B Jia

• A Rusinek

S Bahi

• R Bernier

R Pesci

• A Bendarma

Perforation Behavior of 304 Stainless Steel Plates at Various Temperatures

Keywords: Perforation, Low and elevated temperatures, Failure mode, Energy absorption, Martensitic transformation

The effect of temperature on perforation behavior of 304 austenitic stainless steel plates was investigated experimentally. Perforation tests have been conducted at velocities from 80 to 180 m/s and temperatures between -163 and 200 °C. Low temperatures were obtained using a specific designed cooling device and the temperature distribution on the specimens was verified to be uniform. Based on the experimental results, the failure mode, the initial-residual velocity curves, the ballistic limit velocities and the energy absorption capacity under different temperatures were analyzed. It was found that petalling was the main failure mode during the perforation process. The average number of petals was three at 20 °C or 200 °C and was increasing continuously to five at -163 °C. The ballistic limit velocity V bl was also affected by the initial temperature. It increased slightly from 93 m/s at 200 °C to 103 m/s at -20 °C and then remained constant at lower temperatures. The material showed better energy absorption capacity at low temperatures and this came not only from the temperature sensitivity of the material but also from the strain-induced martensitic transformation effect. According to martensite measurement by X-ray diffraction technique, the martensite fractions along the fracture surface of petals were 87.1%, 66.2%, 52.8% and 32.4% respectively for initial temperatures of -163 °C, -60 °C, -20 °C and 20 °C.

Introduction

As a representative of transformation induced plasticity (TRIP) steels [START_REF] Fischer | Transformation-induced plasticity (TRIP)[END_REF][START_REF] Fischer | A new view on transformation induced plasticity (TRIP)[END_REF], 304 austenitic stainless steel (ASS) has high resistance to corrosion and oxidation as well as a unique combination of high strength and high ductility. Its beneficial mechanical properties come from strain-induced martensitic transformation (SIMT), which means that upon plastic deformation the initial austenite phase ( γ ) transforms into stable martensite phase ( ′ ); thus both increased work hardening rate and significantly enhanced ductility can be achieved. These properties make 304 ASS extensively used in many areas such as civil engineering, navigation and transportation. During its working and manufacturing process such as liquid natural gas storage and transportation at low temperature [START_REF] Kim | Charpy impact properties of stainless steel weldment in liquefied natural gas pipelines: effect of low temperatures[END_REF][START_REF] Park | Comparative study on mechanical behavior of low temperature application materials for ships and offshore structures: part I-experimental investigations[END_REF] and sheet metal forming at elevated temperatures [START_REF] Jayahari | Investigation of high temperature forming of ASS 304 using BARLAT 3-parameter model[END_REF][START_REF] Sun | On the influence of deformation mechanism during cold and warm rolling on annealing behavior of a 304 stainless steel[END_REF], it may be subjected to impact loading over a wide range of temperatures.

As impact loading is encountered from time to time in so many areas, a considerable amount of work has been done over the last decades to study the impact behavior of materials [START_REF] Børvik | Ballistic penetration of steel plates[END_REF][START_REF] Rusinek | Experimental and numerical study on the perforation process of mild steel sheets subjected to perpendicular impact by hemispherical projectiles[END_REF]. According to the projectile velocity, investigation on impact behavior of materials can be divided into 3 categories. The first category refers to low velocity impact (< 50 m/s), where thin plates are commonly perforated by a drop weight tower [START_REF] Antoinat | Perforation of aluminium alloy thin plates[END_REF]. The second covers sub-ordnance and ordnance velocity range between 50 and 1300 m/s, where projectiles are usually accelerated by a compressed air gas gun to perforate or penetrate plates [START_REF] Børvik | Ballistic penetration of steel plates[END_REF][START_REF] Bendarma | Experimental and numerical analysis of the aluminum alloy AW5005 behavior subjected to tension and perforation under dynamic loading[END_REF][START_REF] Rodríguez-Millán | Experimental study on the perforation process of 5754-H111 and 6082-T6 aluminium plates subjected to normal impact by conical hemispherical and blunt projectiles[END_REF]. The last category refers to hypervelocity impact (> 1300 m/s), a velocity range often encountered in outer space impact such as debris hitting spacecrafts [START_REF] Numata | HVI tests on CFRP laminates at low temperature[END_REF]. The second velocity regime aims at investigating the dynamic impact behavior of shell structures in many engineering areas and therefore has received the most attention. Mostly, sub-ordnance impact tests were performed with thin plates and non-deformable projectiles at room temperature to study its ballistic resistance performance, energy absorption capacity and failure mode. Those studies mainly focus on the influence of target thickness [START_REF] Børvik | Effect of target thickness in blunt projectile penetration of Weldox 460 E steel plates[END_REF], impact obliquity [START_REF] Goldsmith | Normal and oblique impact of cylindro-conical and cylindrical projectiles on metallic plates[END_REF], multilayer plates combination [START_REF] Alavi Nia | Experimental study of perforation of multi-layered targets by hemispherical-nosed projectiles[END_REF] and projectile nose shape [START_REF] Børvik | Perforation of 12 mm thick steel plates by 20 mm diameter projectiles with flat, hemispherical and conical noses: part II: numerical simulations[END_REF][START_REF] Arias | Numerical simulations of impact behaviour of thin steel plates subjected to cylindrical, conical and hemispherical non-deformable projectiles[END_REF][START_REF] Børvik | Perforation of 12 mm thick steel plates by 20 mm diameter projectiles with flat, hemispherical and conical noses: part I: experimental study[END_REF] on perforation behavior of materials. Børvik et al. [START_REF] Børvik | Effect of target thickness in blunt projectile penetration of Weldox 460 E steel plates[END_REF] investigated perforation and penetration behavior of Weldox 460E steel plates with target thickness varying between 6 and 30 mm. The slope of the initial-residual velocity curves decreases with increasing target thickness and the target deformation mode changes from global deflection to shear localization. Rodríguez-Martínez et al. [START_REF] Rodríguez-Martínez | Experimental and numerical analysis of the martensitic transformation in AISI 304 steel sheets subjected to perforation by conical and hemispherical projectiles[END_REF] found that the ballistic limit velocity of 304 ASS thin plates was affected by a combination of target thickness and projectile shape. Alavi Nia and Hoseini [START_REF] Alavi Nia | Experimental study of perforation of multi-layered targets by hemispherical-nosed projectiles[END_REF] compared ballistic resistance performance of monolithic, in-contact layered and spaced layered aluminum plates and found the monolithic target behaved the best. Børvik et al. [START_REF] Børvik | Perforation of 12 mm thick steel plates by 20 mm diameter projectiles with flat, hemispherical and conical noses: part II: numerical simulations[END_REF][START_REF] Børvik | Perforation of 12 mm thick steel plates by 20 mm diameter projectiles with flat, hemispherical and conical noses: part I: experimental study[END_REF] conducted perforation tests of Weldox 460E steel plates using blunt, conical and hemispherical projectiles. It was found that both the energy absorption capacity and the failure mode of the material were affected by the projectile nose shape. The failure mode for the blunt projectile was shear banding while conical and hemispherical projectiles perforated the target by pushing the material aside, corresponding to a failure by ductile hole enlargement.

While material properties at low or elevated temperatures differ significantly from those at room temperature [START_REF] Rusinek | Experimental characterisation and modelling of the thermo-viscoplastic behaviour of steel AISI 304 within wide ranges of strain rate at room temperature[END_REF], quite a limited number of studies concerning perforation behavior of materials under various temperatures can be found. Mostly, the low temperature impact studies focus on hypervelocity impact of lightweight alloys and composites at around -173 °C [START_REF] Ohtani | A study of hypervelocity impact on cryogenic materials[END_REF][START_REF] Tanaka | High-speed penetration of a projectile into aluminum alloys at low temperatures[END_REF][START_REF] Numata | Experimental study of hypervelocity impacts at low temperatures[END_REF][START_REF] Tanaka | Hypervelocity crater formation in aluminum alloys at low temperatures[END_REF]. Also, with a specially designed drop weight tower, the low velocity impact behavior of AA 2024-T3 aluminum and TRIP 1000 steel at -60 °C was investigated by Rodríguez-Martínez et al. [START_REF] Rodríguez-Martínez | Thermoviscoplastic behaviour of 2024-T3 aluminium sheets subjected to low velocity perforation at different temperatures[END_REF][START_REF] Rodríguez-Martínez | Thermo-mechanical behaviour of TRIP 1000 steel sheets subjected to low velocity perforation by conical projectiles at different temperatures[END_REF]. As for studies concerning impact behavior of materials under subordnance or ordnance velocity at different temperatures, high temperature perforation by several authors were found [START_REF] Rusinek | New devices to capture the temperature effect under dynamic compression and impact perforation of polymers, application to PMMA[END_REF][START_REF] Liu | Ballistic performance and energy absorption characteristics of thin nickel-based alloy plates at elevated temperatures[END_REF][START_REF] Erice | An experimental and numerical study of ballistic impacts on a turbine casing material at varying temperatures[END_REF][START_REF] Klosak | Experimental study of brass properties through perforation tests using a thermal chamber for elevated temperatures[END_REF]. Rusinek et al. [START_REF] Rusinek | New devices to capture the temperature effect under dynamic compression and impact perforation of polymers, application to PMMA[END_REF] developed a heating chamber coupled to the ballistic impact device to investigate perforation behavior of poly(methyl methacrylate) (PMMA). With the thermal chamber, Klosak et al. [START_REF] Klosak | Experimental study of brass properties through perforation tests using a thermal chamber for elevated temperatures[END_REF] studied perforation behavior of brass alloy plates under temperatures ranging from 20 to 260 °C. Results showed that the energy absorption capacity decreased with increasing temperature. There was also some changes in the petalling failure mode: the number of petals increased from 3 to 6 within the testing temperature regime. Liu et al. [START_REF] Liu | Ballistic performance and energy absorption characteristics of thin nickel-based alloy plates at elevated temperatures[END_REF] investigated the ballistic performance of GH4169 alloy at temperatures ranging from 25 to 600 °C. A similar conclusion was obtained: larger plastic deformation of specimens and lower ballistic limit velocities at higher temperature. For sub-ordnance and ordnance velocity impact of materials under low temperatures, so far as we know, has not been investigated yet in the open literature.

According to literature review above, although 304 ASS may suffer from impact loading over a wide range of temperatures, the sub-ordnance and ordnance velocity impact behavior remains unclear. The purpose of this study, therefore, is to study the effect of the initial temperature on the perforation behavior of 304 stainless steel. First, an original cooling device was developed to test the structure behavior at low temperatures, ranging from -163 to -20 °C. The temperature distribution along the specimen surface was measured and simulated to be sure about the uniform temperature distribution. Then, ballistic impact tests of 304 steel plates were carried out under temperatures from -163 to 200 °C using the newly developed cooling device and the heating chamber by Rusinek et al. [START_REF] Rusinek | New devices to capture the temperature effect under dynamic compression and impact perforation of polymers, application to PMMA[END_REF]. The target thickness was 1.5 mm and the projectile velocity was varying between 80 and 180 m/s. The experimental setup allowed measuring the initial velocity V 0 and residual velocity V R curves of the 3D structure. In addition, martensite fraction in perforated specimens was measured by X-ray diffraction technique to explain the improved energy absorption capacity of 304 ASS at low temperatures.

Material Behavior and Apparatus Description

Ballistic Impact Set-up

Ballistic impact tests of 304 stainless steel were performed using a pneumatic gas gun shown in Fig. 1. During the tests, the projectile is launched using a pneumatic gas gun and goes through the gas gun tube. By changing the initial pressure P 0 , the impact velocity V 0 of the projectile may change up to 180 m/s for a mass of 29 g.

A detailed description of the target fixation with holders is shown in Fig. 1b. The target is sandwiched tightly between two holders by eight screws. These holders are made of high strength steel to be sure that no plastic deformation occurs during the testing process. After the impact process, three states named no perforation, critical perforation and complete perforation may be observed. If a complete perforation occurs, a residual velocity V R is measured by a method similar to the one described previously. Finally, the projectile catcher acts as a buffer to avoid damage.

Experimental results such as target deflection, fracture pattern and energy absorption capacity of materials are all influenced by the shape and mechanical properties of the projectile. Therefore, the dimensions and the mechanical properties of the projectile and of the target are introduced in the next section.

Projectile and Target Description

The dimensions of the projectile used in this study are shown in Fig. 2. It is a cylinder with a diameter of 12.8 mm and a height of 25 mm corresponding to a mass of 29 g. At the top of the cylinder, a conical nose with an angle of 72° is machined as reported in [START_REF] Rodriguez-Millan | Perforation mechanics of 2024 aluminium protective plates subjected to impact by different nose shapes of projectiles[END_REF]. The projectile is made of Maraging steel with a hardness of 640 HV and a yield stress of 2 GPa. The hardness and the strength are so high that the projectile is assumed to be rigid during experiments and numerical simulations.

The specimens were delivered as thin plates of 130 mm side length and 1.5 mm thickness. Their initial microstructure was 100% austenite phase. They were tightly fixed by the target holder to avoid sliding.

Ballistic impact of materials under low or elevated temperatures is encountered from time to time in many engineering areas such as liquefied natural gas (LNG) transportation and sheet metal forming. Hence, it is necessary and interesting to study temperature effect on the perforation behavior of materials. In the next section, the thermal chamber for high temperature testing is introduced.

Thermal Chamber for High Temperature Perforation Tests

In order to perform perforation tests at elevated temperature (200 °C), a thermal chamber developed by Rusinek et al. [START_REF] Rusinek | New devices to capture the temperature effect under dynamic compression and impact perforation of polymers, application to PMMA[END_REF] was adopted, Fig. 3. A furnace is used to heat up the air inside the chamber and then hot air is flowing around using a fan. By thermal conductivity and after a certain waiting time, the specimen reaches the expected temperature. Two thermocouples are fixed in the chamber: one to monitor the temperature inside the oven, another one on the center of the specimen to calibrate the temperature difference between the specimen and the air in the thermal chamber. In order to reach a uniform temperature distribution in the specimen, a waiting time of 20 min is imposed.

To cover perforation tests at not only elevated temperatures but also low temperatures, a cooling device for low temperature testing is developed and introduced in the next section.

A Specific Cooling Device for Low Temperature Perforation Tests

To conduct perforation tests at low temperatures, a cooling device has been designed and developed. As shown in Fig. 4a, the cooling box is fixed on the ballistic impact device. On one hand, the cold nitrogen gas flows from a liquid nitrogen tank through an aluminum pipe into the cooling box; on the other hand, the temperature inside the cooling box is monitored by a thermocouple connected to a temperature controller. Once the temperature inside the box reaches the set value, the temperature controller cuts off the power of the pump to stop the nitrogen gas flow. By this method, low temperatures between -90 and -20 °C can be obtained. In addition, by filling the cooling box with liquid nitrogen directly, -163 °C can be obtained, Fig. 4b.

The arrangement of the cooling device on the ballistic impact device is shown in Fig. 5. The 304 ASS plate is fixed on the target holder, then the cooling box is fixed on the target with a cooling box holder. The box is made of aluminum alloy to ensure a good heat transfer. In addition, four screws are used to apply a reduced force on the cooling box surface to ensure a correct contact between the plate and the cooling box. Therefore, the cooling box and the target are in tight contact with each other during the whole testing process.

Since the cooling device for low temperature perforation tests was newly developed, its reliability should be verified before testing. Unlike the device for high temperature perforation tests where the specimen is sealed firmly in a thermal chamber and a uniform temperature distribution is obtained easily, cryogenic temperatures present an experimental complexity to be performed, especially in terms of stabilization at extreme low temperatures, close to -163 °C. In addition, due to the decreasing thermal conductivity k(T) and the specific heat capacity C p (T) under extreme low tem- peratures Fig. 6, it is important to verify that the impacted zone is deformed in a uniform temperature environment.

Therefore, in the next section, temperature distribution on the specimen is presented and further analyzed by a FEM model based on thermal heat transfer approach.

Calibration and Heat Transfer Modeling of the Cooling Device

To verify the reliability of the cooling device, the temperature evolution and distribution in the target are measured and analyzed. First, as shown in Fig. 7, the temperature of three radii on the target surface is measured (target center, 14 mm radius and 28 mm radius). The results are shown in Fig. 8; by setting temperatures of the coolant to be -38 °C, -64 °C and -88 °C, the corresponding temperatures on the target surface are -20 °C, -40 °C and -60 °C, respectively. The temperature of -163 °C is obtained directly by filling liquid nitrogen into the cooling box without using the temperature controller device. The temperatures of the three locations are not exactly the same with T 14mm < T center < T 28mm : it is due to the hole in the center of the cooling box, that is the path for the projectile to go through the cooling box and impact the target. Hence, the center of the target can not be cooled by surface contact with the cooling box and its temperature is slightly higher. In fact, within an area 56 mm in dimater, the maximum temperature fluctuation is only 4 °C. As the projectile diameter is only 12.8 mm, it can be assumed that the impacted zone is deformed in a uniform temperature environment. To analyze the temperature uniformity on the target in details, numerical simulation using COMSOL Multiphysics has been performed [START_REF] Rusinek | New devices to capture the temperature effect under dynamic compression and impact perforation of polymers, application to PMMA[END_REF]. The thermal transfer is described by the generalized transient heat equation, Eq. 1.

The thermal conductivity k(T) and the specific heat C p (T) of the materials used in the numerical simulation are shown in Fig. 6. The two parameters are strongly depending on the temperature, especially at very low temperatures.

The boundary conditions of the simulation are defined as follow, see Eq. 2 and Fig. 9: • Natural convective heat flux q c on the free surface of the device. • Forced convective heat flux q f through the inner free sur- faces of the cooling box, due to the flow of the nitrogen gas. • Thermal contact heat q int between different interface of contacts in the device.

(1) ⋅K). The simulation results are shown as dots in Fig. 8. In Fig. 8a,b: the numerical results and experiments are in good agreement. In Fig. 8c, the experimental curve decreases slower than the simulated one as the flow speed of nitrogen gas is slightly smaller. But after a certain waiting time around 500 s, the experimental values are consistent with the simulation results. In Fig. 8d, during the initial cooling stage, the experimental curve is comparatively higher. This

⋅ C p (T) ⋅ T t -∇ ⋅ [k(T) ⋅ ∇T] = 0
(2) ⎧ ⎪ ⎨ ⎪ ⎩ q c = -h c � T -T 0 � in c q f = -h f � T -T 0 � in f q int = h int � T -T 0 � in int
is mainly because during experiments, it takes time to fill the cooling box with liquid nitrogen while it is assumed that the box is full of liquid nitrogen since the beginning of the simulation. In Fig. 8a-c, the temperature fluctuation caused by the temperature controller is observed. In fact, the delay of the temperature controller is set 3 °C and it causes a temperature fluctuation of ± 3 °C. In Fig. 8d, the target is cooled by filling the box with liquid nitrogen and the temperature fluctuation phenomenon is not observed any more.

Based on numerical simulation, it is observed that the temperature distribution on the target surface at 1200 s may be assumed as uniform in the impact zone Fig. 10. In fact, within the perforation zone, the maximum temperature differences in the four figures are 2 °C, 3 °C, 4 °C and 8 °C, respectively. For testing 304 ASS by a conical projectile, target deformation occurs mainly in the target center with a radius of 20 mm. Under this condition, the maximum temperature differences are 0.3 °C, 0.4 °C, 0.6 °C and 1.4 °C, respectively. Therefore, the temperature distribution on the target surface is pretty uniform during the perforation process.

In addition, the temperature evolution from the left edge to the right edge of the target (the black line in Fig. 10a) is shown in Fig. 11. The temperature in the center of the target is slightly higher than the surrounding area, consistent with the experimental data in Fig. 8. Therefore, the simulation is in good agreement with experiments.

According to the temperature distribution analysis in this section, the temperature uniformity is obtained within the perforation area. Therefore, in the next section, perforation tests of 304 ASS under low, room and elevated temperatures are performed and the results are presented.

Influence of Testing Temperature on the Perforation Process

In this section, perforation experiments were performed by a conical projectile for five initial temperatures: -163 °C, -60 °C, -20 °C, 20 °C and 200 °C. The tests were conducted over a wide range of initial impact velocities, ranging from 80 to 180 m/s, to obtain a complete ballistic curve of the material. Therefore, the effect of the initial temperature on the perforation process is analyzed.

Effect of Testing Temperature on Failure Mode

The influence of the initial temperature on the failure mode of 304 ASS is shown in Fig. 12. For the tests at 200 °C, failure by ductile petalling, resulting from radial necking during the piercing process [33] is observed. A representative petal pattern of the material at 20 °C is shown in Fig. 12b, with a number of three triangle-shaped petals form. Compared to the petals formed at 200 °C, the shape of petals at 20 °C remain unchanged but the fracture surface becomes slightly rough. In addition, several secondary cracks are observed on the bigger petals; if the crack propagates until the end of the petals, failure pattern with four petals are observed. With a further decrease in testing temperature to -20 °C, the average number of petals increases to four and the petal shape becomes irregular. One thing should be noticed is that no debris was found for the three testing temperatures above. Concerning perforation tests at -60 °C and -163 °C, the breaking patterns become even rougher including debris ejection. The average number of petals increased continuously to five at -163 °C, higher than that of -20 °C, 20 °C and 200 °C. The end of the petals became pretty rugged and discontinuous. Moreover, a lot of small cracks were observed on the petal surfaces.

Except failure pattern, the ballistic curve V R -V 0 and the ballistic limit velocity V bl are also used to characterize the ballistic impact behavior of materials. In the next section, the evolution of V R -V 0 curves and that of V bl with the testing temperature are presented.

Effect of Testing Temperature on the Ballistic Curves

V R -V 0
Results in terms of ballistic curves V R -V 0 are presented in Fig. 13. The curves are then fitted to the relation proposed by Recht and Ipson [START_REF] Recht | Ballistic perforation dynamics[END_REF], Eq. 3, in which the residual velocity of the projectile is calculated as a function of the initial velocity and the ballistic limit velocity.

where V 0 and V R are the initial and residual projectile velocities, V bl is the ballistic limit velocity and α is a fitting parameter.

The ballistic limit velocities and fitting parameters for tests at different temperatures are given in Table 1. As shown in Fig. 13, the equation defines the ballistic curve shape properly. For testing at -20 °C, -60 °C and -163 °C, the ballistic limit velocity remains the same, 130 m/s. For testing at higher temperatures, V bl decreases with increasing temperature. At testing temperature of 200 °C, the ballistic limit velocity is 93 m/s. While the ballistic limit velocity at 200 °C is lower than that at 20 °C, the difference between the two curves decreases with increasing impact velocity. At ballistic impact velocities higher than 150 m/s, the two curves coincide. The fitting parameter α is also affected by testing temperature: the value at room and elevated temperatures is obviously lower than that at lower temperatures. A lower means better impact resistance against projectile and higher ballistic limit velocity [START_REF] Børvik | Perforation of 12 mm thick steel plates by 20 mm diameter projectiles with flat, hemispherical and conical noses: part I: experimental study[END_REF]. Therefore, the evolution of experimentally obtained V bl and with testing tempera- ture are consistent with each other.

Based on the ballistic curves V R -V 0 , the energy absorbed by the specimens can be calculated. In the next section, the effect of testing temperature and the initial projectile velocity V 0 on the energy absorption capacity of 304 ASS is analyzed.

Effect of Testing Temperature on the Energy Absorption Capacity of 304 ASS

During the perforation process, part of the kinetic energy of the projectile is absorbed by the plate. Knowing the initial projectile velocity V 0 and residual projectile velocity V R , energy absorbed by the plate W Total Plate can be calculated as follows:

where M p is the mass of the projectile and equal to 29 g.

A part of the kinetic energy, Eq. 4, is transfer to the plate during the process of impact or perforation. However and as discussed in [START_REF] Rodriguez-Millan | Perforation mechanics of 2024 aluminium protective plates subjected to impact by different nose shapes of projectiles[END_REF][START_REF] Duthil | Material properties at low temperature[END_REF], the energy lost due to elastic deformation of the plate, friction between the projectile and the target and those transferred to the ejected debris can be neglected. Therefore, the energy absorbed by the plate is then written as follows:

where W h is the dissipated energy as heat, W pb is the plastic bending energy of the target, W ps is the plastic stretching energy of the target, W p is the plastic bending energy of the petals and W c is the crack formation and propagation energy.

Energy absorption results as a function of V 0 for tests at different temperatures are presented in Fig. 14. First, energy absorption for tests at low temperatures (-163 °C, -60 °C (4)

W Total Plate = 1 2 M p V 2 0 -V 2 R ( 5
)
W Total Plate = 1 2 M p V 0 -V 2 R = W h + W pb + W ps + W p + W c
and -20 °C) is significantly higher than at 20 °C, while energy absorption at 200 °C is the lowest. This is consistent with the fracture pattern observed: more petals and cracks at low temperatures. A similar phenomenon was observed during low velocity perforation test of TRIP 1000 steel by Rodríguez-Martínez et al. [START_REF] Rodríguez-Martínez | Thermo-mechanical behaviour of TRIP 1000 steel sheets subjected to low velocity perforation by conical projectiles at different temperatures[END_REF]: the ballistic limit velocity V bl changed from 2.6 to 3.1 m/s when testing temperature decreased from 15 to -60 °C. According to the analysis of Rodríguez-Martínez, the improved energy absorption at low temperature came from temperature sensitivity of the TRIP 1000 steel and no martensitic transformation was observed during the perforation process.

Another interesting phenomenon concerning energy absorption capacity of 304 ASS is the evolution of the absorbed energy with impact velocity. For the tests at low temperatures (-163 °C, -60 °C and -20 °C), the initial projectile velocity V 0 does not affect the absorbed energy in the tested impact velocity range. In fact, the averaged energy absorption for the three previous temperatures are 156 J, 154 J and 155 J, respectively. However, for the tests at 20 °C and 200 °C, the absorbed energy decreases linearly with increasing impact velocity. Although the absorbed energy at 20 °C is slightly higher than that at 200 °C, it decreases faster and becomes lower at impact velocities larger than 137 m/s.

As discussed using Eq. 5, a part of the kinetic energy is induced to the plate to generate no perforation, partial or complete perforation depending on the quantity transferred to it. In order to analyze the effect of temperature on energy absorption mechanisms of 304 ASS in more details, the permanent deflection and bending of the impacted specimens at different temperatures was measured, Fig. 15. It is clear that the target deflection increases with increasing testing temperature. Plastic bending of targets is larger at higher temperatures due to thermal softening [START_REF] Pérez-Castellanos | Temperature increase associated with plastic deformation under dynamic compression: application to aluminium alloy Al 6082[END_REF]. Next, this view is further verified by petal thickness measurement.

To study the evolution of petal thickness with testing temperature, the perforated specimens at impact velocity around 146 m/s were cut and then the thickness of the petals was measured. As shown in Fig. 16, with increasing testing temperature from -163 to 20 °C, the thickness first remains constant at around 1.26 mm and then decreases continuously to 0.60 mm (the initial plate thickness t 0 was 1.5 mm). A smaller thickness at high temperatures indicates a bigger plastic strain to fracture and therefore a larger plastic deformation of the specimens [START_REF] Park | Strain-rate effects on the mechanical behavior of the AISI 300 series of austenitic stainless steel under cryogenic environments[END_REF].

Although the plastic deformation of specimens declines with decreasing temperature, the targets absorb more energy at lower temperatures. Therefore, it is supposed that the improved energy absorption capacity of 304 ASS at low temperatures comes from its temperature sensitivity or the SIMT effect. According to martensite fraction measurement (presented in the next section), much martensite was found in the petal area under low temperatures. Hence, different from the results of Rodríguez-Martínez et al. [START_REF] Rodríguez-Martínez | Thermo-mechanical behaviour of TRIP 1000 steel sheets subjected to low velocity perforation by conical projectiles at different temperatures[END_REF], the improved energy absorption of 304 ASS at low temperatures is related not only to the temperature sensitivity but also the SIMT effect.

X-ray Diffraction Analysis of Specimens Perforated at Different Temperatures

A notable phenomenon during the deformation process of 304 ASS is martensitic transformation. The transformation helps to increase not only the flow stress but also the ductility of the material. This phenomenon is often observed during quasi-static deformation tests of 304 ASS. However, studies concerning SIMT under impact loading are rarely published. To verify if martensitic transformation occurs during ballistic impact tests and also to study its influence on the perforation behavior of 304 ASS, martensite fraction in post-mortem specimens was measured by X-ray diffraction (XRD) technique.

Determination of Phases Volume Fraction by X-ray Diffraction (XRD) Technique

After perforation tests, the microstructure of specimens changed from pure austenite to a combination of martensite ′ (body-centered tetragonal phase) and austenite (facecentered cubic phase). The volume fraction of each phase is proportional to the integrated diffraction peak intensity. Hence, using a portable PROTO goniometer and by comparing the integrated X-ray diffraction intensities of the two where I ′ and I are the integrated diffraction peak intensities of martensite and austenite phases, respectively. R ′ and R are parameters depending on the phase composition, crystal structure, interplanar spacing (hkl) and the Bragg angle. Two peaks for each phase were considered: {211} and {200} for martensite, {220} and {200} for austenite [START_REF] Standard | E975-03: standard practice for X-ray determination of retained austenite in steel with near random crystallographic orientation[END_REF].

Effect of Testing Temperature on Martensitic Transformation of 304 ASS

Martensitic transformation occurs in 304 ASS under certain conditions, it usually affects deformation and fracture behavior significantly. Martensite is often observed in austenite stainless steel under quasi-static strain rate or cryogenic temperatures [START_REF] Beese | Effect of stress triaxiality and Lode angle on the kinetics of strain-induced austenite-to-martensite transformation[END_REF][START_REF] Byun | Temperature dependence of strain hardening and plastic instability behaviors in austenitic stainless steels[END_REF][START_REF] Talonen | Formation of shear bands and strain-induced martensite during plastic deformation of metastable austenitic stainless steels[END_REF][START_REF] Hamada | Contribution of deformation mechanisms to strength and ductility in two Cr-Mn grade austenitic stainless steels[END_REF]. However, for martensitic transformation (6)

V � = I � ∕R � I � ∕R � + I ∕R (7) V = I ∕R I � ∕R � + I ∕R (8) V � + V = 1
under dynamic loading, few studies can be found [START_REF] Rodríguez-Martínez | Experimental and numerical analysis of the martensitic transformation in AISI 304 steel sheets subjected to perforation by conical and hemispherical projectiles[END_REF][START_REF] Zaera | A constitutive model for analyzing martensite formation in austenitic steels deforming at high strain rates[END_REF]. To investigate its influence on perforation behavior of 304 ASS, martensite distribution in perforated specimens under different temperatures was measured by X-ray diffraction technique. First, the plates impacted at around 146 m/s under -163 °C, -60 °C, -20 °C and 20 °C were cut, Fig. 17. The parameter M d is the temperature above which no martensitic transformation occurs even with large plastic deformation [START_REF] Rodríguez-Martínez | Experimental survey on the behaviour of AISI 304 steel sheets subjected to perforation[END_REF]: it is measured for our material to be 140 °C. So there is no martensite formed under perforation tests at 200 °C (hence not considered). Then, martensite fraction on the fracture surface of the petals, Fig. 17a, and along the crosssection of the plates, Fig. 17b,was measured.

Martensite fraction on the fracture surface of the petals is shown in Fig. 18a. It is clear that the martensite fraction decreases with increasing testing temperature. A high amount of martensite of 87.1% was found at -163 °C and this value decreases continuously to 32.4% at 20 °C. As martensitic transformation is pretty sensitive to temperature, lower transformation rate at higher temperatures is often observed in quasi-static tension or shear tests [33,[START_REF] Recht | Ballistic perforation dynamics[END_REF][START_REF] Nazeer | Analysis of conical tool perforation of ductile metal sheets[END_REF][START_REF] Kpenyigba | Influence of projectile shape on dynamic behavior of steel sheet subjected to impact and perforation[END_REF].

Martensite distribution along the cross-section of the plates is shown in Fig. 18b. Transformation occurs mainly in the petals and martensite fraction in the plate deflection part is comparatively lower. With increasing temperature from -163 to 20 °C, martensite fraction in the petal part decreases continuously but the value is almost constant in Fig. 17 Martensite measurement positions of the perforated specimens: a on the fracture surface of the petals and b along the cross-section of the plates the plate deflection part. What is more, in the petal part martensite fraction decreases quickly from the top of the petal to its bottom. Compared to the martensite fraction on the fracture surface of the petals, martensite inside the petals is obviously lower. This is mainly because the fracture surface corresponds to the maximum plastic deformation until failure while plastic deformation inside the petals is comparatively smaller.

Conclusions and Remarks

With a newly developed cooling device for ballistic impact device, influence of the temperature on perforation behavior of thin 304 ASS plates impacted by a conical projectile in the velocity range of 80 and 180 m/s was studied. Based on experimental observations, the following conclusions are drawn:

• The cooling device helps to perform perforation tests at low temperatures ranging from -163 to -20 °C. Temperature uniformity on the plate surface is verified both experimentally and by numerical simulation. After a waiting time of 1200 s, the maximum temperature fluctuation on the plate surface are 2 °C, 3 °C, 4 °C and 8 °C for testing temperature -20 °C, -40 °C, -60 °C and -163 °C, respectively. If only the plate center (with a radius of 20 mm) where plate deformation mainly occurs is considered, the maximum temperature fluctuations are 0.3 °C, 0.4 °C, 0.6 °C and 1.4 °C, respectively. • In the testing temperatures considered, petalling is the failure mode of the perforation process. The number of petals increases with decreasing testing temperature. The average number of petals is three at 20 °C or 200 °C, and increased continuously to five at -163 °C. The shape of the petals is also affected by the testing temperature: it looks like a regular triangle at 20 °C and 200 °C while becomes rugged and discontinuous at lower temperatures. • The ballistic limit velocity V bl is also affected by testing temperature. It increases slightly from 93 m/s at 200 °C to 103 m/s at -20 °C and then remains constant at even lower temperatures. Also, the ballistic curves V 0 -V R are influenced by testing temperature. For 20 °C and 200 °C, the ballistic curves seem to coincide when the initial projectile velocity is much higher than the ballistic limit velocity. The ballistic curves at -20 °C, -60 °C and -163 °C are almost the same and slightly lower than that at 20 °C and 200 °C.

• Although the plastic deformation of target is smaller at lower testing temperatures, the energy absorption capacity of 304 ASS at low temperatures is obviously higher than at room or elevated temperatures. A high amount of martensite was observed in the perforated specimens, especially in the petals. The improved energy absorption capacity of 304 ASS at low temperatures comes from not only temperature sensitivity of the material but also the SIMT effect.

Fig. 1 Fig. 2

 12 Fig. 1 Schematic view of the ballistic impact device: (1) gun barrel, (2) initial velocity measurement, (3) target, (4) target holder, (5) residual velocity measurement, (6) projectile catcher

Fig. 3 Fig. 4

 34 Fig.3Apparatus for high temperature perforation testing: a general view of the ballistic impact device and b schematic view of the thermal chamber[START_REF] Rusinek | New devices to capture the temperature effect under dynamic compression and impact perforation of polymers, application to PMMA[END_REF]

Fig. 5 Fig. 6

 56 Fig. 5 The arrangement of the cooling device on the ballistic impact device: a isometric view and b sectional view: (1) thermocouple, (2) pipe, (3) target, (4) cooling box holder, (5) cooling box, (6) target holder

Fig. 8 Fig. 9

 89 Fig. 8 Temperature distribution and evolution of the target with the coolant temperature set at: a -38 °C, b -63 °C, c -87 °C and d -183 °C

Fig. 10

 10 Fig. 10 Temperature distribution on target surface at 1200 s with the cooling box temperature set at: a -38 °C, b -63 °C, c -87 °C and d -183 °C

( 3 )Fig. 11

 311 Fig. 11 Temperature from the left side to the right side of the target

Fig. 12

 12 Fig. 12 (continued)

Fig. 14

 14 Fig.[START_REF] Goldsmith | Normal and oblique impact of cylindro-conical and cylindrical projectiles on metallic plates[END_REF] Energy absorbed by the target as a function of the initial projectile velocity for different testing temperatures

Fig. 15

 15 Fig. 15 Deflection of postmortem specimens tested at around 125 m/s under different temperatures

Table 1

 1 The ballistic limit velocities V bl and the fitting parameter of 304 ASS under different testing temperatures

	Testing	V bl (m/s)
	temperature
	(°C)
	-163	103	2.006
	-60	103	2.028
	-20	103	2.005
	20	96	2.712
	200	93	2.660

Acknowledgements This work of the first author was financially supported by China Scholarship Council under Grant 201606220056.

Author from Agadir thanks M. Tomasz Libura from IPPT to participate in the construction of the device allowing to reach high temperatures under dynamic impact and perforation. The system has been developed in agreement between IPPT, Universiapolis Agadir and Lorraine University, patent no. 41383 Kingdom of Morocco (2017).


Sarah Leclaire
email: sarah.leclaire@free.fr

Marion Chatelain

Anaïs Pessato

Bruno Buatois

Adrien Frantz

Julien Gasparini

Pigeon odor varies with experimental

Keywords: Scent, Zinc, Lead, Birds, Dove

teaching and research institutions in France or abroad, or from public or private research centers.

Introduction

Trace metals (e.g. lead, iron, copper, zinc) are normally present in very low levels in the environment and small amounts are essential part of physiology in animals and plants (Powell 2000, Prasad 2009, Plum et al. 2010). For instance, zinc is essential in regulating homeostasis, immune responses, and oxidative stress [START_REF] Stefanidou | Zinc: a multipurpose trace element[END_REF], and zinc deficiency has serious consequences that have been recognized for many years and that include oxidative damages, diabetes and congenital malformations [START_REF] Fosmire | Zinc toxicity[END_REF][START_REF] Oteiza | Zinc deficiency causes oxidative damage to proteins, lipids and DNA in rat testes[END_REF][START_REF] Prasad | Zinc deficiency: has been known of for 40 years but ignored by global health organisations[END_REF]. However, excessive quantities of trace metals can be toxic. The large amount of trace metals that have been released in the environment by anthropogenic activities [START_REF] Nriagu | Global inventory of natural and anthropogenic emissions of trace metals to the atmosphere[END_REF] have well-known noxious effects on humans [START_REF] Järup | Hazards of heavy metal contamination[END_REF][START_REF] Tainer | Heavy metal toxicity and the environment[END_REF]) and wildlife [START_REF] Demayo | Toxic effects of lead and lead compounds on human health, aquatic life, wildlife plants, and livestock[END_REF][START_REF] Goutte | Demographic consequences of heavy metals and persistent organic pollutants in a vulnerable long-lived bird, the wandering albatross[END_REF][START_REF] Scheuhammer | Effects of environmental methylmercury on the health of wild birds, mammals, and fish AMBIO: a[END_REF]). The binding potential [START_REF] Godwin | The biological chemistry of lead[END_REF][START_REF] Tainer | Heavy metal toxicity and the environment[END_REF]) and the propensity of trace metals to catalyze oxidation reactions [START_REF] Koivula | Metal-related oxidative stress in birds[END_REF] are responsible for immunosuppression (Chatelain et al. 2016c;[START_REF] Snoeijs | The combined effect of lead exposure and high or low dietary calcium on health and immunocompetence in the zebra finch (Taeniopygia guttata)[END_REF][START_REF] Snoeijs | Heavy metal exposure affects the humoral immune response in a free-living small songbird, the great tit (Parus major)[END_REF]), oxidative stress [START_REF] Berglund | Oxidative stress in pied flycatcher (Ficedula hypoleuca) nestlings from metal contaminated environments in northern Sweden[END_REF]) and endocrine disruptive effects [START_REF] Baos | Adrenocortical response to stress and thyroid hormone status in free-living nestling white storks (Ciconia ciconia) exposed to heavy metal and arsenic contamination[END_REF][START_REF] Meillère | Corticosterone levels in relation to trace element contamination along an urbanization gradient in the common blackbird (Turdus merula)[END_REF]. For instance, lead, whose widespread use has caused extensive environmental contamination [START_REF] Nriagu | A history of global metal pollution[END_REF], is associated with impaired learning in herring gulls (Larus argentatus) [START_REF] Burger | Effects of lead on learning in herring gulls: an avian wildlife model for neurobehavioral deficits[END_REF], reduced sperm quality in red deer (Cervus elaphus) [START_REF] Castellanos | Increased chromatin fragmentation and reduced acrosome integrity in spermatozoa of red deer from lead polluted sites[END_REF], and higher oxidative stress in Mozambique tilapias (Oreochromis mossambicus) [START_REF] Kaya | Effects of waterborne lead exposure in mozambique tilapia: oxidative stress, osmoregulatory responses, and tissue accumulation[END_REF].

Sexual ornaments are conspicuous phenotypic traits that can take various forms such as large antlers, bright plumage, or elaborated song, and that are displayed to attract mates.

Because their expression can be influenced by genotype and environment, including various stress factors, such as food availability, extreme temperatures, noise and parasites [START_REF] Mcgraw | Different colors reveal different information: how nutritional stress affects the expression of melanin-and structurally based ornamental plumage[END_REF][START_REF] Troïanowski | Sublethal Pb exposure produces season-dependent effects on immune response, oxidative balance and investment in carotenoid-based coloration in red-legged partridges[END_REF][START_REF] Von Schantz | Good genes, oxidative stress and condition-dependent sexual signals[END_REF], they often honestly reveal the quality of the bearer [START_REF] Zahavi | The handicap principle: a missing piece of Darwin's puzzle[END_REF]. By inducing physiological stress, trace metals can also alter ornament expression, as shown for the plumage coloration and singing behaviour of birds [START_REF] Chatelain | Do trace metals influence visual signals? Effects of trace metals on iridescent and melanic feather colouration in the feral pigeon Oikos Chilgren[END_REF][START_REF] Giraudeau | Metal exposure influences the melanin and carotenoid-based colorations in great tits[END_REF][START_REF] Gorissen | Heavy metal pollution affects dawn singing behaviour in a small passerine bird[END_REF]allverd -Coll et al. 2015). Odor cues are also subjected to physiological and environmental influences [START_REF] Ferkin | Attractiveness of scents varies with protein content of the diet in meadow voles[END_REF][START_REF] Johnston | Endocrine control of female mouse odors that elicit luteinizing hormone surges and attraction in males[END_REF][START_REF] Penn | Chemical signals and parasite-mediated sexual selection[END_REF], and are major secondary sexual traits in a wide range of species [START_REF] Blaustein | Sexual selection and mammalian olfaction[END_REF][START_REF] Havlicek | Women's preference for dominant male odour: effects of menstrual cycle and relationship status[END_REF][START_REF] Martín | Links between male quality, male chemical signals, and female mate choice in Iberian rock lizards[END_REF][START_REF] Wyatt | Pheromones and animal behavior: chemical signals and signatures[END_REF]. However, the effects of environmental pollution on their expression are largely unknown.

In this study, we tested the hypothesis of the impact of trace metals exposure (zinc and lead) on plumage odor in feral pigeon (Columbia livia) by an experimental supplementation approach. Although birds have long been considered to rely primarily on visual and acoustic cues, the past decade has revealed that birds use olfaction in various contexts including mate choice or parental care (reviewed in [START_REF] Caro | The perfume of reproduction in birds: Chemosignaling in avian social life[END_REF]. Bird odors, like mammal odors, can vary with genotype (Leclaire et al. 2014b), sex-hormone levels [START_REF] Whittaker | Songbird chemical signals reflect uropygial gland androgen sensitivity and predict aggression: implications for the role of the periphery in chemosignaling[END_REF]) and diet [START_REF] Thomas | Use of TLC-FID and GC-MS/FID to examine the effects of migratory state, diet and captivity on preen wax composition in White-throated Sparrows Zonotrichia[END_REF], and are mainly composed of alkanes, alkanols, esters and fatty acids (reviewed in [START_REF] Campagna | Potential semiochemical molecules from birds: a practical and comprehensive compilation of the last 20 years studies[END_REF]) that may be energetically costly to produce. Using a similar experimental design, we have already described beneficial effects of zinc on body mass, immunity and female investment into eggs (Chatelain et al. 2016b;Chatelain et al. 2016c), and detrimental effects of lead on iridescent coloration, the maternal transfer of antibodies into eggs and nestling growth (Chatelain et al. 2016b;[START_REF] Chatelain | Do trace metals influence visual signals? Effects of trace metals on iridescent and melanic feather colouration in the feral pigeon Oikos Chilgren[END_REF]. We predict therefore that if pigeon odor is condition-dependent, it should vary in opposite direction with experimental exposure to lead and zinc.

Material and methods

Capture and housing

From February 20 to March 27 2013, we captured 96 free-living adult feral pigeons in several locations in Paris. We immediately transferred them in 8 outdoor aviaries (2.20m x 2.20m) at the CEREEP field station (Centre d'Ecologie Expérimentale et Prédictive -Ecotron Ile-de-France, UMS 3194, Ecole Normal Supérieure, Saint-Pierre-lès-Nemours, France).

Metal supplementation

This study is part of a bigger study investigating the effect of zinc and lead on several life history traits of pigeons. Details on the protocol have thus been published elsewhere (Chatelain et al. 2016b;Chatelain et al. 2016c;[START_REF] Chatelain | Do trace metals influence visual signals? Effects of trace metals on iridescent and melanic feather colouration in the feral pigeon Oikos Chilgren[END_REF]. Briefly, on April 8 2013, after an acclimation period, aviaries were divided into 4 metal treatments (2 aviaries per treatment). Birds were evenly distributed among aviaries according to their flock, sex and plumage eumelanin level (Chatelain et al. 2016b). Treatments were lead supplementation (tap water enriched with 1 ppm lead acetate), zinc supplementation (tap water enriched with 10 ppm zinc sulphate), lead and zinc supplementation (tap water enriched with 1 ppm lead acetate and 10 ppm zinc sulphate) and no supplementation, i.e. control (tap water with no metal added). We chose these concentrations based on the lead concentration measured in blood of urban birds [START_REF] Roux | The presence and impact of environmental lead in passerine birds along an urban to rural land use gradient[END_REF] and on the gastrointestinal absorption rate of lead in zebra finches [START_REF] Dauwe | Tissue levels of lead in experimentally exposed zebra finches (Taeniopygia guttata) with particular attention on the use of feathers as biomonitors[END_REF]. Zinc concentrations were approximated using the zinc/lead concentration ratio measured in the environment and in bird feathers (on average, zinc is 10 times more concentrated than lead; [START_REF] Azimi | Sources, distribution and variability of hydrocarbons and metals in atmospheric deposition in an urban area (Paris, France)[END_REF][START_REF] Chatelain | The adaptive function of melanin-based plumage coloration to trace metals[END_REF][START_REF] Frantz | Contrasting levels of heavy metals in the feathers of urban pigeons from close habitats suggest limited movements at a restricted scale[END_REF]. Drinking bowls and baths were filled every other day with treated water. The efficiency of our supplementation protocol to increase bird exposure to lead and zinc has been previously validated (Chatelain et al. 2016b).

Sample collection

After 20 weeks of treatment, a few feathers (~33 mg) were cut in the lower back region of 74 birds (37 males and 37 females), and stored in 4 ml PTFE-faced septum sealed glass vials at -80°C until analysis. Birds from five aviaries (two lead-supplemented aviaries, one zincsupplemented aviary, one zinc-and-lead-supplemented aviary and one control aviary) were sampled on 27 August 2013, while birds from the remaining three aviaries (one zincsupplemented, one zinc-and-lead supplemented and one control aviaries) were sampled on the next day. The 74 birds sampled included 20 control birds, 19 lead-supplemented birds, 17 zinc-supplemented birds and 18 zinc-and-lead-supplemented birds.

Chemical analyses

We extracted chemical compounds with solid phase microextraction (SPME) using a stableflex 50/30μm D B (divinylbenzene) / CAR (carboxen) / PDMS (polydimethylsiloxan) fiber (Supelco, Sigma-Aldrich, Bellefonte, PA, USA). To clear headspace for SPME extraction, we packed feathers at the bottom of the vial, using a glass rod cleaned with dichloromethane and ethanol between samples. The sample was then placed for 20 min in a laboratory incubator maintained at 34°C. We chose 34°C to mimic the natural temperature of bird body surface during a Parisian summer. During the summer 2013, the average daily maximal temperature in Paris was 25.3°C (public data of Meteo France). At 25°C ambient temperature and low humidity, surface temperature of birds is 33.4°C (Chilgren and King 1973). The SPME fiber was then exposed to the headspace (without touching the feathers) for 25 minutes at 34°C, after which the fiber was retracted and the adsorbed chemicals were analyzed by gas chromatography-mass spectrometry (GC-MS). Only one SPME fiber was used in the study.

We analyzed the samples using a Shimadzu GCMS-QP2010plus (Shimadzu Scientific Instruments, Kyoto, Japan) equipped with an Optima®-5MS column (30 m x 0.25 mm x 0.25 μm, Macherey-Nagel, Düren, Germany) at the Plateforme d'Analyses Chimiques en Ecologie (PACE), technical facilities of the LABoratoire d'Excellence named Centre Méditerranéen de l'Environnement et de la Biodiversité (CEMEB). Helium was used as the carrier gas, at a constant 1ml/min flow rate. Fibers were exposed in a 200 °C injection port to desorb volatiles.

We ran the following temperature protocol: 40° for 5 min, 40°C to 180°C ramped at 5°C/min, 180°C to 250°C ramped at 10°C/min, and finally held at 250°C for 1 min. The ion source and transfer line temperatures was 200°C and 250°C respectively. Mass were scanned from 38 -350 m/z in electron ionization mode. Fibers were removed from the injection port after 10 min of desorption. We regularly interspersed blanks throughout the sample analyses to detect contamination. We ran an alkane standard solution (Alkanes standard solution, 04070, Sigma Aldrich®, Germany) under the same conditions for retention index calculation. The samples from 9 controls, 9 lead-supplemented pigeons, 10 zinc-supplemented birds and 11 zinc-andlead-supplemented birds were ran in March 2015, while the other samples were ran in June 2015. Within each analysis time period (March vs. June 2015), samples were ran randomly.

Consequently, there was no difference in date of analysis between zinc-supplemented and non-zinc supplemented samples or between lead-supplemented and non-lead supplemented samples (all P > 0.15). Ten samples had very low GCMS signal and were not included in the statistical analyses.

To confirm that the feather odor captured from captive pigeons was representative of that of wild pigeons, we compared the chromatograms with those of two samples of back feathers collected just after capture in February 2013. In addition, to shed light on the potential origin of plumage chemicals, we compared the feather profiles to those of preen secretion samples collected on two pigeons just after capture.

Chromatograms were analyzed blind to treatment using the Shimadzu GC-MS Solution software. Analytes were identified by comparison of mass spectral data using the NIST library 2011 and cross-checking linear retention index of the spectral match found in the literature (PubChem database) with the calculated linear retention index of the analytes.

Four monoterpenes were detected in pigeon feathers (α-Pinene, β-Pinene, p-Cymene and Bornyl acetate; Table 1). Most probably of plant origin [START_REF] Charpentier | Critical thinking in the chemical ecology of mammalian communication: roadmap for future studies[END_REF], they were excluded from the analyses. One peak corresponded to a co-elution of several compounds including one contaminant (Table 1), and it was also not included in the analyses.

To ascertain that our GCMS analyses were repeatable, 5 samples were analyzed twice with a few days interval (range: 5 -8 days). Results show that total chromatogram area was lower in the second analysis than in the first analysis (paired t-test: t 4 = 5.32, P = 0.006) but that the composition of the two samples from the same bird was repeatable (PERMANOVA analyses with bird identity as fixed factor: F 4,4 = 8.93, P = 0.001).

Statistical analyses

Because, for most samples, we did not control for the amount of feathers collected, and because an internal standard was not included in the GCMS analyses, we did not rely on the absolute abundance of chromatogram peaks for the analyses of chemical composition; rather, we quantified each peak as the proportion of the peak size relative to the total area of the chromatogram. Proportions of chemical compounds were then transformed using isometric log ratio "ilr" [START_REF] Brückner | A chemo-ecologists' practical guide to compositional data analysis[END_REF]). In addition, to limit the effect of the differences in the amount of feathers on odor composition, we restricted the odor composition analyses on the peaks that each represented on average more than 0.75% of the chromatogram (n = 19 peaks representing on average 91.0 % ± 0.3 % of the total chromatogram areas, Table 1). All of these peaks, except one, were present in all birds.

To determine if the chemical composition of plumage odor was related to lead and zinc exposition, we used a PERMANOVA with 5000 permutations i.e. nonparametric multivariate analysis of variance, (Adonis2 function, "vegan" package in R; [START_REF] Oksanen | Package 'vegan' Community ecology package, version 2[END_REF], based on Euclidean distance. The two-way interaction between lead and zinc, the bird sex and the analysis time period (March vs. June 2015) were also included in the model. The date of feather sampling was used as strata, thus restricting permutations within date of sampling. In Adonis, significant differences may be caused by different within-group dispersion instead of different mean values of the groups [START_REF] Warton | Distance-based multivariate analyses confound location and dispersion effects Methods in[END_REF]. For significant effects, we therefore tested difference in dispersion between groups (Betadisper function, "vegan" package; [START_REF] Oksanen | Package 'vegan' Community ecology package, version 2[END_REF]).

To determine the chemical compounds that were preferentially associated with a significant fixed effect, we used correlation indices (function "multipatt" in the "indicspecies" package in R; De [START_REF] Caceres | Associations between species and groups of sites: indices and statistical inference[END_REF] with the point biserial coefficient of association on proportions of chemical compounds (De Cáceres and Legendre 2009). We restricted permutations within date of sampling. This method uses permutation tests to test significance of value, but does not correct for multiple tests. Multiple testing corrections are flawed because they assume multiple tests are independent, whereas multiple volatiles can show correlated effects (Fig. S1) because of the nature of the data (relative abundance vs. absolute abundance) and because the biosynthesis pathway of these volatiles may be related. Therefore, we also performed robust principal component analyses (RPCA) on the ilrtransformed dataset [START_REF] Filzmoser | Principal component analysis for compositional data with outliers Environmetrics: The official[END_REF]) to identify the subset of compounds that varied with treatment. RPCA axis scores were compared between zinc-supplemented and non-zinc supplemented birds using linear mixed models with zinc treatment and analysis time period as fixed effects, and date of feather sampling as a random factor.

We performed all statistical analyses using the R statistical software (R Core Team 2017) and used a significance level set at α = 0.05.

Results

We detected a total of 45 peaks in the 63 odor profiles of pigeons, that included alkanes, aldehydes, alcohols, ketones, and esters (Table 1 and Fig. 1). Only 6 (13%) of these compounds were also detected in preen secretions (Table 1). In contrast, 39 of them (87%) were also detected in the plumage of wild Parisian pigeons, suggesting that captivity had only slight influence on the identity of feather volatiles.

Chemical composition varied with zinc supplementation (F 1,61 = 4.27, P = 0.016; Fig. 2 and Fig. S2), but not with lead supplementation (F 1,60 = 0.62, P = 0.89) or the interaction between zinc and lead (F 1,59 = 1.22, P = 0.56). It did not vary with sex (F 1,60 = 0.14, P = 0.99), but varied with the period when the samples were analyzed (F 1,61 = 13.20, P < 0.001). Multivariate dispersion in chemical composition between zinc-supplemented and nonzinc supplemented birds was similar (F 1,62 = 0.59, P = 0.45). Correlation indices showed that the proportions of the two hydroxy esters and 3-Hexanone were increased in zinc supplemented birds, while the proportion of Nonanal was reduced (Fig. 3). Similarly, a robust PCA showed that the second principal component, which accounted for 26% of the variance and which was positively related to the two hydroxy esters and the two ketones, was higher in zinc-supplemented birds than in non-zinc supplemented birds (χ² 1 = 7.14, P = 0.008; Fig. 4).

Discussion

By analyzing feather volatiles of feral pigeons chronically exposed to zinc and/or lead, we provide evidence for an effect of zinc on pigeon odor composition. In particular, we found that zinc mainly increased the relative abundance of the two hydroxy esters and 3-Hexanone.

All the compounds affected by zinc supplementation may potentially be produced endogenously by the bird. However, in contrast to several species, where most plumage chemicals originate from the preen gland [START_REF] Leclaire | An individual and a sex odor signature in kittiwakes? Study of the semiochemical composition of preen secretion and preen down feathers[END_REF][START_REF] Mardon | From preen secretions to plumage: the chemical trajectory of blue petrels' Halobaena caerulea social scent[END_REF][START_REF] Zhang | Uropygial gland volatiles may code for olfactory information about sex, individual, and species in Bengalese finches Lonchura striata[END_REF], only the two hydroxy-esters were detected in pigeons' preen secretions. Although the sample size used to study the difference between preen secretion and feather volatiles was low (n = 2 birds sampled for preen secretions), this result is consistent with a study on common wood pigeons (Columba palumbus), where 93% of the whole plumage lipids do not originate from preen secretions [START_REF] Jacob | Gefiederlipide der ringeltaube Columba palumbus/Plumage lipids from the ring dove (Columba palumbus)[END_REF].

The other compounds affected by zinc exposure were not detected in preen secretions, and may rather originate from other lipid-producing areas in the integument [START_REF] Jacob | The uropygial gland[END_REF] or from feather bacteria metabolism [START_REF] Whittaker | Bacterial communities associated with junco preen glands: Preliminary ramifications for chemical signaling[END_REF]. The role of bacteria in animal olfactory signaling has recently received renewed interests [START_REF] Archie | Animal behaviour meets microbial ecology[END_REF][START_REF] Douglas | New synthesis: animal communication mediated by microbes: fact or fantasy?[END_REF][START_REF] Ezenwa | Microbes and animal olfactory communication: where do we go from here?[END_REF] and a few experimental studies in mice, elephants (Loxodonta africana), Indian mongooses (Herpestes auropunctatus) and European hoopoes (Upupa epops) have shown that some major odorants require commensal bacteria for their production [START_REF] Goodwin | The role of bacteria in chemical signals of elephant musth: Proximate causes and biochemical pathways[END_REF][START_REF] Gorman | Analysis of contents of anal scent pockets of Herpestes auropunctatus (Carnivora-Viverridae)[END_REF][START_REF] Li | Synchronous evolution of an odor biosynthesis pathway and behavioral response[END_REF][START_REF] Martin-Vivaldi | Antimicrobial chemicals in hoopoe preen secretions are produced by symbiotic bacteria[END_REF]. Methyl-branched alkanes, methyl ketones, and aldehydes can all be released by bacterial degradation of fatty acids [START_REF] Schulz | Bacterial volatiles: the smell of small organisms[END_REF]. Feathers of zinc-supplemented pigeons have lower bacterial load and different bacterial assemblage than feathers of birds non-exposed to zinc (Chatelain et al. 2016a), which may have led to differences in the composition of feather volatiles between birds.

Further studies on the mechanisms of avian odor production are clearly needed to understand how environment can affect feather volatiles.

We found that zinc increased the proportion of preen-gland-related compounds, which might suggest higher production of preen oil in zinc-exposed birds. Several studies have suggested that birds in higher condition are able to produce higher quantity of preen oil. For instance, experimental infections and immune challenge reduce the size of the preen gland in house sparrows (Passer domesticus) and tawny owls (Strix aluco) (Moreno-Rueda 2015; [START_REF] Pap | Chronic coccidian infestation compromises flight feather quality in house sparrows Passer domesticus[END_REF][START_REF] Piault | Experimental support for the makeup hypothesis in nestling tawny owls (Strix aluco)[END_REF]. Zinc provided at low dose is essential for many biological processes [START_REF] Stefanidou | Zinc: a multipurpose trace element[END_REF], and accordingly, zinc supplementation has various beneficial effects on pigeon condition (Chatelain et al. 2016b;Chatelain et al. 2016c).

Therefore, zinc, through its positive effects on bird condition, may increased the ability of bird to produce wax esters and other costly volatiles. Although avian odors have reapeatedly been shown to vary with genetics (i.e., [START_REF] Krause | Olfactory kin recognition in a songbird[END_REF][START_REF] Leclaire | Odour-based discrimination of similarity at the major histocompatibility complex in birds[END_REF]) and physiological status [START_REF] Douglas | Seasonal covariation in progesterone and odorant emissions among breeding crested auklets (Aethia cristatella)[END_REF][START_REF] Whittaker | Songbird chemical signals reflect uropygial gland androgen sensitivity and predict aggression: Implications for the role of the periphery in chemosignaling[END_REF], a very few studies have shown the effect of environmental stressors on their expression [START_REF] Bombail | In search of stress odours across species: Behavioural responses of rats to faeces from chickens and rats subjected to various types of stressful events[END_REF]. For instance, infection influences the wax ester profiles of preen oil in song sparrows (Melospiza melodia) and the volatile profiles of feces in mallards (Anas platyrhynchos) [START_REF] Grieves | Malarial infection alters wax ester composition of preen oil in songbirds: Results of an experimental study[END_REF][START_REF] Kimball | Avian influenza infection alters fecal odor in mallards[END_REF]. Our study, which shows that the volatile profiles of feather is related to zinc levels in the environment, is therefore consistent with the stress-induced effects on odors demonstrated in other birds.

In several avian species, odor plays a crucial role in social interactions [START_REF] Balthazart | Pheromones are involved in the control of sexual-behavior in birds[END_REF][START_REF] Balthazart | The underestimated role of olfaction in avian reproduction?[END_REF][START_REF] Hirao | The role of uropygial gland on sexual behavior in domestic chicken Gallus gallus domesticus[END_REF]. Zinc-induced changes in pigeon scent might therefore influence the behavior of conspecifics, with, for instance, females avoiding the scent of zinc-deficient males. However, although pigeons are wellknown to use their olfactory abilities for homing (reviewed in [START_REF] Wallraff | Avian navigation: pigeon homing as a paradigm[END_REF], it is unknown whether they use also olfaction for social communication. Feather chemicals may play also a role in social communication by affecting feather coloration or condition (Moreno-Rueda 2017). For instance, in house finches (Carpodacus mexicanus), preen oil enhances the coloration of red feathers [START_REF] López-Rull | Cosmetic enhancement of signal coloration: experimental evidence in the house finch[END_REF]. In pigeons, changes in the coloration of melanic feathers have been induced by zinc supplementation [START_REF] Chatelain | Do trace metals influence visual signals? Effects of trace metals on iridescent and melanic feather colouration in the feral pigeon Oikos Chilgren[END_REF], and might therefore be mediated by changes in the chemical composition of feather volatiles.

Feather chemicals do not only play a role in social communication; they can protect feathers from wear and act as chemical defenses against parasites [START_REF] Douglas | Colonial seabird's paralytic perfume slows lice down: An opportunity for parasitemediated selection?[END_REF][START_REF] Hagelin | Bird odors and other chemical substances: A defense mechanism or overlooked mode of intraspecific communication?[END_REF]. For instance, in rock pigeons, preen oil has positive effects on plumage condition [START_REF] Moyer | Experimental test of the importance of preen oil in rock doves (Columba livia)[END_REF]). 3-Hexanone, which was increased in zinc-exposed birds, is a common urine component of mammals, including humans [START_REF] Burger | Elemental sulfur identified in urine of cheetah, Acinonyx jubatus[END_REF][START_REF] Delbarco-Trillo | Night and day: the comparative study of strepsirrhine primates reveals socioecological and phylogenetic patterns in olfactory signals[END_REF][START_REF] Raymer | Chemical scent constituents in urine of wolf (Canis lupus) and their dependence on reproductive hormones[END_REF][START_REF] Smith | A comparative study of the analysis of human urine headspace using gas chromatographymass spectrometry[END_REF], and has been detected in the feathers of white Leghorn chickens (Gallus gallus domesticus L.) [START_REF] Bernier | Volatile compounds from the integument of white leghorn chickens (Gallus gallus domesticus L.): candidate attractants of ornithophilic mosquito species[END_REF]) and king penguins [START_REF] Gabirot | Odour of King Penguin feathers analysed using direct thermal desorption discriminates between individuals but not sexes Ibis[END_REF]. It belongs to the volatile methyl-ketone family, whose some compounds can provide protection against parasites [START_REF] Borges | Identification of non-host semiochemicals for the brown dog tick, Rhipicephalus sanguineus sensu lato (Acari: Ixodidae), from tick-resistant beagles, Canis lupus familiaris Ticks and tick[END_REF][START_REF] Kirillov | Chemical composition of the essential oil from carnation coniferous (Dianthus acicularis Fisch. ex Ledeb) growing wild in Northern Kazakhstan Natural product[END_REF]. According to the parasite-repellent function of volatile ketones, volatile methyl-ketones of dark-eyed juncos (Junco hyemalis) are increased during the breeding season [START_REF] Soini | Seasonal variation in volatile compound profiles of preen gland secretions of the dark-eyed junco (Junco hyemalis)[END_REF]), when the need for protection against nest parasites is high. Zinc, by inducing higher production of hydroxy-esters and 3-hexanone, might thus enhance plumage condition and the anti-parasite properties of feathers, thus adding to the numerous positive effects of zinc on pigeon (Chatelain et al. 2016b;Chatelain et al. 2016c).

Lead is a toxic trace metal that can impair various biological processes, leading to illness and mortality [START_REF] Papanikolaou | Lead toxicity update[END_REF]. In feral pigeons, lead decreases iridescent coloration, the maternal transfer of antibodies into eggs, and nestling growth (Chatelain et al. 2016b;[START_REF] Chatelain | Do trace metals influence visual signals? Effects of trace metals on iridescent and melanic feather colouration in the feral pigeon Oikos Chilgren[END_REF]. Despite these detrimental effects on pigeon condition, we did not detect effects of lead on pigeon scent. As our experimental exposure to lead underestimated the natural range (lead was 1.5 times less concentrated in the feathers of leadsupplemented pigeons than in those of wild feral pigeons; Chatelain et al. 2016b), we cannot exclude that, in urban areas, the high concentration of lead negatively impacts odor production in birds.

We did not find evidence for a sex signature in the feather odor of wild-caught pigeons kept in captivity. Similarly, no sex-differences in preen secretion composition have been detected in non-breeding rock pigeons [START_REF] Montalti | The chemical composition of the uropygial gland secretion of rock dove Columba livia[END_REF]. Although sexual dimorphism in chemical cues from feathers or preen secretions has been reported in several bird species [START_REF] Amo | Sex recognition by odour and variation in the uropygial gland secretion in starlings[END_REF][START_REF] Leclaire | An individual and a sex odor signature in kittiwakes? Study of the semiochemical composition of preen secretion and preen down feathers[END_REF][START_REF] Whittaker | Songbird chemosignals: volatile compounds in preen gland secretions vary among individuals, sexes, and populations[END_REF][START_REF] Zhang | Uropygial gland-secreted alkanols contribute to olfactory sex signals in budgerigars[END_REF], it is not ubiquitous. For instance, king penguins (Aptenodites patagonicus) and Cory's shearwaters (Calonectris borealis) seem to exhibit no sex-differences in scent [START_REF] Gabirot | Odour of King Penguin feathers analysed using direct thermal desorption discriminates between individuals but not sexes Ibis[END_REF][START_REF] Gabirot | Chemical labels differ between two closely related shearwater taxa[END_REF]. Pigeons display sexual polymorphism in iridescent coloration of neck feathers [START_REF] Chatelain | Do trace metals influence visual signals? Effects of trace metals on iridescent and melanic feather colouration in the feral pigeon Oikos Chilgren[END_REF]Leclaire et al. 2014a), and might, therefore, use this other trait to discriminate between sexes. Alternatively, because the molecules detected using GC-MS depend on the analysis strategy (e.g., extraction protocol and GC column), the profile of chemical composition presented here is not exhaustive. For instance, carboxylic acids, which are better resolved using a polar column [START_REF] Whelan | Short-chain carboxylic acids from gray catbird (Dumetella carolinensis) uropygial secretions vary with testosterone levels and photoperiod Comparative[END_REF]), might encode sex information.

In conclusion, our study showed that exposure to a moderate amount of zinc induced changes in the feather volatile composition of feral pigeons. Future studies should now evaluate the role of these volatiles in social communication, protection against parasites or feather wear to further predict how trace metals, which are chemical pollutants of prime concern in cities, can impact bird fitness.

Figure captions

Figure 1 :

 1 Figure 1: Representative chromatogram of feather volatiles in captive pigeons. Letters refer

Figure 2 :

 2 Figure 2: Partial unconstrained distance-based redundancy analysis plot showing separation

Figure 3 :

 3 Figure 3: Mean ± SE proportion of the four chemical compounds that varied with zinc

Figure 4 :

 4 Figure 4: Robust principal component analysis plot showing separation between pigeons by

Table 1 :

 1 figure

Acknowledgements

We thank the "Mairie de Paris" (Thomas Charachon) for allowing the capture of birds and the Centre de Recherche en Ecologie Experimentale et Predictive (CEREEP) which provided logistic support for the field work of this study. We are very thankful to T. Gayet, S. Pollet, S.

Hasnaoui, F. Lorente, S. Perret and B. Decenciere for their help in field work.

Funding

This work was financed by grants from the local government (Ile-de-France: Sustainable Development Network R2DS, No. 2012-11 to J.G.), and from the "Agence Nationale de la Recherche" (No. ANR-13-PDOC-0002 to S.L.).

Conflict of interest

The authors declare that they have no conflict of interest.

Compliance with Ethical Standards

All experiments were carried out in strict accordance with the recommendations of the "European Convention for the Protection of vertebrate Animals used for Experimental and Other Scientific Purposes" and were conducted under the authorizations of the "Ministère de l'éducation nationale, de l'enseignement supérieur et de la recherche" (authorization N_00093.02) and the "Direction Departementale des Services Veterinaires de Seine-et-Marne" (authorization N_ 77-05). The authors declare no conflict of interest.

Fatty acids

Nonanoic acid 1273 x

Unknown