | Intraspecific phylogeography: the mitochondrial DNA bridge between population genetics and systematics[END_REF][START_REF] Diniz-Filho | Mapping the evolutionary twilight zone: molecular markers, populations and geography[END_REF][START_REF] Avise | Phylogeography: retrospect and prospect[END_REF]) has been very rarely the aim of the studies dealing with genetic diversity of MNEs. Moreover, "up to date" analytical methods based on multi-locus approaches and taking into account the stochasticity of genetic processes (statistical phylogeography: Knowles & Madisson 2002) were not used either, despite the fact that 77% of the studies were published after 2003. Statistical phylogeography allows to decipher the impact of past environmental changes on key demographic and biogeographical processes (local persistence, range shift, or adaptation) which are crucial for our understanding of MNEs' responses to future environmental changes. Of course, not all MNEs have their genetic diversity geographically structured and some are characterized by a lack of genetic differentiation (first quartile below 9 %, Table 1) and one can doubt about the relevance of phylogeography for MNEs. However, half of the studied species have moderate to high genetic diversity (median 17.5%, Table 1) within their populations, and gene flow appeared to be limited in majority of the case studies with 56% of the studies reporting a geographical structure. Such trends indicate that MNEs are not "evolutionary dead-ends" but are sheltering a strong evolutionary legacy that could fuel a high potential for diversification and further evolution. Moreover genetic differentiation among populations can also result from strong genetic drift due to limited gene flow and local population size decrease. In this case, genetic differentiation reflects the vulnerability of local populations to population size reduction which is a relevant issue for conservation.

Conservation units of MNEs: a debriefing and perspectives

All these results indicate that phylogeography and the design of conservation units could be a relevant issue for the long-term conservation of MNEs. But our review reported that only 27% of the studies (i.e. n=15) mentioning a geographical structure used explicitly this information to set conservation priorities of some populations. It is worrying since two thirds of these endemics represent threatened taxa sensu IUCN (CR + EN + VU categories). Analysis of these 15 cases studies revealed that the ESU concept was the most frequently used: seven papers have used it explicitly, and three used a terminology very close to that of ESU such as Significant evolutionary heritage [START_REF] Christe | Footprints of past intensive diversification and structuring in the genus Zelkova (Ulmaceae) in south-western Eurasia[END_REF]; these ten studies have inferred ESUs from genetic clusters. Conservation unit (CU) was used four times and Management unit (MU) only two times. Only two papers developed their own approach, the first one with the definition of Relevant genetic units for conservation (RGUC) (Pérez-Collazos et al. 2008), and the second one including a precise methodology to design how many and which populations to conserve for preserving 99% of genetic diversity (Caujapé-Castells & Pedrola-Montfort 2004). Only two papers introduced their issue with a short review of these concepts of conservation genetics (Pérez-Collazos et al. 2008[START_REF] Pouget | Conservation unit allows assessing vulnerability and setting conservation priorities for a Mediterranean endemic plant within the context of extreme urbanization[END_REF].

Phylogeography, a neglected tool for conservation planning of endemic plants

The general picture from this review is that the design of conservation units was generally overlooked and was not a priority issue, rather a way to enhance the scope of genetic diversity analyses. This is surprising since most of these studies were performed after the publication of the main review papers dealing with the relevance of conservation units [START_REF] Ryder | Species conservation and systematics: the dilemma of subspecies[END_REF][START_REF] Moritz | Defining ''Evolutionarily Significant Units'' for conservation[END_REF][START_REF] Crandall | Considering evolutionary processes in conservation biology[END_REF], Frazer & Bernatchez 2001). This finding is not specific to MNEs because a more general search of literature applied to plants shows that few studies in the world are based on the use of the conservation unit concept.

In the context of weak financial support for biodiversity conservation in the MBR (see the strong geographical bias shown here), the efficiency of conservation science must be optimized. As emphasized by [START_REF] Avise | Phylogeography: retrospect and prospect[END_REF] phylogeography fills the gap between diversification that occurred long time ago, e.g. before the Pleistocene, and population divergence that occurred more recently, e.g. in the late Pleistocene or the Holocene: this is a "grey zone" where speciation is not fully accomplished. One role of phylogeography is to furnish a precise message to conservationists to help them to incorporate "microevolutionary processes" at a moderate financial cost [START_REF] Diniz-Filho | Mapping the evolutionary twilight zone: molecular markers, populations and geography[END_REF]. Overlooking existence of phylogeography structure within species, for example by considering all populations as equal and exchangeable, will lead to an overlook and a risk of lost of irreplaceable evolutionary legacy and the "fuel" for responses to future environmental changes. Indeed, the concept of ESU was created to overcome this latter issue [START_REF] Ryder | Species conservation and systematics: the dilemma of subspecies[END_REF]). On the other hand, the lack of comprehensive researches in systematics and uncertainties on evolutionary units can lead to an excessive splitting during the delineation of taxa that weaken research and conservation. Such problem is recurrent in some genera of the MBR, such as Anchusa, Centaurea, or Limonium and this eventually leads us to not include some case studies in this review because of incongruence between taxa swarm and genetic clusters, e.g. the endemic taxa of Aquilegia in Sardinia [START_REF] Garrido | Spatial genetic structure of Aquilegia taxa endemic to the island of Sardinia[END_REF] or the Genista ephedroides complex [START_REF] De Castro | Genetic structure in the Genista ephedroides complex (Fabaceae) and implications for its present distribution[END_REF]. At this level, delineation of conservation units meets the issue of delineation of taxa, notably at the species rank (Frankham et al. 2012).

Beyond case-by-case studies, geographical trends revealed by "comparative phylogeography" are the clues needed to define areas having a pivotal role for persistence (refugia), diversification (evolutionary cradle) or dispersal (large scale barriers or corridors), i.e. a crucial information for conservation planning at a regional or continental level [START_REF] Carnaval | Stability predicts genetic diversity in the Brazilian Atlantic forest hotspot[END_REF][START_REF] Lexer | Next generation' biogeography: towards understanding the drivers of species diversification and persistence[END_REF][START_REF] Avise | In the light of evolution X: comparative phylogeography[END_REF]). But such approach was never applied to Mediterranean plants. It could be used (i) to link refugia biogeography [START_REF] Médail | Glacial refugia influence plant diversity patterns in the Mediterranean Basin[END_REF] with persistence and migration processes induced by past or current climatic changes, or (ii) to search for spatial congruence between phylogeographical approaches and those examining relative phylogenetic endemism (RPE: see [START_REF] Mishler | Phylogenetic measures of biodiversity and neo-and paleo-endemism in Australian Acacia[END_REF], a metric allowing the distinction between centres of palaeo-and neo-endemism. But if a macroecological approach merging various metrics of evolutionary diversity (phylogenetic diversity, phylogenetic divergence and contrast between palaeo-and neo-endemism) was very rarely applied in the Mediterranean region (but see [START_REF] Molina-Venegas | Climatic and topographical correlates of plant palaeo-and neoendemism in a Mediterranean biodiversity hotspot[END_REF], it could be seriously weakened and biased according to taxonomy. Thus, again, a strong integrative systematics remains a crucial aspect for a robust conservation of Mediterranean plants [START_REF] Simón Porcar | Using floristics, modern systematics and phylogenetics for disentangling biodiversity hotspots across scales: a Mediterranean case study[END_REF].

Because of the very low number of MNEs studied, compared to their huge total diversity in the MBR, the design of conservation units according to an ideal scientific background evaluating genetic and ecological exchangeabilities as well as criteria associated to population fitness will be necessary impossible at the level of the Mediterranean region. Then, other approaches like the focus on species' adaptive variation contained in peripheral isolates (e.g. [START_REF] Macdonald | Peripheral isolates as sources of adaptive diversity under climate change[END_REF]) should be used to set conservation priorities in term of vulnerability and biogeography without requiring conservation genetics for each narrow endemic. For such issues, results from phylogeographies can be used to evaluate the efficiency of new methods to target key zones (ancestral areas, refugia, peripheral isolates and more generally source and sink of diversification) for the conservation of evolutionary diversity [START_REF] Pouget | Spatial mismatches between plant biodiversity facets and evolutionary legacy in the vicinity of a major Mediterranean city[END_REF].

What are the urgencies for the conservation genetics of narrow endemic plants in the Mediterranean hotspot?

Narrow endemic plants of the Mediterranean hotspot are mainly composed of a contingent of stress-tolerant taxa sensu Grime (Médail & Verlaque 1997), able to overcome several environmental changes. This is illustrated sometimes by their local persistence in highly constrained ecological situations, for example along elevational gradient (e.g. [START_REF] Pouget | Phylogeography sheds light on the central-marginal hypothesis in a Mediterranean narrow endemic plant[END_REF]). However, the concept of genetic and ecological exchangeability [START_REF] Crandall | Considering evolutionary processes in conservation biology[END_REF] was very rarely used to design conservation priorities [START_REF] Pouget | Phylogeography sheds light on the central-marginal hypothesis in a Mediterranean narrow endemic plant[END_REF][START_REF] Pouget | Conservation unit allows assessing vulnerability and setting conservation priorities for a Mediterranean endemic plant within the context of extreme urbanization[END_REF] despite the importance of environmental heterogeneity and local adaptation for conservation issues within the MBR (Médail & Quézel 1997).

The importance of the persistence process for the maintenance of local biodiversity is generally underestimated, or even not taken into account, in studies that model the biogeographical destiny of species in the face of global changes (but see [START_REF] Randin | Climate change and plant distribution: local models predict high-elevation persistence[END_REF]. The persistence by longevity and/or vegetative reproduction for long-lived plants in stable habitats such cliffs or rocky outcrops is a major biological trait that explain the long-term viability of relict narrow endemics in the MBR [START_REF] García | Persistence, multiple demographic strategies and conservation in long-lived Mediterranean plants[END_REF][START_REF] García | Life history and population size variability in a relict plant. Different routes towards long-term persistence[END_REF]. Some recent studies that focused on the response of single species (e.g., [START_REF] Fazan | Imprisoned in the Cretan mountains: how relict Zelkova abelicea (Ulmaceae) trees cope with Mediterranean climate[END_REF], about the narrow Cretan endemic tree Zelkova abelicea) or on entire plant communities (e.g., [START_REF] Henne | Reviving extinct Mediterranean forests communities may improve ecosystem potential in a warmer future[END_REF] underline also that we may have underestimated the ecological amplitude and tolerance to climatic changes of many socalled 'temperate' species that thrive on Mediterranean mountains but could tolerate drier and warmer climates. These patterns probably explain why most of the analyzed studies have focused on the long-lived MNEs occuring on stable ecosystems, notably cliff and other rocky habitats. We may note that there could be a bias related to the presence of MNEs in stable and "protected" habitats such as cliffs, possibly because recurent overgrazing by goats and sheeps has already dramatically reduced in more exposed habitats the populations of (other) MNEs devoid of efficient enough adaptation to resist to herbivory pressures. Feral or domesticated herbivores may be indeed a serious problem in some areas, especially on small Mediterranean islands (e.g., [START_REF] Bergmeier | The vegetation of islets in the Aegean and the relation between the occurrence of islet specialists, island size, and grazing[END_REF]. Therefore, there is a crucial need to focus on other plant functional groups, especially on short-lived endemics (annuals and biennials) localized in other habitats than the rupicolous ones. To fill these functional gaps would allow us to better understand the biological and ecological response of the other contingents of MNEs to the current environmental changes. Narrow endemics linked to highly vulnerable and threatened habitats, particularly those in coastal areas and low altitude wetlands, deserve a particular focus because these territories directly suffer the profound human impacts induced by the artificialisation and fragmentation of ecosystems [START_REF] Blondel | Biodiversity and conservation[END_REF][START_REF] Beltrame | Mediterranean wetlands: outlook. First Mediterranean wetlands observatory report[END_REF]). Indeed, demands on biocapacity now exceed the Mediterranean region's supply by more than 150 percent [START_REF] Galli | Mediterranean ecological footprint trends[END_REF], and this hotspot area has lost at least c. 50% of the wetlands that existed in 1900 [START_REF] Beltrame | Mediterranean wetlands: outlook. First Mediterranean wetlands observatory report[END_REF]. This review demonstrates also the deep geographical unbalance between the western and eastern basins, since a large majority of studies (90.5%) concern the western part whereas only seven narrow endemics of the eastern Mediterranean could be included in this analysis. This distortion does not reflect at all the distribution of the major hotspots of Mediterranean endemism. There is in fact a bipolar concentration of plant endemism on both sides of the Mediterranean Basin (Médail & Quézel 1997) ). We thus note the enormous gaps in the knowledge of the genetic structures of this still little studied group, and the consequences that this entails in the conservation prioritization.

It is therefore necessary to develop research on the least studied functional groups of narrow endemics (annuals and biennial herbs), on the less stable habitats which are the most threatened by landscape dynamics (grasslands, wetlands, forest edges), and on the least known regional hotspots of the MBR (Maghreb, Balkans, Levant, Turkey).

Conclusions

Our review, although based on almost all the studies related to the genetic structure of narrow endemic plants of the Mediterranean hotspot, clearly shown that a strong genetic differentiation is potentially present within these taxa and that phylogeography is a highly relevant approach to set conservation priorities. However systematics, phylogeography and conservation units must be linked by the need to clearly delineate evolutionary entities. Profound progresses on these three issues are needed for a better knowledge and a robust conservation biogeography of narrow endemics. Integrative approaches based on biological, ecological, genetic and even genomic data are also needed, in relation with a good characterization of the environmental heterogeneity of habitats on the local and micro scales (e.g. [START_REF] Mee | The ecological and evolutionary implications of microrefugia[END_REF]. Added to the functional and geographic gaps shown here, the challenge is therefore outrageously high. Conservation biogeography researches dealing with ecological and biological uncertainties associated to our lack of knowledge on cryptic evolutionary legacy are therefore needed for one of the largest and hottest biodiversity hotspot of the world.

Fig. 2 .

 2 Fig. 2. Annual records of publications reporting the use of genetic data for Mediterranean narrow endemic plants (total nuber of studies = 84, corresponding to 83 studied taxa).

Table 1

 1 Summary statistics of genetic diversity and differentiation for the 83 case-studies of Mediterranean narrow endemic plants (MNEs).

			Number	Mean population		Genetic
			of	genetic diversity		differentiation
			markers
				All	Isozyme	AFL	All	Isozyme	AFL
				marker	s	P	marker	s	P
				s			s
	Minimum		1	0	0	0	0	0	0.01
	1st quartile		1	0.09	0.06	0.10 0.09	0.03	0.12
	Median		1	0.17	0.11	0.17 0.22	0.16	0.25
	Mean		1.5	0.21	0.17	0.21 0.24	0.19	0.27
	3rd quartile		2	0.24	0.23	0.24 0.35	0.27	0.39
	Maximum		4	0.76	0.56	0.74 0.85	0.85	0.58
	Number	of	83	67	24	24	72	24	23
	studies

  : (i) in the eastern basin, in Turkey (including Irano-Turanian and Euro-Siberian regions) with ca. 3100 endemic species (endemism rate of 31%: Şekercioğlu et al. 2011, modified) and in Greece with 1459 endemic taxa (endemism rate of 22 %: Dimopoulos et al. 2016); (ii) in the western basin, in Spain (mainland + Balearics islands) with 1335 endemic taxa (endemism rate of 19.3%: Aedo et al. 2013), and Morocco with 879 endemic taxa (endemism rate of 18.7%: Rankou et al. 2013

Acknowledgments

We thank Prof. François Munoz for his invitation to write this review for this special issue of Biological Conservation "Making sense of the multiple facets of rarity for conservation" and for his comments and improvements on the preliminary drafts of the manuscript, to Dr. Katia Diadema and Manuel Cartereau for some unpublished data concerning the Mediterranean narrow endemics analyzed in this review.


Joao Santos
email: santosjmo@gmail.com

Veronique Cerezo

Khedoudja Soudani

Sara Bressi

A comparative life cycle assessment of hot mixes asphalt containing bituminous binder modified with waste and virgin polymers

Keywords: Life cycle assessment, Waste nitrile rubber (NBR), EthyleneVinyl-Acetate (EVA), Modified bituminous binder

This paper presents the results of a life cycle assessment undertaken to compare the potential environmental impacts associated with the use of asphalt surface mixtures produced with polymer modified bitumen with those of a conventional asphalt surface mixture. Seven types of hot mix asphalt mixtures to be used in the surface course are compared, among which one produced with virgin materials and conventional binder, which is used as a reference, and six alternative mixtures containing a bituminous binder modified with different percentages of waste nitrile rubber from shoe sole and Ethylene-Vinyl-Acetate polymer. The ILCD impact assessment method at midpoint level was adopted to assess the environmental performance of the wearing course of a French pavement structure over a 30-year project analysis period considering the following pavement life cycle phases: (1) extraction of raw materials, modification of the bituminous binder and mixtures production; (2) transportation of materials, and; (3) construction and maintenance and rehabilitation.

Introduction

Recently, increased traffic levels, heavier loads, and extreme weather conditions have urged road authorities to develop new, or advance existing solutions, in order to improve the resistance of the road pavements to the adverse effects of mechanical and environmental loading.

Concomitantly, as greenhouse gas (GHG) and their effect on the climate are increasingly in the spotlight with respect to policy, legislation and general public's concern, the pavement industry and scientific community have been challenged to improve conventional asphalt mixtures materials by developing more sustainable technologies.

Asphalt binder modification by means of polymer addition, either virgin or recycled and used individually or in a blend mode, has proved to have the potential to enhance the bituminous properties, most typically those related to high temperature performance characteristics, thereby postponing the effects of permanent deformation and fatigue damage [START_REF] Airey | Fundamental binder and practical mixture evaluation of polymer modified bituminous materials[END_REF][START_REF] Kumar | Strength characteristics of polymer modified mixes[END_REF]. However, depending on their nature, the manufacturing of these polymers can also increase the environmental impacts of the asphalt binder in the mixture [START_REF] Van Dam | Towards sustainable pavement systems-a reference document[END_REF].

Therefore, it is of paramount importance to objectively quantify the extent to which the potential environmental benefits associated with the decreasing amount of materials needed over the pavement life cycle as a consequence of the increased service life of pavements is not offset by the environmental impacts originated by their production.

To the best of the authors' knowledge, the only study existing in the literature assessing the environmental impacts resulting from using polymers to modify asphalt binder was performed by [START_REF] Butt | Considering the benefits of asphalt modification using a new technical life cycle assessment framework[END_REF]. However, neither the waste nitrile rubber (NBR) nor the Ethylene-Vinyl-Acetate (EVA) polymers were among those studied.

Given the circumstances above stated, the study presented in this paper aims to investigate the extent to which the use of polymer modified bitumen (PMB) in asphalt mixtures applied in the wearing course of flexible road pavements is beneficial from an environmental point of view.

Nomenclature

Methodology

A comparative process-based life cycle assessment (LCA) study was performed according to the ISO 14040 series [START_REF]International Standard ISO 14040: environmental management -life cycle assessment: principles and framework[END_REF][START_REF]environmental management -life cycle assessment: requirements and guidelines[END_REF] and the University of California Pavement Research Center [START_REF] Harvey | Pavement life cycle assessment workshop: discussion summary and guidelines[END_REF] to compare the potential environmental impacts of different asphalt mixtures adopted in the construction, maintenance and rehabilitation (M&R) of a road pavement section during its life cycle.

The stages adopted in this study include goal and scope definition, inventory analysis, impact assessment, and interpretation.

Goal and scope definition

Goal

The main goal of this study is to compare the potential life cycle environmental impacts arising from the use of asphalt surface mixtures containing different percentages and types of PMB with those of a conventional asphalt surface mixture in the construction and M&R of wearing courses of flexible road pavements (baseline scenario).

Furthermore, in order to assess the robustness of the results to changes in the methodological assumptions, a scenario analysis was performed by considering two alternative scenarios. In the first one, hereafter named AS1, it is assumed that the use of any type of PMB leads to an increase in the durability of the pavement equal to 20%, or in other words 2 years. In the second one, hereafter named AS2, it is assumed that the asphalt plant is run by natural gas rather than by heavy fuel oil (HFO). This is a plausible scenario that stands the best chance of becoming the actual and near-term future practice, thereby replacing the baseline scenario, as new asphalt plants are increasingly switching to natural gas because of its general lower price and cleaner-burning properties.

The comparative findings of this study are intended to be used by highway agencies and pavement practitioners to make more assertive judgments on the pros and contras associated with the use of binder modifiers agents for enhancing the performance of road pavements throughout their life cycle.

System description and boundaries

The system boundaries define the unit processes considered in the LCA study and were drawn to cover the following pavement life cycle phases, modeled through individual but interconnected modules: (1) extraction of materials and mixtures production, consisting of the acquisition and processing of raw materials, and the mixing process of asphalt mixtures in plant; (2) construction and M&R, including all construction and M&R procedures and related construction equipment usage; (3) transportation of materials, accounting for the transportation of materials to and from the construction site and between intermediate facilities; and (4) end-of-life (EOL), which models the destination of the pavement structure after the project analysis period (PAP).

The upstream emissions and resources consumption associated with the production of the energy sources used to power the different processes and construction equipment were also included in the system boundaries. On the other hand, construction equipment, road-related safety and signaling equipment (including road marking), road accessories (fences, road lighting software, etc.), construction and M&R of the remaining layers of the pavement structure and the earthworks required to build the platform over which the pavement foundation will be built were not included in the system boundaries. The environmental impacts resulting from the construction and M&R of the remaining layers were disregarded because their geometry and type of mixture is the same regardless of the type of the mixture considered in the wearing course.

Functional unit

The functional unit is the central core of any LCA and forms the basis for comparisons between different systems with the same utility for the same function. In the pavement domain, this means a unit of pavement that can safely and efficiently carry the same traffic over the same PAP. Then, it is defined by their geometry, service life and level of traffic supported.

The functional unit of the case study presented in this paper is a typical French highway section of 1-km length, composed of two independent roadways, each with 2 lanes with an individual width of 3.5m. The PAP is 30 years, starting in 2015. The initial two-way average annual daily traffic (AADT) was considered to be equal to 6500 vehicles/day, of which 33% are heavy duty vehicles (HDV) equality divided between rigid HDV and articulated HDV.

The geometric characteristics of the wearing course considered are presented in Figure 1. As for pavement maintenance, a pavement M&R strategy derived from French practice was considered in the baseline scenario [START_REF] Laurent | Evaluation économique des chaussées en béton et classiques sur le réseau routier national français[END_REF]. It is based on the assumption that asphalt mixtures using conventional binder and PMB will perform likewise. The features of the M&R activities undertaken in the baseline scenario, as well as the application timing are displayed in Figure 1. In the AS1 the increase in the durability of the pavement equal to 20% means that the first and second M&R activities will take place in the years 2026 and 2036. Therefore, there will be no need of applying the 3 rd M&R activity, as scheduled in the baseline scenario. In the AS2 a pavement M&R strategy equal to that of the baseline scenario is adopted.

Table 1 presents the formulations of the PMB adopted in the alternative asphalt surface mixtures applied in the wearing courses, as determined by [START_REF] Soudani | Contribution à l'Etude du Comportement Rhéologique des Bitumes Modifiés par le Déchet Polymérique NBR -Influence sur les Propriétés Mécaniques et d'Adhérences des Enrobés[END_REF]. The design of all mixtures consists of 94.52% aggregates and 5.48% bituminous binder by weigh of asphalt mixture.

Data sources and data quality requirements

The inventory data required to perform a LCA study are classified into two categories: primary and secondary data. Primary data are those specific to the production processes for the product or service studied in the LCA. In turn, secondary data represent generic or average data for the product or service subject to analysis [START_REF]International reference life cycle data system (ILCD) handbook -general guide for life cycle assessment -detailed guidance[END_REF].

In this study the data sources were selected in order to be as much time, geographical and technological representative as possible. That means that the most recent and truthful data representing French processes and conditions were used as inputs and outputs when modelling the processes covered by the several sub-components integrating the system boundaries. Specifically, the primary data include mainly: (1) the composition of the mixtures; (2) the annual fuel consumption, production and life period of asphalt mixing plants; [START_REF] Van Dam | Towards sustainable pavement systems-a reference document[END_REF] transportation distances; and (3) construction vehicles fleet composition.

Regarding the secondary data, they are mainly related to the inventory analysis of (1) raw materials, (2) fuels, and (3) construction, transportation, and on-road vehicles operation, and were obtained from existing publicly available reports and the ecoinvent database version 3.2, but modified whenever possible and suitable to best approximate French conditions by using French energy inputs/mixes.

Life cycle inventory

The life cycle inventory (LCI) stage consists of the real data collection and modelling of the system. In addition to the data sources, it relies on the several models selected for modelling the processes analyzed by the several considered sub-systems that make up the whole system.

Materials extraction and mixtures production phase

This phase addresses the environmental burdens arising from the acquisition and processing of the materials applied during the initial construction and future M&R of a road pavement segment. This includes all materials manufacturing processes, from extraction of raw materials to their transformation into a pavement input material (material extraction sub-phase), ending with the mixture production at a mixing plant (materials production sub-phase). The latter subphase accounts for the environmental burdens associated with the operation of the (1) mixing plant (i.e., dryer, hot screen, mixers, etc.), (2) wheel loader during the movement of aggregates from the stockpiles to the feed bins and (3) electronic group of the asphalt plant setup.

Materials extraction sub-phase

The aggregates required to produce the asphalt mixtures were modelled as gravel and the LCI data associated with their production were obtained from the unit process "gravel, crushed | gravel production, crushed" of the ecoinvent database. Also taken from the aforementioned database was the LCI data corresponding to the production of asphalt binder ("pitch | petroleum refinery operation") and EVA polymer ("ethylene vinyl acetate copolymer | ethylene vinyl acetate copolymer production"). The EVA polymer is assumed to be posteriorly transported to the asphalt plant facility where is located a PMB production plant.

As for the NBR processing (sole shoe milling), a representative production rate of a Comas FD1-A milling shoe sole machine equal to 90 kg milled shoe sole/hour was considered when determining the electricity consumption requirements (200 kWh/ton milled shoe sole) [12].

Furthermore, from the perspective of the LCA methodology, the consideration of NBR as pavement material was made on the basis of the "cut-off" approach [START_REF] Schrijvers | Developing a systematic framework for consistent allocation in LCA[END_REF]. The production of the several types of PMB (Table 1) is assumed to be performed according to the average conditions allowed by an IKA DISPAX-REACTOR® DR 2000/50 PMB production plant [14]. They corresponds to a production rate of approximately 26 ton PMB/hour, to which it is associated an electricity consumption of 7.1 kWh/ton of PMB and an energy consumption for thermal oil heating of approximately 98 MJ/ton of PMB.

The LCI data related to the production and distribution of those energy resources representing the French conditions were posteriorly taken from the ecoinvent database.

Mixtures production sub-phase

This sub-phase addresses the LCI of the asphalt mixtures production process. In this case study it was assumed that all asphalt mixtures were produced through a conventional heavy fuel oil-fired batch mixing plant. The energy required for producing 1 ton of asphalt mixture and storing the binder in the asphalt plant were respectively 255 and 40 MJ. The fuel consumed by the wheel loader was 0.194 liters and the electricity consumed by the electric group of the asphalt plant was 5 MJ per ton of asphalt mixture produced. The values presented above correspond to the average French practices. Finally, the LCI data corresponding to the process "heat production, heavy fuel oil, at industrial furnace 1MW | heat, district or industrial, other than natural gas | cut-off, U" existing in the ecoinvent database was taken as reference when modelling the operation of the burner existing in the asphalt plant during the production of the mixtures.

Construction and M&R phase

In the construction and M&R phase, the environmental burdens are due to the combustion-related emissions from the construction machinery operation. The consumption-related emissions associated with the operation of each construction equipment were determined by combining the LCI data corresponding to the ecoinvent database process "machine operation, diesel, >= 74.57 kW, high load factor | machine operation, diesel, >= 74.57 kW, high load factor" with the typical productivity of each operation involved in pavement construction and maintenance activities. Furthermore, it was considered that the paving operations were equal for all the mixtures analyzed.

Transportation of materials phase

The environmental impacts resulting from the transportation of materials are due to the emissions released during the combustion process of the transportation vehicles when performing two-way trips from the site where the extraction of the raw materials takes place to the asphalt plant and from the asphalt plant to the work site. All materials and mixtures were assumed to be hauled by HDV, and a modified version of the ecoinvent database process "transport, freight, lorry >32 metric ton, EURO4 | transport, freight, lorry >32 metric ton, EURO4 | cut-off, U" was used to determine the environmental burdens associated with the transportation of materials movements. The original process was modified in order to disregard the inventory corresponding to the construction of the road infrastructure that is considered by the original process. The transportation distances considered for each material and mixtures used in this case study are representative of the French conditions and are shown in Table 2.

EOL phase

When a road pavement reaches the end of the PAP, it can be given two main destinations: (1) remain in place; or (2) be removed. In this case study, it is assumed that the pavement remains in place and undergoes the M&R activity illustrated in Figure 1. The environmental burdens assigned to this phase are due to the materials extraction and mixtures production and the combustion-related emissions from the use of the construction equipment and transportation HDV.

Life cycle impact assessment

The life cycle impact assessment (LCIA) stage of the standardized LCA methodology comprises several steps, namely, classification, characterization, normalization, group and weighting [START_REF]International Standard ISO 14040: environmental management -life cycle assessment: principles and framework[END_REF]. Among these steps, only classification and characterization were undertaken in this study. The calculation of the impact category indicator scores was performed at midpoint level by applying the ILCD 1.0.8 2016 method [START_REF]International reference life cycle data system (ILCD) handbook -recommendations for life cycle assessment in the European context -detailed guidance[END_REF]. Finally, the OpenLCA software version 1.5.0 was used for modelling the processes analyzed in this case study [START_REF] Greendelta | Open LCA[END_REF].

Results

Baseline scenario

Table 3 summarizes the LCIA results for the baseline and alternatives scenarios in which the conventional bituminous binder (no modified) is used in the production of the asphalt surface mixtures. Figure 2 shows the relative variation of the impacts scores for each asphalt surface mixture produced with PMB in relation to those associated with the conventional asphalt surface mixture. The relative values should be understood as follows: positive relative numbers mean that the use of PMB improves the LCIA results in relation to those associated with the conventional asphalt surface mixture. In turn, negative numbers represent a worsening of the environmental profile. The analysis of the results presented in this figure shows that overall the use of EVA polymer as a modifier agent leads to a deterioration of the life cycle environmental profile of the pavement structure in relation to that corresponding to the use of conventional (no modified) binder. The only exception to this general trend is observed for the impact categories ionizing radiation, ozone layer depletion and land use. Given those results, it is not surprising that the greatest negative relative variation of the impacts scores are observed when the binder is modified by means of the use of 5% of EVA polymer by weight of binder. In these circumstances, the greatest relative variation in absolute value (approximately 12%) is registered in the impact category freshwater eutrophication.

In turn, the use of NBR polymer as a modifier agent leads to an improvement of the life cycle environmental profile of the pavement structure in relation to that corresponding to the use of conventional (no modified) binder. That improvement is observed for all impact categories and can be as high as approximately 4% (ozone layer depletion) whereas the lowest and marginal improvement is observed in the impact category land use (approximately 0.1%). Notwithstanding the environmental benefits obtained with the use of the NBR polymer, when it is combined with the EVA polymer in a formulation in which the total amount of polymer is equal to 2%, it is not enough to offset the negative effects resulting from the use of the EVA polymer. However, when the total amount of polymer totals 5% and the percentages of NBR and EVA polymers are equal to 75% and 25% of that value, in 9 out of 15 impact categories the positive environmental effects resulting from the use of NBR polymer cancels out the negative environmental effects associated with the use of EVA polymer. The greatest relative improvement is once again observed in the impact category ozone layer depletion (approximately 4%), whereas the most tenuous is registered in the impact category respiratory effects.

Alternative scenarios

Figures 3 and4 show respectively for AS1 and AS2 the relative variation of the impacts scores for each alternative asphalt surface mixture in relation to those corresponding to the use of the conventional asphalt surface mixture in the baseline scenario.

The analysis of Figure 3 reveals that if the use of PMB in asphalt mixtures has the capacity to improve pavement performance according to the conditions considered in this case study, then considerable environmental benefits can be obtained regardless of the type of PMB used in the asphalt mixture applied in the wearing course of the pavement structure and subsequent M&R activities. The most prominent reduction in the impacts scores are obtained when the total percentage of polymer in the PMB amounts to 5%, and particularly when that percentage is fully made up of NBR polymer. The greatest life cycle impacts saving is equal to approximately 21% (impact category ozone layer depletion in the asphalt mixture adopting PMB 5%(100NBR%;0%EVA)) while the lowest life cycle impacts saving is equal to approximately 9% (impact category freshwater eutrophication in the asphalt mixture adopting PMB 5%(0NBR%;100%EVA)). When analyzing the extent to which the alternative asphalt plant fuel affects the variation of the scores of a given impact category across the alternatives asphalt surface mixtures, Figure 4 shows that overall shifting from HFO to natural gas offers considerable potential and uniform environmental benefits across the six alternative asphalt surface mixtures. The most expressive improvements in the environmental profiles are observed in the impact category respiratory effects (approximately 30%) followed by the impact category acidification (approximately 28%). The only exception to the global improvement in the environmental profiles resulting from changing the type of fuel consumed by the asphalt plant is observed for the impact category freshwater eutrophication, which is expected to see its scores increased by roughly 4%.

Summary and conclusions

In this paper, the results of a process-based LCA of the construction and M&R of the wearing course of a French road pavement section using PMB asphalt surface mixtures, were presented and compared with those in which a conventional asphalt surface mixture (no modified binder) is alternatively applied. Furthermore, two alternative modelling scenarios were considered. In the baseline and second alternative scenarios the conventional asphalt surface mixture (using no modified binder) and the six alternative asphalt surface mixtures (using PMB) were assumed to perform likewise throughout the pavement life cycle. In turn, in the first alternative scenario, the six alternative asphalt surface mixtures were assumed to extend the service life of the wearing course, comparatively to that provided by conventional asphalt surface mixture.

The life cycle of the road pavement sections was divided into four main phases: (1) materials extraction and mixtures production; (2) construction and M&R; (3) transportation of materials; and (4) EOL. The LCI of inputs and outputs associated with the processes considered by the several pavement life cycle phases was determined by combining primary data, representing the current French practices and conditions, with secondary data taken primarily from the ecoinvent database version 3.2. The ILCD 1.0.8 2016 impact assessment method was adopted to characterize the environmental performance of the road pavement section.

The LCIA results of the case study showed that in the baseline scenario the use of EVA polymer as a modifier agent leads to a deterioration of the life cycle environmental profile of the pavement structure in relation to that corresponding to the use of conventional (no modified) binder. This result contrast with that in which the NBR is the binder modifier agents, as it was found to improve the life cycle environmental performance of the road pavement section.

However, if the use of PMB asphalt mixtures succeed in improving pavement performance as proven in literature, thereby reducing the frequency of the application of M&R activities, then considerable environmental benefits can be obtained regardless of the type of PMB used in the asphalt mixture implemented in the wearing course of the pavement structure and subsequent M&R activities.

Fig. 1 .

 1 Fig. 1. Geometric characteristics of the flexible pavement structure and M&R strategy adopted in the baseline scenario. Key: AC-asphalt concrete.

Table 1 .

 1 Formulation of the several PMB considered in the case study.

					PMB ID
		1	2	3	4	5	6
	Total % of polymer (by weight of binder)	2	2	5	5	5	5
	% of NBR (by weight of polymer)	50	75	50	75	100	0
	% of EVA (by weight of polymer)	50	25	50	25	0	100

Table 2 .

 2 Transportation distances considered in the case study.

	Type of material	One-way trip distance (km)
	Aggregates	20
	Binder	100
	Asphalt mixtures 20
	EVA	100
	NBR	100

Table 3 .

 3 LCIA results referring to the case in which the bituminous binder used in the production of the asphalt surface mixtures is not modified.

	Impact category	Reference unit	Baseline scenario	Alternative scenario AS1 AS2
	CC	kg CO2-Eq	453306	371038	406746
	Ac	mol H+-Eq	4455	3647	3186
	FEco	CTUh.m3.yr	2465819	2018343	2375463
	FEu	kg P-Eq	63	52	66
	EQ-IR	mol N-Eq	8.3E-1	6.8E-1	7.7E-1
	MEu	kg N-Eq	520	425	440
	TEu	mol N-Eq	5768	4721	4884
	CE	CTUh	2.1E-2	1.7E-2	2.1E-2
	HH-IR	kg U235-Eq	136657	111858	128349
	NoCE	CTUh	9.2E-2	7.5E-2	8.7E-2
	OLD	kg CFC-11-Eq	3.2E-1	2.6E-1	3.1E-1
	POC	kg ethylene-Eq	2138	1750	1842
	RE	kg PM2.5-Eq	378	309	262
	LU	kg Soil Organic Carbon	5277339	4319656	4907831
	MFR	kg Sb-Eq	29	23	29

  Relative variation of the LCIA results obtained in the AS1 in relation to those corresponding to the use of conventional asphalt surface mixture (no PMB) in the baseline scenario. Key: Polymer%(NBR%;EVA%). Fig.4. Relative variation of the LCIA results obtained in the AS2 in relation to those corresponding to the use of conventional asphalt surface mixture (no PMB) in the baseline scenario. Key: Polymer%(NBR%;EVA%).

	2%(50%;50%)		2%(75%;25%)	5%(50%;50%)
	5%(75%;25%)		5%(100%;0%)	5%(0%;100%)
	LU	MFR	-5% 0% 5%	CC	Ac	FEco	0% 5%(50%;50%) 5%(0%;100%)	MFR	Ac 2%(50%;50%) 25% CC 5%(75%;25%)	2%(75%;25%) 5%(100%;0%)
	POC RE	-15% -10%			EQ-IR FEu	RE	LU		5% 10% 15% 20%	FEu FEco
	OLD					MEu	POC			0%	EQ-IR
	NoCE				TEu
		HH-IR	CE			OLD			MEu
								NoCE		TEu
	Fig. 2. Relative variation of the LCIA results in the baseline scenario for each			HH-IR	CE
	alternative asphalt surface mixture. Key: Polymer%(NBR%;EVA%).
							0% 10% 20% 30% 40% 2%(50%;50%) CC Ac 5%(75%;25%) Fig. 3. -10% POC RE LU MFR 0% 5%(50%;50%) 5%(0%;100%)	FEco FEu EQ-IR 2%(75%;25%) 5%(100%;0%)
							OLD			MEu
								NoCE		TEu
									HH-IR	CE


A Weiss
email: aweiss@isae.fr

L Michel
email: lmichel@isae.fr

S Mahdi

E Cros

J J Barrau
email: barrau@cict.fr

Influence of ply-drop position in thickness direction on static and fatigue loading behaviour of carbon fibre epoxy laminates

Keywords: ply-drop, fatigue, damage modes, numerical simulation, composite

The influence of ply-drop position in thickness direction under fatigue loading (R = -1) has been studied. Two configurations with different ply-drop positions which drop from 20 to 12 plies have been compared. Fatigue tests and finite element simulations were performed to identify the damage modes, spots and kinetics before final failure.

INTRODUCTION

Recent developments in structural design show an increasing application of laminated composite materials in complex structures. To match structural requirements, the thickness of structural parts has to vary depending on the introduced loads. These thickness variations are realised by so called ply drop-offs. In these zones the state of stress changes and stress concentrations are expected. Studies about damage modes and propagation in these zones caused by stress concentrations have already been carried out [START_REF] Murri | Fatigue and damage tolerance analysis of a hybrid composite tapered flexbeam[END_REF][START_REF] Steeves | Compressive strength of composite laminates with terminated internal plies[END_REF][START_REF] Thomsen | Interface failure at ply drops in CFRP/sandwich panels[END_REF][START_REF] Ganesan | Progressive failure and post-buckling response of tapered composite plates under uni-axial compression[END_REF]. For a simple specimen geometry some design guidelines have been proposed to avoid or minimise damage [START_REF] Shim | Role of delamination and interlaminar fatigue in the failure of laminates with ply dropoffs[END_REF][START_REF] Mukherjee | Design guidelines for ply drop-off in laminated composite structures[END_REF]. These basic studies give an idea about the influence of ply-drops on the static and fatigue load behaviour. The present study goes further in terms of mechanical behaviour of a specimen with several ply-drops. The objective of the study is to evaluate the influence of ply-drop position on structural resistance on static and fatigue loading and finally on the SN curve. To do so two ply-drop configurations representative of industrial applications with different ply-drop positions have been tested. Damage spots as well as the damage kinetics before final failure were observed during fatigue mechanical tests. The observed phenomena and kinetics contributes to explain the differences in fatigue lifetime of the two tested configurations. Some phenomena observed experimentally are also analysed with the help of the interlaminar stress state calculated with a finite element model.

SPECIMEN SETUP AND CONTROL DURING TEST

Testing material and specimens' dimensions

The used material is a pre-impregnated carbon fibre/epoxy composite delivered by Hexcel (T700-M21 268 g), the thickness of the UD plies is 0.25 mm. The ply-drop zone is non symmetrical with an angle of 7°.

Stacking sequence

Test specimen drop from 20 to 12 plies by respecting the usual guidelines for ply-drop zones. The base laminates of the thick and the thin section are strongly orientated with 50% of the plies at 0°. These base laminates and the dropped plies are the same for the two tested configurations. As the first two ply-drops close to the thin section are considered the most critical for damage initiation, the position of these ply-drops has been varied. To vary this position the order of ply-drops has been permuted which leads to a configuration with ply-drops 1and 2 close to the mid plane (V int ) and another one with ply-drops 1 and 2 far from the mid plane (V ext ) as shown in Figure 2.

V int V ext

Testing conditions and damage observation

All static and fatigue tests have been carried out on a hydraulic Instron testing machine with a maximal capacity of 300 kN. Under compression load the specimens' non symmetry creates a bending moment which causes a displacement of the specimen perpendicular to the load direction. To avoid this deflection which is not representative of a panel structure under real conditions, the specimens have been clamped in an antibuckling device (Figure 3). Furthermore this device maintains the specimens' position while tested. Teflon pads have been inserted to minimise friction in the non-clamped zone.

Figure 3: Specimen clamped in the anti-buckling device

To monitor the damages during fatigue loading the tests have been stopped regularly and both sides have been observed with an optical microscope. As the specimen have not been removed from the testing machine when the tests have been stopped it has been possible to apply a static load. This makes easier the detection of crack damages by the optical observation.

Damage kinetics during the last cycles before final failure have been filmed by a camera (60 images/s) on one side of the specimen. The specimen's displacement has also been recorded by an extensometer placed on the anti-buckling device.

FINITE ELEMENT MODELLING

To calculate the state of interlaminar stress a global-local 3D finite element model has been implemented with the code Samcef©. The local model is designed to correctly represent the free-edge effects.

Geometry and boundary conditions of the global-local model

The global model represents the complete ply-drop zone plus the unclamped part of the base laminates in the thick and the thin section including the teflon part of the antibuckling device. To simulate the conditions of a specimen clamped in the testing machine, on the right side all degrees of freedom are restrained, idem on the left side except for the displacement in load direction. The laminate is modelled ply by ply with 20 nodes cubic elements with one element in the plies' thickness direction. The triangular zones at the ply-drops are modelled by splines to avoid a brutal changing in Teflon pads Specimen the slope. 15 nodes elements and the material data of the matrix material have been used in these triangles. The anti-buckling device is meshed by 15 nodes elements with material data for usual steel. No friction has been considered for modelling the contact between the specimen and the anti-buckling device. The meshing in the specimen's width direction has been optimised in terms of computational time and required precision. Edge effects cause high stress variations close to the specimen's sides therefore the elements close to the borders are cubic. In the specimen's middle stress variations being comparatively smaller; a coarser meshing has been chosen along width direction.

In order to refine the meshing without immoderately increasing computational time,

Method for calculation of representative values

There are two main problems when calculating numerical stresses around ply-drops, the stress singularity because of the dropped ply and the edge-effect at the free edges.

Convergence studies have shown that the average stress values calculated for a squared zone of 0.25 x 0.25 mm already converge for a meshing of 2 elements per ply. The dashed regions in Figure 5 show the zones of the interface elements used for calculating the average stress values. With this method it is possible to quickly and directly compare the stress values on both edges, in the middle and between different ply-drops.

Figure 5: Zones used to averaged the interlaminar stresses at the specimens' edges

RESULTS AND ANALYSIS

Static tests

The static compression tests have been performed with a loading rate of 20 kN/min, and on tension equal to 40 kN/min. Failure stresses in the thin section and the standard deviations are given in Table 1. Four tests on tension/compression have been achieved for the V int , five for the V ext . It is important to notice that the failure stresses under compression and tension load of the V int are lower than the V ext . Furthermore for both configurations the failure stresses on tension load are nearly two times higher than on compression, thus the most critical value is the compression load of the V int .

Fatigue test parameters

The load ratio is R = -1, the frequency has been fixed to 5 Hz. To allow a direct comparison of fatigue lifetime, the load levels are the same for both configurations, they are fixed as a function of the static compression failure stress of the V int (70, 60, 50 and 45%).

Zone 1

Zone 2

Distribution of fatigue damages observed on both specimen sides

By stopping the fatigue tests regularly and observing the two edges of the specimen, the damage modes and spots have been identified. A non-symmetrical distribution of delaminations on the specimen's edges has been found. An example is given in Figure 6 which shows the 2 nd ply-drop of the V int and V ext . For both configurations there are delaminations on side 2 but no visible damage on side 1. It is important to notice that for the V int delaminations appear preferably on side 2, at this side the opening stress 33 and shear stress 13 are higher than on the other side. The No damage same for the V ext , the delaminations appear preferably on side 2, there the shear stress 13 is higher but the opening stress 33 is lower than on the other side.

Comparing the test results with the numerical values shows that on the preferred side for delaminations the shear stress 13 is always higher. The opening stress 33 doesn't show this clear tendency but its absolute values are low compared to the shear stress 13.

Critical delamination spots in the ply drop-off zone

Observations during fatigue tests show that for the configuration V int as well as V ext the delaminations initiate around ply-drops 1 and 2.Figure 7 shows side 2 of a specimen V int where delaminations initiated around ply-drop 1 and 2. For the V int , at ply-drops 1 and 2 the opening stress 33 is slightly higher but the shear stress 13 is much higher. In general the absolute values of the shear stress 13 are much higher than the opening stress 33. For the V ext (Figure 9), the opening stress has its highest value at ply-drop 1, but decreases at ply-drop 2 to a value even lower than at ply-drops 3 and 4. The shear stress 13 at ply-drops 1 and 2 is at least on one side higher than at ply-drops 3 and 4. The general level of absolute values of shear stress is much higher than the opening stress 33 as for the V int .

The mechanical tests have shown that for both configurations the critical spots of delamination initiation are the ply-drops 1 and 2. The numerical simulations show that at ply-drops 1 and 2 the interlaminar shear stress 13 is higher than at the other ply-drops with a general high level compared to the other interlaminar stress components. The opening stress 33 presents lower absolute values and doesn't show a clear tendency.

Final failure on fatigue loading

To study the damage kinetics one side of the specimen has been filmed with a camera. The observations show that there are three stages during cyclic loading, cycling without damage, damage initiation with stable propagation and finally unstable propagation until final failure.

The films show that before final failure both configurations are separated in three sublaminates, resulting from the delamination propagation around ply-drops 1 and 2. Figure 10 sketches the separation of the two configurations. The V int is separated in three sub-laminates of same size which transfer nearly the same load. During cyclic loading these sub-laminates bend independently until one of them breaks which causes a load-redistribution to the other two sub-laminates and quickly the final failure. The V ext is separated in two small sub-laminates on top and bottom and one big in the middle. This big sub-laminate transfers most of the load; its breakage leads to final failure.

The differences between the two tested configurations in fatigue lifetime are shown with a semi-logarithmical SN curve in Figure 11. The value at N = 1 is the static compression failure stress. The two curves are nearly parallel, the slope of the V ext is slightly flatter, but the V int is vertically shifted to lower values. The results show that the vertical position of the plydrops influences the fatigue lifetime. Contrary to the general guideline to place the plydrops close to the mid-plane, the tested configuration with the ply-drops next to the thin section far from the mid-plane presents a longer fatigue lifetime.

Mid-plane

Ply-drop

Delaminations

CONCLUSIONS AND PERSPECTIVES

Two ply-drop configurations with different positions of ply-drops in thickness direction have been tested, one with the ply-drops close to the thin section close to the mid-plane (V int ) the other one far from the mid-plane (V ext ). A finite element 3D global-local model has been developed to calculate the interlaminar opening 33 and shear stress 13 around the ply-drops. By observation it has been found that delamination initiates preferably on one particular side of the specimen. Comparing the experimental observations with the numerical values shows that the side with higher frequency of delaminations has higher shear stress 13. The opening stress 33 doesn't show a clear tendency. Another observed phenomenon is that during cyclic loading for both configurations the delaminations initiate around ply-drops 1 and 2. It has been found that the numeric values of shear stress 13 are higher at ply-drop 1 and 2 than at 3 and 4, the general level of opening stress 33 is lower with no hard peaks.

Damage kinetics before final failure have been observed with a camera. After delamination propagation around ply-drop 1 and 2, the specimens divide into three sublaminates which buckle independently. The V int separates in three sub-laminates of nearly same size and finale failure occurs when one of them breaks. The V ext separates in two small and one big sub-laminate, breakage of the big one leads to final failure.

The experimental results show that initial damage are delaminations at ply-drop 1 and 2 for both configurations. Contrary to what was expected, the configuration with the plydrops next to the thin section far from the mid-plane (V ext ) presents a longer fatigue lifetime compared to the configuration with the ply-drops close to the mid-plane (V int ).

Other ply-drop configurations are under study to confirm the identified tendencies. Not only first damage and final failure but also damage kinetics will be studied more in detail. A finite element model will be developed in order to simulate delamination propagation initiated at one side in width direction.

Figure 1 :

 1 Figure 1: Geometry of ply-drop specimen

Figure 2 :

 2 Figure 2: Stacking sequence for the two tested ply-drop configurations with ply-drops 1 and 2 close the mid plane (V int ) and far from the mid plane (V ext )

  a local model has been developed. This local model represents the zone around a ply-drop with one ply on top and one on bottom of the dropped ply, thus there are as many local models as ply-drops. The displacements of the global model around the zone formed by the local model have been saved. With these global displacements, the displacements for the local models have been interpolated with a local linear Matlab function. To minimise problems with stress singularities the interlaminar stresses has been calculated within interface elements with infinite rigidity which have been inserted between the plies. A mesh refinement study has shown that stress values are correctly represented with one element per ply for the global model and 2 elements per ply for the local model by keeping elements cubic at the specimen edges. All material laws are linear elastic, the solver module is non-linear.

Figure 4 :

 4 Figure 4: Global-local model of a specimen

Figure 6 :

 6 Figure 6: Delamination at 2 nd ply-drop of V int (top) and V ext (bottom) tested at 50%, interrupted at 8000 cycles

V int side 1 V int side 2 V

 12

Figure 7 : 2 Figure 8 Figure 8 :

 7288 Figure 7: Side 2 of a V int tested at 50% and stopped at 10000cycles with delaminations around ply-drop 1 and 2

Figure 9 :

 9 Figure 9: Interlaminar stresses in zone 1 at ply-drop 1 to 4 for the V ext

Figure 10 :

 10 Figure 10: Separation of V int (left) and V ext (right) in three sub-laminates before failure

Figure 11 :

 11 Figure 11: Semi-logarithmical SN curve for the two tested configurations V int and V ext

Table 1 :

 1 Static failure stresses (Standard deviation)

	Configuration	V int	V

ext Average stress for compression load (MPa)

		-452 (±14)	-473 (±6)
	Average stress for tension load (MPa)	819 (±46)	922 (±24)

Table 2 :

 2 Frequency of delaminations observed in fatigue tests and calculated interlaminar stresses

		V int		V ext
		Side 1	Side 2	Side 1	Side 2
	Observed frequency
	of delaminations /	1 / 5	4 / 5	0 / 7	7 / 7
	Total number of tests
	Opening 33 (MPa)	44	51	26	15
	Shear 13 (MPa)	65	145	131	191