1 María Isabel RIVAS GINEL -La traducción automática en la localización de videojuegos Resumen La productividad es crucial en el sector de la localización videolúdica y por ello las herramientas de traducción asistida por ordenador, de gestión terminológica y la traducción automática son cada vez más adoptadas como una solución a la creciente demanda de contenido para traducir. En los últimos años el rendimiento de estos sistemas se ha mejorado gracias, entre otros motivos, a la aparición de nuevos programas informáticos o a la traducción automática neuronal. Sin embargo, ¿cuál es la situación de la traducción automática en la localización de videojuegos? El presente trabajo muestra los resultados de un cuestionario en línea redactado en inglés creado para determinar cuáles son las herramientas más utilizadas y cuál es la tasa de adopción de las nuevas tecnologías a través de una encuesta constituida por preguntas cerradas repartidas en bloques en función del tipo de herramienta. El análisis de dichos resultados muestra que 11.6% de los encuestados la utilizan de forma regular, 37.1% de vez en cuando y 51,29% no la utilizan en absoluto.

Résumé

La productivité est cruciale dans le secteur de la localisation, c'est pour cela que la traduction automatique ainsi que les logiciels de traduction assistée par ordinateur ou de gestion terminologique, sont de plus en plus adoptés comme une solution à la demande grandissante de contenus à traduire. Au cours des dernières années, la performance de ces systèmes s'est améliorée, notamment grâce à l'apparition de la traduction automatique neuronale. Mais quelle est la situation de la traduction automatique dans la localisation des jeux vidéo ? Nous avons basé notre travail sur une enquête par questionnaire en ligne rédigée en anglais, qui vise à déterminer quels sont les outils les plus utilisés et à évaluer l'adoption de ces nouvelles technologies à travers une enquête constituée de questions fermées réparties en blocs en fonction du type d'outil. En analysant les résultats nous constatons que 11.6% utilisent de façon régulière la traduction automatique, 37.1% en font usage de temps en temps, et que 51.29% ne l'utilisent pas.

Abstract

Productivity is crucial in the video game industry and therefore, in video game localisation. During the past years, tools used for computer assisted translation, terminology management and machine translation have become essential to the trade as a solution to the increasing amount of content that needs to be translated. These systems' performance has improved thanks, among other things, to the appearance of neural machine translation. However, what are the current practices regarding machine translation in the video game localisation sector? This paper presents the results of an online survey created in order to determine business practices, the tools that are most commonly used and whether new technologies have been fully adopted by the professionals of the field. The analysis of the data shows that only 11.6% of the respondents use machine translation regularly, 37.1% sometimes and 51.29% do not use it at all.

Introducción

La industria del videojuego es un sector de actividad en constante crecimiento desde su nacimiento en los años 70, tanto en el plano económico como profesional. Un estudio recientemente publicado en la página web especializada Newzoo.com (Newzoo 2020) muestra que, a pesar de la crisis económica de los últimos años y la actual pandemia mundial, los ingresos generados por el sector podrían alcanzar los 159.3 mil millones de dólares en 2020, sobrepasando una vez más la industria cinematográfica como lo viene haciendo en los últimos 10 años. Basta con observar el éxito alcanzado por consolas como PlayStation, Xbox o Wii en el curso de los últimos 30 años y la actual difusión de videojuegos creados específicamente para smartphones para poder apreciar la verdadera expansión de estos productos de entretenimiento y, por consiguiente, el aumento de los beneficios que producen. En otras palabras, el estereotipo de jugador de videojuegos como un joven estudiante o adolescente ya no corresponde con la realidad: hoy en día, los adeptos a los video games se encuentran en todas las categorías de edad y están repartidos por todo el mundo. En su informe publicado a principios de año dirigido a los profesionales del desarrollo de videojuegos, Newzoo incluye las previsiones para 2020 sobre el número de jugadores a nivel mundial. Dicho documento estima que para finales de 2020 habrá 2,7 mil millones de jugadores en todo el mundo, un aumento de más de 135 millones comparado con el año anterior (Newzoo 2020).

Todos estos datos evidencian la necesidad de productos adaptados a los diferentes mercados a nivel internacional y confiere a la localización un lugar muy importante en el sector. Como lo indican numerosos informes de pérdidas y ganancias realizados por estudios de producción, profesionales del sector, así como un considerable número de post-mortems; más de la mitad de los ingresos generados por un juego pueden provenir de las ventas en los mercados extranjeros. Sergei Klimov [START_REF] Klimov | 2 Years of Gremlins, Inc.: demographics[END_REF]) muestra el porcentaje de copias vendidas por país del juego Gremlins, Inc. tras 18 meses en el mercado en un artículo publicado en el blog Gamasutra. En los gráficos que aparecen en dicho artículo se puede observar que más del 60% de las unidades habían sido vendidas a nivel internacional. Además, el autor concluye afirmando que alrededor del 70% de los jugadores jugaba a Gremlins, Inc. en una lengua diferente del inglés. Por consiguiente, es primordial conocer las necesidades reales de los profesionales de la localización para poder buscar soluciones adaptadas a las prácticas de la industria e intentar crear sistemas interactivos centrados en el usuario para aumentar la productividad, la calidad del trabajo, y al mismo tiempo simplificar el proceso de traducción en todas sus etapas.

Metodología

El presente trabajo muestra los resultados de un cuestionario en línea redactado en inglés creado para determinar cuáles son las prácticas del sector, el acceso a material de referencia y a un entorno visual, las herramientas más utilizadas y la tasa de adopción de nuevas tecnologías así como las funcionalidades de un hipotético programa informático perfecto. Dicha encuesta fue creada siguiendo un modelo estructurado constituido por preguntas cerradas repartidas en bloques. El primer bloque se centra en recolectar datos relevantes sobre el perfil de los participantes, el segundo tiene por objetivo analizar las prácticas actuales del sector y el resto del cuestionario está dividido en función del tipo de herramienta utilizado: extracción e integración, gestión terminológica, glosarios, corpus, memorias de traducción, traducción automática, etc.

El cuestionario, que cuenta con 19 páginas y un total de 46 preguntas (lo que equivale a un tiempo de 9 minutos mínimo por respuesta en caso de que todas sean positivas), fue publicado el 10 de abril de 2020 y se cerró el 3 de agosto del mismo año recibiendo 620 respuestas completas de un total de 1000 en poco menos de 4 meses. El método de distribución que probó ser el más eficaz para obtener respuestas fue dirigirse directamente a los profesionales a través de la red social especializada en el mundo laboral LinkedIn utilizando los parámetros de búsqueda o bien filtrando los perfiles de los empleados que aparecían en la plantilla en línea de empresas dedicadas tanto a la creación de videojuegos así como en agencias de traducción especializadas en el sector de la localización.

Análisis de los resultados

En lo referente al perfil de los encuestados, se puede observar una edad media de entre 25 y 34 años (55,81%); los participantes provenían de 113 países diferentes de entre los cuales España era el más representado con 170 personas; y además se pueden contabilizar más de 40 lenguas de trabajo. Otro dato que puede ser relevante para la tasa de adopción de herramientas especializadas es que más del 60% de los encuestados había seguido algún tipo de estudios universitarios en el sector de la traducción con 236 estando en posesión de un máster, 142 de una diplomatura y 10 de un doctorado.

En cuanto a su experiencia laboral, nuevamente más del 60% contaba con una trayectoria profesional en el sector de al menos 5 años, el 85,48% tenía la traducción como actividad principal y un 56,13% de los participantes indicaron que más de tres cuartas partes del total de su actividad provenían de la localización de videojuegos. La gran mayoría de los encuestados trabajaba por cuenta propia: 28,06% como freelancers; 25,32% a la vez de forma independiente y con agencias; 16,77% como freelancers pero para una o varias agencias; 12,42% en plantilla en una empresa de traducción; 12,10% en plantilla en una empresa fuera del mundo de la traducción; 2,9% eran estudiantes; y 2,42% eran aficionados.

Tras analizar los resultados de las preguntas referentes al uso de la traducción automática, se puede observar que el 51,29% (Tabla 1) afirma no utilizarla, esta cifra resulta de la suma de aquellos que respondieron que nunca habían oído hablar de la TA (0,48%) y aquellos que marcaron que la conocían pero que no la utilizaban (50.81%). Si comparamos esta cifra con los resultados obtenidos en encuestas anteriores como aquellas llevadas a cabo por investigadores como Doherty et al. en 2013 et Zaretskaya et al. en 2017 (Tabla 2) sobre la adopción de nuevas tecnologías por traductores profesionales en el mundo de la traducción en general se puede observar un número mayor de usuarios en el sector de la localización de videojuegos; si bien solamente 11.6% declaran usarla de forma regular y 37,1% de forma ocasional. Como parte de la encuesta se creó una pregunta compuesta por diferentes enunciados que los participantes debían valorar a través de escalas de preferencias para discernir las funcionalidades que deberían incluirse en un hipotético programa informático específicamente dedicado a la localización de videojuegos. Los participantes debían valorar cada funcionalidad propuesta como «molesta», «poco importante», «no muy útil», «útil» o «esencial». La lista contenía 14 funcionalidades (Gráfico 1) aunque en este artículo solo se discutirán aquellas relacionadas con la TA: «incluir la traducción automática» y «la posibilidad de combinar corpus, herramientas TAO y la traducción automática».

Opciones

Se puede observar a primera vista que estas dos funcionalidades son las menos apreciadas de la lista con 170 personas que señalan la traducción automática como «molesta»; 154 como «poco importante»; 195 como «no muy útil»; 105 como «útil» y solamente 16 de los participantes la consideran como esencial. «La posibilidad de combinar corpus, herramientas TAO y la traducción automática» es la segunda función con el menor número de votos positivos.

Si se observan las respuestas de aquellos que decidieron dejar un comentario en la pregunta sobre las funcionalidades, se puede ver que los profesionales del sector consideran que debería darse prioridad a herramientas que faciliten una mejor comunicación, la posibilidad de tener acceso al entorno visual en el caso de problemas de espacio o herramientas de gestión de límite de caracteres y mejores programas de revisión de ortografía. De hecho, esta actitud negativa frente a la traducción automática aparece también reflejada en parte de las respuestas de aquellos participantes que marcaron la opción «otro» en la anterior pregunta sobre el tipo de programa informático utilizado. Algunos comentarios incluyen términos como «hindrance» (estorbo) o «counterproductive» (contraproducente) y una amplia variedad de comentarios extremadamente negativos.

Gráfico 1: Funcionalidades de un hipotético programa informático

Conclusión

Los resultados obtenidos reflejan la complejidad del sector ya que el traductor especializado en la localización de videojuegos debe enfrentarse a un contenido extremadamente ecléctico que combina diálogos, instrucciones, narrativa e interfaces de usuario con imágenes en movimiento. Además, se trata de un sector que requiere el dominio de un vocabulario cada vez más específico no solo para cada una de las diferentes subcategorías sino también para las consolas en las que se lanzará el videojuego, las diferentes sagas, los neologismos creados específicamente para un juego en concreto, etc. Asimismo, como bien indica el término «localización», el traductor debe dedicarse continuamente a la adaptación cultural del contenido, tanto a nivel no textual (unidades de medida, divisas, leyes, etc.) como cultural (referencias, juegos de palabras, bromas, etc.) para poder lograr la inmersión del jugador, uno de los aspectos clave y de las principales características del mundo del videojuego y que tiene un papel fundamental en el éxito de este producto.

Otra particularidad del sector son las etiquetas, las variables y la concatenación, es decir, una unidad formada a la vez por palabras y por variables. Las variables y la concatenación contribuyen notablemente a la anteriormente mencionada inmersión del jugador, ya que pueden ayudar a crear la sensación de que el juego se dirige directamente al jugador y está hecho a su medida. También cumplen la función de reducir la cantidad de texto, un ejemplo sería la frase siguiente:

Entrar en %lugar para recuperar %objeto Las variables %lugar y %objeto serán reemplazadas automáticamente en el juego por palabras o segmentos de frases que se encuentran almacenadas en la base de datos de lugares y de objetos respectivamente. Este fenómeno se aplica a toda categoría de juego para incluir igualmente el nombre de los personajes o combinaciones de acciones en frases pre-traducidas. Estas variables, que también aparecen en la localización de programas informáticos en general, no pueden ser modificadas, eliminadas o ignoradas ya que más adelante serán reemplazadas y la posición que ocupen en el texto meta debe ser coherente y tener en cuenta las eventuales modificaciones ligadas al género, número, y su posición al interior de la frase. Las etiquetas tienen la función de modificar la forma, disposición, orden o el tiempo en el que el texto aparecerá en pantalla. Puesto que no existe una forma estándar de crear etiquetas, a veces puede resultar muy difícil diferenciar los segmentos que pueden ser traducidos de los que no o a qué hacen referencia, entre otras cosas.

Otro fenómeno típico del sector son las deficiencias en la fase de internacionalización del videojuego, etapa en la que el desarrollador prepara el programa para que pueda ser traducido sin problemas. Durante este paso los programadores deben asegurarse de que los caracteres especiales aparecen en pantalla, que todo el texto que debe traducirse haya sido extraído y no se encuentre incrustado o que las interfaces contengan suficiente espacio (o puedan adaptarse) para mostrar la traducción. Si bien los grandes estudios suelen pensar en la internacionalización durante la fase de preproducción, un gran número de desarrolladores independientes no tienen en cuenta dicho paso durante la fase de creación del proyecto, olvidando así de implantar ciertas medidas que posibilitan el trabajo del localizador. Esta falta de atención está intrínsecamente vinculada a la tasa de expansión de los idiomas y priva a menudo al traductor del espacio necesario para los términos que debe traducir. Este hecho se ve agravado por la práctica ampliamente extendida de enviar los segmentos que se deben traducir en documentos Excel, sobre todo en el caso de traductores que trabajan como freelance y a los que no se les suele dar acceso al videojuego en sí. Este hecho provoca una escisión entre la imagen y el texto que es característica de la localización de videojuegos, un sector en el que por norma general el profesional debe trabajar a ciegas.

Los dos últimos gráficos muestran hasta qué punto los localizadores deben hacer malabarismos con el texto debido a las restricciones técnicas. El gráfico número 2 muestra que casi el 47% del tiempo la única indicación con la que cuentan los profesionales del sector para evitar truncamientos, solapamientos y desbordamientos son el número máximo de caracteres permitidos por línea. Por ello, los localizadores se ven obligados a encontrar soluciones creativas y a prestar especial atención a estos elementos. El gráfico número 3 muestra que, además de las restricciones de espacio, deben recurrir al lenguaje controlado debido a variables y a la concordancia a menudo, ya sea de


Flavien Denis
email: flavien.denis@isl.eu

Robert Hruschka
email: robert.hruschka@isl.eu

Myriam Bastide
email: myriam.bastide@isl.eu

Berthold Sauerwein
email: berthold.sauerwein@isl.eu

Yannick Hoarau
email: hoarau@unistra.fr

Robert Mose
email: mose@unistra.fr

Experimental Investigation of Thermal Ablation by Melting in a Hypersonic Shock Tunnel

In this paper a novel approach for the experimental measurement of the ablation by melting is presented. The experiments are carried out with a hypersonic shock tunnel. The tested models are made of gallium, which melts at low temperatures, allowing the generation and then the measurement of the ablation profiles within only two milliseconds. By means of an optical visualization, the model surface changes are recorded during the experiment. An image post-processing algorithm computes the ablation profiles. The results for two hemisphere-cone models and one hemisphere-cylinder model in a Mach 4.5 nitrogen flow are presented. It is found that the maximum ablation occurs at an angular position of around 30 degrees off-axis rather than at the stagnation point. This is most likely a consequence of the laminar-turbulent transition on the model forebody.

Nomenclature

Cp

= Heat capacity at constant pressure heat transfer between the flow and the hypersonic object. The stagnation surfaces in particular are exposed to a large heat load, which degrades and eventually destroys them. This phenomenon is called ablation. As a sub-category, liquid ablation occurs when the metallic surface of a hypersonic object is heated beyond the melting point, being successively either advected downstream by the flow or evaporated.

[J • kg -1 • K -1 ] d = Surface displacement [mm] h = Enthalpy [J • kg -1 ] ∆H f = Latent heat of fusion [J • kg -1 ] M = Mach
The subject is of renewed interest owing to the development of new hypersonic applications involving the ablation of metallic parts, such as the disintegration prediction of end-of-life LEO satellites and space debris into the atmosphere ( [START_REF]Space debris mitigation guidelines of the Committee on the peaceful uses of outer space[END_REF], [START_REF] Grassi | Design for demise techniques for medium/Large LEO satellites reentry[END_REF] and [START_REF] Annaloro | Space debris atmospheric entry prediction with spacecraft-oriented tools[END_REF]) and the design of hypersonic projectiles accelerated by rail-gun-launchers [START_REF] Hundertmark | A scenario for a future European shipboard railgun[END_REF]. Moreover, liquid ablation is also relevant for the modelling of meteorite atmospheric entry.

In the present study, liquid ablation, i.e. ablation by melting is investigated. This fundamentally differs from the ablation by pyrolysis and gas blowing, which occurs for example on the heat-shields of re-entry-vehicles. Liquid ablation consists of two distinct phases: a heating phase during which the surface heats up from the initial temperature to the melting temperature and a successive melting phase. Also, a thin liquid layer develops along the body surface, which is sheared off by the flow. Thus, over time, this type of ablation leads to an important mass loss and a significant surface shape change, which in turn changes the overall aerodynamics of the object and may finally destroy it. Secondary phenomena, such as surface oxidation and radiative heat transfer can also influence the speed of the ablation process.

Most of the experimental data for liquid ablation have been obtained in the 1950s and 1960s, when metallic thermal protection systems were investigated ( [START_REF] Bridges | An experimental investigation of the re-entry behavior of metals and alloys[END_REF], [START_REF] Chang | Analysis of the aerodynamic ablation of a metal sphere[END_REF] and [START_REF] Luneau | Contribution à l'étude des phénomènes aérothermiques liés au vol hypersonique décéléré[END_REF]). Today, arcjet facilities are usually used for experimental studies of ablation. While they are well suited for material analysis under stagnation point flow conditions, they are not able to reproduce the aerothermodynamics experienced by a hypersonic model. However, some recent ablation experiments were carried out in hypersonic shock tunnels but they used low temperature ablators such as water ice [START_REF] Silton | Ablation onset in unsteady hypersonic flow about nose tip with cavity[END_REF] or dry ice [START_REF] Callaway | Photogrammetric measurement of recession rates of low temperature ablators in supersonic flow[END_REF] instead of an ablating metal.

Here, we present a new experimental method suitable to quantify liquid ablation of a metallic ablator that is purely melting under realistic flow conditions by using a hypersonic high-enthalpy shock tunnel.

II. Experimental procedure A. Hypersonic facility

The ablation experiments are conducted within the ISL hypersonic high-enthalpy shock tunnel STB shown in Figure 1. The high-pressure part is 2.5m and the driven part 18.4m long. The inner diameter is 100mm for both parts.

The facility is described in more details in [START_REF] Gnemmi | Shock tunnels at ISL[END_REF]. For this study, the hypersonic flow is generated by expanding the test gas in a Mach 4.5 contoured nozzle. Nitrogen is used as test gas in order to avoid interference by oxidation on the ablation process. The dissociation of molecular nitrogen is negligible since the maximum stagnation temperature is below 1600K. Therefore, the model ablation is expected to be solely a combination of material melting and erosion.

Four flow conditions, summarized in Table 1 are tested. The freestream conditions are computed from the following experimentally measured parameters: the initial driver and test gas pressures, the shock propagation velocity in the shock tube and the stagnation pressure after shock reflection at the nozzle throat. The steady flow duration for these conditions is about two milliseconds, during which the stagnation pressure varies by 15 %. For all flow conditions the Knudsen number is lower than 0.03, so that the flow can be considered as a continuum flow without surface slip [START_REF] Moss | Direct simulation of transitional flow for hypersonic reentry conditions[END_REF].

Due to the combination of the noisy shock tunnel flow and the freestream Reynolds number [START_REF] Oertel | Strömungsmechanik: Methoden und Phänomene[END_REF], the boundary layer is expected to become turbulent in the vicinity of the stagnation point region.

In the measurement section, the model is mounted 65mm downstream of the nozzle. Two side windows allow optical access for model illumination and image recording. Before the test, the pressure in the measurement section is about 5Pa.

B. Low temperature ablating metal

Due to the short steady flow duration of only two milliseconds, metals commonly used for aerospace applications such as steel or titanium cannot be used in this ablation study, as they do not have enough time to melt. For this reason, a low-temperature ablation material has to be used. Materials used in previous studies are not well-suited for liquid ablation experiments in shock tunnels. Naphthalene [START_REF] Combs | Heat-shield ablation visualized using naphthalene planar laser-induced fluorescence[END_REF] and camphor [START_REF] Stock | Hypersonic low temperature ablation an experimental study of cross-hatched surface patterns[END_REF] are ablated by sublimation, while models made of water ice [START_REF] Silton | Ablation onset in unsteady hypersonic flow about nose tip with cavity[END_REF] or CO2 ice [START_REF] Callaway | Photogrammetric measurement of recession rates of low temperature ablators in supersonic flow[END_REF] melt and also evaporate during the test preparation.

Condition M ∞ Re/x u ∞ T ∞ P ∞ T 0 h 0 P 0 n • (-) (m -1 ) (m • s -1 ) (K) (Pa) (K) (J • kg -1 ) (W • m -2 )
Gallium has been chosen as substitute ablation material for the following reasons. It is a grey, non-toxic metal with a low melting temperature of 302.9K and a high boiling point of 2477K, so a purely liquid ablation can be expected during the test. Gallium properties at 10 5 Pa are given in Table 2. No significant difference to these values is observed at lower pressures. As is the case for other metals, a thin oxide layer covers the surface of the gallium model. Despite a high melting temperature of 2070K for the gallium oxide β-Ga 2 O 3 [START_REF] Víllora | Electrical conductivity and carrier concentration control in β-Ga2O3 by Si doping[END_REF], the experiments described later demonstrate that the oxide layer has a minor influence on the ablation process and is quickly stripped off by the flow.

With the exception of its low melting point, the thermal properties of gallium are of the same order of magnitude as those of other commonly used metals. The gallium ablation process can thus be considered similar to theirs.

Table 2 Gallium properties for the solid phase (from [START_REF]Thermochemical tables, Gallium (Ga)[END_REF] and [START_REF] Ayrinhac | Compilation of thermodynamic and elastic data of liquid Ga[END_REF])

ρ Ga Cp Ga λ Ga ∆H f ,Ga T f ,Ga T s,Ga (kg • m -3 ) (J • kg -1 • K -1 ) (W • m -1 • K -1 ) (J • kg -1 ) (K) (K)
5904 370 41 80200 302.9 2477

C. Model geometry

Three model geometries are used for the experiments: an 8mm-hemisphere-cone-cylinder, a 10mm-hemispherecone-cylinder and a 14mm-hemisphere-cylinder, which are mounted on a rearward-facing sting. Due to the short duration of the experiment, the heating is expected to occur exclusively within the model. The geometries with their dimensions are shown in Figure 2. Pictures of the model production tools are presented in Figure 3.

A summary of the performed runs with the corresponding Reynolds number is given in Table 3. First, one reference image is selected for each magnification level, taken at the same time when the flow is established but the ablation process is not started yet. The model profile extracted corresponds to the initial state of the ablation profiles.

The displacement of the model caused by sting vibration is tracked for each frame of the low-magnification video using a cross-correlation with the reference image. The obtained correction for vibration is then applied to both video recordings, taking into account the magnification ratio.

IV. Experimental results and discussion

A. Qualitative description

Ablation is observed for all flow conditions and for all models, primarily in the vicinity of the stagnation point region on the hemispherical part. The generated liquid layer is then sheared off along the model surface by the flow. No ablation by erosion was observable in any video, confirming that the main ablation process in this experiment is ablation by melting.

The model images presented in Figure 6 do not show the ablated region but rather the region affected by the liquid layer, which re-solidified once the steady flow had passed. They do, however, provide a first indicator of the ablation intensity. As expected, the ablation is most significant at the conditions with the highest flow enthalpy, namely flow conditions 1 and 2. For condition 4, on the contrary, the region contaminated by the re-solidified ablation layer is restricted to the model hemisphere only, thus indicating only weak ablation.

B. Effect of the flow condition on the heating phase duration

The heating phase duration is defined as the period between the steady flow establishment and the ablation start.

During this phase the model wall temperature increases from the initial temperature to the melting temperature of the material.

The heating phase durations extracted from the test videos are presented in Table 4. The measurement uncertainty is about 0.2ms. It is mainly caused by the uncertainty in the nozzle steady flow starting point. For the high enthalpy conditions 1 and 2, the ablation starts earlier compared to the lower enthalpy conditions due to the increased surface heat transfer. The differences in the measured ablation start times for the high enthalpy conditions 1 and 2 are within the measurement uncertainty, so that no conclusion can be drawn regarding the dependencies on the model diameter or on the flow parameters.

C. Ablation profiles as a function of time

The present method allows the tracking of the ablation profiles as a function of time for the high enthalpy flow cases.

An example of ablation profiles as a function of time is given in Figure 7. At the early phase of ablation the measured profile is somewhat asymmetric. From 1.5ms onwards, the profile asymmetry is less pronounced. Even though some local asymmetries remain, the observations show that the ablation globally preserved the model symmetry. It is to note that the ablation profile at 0ms is not zero for all angles, as it is time-averaged over seven frames and also the model already vibrates before the nozzle flow is established, causing some position uncertainty.

It can be observed that from the ablation start time onwards the profiles are similar in their overall shape, featuring two local maximums: the first located at the stagnation point and the second near 20 • -40 • . These maximums are even more pronounced at 2ms. Similar ablation profiles were observed on water ice models in supersonic wind tunnel tests [START_REF] Silton | Ablation onset in unsteady hypersonic flow about nose tip with cavity[END_REF] as well as on aluminium spheres in a ballistic hypersonic free flight tunnel [START_REF] Luneau | Contribution à l'étude des phénomènes aérothermiques liés au vol hypersonique décéléré[END_REF]. In these experiments, the location for maximum ablation was related to the position of the laminar-turbulent transition of the boundary layer.

D. Effect of the hemisphere diameter on the ablation profiles

The ablation profiles obtained at 2ms at flow condition 1 for the three different hemispheres are presented in Figure 8. Despite some scattering in the data and a surface displacement of less than 0.1mm, the characteristic ablated shapes with the two ablation maxima are clearly observable for each tested geometry.

The first ablation maximum is located at the stagnation point, where the laminar wall heat flux is maximal. The second ablation maximum corresponds to the position of maximal ablation and is always observed near 30 • from the symmetry axis. The higher ablation rate results from an increase in the melting rate due to an increasing wall heat flux caused by the boundary layer transition. The Reynolds number regime and the noisy shock tunnel flow suggest that the boundary layer may become turbulent somewhere along the hemisphere surface. This would lead to an increase in wall heat flux just downstream of the laminar-turbulent transition. Following this hypothesis, the position of the ablation maximum should also depend on the model hemisphere diameter. However, this location shift is too small to be observed within the scatter of the data, i.e. it is within the measurement uncertainty.

What can be observed in Figure 9a, however, is a shift of the ablation local minimum located between 10 • and 20 • .

It is shifted from 15 • -20 • for the 8mm-hemisphere to 10 • for the 14mm-hemiphere, thus it is in agreement with the expected direction of the displacement of the laminar-turbulent transition. It should be noted that the ablation profiles are similar irrespective of the hemisphere diameter. Therefore and by also considering the measurement uncertainty, no conclusion can be drawn regarding the dependency of the magnitude of ablation on the hemisphere diameter.

The ablation profiles for condition 2 are similar in shape with the two ablation maxima located at nearly the same position on the hemisphere, thus suggesting no change in the overall physics of the ablation process. The scattering in the data prevents the determination of the precise location in the ablation minima and maxima, making it impossible to verify the aforementioned hypothesis of a transition-related shift of maximum ablation.

V. Conclusion

Using a hypersonic shock tunnel facility, a new experimental method for quantitative ablation rate measurements under realistic flow conditions in terms of Mach and Reynolds numbers is presented. The use of gallium as low melting point metal allows to measure ablation profiles within less than 2ms. Gallium as a substitute ablator is chosen, since its ablation behaviour is similar to other metals classically used in aerospace applications. The surface displacement is determined with a combination of a direct visualization system and an image processing algorithm, allowing surface displacements caused by ablation of less than 0.1mm to be detected.

Quantitative measurements are only possible for high-enthalpy flow conditions when the ablation rate is high enough.

Uncertainties due to measurement scatter remain high but are reduced by performing multiple runs.

The ablation profiles obtained with this method show similarities with previous observations made using different facilities. The ablation maximum is never located at the stagnation point but at a location near 30 • from the symmetry axis. The proposed explanation is that the boundary layer becomes turbulent just upstream of this position and increases the local wall heat flux, enhancing the ablation process.

Finally, we described a new measurement method able to investigate the liquid ablation phenomena under realistic flow conditions in terms of Mach and Reynolds numbers. The method has the potential to enhance the database for ablation for hypersonic dense flows, which can be used for the validation of a future CFD solver dedicated to the simulation of liquid ablation.

  number [-] P = Pressure [Pa] or Power flux [W • m -2 ] Re = Reynolds number [-] Re/x = Reynolds number per meter [m -1 ] T = Temperature [K] u = Velocity [m • s -1 ] θ = Angle with the model symmetry axis [-]λ = Heat conductivity [W • m -1 • K -1 ] ρ = Density [kg • m-3 ] are characterized by high temperatures in the shock layer, resulting in substantial

  -, 10mm-and 14mm-hemisphere-cylinder geometriesDue to the low melting temperature of gallium the model cannot be machined. Thus a custom model generation technique based on low temperature casting has been developed. First, gallium is melted and heated up to 333K. At the same time, the 3D-printed molds are assembled on the model sting and preheated to 333K for 15min. Then the liquid gallium is cast into the molds and left to cool down at ambient temperature during 15min. Thereafter, the model is cooled down to near 273K until the gallium solidifies. Finally, the molds are removed and the model is polished before being used in the experiment. This procedure ensures that the model is free of blowholes and is mechanically smooth.

Fig. 3

 3 Fig. 3 Model generation tools

Fig. 4

 4 Fig. 4 Optical setup

  Before model surface contour extraction, each frame is normalized by applying a median blur, a gray scale dilation and a threshold. The ablation profiles are computed by determining the local minimum distance between the reference image contour and the actual frame contour. The post-treatment uncertainty is one pixel when the surface visualization is undisturbed. It is determined by applying the post-treatment algorithm to a non-ablative model. During the test, ablation trails flow around the model and partially shroud the model surface, inducing some additional error in the model vibration correction and consequently in the calculation of the ablation profiles. To compensate for this uncertainty, the ablation profiles shown in the following sections are temporally averaged over seven consecutive frames. Additionally, they are spatially averaged, retaining a resolution of 0.05mm.

Fig. 5

 5 Fig. 5 Images from the post-processing

Fig. 6

 6 Fig. 6 10mm-hemisphere-cone models before and after testing

Fig. 7

 7 Fig. 7 Ablation profiles for the 8mm-hemisphere at the flow condition 1

Fig. 8

 8 Fig. 8 Ablation profiles at t = 2ms for condition 1 with the uncertainties represented by the blue areas

Fig. 9

 9 Fig. 9 Comparison of ablation profiles at t = 2ms

Schematic of the ISL shock tunnel Table 1 Flow conditions

	O100mm	2.5m	18.4m	30m
		1st diaphragm	2nd diaphragm
				Nozzle	Model
		Driver gas tube H2 + N2	Test gas tube N2	Measurement section	Dump tank

Fig.

1

Table 3 Test summary

 3

	Condition n •	Model geometry	Number of runs	Re D
	1	Hemisphere ∅8mm-cone	6	3.96 E+5
	1	Hemisphere ∅10mm-cone	8	4.95 E+5
	1	Hemisphere ∅14mm-cylindre	6	6.92 E+5
	2	Hemisphere ∅10mm-cone	2	9.06 E+5
	3	Hemisphere ∅10mm-cone	3	5.86 E+5
	4	Hemisphere ∅10mm-cone

Table 4 Heating phase durations

 4

	Condition n •	Model geometry	Heating phase duration (ms)
	1	Hemisphere ∅8mm-cone	0.5
	1	Hemisphere ∅10mm-cone	0.5
	1	Hemisphere ∅14mm-cylindre	0.4
	2	Hemisphere ∅10mm-cone	0.4
	3	Hemisphere ∅10mm-cone	0.9
	4	Hemisphere ∅10mm-cone	n/a

Acknowledgments

This work is supported in the framework of a Ph.D. thesis by the French-German Research Institute of Saint-Louis, the Icube Laboratory (UMR7357) of Strasbourg University and the doctoral school 269 Mathematics, Sciences of the Information and the Engineer of Strasbourg. The authors gratefully acknowledge the technical support of Robert BERNHARD, Remy KEMPF, Kevin MARTINEK and Jean-Marc WUNDERLY.

Funding Source

This work is funded by the French-German Research Institute of Saint-Louis.


Taha Elgayyar

Ranin Atwi

Alain Tuel

Laurence Burel

Yves Schuurman

Frederic C Meunier

Evidencing Pt-Au alloyed domains on supported bimetallic nanoparticles using CO desorption kinetics

Keywords:

We explored here the possibility to use CO desorption kinetics monitored by IR spectroscopy as a tool to characterize the mixing of Au and Pt atoms at the surface of supported nanoparticles. CO desorption of CO monitored at 50 °C showed that a bimetallic sample exhibited an intermediate behavior between those of monometallic Au and Pt samples, proving the alloyed nature of the nanoparticles. Our data also suggest that alumina-supported Au nanoparticles are active for Boudouard reaction (2 CO → C + CO 2 ) at 50 °C, in contrast to the Pt-based sample.

Introduction

Gold has often been associated with other metals to modify the structural properties or the catalytic activity of metal nanoparticles. For instance, an enhanced resistance against oxidation/sulfidation of Pt and Pd was reported over Pt-Au 1 and Pd-Au 2 nanoalloy catalysts. Synergistic effects were observed over Pd-Au and Cu-Au/TiO 2 catalysts used for glucose oxidation 3 . Alloying Pd with Au markedly modified CO adsorption on Pd 4 : high dilution of Pd resulted in the loss of bridged CO species adsorbed on Pd-Pd pairs at the benefit of linear Pd-CO species through single atom alloy (SAA) formation.

The thorough characterization of supported bimetallic nanoparticles to determine the extent of mixing of Au with another metal is difficult. Powder X-ray diffraction is not informative when the nanoparticles are smaller than about 3 nm or if the two metals are poorly miscible like Au and Pt. On the contrary, EXAFS can provide detailed averaged coordination numbers of Au to the other metals, although this requires access to synchrotron facilities. Advanced TEM analyses such as STEM-XEDS 5 can provide detailed local information on a few selected nanoparticles, but such limited sampling may not be statistically representative of the whole material.

The characterization by CO adsorption can give information about the structure of the surface of the nanoparticles and possibly the presence of alloyed phases, as mentioned above for the case of Au-Pd systems. Several groups studied CO adsorption over bimetallic Pt-Au catalyst 6,7,8 . IR spectral analysis of Au-Pt systems is complex, because of the significant overlap existing between the frequency ranges of bands observed on Au and Pt sites, which has been discussed in a recent review 9 .

Linearly adsorbed CO on reduced Pt species exhibit bands in the range 2100-1990 cm -1 depending on site coordination number and surface coverage 10,11 . Regarding reduced gold, bands in the range 2120-2080 cm -1 have typically been assigned to CO adsorbed at steps or defects on neutral Au nanoparticles 12 while those located within 2080-2000 cm -1 have been assigned to CO adsorbed on negatively charged Au clusters 13,14 . Some authors have proposed that COrestructured Au surface exhibited bands around 2070-2060 cm -1 15 , though others claimed that the observed restructuration could have actually arisen from Au contamination with Ni(CO) 4 16 .

Whatever the true origin(s) of Au-CO bands below 2080 cm -1 , these reports show that assigning IR band of CO(ads) in the case of Au-Pt samples is not straightforward. upon exposure at very low CO pressures (ca. 3 x 10 -10 torr) at room temperature 17 . The surface of bimetallic Au-Pt nanoparticles would typically be enriched in Au under vacuum conditions or an inert gas, due to the lower surface tension of Au (1145 mN/m) as compared to that of Pt (1763 mN/m). A complete Au shell around a Pt-rich core would be formed for Au concentrations greater than 50 wt.% 17 . Pt would then migrate and be trapped at the surface in the presence of CO, due to the thermodynamically favored formation of strong Pt-CO bonds.

The rate of desorption of CO might be a more practical way of assessing the surface composition of Au-Pt bimetallics, rather than the often-unequivocal wavenumbers of IR bands.

CO adsorption on Au, Pt and Au-Pt bimetallics supported on silica 7 and zirconia 6 has been investigated and CO desorption was also discussed, though very briefly. A fast desorption of the bands associated with Au (above 2100 cm -1 ) could be observed at room temperature, while those mostly and apparently associated with Pt (below 2100 cm -1 ) were stable. None of these groups could yet provide any clear evidence of the presence of alloyed nanoparticles, which would probably exhibit an intermediate behavior, since Au could possibly serve as a porthole for CO desorption after surface diffusing from Pt sites. The present contribution investigates CO desorption on monometallic Au, Pt and bimetallic NPs supported on alumina, which were also fully characterized by XRD and TEM-EDX.

Experimental section.

Catalyst preparation.

The monometallic Au and Pt samples were prepared using a wet impregnation method 18 . The aqueous solutions of the precursors of Au (Chloroauric acid, HAuCl 4 .3H 2 O) or Pt (Platinic chloride hexahydrate, H 2 PtCl 6 .6H 2 O) both from Sigma Aldrich were added to a suspension of 1 g of γ-alumina support (from Saint Gobain, γ-Alumina (NORPRO), BET=208 m 2 /g) in 35 mL of distilled water, and the stirring for 45 min at room temperature. A freshly prepared solution of NaBH 4 in 10 mL distilled water was added dropwise, and the stirring was kept for 25 min. The NaBH 4 /metal ratio was 20 for Pt and 10 for Au. Finally, the suspension was filtered and washed several times with distilled water, and finally dried in an oven overnight at 90 °C. The bimetallic Pt-Au sample was prepared using a co-wet impregnation method, i.e. the impregnation of the two metals was performed simultaneously in a one-pot synthesis, similarly to the aforementioned procedure.

Supported gold nanoparticles may undergo sintering due to the exposure to light or moisture 19,20 . The samples were thus stored, preferably without reduction, in dry conditions and away from day light in a refrigerator at 5 °C. Any reduction was typically carried out just before characterization.

Catalyst characterization.

The elemental analysis analysis was performed by ICP-OES using an ACTIVA spectrophotometer from Horiba JOBIN YVON. TEM images were obtained using a JEOL 2010 microscope (resolution of 0.19 nm) operating at 200 kV. The particle diameters (d i ) were measured using Image J software. The surface weighted diameter was calculated as (d SW = ∑n i d i 3 / ∑n i d i 2 ). The dispersion was calculated depending on the d SW using the calculation methodology reported by Van Hardeveld and Hartog 21 considering a cuboctahedral model. The corresponding EDX analysis was performed on the same microscope on several particles using Link ISIS analyzer (from Oxford Instruments).

Powder X-ray diffraction (XRD) patterns of the samples were recorded using a diffractometer D8 advance A25 from Bruker equipped with a nickel filter, a copper tube ( = 1.54184 Å) and a multi-channel fast detector (LynxEye 192 channels an active length of 0.2947 nm). Diffractograms were collected at room temperature under air, over the range 4-70 ° with 0.02 ° step and 2 s per step.

The DRIFTS experiments were carried in a Praying Mantis DRIFTS reaction cell (from Harrick). The cell was placed in a collector assembly and fitted in a Nicolet 8700 FTIR spectrometer (from Thermo Fischer Scientific) equipped with a mercury cadmium telluride (MCT) detector cooled with liquid nitrogen. The gas flows were controlled by independent mass flow controllers (from Brooks). High-purity gases were purchased from Air Liquide (He and 5% CO/He) and Messer (H 2 ), and further purified using a liquid N 2 trap connected to the inlet gas line in the set-up. The catalysts were in situ reduced in a flow of 50 mL/min of 60% H 2 /He at 300 °C for 30 min. The system was heated from room temperature to 300 °C at a heating ramp of 10 °C/min. After the reduction treatment, H 2 was stopped, and the system was cooled down to 50°C in a He flow. The CO adsorption test was then carried out under a flow of 50 mL/min of 1% CO/He for 15 min. Finally, CO was stopped, and the CO desorption was monitored in a flow of 100 mL/min of He at 50 °C for 5 h.

The DRIFTS spectra were collected using the OMNIC software with a resolution of 4 cm - 1 and analyzed using OPUS software. The sample spectrum collected at 50 °C under He before CO adsorption was used as background. The DRIFTS spectra are reported as log (1/R), where R is the sample reflectance. This pseudo-absorbance gives a better linear representation of the band intensity against surface coverage than that given by the Kubelka-Munk function for strongly absorbing media such as those based on metals supported on oxides 22 . The contribution of gasphase CO was subtracted using a CO(g) spectrum collected at the reaction temperature over KBr, which enabled a perfect correction 23 .

Results and discussion

The measured metal loadings of the monometallic samples and that of the bimetallic material are shown in Table 1. The Pt loading was chosen to be larger than that of Au on the bimetallic sample to favor the presence of Pt at the surface on bimetallic NPs, if any. a Mean crystallite size estimated using Scherrer equation.

b Surface-weighted mean diameter (200 nanoparticles counted).

c Metal dispersion is estimated using a cubo-octahedral model and a calculation method described in reference 21 .

The XRD patterns collected over the three samples showed peaks related to Au or Pt phases and the alumina support (Figure 1A). The signal related to alumina was subtracted to better evidence the peaks associated with the metals and a zoom in the range corresponding to the (111) diffraction peaks of Au and Pt is shown in Figure 1B. The peaks were quite broad and the size of the crystallite domains of the monometallic samples were 7.5 nm for Pt and 3.2 nm for Au Figure 2 shows TEM images and the corresponding particles size distributions of the monometallic Pt and Au samples. The distribution of sizes was highest between 2 and 3 nm for both samples, but the Pt-based material exhibited much larger NPs, some of which were agglomerated in larger bunches (Figure 2 B). The corresponding surface-weighted diameters d SW were 3.9 nm for Au and 7.1 nm for Pt, values that were coherent with those determined from the XRD analysis (Table 1).

Figure 3 shows a representative TEM image and the corresponding particles size distributions of the bimetallic Pt-Au material. The sample displayed a rather narrow particle size distribution (up to 6 nm) with a d SW of 3.4 nm, similar to that of the monometallic Au sample (Table 1). The EDX analysis collected over several particles of the bimetallic sample showed the presence of both metals in most particles analyzed (Figure 4). The average Au/Pt atomic ratio determined by the EDX analysis (i.e. 0.13) over this limited set of data was similar to that obtained by the global ICP analysis (i.e. 0.14). In conclusion, the XRD and TEM-EDX data show that the metals are well-dispersed over the alumina support and that most NPs of the bimetallic sample contained both Au and Pt. Steady-state DRIFTS spectra were obtained after 15 min under a flow of 1% CO/He over the three materials (Figure 5). Only the wavenumber range below 2200 cm -1 exhibited bands. No hydroxyl groups characterized by bands over 3000-3800 cm -1 were formed upon CO introduction, stressing that the concentration of water in the feed was negligible. No bands related to C-H stretching bands could be observed around 3000 cm -1 , showing that the level of hydrocarbon contaminants was also negligible. It can be assumed that the optical path length was the same for the three materials, since the same support and low metal loadings were used (Table 1). Significant differences were observed between the monometallic Au material and the two Pt-containing samples. A single sharp band located at 2106 cm -1 typical of CO adsorbed linearly on low coordination metallic gold sites (referred to as Au L-CO) was observed on Au/alumina (Figure 5, inset) 4,12,24,25,26 . In contrast, much larger and broader bands band assigned to linearly adsorbed CO on metallic Pt sites (referred to as Pt L-CO) 27 were observed between 2100 and 1950 cm -1 for both the monometallic Pt and the bimetallic Au-Pt samples. Bridged CO species (referred to as Pt B-CO) 27 between 1950 and 1700 cm -1 were also observed for the Pt-containing materials. These observations are consistent with the stronger affinity of CO for Pt ( ads H 0 ca. -150 to -180 kJ mol -1 ), leading to all Pt surface atoms being covered at 50 °C, while only a minor fraction of sites would be in the case of Au ( ads H 0 ca. -80 to -60 kJ mol -1 ) 9,28 .

Interestingly, the signal measured over the bimetallic sample, which can be mostly assigned to CO adsorbed on Pt atoms, was significantly lower than that measured over the Pt/alumina, in spite of a higher Pt loading and higher metal dispersion. It shows that the minor fraction of Au present decreased CO adsorption on Pt, most likely due to the preferential presence of Au at the surface of the bimetallic NPs, as discussed in the introduction. The band of linear CO of the bimetallic sample did not exhibit the low frequency part between 2000-1950 cm - 1 observed in the case of the monometallic Pt band, suggesting that the sites occupied by Au on the NPs surface were those with lowest coordination numbers. This is expected because Au, which exhibits a much lower surface tension than Pt, will preferentially occupy defective positions.

A pair of bands at 1650 cm -1 and 1430 cm -1 was noted only in the case of the Au/alumina and corresponding to adsorbed hydrogenocarbonates 29 . These bands were not due to any contamination by CO 2 of the gas feed as those were not observed in the cases of the Pt-containing samples. These observations could be repeated in additional experiments carried out on all the samples. This shows that CO 2 was formed in situ over the monometallic gold material, possibly through the water-gas shift reaction (WGS, CO + H 2 O → H 2 + CO 2 ) due to water traces or through the dissociation of CO via Boudouard reaction (2 CO → C + CO 2 ). We favor the latter route, because of the dryness of our feed and the fact that CO dissociation on Au(110) was already observed at room temperature 12,[START_REF] Santos Aires | Proc. Int. Conf. Phys. Surfaces Interfaces[END_REF] .

The desorption of CO from the surface of the NPs was then monitored by DRIFTS at the same temperature of 50 °C under a stream of He. The Au L-CO observed over the monometallic Au sample had essentially vanished only 2 min after removing CO from the feed (Figure 6A).

This observation is consistent with the reported low heats of adsorption of CO on Au, i.e.  ads H 0 ca. -80 to -60 kJ mol -19, 28 . In contrast, the bands of the adsorbed carbonates (likely located on the support) showed a slight decrease in intensity over the same period.

10 Figure 6: DRIFTS spectra recorded over Au/alumina after 2 and 3 min following the removal of CO from the feed. Feed: He, 100 mL/min, 50 °C. The spectrum noted 0 min was recorded just before removing CO.

The case of the monometallic Pt sample was more complex. The B-CO bands rapidly lost about 20 % intensity within 10 minutes and then decreased more slowly, resulting in a loss of about 40 % intensity after 5 h (Figure 7). The slowing down of the B-CO desorption rate is likely related to the increased adsorption strength with lower surface coverage, as B-CO adsorption heats were shown to vary from -45 kJ mol -1 at full coverage to -94 kJ mol -1 at zero coverage [START_REF] Bourane | [END_REF]32 .

In parallel, the shape of the L-CO band was modified, by losing intensity between 2090-2050 cm -1 and gaining intensity between 2040 and 1950 cm -1 (Figure 7A). This red-shift of the L-CO actually resulted in a slight increase in the band area over the first few minutes after stopping CO, which then remained stable for 5 h (Figure 7B). The origin of this moderate L-CO intensity increase is unclear, it could correspond to an increase of L-CO surface coverage (through conversion of some B-CO into L-CO), a reconstruction of the surface or a difference of molar absorption coefficient. In any case, these data show that Pt L-CO were strongly held at the surface of the Pt NPs, showing no desorption for up to 5 h. The evolution of the carbonyl signal on the bimetallic Au-Pt sample (Figure 8) following CO removal from the feed was quite different from that observed on the monometallic Pt (Figure 7). L-CO (2068 cm -1 ) and B-CO (1836 cm -1 ) species, mostly adsorbed on Pt sites, exhibited first a rapid decay over the first 20 min, followed by a slower gradual decrease (Figure 8). After 4 h, the decay of both L-CO and B-CO signals appeared to accelerate and B-CO species were essentially no longer visible after 5 h (Figure 8B). The L-CO signal had decreased by about 50 % after 5h (Figure 8B), which is a marked difference from the monometallic Pt sample that showed no desorption over the same period (Figure 7B).

The initial fast L-CO decay was likely related to CO desorbing from Au-rich NPs. A minor fraction of pure Au NPs was also present initially, as indicated by the band at 2106 cm -1 (Figure 8A). Two models can be put forward to explain the faster desorption from Pt-CO in the case of the bimetallic sample. First, CO strongly adsorbed on Pt sites could surface-migrate towards an adjacent Au site, from which facile desorption would occur (Scheme 1A). Second, direct desorption from Pt sites could also possibly occur if neighbouring Au atoms would exert a sufficient electronic effect on Pt to decrease the Pt-CO bond strength (Scheme 1B). The first and second models relate to geometric and electronic effects, respectively. The slower decay observed between 1 and 4 h would then be due to desorption from NPs less rich in Au. The final section between 4 and 5 h showing an acceleration of desorption could possibly be relating to a restructuring of the surface of the NPs. Surface Pt liberated from CO would become less thermodynamically favoured at the surface and exchange with Au atoms, which display a lower surface tension (Scheme 1C). Such surface segregation effects in the addition/removal of CO would be similar to those reported by Tenney et al. for Au-Pt surfaces 17 .

Whatever the true effect of Au on Pt, the decay of the DRIFTS signal of the L-CO species observed over the three materials show that desorption of CO from Pt atoms in the bimetallic

Conclusions

Al 2 O 3 -supported Pt-Au nanoparticles around 3 nm in diameter were prepared and the formation of bimetallic nanoparticles could be evidenced by TEM-EDX. The nature of the IR spectra could not directly reveal the alloyed state of the surface. In contrast, the kinetics of desorption of CO monitored at 50 °C showed that the bimetallic sample exhibited an intermediate behavior between that of the monometallic Au and Pt samples, proving the alloyed nature of the nanoparticles. It is not yet clear whether the faster desorption from Pt was due to an electronic effect weakening the Pt-CO bond or to the migration of CO from Pt to the adjacent Au on which faster CO desorption occurs. This work stresses the benefit of using CO desorption kinetics as a means to characterize Au-Pt nanoalloys.

Conflicts of interest :

There are no conflicts of interest to declare.

Figure 1 :

 1 Figure 1: (A) X-ray diffraction patterns of the alumina-supported monometallic and bimetallic materials samples and the alumina support. (B) Zoom in on the XRD patterns of the monometallic and bimetallic materials. The pattern of the alumina support was subtracted. The dotted lines are associated with the diffracting planes as reported in the JCPDF files for Au (01-071-4616) and Pt (00-001-1311).

Figure 2 :

 2 Figure 2: TEM images and particle size distributions of the alumina-supported (A) Au, and (B) Pt.

Figure 3 :

 3 Figure 3: TEM image and the corresponding particle size distribution of the alumina-supported bimetallic Pt-Au material.

Figure 4 :

 4 Figure 4: EDX analysis over several particles of the alumina-supported bimetallic Pt-Au sample.

Figure 5 :

 5 Figure 5: DRIFTS spectra recorded at 50 °C over the reduced Pt, Au and Pt-Au supported on alumina after exposure to CO for 15 min. Feed: 1% CO/He, 50 mL/min. The inset is a zoom in on the low frequency region. The spectrum collected under He before CO introduction was used as background.

Figure 7 :

 7 Figure 7: (A) DRIFTS spectra recorded over Pt/alumina after various times following the removal of CO from the feed. The spectrum noted 0 min was recorded just before removing CO and (B) corresponding evolution of the relative areas of the L-CO and B-CO DRIFTS bands. Feed: He, 100 mL/min, 50 °C.

Figure 8 :

 8 Figure 8: DRIFTS spectra recorded over the Au-Pt/alumina after various times following the removal of CO from the feed. Feed: He, 100 mL/min, 50 °C. The spectrum noted 0 min were recorded just before removing CO.

  between that of the monometallic samples (Figure9). It is therefore concluded that CO desorption kinetics can be used as a tool to investigate the alloyed state of Au-Pt bimetallics.

Figure 9 :

 9 Figure 9: Evolution of the relative areas of the L-CO DRIFTS bands following the removal of CO from the feed. Feed: He, 100 mL/min, 50 °C.

Table 1 : Properties of the Al 2 O 3 -supported Au, Pt and Pt-Au samples. Sample Metal loading (wt.%) Au/Pt atomic ratio XRD particle size a (nm) TEM particle size b (nm) Metal Dispersion c (%)

 1

		Pt	Au
	Pt	1.3	----	0	7.5 ± 1	7.3 ± 1	20
	Au	----	2.13	----	3.2 ± 1	3.9 ± 1	35
	Pt-Au	2.1	0.3	0.14	5.6 ± 1	3.4 ± 1	38

Acknowledgements: TE acknowledges funding from the ANR project DECOMPNOx (ANR-18-CE07-0002-01). RA acknowledges funding from the University of Lyon.


Keywords: instrumented section, platoon, wandering, fatigue

Comparison of the response of an instrumented pavement section under individual and platoon truck loading

Trucks transiting in platoon configurations promise a reduction in traffic congestion, an improvement in road safety, as well as lower fuel consumption and greenhouse gases emissions. Considering that truck platoon configurations are new, it is necessary to describe the structural responses they can produce in the pavement. In this sense, the present study shows the results obtained from a fullscale experiment designed to evaluate the structural responses of a test track subjected to the passage of trucks under two configurations: individual and platoon. The experiment consisted of measuring the transversal and longitudinal strains produced at the level of the bituminous layers of the pavement structure when trucks under both test configurations are transiting at speeds of 40 km/h, 60 km/h, 70 km/h, and 80 km/h. In order to consider the weather influence, two test campaigns were performed, one during winter and the other in summer. The main conclusion of the study is that by managing inter-vehicle distances, truck speeds, wandering, time, and periods of circulation, it is possible to minimize the impact of truck platoon configurations on the pavement's structure performance.

Introduction

Before developing fully automated vehicles, truck platooning represents a promising solution to optimize road transportation. Allowing several trucks in convoy, to follow automatically a human-driven leader truck can present promising benefits in terms of reducing congestion for a better traffic flow, improving the braking/acceleration abilities of the vehicles, reducing fuel consumption, and more generally reducing operating costs of the vehicles and enhancing road safety [START_REF] Alam | Heavy-Duty Vehicle Platooning for Sustainable Freight Transportation: A Cooperative Method to Enhance Safety and Efficiency[END_REF][START_REF] Bonnet | Fuel Consumption Reduction in a Platoon: Experimental Results with Two Electronically Coupled Trucks at Close Spacing[END_REF][START_REF] Eilers | COMPANION-Towards Co-Operative Platoon Management of Heavy-Duty Vehicles[END_REF]Gungor et al. 2020;Gungor and Al-Qadi 2020a;[START_REF] Hoque | Highway Cost Analysis for Platooning of Connected and Autonomous Trucks[END_REF][START_REF] Humphreys | An Evaluation of the Fuel Economy Benefits of a Driver Assistive Truck Platooning Prototype Using Simulation[END_REF][START_REF] Konstantinopoulou | Specifications for Multi-Brand Truck Platooning[END_REF][START_REF] Ladino | Cross-Platform Simulation Architecture with Application to Truck Platooning Impact Assessment[END_REF][START_REF] Lammert | Effect of Platooning on Fuel Consumption of Class 8 Vehicles Over a Range of Speeds, Following Distances, and Mass[END_REF][START_REF] Noorvand | Autonomous Vehicles: Assessment of the Implications of Truck Positioning on Flexible Pavement Performance and Design[END_REF][START_REF] Robinson | Operating Platoons on Public Motorways: An Introduction to the SARTRE Platooning Programme[END_REF][START_REF] Suzuki | A New Truck-Routing Approach for Reducing Fuel Consumption and Pollutants Emission[END_REF][START_REF] Thunberg | Vehicle-to-Vehicle Communications for Platooning: Safety Analysis[END_REF][START_REF] Tsugawa | Results and Issues of an Automated Truck Platoon within the Energy ITS Project[END_REF][START_REF] Tsugawa | A Review of Truck Platooning Projects for Energy Savings[END_REF]. However, platooning also leads to channelization of traffic and a reduction of the recovery time between successive loads, which could lead to an increase of damage of pavement structures, reducing their service life and increasing their maintenance and rehabilitation costs [START_REF] Chen | Assess the Impacts of Different Autonomous Trucks' Lateral Control Modes on Asphalt Pavement Performance[END_REF][START_REF] Chen | A Lateral Control Scheme of Autonomous Vehicles Considering Pavement Sustainability[END_REF][START_REF] Marsac | Optimization of Truck Platoon Wander Patterns Based on Thermo-Viscoelastic Simulations to Mitigate the Damage Effects on Road Structures[END_REF][START_REF] Noorvand | Autonomous Vehicles: Assessment of the Implications of Truck Positioning on Flexible Pavement Performance and Design[END_REF][START_REF] Zhou | Optimization of Lateral Wandering of Automated Vehicles to Reduce Hydroplaning Potential and to Improve Pavement Life[END_REF].

To address this issue, recent research studies mainly based on numerical simulation, have started to analyze the impact on pavement performance of the variation of specific variables representative of truck platoon configurations: distribution of the lateral wheel wander [START_REF] Chen | Assess the Impacts of Different Autonomous Trucks' Lateral Control Modes on Asphalt Pavement Performance[END_REF][START_REF] Chen | A Lateral Control Scheme of Autonomous Vehicles Considering Pavement Sustainability[END_REF]Gungor et al. 2020;Gungor andAl-Qadi 2020b, 2020a;[START_REF] Marsac | Optimization of Truck Platoon Wander Patterns Based on Thermo-Viscoelastic Simulations to Mitigate the Damage Effects on Road Structures[END_REF][START_REF] Noorvand | Autonomous Vehicles: Assessment of the Implications of Truck Positioning on Flexible Pavement Performance and Design[END_REF][START_REF] Rana | Simulation of Autonomous Truck for Minimizing Asphalt Pavement Distresses[END_REF][START_REF] Song | Organization of Autonomous Truck Platoon Considering Energy Saving and Pavement Fatigue[END_REF][START_REF] Zhou | Optimization of Lateral Wandering of Automated Vehicles to Reduce Hydroplaning Potential and to Improve Pavement Life[END_REF], inter-vehicle distances [START_REF] Gungor | Development of Adjustment Factors for MEPDG Pavement Responses Utilizing Finite-Element Analysis[END_REF](Gungor et al. , 2020;;Gungor and Al-Qadi 2020a) and traffic distribution scenarios [START_REF] Hoque | Highway Cost Analysis for Platooning of Connected and Autonomous Trucks[END_REF][START_REF] Rana | Simulation of Autonomous Truck for Minimizing Asphalt Pavement Distresses[END_REF]. By using a mechanistic-empirical approach, some of those researchers determined the damage produced by platoons in terms of fatigue and accumulated deformation. The analysis agrees upon the need to control wandering (Gungor andAl-Qadi 2020a, 2020b;[START_REF] Rana | Simulation of Autonomous Truck for Minimizing Asphalt Pavement Distresses[END_REF][START_REF] Zhou | Optimization of Lateral Wandering of Automated Vehicles to Reduce Hydroplaning Potential and to Improve Pavement Life[END_REF], intervehicle distances (Gungor et al. 2020) and to define exclusive lanes or times of the day for the transit of platoons [START_REF] Hoque | Highway Cost Analysis for Platooning of Connected and Autonomous Trucks[END_REF][START_REF] Rana | Simulation of Autonomous Truck for Minimizing Asphalt Pavement Distresses[END_REF] in order to preserve the fatigue performance and self-healing capacities of the pavement, and reduce maintenance costs (Gungor et al. 2020;Gungor and Al-Qadi 2020a).

In the same line, and as a previous reference for the present study, [START_REF] Marsac | Optimization of Truck Platoon Wander Patterns Based on Thermo-Viscoelastic Simulations to Mitigate the Damage Effects on Road Structures[END_REF] developed a model to evaluate the effects of multiple loads of truck platoons on road structures for different configurations of wander patterns, using the software ViscoRoute© 2.0. This software's capacity to evaluate the viscoelastic response under dynamic solicitations allowed to evaluate the aggressiveness produced by different truck platoon configurations, in order to define a wandering pattern to be tested on a test track on site. According to the results of the study, including a wandering of 25 cm can reduce fatigue damage caused by a platoon configuration composed of three trucks by a ratio of 6, compared to a configuration without wandering.

In this context, this paper presents the results of a full-scale experiment carried out in Tarragona, Spain, to study the response of an instrumented pavement section, subjected to the transit of trucks traveling in individual and platoon configurations. A laser system was used to verify the speed, wandering, and inter-truck distances applied during testing.

The platoon configuration used consisted of three trucks traveling in the same lane at inter-vehicle distances defined by a time gap of 0.8s. The test protocol involved the variation of the truck speed and the lateral wheel wander. Two test campaigns were carried out, the first one in the winter (January 2020) and the second one in the summer (August 2020), to consider the two extreme temperature conditions that characterize the weather in Tarragona.

This study is part of the research conducted in the European Project ENSEMBLE (ENeabling SafE Multi-Brand pLatooning for Europe), which main goal is "to pave the way for the adoption of multi-bran truck platooning in Europe to improve fuel economy, traffic safety and throughput" (Anon n.d.; [START_REF] Mascalchi | Specifications for Multi-Brand Truck Platooning[END_REF].

Materials and methods

2.1.Pavement test section

The test track used in this research is located in the facilities of the automotive company Applus IDIADA, in Tarragona, Spain. Applus IDIADA test track has been considered as the best in Europe, specially used in the design and development of test sections for automotive industries from everywhere in the world. As shown in Fig 1, a 12 m long instrumented test section was built on this test track. The corresponding pavement structure is composed by three asphalt layers (4 cm thick wearing course, 6 cm thick base course and 15 cm thick subbase course) placed over a cement stabilized layer. The instrumentation placed in the test section consists of 24 strain gauges type KM-l00HAS Tokyo Sokki Kenkyujo, for the measurement of the longitudinal strains (12 gauges) and transversal strains (12 gauges) at 10 cm and 25 cm depth from the pavement surface and 6 thermocouples, 3 of them placed at the bottom of each asphalt layer for two different locations. The strain gauges were placed at 0.85m, 1.05m, and 1.25m from the border of the lane. This configuration was selected to enable the lateral truck offset and wandering presented in Fig. 2, which was measured by using the laser system shown in Fig. 3. The laser system shown in Fig. 3 was composed by 4 lasers configured to manage and measure inter-truck distances, trucks speeds and wandering applied during testing. The instrumentation design was based on the evaluation of the bottom-up fatigue phenomena under individual and platoon truck configurations, which is related to the maximum strain in tension in the asphalt surface layers.

2.2.Data collection details: testing protocol

The objective of the tests was to measure and compare the response of the pavement under

Discussion of results

3.1.Description of the measured strain signals

This paper will present only the data collected by the strain gauges placed at the bottom of the sub-base layer. observed in a study conducted on a real pavement [START_REF] Duong | Continuous Strain Monitoring of an Instrumented Pavement Section[END_REF]. In this study, high extension strains have been measured at the bottom of the bituminous layer at high temperature (30°C), and similarly, a slow return to zero of the strains after loading was observed.

3.2.Data repeatability and reproducibility

To evaluate the repeatability of the data, as shown in Fig. 10, the strain measurements from runs 5 and 26 were selected to represent the following test conditions: use of the same operator (Truck 1), individual configuration, same lateral distance from the strain gauge, same test truck, similar strain gauges (T1, T2, L1 and L2) and short intervals of time between passages (run 5: 11/01/2020, 16:02; run 26: 12/01/2020, 10:11). The Analysis Of VAriance (ANOVA) performed according to the null hypothesis of equality among the average values of the maximum strains obtained for each axle, led to conclude that for a confidence level of 99%, there is a significant influence of the axle type but not of the run number (5 or 26), meaning good data repeatability during the test.

3.3.Verification of test conditions using instrumentation and laser measurements

With the purpose to verify the correspondence between the truck speed applied during testing and the real speed conditions obtained, Fig. 12 and Fig. 13 show a comparison of the truck speeds obtained from the data collected with the laser system and the strain gauges, for both test campaigns. The visual analysis of the figure shows higher variation for a truck speed of 80 km/h which can be caused by the fact that platoon truck configurations are being produced by human drivers. Therefore, as test speeds increase, the difficulty for the truck drivers to keep constant inter-trucks distances increases. The ANOVA of the data concludes that for a confidence level of 95% the values are significantly the same for both systems. Table 2 shows the corresponding coefficients of Considering that the lateral distance between each strain gauge and each truck has certain variations during testing, Fig. 14 shows the corresponding histogram graph of lateral tire positions, which reveals that a high number of runs were done at positions between -30 cm and 0 cm, with a certain tendency to be located on the left side. By the form of the graph and considering the central limit theorem which establishes that the distribution of a sample tends to be normal as the size becomes larger, these values can be considered to follow a normal distribution centered on -17 cm with a standard deviation value of 20 cm.

In this case, the values were only verified by the measurements done with the laser system. Finally, Fig. 15 displays inter-truck distances measured with the laser system and computed from the strain signals obtained from pavement instrumentation. As shown in Fig. 16, these inter-truck distances have been obtained by maintaining a time gap of approximately 0,8 s between the trucks in the platoon configuration. As a consequence, inter-truck distances increase with increasing truck speed. The higher variation of intertruck distances observed at higher truck speeds can be due to the fact that the vehicles were not automated but driven by human drivers, which had more difficulty to maintain constant inter-truck distances at high speed. Table 3 indicates the coefficients of variation of these inter-truck distances, which values are higher during the Summer Campaign. This can only be explained by the change of the truck drivers participating to the test campaigns.

3.4.Influence of the Platoon Configuration on pavement performance

Fig. 17 shows the critical tensile strain levels defined in this study, to treat the signals obtained from testing. These points were proposed to analyze ideal Platoon Configurations that could be implemented, with a view to avoid the reduction in the fatigue performance defined for the pavement structure during design. The first and second points correspond to the maximum values obtained under the pass of the first and second axle. The third point corresponds to the maximum strain under the pass of the tridem axle, and the fourth point to the accumulated strain registered 0.7 s after the passage of each truck. The value of 0.7 s was selected to enable comparisons with the Platoon configuration because the inter-truck time interval can be less than 0.8 s in some cases due to the variability experienced during the test. In this sense, the distance from the center of each point was corrected by three factors:

(1) the truck location, (2) the strain gauge location, and (3) the width of the tire in the sense of traffic.

With this correction, both figures show the decrease of the different critical strain values when the lateral position of the tires varies relatively to the position of each gauge. These figures show that for Axle 1 and Axle 3, the maximum strains were obtained at approximately ±11.25 cm from the strain gauge, which fits with the 22,5 cm of the tire wide. The same behavior is produced for Axle 2 which by the dual configuration are in a wider area, approximately ±23 cm. Going longer than this distance, the strain values decrease reaching zero at approximately 25 cm apart from the border of the tire. This means that increasing the lateral wandering can reduce significantly the number of times that the highest strain solicitations due to the pass of the truck, will be applied at a given point. The figures also show that, in comparison to the individual configuration, there are not considerably difference in the values obtained during the winter campaign. Similarly, Fig. 20 and Fig. 21 show the critical strain values obtained during the summer campaign, for each critical strain level, relatively to the line at 0.85m from the centerline of the lane. In comparison to the winter campaign, the curves show the same tendencies.

However, the values have considerably increased, revealing differences between individual truck and platoon truck configurations. The maximum critical strain level is about 100 strains for individual trucks and 140 strains for platoon configuration and

Data comparison and statistical analysis for the critical strain levels

With the purpose to compare the effect of individual trucks and platoon configurations, Fig. 22 and Fig. 23 show the average strain values obtained from both winter and summer test campaigns, by selecting only runs with a lateral deviation of ±15 cm from the strain gauge, i.e. a wandering of ±15 cm. Fig. 24 shows the percentage of increase of strains -The critical strain values under Axle 1 and Axle 2 have similar values; The strains produced by Axle 3 are approximately 20% higher, for both test campaigns.

-Compared with the individual configuration, platoon configurations lead to an increase of strain levels only in the summer campaign. The inter-truck distances also affect the summer results. Here it is important to note that for the winter campaign, the slight reduction in strain values obtained for the platoons can be result of the temperature conditions during testing, which were a little lower in this case.

-The percentage of change obtained from comparing the strain values associated to each strain level, reveals big increases for the summer campaign in comparison to the winter one. The effect is even more noticed for the accumulated strain values.

The Multivariable Analysis of Variance (MANOVA) [START_REF] Montgomery | Applied Statistics and Probability for Engineers[END_REF] of this data helps to support the previous comments, since it is concluded that for a confidence level of 95% and the winter campaign, the strain values do not change by varying the (1) the load configuration, (2) the truck speed, and (3) the truck number, while in the summer campaign the variations of all these parameters change the strain values.

Therefore, in order to identify the conditions producing differences for the summer campaign, the Tukey's honest significance tests for a confidence level of 95% revealed that:

-In relation to the axle number, the average strains obtained under the passage of Axle 1 and Axle 2 are significantly the same and differ from the ones obtained under the passage of Axle 3, which are around 20% higher.

-In relation to the truck speed, the values differ for all the cases between the individual and platoon configuration, except for the speed of 40 km/h, where the difference is not significant.

-In relation to the truck number, the strain values are the same for all the conditions, except for the accumulated strain.

Effect of an increased wandering for the summer campaign

Considering the significant influence of platoon configurations in the summer campaign, Fig. 25 shows the average strain values obtained by including all truck passages with a wandering interval of [-25cm,+25cm] from the strain gauge, as well as the percentages of change in reference to the original wandering interval. As can be observed, the

Conclusions

This study presents an analysis of the horizontal strains induced in the asphalt layers of a pavement test section subjected to the passage of trucks in individual and platoon configurations. The objective of the study was limited to evaluate the impact of the passage of trucks in platoon mode, over the fatigue strain values obtained from an instrumented pavement section located in the test track facilities of Applus IDIADA, in Tarragona, Spain. Fatigue life predictions associated with this research project will be complementary addressed in further research documents related to this research line. The test protocol used for this study included the following configuration:

-two loading scenarios, trucks travelling in individual and platoon configurations;

-two test campaigns, the first in winter and the second in summer;

-for each campaign, different test speeds and lateral deviations of the trucks from the strain gauges (wandering).

From the results obtained, the following conclusions can be drawn:

• The instrumentation designed and placed on-site allows collecting data with good reproducibility and repeatability. The data collected by each sensor were of good quality, and there was no loss of sensors during the construction process of the test section on IDIADA facilities.

• The agreement between the test speeds and the inter-truck distances measured from the instrumentation and the laser system, as well as its good homogeneity and consistency, proved that the defined test protocol allowed an adequate reproduction of platoon truck configurations. o Extending the interval of lateral wheel wandering from [-15cm, +15cm] to [-25cm, +25cm] could reduce the average critical tensile strain values in the pavement structure. This means that a small increase in the wandering among the trucks in platoon mode can significantly reduce the fatigue damage of the pavement, distributing the highest tensile strains are over a larger area.

In summary, the study has proved that the effect of Platoon Configurations could be potentially reduced by means of: (1) the management of their distribution along the year and along the day, with a preference for highest transit rates during the winter months or for further research, avoiding the warmest hours during the day (2) the increase of the lateral wandering between the trucks inside the platoon, as well as between platoons, particularly when the temperatures are high. , and (3) for further research, the increase in the inter-truck distances, especially during the summer season. In reference to previous studies, based mostly on modeling with damage predictions, traffic modeling and some real scale testing [START_REF] Chen | Assess the Impacts of Different Autonomous Trucks' Lateral Control Modes on Asphalt Pavement Performance[END_REF][START_REF] Chen | A Lateral Control Scheme of Autonomous Vehicles Considering Pavement Sustainability[END_REF]Gungor et al. 2020;Gungor andAl-Qadi 2020a, 2020b;[START_REF] Hoque | Highway Cost Analysis for Platooning of Connected and Autonomous Trucks[END_REF][START_REF] Marsac | Optimization of Truck Platoon Wander Patterns Based on Thermo-Viscoelastic Simulations to Mitigate the Damage Effects on Road Structures[END_REF][START_REF] Noorvand | Autonomous Vehicles: Assessment of the Implications of Truck Positioning on Flexible Pavement Performance and Design[END_REF][START_REF] Rana | Simulation of Autonomous Truck for Minimizing Asphalt Pavement Distresses[END_REF][START_REF] Song | Organization of Autonomous Truck Platoon Considering Energy Saving and Pavement Fatigue[END_REF][START_REF] Zhou | Optimization of Lateral Wandering of Automated Vehicles to Reduce Hydroplaning Potential and to Improve Pavement Life[END_REF] this proper management could be reflected in terms of: (1) longer fatigue cracking/permanent deformation life, (2) lower pavement structure design thicknesses and (3) later rehabilitation/maintenance treatments.

Acknowledges

The research presented in this paper is part of the ENSEMBLE project, co-funded by the European Union under the Horizon2020 Research and Innovation Program, grant agreement No 769115.

Fig. 1 .Fig. 2 .

 12 Fig. 1. IDIADA test track and instrumented test section.

Fig. 3 .

 3 Fig. 3. Laser system used in the test.

  individual and platoon truck configurations. The tests were performed with three 5-axle semi-trailer trucks of similar characteristics, with a total weight of ~40 tons, which details are shown in Fig 4.

  were performed: the first one in winter, in January 2020, and the second one in summer, in August 2020. During each campaign, the tests were carried out varying the truck speeds (40, 60, 70, and 80 km/h) and wandering (naturally produced along with testing), for the two truck configurations shown in Fig.5:(1) Individual and (2) Platoon. In the Platoon truck configuration, a time gap of 0.8 s between the vehicles was applied to represent the minimum commercial values used in the adaptative cruise control systems, leading to inter-vehicle distances varying with the test speed.During each test campaign, a total of 24 runs were performed with the trucks in the individual configuration and 20 passages in the platoon configuration. As shown in

Fig. 5 .

 5 Fig. 5. Test scenarios: Individual and Platoon.Inter-truck distance, trucks 1 and 2 (L 2 )

  Fig. 6 and Fig. 7 show examples of longitudinal and transversal strain signals recorded, by the passage of Truck 1 in individual configuration and during winter and summer campaigns (i.e. low and high temperature conditions), when the test speed was 60 km/h. Twelve strain gauges, located at the bottom of the subbase layer, are presented: 6 transversal and 6 longitudinal, but located at different lateral positions. The results indicate that (1) the measured transversal strain signals change with the change in the lateral position of the wheels, even change its sign (extension or contraction) when a vehicle passes at different lateral positions relatively close to the sensors, (2) at high temperatures, the strain signals change significantly showing amplitudes much more higher, with transversal strains reaching values of approximately 110 µstrains at the bottom of the sub-base layer and closed to the wheels passage, (3) the shape of the transverse strain signals at the bottom of the bituminous layers is also modified, with a slow return to zero after loading, this delayed response is due to the viscoelastic behavior of the bituminous layers, and is more pronounced under the three rear axles of the vehicle and at high temperatures. Similar results have been previously

Fig. 6 .Fig. 7 .Fig. 9 .

 679 Fig. 6. Horizontal strains obtained from run 8 for Truck 1 under individual configuration

  To evaluate data reproducibility, as shown in Fig.11, the parameters considered were the following: individual configuration, same test section, same wandering, different operators (truck 1, truck 2, and truck 3), and different strain gauges. The ANOVA of the maximum strain values obtained for each axle led to conclude that, for a confidence level of 99%, there is a significant influence of the type of axle but not of the run number. This indicates a good reproducibility of the data collected.

Fig. 10 .Fig. 11 .

 1011 Fig. 10. Analysis 1: Repeatability, Truck 1, Truck speed = 60 km/h, Individual

  Fig. 12. Truck speeds for both individual and platoon configurations (winter campaign).

Fig. 13 .

 13 Fig. 13. Truck speeds for both individual and platoon configurations (summer campaign).

Fig. 14 .

 14 Fig. 14. Distance between the strain gauge and the front tire for the whole test

Fig. 15 .

 15 Fig. 15. Inter-truck distances for a time gap of 0.8 s in the platoon configuration.

Fig. 16 .

 16 Fig. 16. Inter-truck time gaps in the platoon configuration.

Fig. 17 .

 17 Fig. 17. Points of analysis of the strain signals used in this study.

Fig. 18 .

 18 Fig. 18. Transversal strains at each point of analysis for the individual truck scenario

Fig. 19 .

 19 Fig. 19. Transversal strains at the points of analysis for the platoon configuration (winter campaign).

  Fig. 20. Transversal strains at the points of analysis for individual configuration (summer campaign).

Fig. 21 .

 21 Fig. 21. Transversal strains at the points of analysis for the platoon configuration (summer campaign).

  and summer campaign, for individual trucks, and for platoons. The results indicate that:

Fig. 22 .

 22 Fig. 22. Comparison of the strains at the different points of analysis for the individual and platoon configuration, when wandering is limited to 30 cm, in the winter campaign.

Fig. 24 .

 24 Fig. 24. Strain increase (in percentage) for each point of analysis comparing winter and summer campaigns

Fig. 25 .

 25 Fig. 25. Effect produced by the extension of the wandering for the summer campaign.

•

  Although there were slight temperature variations during both individual and platoon configuration tests for each test campaign, the values were significantly enough comparable. The inspection and statistical comparison of the strain values associated with the critical strain levels indicated that: o There is not considerable effect of using platoon truck configurations during the winter campaign, due to the predominantly elastic performance of the material at low temperatures. o The passage of trucks in platoon configuration during the summer campaign considerable affects the fatigue strain as result of a mostly viscoelastic behavior of the asphalt layers at the higher temperatures present during summer. o The accumulated strain values were not considerably affected during the winter campaign due to the mostly elastic performance of the asphalt materials in the surface layers. The contrary situation happened during the summer campaign showing that the accumulated strain values were significantly different between individual and platoon configurations due to the thermo-viscoelastic performance of the materials in the asphalt layers.

Table 1 ,

 1 the pavement response parameters measured were the longitudinal and transversal strains at the bottom of the base asphalt layer and of the subbase asphalt layer, as well as the pavement layer temperatures.

	Ax1
	315/80 R=22.5		Origin	Y
	Ax2
	315/80	X1
	R=22.5
	Ax3
	385/65
	R=22.5
		Left	Right
		X

Table 1 .

 1 Parameters evaluated in the experimentation.

	Parameter		Characteristics
	Pavement responses:	Depth of measurement: 10 cm (4cm of a wearing course + 6cm of a base layer) and
	horizontal strains	25 cm (4cm of a wearing course + 6cm of a base layer + 15 cm of a subbase layer).
		Direction of measurement: longitudinal (L) and transversal (T).
	Truck speed	40, 60, 70 and 80 km/h.
		Measured with a laser system and verified from processing the strain data.
	Wandering (w)	30 cm and 50 cm.
	Lateral offset (LO)	From the center line of the lane: 0 cm.
	Truck test scenarios	Individual configuration and platoon configuration.
	Inter-truck distance (L)	Adjusted to a time gap of 0.8 s between trucks.
	Campaigns	Winter:
		•	11 of January 2020:
			o Individual Trucks
			o Temperature: 10.3°C and 11.3°C respectively at 25cm and 10cm
			depth from the pavement surface.
		•	12 of January 2020:
			o Platoon Configuration.
			o Temperature: 8.3°C and 6.1°C respectively at 25cm and 10cm
			depth from the pavement surface.
		Summer:
		•	29 of August 2020:
			o Individual Trucks
			o Temperature: 27.5°C and 25.9°C respectively at 25cm and 10cm
			depth from the pavement surface.

Table 2 .

 2 Coefficients of variation of truck vehicle speeds.

				Coefficient of variation
	Configuration	Winter campaign 40 60 70	80	Summer campaign 40 60 70 80
		km/h	km/h	km/h	km/h	km/h	km/h	km/h	km/h
	Individual -Laser	1% 1% 1% 1% 1% 1% 1% 1%
	Individual -Computed 3% 3% 4% 5% 3% 4% 4% 4%
	Platoon -Laser	2% 2% 1% 11% 2% 1% 2% 2%
	Platoon -Computed 2% 2% 11% 6% 6% 5% 6% 3%

Table 3 .

 3 Coefficient of variation for inter-truck distance.

					Coefficient of variation
	Configuration	40	Winter campaign 60 70	80	Summer campaign 40 60 70 80
		km/h	km/h	km/h	km/h	km/h	km/h	km/h	km/h
	Platoon -Laser	8% 8% 15% 17% 10% 25% 15% 15%
	Platoon -Computed 7% 7% 14% 20% 13% 29% 15% 41%