domain: qfin scco sde sdv shs

text:
Armelle Mazé
email: armelle.maze@inrae.fr

Aida Calabuig Domenech

Isabelle Goldringer

Restoring cultivated agrobiodiversity: The political ecology of knowledge networks between local peasant seed groups in France

This article, using an institutional and political ecological perspective, analyses the role of knowledge networks supporting peasant seed groups in France. These groups promote a dynamic approach to agrobiodiversity restoration, developing new models of collaborative "peer-to-peer genetics" and distributed participatory breeding. Our analysis focuses here on the small grain cereal participatory breeding group. Based on detailed qualitative surveys and a network formalization, our study provides a better understanding of how these peasant seed groups self-organized and of how their horizontal and distributed network structure favors the dynamics of collective learning and knowledge spillovers. Further directions for policy making are discussed in support of more resilient plant breeding and agrobiodiversity restoration in European agricultural landscapes.

Introduction

Agrobiodiversity conservation is a major challenge in the European context, where there has been a massive loss of collective knowledge about landraces and ancient varieties after several decades of agricultural modernization (Brac de la Perrière, 2014; [START_REF] Velten | Rethinking biodiversity governance in European agricultural landscapes: acceptability of alternative governance scenarios[END_REF]Zimmerer and Haan, 2019). Among European countries, France is said to be one of the countries with a larger loss of cultivated agrobiodiversity, especially for main crops such as small grain cereals [START_REF] Bonnin | Agriculture, explaining the decrease in the genetic diversity of wheat in France over the 20th century[END_REF]. Recent studies have also stressed the decline in climate resilience of European wheat as a result of the priority given to cropyield-related tipping points, the main drivers of agricultural modernization [START_REF] Kahiluoto | Decline in climate resilience of European wheat[END_REF]. Over the past few years, a number of peasant seed initiatives have played a key role in France raising awareness about the benefits of restoring cultivated agrobiodiversity and promoting the reintroduction of landraces and ancient varieties, as well as their on-farm conservation [START_REF] Demeulenaere | A political ontology of seeds: the transformative frictions of a farmer' movement in Europe[END_REF][START_REF] Demeulenaere | Free our seeds! 'Strategies of farmers' movements to reappropriate seeds[END_REF]. 1 However, restrictive seed laws and persistent misconceptions impose severe restrictions to their integration into policy promoting farm agrobiodiversity restoration and conservation [START_REF] Pascual | The economics of agrobiodiversity conservation for food security under climate change[END_REF][START_REF] Coomes | Farmer seed network make a limited contribution to agriculture? Four common misconceptions[END_REF][START_REF] Velten | Rethinking biodiversity governance in European agricultural landscapes: acceptability of alternative governance scenarios[END_REF][START_REF] Gevers | Peasant seeds in France: fostering a more resilient agriculture[END_REF]. Until now, valuable lessons from these studies have not reach the mainstream debates and policy making on the future of agriculture in the European context.

This article intends to contribute to these debates by exploring the specific features characterizing the knowledge networks that support a French peasant seed network, the Réseau Semences Paysannes (hereafter RSP), and especially its 'participatory wheat breeding group' (PWBG). Our analysis is based on the recent advances in the IAD/SESF 2 literature on polycentric and networked forms of collective action [START_REF] Ostrom | A general framework for analyzing sustainability of social-ecological systems[END_REF][START_REF] Ostrom | Moving beyond panaceas: a multi-tiered diagnostic approach for social-ecological analysis[END_REF][START_REF] Epstein | Institutional fit and the sustainability of social-ecological systems[END_REF]. In the IAD/ SES literature, the importance of network governance has increased considerably over recent years [START_REF] Janssen | A network perspective on the resilience of social-ecological systems[END_REF][START_REF] Newig | Synapses in the network: learning in governance networks in the context of environmental management[END_REF]Bodin and Prell, 2011). Sustainability studies have highlighted the role of interindividual and social networks in facilitating information and knowledge-sharing among resource users [START_REF] Ostrom | Moving beyond panaceas: a multi-tiered diagnostic approach for social-ecological analysis[END_REF]Bodin and Prell, 2011). However, recent advances in network research have also suggested that social and knowledge networks are not isomorphic, the latter meriting specific investigations (Glückler et al., 2017).

Our contribution is twofold. First, we provide a detailed original qualitative dataset and formalization of knowledge networks https://doi.org/10.1016/j.ecolecon.2020.106821 Received 11 November 2019; Received in revised form 12 August 2020; Accepted 13 August 2020 supporting the PWBG, which supplements previous studies on the political ecology of seed commons [START_REF] Demeulenaere | A political ontology of seeds: the transformative frictions of a farmer' movement in Europe[END_REF][START_REF] Demeulenaere | Free our seeds! 'Strategies of farmers' movements to reappropriate seeds[END_REF][START_REF] Demeulenaere | Participatory plant breeding and scale issues. On a collaboration between farmer plant breeders and field geneticists[END_REF][START_REF] Mazé | Commoning the seeds: alternative models of collective action and open innovation for recreating local knowledge commons in France[END_REF]. Second, our analysis suggests important insights about the models of collective action and the functioning of peasant-led community-based breeding adopted by the PWBG to support on-farm agrobiodiversity restoration. In the face of a blatant lack of research towards more resilient and hardy seed varieties, the PWBG also developed innovative strategies, including experiments and research activities, proposing a different route to innovation defined by [START_REF] Bonneuil | Vers une génétique de pair à pair ? L'émergence de la sélection participative[END_REF] as new models of "peer-to-peer genetics" and community-based breeding that are blending more effectively the experiential and situated knowledge of peasants with the most recent advances in population genetics [START_REF] Dawson | Collaborative plant breeding for organic agricultural systems in developed countries[END_REF][START_REF] Goldringer | Recherche participative pour des variétés adaptées à une agriculture à faible niveau d'intrants et moins sensibles aux variations climatiques[END_REF][START_REF] Rivière | Mise en place d'une méthodologie de sélection participative sur le blé en France[END_REF].

This article is organized as follows. After providing a short overview of the legal context and the organizational innovations introduced by PWBG in section 2, section 3 develops the IAD/SES analytical framework with knowledge networks. Section 4 details the methodology and data set used to characterize peasant seed and knowledge activities. Section 5 presents our findings about the horizontal and distributed structure of knowledge networks involved in the PWBG and their role in knowledge accumulation and collective learning. Section 6 discusses remaining challenges and some policy implications to build more resilient collaborative peasant breeding and agrobiodiversity restoration in European agricultural landscapes.

Agrobiodiversity restoration in European agricultural landscapes

The rise of peasant seed networks in European countries marks a possible fundamental transformation in the governance of breeding and seed activities towards a more decentralized and pluralistic governing process of natural and environmental resources [START_REF] Chable | Embedding cultivated diversity in Society for Agro-Ecological Transition[END_REF].

The political rise of peasant seed networks in France

In the face of an increasingly concentrated seed industry by large national and international companies, the emergence of peasant seed groups in France was first motivated by the threats posed by the dissemination of GMOs3 through supposedly GMO-free certified seeds; these groups took the initiative to better secure their own seed sourcing and to reassert their autonomy and independence from the seed industry [START_REF] Bonneuil | Vers une génétique de pair à pair ? L'émergence de la sélection participative[END_REF]. Set up as a political and protest movement against restrictive national and European seed laws, they defend the peasant rights to breed and exchange seeds, preventing them from being patented, genetically modified, owned or controlled by small or large seed companies. The RSP actively defends peasant seed rights through the activist movement "liberate our seeds" [START_REF] Demeulenaere | A political ontology of seeds: the transformative frictions of a farmer' movement in Europe[END_REF][START_REF] Demeulenaere | Free our seeds! 'Strategies of farmers' movements to reappropriate seeds[END_REF]Girard and Frison, 2018).

In France, the RSP was formally created in 2003 as a grassroots association (under the so-called French Law of 1901) in southwest France, forming an inter-organizational web of more than 90 local collective groups situated in all main French regions, 12 of them being involved with the PWBG. The political project pursued by the RSP aimed to give a sense of resistance and self-empowerment of peasants against the hegemonic techno-scientific genetic discourses [START_REF] Demeulenaere | A political ontology of seeds: the transformative frictions of a farmer' movement in Europe[END_REF]. 4 The RSP rapidly started to promote the reintroduction of more diversified landraces and ancient varieties [START_REF] Osman | Inventory of initiatives on seeds of landraces in Europe[END_REF][START_REF] Brac De La Perrière | Decentralized-participatory plant breeding: an example of demand driven research[END_REF]. Facing a blatant lack of research by the conventional seed industry on developing more resilient and hardy seed varieties, the RSP also found additional yardsticks for developing further experimental and research activities using landraces and ancient varieties [START_REF] Goldringer | Recherche participative pour des variétés adaptées à une agriculture à faible niveau d'intrants et moins sensibles aux variations climatiques[END_REF][START_REF] Demeulenaere | Participatory plant breeding and scale issues. On a collaboration between farmer plant breeders and field geneticists[END_REF].

Recreating a shared collective knowledge about landraces and ancient varieties

In European countries, agricultural modernization has deeply transformed local peasant communities. Oftentimes, there is no remaining individual knowledge of landraces or ancient varieties, nor of their specific seed attributes and adaptive responses to their soil-climate environments. Recreating rapidly a shared collective knowledge about landraces and ancient varieties was a priority (Brac de la Perrière, 2014; [START_REF] Lewrouw | Maisons de semences paysannes. Regards sur la gestion de la biodiversité cultivée en France[END_REF]. However, as stressed by [START_REF] Berkes | Rediscovery of traditional ecological knowledge as adaptive management[END_REF], the rediscovery of local ecological knowledge (LEK) has not been mired in the past but remains an evolving and creative knowledge-practice-belief complex. Inspired by other experiments conducted in Syria [START_REF] Brac De La Perrière | Decentralized-participatory plant breeding: an example of demand driven research[END_REF], a small group of cereal producers within the RSP started, with the methodological and technical assistance of a research team of population geneticists, a farmer-led participatory breeding for small grain cereals [START_REF] Dawson | Collaborative plant breeding for organic agricultural systems in developed countries[END_REF][START_REF] Goldringer | Recherche participative pour des variétés adaptées à une agriculture à faible niveau d'intrants et moins sensibles aux variations climatiques[END_REF][START_REF] Rivière | Mise en place d'une méthodologie de sélection participative sur le blé en France[END_REF]. These peasants also started to reclaim their status as "peasant-researchers" [START_REF] Demeulenaere | A political ontology of seeds: the transformative frictions of a farmer' movement in Europe[END_REF]. Traditionally, farmers have always practiced what is called a "massal selection" based on observing the phenotypic traits of their plant populations, as well as their adaptation to their local terroir and specific soil-climate environment. Through their-day-to-day activities, accumulation of situated observations, continuous testing and experimentation, peasants are developing new knowledge and innovating [START_REF] Demeulenaere | Participatory plant breeding and scale issues. On a collaboration between farmer plant breeders and field geneticists[END_REF].

Since its inception, the PWBG self-organized in a way to favor a collaborative approach between peasants and researchers; in which scientists, in this case, population geneticists, positioned themselves on an equal footing with the peasants, letting them express the diversity of their breeding preferences depending of their specific needs, farming practices and local soil and climate environment [START_REF] Goldringer | Recherche participative pour des variétés adaptées à une agriculture à faible niveau d'intrants et moins sensibles aux variations climatiques[END_REF][START_REF] Rivière | Mise en place d'une méthodologie de sélection participative sur le blé en France[END_REF]. The PWBG has also innovated by developing new peasant population varieties that are better suited to the peasants' needs and their specific agro-ecological environment. By blending more effectively peasants' experiential knowledge with scientific knowledge in population genetics, the PWBG has thus given peasants the opportunity to further their knowledge about landraces and ancient varieties [START_REF] Demeulenaere | A political ontology of seeds: the transformative frictions of a farmer' movement in Europe[END_REF]. It also suggests a possible paradigm shift from the dominant trait-based breeding adopted by the seed industry as population (peasant) varieties are looking for a better social-ecological fit (alignment), as defined by [START_REF] Epstein | Institutional fit and the sustainability of social-ecological systems[END_REF], with the spatial and temporal heterogeneity of the local environment and promoting a better co-adaptation and dynamic population conservation.

Towards innovative models of peer-to-peer community-based breeding

A major organizational innovation introduced by the PWBG is its efforts towards more collaborative, open, distributed and contextualized ways of organizing dynamic on-farm agrobiodiversity conservation. A growing number of studies consider these alternative peasant seed initiatives as "seed commons", e.g. collectively managed shared collective resources by local place-based peasant groups shaped by common values, knowledge ecosystems and rules of sociability [START_REF] Demeulenaere | A political ontology of seeds: the transformative frictions of a farmer' movement in Europe[END_REF][START_REF] Mazé | Commoning the seeds: alternative models of collective action and open innovation for recreating local knowledge commons in France[END_REF]. Through its distributed and networked model of community-based breeding, the PWBG is thus proposing a break with the current corporate and entrepreneurial models adopted by the seed industry and their vertical labor division between peasants and breeders [START_REF] Bonneuil | Gènes, pouvoirs et profits[END_REF]. Fig. 1 illustrates the differences between the current organizational model of the seed industry (left side) and the networked and distributed model adopted by the PWBG (right side). landrace varieties retained by the National Seed Collection managed by the public research institute INRA in Clermont-Ferrand, in charge of the ex situ conservation of small grain cereals in France [START_REF] Demeulenaere | A political ontology of seeds: the transformative frictions of a farmer' movement in Europe[END_REF].

As part of their models of collective action, the strong ethical values conveyed within the PWBG about peasant's self-empowerment, equity, reciprocity and trust were mirrored by the distributed and horizontal network structure, and democratic decision making they adopted [START_REF] Dawson | Collaborative plant breeding for organic agricultural systems in developed countries[END_REF][START_REF] Demeulenaere | A political ontology of seeds: the transformative frictions of a farmer' movement in Europe[END_REF][START_REF] Mazé | Commoning the seeds: alternative models of collective action and open innovation for recreating local knowledge commons in France[END_REF]. Within the RSP, the peasants are no longer considered just as users of breeding innovations designed by others: now, they are themselves the actors and the innovators involved in a peer-to-peer process of knowledge, cocreation and open innovation, as defined by Von [START_REF] Hippel | Democratizing innovation[END_REF]. By giving more autonomy to peasants to develop their own rules, the network governance adopted by the PWBG aimed enhancing the reflexive individual and collective learning needed in the context of knowledge restoration [START_REF] Demeulenaere | Participatory plant breeding and scale issues. On a collaboration between farmer plant breeders and field geneticists[END_REF][START_REF] Mazé | Commoning the seeds: alternative models of collective action and open innovation for recreating local knowledge commons in France[END_REF]. A better understanding of the specific conditions under which these innovative peasant seed networks operate is still needed.

Analytical framework

To substantiate our analysis, we build upon the IAD/SES framework developed by Elinor [START_REF] Ostrom | A general framework for analyzing sustainability of social-ecological systems[END_REF] and recent analytical advances on the role of knowledge and collective learning in polycentric and networked forms of governance of SES [START_REF] Janssen | A network perspective on the resilience of social-ecological systems[END_REF][START_REF] Andersson | Analysing decentralized resource regimes from a polycentric perspective[END_REF][START_REF] Newig | Synapses in the network: learning in governance networks in the context of environmental management[END_REF]Bodin and Prell, 2011). 5

Knowledge ecosystems, collective action and self-governance

In the sustainability literature, a growing body of studies has suggested that because polycentric and networked forms of governance enable actors to develop their own rules and organizations at multiple levels, they also possibly work more effectively and achieve better suited social-ecological fit in solving local environmental issues due to lower coordination costs and facilitated knowledge transmission and monitoring strategies [START_REF] Ostrom | Moving beyond panaceas: a multi-tiered diagnostic approach for social-ecological analysis[END_REF]Bodin and Prell, 2011;[START_REF] Epstein | Institutional fit and the sustainability of social-ecological systems[END_REF]. Here, individual and collective learning are clearly conceived as a collective action problem themselves, which involves sharing experiences and engaging in collaborative governance. In their study, [START_REF] Ostrom | Moving beyond panaceas: a multi-tiered diagnostic approach for social-ecological analysis[END_REF] suggested that such network governance can under some conditions outperform the classic top-down and hierarchical governance models, but possibly at the expense of higher coordination costs. Special attention must thus be paid to the balance between these different dimensions [START_REF] Ostrom | A general framework for analyzing sustainability of social-ecological systems[END_REF][START_REF] Ostrom | Background on the institutional analysis and development framework[END_REF][START_REF] Pahl-Wostl | A conceptual framework for analyzing adaptive capacity and multilevel learning processes in resource governance regimes[END_REF].

Over the past few years, network analysis offered a fruitful framework for theorizing and empirically investigating important socialecological interactions in the context of complex social-ecological systems [START_REF] Ostrom | A general framework for analyzing sustainability of social-ecological systems[END_REF]Bodin and Prell, 2011). Several studies considered the role of informal social networks between individuals as primary channels for knowledge flows on the co-management of natural resources. While informal networks play a crucial role in transferring and sharing knowledge, [START_REF] Folke | Adaptive governance of social-ecological systems[END_REF] also suggested that "bridging organizations", such as the PWBG, may help lower the costs of collaboration, reduce behavioral uncertainty and favor mutual collective learning processes. A major challenge is thus, as stressed by [START_REF] Hess | Understanding Knowledge as a Commons: From Theory to Practice[END_REF], to blend more effectively systems of rules and norms, to deal with the knowledge gaps and to guarantee general access to knowledge while ensuring recognition and support for those who create knowledge in its various forms. A better understanding of the way knowledge ecosystems, collective action and self-governance function is needed.

Collective learning and the selective nature of knowledge networks

Due to the loss of traditional knowledge about landrace and ancient varieties, recreating a shared collective knowledge and social memory about cultivated agrobiodiversity couldn't be based on maintained 5 In the IAD/SES classificatory system [START_REF] Ostrom | A general framework for analyzing sustainability of social-ecological systems[END_REF], the network structure is represented by the variable GS3 and knowledge of SES by the variable U7. Evaluative criteria includes: i) economic efficiency and coordination costs; ii) fiscal equivalence; ii) redistributional equity; iii) accountability; iv) conformance to values of local actors; and v) sustainability over the long term [START_REF] Ostrom | Background on the institutional analysis and development framework[END_REF]. traditional knowledge anymore. Furthermore, it requires special effort to recreate a shared, common and collective knowledge (Brac de la Perrière, 2014). A fundamental approach of the PWBG has been to recreate a collective space of knowledge exchanges and practices to harness both individual and collective learning through the day-to-day practice of seed handling and experimentation (Brac de la [START_REF] Brac De La Perrière | Decentralized-participatory plant breeding: an example of demand driven research[END_REF][START_REF] Demeulenaere | A political ontology of seeds: the transformative frictions of a farmer' movement in Europe[END_REF][START_REF] Mazé | Commoning the seeds: alternative models of collective action and open innovation for recreating local knowledge commons in France[END_REF].

In the innovation literature, many studies have also observed that knowledge diffusion operates in a selective and uneven way, suggesting the influence of structural properties of knowledge networks [START_REF] Giuliani | The selective nature of knowledge networks in cluster: evidence from the wine industry[END_REF]Glückler et al., 2017). 6 The structures of knowledge networks reflect the different internal capabilities to transfer and absorb knowledge at the local level and the heterogeneity of "knowledge bases" of network members, defined by their specific skills, know-how and knowledge domains. Hence, the collective groups with stronger "knowledge bases", which enhance their absorptive capacity, are more likely to be more central in the knowledge networks [START_REF] Giuliani | The selective nature of knowledge networks in cluster: evidence from the wine industry[END_REF]. If knowledge is not randomly distributed and follows structured patterns of knowledge diffusion and generation, a small subset of actors may actively participate in knowledge creation and absorption patterns. Stronger relational density can reflect stronger cohesiveness through knowledge exchanges, trust and social proximity. If some members value isolated strategies or outside partnerships more, weaker cohesiveness will be observed [START_REF] Giuliani | The selective nature of knowledge networks in cluster: evidence from the wine industry[END_REF]. We thus can assume that the heterogeneity of knowledge bases among the PWBG, and the complementarities of their know-how and fields of expertise, influences the structure of knowledge networks and the dynamics of collective learning and their innovative capacities.

Network cohesion, modularity and dynamics of spatial clustering

The analysis of network topology is thus useful to better understand the formation and evolution of network dynamics and their determinants (Glückler et al., 2017). In the IAD/SES literature, the network structure has been primarily characterized by variables such as centrality, modularity and connectivity (Bodin and Prell, 2011;[START_REF] Cox | Applying a social-ecological system framework to the study of the Taos Valley irrigation system[END_REF]. The modularity of the network structure is characterized by the existence of subgroups with denser links between their individual members. A modular conformation of networks takes advantage of the geographical proximity and the local embeddedness in their environment, facilitating collective problem solving and reducing transaction costs [START_REF] Cox | Applying a social-ecological system framework to the study of the Taos Valley irrigation system[END_REF]. In knowledge networks, nodes are viewed as repositories of knowledge. Due to the power of spatial clustering and localized knowledge spillovers for learning and increasing innovative capacity, clustered groups often show a higher innovative capacity than isolated actors [START_REF] Giuliani | The selective nature of knowledge networks in cluster: evidence from the wine industry[END_REF]Glückler et al., 2017).

If geographical proximity and the embeddedness of the actors in local networks positively affect learning and innovative processes, the diffusion and generation of knowledge is likely to be differentiated depending on existing asymmetries and the heterogeneity of the actors' knowledge base [START_REF] Giuliani | The selective nature of knowledge networks in cluster: evidence from the wine industry[END_REF]. Network segregation also indicates that there are incentives for actors to occupy strategic positions between fragmented communities. The centrality of an organization's position is influenced by its size, its absorptive capacities, or the openness of its organizational model [START_REF] Giuliani | The selective nature of knowledge networks in cluster: evidence from the wine industry[END_REF]. Networks are also subject to possible trade-offs and tensions between triadic closure and openness, which can affect the knowledge spillovers, spatial dynamics and network sustainability. In the next sections, we analyze more precisely the structure of knowledge networks supporting the PWBG and their role in the dynamics of knowledge accumulation and dissemination.

Methodology and data

Our field study focuses on the RSP's cereals group and its Participatory Wheat Breeding Group (PWBG), forming a web of 14 local peasant seed groups (and a few isolated individuals) located in different regional areas in France (Fig. 2).7 By covering noncontinuous territories the PWBG is operating as an open and horizontally distributed network connecting independent local peasant seed groups in different regions with a large range of soil-climate environments: from the Atlantic coast, the Parisian Basin and northern France/Belgium to the mountainous Massif Central and the Alps, and the southwest and Mediterranean areas.

Methodology

To analyze the knowledge networks supporting the PWBG and their role in the dynamics of knowledge co-creation and collective learning, we have adapted and extended the methodology developed by [START_REF] Giuliani | The selective nature of knowledge networks in cluster: evidence from the wine industry[END_REF]. In her study, the importance of "knowledge base" is defined by R&D investments and qualified personnel, the levels of education and training, and the nature and intensity of experimentation [START_REF] Giuliani | The selective nature of knowledge networks in cluster: evidence from the wine industry[END_REF]. In the case of the PWBG, the seed group's activities are not restricted to seed exchanges but also involve a diversity of other seed activities, such as prospection, conservation, and population breeding. Our interviews showed that two dimensions were particularly important for the local seed groups in developing their own "knowledge base" on landraces.

-The diversity of seed activities and the degree of day-to-day involvement, training and experimentation in seed handling are central in fostering knowledge accumulation about the adaptive comanagement of landraces and ancient varieties and new peasant breeds with their local climate/soil environment. The more diversified seed activities they have, the more opportunities they have for knowledge co-creation and capability building. -The level of participation in the PWBG is a key aspect to foster knowledge accumulation and reflexive learning. Thus, the more involvement there is in the participatory breeding activities, the more opportunities for individual and collective learning and knowledge co-creation, leading to a more central position in the network.

Our methodology involved the following steps. Conceptually, we first started by characterizing the different seed activities performed by the different groups, differentiating seed prospection, ex situ and in situ conservation, selection, multiplication, dissemination, and their role in knowledge accumulation and dissemination. In the second step, we identified the links connecting the network members with each other and with their local research environment. Thus, the intensity and quality of knowledge co-creation and exchanges were assessed through different proxies defined in Table 1, such as their degree of involvement in the PWBG and their participation in training, workshops, seminars or other activities such as farm visits, technical support, meetings, research projects, experiments or breeding activities. Table 1 summarizes the scale and criteria to assess their involvement in the PWBG activities.

Data collection

Our study combines qualitative data on the historical development of the PWBG, the characteristics of member groups and individuals, their different seed activities, their degree of involvement and the interactions with researchers through specific regional, national or European research projects. The survey was carried out between March and June 2017 through twenty semi-structured interviews with representatives (facilitator or key peasant) of all the groups involved in the PWBG and researchers. Secondary data were collected from internet websites and internal documentation on research projects and other activities (open days, training). Last, a triangulation of data was carried out through a "roster recall" method about the nature and intensity of interactions and involvement of each local group or individual with the PWBG.

Network nodes and links

The network formalization follows classic procedures in network analysis, including the identification of the structural configurations and network characteristics (nodes, links and attributes). We also elaborated specific metrics and criteria to characterize the nature and intensity of the relationships between the local peasant's seed groups involved in the PWBG. 8 Each network node represents a specific collective peasant seed group/individual and an oriented tie from one association (i) to another (j), indicating that the collective group has been exchanging information and knowledge about its seed collections, experiments, and research activities. Each collective group member of the PWBG was also characterized by the following attributes defining its "knowledge base": i) the size of their collection, including the number and type of varieties, ii) the nature of breeding and experimentation platforms, iii) their seniority in the network in relation to the year of joining PWBG (Annex 1), iv) their degree of specialization related to the type of seed activities or plant species, v) the degree of involvement in the PWBG and related seed activities, and vi) their participation in EU funded or national research projects.

Results and analysis

In the next sections, we present the main findings characterizing the influence of the knowledge base of local peasant seed groups on the network structure (5.1), the nature and intensity of knowledge exchanges within the PWBG activities (5.2), the differentiated involvements of the local groups in the PWBG (5.3), the specific local knowledge ecosystems they created (5.4), and the dynamics of the network (5.5).

The influence of the knowledge base on the network structure

An important outcome of the network analysis was to highlight the specific preferences of each local group in developing their own seed activities and how it influences their specific knowledge accumulation about landrace and ancient varieties. During our interviews, several groups mentioned that the management of the breeding and propagation platforms requires much time and effort, which is proportional to the number of varieties cultivated each year. The capacity of the collective groups to manage such a large number of varieties within a collection of 200-300 varieties is a key dimension characterizing the primary groups in the network (ARDEAR Rhone Alpes (AD R-A), Triptolème (Trp), CETAB (CTB), Graine de Noé (GdN), Limestère (LM)) and their respective experience with and knowledge about how to do this, defining their respective knowledge capital in handling seed activities.

Fig. 3 provides an overview of the interactions within the network and shows the groups with the strongest knowledge base by the experience that they accumulated through day-to-day manipulation of and experimentation with ancient and landrace varieties. Each group is represented by three elements: i) the size of the seed collection and the number of varieties conserved (up to 300 varieties); ii) their degree of involvement in the PWBG; and iii) the types of varieties that they are interested in, e.g., only ancient landrace varieties conservation (purple color), newly designed peasant varieties as an outcome of the PWBG (blue color) or a mix of the two ancient landrace varieties conservation and newly designed participatory peasant variety developed by the PWBG (black color).

In Fig. 3, the group of Graine de Noé (GdN) occupies a central position (node with a purple square) in comparison to the others, which is linked to its seniority in terms of working with landrace seed varieties, the size of its collection and its key role in the on-farm conservation of

Table 1

The scale and criteria used to establish these relationships are as follows: seed collections. In contrast to the other core groups, GdN is less involved in new breeding activities, prioritizing seed conservation and knowledge dissemination about landraces and ancient varieties. The other primary groups CETAB (CTB), ARDEAR Rhone Alpes (AD RA), and Triptolème (Trp) (see Fig. 3) have more diversified seed activities in relation to ancient and landrace seed varieties, also giving more space to innovative breeding strategies. Knowledge exchanges seem to be more important (scale 4 to 6) among these primary groups, as represented in Fig. 3, that also have the most important seed collections. Thus representing a good indicator of their knowledge base about landraces and ancient varieties in relation to the diversity of seed activities that they are conducting.

Each primary group can thus be considered a "knowledge leader" in a specific field due to the know-how acquired in different domains, such as the conservation of ancient and landrace varieties (GdN), the dynamic management of crop populations and seed mixtures (Trp) or cross-breeding and bread-making (CETAB). By developing complementary, but still overlapping, domains of know-how and expertise, the different groups fostered peasant interests, knowledge dissemination and training within the network. The other less central groups in the network dynamics (MSP MSP46, G65, AB04, C32, etc.) appear to play the role of "knowledge takers", as they prioritize specific valueoriented models (bread-making with landraces, eco-tourism, etc.), with a lower investment in the core collective activities of the PWBG, while still participating in information dissemination.

Knowledge cocreation and exchanges: differentiated positions

The nature and intensity of knowledge exchanges defined the horizontal links between the groups that were more involved in the network (only scales 5 and 6) in comparison to the overall interactions (scales 1 to 6). Fig. 4 highlights the differentiated position of each group in relation to its respective knowledge base and specific domain of expertise about ancient and landrace seed varieties. Fig. 4a shows the groups with the strongest relationships (scales 5 to 6), compared to Fig. 4b representing all the interactions, including the weakest (scales 1 to 6). The primary groups identified in Fig. 3 (GdN, AD R-A, Trp, CETAB) with the largest size of seed collections and stronger internal capabilities and knowledge base about ancient and landraces varieties are also more central in Fig. 4a and Fig. 4b.

The more peripheral groups, as illustrated in Fig. 4b, are on the right side: these are the collective groups that are more interested in a valueoriented model (specialty bread, branding strategies, etc.), such as the AB04, G65, and MSP 46 groups that work more independently9 . On the left side are the few individuals (BV, JSG, SPT) involved in conservation and breeding activities but who interact less with the rest of groups due to personal time constraints. Even if less central, both subgroups contribute to the PWBG activities through data collection, breeding and conservation activities, as well as knowledge dissemination outside of the network. Weak ties still play an important role in information gathering and dissemination.

The dynamic and evolving nature of participation in network activities is also a key aspect. While early participation in the first research projects was ranked 2 or 3 (see Table 2 below), participation in more recent research projects was ranked 4 and 5 as the knowledge contribution of the groups was greater in the latter projects due to a stronger knowledge base and collective experience acquired by the participants. New PWBG members also appear to be more active in the collective network activities, indicating the importance of the initial learning process and knowledge acquisition before becoming more selfsufficient and autonomous in their own breeding activities and experiments. 10 Fig. 3. The PWBG knowledge network and its primary groups (scale 4 to 6). In -Fig. 3, the node's size represents the size of the seed collection managed by the different collective groups and the shape of the node represents their degree of involvement in the PWBG: from the strongest 4-Circle, 3-Square (Box), 2-Upward pointing triangle, 1-Downward pointing triangle, to the weakest 0-Diamond. The colors represent the different focus in seed activities: conservation (purple), new breeding activities (blue), or both (black).

Differentiated involvements in the PWBG activities

A major driver of the network's development was the involvement of the PWBG in formal research projects which influenced its progressive structuration over time. Research projects provided additional financial resources and fostered knowledge co-creation and dissemination among the groups. This was achieved through joint activities stimulating mutual aid and collaborative arrangements among the participants. A more precise analysis showed that, in fact, this participation varies across time and depends on the specific interests of each local peasant seed group, as reflected in Table 2.

Table 2 also shows the differentiated positions between the primary groups and those more at the periphery within the collective dynamics. Whereas a primary group (scale 4) is strongly involved in all PWBG activities, with a specific interest in breeding, another group is more involved in seed conservation (scales 2 and 3). The last two groups show a lower level of involvement due to time constraints (scale 1) or their specific interest in specific landrace varieties and strategies of value creation (specialty bread, local brand names, etc.). Differences in knowledge bases and field expertise accumulated by the different groups also influence the nature of their involvement in the PWBG activities.

Considering this heterogeneity is important in terms of managing the network dynamics and the collective strategies for agrobiodiversity restoration. It reflects differences in learning trajectories and building capabilities: those that joined the group more recently and are more interested in breeding activities, have stronger needs in terms of knowledge acquisition and are thus the most involved. However, once a minimum knowledge base has been acquired, some groups have less interest in continuing, resulting in a lower participation in the PWBG and a potential reduction of knowledge spillovers among the participating groups. This questions the overall network dynamic and sustainability over time.

The development of local knowledge ecosystems

Whereas the RSP played a key coordination role, the ability of the local peasant seed groups to mobilize other resources and to develop their own knowledge ecosystems and new forms of collaborations with local research partners was also crucial (Fig. 5). The connections with the research team INRA Le Moulon (Isabelle Goldringer) specializing in population genetics were central in the progressive structuration, dayto-day coordination and facilitation of the PWBG and in providing the genetic technologies and related analytical and mathematical tools needed for the design of field experiment settings, as well as identifying and taking into account the specific peasants' organizational constraints [START_REF] Dawson | Collaborative plant breeding for organic agricultural systems in developed countries[END_REF][START_REF] Goldringer | Recherche participative pour des variétés adaptées à une agriculture à faible niveau d'intrants et moins sensibles aux variations climatiques[END_REF][START_REF] Rivière | Méthodologie de la sélection décentralisée et participative sur le blé tendre[END_REF]). 11 This central position is clearly defined in Fig. 5 and highlights the modular structure of the PWBG and the connections between the peasant groups and local research partners (INRA centers, ITAB, agricultural engineering colleges).

Joint participation in research projects has created the conditions and space for knowledge exchanges and coproduction between researchers, facilitators and peasants. By functioning as a "knowledge platform", the PWBG generates positive knowledge spillovers towards its members located in different regions. Research was conceived to be supportive in different ways: i) in the search for varieties at public genetic resource centers, ii) as a source of scientific knowledge, and iii) in the organization of evaluation and selection trials [START_REF] Goldringer | Recherche participative pour des variétés adaptées à une agriculture à faible niveau d'intrants et moins sensibles aux variations climatiques[END_REF][START_REF] Rivière | Méthodologie de la sélection décentralisée et participative sur le blé tendre[END_REF]. While peasant seed groups are keen to reclaim their autonomy and independence from public research institutions, our study shows that they still benefit from local research support, which provided useful help to conduct experiments and trials [START_REF] Rivière | Mise en place d'une méthodologie de sélection participative sur le blé en France[END_REF]. This includes specific projects conducted with a technical institute on organic production (ITAB) 12 and several local INRA research laboratories in Rennes (Brittany), Orléans (Central and Loire Vallée Fig. 4. The core collective groups within the PWBG. 10 The groups with small collections still play an important role in seed and knowledge dissemination. When joining the PWBG, it is observed that peasants often start experimenting and developing their own seed collection. Thus, such decentralization and dissemination increase the potential social-ecological fit and adaptability of population (peasant) varieties to different farming practices, and soil and climate conditions.

11 Added analytical tools developed by the research team include specific mathematical tools that help visualize and objectivize the genetic outcomes of the phenotypic breeding strategies and the so-called "massal selection" practiced by the peasants involved in the PWBG [START_REF] Rivière | Mise en place d'une méthodologie de sélection participative sur le blé en France[END_REF].

12 For example, ITAB carries out studies regarding the nutritional and organoleptic qualities of organic bread derived from peasant varieties and gives technical support on organic farming. Moreover, one of its experimental sites is located in one of the member groups in the network (BV).

regions) and agricultural colleges (such as in Purpan near Toulouse). They provided experimental platforms both for in situ conservation of seed collections and for plot experiments. Thus, rather than operating in isolation, one of the PWBG strengths is related to its capacity to develop new alliances and local knowledge ecosystems aligned with their specific needs and objectives [START_REF] Mazé | Commoning the seeds: alternative models of collective action and open innovation for recreating local knowledge commons in France[END_REF].

Network modularity and the dynamics of local spatial clustering

A key aspect of the PWBG is being able to connect collective groups that were not located in the same region around a shared activity, e.g., the development of collaborative breeding that facilitated knowledge exchange and empowered the peasants to develop their own breeding activities [START_REF] Mazé | Commoning the seeds: alternative models of collective action and open innovation for recreating local knowledge commons in France[END_REF]. Characterized by territorial discontinuities, the modular network structure adopted by the PWBG has been positively enhancing knowledge spillovers between the different peasant seed groups, as well as strengthening their capabilities to maintain, over time and across places, dynamic seed collections covering hundreds of different varieties and thus ensuring their socialecological fit to various soil and climate environments. Over time, different spatial dynamics were nevertheless observed within the PWBG in relation to endogenous group dynamics and to the specific financial and political support they received from regional public authorities: -In the southwest, covering the regions of Aquitaine and Occitanie, the connections with the local collective group AgroBio Périgord (ABP), another RSP member, developed a strong know-how, matured expertise in the maize breeding, and stimulated knowledge spillovers across species (maize, cereals, vegetable, perennials) among the groups. -In Burgundy and Rhone-Alpes, the local groups were more motivated by the conservation of patrimonial heritage and faced a rapid demand increase for organic products, -In the west of France (Brittany, Loire Vallée), strong connections with the research laboratory INRA Rennes (Véronique Chable), operating as an EU project leader, provided facilitated access to funding and created positive knowledge spillovers.

These local dynamics favored the formation of local clusters, giving more visibility to the promotion of peasant seeds and their dissemination. Geographical proximity facilitates knowledge spillovers through spatially close groups and embedded in local networks of informal contacts. Nonetheless, in regions where conventional agriculture is dominant or where peasants are isolated, the connection with the PWBG activities at the national level remains crucial.

Discussion

Our findings provide important information on how the PWBG and its local seed groups collectively self-organized and developed a set of innovative peasant seed activities. Whereas dynamic in situ conservation of landraces and ancient varieties is central, our analysis suggest that this is the combination of different seed activities, including collaborative breeding and joint research on new population varieties that stimulate both individual and collective learning dynamics. It has generated new sources of diversification for peasants looking for varieties better suited to their specific needs and value creation in the context of low input and organic systems. Our analysis of the knowledge networks supporting the PWBG shows the value of the horizontal and distributed network approach they adopted by linking separate networks segments and helping new members to develop their capabilities in handling landraces and ancient varieties. In the network literature, a bridging actor often assumes a broker position that favors the development of strong ties, producing higher private and social returns to its members. This network cohesion has thus a positive effect on knowledge sharing and co-creation among heterogeneous actors but also in connecting and generating positive knowledge spillovers in favor of isolated groups or individuals. It thus shows the importance of the network dynamism in terms of ensuring that new knowledge is available and in creating complementarities and synergies between different local peasant seed groups. The overall network dynamic is nevertheless also subject to the risks of classical trade-offs between network closure and openness.

A major remaining challenge for the PWBG and its local peasant seed groups is the lack of stable financial and human resources, and the local political instability which is affecting their access to financial support. 13 To give more visibility to their seed activities, some members of the RSP launched the concept of "peasant seed houses" (PSH), inspired by Brazilian experiences of "casas de sementes criollas" [START_REF] Lewrouw | Maisons de semences paysannes. Regards sur la gestion de la biodiversité cultivée en France[END_REF]. This initiative facilitated their access to EU funding through the 2nd pillar of the Common Agricultural Policy (CAP) on territorial development. However, such EU funding is managed at the regional level creating disparities between the different

Table 2

The degree of involvement of collective groups in the PWBG. 13 For example, the change of political majority in the region Rhone-Alpes (South East) after the regional election in 2015, led to a drop in the public support provided to organic production and related advisory services, such as the ARDEAR Rhone-Alpes, a member of the PWBG.

PWBG groups. It also prevented the RSP, coordinating the PWBG at the national level, to have access to these financial resources.

Favoring some regions, especially in the South-West where the peasant seed groups are currently forming an experienced cluster, creates incentives for these groups to invest less in the PWBG activities. Over time, this situation could negatively affect the other groups located in less favored regions where conventional agriculture is dominant and where the knowledge spillovers created by the PWBG are needed the most. Several studies have highlighted the current limitations of economic mechanisms supporting agrobiodiversity restoration, mainly through individual or collective agro-environmental incentive schemes [START_REF] Pascual | The economics of agrobiodiversity conservation for food security under climate change[END_REF]. Other policy toolboxes such as "community seed banks" (CSB) have remained out of the CAP instruments. Investing in such agro-ecological infrastructures, as CSB or PSH promoted by the RSP, was recently suggested by several European peasant seed initiatives and could be a substantial lever of action for policy changes [START_REF] Koller | Community Seed Banks in Europe[END_REF]). 14

Conclusion

Our study provides several important insights to rethink dynamic on-farm agrobiodiversity governance in the European agricultural landscape, and overcome a number of misconceptions that have prevented the recognition and integration of peasant seed groups into European public policies [START_REF] Mc Guire | Making seed systems more resilient to stress[END_REF][START_REF] Coomes | Farmer seed network make a limited contribution to agriculture? Four common misconceptions[END_REF]. Conflict and power struggles about seeds involve tremendous asymmetric forces and resistances. As suggested by [START_REF] Ostrom | Governing the Commons: The Evolution of Institutions for Collective Action[END_REF][START_REF] Ostrom | A general framework for analyzing sustainability of social-ecological systems[END_REF], this is only because the seed commons are organized collectively that they can be a viable alternative to markets or state management. In the face of growing criticism towards the linear and top-down model of breeding innovation in agriculture, a number of studies called for a paradigmatic reorientation of plant breeding systems, giving more space to the coexistence with alternative community-based breeding [START_REF] Bonneuil | Gènes, pouvoirs et profits[END_REF][START_REF] Vanloqueren | How agricultural research systems shape a technological regime that develops genetic engineering but locks out agroecological innovations[END_REF]. Challenging the dominant view of peasant-led collaborative breeding as a form of low-tech and frugal innovation requiring low investment, our findings show that the PWBG stimulated innovative forms of collective experimentation and research in the search for better suited population varieties for organic and low-input farming systems. Acknowledging and financially supporting these farmer-led collaborative breeding as part of local agro-ecological infrastructures could be an important step towards agrobiodiversity restoration in European agricultural landscapes.

Fig. 1 .

 1 Fig. 1. The differences of organizational models between the seed industry (left) and the distributed model of the PWBG (right) (adapted from Bonneuil and Thomas, 2009).

Fig. 2 .

 2 Fig. 2. Localization of the local peasant seed groups of the PWBG.

0-

  No specific relationship between association members 1 -Participation in the same project: Programme Leonardo, GROW, RSP meetings, 2 -Exchange of information and data through different projects: Routine selection projects, database, national meetings, PhD thesis (first projects) 3 -Exchange of information and knowledge: National (Renabio, semaine SP) and international meetings (Belgium 2017, Let's liberate diversity), SOLIBAM meetings, relations with research (ITAB, INRA-results of projects) 4 -Exchange of knowledge (between associations): Speeches, training (baking), workshops, farm visits, technical support, counseling, SOLIBAM evaluation (structuring of projects, exchange of knowledge and results, meetings); seed exchanges/distribution without a close relationship, such as C32, CTB, AD N, Trp 5 -Project ECOAGRI (which helped structure the group): Exchange of seeds, knowledge and experiences; meetings; relationship reinforcement between participants 6 -Close relation-partnership: Exchanging seeds; working together frequently; sharing seed samples, varieties, mixtures; seed collection; working together on local projects 8 The network formalization was completed using the software program UCINET (version 6.631).

Fig. 5 .

 5 Fig. 5. The knowledge ecosystems surrounding the PWBG (values from 3 to 6) Legend: The circle represents the INRA Le Moulon and the triangle represents the local INRA centers, ITAB and agricultural engineering colleges.

GMOs: genetically modified organisms.

A key step was that the RSP members reclaimed access rights to the ancient

In classic social network analysis, centrality of some actors is defined by "high ranking nodes that have a significantly higher than average social capital, leadership or mutual trust, and thus, a higher number of links and/or links stretching beyond their local network neighborhood.

Annex 1 presents all the local peasant seed groups participating in the PWBG, their acronyms in figures and their main characteristics.

In Figure4b, network density is higher on the right side, indicating stronger interactions between collective groups, as compared with individual members who interact less with the rest of the groups due to lack of time (situated to the left side).

Acknowledgements

This work was part of the research project "SENAC" (Social-Ecological Networks in A Changing world) supported by a grant overseen by the French National Research Agency (ANR) as part of the "Investments d'Avenir" Programme LabEx BASC (ANR-11-LABX-0034). Additional funding was provided by the INRAE through its Métaprogramme SusMcrop. We thank all the peasants and local facilitators who provided their valuable time to answer our questions, and especially to Pierre Rivière, Alexandre Hyacinthe for their fruitful suggestions, and to Etienne Polge for his support with the Ucinet software. We also thank the reviewers who help us to significantly improve our manuscript. Any errors remain ours.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ecolecon.2020.106821.


Andres Sela
email: asela@mondragon.edu

Gorka Ortiz-De-Zarate

Daniel Soler

Patxi Aristimuño

Denis Soriano

Guénaël Germain

François Ducobu

Pedro José Arrazola

Paul Stief
email: paul.stief@ensam.eu

Jean-Yves Dantan

Alain Etienne

Ali Siadat

Surface drag analysis after Ti-6Al-4V orthogonal cutting using grid distortion

Keywords: Assembly, Design method, Family identification Surface drag, surface integrity, orthogonal cutting, Ti-6Al-4V Surface drag, surface integrity, orthogonal cutting, Ti-6Al-4V

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

Introduction

Due to the fast development in the domain of communication and an ongoing trend of digitization and digitalization, manufacturing enterprises are facing important challenges in today's market environments: a continuing tendency towards reduction of product development times and shortened product lifecycles. In addition, there is an increasing demand of customization, being at the same time in a global competition with competitors all over the world. This trend, which is inducing the development from macro to micro markets, results in diminished lot sizes due to augmenting product varieties (high-volume to low-volume production) [START_REF] Ortiz-De-Zarate | Experimental and FEM analysis of surface integrity when broaching Ti64[END_REF]. To cope with this augmenting variety as well as to be able to identify possible optimization potentials in the existing production system, it is important to have a precise knowledge of the product range and characteristics manufactured and/or assembled in this system. In this context, the main challenge in modelling and analysis is now not only to cope with single products, a limited product range or existing product families, but also to be able to analyze and to compare products to define new product families. It can be observed that classical existing product families are regrouped in function of clients or features. However, assembly oriented product families are hardly to find.

On the product family level, products differ mainly in two main characteristics: (i) the number of components and (ii) the type of components (e.g. mechanical, electrical, electronical).

Classical methodologies considering mainly single products or solitary, already existing product families analyze the product structure on a physical level (components level) which causes difficulties regarding an efficient definition and comparison of different product families. Addressing this

Introduction

Titanium alloys, especially Ti-6Al-4V, are widely used in industry, being relevant in aeronautical applications, as well as automotive, chemical and medical ones [START_REF] Ortiz-De-Zarate | Experimental and FEM analysis of surface integrity when broaching Ti64[END_REF]. Ti-6Al-4V is an 𝛼𝛼+𝛽𝛽 alloy which represents more than 50% of the production of the titanium alloys [START_REF] Arrazola | Machinability of titanium alloys (Ti6Al4V and Ti555. 3)[END_REF]. It is employed in industry because of the combination of good properties such as low density, high strength at elevated temperatures and corrosion resistance [START_REF] Cedergren | Influence of work material microstructure on vibrations when machining cast Ti-6Al-4V[END_REF][START_REF] Saoubi | High performance cutting of advanced aerospace alloys and composite materials[END_REF].

However, in spite of these advantages, it is known as difficult-to-cut material due to its poor machinability because of its low thermal conductivity and high chemical activity, which is traduced in chip tool adhesion and segmentation, affecting the final surface of the workpiece [START_REF] Saoubi | High performance cutting of advanced aerospace alloys and composite materials[END_REF][START_REF] Courbon | Tribological behaviour of Ti6Al4V and Inconel718 under dry and cryogenic conditions-Application to the context of machining with carbide tools[END_REF][START_REF] Ulutan | Machining induced surface integrity in titanium and nickel alloys: A review[END_REF].

There exist different ways to evaluate the final surface state, paying attention to material properties (hardness or residual stress), metallurgical and microstructural aspects (phase transformation, tearing or surface drag) or topological ones (roughness) [START_REF] Ulutan | Machining induced surface integrity in titanium and nickel alloys: A review[END_REF][START_REF] Thakur | State-of-the-art in surface integrity in machining of nickel-based super alloys[END_REF][START_REF] Liang | State-of-the-art of surface integrity induced by tool wear effects in machining process of titanium and nickel alloys: A review[END_REF]. This state is directly related to the final behavior of the workpiece and, especially, to its fatigue life [START_REF] Javidi | The effect of machining on the surface integrity and fatigue life[END_REF]. Thus, special attention should be paid to finishing operations, as they define the surface state of the manufactured component.

Based on literature review, the surface integrity of aeronautical alloys such as Ti-6Al-4V has been widely studied, with special focus on aspects such as roughness, hardness or residual stress [START_REF] Ulutan | Machining induced surface integrity in titanium and nickel alloys: A review[END_REF][START_REF] Thakur | State-of-the-art in surface integrity in machining of nickel-based super alloys[END_REF][START_REF] Liang | State-of-the-art of surface integrity induced by tool wear effects in machining process of titanium and nickel alloys: A review[END_REF]. Other important aspects such as microstructural alterations or surface drag, depth of the affected machining zone, are usually not considered and few examples can be found in literature.

Available online at www.sciencedirect.com ScienceDirect

Introduction

Titanium alloys, especially Ti-6Al-4V, are widely used in industry, being relevant in aeronautical applications, as well as automotive, chemical and medical ones [START_REF] Ortiz-De-Zarate | Experimental and FEM analysis of surface integrity when broaching Ti64[END_REF]. Ti-6Al-4V is an 𝛼𝛼+𝛽𝛽 alloy which represents more than 50% of the production of the titanium alloys [START_REF] Arrazola | Machinability of titanium alloys (Ti6Al4V and Ti555. 3)[END_REF]. It is employed in industry because of the combination of good properties such as low density, high strength at elevated temperatures and corrosion resistance [START_REF] Cedergren | Influence of work material microstructure on vibrations when machining cast Ti-6Al-4V[END_REF][START_REF] Saoubi | High performance cutting of advanced aerospace alloys and composite materials[END_REF].

However, in spite of these advantages, it is known as difficult-to-cut material due to its poor machinability because of its low thermal conductivity and high chemical activity, which is traduced in chip tool adhesion and segmentation, affecting the final surface of the workpiece [START_REF] Saoubi | High performance cutting of advanced aerospace alloys and composite materials[END_REF][START_REF] Courbon | Tribological behaviour of Ti6Al4V and Inconel718 under dry and cryogenic conditions-Application to the context of machining with carbide tools[END_REF][START_REF] Ulutan | Machining induced surface integrity in titanium and nickel alloys: A review[END_REF].

There exist different ways to evaluate the final surface state, paying attention to material properties (hardness or residual stress), metallurgical and microstructural aspects (phase transformation, tearing or surface drag) or topological ones (roughness) [START_REF] Ulutan | Machining induced surface integrity in titanium and nickel alloys: A review[END_REF][START_REF] Thakur | State-of-the-art in surface integrity in machining of nickel-based super alloys[END_REF][START_REF] Liang | State-of-the-art of surface integrity induced by tool wear effects in machining process of titanium and nickel alloys: A review[END_REF]. This state is directly related to the final behavior of the workpiece and, especially, to its fatigue life [START_REF] Javidi | The effect of machining on the surface integrity and fatigue life[END_REF]. Thus, special attention should be paid to finishing operations, as they define the surface state of the manufactured component.

Based on literature review, the surface integrity of aeronautical alloys such as Ti-6Al-4V has been widely studied, with special focus on aspects such as roughness, hardness or residual stress [START_REF] Ulutan | Machining induced surface integrity in titanium and nickel alloys: A review[END_REF][START_REF] Thakur | State-of-the-art in surface integrity in machining of nickel-based super alloys[END_REF][START_REF] Liang | State-of-the-art of surface integrity induced by tool wear effects in machining process of titanium and nickel alloys: A review[END_REF]. Other important aspects such as microstructural alterations or surface drag, depth of the affected machining zone, are usually not considered and few examples can be found in literature.

For instance, focused on titanium alloys, Che-Haron (2001) [START_REF] Che-Haron | Tool life and surface integrity in turning titanium alloy[END_REF] and Grintin and Nouari (2009) [START_REF] Ginting | Surface integrity of dry machined titanium alloys[END_REF] observed severe tearing and plastic deformation when turning Ti-6246, especially after prolonged machining and under dry conditions. In addition, when the tool failed, notable deformation of the microstructure was observed. Thus, phenomenon such as surface drag was observed to increase by using worn tools [START_REF] Che-Haron | Tool life and surface integrity in turning titanium alloy[END_REF]. The same conclusions were obtained by Che-Haron and Jawaid (2005) for Ti-6Al-4V [START_REF] Che-Haron | The effect of machining on surface integrity of titanium alloy Ti-6% Al-4% V[END_REF]. Puerta-Velásquez et al. ( 2010) studied the evolution of the microstructure of Ti-6Al-4V, for high cutting speeds (up to 600 m/min) [START_REF] Velásquez | Sub-surface and surface analysis of high speed machined Ti-6Al-4V alloy[END_REF]. They demonstrated that the thickness of the machining affected zone increased with cutting speed almost linearly. [START_REF] Joshi | Microstructural characterization of chip segmentation under different machining environments in orthogonal machining of Ti6Al4V[END_REF] [START_REF] Joshi | Microstructural characterization of chip segmentation under different machining environments in orthogonal machining of Ti6Al4V[END_REF] demonstrated that different environments (dry, cooling or LN2) for orthogonal cutting of Ti-6Al-4V have effect on machining affected zone. Li et al. (2017) analyzed microstructural surface evolution when high speed machining Ti-6Al-4V via FEM [START_REF] Li | Prediction of the critical cutting conditions of serrated chip in high speed machining based on linear stability analysis[END_REF].

The majority of these studies were carried out at high speeds, not including broaching conditions on the analysis. However, broaching is a widely used operation in aerospace due to the good surface integrity condition obtained [START_REF] Ortiz-De-Zarate | Experimental and FEM analysis of surface integrity when broaching Ti64[END_REF]. Thus, it is important to know the state of the machined surface at broaching conditions, usually characterized by low cutting speeds and feeds [START_REF] Mo | An example of selection of the cutting conditions in broaching of heat-resistant alloys based on cutting forces, surface roughness and tool wear[END_REF], [START_REF] Sela | A mechanistic model to predict cutting force on orthogonal machining of Aluminum 7475-T7351 considering the edge radius[END_REF]. Nevertheless, it has barely been studied in comparison to other machining operations such as turning or milling. In addition, [START_REF] Childs | Ti6Al4V metal cutting chip formation experiments and modelling over 6 A. Sela / Procedia CIRP 00 (2019) 000-000 a wide range of cutting speeds[END_REF] [START_REF] Childs | Ti6Al4V metal cutting chip formation experiments and modelling over 6 A. Sela / Procedia CIRP 00 (2019) 000-000 a wide range of cutting speeds[END_REF] introduced a failure model, validated at low and high cutting speeds, to model the orthogonal cutting of Ti-6Al-4V. Some results of numerical surface drag were shown, but, as surface integrity was out of the scope of the paper, this issue was not deeply studied. Also an experimental and numerical comparison of surface drag for Ti-6Al-4V can be found in [START_REF] Ortiz-De-Zarate | Experimental and FEM analysis of surface integrity when broaching Ti64[END_REF].

To the best of our knowledge, the study of surface drag is usually based on optical microstructure analysis and its effectiveness could depend on the microstructure obtained. Thus, this paper presents a methodology to measure the surface drag with an optical method, based on the distortion of a mechanical grid machined in the workpiece. In addition, these measurements were carried out at 7.5 m/min (broaching conditions) and were validated with FEM simulations and microstructure based measurements.

The paper is organized as follows. First, the experimental set-up is explained. Then, the numerical model is briefly shown. After that, numerical and experimental results are analyzed and discussed and some conclusions are finally drawn.

Experimental set-up

Orthogonal linear cutting tests were carried out on a vertical machine center Lagun CNC 8070. The workpiece material was Ti-6Al-4V. To prevent side flow problems it has been decided to machine two workpieces of 2 mm clamped together with the grid between two halves [START_REF] Ghadbeigi | Determination of micro-scale plastic strain caused by orthogonal cutting[END_REF]. The workpiece was clamped to the machine table whereas the tool holder was set in the spindle. The set-up used is explained in the previous publication [START_REF] Ortiz-De-Zarate | Methodology to establish a hybrid model for prediction of cutting forces and chip thickness in orthogonal cutting condition close to broaching[END_REF].

The insert used was an uncoated TPUN 160308, with a nominal edge radius of 25 µm (see Fig. 1), measured in an optical 3D measurement device (Alicona Infinite Focus IFG4), with an accuracy of 1 µm and a magnification of 10X.

The effect of the feed (uncut chip thickness) on the surface integrity state was studied varying from 0.1 to 0.4 mm. The cutting speed was 7.5 m/min, typically used in broaching operations. The experimental plan, tool geometry and material are summarized in Table 1. To measure the surface drag, a mechanical grid was created using a KERN-EVO micromilling machine. The ball end mill used had 0.1 mm diameter, and a grid with a line separation of 0.04 mm was obtained. Fig. 2 shows the undeformed grid measured in a macroscopy device with a magnification of 6X. The grid state after the machining process was measured with the Alicona IFG4 with 50X magnification, to have enough resolution to carry out the measurement of the surface drag. The length of cut was 24 mm and notable repeatability was obtained along the whole length on surface drag measurement. For each cutting test new tool was used to avoid tool wear effects.

Numerical model

Material model

The measurements of the surface drag were compared with numerical results. In a previous publication, flow stress and damage law for Ti-6Al-4V were analyzed and determined [START_REF] Ortiz-De-Zarate | Evaluation of different flow stress laws coupled with a physical based ductile failure criterion for the modelling of the chip formation process of Ti-6Al-4V under broaching conditions[END_REF]. 2, takes into account the effects of isotropic hardening, strain rate hardening and thermal softening as showed in [START_REF] Ortiz-De-Zarate | Evaluation of different flow stress laws coupled with a physical based ductile failure criterion for the modelling of the chip formation process of Ti-6Al-4V under broaching conditions[END_REF]. [START_REF] Childs | Ti6Al4V metal cutting chip formation experiments and modelling over 6 A. Sela / Procedia CIRP 00 (2019) 000-000 a wide range of cutting speeds[END_REF] and it was demonstrated to be accurate at low and high cutting speeds, at least to predict cutting forces, chip morphology and temperature [START_REF] Childs | Ti6Al4V metal cutting chip formation experiments and modelling over 6 A. Sela / Procedia CIRP 00 (2019) 000-000 a wide range of cutting speeds[END_REF], [START_REF] Ortiz-De-Zarate | Evaluation of different flow stress laws coupled with a physical based ductile failure criterion for the modelling of the chip formation process of Ti-6Al-4V under broaching conditions[END_REF]. At high cutting speeds, the failure is supposed to be due to thermal softening, which causes adiabatic shearing along the primary shear zone. However, at low cutting speeds as the one used in this research, the failure mechanism is governed by the lack of ductility, causing the segmentation of the chip. Thus, the failure model implemented had two different stages, decomposed into the initiation of the failure and the flow stress reduction due to this failure.

The Mohr-Coulomb failure law was chosen to represent the failure initiation [START_REF] Childs | Ti6Al4V metal cutting chip formation experiments and modelling over 6 A. Sela / Procedia CIRP 00 (2019) 000-000 a wide range of cutting speeds[END_REF], [START_REF] Ortiz-De-Zarate | Evaluation of different flow stress laws coupled with a physical based ductile failure criterion for the modelling of the chip formation process of Ti-6Al-4V under broaching conditions[END_REF]. This model considers the accumulation of damage (D) along a streamline, where the damage is modelled as a function of the stress triaxiality (𝜂𝜂) and failure coefficients (𝜀𝜀 𝑓𝑓,0 and c). The effect of temperature was also taken into account, as it was observed to affect ductile failure.

The mechanical and thermal properties of the material and the tool were taken from literature [START_REF] Ortiz-De-Zarate | Evaluation of different flow stress laws coupled with a physical based ductile failure criterion for the modelling of the chip formation process of Ti-6Al-4V under broaching conditions[END_REF].

Model description

The simulations were carried out in AdvantEdge 2D, which has a coupled thermo-elasto-plastic Lagrangian code with continuous remeshing and adaptive meshing [START_REF] Ortiz-De-Zarate | Evaluation of different flow stress laws coupled with a physical based ductile failure criterion for the modelling of the chip formation process of Ti-6Al-4V under broaching conditions[END_REF]. The mesh was created with a minimum element size of 2 µm to ensure accurate results. The residual stress analysis was activated to achieve a fine mesh in the machined surface.

The material model previously explained was implemented in the software by user-defined subroutine developed in Fortran language.

To reduce the time to reach the steady state, the heat capacity of the tool was set to a value much lower than the physical one, as it was stated in [START_REF] Arrazola | Metal cutting experiments and modelling for improved determination of chip/tool contact temperature by infrared thermography[END_REF].

Finally, to represent the tool-chip friction law, a stickingsliding model was chosen and, due to the low cutting speeds, the friction coefficient was set to 1 according to [START_REF] Ortiz-De-Zarate | Evaluation of different flow stress laws coupled with a physical based ductile failure criterion for the modelling of the chip formation process of Ti-6Al-4V under broaching conditions[END_REF].

The simulations were carried out following the same experimental plan as shown in Table 1.

Results and discussion

The main idea to measure the surface drag is to observe the affected zone with the Alicona IFG4 with a magnification of 50X. Due to the existence of a physical 3D grid in the workpiece it is possible to see whether it has been deformed by the machining process, and therefore to measure its depth. The red line in Fig. 3 shows Alicona's image of the workpiece after being machined with a feed of 0.2 mm. The red line marks the limit of the surface drag, placed at 47 µm of the machined surface.

These results can be compared with those obtained by numerical simulations (see Fig. 4). In this case, to obtain the surface drag, the strain was measured along different lines orthogonal to the machined surface.

The plastic strain of each line for each cutting condition was plotted against depth (Fig. 5). Due to the asymptotical behavior observed, it was decided that a strain of 0.02 is the threshold value to determine the limit of the drag zone [START_REF] Ghadbeigi | Quantitative strain analysis of the large deformation at the scale of microstructure: comparison between digital image correlation and microgrid techniques[END_REF]. Taking into account that the numerical measurements could be fitted with an exponential function, it has been stated that the surface drag in this case was 54 µm (7 µm of difference between numerical 3. Fig. 5 shows that at low feeds (f = 0.1 and 0.2 mm), the obtained plastic strains were similar, being close to 7, at low depths (near the machined surface) and then it decreases drastically with depth. On the contrary, at the highest feed (0.4 mm), the strain at the machined surface was lower than 4 and the slope was less pronounced. This effect could be relevant on the quality of the machined surface.

In addition, experimental values were also compared with common measurements of surface drag based on microstructure alterations. For that, the sample was polished and chemically etched with Kroll. Then, the measurements were carried out in an optical microscope Leica with magnification of 50X (see Fig. 6). The measurements were carried out within the meshed workpiece, in the middle of this workpiece and located one millimeter away from the grid.

The measurement for each condition was repeated twice and the variations between each repetition were lower than 3 µm.

In Table 3 all the results are summarized, including grid based, microstructural based (optical) and numerical values. The uncertainty of the measurements done with Alicona IFG4 device was of ± 5 µm based on grid dimensions and repeatability of the results along the length of cut. The results against feed are shown in Fig. 7. The error bar in the numerical results comes from the fact that strain was measured in several lines.

In general, it was observed that the values reported by microstructure based measurements are notably lower than the ones measured with grid distortion and lower than the values predicted by the model, especially at high feeds.

Comparing both experimental techniques, at f = 0.1 mm, the difference was 5 µm, which is inside the range of uncertainty. However, these differences are more remarkable at higher feeds (being 22 µm for f = 0.2 mm and 53 µm for 0.4 mm). If these differences are plotted against feed, a behavior totally linear was observed (see Fig. 8). This suggests that the new method is more sensitive to material deformation than the microstructural change based method. Between numerical and new method results, the differences were lower. It was around 8 µm for 0.1 and 0.2 mm and 15 µm for 0.4 mm, which could be considered a good agreement.

In addition, for the used cutting speed, new method results show a linear trend between surface drag and feed. A slope of 226 µm/mm (surface drag/feed) was obtained for the grid measurements.

Conclusions

A new technique to measure the surface drag was presented based on a mechanical grid. With this technique, it is not necessary to use the microstructure to obtain the surface drag, avoiding the intrinsic randomness associated with microstructural aspects.

Comparing both techniques, it was observed that microstructure is not able to reproduce the total amount of strain achieved as the machining affected zone is higher than the one measured using microstructural distortion.

As the accuracy of the technique presented depends on the dimension of the grid, the obtained value could be considered as a lower limit value of the surface drag and the machining affected zone could be even higher than the one measured with this method.

This technique was applied to characterize the surface drag of Ti-6Al-4V after orthogonal machining under cutting conditions close to broaching. The experimental results were compared with numerical ones achieving a good agreement, with errors about 8 µm at low feeds and 15 µm at the highest one.

The effect of the feed on surface drag was analyzed, and it was shown that there exists an increasing trend on surface drag when feed is increased. This trend is important, as the cutting speed tested is typical in the broaching process, which is commonly used as a finishing process. This confirms that to achieve good surface integrity low feeds should be used.

However, the plastic strain numerically observed was higher for low feeds, which can also be considered a bad point in terms of surface integrity.

Fig. 1 .

 1 Fig. 1. Measurement of the edge radius

Fig. 2 .Fig. 3 .

 23 Fig. 2. Experimental surface state for f = 0.2 mm

Fig. 7 .Fig. 6 .Fig. 5 .Fig. 4 .

 7654 Fig. 7. Variation of surface drag against feed

Fig. 8 .

 8 Fig. 8. Differences on surface drag measurements against feed between the two experimental techniques

Table 1 .

 1 Experimental plan ToolRef.

		Coating	Uncoated
		Edge radius, 𝑟𝑟 𝜀𝜀 (µm)	25
	Cutting conditions Cutting speed, Vc (m/min)	7.5
		Feed, f (mm)	0.1, 0.2, 0.4
	Lubrication	Type	Dry
	Workpiece	Material	Ti-6Al-4V

TPUN 160308

Rake angle, 𝛾𝛾 (º) 6

Relief angle, 𝛼𝛼 (º) 5

Table 2 .

 2 Adjusted flow stress model parameters[START_REF] Ortiz-De-Zarate | Evaluation of different flow stress laws coupled with a physical based ductile failure criterion for the modelling of the chip formation process of Ti-6Al-4V under broaching conditions[END_REF]

	A (MPa) B (MPa)	C	n	m	𝜀𝜀 0 ̇
	1130	530	0.0165	0.39	0.61	1
	To represent the typical chip segmentation during
	machining of Ti-6Al-4V it is necessary to introduce a ductile
	failure model. The ductile failure model used was reported by
	Childs et al. (2018)

Table 3 .

 3 Comparison of the results

		Depth of the surface drag (µm)
	f (mm)	Grid distortion	Optical	Numerical
	0.1	20	15	28
	0.2	47	25	54
	0.4	90	37	75

Procedia CIRP 00 (2019) 000-000 www.elsevier.com/locate/procedia

A. Sela / Procedia CIRP 00 (2019) 000-000

Acknowledgements

The authors would like to thank the projects NG18 (KK-2018/00001), SURFNANOCUT (RTI2018-095463-B-C21), MECAERO (PIBA 2018-85) and the grant for Education and Training of Research Staff (FPU 17/02498).