domain: sde sdv sde.be

text:
Pierre Hiernaux
email: pierre.hiernaux2@orange.fr

Kalilou Adamou
email: adamou.kalilou@yahoo.fr

Oumarou Moumouni

Matthew D Turner
email: mturner2@wisc.edu

Xiaoye Tong

Patrice Savadogo
email: p.savadogo@cgiar.org

Eric Mougin
email: eric.mougin@get.omp.eu

Oumarou Malam
email: oumarou.malamissa@ird.fr

Expanding networks of field hedges in densely populated landscapes in the Sahel

Keywords: agroforestry, hedgerow, land tenure, deforestation, bocage, tropical Africa

Changing woody plant density across agricultural landscapes of the Sudano-Sahelian region is a debated issue. This paper reports the results of an investigation on the contribution of field hedges to overall woody plant density. Hedges separating individual cropped parcels were studied within village agropastoral territories in the Dantiandou district in western Niger. In 1992, field hedges were mapped over the study area using aerial photography and in 2016, using high resolution Google Earth imagery. In 1992, field hedge length was equal to 1006 km within 435 km² of croplands, equivalent to 2.3 km km -2 . In 2016, 17.5% of these hedges had disappeared, but 1591 km of new hedges were observed resulting in an increased density of hedges to 5.6 km km -2 . In 24 years, hedges had increased at a mean annual rate of 3.7% likely associated with the splitting of crop fields by inheritance.

The composition and productivity of hedges were also monitored in 1996, 2010 and 2015. All trees (maximum height ≥4m), shrubs (<4m) and tussock perennial grasses were recorded within twelve field hedge samples of 200m each. Field measurements were used to estimate basal area, crown area, foliage mass and wood mass of each woody plant within the sampled hedges. No significant trends were found between 1996 and 2015 in woody plant density, basal area, crown cover, wood and foliage masses across all monitored hedges. However, overall means hide a slight decrease in tree contributions, while shrub contributions first increased and then decreased. They also mask contradictory trends among sites most likely related to different rates of shrub coppicing and tree cutting. The woody species composition of the hedges is poor with an increase of Combretum glutinosum to the detriment of Guiera senegalensis over the study period.

Multiplying the hedge sample statistics by the changing lengths of field hedges in the study area provides estimates of the contribution of the hedge woody plants to the woody plant population at a landscape scale. Between 1992 and 2016, field hedges contributed to increases of woody plant density by 3.9%, basal areas by 5.4% , crown cover by 2.7% , leading to 6.1% annual increase in foliage mass and 8.8% increase in wood mass.

Introduction

The trend of woody plant populations in Sahelian West Africa is a topic of considerable uncertainty and controversy. Many publications have referred to alarming deforestation [START_REF] Chamard | La forêt sahélienne menacée[END_REF] caused by cropping or forestry exploitation to provide fuel to urban centers [START_REF] Peltier | Bilan après quinze ans de gestion communautaire d'une forêt villageoise de l'Ouest nigérien[END_REF]). However, a recent remote sensing work has found that woody plant cover has generally increased across Sahel from 2000 to 2014 with a few exceptions [START_REF] Brandt | Assessing woody vegetation trends in Sahelian drylands using MODIS based seasonal metrics[END_REF]. Within this debate, a major area of uncertainty is how woody plant populations have changed on agricultural landscapes that represent a significant proportion of the land area in the region.

The landscapes of the southern Sahel are strongly shaped by a long history of agrarian management [START_REF] Raynaut | Societies and nature in the Sahel: ecological diversity and social dynamics[END_REF][START_REF] Ballouche | L'homme, artisan des paysages de savane[END_REF]. Human management reshapes rural landscapes through land use (Fig. SI 1) with a major distinction being between rangelands and agricultural lands, reviving the ancient distinction between saltus and ager [START_REF] Poux | Réinvestir le saltus dans la pensée agronomique moderne: vers un nouveau front éco-politique?[END_REF]. Rangelands are defined by their pastoral management under communal access, but also by their marginal potential for cropping associated with shallow or infertile soils, or with increased flood risks in low lying areas. Moreover, rangelands cannot be distinguished from agricultural lands simply by the presence of grazing since all portions of the rural landscape are open to grazing after crop harvest [START_REF] Turner | The Distribution of grazing pressure in relation to vegetation resources in semiarid West Africa: The Role of Herding[END_REF]. Gathering and wood collection also extend to the whole landscape and are regulated by specific use rights [START_REF] Sidikou | Droits d'usages traditionnels locaux et demande externe des populations urbaines au Niger[END_REF].

Agricultural lands include all the arable lands cropped or fallowed centered around villages.

Two components constitute the woody population in agricultural lands: 1. the large trees and coppiced bushes of the agrarian parkland within fields and fallows [START_REF] Boffa | Agroforestry Parkland Systems in Sub-Saharan Africa[END_REF], and 2. the web of hedges that delineates fields, tracks and paths. Less spectacular and studied than the trees [START_REF] Gijsbers | Dynamics and natural regeneration of woody species in farmed parklands in the Sahel region (Province of Passoré, Burkina Faso)[END_REF][START_REF] Boffa | Productivity and management of agroforestry parklands in the Sudan zone of Burkina Faso[END_REF][START_REF] Nouvellet | Le parc à Karités au Mali : inventaire, volume, houppier et production fruitière[END_REF][START_REF] Yossi | Les haies vives au Sahel. Etat des connaissances et recommandations pour la recherche et le développement[END_REF]) and coppiced bushes [START_REF] Diack | Decomposition of a native shrub (Piliostigma reticulatum) litter in soils of Semiarid Senegal[END_REF][START_REF] Achard | Les jachères fourragères naturelle et améliorée en Afrique de l'Ouest. In : La jachère en Afrique tropicale : de la jachère naturelle à la jachère améliorée[END_REF][START_REF] Dick | The native shrubs Piliostigma reticulatum and Guiera senegalensis: The unrecognized potential to remediate degraded soils and optimize productivity of Sahelian agroecosystems[END_REF][START_REF] Diakhaté | How the native woody shrub Piliostigma reticulatum affects soil microbial communities when growing millet in semi-arid Senegal[END_REF] ) of the agrarian parkland, the woody field hedges frame some of the most densely populated agropastoral landscapes in West Africa [START_REF] Pélissier | Les paysans Sérères. Essai sur la formation d'un terroir du Sénégal[END_REF][START_REF] Portères | Le Caractère magique originel des haies vives et de leurs constituants[END_REF][START_REF] Seignobos | Des fortifications végétales dans la zone soudano sahéliennes (Tchad et Nord Cameroun)[END_REF][START_REF] Levasseur | L'utilisation des haies vives améliorées dans le cercle de Ségou, au Mali : Le signe d'une société en mutation[END_REF].

The objective of this paper is to quantify changes over the past two decades in the woody population of field hedges within a study area located within western Niger. Western Niger is one of the areas of the Sahel where woody plant cover has been found not to have increased from 2000 to 2014 [START_REF] Brandt | Assessing woody vegetation trends in Sahelian drylands using MODIS based seasonal metrics[END_REF]. Within this area, woody plant cover has declined on plateau rangelands1 , declined slightly or remained the same on slopes, and increased slightly in valleys and near villages. While slight positive trends have been observed locally [START_REF] Emeterio | Changements socioenvironnementaux et dynamiques des paysages ruraux le long du gradient bioclimatique nord-sud dans le sud-ouest du Niger (régions de Tillabery et de Dosso)[END_REF], one would expect a decline of woody cover on agricultural land due to the progressive expansion of the land cropped to the detriment of fallows [START_REF] Moussa | Les changements d'usage des sols et leurs conséquences hydrogéomorphologiques sur un bassin versant endoréique sahélien[END_REF][START_REF] Tong | Revisiting the coupling between NDVI trends and cropland changes in the Sahel drylands: la case study in western Niger[END_REF]. Still, the coarse resolution of the satellite imagery used in assessments of woody plant cover at the regional scale does not allow a distinction of the contribution of field hedges from that of the parkland within field or fallows.

In Western Niger, the trees, shrubs, and certain perennial grasses, which constitute the field hedges, are not planted like in other regions of the Sahel [START_REF] Yossi | Les haies vives au Sahel. Etat des connaissances et recommandations pour la recherche et le développement[END_REF]. Instead, hedges are formed by selective sparing of woody plants at field edges when fields are cleared (Fig. SI 2). The initial objective of this protection is to mark the field limits. The management of hedges is conducted largely independently of the management within the surrounding fields or fallows.

Landscape-level changes in the woody population of field hedges were estimated by combining detailed ecological monitoring work of the composition and density of field hedges with their mapping, using aerial photos and high resolution satellite imagery. Woody populations of rangelands, fields and fallows in the region have most commonly been assessed through either exhaustive census on sampled areas [START_REF] Hiernaux | Woody plant population dynamics in response to climate changes from 1984 to 2006 in Sahel (Gourma, Mali)[END_REF] or by distance methods [START_REF] Picard | Estimation of the density of a clustered point pattern using a distance method[END_REF]. However, the linear and dense arrangement of the woody plants along field hedges is not compatible with either of these methods and therefore, a more appropriate assessment method was developed for this research, as reported below. Finally, observed changes in the spatial pattern of hedges will be related to changes in land use and land tenure as documented by surveys of families farming fields in the study area.

Materials and method

The study site

To better understand the evolving ecology and contribution of field hedges to the overall woody population in the Sahel, research was conducted within a 499 km 2 area in the Dantiandou district of southwestern Niger (Fig. 1). In the study area, rangelands are confined to the Oligocene ferricrete that caps the sedimentary plateaus at the highest elevations of the landscape. Rangeland also extends along the edges of the plateau and on a few shallow soils on the Pleistocene ferricrete downslopes (d'Herbès and Valentin 1997). Cropping is prevalent on the sandy soils on the slopes and in the valleys, and on sandy clay soils lying in flats and stream beds [START_REF] Turner | The effects of management history and landscape position on inter-field variation in soil fertility and millet yields in southwestern Niger[END_REF]. Agricultural lands are composed of a minority of permanently cropped fields, close to villages and to pastoral camps, and a majority of shifting crop fields and fallows further away [START_REF] Osbahr | Livelihood strategies and soil fertility at Fandou Béri, southwestern Niger[END_REF]. Indeed, permanent cropping requires fertilization that is most commonly achieved by corralling livestock on the fields during the dry season [START_REF] Hiernaux | Livestock: recyclers that promote the sustainability of smallholder farms[END_REF] with or without the application of domestic wastes carried to the field by carts. The capacity to manure depends on the size of the family herd and on the extent of seasonal transhumance of the herd away from the village. The soil fertility of shifting fields is managed by alternating cropping for five years with fallowing that lasts typically three years but may last much longer especially on lands marginal for cropping [START_REF] Hiernaux | The influence of farmer and pastoralist management practices on desertification processes in the Sahel[END_REF]. Fallow duration also depends on the field's tenure status (Turner and Moumouni 2018a). Agricultural lands are clustered around villages the locations of which are where lands were free to crop and ground water was accessible to hang-dug wells (more recently, boreholes) at the time of settlement [START_REF] Favreau | Estimate of recharge of a rising water table in semiarid Niger from 3 H and 14 C Modeling[END_REF].

The study area's population is dominated by the Djerma who also control agricultural land through village chieftaincies. Although Djerma families hold no formal land title, they have legal customary rights to their cropped field; rights which are generally inheritable by sons [START_REF] Ph | rural Africa : Insights into the dynamics of institutional innovation[END_REF]Turner and Moumouni 2018a). The founding Djerma families of the village enjoy chieftaincy rights on lands often arranged in sectors radial from the village. Among them, the village chief manages usufruct rights to the chieftaincy common lands. These usufruct rights are assignable by contract to new settlers, including agropastoral Fulani families who have settled in the vicinity of the villages within the study area dating back to the severe 1973-74 drought. These rights are restricted to cropping activity without exclusive rights to exploit woody plants or to graze livestock on the agricultural land.

The landscape is further structured by a web of tracks that radiate from each village to access the fields and travel to neighboring villages. In addition, there is a web of livestock paths that allow local livestock access to water points and rangelands, especially during the wet season when they are not authorized to cross cropped fields. These livestock paths also connect to a web of regional livestock paths allowing regional and international transhumance [START_REF] Turner | The role of livestock mobility in the livelihood strategies of rural peoples in semi-arid West Africa[END_REF]. The pastoral camps of settled Fulani families are positioned along these livestock paths just outside the edge of permanently cropped fields surrounding each village.

Their positions reflect that cropped fields are off limits to livestock during the growing season and that pastoral families depend on village water resources during the dry season. The study area's climate is typical for the semi-arid monsoonal tropics with an average annual rainfall of 494 ± 30mm (1990-2018) at Banizoumbou, a village within the study area, N distributed from June to October in 35 ± 6 rainy days most of which as convective storms [START_REF] Panthou | Recent trends in the regime of extreme rainfall in central Sahel Int[END_REF]. There are large rainfall variations between years with series of wetter and dryer years such as the regional droughts of the early 1970 's, mid 1980's and more recently between 2009and 2011[START_REF] Lebel | Recent trends in the Central and Western Sahel rainfall regime (1990-2007)[END_REF]. However, there is no overall trend in annual rainfall over the 1990-2018 period that encompass the monitoring period for this study (Fig. 2).

Figure 2 Annual rainfall departure from the mean over 1990 to 2018 (494 ± 30 mm) at Banizoumbou (13°32'N, 2°40'E).

Mapping land use and field hedges

Woody field hedges were initially mapped by stereoscopic photointerpretation of 1992 panchromatic aerial photos (Table SI 1) over the study area. The map was digitized in ATLAS-GIS and geometrically corrected using 150 GPS readings (Trimble Pathfinder Basic Plus GPS receiver) under a UTM projection with a WGS-84 datum. Yet, when the file was overlaid on Google Earth small shifts in random directions were observed and these were corrected manually. The woody field hedges in 1992 were mapped along with six classes of land use (built-up area, manured cropland, unmanured cropland, recent fallow, old fallow, rangeland) together with tracks, cattle paths, pastoral camps, granaries, isolated trees, ephemeral water courses, ponds, wells and boreholes [START_REF] Hiernaux | The Fakara: a semi-arid agro-ecosystem under stress. Report of research activities of the DMP-GEF programme[END_REF]. The woody field hedges in 2013-16 were mapped using available Google Earth imagery (Table SI 1). The hedges mapped in 1992 were first overlaid on the google map imagery and the hedges that had disappeared were identified and erased from the 2016 file. Then the hedges not mapped in 1992 but present in 2013-2016 were mapped in a separate file. To document the land use changes, the categories of land use mapped in 1992 plus an additional category of

Monitoring woody plant and tussock grasses in field hedge samples

Twelve samples of field hedges were selected in 1996: three on plateau dunes or on sandy upper slopes; three on sandy mid-slopes; three on loamy-sand bottom-slopes; and three on alluvial soils in valleys (Fig. 1). Only one of the sites (K23) is located along permanently cropped fields.All the others are along fields that are managed in a crop-fallow cycle. First measured in 1996, the hedges were revisited in 2010 and 2015. One of the hedges described in 1996 had disappeared in 2010 due to land erosion on a stream bank (B17) and was replaced by a different hedge (BZ26).

The monitored field hedge sections were 200m long, subdivided into eight segments of 25m. The perennial grasses (Andropogon gayanus, Aristida sieberiana), shrubs (maximum crown height below 4m) and trees (crown height above 4m) along the hedges are systematically measured. For each woody plant, the following were recorded: the species, crown height, largest crown diameter (D, m), crown diameter perpendicular to the largest diameter (d, m), and the circumference (C, cm) of each stem at 10 cm above ground level.

The density of shrubs and trees was calculated by simply counting individual plants along the hedge section. Errors are very unlikely, limited to the possibility of inadequately distinguish individual shrubs when highly intertwined. The basal area (Ba, cm²) of each woody plant was calculated by summing the basal area of each stem calculated as :

Ba = C 2 /(4.π) (1)
Risks of error in assessing basal areas are minor as well, limited to missing small stems, especially after recent shrub coppicing, and to imprecise measurements of the stem circumferences, especially for low branching trees or shrubs.

The foliage dry mass (Fm) and wood dry mass (Wm) are estimated using existing species-specific allometric functions of the circumference of each stem [START_REF] Cissé | Production fourragère de quelques arbres sahéliens : relation entre la biomasse foliaire maximale et divers paramètres physiques[END_REF][START_REF] Henry | Estimating tree biomass of sub-saharan African forests: a review of available allometric equations[END_REF]:

Fm = α.C β (2) Wm = γ.C δ (3)
The species-specific coefficients α, β, γ and δ are provided in Table SI 2 . Error associated with the use of allometric functions such as these is likely if the sizes of woody plants sampled fall outside the range of circumferences used to generate the function. Unfortunately, such ranges are not systematically documented for the allometric functions used but it is unlikely that the size distribution of woody plants sampled here are outside the ranges used to develop these functions.

The crown area (Ca, m²) is estimated as the area of the ellipsoid shape defined by the largest crown diameter (D, m) and the perpendicular diameter (d, m):

Ca = π . (D.d)/4 (4)
There is a possible observer bias tied to the crown diameter estimation being either measured in 1996 and 2015, as the extreme longest diameters, between the more remote branchlets, while in in 2010, the diameter estimated from a rounded ellipsoid fitting the crown.

Densities, basal areas, crown areas, wood and foliage masses are expressed per 100m of hedge. The contribution of field hedge woody plants to the overall woody plant population is assessed in two different ways. The first approach estimates the contribution per unit area (ha) by relating the measurements expressed by 100m of hedge to the 10 m-wide band surrounding the hedge, i.e. 5 meters on either side of the hedge axis. This is justified by the observation that the maximum crown diameter of the trees in monitored hedges did not exceed 10 meters.

Using this approach, leaf and wood masses per hectare equivalent are estimated by simply multiplying by 10 the values given for 100 meters of hedge. The second way is to estimate the absolute number of woody plant individuals contributed by hedges by multiplying the hedge woody plants values expressed by 100 meter of hedge by the length of field hedges mapped on the same date.

Surveying land tenure and human population

The land-use maps in 1994 to 1996 were used as background to survey and map land tenure, identifying the family managing each field and under which tenure status: primary rights to the land generally held by members of chieftaincy lineage and the few purchases of land, or secondary rights which characterizes the rights held by those outside of the chieftaincy lineage [START_REF] Hiernaux | The Fakara: a semi-arid agro-ecosystem under stress. Report of research activities of the DMP-GEF programme[END_REF]). An additional survey was conducted in 1998 on the

Results

Field hedge and land use mapping

Field hedge mapping

The total length of woody field hedges in the study area in 1992 equals 1006 km over 499 km² of land (equivalent to 2.02 km km -2 ) and over 435 km² of agricultural land (2.31 km km -2

) when the unarable rangelands, without field hedges, are excluded (Fig. 3, Table 1). In 2016, 174.1km (17.5%) of these 1992 field hedges had disappeared, but 1591.3 km of new hedges were observed so that the resulting density of woody field hedges per area of agricultural land reached 5.56 km km -2 (Fig. 4). Woody field hedges had thus increased by a factor 2.4 equivalent to a mean annual increase rate of 3.7% from 1992 to 2016.

Land tenure mapping and recording field management history

Using the 1994 land-use map, interviews of farmers produced a land tenure map as described above. Figure 6 shows a portion of the land tenure map, overlaid on the map of the woody field hedges for an area lying northwest of a village in the center of the study area (see inset of

2.2.Field hedge monitoring

Woody plant density, basal area, crown cover and foliage masses in the field hedges.

Mean woody plant density of surveyed hedges varies little between 15.9 in 1996, 20.7in 2010 and 16.6per 100m of hedge in 2015 (Table 2; Table 2 shows that across the years 1996, 2010, 2015, basal area and leaf mass of all woody plants first increased and then decreased as plant density while wood mass increases throughout the monitoring period. The opposite trend is seen for cumulative crown cover which first declines and then partially recovers over the same time period . This observation likely results from the bias in crown diameter measurements in 2010 described above and should be thus disregarded.

Among sites, contrasting trends were observed throughout the period. Woody plant density decreased at two sites, remained relatively stable at four sites and increased from 1996

Tree and shrub contributions

Crown cover, basal area, leaf mass, and wood mass show the same general trends for trees and shrubs across the monitored period. However, different trends in plant density were observed. The slight increase in woody plant density hides a decrease in the mean density of trees while shrub density strongly increased between 1996 and 2010, and then declined (Fig

SI 4a

). However, looking at each hedge, the overall decrease in tree density does not apply to sites K23, TT16 and BZ29 where tree density and aggregated basal area increased between 2010 and 2015 probably because some of the shrubs passed the threshold of 4m in height and were counted as trees (Fig SI 6 a,c). For the other hedges, the decrease in tree density, basal area, with associated decreases in tree wood and leaf masses could be due to selective logging, especially at B24, K16, KZ7 and ND2 (Fig 6 a,c 7a,c). Shrub coppicing, such as that experienced at hedges B24, TT16 and TT17 between 2010 and 2015 did not affect shrub density but reduced basal areas, wood and leaf masses. In contrast, the KA5 and ND2 hedges that were not coppiced during the same period, show opposite trends (Fig 6 b,d 7b,[START_REF] Herbès | La brousse tigrée au Niger: synthèse des connaissances acquises. Hypothèses sur la genèse et les facteurs déterminant les différentes structures contractées[END_REF].

Botanical composition of field hedge woody populations.

The number of woody plant species recorded in the 12 monitored hedges (Table SI 3) varied between 9 in 1996, 14 in 2010 and 12 in 2015 among which, three species largely dominate, together representing 93-98% of the total value depending on the parameter considered (Fig.

8):

Guiera senegalensis, Combretum glutinosum and Piliostigma reticulata. As a shrub, G.

senegalensis ranks first in density and crown cover. C. glutinosum, which may grow as a tree, ranks first in wood mass. P. reticulata, a small tree which is often coppiced, always ranks third. Although plant density did not change much over time, C. glutinosum increased in crown cover and basal area leading to increases in leaf and wood masses. On the contrary G.

senegalensis decreased in both crown cover and basal area leading to decreases in leaf and wood masses from 1996 to 2015.

Two perennial tussock grasses in the field hedges

Two perennial grass species are common in some of the field hedges, growing between woody plants. Andropogon gayanus, a 2-3m tall tussock grass, is protected by farmers in fields and particularly in the hedges because of its multiple uses (woven mats, thatch, chicken coops, beehives…) and the good fodder value of its dry season regrowth. Aristida sieberiana is not protected but its seed dispersion and germination is favored by soil disturbance associated with cropping. Both species' densities decreased during the study period with a faster reduction of A. sieberiana than A. gayanus (Fig. 9). The large standard deviation of

2.3.Contribution of field hedges to woody plant populations at the landscape scale

Relating the woody plants described for 200m of field hedge to a band of land of 10m wide provides an estimate of the field hedge contribution to woody plant density, basal area, cover, leaf and wood masses per unit land. These contributions expressed per hectare are calculated by multiplying by 10 from the statistics expressed per 100 m of hedge (Table 2). Expressing hedge contributions in this way allows one to compare these values with those estimated within cropped or fallowed fields of the study area. The woody plant populations of the field hedges are much denser than that of cropped and fallowed fields (San Emeterio Cabañes et al. 2013;[START_REF] Brandt | Assessing woody vegetation trends in Sahelian drylands using MODIS based seasonal metrics[END_REF], with densities and cover closer to that of Sahelian open woodlands [START_REF] Hiernaux | Woody plant population dynamics in response to climate changes from 1984 to 2006 in Sahel (Gourma, Mali)[END_REF]).

Alternatively, multiplying the hedge statistics from field observations in 1995 (Table 2) by the density of hedges mapped in 1992 (Table 1) provides estimates of contribution of hedges over the study area's agricultural land in 1992-95 (Table 3). Similarly 2015 field statistics and 2013-2016 hedge map allows one to estimate the woody hedge contributions in 2013-16. Estimates of the parameter contributions of woody plants found within hedges across the study area all increased over the 24 years with mean annual growth rates varying between 3.9% for density, 5.4% for basal area, 2,7% for crown cover, resulting in annual increases by 6.1% in foliage mass and 8.8% wood mass. Shrubs are driving the increases in density, crown cover and foliage mass, while trees dominate the increase of basal area and wood mass.

Table 3. Mean and standard deviation of the contribution of the field hedges to the density (ha - 1 ), basal area (m² ha -1 ), cover (%)), foliage and wood masses (kg ha -1 ) of woody plant population on agricultural land within the study area in 1992-2005 and 2013-2016.

Discussion

Literature on field hedges is scarce in the Sahel. Their existence in the agrarian landscape is recognized [START_REF] Pélissier | Les paysans Sérères. Essai sur la formation d'un terroir du Sénégal[END_REF][START_REF] Sanogo | Adoption de la haie vive dans le bassin arachidier du Sénégal[END_REF][START_REF] Levasseur | L'utilisation des haies vives améliorées dans le cercle de Ségou, au Mali : Le signe d'une société en mutation[END_REF] but there have been no

quantitative descriptions of densities, cover or wood masses. The literature has focussed more on the impact of woody hedges on biodiversity or agroecosystem processes [START_REF] Burel | Hedgerows and their role in agri-cultural landscapes[END_REF], with a particular attention to their potential to control either wind [START_REF] Banzhaf | Soil tillage and windbreak effects on millet and cowpea: I. Wind speed, evaporation and wind erosion[END_REF][START_REF] Michels | Effects of windbreak species and mulching on wind erosion and millet yield in the Sahel[END_REF] or run-off erosion [START_REF] Valet | Nouvelle stratégie d'éco-développement durable par la gestion et la valorisation du report hydrique[END_REF][START_REF] Smolikowski | Influence of soil protection techniques on runoff, erosion and plant production on semi-arid hillsides of Cabo Verde[END_REF]. The role of hedges serving as land tenure markers [START_REF] Pélissier | Campagne africaine en devenir[END_REF]) is also mentioned. Moreover, there is an abundant technical literature on the agroforestry methods, best adapted woody species, planting or drafting practices to establish new live hedges, as if they were not part of the agrarian system already [START_REF] Depommier | Propagation et comportement d'espèces à usages multiples en haies vives pour la zone sahélo-soudanienne: résultats préliminaires d'essais menés à Gonse et Dinderesso (Burkina Faso)[END_REF][START_REF] Ayuk | Adoption of agroforestry technology: The case of live hedges in the Central Plateau of Burkina Faso[END_REF][START_REF] Yossi | Les haies vives au Sahel. Etat des connaissances et recommandations pour la recherche et le développement[END_REF]). Reflective of this, the woody plants growing in the hedges are generally excluded from the woody plant surveys carried out in forests and savannas, and also in cropped or fallowed fields. One reason for this disregard is the inability of standard survey methods to assess linear vegetation patterns. Yet, as demonstrated by this research, woody plants found within field hedges contribute significantly to the woody populations of agricultural landscapes.

3.1.Heterogenous structures and homogeneous species composition

Woody plant populations were found to differ significantly among the 12 sampled field hedges with coefficients of variation of the 12 hedge means between 40 and 43% for woody plant density, basal area, crown cover, foliage and wood masses (Table 2). The heterogeneity increases when shrubs and trees are separated with coefficients of variation for shrubs ranging between 55% for density and 71% for crown cover, and for trees between 71% for density and 87% for crown cover. The larger heterogeneity of crown cover could be related to the variations caused by the irregular coppicing of hedge shrubs. Because of their higher density, shrubs contribute about as much as trees to total basal area, crown cover and foliage mass.

These variations are not explained by the geomorphic position nor related run-off/run-on balances (not shown).

The species composition of the field hedges of the study area is relatively poor by comparison with other woody populations in Niger [START_REF] Mahamane | Plants diversity in Niger: state of the present studies[END_REF][START_REF] Idrissa | Biodiversity and Structure of Woody Plants of Sahelian Rangelands of Baban Rafi[END_REF]) with only 16 species registered across the 12 sites over the three years of observation (Table SI 3). This poverty is reinforced by the fact that three species: G. senegalensis, C. glutinosum and P. reticulata together represent 93-94% of plant density, 96-97% of basal area, 97-98% of crown cover, wood and foliage masses. In Western Niger, the trees and shrubs of these field hedges are not planted, nor sown, but result from selective sparing of woody plants at field clearing. They are just the most common species found on the infertile acidic sandy soils in the southern Sahel [START_REF] Hiernaux | Les parcours du Sahel[END_REF] 3.2. Weak trends in hedge woody plant and perennial grasses densities, cover and production over last two decades.

There is no significant consistent trend between 1996 and 2015 in hedge woody plant density, basal area, crown cover, wood and foliage masses across the 12 hedges monitored.

Moreover, the weak trends found for total woody plant population in the hedges result from different trends between trees and shrubs ( ). The weakness of the overall trends, the contradictory trends among sites; between trees and shrubs, and between the two main species all argue in favor of a woody population dynamics more linked to site-specific management by selective shrub coppicing and tree cutting than to long term climate changes. Given that the prime objective of the woody hedge establishment is to delineate field boundaries, one would expect a greater level of conservation of hedge plants than within the field [START_REF] Burel | Hedgerows and their role in agri-cultural landscapes[END_REF][START_REF] Louppe | Les haies vives et défensives en zones sèche et sub-humide d'Afrique de l'Ouest. In : La jachère en Afrique tropicale[END_REF]. Yet, the decreases in tree density and basal area over the last two decades may reflect an intensification of woody plant exploitation [START_REF] Lericollais | La gestion du paysage, Sahélisation, surexploitation et délaissement des terroirs sereer au Sénégal[END_REF]). The trends observed in shrub density confirm population stability, but again the slight downward trend in density that occurred in the last decade and the progressive decrease in shrub basal area is consistent with an increase in coppicing and exploitation pressure in spite of reported adoption by local farmers of "Assisted Natural Regeneration" (RNA) management practices [START_REF] Botoni | La Régénération Naturelle Assistée (RNA) une opportunité pour reverdir le Sahel et réduire la vulnérabilité des populations rurales[END_REF]. The density reduction of the two perennial herbaceous species, A. gayanus and A.

sieberiana, is also consistent with an increase in exploitation pressure, by cutting the haulms of A. gayanus as construction material and grazing the dry season regrowth of both grasses.

Indeed, these declines in perennial density over the last decade is inconsistent with the trend toward more frequent large rainfall events [START_REF] Panthou | Recent trends in the regime of extreme rainfall in central Sahel Int[END_REF]) that should favor perennials to the detriment of annual herbaceous species. (Table 1). Merging neighbor fields may eliminate some hedges but more frequent splitting of fields generates new hedges and 1591 km of hedges were created between 1992 and 2013-shares2016. Splitting a field in two should geometrically increase hedge length by a quarter (in a square shaped field) to a third (in a rectangular field that is twice as long as wide). In 24 years, the overall length proportion of new versus old hedges reaches 1.58 which would correspond to a bit less than two splits of each existing field on average, resulting in the fragmentation of 1996 fields into 3 fields, on average. Indeed, when households divide, it is more common for all fields managed by the family to be split equally among successors to ensure that the quality of land is divided equally (Turner and Moumouni 2018a). Yet not all field splits result in a woody field hedge, a tacit agreement on a demarcation line may suffice.

This appears to apply in the fields cropped by Fulani agro-pastoralists who do not have primary rights on the land they crop [START_REF] Sidikou | Droits d'usages traditionnels locaux et demande externe des populations urbaines au Niger[END_REF].

3.4.The increased contribution of field hedges to agricultural land woody plant population.

The increased contribution of the hedges to the woody plants population at the landscape scale mainly comes from the densification of the web of hedges with their total length increasing by a factor of 2.4 in 24 years but is also influenced by the small changes in woody population structure as assessed on the sample of twelve hedge monitored from 1996 to 2015.

The resulting contribution of hedges to woody plant density increased by 2.5, from 4.6 in 1995 to 9.2 plant ha -1 in 2016, mostly due to increasing shrub numbers with crown cover only increasing by a factor of 1.9 (Table 3). The hedge contribution to woody plant basal area tripled, so that leaf mass multiplied by 4.

Conclusions

In the monitoring of 12 sampled hedges from 1996 to 2015 no significant overall trends were found for mean density, crown cover or basal area of the woody plants. Instead opposite trends were observed between tree and shrubs, between dominant species and, above all, among hedge sites. Selective cutting and coppicing would explain these locally specific trajectories that are not explained by rainfall variations but likely from variable exploitation pressure over the last decade. Indeed, the hedge's function as a land tenure marker that ensures long life to most hedges does not prevent their exploitation through gathering (fruits, dead wood, barks), pruning, and selective coppicing or cutting. The steady increase in rural human population density is leading to a fragmentation of the agricultural lands through the splitting of fields among successors at each generation. This fragmentation has led to an expansion of woody field hedges by a factor 2.4 over the 24 years of the survey, equivalent to a mean annual increase of 3.7%. The densification of the web of field hedges drives mean annual increases in the contribution of field hedges to the woody plant population of the agricultural land in the study area extending over 435 km². The field hedge woody plant increased density by 3.9%, basal areas by 5.4% , crown cover by 2.7% , leading to 6.1% annual increase in foliage mass and 8.8% increase in wood mass.

Figure 1

 1 Figure 1 Geographic location of the study area (499km²) in the Dantiandou District of Niger. The three lines of the small maps figure isohyets 150, 300 and 600mm from CHIRPS data 1982-2014 delineating northern, central and southern Sahel bioclimates. Twelve field hedges monitored since 1996 (+ B17 only observed in 1996) are located relative to the main edaphic situations: hard pan plateau, top sandy slope, mid and bottom sandy slopes and valleys (Hiernaux and Ayantunde 2004). The black square frame locates the more detailed map of Fig 6.

  " (no sign of cropping at least during the previous century) were mapped over the same area covered by historical aerial photo coverages of 1950 (IGN ) and 1975 (IGNN). Land use mapping was also performed on low altitude aerial color slides taken in1994, 1995 and 1996. Finally, land use mapping was extended to high resolution multispectral satellite data (SPOT images) by supervised classification in2007, 2008, 2009, and 2010.

  history of each field, establishing the ancestor of the actual family head that had first cleared the field for cropping, the year of that clearing and the list of the names of the successive managers since clearing. The land tenure status of crop fields and land rights of families in three major villages have been further studied and characterized recently(Turner and Moumouni 2018a, b). The composition of families were surveyed[START_REF] Gijsbers | Dynamics and natural regeneration of woody species in farmed parklands in the Sahel region (Province of Passoré, Burkina Faso)[END_REF][START_REF] Michels | Effects of windbreak species and mulching on wind erosion and millet yield in the Sahel[END_REF][START_REF]Présentation des résultats prélimaires du quatrième (4 ième ) recensement général de la population et de l'habitat (RGP/H) 2012. Min. Finances[END_REF]in (Zezza et al. 2016) that allows an assessment of demographic growth and changes in family composition. These population surveys were compared with the more recent administrative population census performed in2008 and 2013 (INS-Niger 2013).

Figure 3 .

 3 Figure 3. Map of the woody field hedges in 1992 overlaid on Google Earth image. Hedges that were eliminated (white) and maintained (purple) by 2013-16 are delineated with the study area delineated in yellow.

Figure 4 .

 4 Figure 4. Map of the woody field hedges in 2013-16 from Google earth imagery of 11/11/2016 (70% East side), 31/01/2016 (10%, Mid-Center), or 28/09/2013 (20%, West).Hedges that were already present in 1992 (purple), new hedges (blue) are delineated with study area delineated in in yellow.

Figure 5 .

 5 Figure 5. Changes in land use over the study area (499km²). Areas are estimated from classification of aerial photos (1950-1996) and supervised classifications of SPOT imagery (2007 onwards). The cropland is categorized as either shifting crop fields or permanently cropped fields.

Figure 1 Figure 6 .

 16 Figure1for this figure's location within study area). The process of agricultural land splitting through intergenerational inheritance is illustrated by a few fields identified on this portion of the land tenure map (with fields attached to certain families designated by family codes 19 to 26 described more fully in Fig SI9). In 1998 these fields were separately managed by eight families (seven brothers and one of their sons) --all descendants from a common ancestor, Boulymane K., grandson of Koutoubi H. who founded the village in 1906(Turner and Moumouni 2018a). Boulymane K.'s only son, Moussa B., had seven sons who shared his rights to the land he inherited (Fig.SI 9). The next generation currently in charge of field management is composed of 26 family heads, associated with further splits of the land parcels by a factor 3.7 on average. Some of these plot partitions generate new field hedges, either already established in 2016 (in light green in Fig.6) or just outlined (in light blue). Indeed, the split of fields between brothers is sometimes initiated before the father's death (8.1± 4.7 years before, on average) but more commonly. after the father's death (13.4±16.6 years after, on average)(Turner and Moumouni, 2018b).

  Fig. SI 4a). In other words, woody plants, on average, are standing 5 to 6 meters apart along the hedge (Fig. SI 1).

  to 2010 and then slightly decreased in 2015 at the other four sites (Fig. SI 5a). Similarly, the mean decrease in basal area hides an increase at five sites and a decrease at the other seven (Fig. SI 5c). Cumulative crown cover, leaf mass, and wood mass display much less inter-site variation with only one site in each case showing contrasting trends (Figures S1 5d and S1 5e).

Figure 8 .

 8 Figure 8. Species contribution (%) to the mean a) crown cover , b) total basal area, c) wood mass, d) leaf mass and e) woody plant density along 12 field hedges in 1996, 2010 and 2015.

  different values among sites, the perennial grasses being more frequent either in more silty sands at the top of slopes or in depressions (Fig.SI 8). However a decrease in perennial grasses occurred in all sites.

Figure 9 .

 9 Figure 9. Mean and standard deviations of density (#/100 m of field hedge) of Andropogon gayanus and Arisitida sieberiana tussocks along 12 field hedges in 1996, 2010 and 2015.

Fig

  3.3.A denser web of field hedges resulting from land tenure fragmentation. Among the 12 sample hedges observed in 2015, all were already in place in 1992, 8 in 1975 and 3 in 1950 (Fig SI 2), field hedges are thus persistent features of the landscape. Only one of the hedges sampled in 1996 was dismantled by 2010 because of local soil erosion, and among the 1006 km of hedges mapped in 1992 only 176 km were not observed in 2013-2016

  1 and wood mass multiplied by 12.7. Whether these increased contributions of field hedges to woody plant population compensate or not the reduction of trees and shrubs within fields and fallows still needs to be assessed. And if hedges do compensate for the losses within fields and fallows , would they further compensate, at the landscape scale, the decimation of the woody plants of the patterned vegetation of the plateau rangelands (Bouzou Moussa et al 2009; Abdourhamane Touré et al. 2010)?

Table 1 .

 1 Inventory of woody field hedges over the study area (499 km² out of which 435 km² of agricultural lands) in 1992 and 2013-2016. The density of hedges is calculated at the full landscape (499 km 2 ) or at the agricultural land (435 km 2 ) scale.

	Year	Type of	Number	Total	Mean	Density	Density
		hedges		length	length	within	in study area's
						study area	agricultural
							land
			#	Km	m	km/km²	km/km²
	1992	Hedges	1981	1005.7	507	2.02	2.31
	2013-16 1992 Persistent	1716	829.6	483	1.66	1.91
	2013-16 New hedges	4682	1591.3	340	3.19	3.66
	2013-16 All hedges	6398	2420.9	378	4.85	5.56

Table 2 .

 2 Mean density, basal area, crown cover, foliage and wood masses per 100m of field

	hedge measured in 12 sites in 1996, 2010 and 2015.
	Parameter for 100m of field hedge	Year	Trees mean Stdev	Shrubs Mean Stdev	All woody mean Stdev
		1996	4.0	1.9	11.8	8.2	15.8	6.8
	Density (#)	2010	2.5	1.8	18.1	9.1	20.5	8.2
		2015	2.2	2.0	14.3	6.8	16.6	6.4
		1996	0.2	0.1	0.3	0.2	0.2	0.1
	Basal area (m²)	2010	0.2	0.2	0.2	0.1	0.4	0.2
		2015	0.2	0.2	0.2	0.1	0.4	0.2
		1996	167.9	115.5	73.3	55.0	234.1	101.9
	Cover ( m²)	2010	52.3	47.4	60.8	38.4	110.5	37.0
		2015	77.2	77.7	112.0	84.0	185.1	85.3
	Leaf mass (kg dry mass)	1996 2010 2015	49.6 59.4 57.8	33.5 78.5 76.7	16.9 61.2 52.5	14.1 38.2 41.7	66.6 120.6 110.3	29.3 48.7 50.1
	Wood mass (kg dry mass)	1996 2010 1576.9 2177.5 678.1 510.2 2015 1821.2 2637.6	148.0 581.0 755.3 1004.0 2576.5 2318.3 127.0 826.1 439.8 409.5 2157.9 1941.6

In western Niger, deforestation on the plateaus has often been observed within the patterned woody vegetation type known as 'tiger bush' or 'spotted bush'(d'Herbes et al. 1997). Degradation of tiger bush has been observed elsewhere in the Sahel(Abdourhamane Touré et al.

2010[START_REF] Trichon | The persistent decline of patterned woody vegetation: The tiger bush in the context of the regional Sahel greening trend[END_REF]).

Acknowledgements

The field work was first carried out under an ILRI (International Livestock Research Institute, Nairobi, Kenya) research project, more recent observations were funded by the AMMA-CATCH observatory (http://www.amma-catch.org). The authors are very grateful to the rural people of the Dantiandou district for their hospitality and for their patience with our endless inquiries. They are also grateful to Laurent Kergoat and two anonymous reviewers for their careful and inspirational review of the manuscript.


Jeremy Vanhelst
email: jeremy.vanhelst@chru-lille.fr

Laurent Beghin

Alain Duhamel

Stefaan De Henauw

Jonatan R Ruiz

Anthony Kafatos

Odysseas Androutsos

Kurt Widhalm

Beatrice Mauro

Michael Sjostrom

Jérémy Vanhelst

Laurent Béghin

Michael Sjöström

Mathilde Kersting

Frédéric Gottrand

J Vanhelst

Do adolescents accurately evaluate their diet quality? The HELENA study

Keywords: Youth, Assessment, Nutrition, Awareness, Epidemiology study

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The prevalence of obesity has tripled in European countries in the last 30 years, and continues to rise at an alarming rate, especially in young people [START_REF] Ncd-Risc | Trends in adult body-mass index in 200 countries from 1975 to 2014: a pooled analysis of 1698 population-based measurement studies with 19•2 million participants[END_REF]. Overweight and obesity have many health consequences, making prevention particularly important [START_REF] Procter | The aetiology of childhood obesity: A review[END_REF]. In children, dietary habits are closely related to overweight and obesity [START_REF] Nicklas | Eating patterns and obesity in children. The Bogalusa Heart Study[END_REF][START_REF] Barlow | Expert committee recommendations regarding the prevention, assessment, and treatment of child and adolescent overweight and obesity: Summary report[END_REF].

Adolescence represents a period during which multiple physiological and psychological changes occur that considerably affect dietary habits [START_REF] Lake | Longitudinal change in food habits between adolescence (11-12 years) and adulthood (32-33 years): the ASH30 Study[END_REF][START_REF] Post-Skagegård | Changes in food habits in healthy Swedish adolescents during the transition from adolescence to adulthood[END_REF]. The rapid physical growth that occurs during this period is associated with an increase in nutritional needs. Adolescence is marked by an increasing intake of energy-dense foods that are low in nutrients such as snacks and sugar-sweetened beverages and a decrease in intake of nutrient-dense foods such as fruits and vegetables [START_REF] Guenther | Most Americans eat much less than recommended amounts of fruits and vegetables[END_REF][START_REF] Nielsen | Trends in energy intake in the U.S. between 1977 and 1996: similar shifts seen across age groups[END_REF][START_REF] Wang | Increasing caloric contribution from sugarsweetened beverages and 100% fruit juices among US children and adolescents, 1988-2004[END_REF]. Intervention or promotion programs for a healthy diet have been shown

to have limited success in childhood and adolescence [START_REF] Luckner | Effectiveness of interventions to promote healthy weight in general populations of children and adults: a meta-analysis[END_REF][START_REF] Hingle | Parental involvement in interventions to improve child dietary intake: a systematic review[END_REF]. Lack of awareness of personal dietary habits has been identified as a major barrier to motivating adults to change to healthier diets [START_REF] Oenema | Feedback strategies to raise awareness of personal dietary intake: results of a randomized controlled trial[END_REF]. We hypothesized that a similar barrier would apply for adolescents. Indeed, adolescents may think that they achieve healthy dietary habits because they wrongly assess their diet quality.

Therefore, the aim of the present study was to examine the diet quality awareness in a large sample of European adolescents. A secondary aim was to investigate factors associated with diet quality awareness.

Methods

Study design

The Healthy Lifestyle in Europe by Nutrition in Adolescence Cross-Sectional Study (HELENA-CSS) is a multicenter study performed in 10 European cities belonging to nine countries. The HELENA-CSS was designed to obtain reliable and comparable data on nutrition and health-related parameters from a sample of European adolescents aged 12.5 -17.5 yr. A sample of 3,528 adolescents met the HELENA general inclusion criteria. A detailed description of the HELENA study's methodology and sampling has been published elsewhere [START_REF] Béghin | Main characteristics and participation rate of European adolescents included in the HELENA study[END_REF][START_REF] Moreno | Design and implementation of the Healthy Lifestyle in Europe by Nutrition in Adolescence Cross-Sectional Study[END_REF][START_REF] Moreno | Assessing, understanding and modifying nutritional status, eating habits and physical activity in European adolescents: the HELENA (Healthy Lifestyle in Europe by Nutrition in Adolescence) study[END_REF].

Written, informed consent was obtained from the adolescent and the parents. The HELENA study was approved by the local ethics committee in each country, and all procedures were performed in accordance with the ethical standards of the Helsinki Declaration of 1975 as revised in 2008 and the European Good Clinical Practices [START_REF] Béghin | Quality assurance of ethical issues and regulatory aspects relating to good clinical practices in the HELENA Cross-Sectional Study[END_REF].

Measurements

Self-rated diet quality

Self-rated diet quality was assessed using a questionnaire. The adolescent was asked the single question: "Your diet is: rather unhealthy, not healthy or unhealthy, rather healthy, healthy, very healthy". A healthy eating was defined as "a healthy diet is a well-balanced diet which contains a lot of fruit, vegetables and dairy products, a good portion of starchy foods like bread, potatoes and pasta, a moderate portion of meat or fish, and not too much fat and sugar. Also the intake of a large amount of fluid is very important in a healthy diet. The energy content of a healthy diet is in accordance with the needs of the human body" [START_REF] Gonzalez-Gross | Manual of operation: The HELENA study[END_REF]. For the assessment of diet quality, the answers were classified a priori into three categories: low when the answer was "rather unhealthy" or "not healthy or unhealthy", medium when the answer was "rather healthy" and high when the answer was "healthy" or "very healthy". This question about awareness was extracted from a healthy diet determinants questionnaire that has been previously found to be reliable and valid, specifically awareness question correlated well with fresh fruit, soft drinks and ascorbic acid [START_REF] Vereecken | Reliability and validity of a healthy diet determinants questionnaire for adolescents[END_REF].

Diet quality assessment

Dietary intake was assessed by two nonconsecutive 24-hour recalls performed on any two convenient days of the week [START_REF] Biró | Selection of methodology to assess food intake[END_REF]. The 24-hour recalls were recorded using a selfadministered, computer-based HELENA Dietary Intake Assessment Tool (HELENA-DIAT)

that has been validated in European adolescents [START_REF] Vereecken | Development and evaluation of a selfadministered computerized 24-h dietary recall method for adolescents in Europe[END_REF]. Detailed descriptions of data collection and analysis have been published elsewhere [START_REF] Vereecken | Development and evaluation of a selfadministered computerized 24-h dietary recall method for adolescents in Europe[END_REF][START_REF] Dehne | The German Food Code and Nutrient Data Base (BLS II.2)[END_REF][START_REF] Béghin | Influence of parental socio-economic status on diet quality of European adolescents: results from the HELENA study[END_REF][START_REF] Vig | De actieve voedingsdriehoek: een praktische voedings -en beweeggids (The Active Food Pyramid: A Practical Guide to Diet and Physical Activity)[END_REF][START_REF] Vyncke | Validation of the Diet Quality Index for adolescents by comparison with biomarkers, nutrient and food intakes : the HELENA study[END_REF].

Participants' characteristics

Weight was measured in light clothes, without shoes, to the nearest 0.1 kg using an electronic scale (SECA 871; SECA, Hamburg, Germany). Height was measured without shoes to the nearest 0.1 cm using a telescopic height-measuring instrument (SECA 225; SECA). Body mass index (BMI) was calculated as weight (kg)/height 2 (m 2 ). Nutritional status was assessed according to the International Obesity Task Force scale [START_REF] Cole | Establishing a standard definition for child overweight and obesity worldwide: international survey[END_REF]. Pubertal status was assessed by a physician through direct observation according to Tanner and Whitehouse [START_REF] Tanner | Clinical longitudinal standards for height, weight, height velocity, weight velocity, and stages of puberty[END_REF].

Maternal educational level was classified into one of four categories using a specific questionnaire adapted from the International Standard Classification of Education (ISCED) (http://www.uis.unesco.org/ Library/Documents/isced97-en.pdf), and was scored as 1:

primary and lower education (levels 0, 1 and 2 in the ISCED classification); 2: higher secondary (levels 3 and 4 in the ISCED classification); and 3: tertiary (levels 5 and 6 in the ISCED classification).

Statistical analysis

Data are presented as percentages for qualitative variables and mean ± standard deviation (SD) for quantitative variables. Normality of distribution was checked graphically and using the Shapiro-Wilk test.

To assess the selection bias related to missing or incomplete data, the main characteristics of the included and nonincluded adolescents were compared using a Student ttest for quantitative variables, a chi-square test for categorical variables and the Mantel-Haenszel trend test for qualitative ordinal variables.

The association of DQI-A with dietary awareness was studied using a linear mixed model including the center as a random effect and diet quality perception level as the fixed effect (treated as an ordinal factor). We performed key subgroup analyses based on gender, pubertal status, nutritional status, maternal educational level and place adolescents used to

Results

Of 3528 adolescents meeting the inclusion criteria, 3389 (96%) were finally included in the statistical analysis after excluding those with missing or incomplete data for self-rated quality of diet. Characteristics of the population studied are presented in Table 1. Except for maternal educational level, there were no significant differences found between the included and nonincluded groups.

As shown in Figure 1, the DQI-A score increased gradually with the adolescent's dietary awareness level (p < 0.0001). The mean DQI-A was 59.0 (SD = 14.8) in adolescents with a low dietary awareness compared with 65.4 (SD = 12.6) in adolescents with a high dietary awareness. Similar results were found for all the DQI components.

When analyses were stratified according to key subgroups, no heterogeneity in the association of DQI-A and dietary awareness level was found for gender, pubertal status or maternal education level (Table 2). We found a significant heterogeneity associated with the lunch location (school or home). The positive association between DQI-A and diet awareness was stronger in adolescents who eat at home than those eating at school (Table 2). In addition, we found also a significant heterogeneity associated with nutritional status, with obese adolescents showing no significant association between DQI-A and dietary awareness, while a positive association was found for the overweight, normal and underweight groups (Table 2). The mean difference in DQI-A between the highest and lowest dietary awareness level was 9.3 in the underweight, 6.9 in those of normal weight, 5.2 in the overweight and 0.5 in the obese. Similar results were found for each DQI component (Table 3).

Discussion

Although several studies have been performed to evaluate the perception of dietary intake in children and adolescents, our study is the first to investigate the relationship between diet quality and the awareness of diet quality in European adolescents [START_REF] Forrestal | Energy intake misreporting among children and adolescents: a literature review[END_REF]. We hypothesized that a lack of awareness of personal dietary habits could be a major barrier for intervention programs aimed at promoting a healthy diet.

Unexpectedly, our main finding was that European adolescents, regardless of gender, pubertal status, maternal educational level and lunch location, correctly assess their own diet quality. While adolescents have been shown to have difficulties in qualifying their daily physical activity (they tend to overestimate their physical activity patterns), our data show that is not the case for their assessment of diet quality [START_REF] Corder | Physical activity awareness of British adolescents[END_REF]. This probably results from education and information about a "healthy" diet in the European countries included in the study.

Gender, pubertal status or educational level did not affect diet quality awareness, whereas these variables were demonstrated to have an influence on physical activity awareness [START_REF] Béghin | Nutritional and pubertal status influences accuracy of self-reported weight and height in adolescents: the HELENA Study[END_REF].

Another important finding from our study is that obese adolescents do not have a valid perception of their diet quality. In addition to underestimating their weight and energy intake, our results show that obese adolescents do not discriminate well between a healthy or unhealthy diet [START_REF] Corder | Physical activity awareness of British adolescents[END_REF][START_REF] Béghin | Nutritional and pubertal status influences accuracy of self-reported weight and height in adolescents: the HELENA Study[END_REF][START_REF] Rangan | Misreporting of energy intake in the 2007 Australian Children's Survey: identification, characteristics and impact of misreporters[END_REF][START_REF] Maximova | Do you see what I see? Weight status misperception and exposure to obesity among children and adolescents[END_REF][START_REF] Murakami | Characteristics of under-and over-reporters of energy intake among Japanese children and adolescents: The Ryukyus Child Health Study[END_REF]. This is an additional factor that could contribute to the failure of intervention programs that aim to reduce obesity. Our observation that obese adolescents misreport their diet quality emphasizes the importance of improving awareness of diet quality, the first step in any intervention to promote a healthy diet. Based on the results presented in our study, regular feedback to obese adolescents on their dietary quality might be beneficial and could motivate them to adjust their own diet throughout the day. New technology, such as nutrition applications for mobile devices, could be used to give regular and rapid feedback on dietary intake quality, and therefore might improve dietary intake quality perception and behaviors [START_REF] Schap | Merging dietary assessment with the adolescent lifestyle[END_REF][START_REF] Lieffers | Dietary assessment and self-monitoring with nutrition applications for mobile devices[END_REF][START_REF] Hebert | Gender differences in social desirability and social approval bias in dietary self-report[END_REF]. This method presents a great opportunity to modify awareness and might instill healthy behaviors, while providing objective information about individual dietary quality might bring about a more realistic estimation of dietary quality by obese European adolescents. Another possible explanation for the misperception of diet quality by obese adolescents is the influence of social desirability (the tendency to respond so as to avoid criticism) and social approval (the tendency to seek praise), which can bias answers in selfreporting [START_REF] Hebert | Gender differences in social desirability and social approval bias in dietary self-report[END_REF].

In our study, we found a stronger positive association between DQI-A and diet awareness in adolescents who eat at home compared to those eating at school. This difference might be due to the influence of the family on healthy diet awareness. However a significant positive between DQI-A and awareness was found both in adolescent eating at home and those eating at school.

The current study has strengths and limitations. The strengths of the study are the large sample size of adolescents in 10 European cities, the use of standardized procedures, and the strong methodology used to assess dietary habits [START_REF] Julián-Almárcegui | Comparison of different approaches to calculate nutrient intakes based upon 24-h recall data derived from a multicenter study in European adolescents[END_REF]. The limitations of the study include the cross-sectional design with observed associations, which cannot be interpreted to reflect causal relationships. In addition, even though the HELENA-DIAT has been validated against dietary recall with an interviewer, the main limitation is the subjectivity, especially in obese people, of the assessment of dietary intake that was evaluated only by the adolescent participants. Then, as this study was performed ten years ago (2006-2007), we could not exclude our results represent the present situation.

Conclusions

Adolescents evaluate well their food quality independent of their pubertal status, gender and parental educational level, except for obese adolescents who are not able to assess accurately their diet quality. Improving dietary awareness in obese adolescents might help to induce behavioral changes.

  have lunch (school or home). Heterogeneity in the association of DQI-A with dietary awareness level across subgroups was assessed by adding a multiplicative term into the linear mixed model. All statistical tests were performed at a 2-tailed α level of 0.05. Data were analyzed using SAS version 9.4 [SAS Institute Inc., Cary, NC 27513, USA].

Figure 1 .

 1 Figure 1. DQI-A according to the adolescents' dietary quality awareness † DQI-A: Diet Quality Index-Adolescents * P for trend adjusted for center using linear mixed effect model (diet perception level was treated as an ordinal factor) (p<0.0001) Values are mean (Standard Deviation)

Table 1 .

 1 Comparison of mean characteristics between the included and non-included adolescents

	Included	Not included	P

Table 2 .

 2 DQI-A according to the adolescents' diet quality awareness and key subgroups

			Diet quality self-assessment
		Low	Median	High	P*	P het
	Gender
	Boys Girls	56.0 (15.6)** 61.5 (13.6)	61.7 (13.8) 63.5 (13.2)	63.4 (13.2) 67.3 (11.7)	<0.0001 <0.0001	0.59
	Nutritional status
	Undernourished	57.0 (12.8)	62.6 (13.6)	66.3 (12.8)	<0.0001
	Normal Weight Overweight	58.7 (14.9) 59.2 (15.1)	62.2 (13.3) 64.0 (14.0)	65.6 (12.3) 64.4 (13.7)	<0.0001 0.002	0.006
	Obese	62.2 (14.3)	67.8 (14.7)	62.7 (13.9)	0.42
	Pubertal status
	II	60.0 (13.6)	63.3 (11.7)	65.4 (13.0)	0.016
	III IV	58.9 (14.9) 58.9 (14.7)	64.4 (12.9) 61.8 (13.8)	65.0 (13.1) 64.5 (13.1)	<0.0001 <0.0001	0.12
	V	58.8 (15.4)	62.7 (13.8)	67.5 (10.9)	<0.0001
	Mother education level
	I	55.7 (15.3)	59.0 (14.3)	59.7 (13.9)	<0.0001
	II	61.3 (13.6)	64.0 (12.8)	66.5 (11.3)	<0.0001	0.27
	III	62.3 (13.4)	65.6 (12.1)	68.9 (10.6)	<0.0001
	Place adolescents
	used to have lunch
	School Home	61.5 (12.4) 58.8 (15.3)	64.3 (12.3) 62.6 (14.0)	66.9 (11.6) 66.1 (12.6)	<0.0001 <0.0001	0.043

P het indicates p-values for heterogeneity in relation to DQI-A and diet awareness level across key subgroups. * P for trend adjusted for center using linear mixed effect model (diet perception level was treated as an ordinal factor). **mean (Standard Deviation)

Table 3 .

 3 DQI components according to the adolescents' diet quality awareness and nutritional status P for trend adjusted for center using linear mixed effect model (diet awareness level was treated as an ordinal factor). Values are mean (Standard Deviation)

			Diet awareness
		Low	Median	High	P*
	Underweight
	DQI-Quality	28.0 (35.4)	38.6 (33.5)	43.4 (33.8)	0.013
	DQI-Equilibrium	37.8 (9.9)	40.5 (11.1)	44.5 (12.4)	0.001
	DQI-Diversity	72.0 (14.4)	76.6 (13.3)	79.4 (12.6)	<0.0001
	DQI-Meal	91.7 (12.1)	94.9 (12.4)	96.5 (8.6)	0.019
	Normal Weight
	DQI-Quality	35.7 (38.5)	40.9 (34.6)	46.7 (32.7)	<0.0001
	DQI-Equilibrium	38.6 (10.0)	40.8 (10.4)	43.1 (10.2)	<0.0001
	DQI-Diversity	70.9 (14.5)	74.1 (14.0)	78.0 (13.2)	<0.0001
	DQI-Meal	90.0 (14.8)	93.1 (12.3)	94.9 (10.9)	<0.0001
	Overweight
	DQI-Quality	40.2 (40.1)	50.1 (35.3)	49.3 (36.4)	0.004
	DQI-Equilibrium	39.1 (10.7)	42.3 (10.5)	42.5 (11.2)	0.0007
	DQI-Diversity	71.0 (14.8)	72.5 (14.9)	74.5 (15.3)	0.045
	DQI-Meal	87.6 (16.0)	91.7 (13.5)	91.5 (13.9)	0.017
	Obese
	DQI-Quality	48.3 (39.5)	54.7 (33.4)	51.8 (32.6)	0.41
	DQI-Equilibrium	40.0 (9.6)	44.6 (12.4)	42.3 (10.2)	0.11
	DQI-Diversity	70.6 (15.2)	75.0 (16.9)	71.0 (16.0)	0.50
	DQI-Meal	90.2 (14.4)	95.4 (10.8)	84.5 (15.0)	0.23

*

Acknowledgements

The authors thank the participants for taking part in the study.

Healthy Lifestyle in Europe by Nutrition in Adolescence (HELENA), Diet Quality Index for Adolescent (DQI-A), HELENA Dietary Intake Assessment Tool (HELENA-DIAT)

Funding sources

The authors thank the participants for taking part in the study. The HELENA study is made possible by the financial support of the European Community Sixth RTD Framework Programme (Contract FOOD-CT-20056007034) and the Spanish Ministry of Science and Innovation (RYC-2010-05957). The content of this paper reflects only the authors' views, and the European Community is not liable for any use that may be made of the information contained therein.

Conflict of interest

The authors do not have any competing interests.

Legends

Figure 1. DQI-A according to the adolescents' diet quality awareness.


J D D Rodríguez Ramírez 1-A

B Castanié
email: bcastanie@insa-toulouse.fr

C Bouvet

ANALYSIS OF NONLINEAR BEHAVIOR ON HONEYCOMB CORES

Keywords: Honeycomb, Shear testing, Buckling, Artificial neural network

This work presents the results of 21 shear tests on three types of honeycomb cores: the HRH-78-3/16-3.0, HRH-10-3/16-6.0 and an aluminium honeycomb core made of 5052 alloy with 3/8" cells. First, a benchmark of the different designs for shear testing of the honeycomb cores is made. Then, several double lap specimens were fabricated and tested while a 3D image correlation system was used to record the tests. An experimental analysis of the post-buckling phenomena of the HRH-78 W is presented. An artificial neural network is developed and used to visually identify the buckles of the test specimens. Finally, a detailed description of the behaviour of the structure including the postbuckling scenario is presented.

INTRODUCTION

Sandwich structures are widely used in many industrial applications like acclimated transportation, aircrafts, aerospace, naval, civil engineering, fluid storage, electronics, etc. These structures offer an exceptional benefit when they are used in aeronautics. The incorporation of this technology into the aircraft structures has proved to be a great solution for mass reduction problems due to their important bending stiffness and low weight, which allows to design lightweight parts.

When a bending load is applied to these panels, the core absorbs almost all the shear component of the force, therefore the shear properties of the core are indispensable for the sandwich design.

Several interesting publications have been made about the modelling of honeycomb cores [START_REF] Bunyawanichakul | Non-linear finite element analysis of inserts in composite sandwich structures[END_REF], [START_REF] Bunyawanichakul | Experimental and numerical analysis of inserts in sandwich structures[END_REF], [START_REF] Seemann | Numerical Modeling of Nomex Honeycomb Sandwich Cores at Meso-Scale Level[END_REF]. This work is a contribution to the understanding of the nonlinear response of honeycomb cores under shear loads. Several experimental tests will be made to describe in detail the scenario before and after the buckling of the honeycomb cells.

BENCHMARK STUDY OF SHEAR SPECIMENS

Four different types of specimens were fabricated and tested to observe the different advantages that each design offers to study the shear properties of a honeycomb material. The aim was to select a design that fits the best to obtain the following features: The average shear stress vs. gamma curve, the shear modulus of the core, the maximal average shear stress of the core, observe the evolution of the nonlinear behavior of the structure, observe the evolution of buckles in the structure, a specimen which fabrication is simple and allows to obtain repetitiveness of the results.

Tests specimens

All the specimens were fabricated using the HRH-78-3/16-3.0 Nomex honeycomb core. According to [START_REF] Hexweb | HexWeb Honeycomb Attributes and Properties[END_REF] the W direction shear modulus is 24.13 MPa and the shear strength is 0.558 MPa. For the L direction, the shear modulus is 31.71 MPa and the shear strength is 0.8549 MPa.

A sandwich beam three-point specimen (Fig. 1) was fabricated. When a load is applied to sandwich beam, the core is submitted almost entirely to shear stresses while the skins absorb the flexural components. Therefore three-point sandwich beams are very often used for shear testing of honeycomb cores. The core was made of the HRH-78-3/16-3.0 Nomex L direction of 20 mm of thickness, the skins where made of aluminum of two mm of thickness. The size of the specimen was of 20mm x 20 mm x 160 mm, finally the length span was of 140 mm.

A double single lap specimen (Fig. 1) was fabricated. This specimen submits the core to shear but also to flexural stresses due to the significant thickness of the core, however the fabrication is simple.

The core is made of HRH-78-3/16-3.0 Nomex W direction of 20 mm of thickness, the skins were made of aluminium of 5 mm of thickness to minimize the flexural deformations.

A double lap specimen (Fig. 1) was also fabricated. As for the previous specimen, this specimen submits the core to shear stress, while the flexural forces are dissipated due to the symmetry of the specimen. The core is made of HRH-78-3/16-3.0 Nomex W direction of 20 mm of thickness, the skins where made of aluminum of 5 mm of thickness.

A two materials double lap specimen (Fig. 1) was also fabricated. This specimen is very similar to the previous one, but two materials are used instead of only the Nomex core. The purpose is to use as secondary material a which is much stronger to concentrate the shear forces on the principal material. The used materials are the HRH-78-3/16-3.0 Nomex W direction honeycomb and plywood, both of 20 mm of thickness, the skins were made of aluminium of 5 mm of thickness.

Results and discussion

All the specimens were tested using a 10 KN Instron machine. The force was recorded from the corresponding signal of the machine, the displacement was measured in two different ways; using a LDVT displacement sensor and using a 3D digital image correlation system (DIC). The average shear stress and gamma are calculated from the applied force, the dimensions of the specimens and the displacement field. The curves are shown in figure 2.

The beam specimen is very simple to fabricate but the results are not very accurate, this can be caused by several factors such as the positioning on the specimen in the setup, causing significant variations when the displacement is measured using the 3D correlation system. This can be compensated by using another technique to measure the displacements, but this should add more complexity to the test setup. The evolution of the buckles is visible, but the imposed displacement creates some difficulties when measuring the displacement field, thus the cells must be inspected one by one which is not practical. The cells near the applied loads are submitted also to compression forces, thus collapsing earlier.

The double single lap specimens are simple to fabricate but the results are not as expected, the effect of the flexural forces is very strong (Fig. 2) and the results are not significant. This effect can be avoided by adding articulations at each end of the specimen, however this will make add some complexity to the fabrication procedure and test setup.

The best results are obtained by the double lap and two-material double lap specimens, the experimental results are reasonably correct. Also, the displacement field can be easily measured. In the other hand, failure of the double lap specimen can aleatorily occur on both sides of the specimen.

For the two-material specimen, failure happens in the honeycomb side because the material is weaker. However, the global shear strength of the honeycomb is weaker than expected, this can be caused by the difference of shear modulus, thus the gamma evolution between the two materials. In conclusion, the specimen design that fits the best for our research is the double lap specimen. To ensure the failure of the specimen in the desired section, one side will be fabricated whit a lightly inferior length, thus increasing the average shear stress in the short side and causing the failure without inducing a significant variation on the measurements.

SHEAR TESTING OF HONEYCOMBS

Once the specimen design was selected, the next step was to fabricate the specimens to study the properties of the HRH-78 3/16-3.0, HRH-10-3/16-6.0 and aluminum 5052 3/8. The study will be in both directions L and W. Additionally, two kinds of test are to be performed, a static test to determine the overall shear nonlinear response and a cyclic test to analyse the elastic and postbuckling domain and of the honeycomb structure under shear forces. To get reliable results and check the reproducibility of the obtained data, every experiment will be reproduced three times. This gives a total of 12 different test conditions (3 types of honeycombs x 2 directions x2 types of tests) reproduced three times resulting in a total of 36 tests.

Tests on the HRH78 Nomex core

For this purpose, 3 specimens of each direction W and L of the honeycomb core HRH-78 were tested, the average shear stress and gamma are extracted and plotted in figure 3. The average shear modulus of the W and L directions where of 25 MPa and 32 MPa and the maximal shear stress were of 0.58 MPa and 0.75 MPa respectively. The failure scenario is shown in figure 3 and was the same for the 6 specimens, it begins with an accentuated deformation of the side whit the smaller area, then there is a big collapse of only one Nomex piece, finally the other one collapses too. Six more specimens were submitted to charging and discharging conditions. At every cycle the maximal imposed displacement was slightly increased to be able to identify the average shear stress when the material starts to be damaged. It is very difficult to show all the 6 curves of each specimen because of the amount of data, however the envelope curves are very similar to those of the figure 3, so only one curve is plotted in figure 4. The cyclic tests revealed that there is a significant decrement in the loading slope after passing the 0.52 MPa in the W direction and 0.65 MPa for the L direction. This could mean that at this point, the honeycomb structure is irreversibly damaged.

Tests on the HRH10 Nomex core

Twelve specimens were fabricated using the HRH-10-3/16-6.0 but only three specimens in the W direction were tested, the failure mode was the debonding of the honeycomb/skins interface, instead of the collapse of the cells as desired. This may be caused by the quality of the redux 609 that was used to fabricate the pieces, or due to the fact that several curing were made. The failure does not happen in the section with the smallest area. The test results don't give any important information beside the shear modulus which is 45 MPa see figure 5.

Figure 5: Average shear stress vs gamma of the HRH-10 specimens, the failure mode was debonding.

Tests on the aluminum 5052 honeycomb core

Twelve specimens were fabricated using the aluminium honeycomb core, but only two specimens in the W direction were tested (Fig 6). The same failure mode of the tests on the HRH10 was found, debonding of the core/skin interface arrives before the collapse of the cells.

ANALYSIS OF THE POST-BUCKLING PHENOMENA OF THE HRH-78 IN W DIRECTION

Using the 3D correlation system, it is possible to look from closer the buckling evolution at the exterior surface of the honeycomb cores. The magnitude of the deformation out of the plane of the buckles varies proportionally to gamma. Also, it can be seen that there are buckles that go out and others that go inside of the structure. The local buckles start to be visible at an average shear stress of 0.24 MPa, and they grow and expands until they are big enough to cause the nonlinear behavior of the curves, finally they are so big that cause the collapse of the cells (Fig. 7).

Figure 7: Analysis of the buckling of the cells at 0.24 MPa and 0.4 MPa

Setting the scale for the analysis

An inspection of the recorded images of the three specimens was made when the applied stress was 0.4 MPa (Fig 7), which is also where the nonlinear behaviour start. We made the hypothesis that the buckles that are formed at this point are big enough to affect the stability of the honeycomb, thus changing the loading slope, the average size of the buckles was 0.1054 mm. It was seen that the buckles that goes out of the structure doesn't grow, while the buckles that go inside the structure continue to grow and propagate until the cells collapses.

Using the previous information, the scale of the recorded images of the three specimens was modified, the deformation was measured between 0 mm and -0.1024 mm. This allowed to see that the collapse of the cells is caused by the creation of local buckles but also due to the propagation of the collapse of the surrounding cells, thus the cells do not necessarily collapse all at the same time.

This could mean that the change in the slope is related to the collapse of the cells and not directly to the buckles, even if the collapse of the cells is related to the local buckles.

Analysis of the images by an artificial neural network

A more detailed analysis of the videos was necessary, the cells must be analysed to determine when they are collapsed and how much influence does this have on the global behavior of the structure. In figure 8 the evolution of the collapse of one cell is shown. The buckles stop to grow once when the vertical borders of the cell are no longer straight in the middle. We considered that the collapse of the cell can be determined by monitoring the growing of the buckles in the cell. To study the collapse of the cells of the specimens approximately 13,334 images needed to be analysed to which represent a big complexity and could lead to subjective inaccuracies if the analysis is performed by a person.

An artificial neural network (ANN) was developed to do the task, this reduced the time of analysis and provides at the same time an objective method to detect when a cell is collapsed.

A pre-treatment of the images was performed on every frame of the videos. First, the cells were cut from all the frames. Then all the cells images where reshaped to the same number of pixels. After this, a color segmentation filter was applied to focalize the analysis in the red tonalities.

The ANR was trained with 29 examples of buckled cells and 50 examples of non-buckled cells that were selected by the author. The ANN was able to classify all of them correctly. The results of the analysis are shown in percentage of buckled cells vs gamma. Both sides of every specimen are analysed. The results are shown in figure 9. The analysis revealed that the nonlinear behavior is certainly related to the buckling of the cells. However, the nonlinear behaviour starts before any cell was detected to be collapsed.

CONCLUSIONS

The formation of buckles in the honeycomb walls is caused by two factors, the displacements of the skins, and because the honeycomb structure is stabilized (clamped) by the skins. A detailed description of the evolution of deformations of the HRH-78 Nomex honeycomb core can be made.

From 0 MPa to 0.24 MPa the magnitude of the buckles is very small, almost undetectable. Form 0.24 MPa to 0.4 MPa the buckles start to be clearly visible without affecting the linear behaviour of the honeycomb structure. The formations of buckles in the honeycomb could be seen as a way to store mechanical energy like a spring (if the buckles remain totally elastic). From 0.46 MPa to 0.52 MPa the buckles are big enough to cause instability of some cells and they start to collapse. However, the cyclic test indicates that the structure remains elastic. This could mean that the buckles of the Nomex paper are still in the elastic domain or that the plastic deformations are very small or not important enough to cause a significant damage of the structure.

Form 0.52 MPa to 0.59 MPa the cyclic test indicates that the structure is permanently damaged. The deformation of the cells is propagated and they continue to collapse. The maximal average shear Xi'an, 20-25 th August 2017 stress is presumably reached when all the cells are collapsed. At this point the Nomex paper is teared at some regions.

Figure 1 :

 1 Figure 1: Different types of specimens: a) Double lap, b) Three-point sandwich beam, c) Double single lap, d) Two-materials double lap

Figure 2 :

 2 Figure 2: Test results: a) Average shear stress vs gamma curves, b) Buckling of the double single lap specimen, c) Buckling of the sandwich beam, d) Buckling o the double lap specimen

Figure 3 :

 3 Figure 3: Test results: a) Average shear stress vs gamma curves, b) Failure scenario of the double lap specimen

Figure 4 :

 4 Figure 4: Average shear stress vs gamma of a cyclic tests Ep7 vs Ep3

Figure 6 :

 6 Figure 6: Average shear stress vs gamma of aluminum honeycomb specimens, the failure mode was debonding.

Figure 8 :

 8 Figure 8: Buckling evolution of a cell

Figure 9 :

 9 Figure 9: The collapse of the cells is related to the reduction of the shear properties.

From 0. 4

 4 to 0.46 MPa the average shear stress vs. gamma curve indicates a strong nonlinear behavior. The cyclic tests presumably indicated that the honeycomb structure remains elastic even if the nonlinear behavior is present. No collapsed cells are detected at this point, so the start of nonlinear behavior can be attributed to the formation of local buckles in the cells.

ACKNOWLEDGEMENTS

This research was developed at the ICA laboratory at Toulouse France and was co-financed by the CONACYT form Mexico.