Michel Cabane
email: michel.cabane@latmos.ipsl.fr

Keywords: Atmosphere, Biomarkers, Chlorohydrocarbons, Curiosity, Derivatization, Gale, GC-MS, In situ, Mars, Mars Science Laboratory, Methane, MSL, Organics, Pyrolysis, Regolith, Rover Definition

Mars Science Laboratory is a large instrumented vehicle (rover) built by ▶ NASA that landed on Mars in August 2012 (launched November 2011) and will operate for more than 1 Martian year (i.e., 2 Earth years). This rover, also called Curiosity, characterizes and interprets the geology of the landing region at all scales, assesses the biological potential of encountered targets, investigates planetary processes of relevance to past habitability, and characterizes radiation arriving at Mars' surface. Aboard MSL, an analytical laboratory is especially devoted to atmospheric analysis and search for organics in the ground.

Overview

Ancient Martian history, from its formation up to about three billion years ago, looks close to Earth's (Hadean-Archaean on Earth,▶ Noachian-EarlyHesperian on Mars). We may then ask, was it possible for life to appear on Mars? One faces three possibilities: (a) life appeared and is extinct, but organic remnants or biominerals can be found; (b) only ▶ complex organic molecules, "bricks of life," were formed, and some of them still exist; and (c) Mars has been absolutely sterile from the beginning.

Viking Results

In 1976, ▶ NASA sent the ▶ Viking orbiters and landers to Mars. On landers Viking 1 and 2, instruments were devoted to the search for biosignatures and molecular analysis of the ground. They were (1) a gas exchange experiment: nutrients were brought to the soil, variations in concentrations of O 2 ,CO 2 ,CH 4 , etc. were searched for [START_REF] Oyama | The Viking gas exchange experiment results from Chryse and Utopia[END_REF]; (2) two experiments using radioactive 14-C: CO 2 exchanges between atmosphere, soil, and nutrient were monitored [START_REF] Horowitz | Viking on Mars: the Carbon assimilation experiment[END_REF] [START_REF] Levin | Recent results from the labeled release experiment[END_REF]; (3) a pyrolysis-gas chromatograph-mass spectrometer (GC-MS) experiment: Mars soil was heated and analyzed [START_REF] Bieman | The search for organic substances and inorganic compounds in the surface of Mars[END_REF].

Ambiguous responses to the first three experiments were understood as due to the presence of nonbiological reactants rather than metabolic activity. One knows, now, that Mars' atmosphere contains gaseous oxidants, such as H 2 O 2 , due to atmospheric photochemistry [START_REF] Atreya | Oxidant enhancement in Martian dust devils and storms: implications for life and habitability[END_REF][START_REF] Zanhle | Photochemical instability of the ancient Mars atmosphere[END_REF]) and that oxidizing compounds may also be present in the soil: Phoenix lander, when characterizing Mars soil near the North pole, observed the presence of perchlorates (ClO 4 À ) [START_REF] Hecht | Detection of Perchlorate and the soluble chemistry of Martian soil at the Phoenix lander site[END_REF].

Concerning the ▶ pyrolysis-GC-MS experiment, no organics were detected, except, at low level, chlorohydrocarbons (CH 3 Cl, and CH 2 Cl 2 ) the source of which was understood as terrestrial: chlorinated solvents, traces of methanol and HCl brought from Earth [START_REF] Bieman | The search for organic substances and inorganic compounds in the surface of Mars[END_REF], although they were not detected at the same level in the blank runs. Nevertheless, organic matter is brought to the Martian surface by micrometeorites ($10 À3 mg/cm 2 /year, Flynn 1996), and the question was, then, about its observability. [START_REF] Benner | The missing organic molecules on Mars[END_REF] showed that, in harsh environments, organic complex molecules degrade into refractory species (e.g., benzene-carboxylic acids) (Fig. 1). So, pyrolysis at moderate temperatures (500 C in Viking ovens) could not produce any volatile organic compound. Later, Navarro-Gonzàlez et al. ( 2010) took into account Phoenix observations and, from the pyrolysis of Mars-like soils (Atacama desert) mixed with perchlorates, obtained the same chlorohydrocarbons as observed by the Vikings. So, organics, at low levels, could be present on Mars surface and revealed by pyro-GC-MS analysis in presence of perchlorates, and this could be an alternative explanation to Bieman et al. results. Following this, exchanges of views occurred between the two teams [START_REF] Bieman | Comment on "Reanalysis of the Viking results suggest perchlorate and organics at midlatitudes on Mars[END_REF][START_REF] Navarro-Gonzàlez | Reply to comment by Bieman and Bada on "Reanalysis of the Viking results suggest perchlorate and organics at midlatitudes on Mars[END_REF].

Consequently, (i) the analytical laboratory of Curiosity, SAM, has been built (2005)(2006)(2007)(2008)(2009) taking into account the fact that detection of organic matter by pyro-GC-MS needs higher temperatures or some

Water on Mars

Since Viking, many orbiters (▶ Mars Express, MGS,▶ Mars Odyssey, MRO, etc. [START_REF] Bibring | Global mineralogical and aqueous Mars history derived from Omega/Mars Express data[END_REF][START_REF] Murchie | Evidence for the origin of layered deposits in Candor Chasma, Mars, from mineral composition and hydrologic modeling[END_REF])) observed Mars and returned strong indications that water circulated on Mars' surface during the ▶ Noachian period (Fig. 2). Spectral analyses show that carbonates, hydrates, sulfates, and phyllosilicates are present on Mars, linked to what could be riverbeds, deltas, or alluvial cones [START_REF] Wray | Phyllosilicates and sulfates at Endeavour Crater, Meridiani Planum, Mars[END_REF], which could be associated with life or its beginning.

Likewise, instrumented vehicles (rovers) have also explored Mars since 1997. Pathfinder-Sojourner (1997) and the MERs (Mars Exploration Rovers) Spirit (2004[START_REF] Thompson | Constraints on the origin and evolution of the layered mound in Gale Crater, Mars, using Mars Reconnaissance Orbiter data[END_REF]) and Opportunity (2004-present) were mainly designed to identify geological patterns (panoramic cameras, microscope) (Fig. 3) and to analyze rocks using APXS (Alpha Particle X-Ray Spectroscopy), Mössbauer spectrometry, and TES (thermal emission spectrometry). Again, one can quote the detection of the same minerals as above [START_REF] Squyres | The opportunity rover's Athena science investigation at Meridiani Planum[END_REF]).

Mars Science Laboratory

From 1999, MEPAG (Mars Exploration Program Analysis Group, NASA) defined four goals linked by the motto "follow the water": (1) Life, (2) Climate, (3) Geology, (4) Human exploration. In this frame, MSL (Mars Science Laboratory) was developed and launched in November 2012 (the vehicle to be landed on Mars surface was named "Curiosity"). MSL project has, in particular, for objectives to assess the past and present habitability, to characterize carbon cycling in its geochemical context, and to assess whether life is or was present (Grotzinger et al. 2012). From numerous (%50) candidates for MSL landing (cf. [START_REF] Golombek | Selection of the Mars Science Laboratory landing site[END_REF], taking into account engineering constraints, and after five science workshops, Gale crater was selected in 2011 (Fig. 4). This crater (150 km diameter) was formed, from a meteoritic impact, about 3.8 Gy ago (Late Noachian to Early Hesperian) (Anderson and Bell, 2010). At the center of the crater, Aeolis Mons (aka Mount Sharp) culminates at more than 5 km. Scientists consider that this mound might be the remnant of sedimentary layers that deposited for some 100 My inside the crater, then were eroded [START_REF] Thompson | Constraints on the origin and evolution of the layered mound in Gale Crater, Mars, using Mars Reconnaissance Orbiter data[END_REF][START_REF] Wray | Gale crater: the Mars Science Laboratory/Curiosity rover landing site[END_REF]. Data analysis show clay-bearing units interstratified with sulfate-bearing strata, indicative of aqueous activity.

Curiosity was successfully landed on the crater's floor in August 2012, at the extremity of what appears to be an alluvial cone, brought to a potential lakebed by the river that drew "Peace Vallis," incised into the crater's rim (Fig. 5).

Basic Methodology MSL for 2012

This rover (Fig. 6) is larger (2.75 Â 3.05 Â 2.13 m) and heavier (900 kg) than Sojourner (11 kg) or MERs (174 kg each); from 2012, after landing (Fig. 7), its goal is to operate for more than 1 Martian year ($669 sols = $687 terrestrial days) and navigate some tens of kilometers, from the landing point -inside the landing ellipse-to the first layers of Mount Sharp. On a sandy flat surface, the velocity of Curiosity lies between 140 m/h for blind drive and 20 m/h for hazard avoidance with visual odometry. It is powered, as were Viking 1 and 2, by radioisotope thermoelectric generators (RTGs; 2.5 kW-h/sol) and is heavily instrumented. Its 80 kg science payload is composed of ten instruments (Grotzinger et al. 2013). They are devoted to (1) observation: cameras or microscopes (MarDI, descent imager; MastCam, on the mast; MAHLI, at contact with the samples); (2) environmental studies: an instrument (RAD) to characterize the broad spectrum of radiation near the surface and a pulsed neutron source and detector (DAN) for measuring ▶ hydrogen or ▶ ice and ▶ water at or near the Martian surface (less than 1 m), a meteorological station and UV sensor (REMS); (3) mineralogy and geochemistry of rocks and soil: APXS (Alpha Particle X-Ray Spectroscopy, at contact with the sample), CheMin (X-Ray diffraction on sampled powders), ChemCam (on the mast, UV to near-IR spectroscopy of gases issuing from laser impacts on targets); (4) molecular analysis of atmosphere and soil samples: SAM for MS, ▶ GC/MS, and IR spectroscopy (see below). Some of these instruments are on the mast (MastCam, ChemCam) or at the end of the 1.9 m length robotic arm (APXS, MAHLI). This arm also carries the scooping or drilling instruments (Anderson et al, 2012) that feed with mineral powder (<150 mm diameter, about 50 mg per dosing) the sample preparation devices for SAM and CheMin, which are located inside the rover.

Operations began as soon as the rover was deposited on Mars surface (see Fig. 7). It can be noticed that the way Curiosity is operated leads to quasi-daily downlink (data from Mars to Earth) or uplink (commands from Earth to Mars), seven days on seven at the beginning and, later, limited to working days; note that operations are linked to the Martian diurnal cycle (the "sol") that slips of about 40 min each "Earth" day. During the first three months (i.e., commissioning), teams were operating exclusively from JPL, then they came back to their laboratories. Results from Curiosity's science payload are embargoed until MSL project decides/authorizes to release them (Press conferences from NASA, international conferences, papers in peer-reviewed journals, for example). 8). These are a quadrupole mass spectrometer (MS, under GSFC/NASA responsibility, Mahaffy 2008), a tunable IR laser spectrometer (TLS, under JPL/NASA responsibility, Webster and Mahaffy, 2012), and a stand-alone gas chromatograph (GC) (under Univ. of Paris/CNES/CNRS responsibility, [START_REF] Cabane | Did life exist on Mars? Search for organic and inorganic signatures, one of the goals for "SAM" (sample analysis at Mars)[END_REF][START_REF] Rodier | Search for organics in extraterrestrial environments by in situ gas chromatography analysis[END_REF]. The goals of SAM are to (1) analyze the atmosphere, search for minor species (e.g., CH 4 , ▶ noble gases), abundances, and isotopic ratios; (2) analyze ground samples, search for organics and their possible ▶ chirality, structural gases in mineral species (CO 2 ,H 2 O, SO 2 , etc.); (3) measure isotopic ratios.

Six different gas chromatographic modules can be used, which cover a range from low-mass permanent gases or low carbon number (C1-C5) hydrocarbons to high masses (> C15), with the possible use of a "chiral" column to search for ▶ enantiomers. Specific detectors (thermal conductivity detectors, TCDs) are used that can measure down to some 10 À10 mol of a given species in a He flow. The Quadrupolar Mass Spectrometer precisely identifies molecular compositions: the mass measurement ranges from 2 to 535 Da, with a detection limit $ 1 ppm ($1 pmole in a He flow) and isotope ratios down to 1 per mil for ▶ noble gases. In the case of GC analysis, the MS can be used as detector to lower the detection level (some pmoles for GC-MS compared to some 10 À10 mol for GC-TCD). The cell of Tunable Laser Spectrometer instrument can detect down to 2 ppb of CH 4 and 2 ppm of water without enrichment and measure down to <10 per mil isotope ratios for C and O. Isotope measurements from TLS and MS will, in particular, help to solve the methane concern [START_REF] Lefèvre | Observed variations of methane on Mars unexplained by known atmospheric chemistry and physics[END_REF]: what are the origins and sinks of the Martian CH 4 , some tens of ppbs observed using teledetection (cf [START_REF] Atreya | Methane and related trace species on Mars: origin, loss, implications for life, and habitability[END_REF], that could originate from well-defined places? Gases sampled in the atmosphere are pumped through GC-MS-TLS; on their path, the Chemical Separation and Processing Laboratory (CSPL: getters, scrubbers, cold traps) may help to eliminate/ separate some species (CO 2 ,H 2 O) or to concentrate the sample.

Soil or rock powder need to be sieved (diameter <150 mm) before introduction into SAM's inlets. They can be heated up to 1,000 C (74 cells potentially reusable, in the Sample Manipulation System, SMS), and exhausted gases are analyzed; in the case of GC analysis, an He flow (pressurized tanks onboard) goes through the oven and pushes the mixture toward GC columns. In case of refractory species, some cups are equipped for wet chemistry (▶ derivatization): a chemical reagent transforms the molecule into a vaporizable one, while preserving the structure [START_REF] Buch | Development of a gas chromatography compatible Sample Processing System (SPS) for the in-situ analysis of refractory matter in the Martian soil: preliminary results[END_REF]). Combustion of samples in O 2 atmosphere (pressurized tank onboard) may be also used to determine the isotopic ratios.

Gases from heating of the sample can be directly analyzed by the MS, using the EGA (Evolved Gas Analysis) mode: the release of typical molecules (H 2 O, CO 2 ,O 2 ,SO 2 ) is monitored by the MS when the temperature of the cell that contains the sample is varied. Then, in a function of temperature, molecules show up and disappear, corresponding to their release from the sample at given temperatures. They can be adsorbed molecules, easily released at low temperatures, or weakly bonded molecules (e.g., H 2 O from Gypsum CaSO 4 (H 2 O) 2 ), or they can be molecules that come, at higher temperatures, from a breakup (e.g., CO 2 from CaCO 3 ) or a dehydroxylation (e.g., H 2 O from kaolinite Al 2 Si 2 O 5 (OH) 4 ). Monitoring organic molecules when heating the sample is very useful, as far as it allows a better understanding of the processes and reactions that can release and/or modify the structure of organic molecules. Moreover, a precise investigation on the temperature of release could help to decide on the (a)biotic origin of some minerals (Stalport, 2005;Stalport, 2007). During the heating process, for a given temperature range, one can decide to send gases to TLS or GC-MS for further analyses.

In order to have a complete understanding of the results that are obtained on Mars, using GC and/or MS and/or TLS, devoted experiments intended to check the possible pathways are performed on Earth. For this purpose, teams can use, in their laboratories, separate instruments, most of them presenting the same characteristics (e.g., spare models) as the ones that are used on SAM; a complete spare model of SAM is also used, as a full testbed, at NASA/GSFC, in Martian conditions.

Key Research Findings

Curiosity landed safely on Mars surface (landing site was named "Ray Bradbury") on August 5, 2012 and, from then, performed measurements, using all the instruments of the Science Payload and moving on Mars surface. The embargo policy does not allow quoting nonvalidated results; Nasa's websites (e.g., ▶ http://mars.jpl.nasa.gov/msl/) allow to follow Curiosity in its progress and results.

The main idea, before landing, was to drive out the landing ellipse, quite rapidly, in direction of sulfateand phyllosilicate-bearing strata (lower part of the mound). Curiosity was deposited near the center of the landing ellipse (see Fig. 5), and it appeared that, at short distances (less than 300 m), there were targets that were worth the visit, in full agreement with the expectations that the team had, from Gale's choice. It took from August 2012 to June 2013 to carry out this task. Here are described the main results, obtained during the first 6 months, that are linked to Astrobiology; other results (Meteorology, Mineralogy, Geology, etc.) may be found on devoted websites and science papers.

Conglomerates

After leaving "Bradbury," on sol 22, Curiosity approached "Link" (sol 27), then "Hottah" (sol 39) (see Curiosity traverse on Fig. 9), and images were obtained from Mastcam (Figs. 10 and 11). One observes that these outcrops are sedimentary conglomerates composed of rounded gravels cemented together. The size of the rounded stones is not compatible with aeolian processes (these pieces of rock are too big to be moved by the wind), and only flowing water can explain what is observed [START_REF] Newsom | Regional and global context of soil and rock chemistry from ChemCam and APXS at Gale crater[END_REF][START_REF] Williams | Martian fluvial conglomerates at Gale crater[END_REF]: pebbles transported for a minimum distance of a few km, by a water flow of velocity from .2 to .75 m/s and depth .03 to .9 m.

Obviously, this conclusion is compatible with the position of Curiosity at this time, at the end of an alluvial fan from Peace Vallis (see Fig. 5), and it strengthens the idea of water flowing on Mars, some billion years ago, in a mild and dense atmosphere.

Atmosphere

SAM used its TLS instrument to search for CH 4 in the atmosphere and compare with the previous results of 10 ppb or more [START_REF] Kerr | Question of Martian Methane is still up in the air[END_REF]) that were obtained from remote observations. Measurements of TLS show a very low level of CH 4 : 0,55 +/À 1.46 ppb, at the 1-sigma level [START_REF] Webster | Measurements of Mars Methane at Gale Crater by the SAM Tunable Laser Spectrometer on the Curiosity Rover[END_REF], which allows to think that, if CH 4 is present, it has a concentration that is well below the ones that one could expect from teledetection. Concerning atmospheric escape that helped to lead to the present 7 mbars, SAM observed a decrease of 36 Ar/ 38 Ar, compared to Solar or Earth values, that is fully compatible with the values observed for Martian meteorites and corroborates the theories of a dense atmosphere for the first billion years of Mars history. The same trend was obtained by measuring D/H ratio in the atmosphere or from H 2 O released when heating soil samples (at Rocknest, see below).

Sand

On her path to the site labeled Glenelg (see Fig. 9), Curiosity stopped (sol 56) at Rocknest, a sand dune (Fig. 12). This allowed the first scooping of Mars soil by Curiosity and analysis by SAM pyro-GC-MS Fig. 11 Comparison between conglomerate Link -on left-and an Earth conglomerate -at right-. The yellow bar corresponds to 1 cm in both cases (Credits: NASA/JPL-CalTech/MSSS) Fig. 12 The sand dune Rocknest (middle right) at the lee side of a rock outcrop (Credits: NASA/JPL-CalTech/MSSS) chain (Leshin et al. 2013). EGA (Evolved Gas Analysis) was used for some analyses of the samples, and the release of typical molecules (H 2 O, CO 2 ,O 2 ,SO 2 ) was monitored using the MS (see an example on Fig. 13).

Chemin observed [START_REF] Vaniman | Data from the Mars Science Laboratory CheMin XRD/XRF instrument[END_REF]) that all of the identified crystalline phases it identifies are volatile free; H 2 O, SO 2, and CO 2 volatile releases are then associated with the amorphous or poorly ordered materials. Water vapor is released at rather low temperatures, corresponding to a mixture of adsorbed or weakly bonded water for the lowest part of the spectrum and more or less tightly bonded water for the highest part. Oxygen release can be attributed to the decomposition of perchlorates. CO 2 comes, at low temperatures, from adsorbed molecules, and at high temperatures from carbonates decomposition, a source for CO 2 may be the action of O 2 on organic matter. Using GC-MS, low amounts of CH 3 Cl and CH 2 Cl 2 are detected, which might correspond to action of perchlorates on organics either present in the samples or brought from Earth. The complexity of the results, the interaction of various species between themselves, leads to cautious interpretations; laboratory experiments are in progress, in order to reproduce and explain the results observed on Mars.

Rock

From sol 157, Curiosity explored a small depression, "YellowKnife Bay," where another kind of rocks was observed (Fig. 14). When shooting at light-toned veins with its laser, and analyzing the emitted light, ChemCam concluded to the presence of elevated levels of calcium, sulfur, and hydrogen. This may correspond calcium sulfates as Gypsum CaSO 4 (H 2 O) 2 or Bassanite CaSO4(H 2 O) 0.5 . These minerals form when stagnating "salty" water infiltrates rocks and fills cracks with mineral deposits. SAM-EGA Analysis applied to the rock powder that was obtained by drilling the rock John Klein (down to about 6 cm, Fig. 15) showed the same molecules as in the case of Rocknest and gave the same patterns for O 2 ,SO 2, and CO 2 . But concerning H 2 O, the pattern is rather different: instead of culminating when the temperature is about 250 C, as seen for Rocknest, it appears (see an example on Fig. 16) that water is also delivered for temperatures from 400 C to 700 C. This may correspond to phyllosilicates, as far as much more energy is needed to obtain H 2 O from hydroxyl radicals in their chemical formula to deliver H 2 O. The same observation came from CheMin where diffraction patterns of phyllosilicates appeared when bombarding the rock powder with X-Rays (Fig. 17).

Concerning sulfur, it was concluded to the presence of hydrogen sulfide (H 2 S, reduced S) and sulfate (SO 4 , oxidized S). Similarly to Rocknest, chlorohydrocarbons are observed by GC-MS, and laboratory experiments are in progress for a better understanding.

One can say, as it was declared at a NASA Press Conference in March 2013, that in this plain, near an alluvial cone, what was found around John Klein, at Yellowknife Bay, is fully representative of what is defined as a past habitability of Mars, that is to say, a rather neutral watery environment (Fig. 18), oxidized and reduced atoms, carbonates and clays, etc.

After the conjunction event, in April 2013, when Mars and Earth were separated by the Sun, Curiosity drilled again, from sol 275, a set of holes in Cumberland rock, not so far from John Klein, in order to finalize its study of Yellowknife Bay, before the beginning of her trip to Mount Sharp (see on Fig. 19 Mount Sharp's panorama as seen by Curiosity). In the frame of its Exomars program, and in cooperation with RosCosmos, ESA will launch, in 2016, the trace gas orbiter, TGO, devoted to spatial and temporal localization, and possible sources of methane and other light hydrocarbons or nitriles but also of nitrogen and sulfur compounds, O 3 , HCl, HO 2 , etc., with required detection sensitivities down to 1 ppt. An Entry Descent and Landing demonstrator module will be associated, which could perform some science measurements. In the same frame, ESA will land, in 2018, the Exomars Rover, Pasteur, equipped with a drill, able to collect samples down to 2 m below the surface, and a science payload mainly devoted to astrobiology. Pasteur will explore Mars for one or more seasons; a surface platform should be associated to Pasteur to investigate Martian environment.

To continue its Mars Exploration program, NASA has in mind to send to Mars, in 2020, a rover with almost the technical capabilities as Curiosity and an enhanced drilling system. The mission would provide an isotopic datation of Mars terrains (instead of ages obtained from craterization rates) and address key questions about the potential for life on Mars. The mission would also provide opportunities to gather knowledge and demonstrate technologies that address the challenges of future human expeditions to In the proximity, one observes a band of dark dunes (cf Fig. 5) and, further on, layered hills at the base of Mount Sharp. The top ridge is about 15 km away from the rover (Credits NASA/JPL-Caltech/MSSS) Fig. 18 On left: Wopmay rock as seen by Opportunity rover in Endurance crater (Meridiani Planum). On right: rocks of the "Sheepbed" unit in Yellowknife Bay (Gale Crater), as seen by Curiosity. Both rocks were formed in the presence of water, but the water at Wopnay was highly acidic and salty, while the water at Sheepbed had a more neutral pH and lower salinity (Credits: NASA/JPL-Caltech/Cornell/MSSS) Mars. The rover could acquire rock cores with a core drill and place the cores in a cache. The cache would be left on the ground, while the rover would continue exploring the planet. This could be the first step of an exploration leading ultimately to the arrival, at Earth, of Mars Sample Return mission.

Fig. 1

 1 Fig. 1 Oxidative degradation of the generic alkane (represented here by pentane) to acetic acid (a), toluene to benzoic acid (b), and kerogen to benzenehexacarboxylic acid (mellitic acid) (c) (From Benner et al. 2000, Copyright National Academy of Sciences, USA)

Fig. 2

 2 Fig. 2 Color-enhanced image of the rim of Jezero Crater (40 km diameter). Ancient river (meander at left) carried phyllosilicates (shown in green) into a lake, forming a delta. CRISM instrument aboard MRO, 2008 (Credits: NASA/JPL/ JHUAPL/MSSS/Brown University)

Fig. 3 Fig. 6

 36 Fig. 3 (artificial colors: 'Earth illumination') Cape St Vincent, on the rim of crater Victoria, as seen by MER Opportunity (June 2007), vertical height $ 8 m (Credits: MER Mission, Cornell, JPL, NASA)

Fig. 7

 7 Fig. 7 Landing of MSL (artist view): at $20 m above the surface the rover is lowered on a tether from the descent stage (v < 1 m/s), and placed directly on the Martian surface (Credits NASA/JPL-Caltech)

Fig. 8

 8 Fig. 8 An artist view of SAM (Credits NASA/GSFC)

Fig. 9

 9 Fig. 9 Traverse path of Curiosity from Bradbury landing site to Glenelg. False colors correspond to the various terrains observed from orbit (Credits NASA/JPL-CalTech/UofA)

Fig. 13

 13 Fig. 13 An example of Evolved Gas Analysis (EGA) performed by SAM on a Rocknest sand sample (signal from the mass spectrometer in function of sample temperature), see also Mahaffy et al. 2013. When T is increased, various molecular species appear when the heating energy allows their release. (Credits NASA-GSFC/JPL-Caltech)

Fig. 15

 15 Fig.15At the center: the hole (diameter 1.6 cm, depth 6.4 cm) drilled by Curiosity into the rock John Klein; the shallower (2 cm depth) hole, on the right, corresponds to a test performed earlier, to test the drill. On can observe the difference between the red-brownish oxidized surface of the rock and the gray-greenish exhumed powder (Credits: NASA/JPL-CalTech/MSSS)

Fig. 17

 17 Fig. 17 Comparison between diffraction images obtained by CheMin experiment for Rocknest and John Klein samples. Phyllosilicates appear clearly for the rock sample; compare also results of EGA in the two cases in Figs. 13 and 16 (Credits NASA-GSFC/JPL-Caltech/Ames)

Fig. 19

 19 Fig. 19 Image taken by 100 mm PanCam aboard Curiosity, looking at S-SW, on August 25 2012. In the proximity, one observes a band of dark dunes (cf Fig. 5) and, further on, layered hills at the base of Mount Sharp. The top ridge is about 15 km away from the rover (Credits NASA/JPL-Caltech/MSSS)


Shenghui Liu

Philippe Beillas

Li Ding

Xuguang Wang
email: xuguang.wang@univ-eiffel.fr

Xiaogai Li

Fengfeng Xi

PIPER adult comfort: an open-source full body human body model for seating comfort assessment and its validation under static loading conditions

Keywords: human body model, finite elements, seating comfort/discomfort, validation, pressure/ force distribution, open source

Introduction:

In this paper we introduce an adult-sized FE full-body HBM for seating comfort assessments and present its validation in different static seating conditions in terms of pressure distribution and contact forces.

Methods:

We morphed the PIPER Child model into a male adult-sized model with the help of different target sources including his body surface scans, and spinal and pelvic bone surfaces and an open sourced full body skeleton. We also introduced soft tissue sliding under the ischial tuberosities (ITs). The initial model was adapted for seating applications with low modulus soft tissue material property and mesh refinements for buttock regions, etc. We compared the contact forces and pressure-related parameters simulated using the adult HBM with those obtained experimentally from the person whose data was used for the model development. Four seat configurations, with the seat pan angle varying from 0 °to 15 °and seat-to-back angle fixed at 100 °, were tested.

Results:

The adult HBM could correctly simulate the contact forces on the backrest, seat pan, and foot support with an average error of less than 22.3 N and 15.5 N in the horizontal and vertical directions, which is small considering the body weight (785 N). In terms of contact area, peak, and mean pressure, the simulation matched well with the experiment for the seat pan. With soft tissue sliding, higher soft tissue compression was obtained in agreement with the observations from recent MRI studies.

Discussion: The present adult model could be used as a reference using a morphing tool as proposed in PIPER. The model will be published openly online as part of the PIPER open-source project (www.PIPER-project.org) to facilitate its reuse and improvement as well as its specific adaptation for different applications.

Introduction

Sustained loads on the soft tissue of the buttocks may cause seating discomfort or even physiological problems such as pressure ulcers [START_REF] Gefen | How much time does it take to get a pressure ulcer? Integrated evidence from human, animal, and in vitro studies[END_REF]. Along with pressure, soft tissue deformations and internal loading in terms of compression, strain, and stress are generally considered relevant for seating discomfort assessment [START_REF] De Looze | Sitting comfort and discomfort and the relationships with objective measures[END_REF]. However, these internal biomechanical parameters cannot be directly measured in vivo. With the development of computational capability, different biomechanical models such as finite element (FE) models [START_REF] Verver | A finite element model of the human buttocks for prediction of seat pressure distributions[END_REF][START_REF] Siefert | Virtual optimisation of car passenger seats: Simulation of static and dynamic effects on drivers' seating comfort[END_REF][START_REF] Grujicic | Seat-cushion and soft-tissue material modeling and a finite element investigation of the seating comfort for passenger-vehicle occupants[END_REF][START_REF] Al-Dirini | Development and validation of a high anatomical fidelity FE model for the buttock and thigh of a seated individual[END_REF][START_REF] Beaugonin | ESI unique human model for seat (dis) comfort evaluation[END_REF], multibody modeling with [START_REF] Rasmussen | Computational analysis of the influence of seat pan inclination and friction on muscle activity and spinal joint forces[END_REF][START_REF] Grujicic | Seat-cushion and soft-tissue material modeling and a finite element investigation of the seating comfort for passenger-vehicle occupants[END_REF] and without [START_REF] Campos | Pressure sensing of an aircraft passenger seat with lumbar control[END_REF][START_REF] Zhong | Three-Dimensional biomechanical modeling for sitting contact stress analysis[END_REF] considering musculoskeletal systems are built for seating dis/comfort assessment or seat design. To estimate the internal biomechanical parameters such as soft tissue deformation, deformable finite element (FE) human body models (HBM) are required (see a review by [START_REF] Savonnet | Finite element models of the thighbuttock complex for assessing static sitting discomfort and pressure sore risk: A literature review[END_REF]. There are mainly two types of FE models: partial thigh-buttocks models built from medical images [START_REF] Verver | A finite element model of the human buttocks for prediction of seat pressure distributions[END_REF][START_REF] Al-Dirini | Development and validation of a high anatomical fidelity FE model for the buttock and thigh of a seated individual[END_REF][START_REF] Moerman | Engineering on the importance of 3D, geometrically accurate, and subject-specific finite element analysis for evaluation of in-vivo soft tissue loads[END_REF][START_REF] Cheng | Considerations and experiences in developing an FE buttock model for seating comfort analysis[END_REF][START_REF] Macron | Development and evaluation of a new methodology for the fast generation of patient-specific Finite Element models of the buttock for sitting-acquired deep tissue injury prevention[END_REF], and full-body models from medical images for skeleton and 3D body scans for skin geometry [START_REF] Siefert | Virtual optimisation of car passenger seats: Simulation of static and dynamic effects on drivers' seating comfort[END_REF][START_REF] Grujicic | Seat-cushion and soft-tissue material modeling and a finite element investigation of the seating comfort for passenger-vehicle occupants[END_REF][START_REF] Du | Simulation of the interaction between driver and seat[END_REF][START_REF] Huang | Modeling of human model for static pressure distribution prediction[END_REF][START_REF] Lazarov | The casimir model for simulation in seating comfort applications -a status update for LS-DYNA[END_REF][START_REF] Beaugonin | ESI unique human model for seat (dis) comfort evaluation[END_REF][START_REF] Huang | Comfort assessment and optimization based on FE simulation for high-speed train seats: Comparison with different design parameters[END_REF]. The partial models can only be used for evaluating the seat pan, and boundary conditions such as external contact forces and position of the bones cannot be easily defined under real seating conditions. To evaluate the comfort of a complete seat, a full human body model (HBM) is needed. A few HBMs were proposed by some researchers [START_REF] Grujicic | Seat-cushion and soft-tissue material modeling and a finite element investigation of the seating comfort for passenger-vehicle occupants[END_REF][START_REF] Du | Simulation of the interaction between driver and seat[END_REF][START_REF] Huang | Modeling of human model for static pressure distribution prediction[END_REF][START_REF] Huang | Comfort assessment and optimization based on FE simulation for high-speed train seats: Comparison with different design parameters[END_REF] as well as by some engineering software companies [START_REF] Siefert | Virtual optimisation of car passenger seats: Simulation of static and dynamic effects on drivers' seating comfort[END_REF][START_REF] Beaugonin | ESI unique human model for seat (dis) comfort evaluation[END_REF]. However, for both existing partial and whole-body models, the validation was performed in comparison with pressure-related parameters under either very simplified conditions or a single specific seat condition. For instance, [START_REF] Huang | Modeling of human model for static pressure distribution prediction[END_REF] developed a HBM based on the Hybrid III 50th Percentile Male dummy to evaluate the comfort and design of automotive driving seats. They only used a rigid chair with two flat rectangular plates representing the seat pan and backrest to validate their model. Without considering soft cushion, human body interaction with the seat cannot be correctly simulated. [START_REF] Grujicic | Seat-cushion and soft-tissue material modeling and a finite element investigation of the seating comfort for passenger-vehicle occupants[END_REF] validated their full HBM with respect to the measured pressure distribution reported by [START_REF] Verver | A finite element model of the human buttocks for prediction of seat pressure distributions[END_REF] based on a male subject sitting on a sitting on a seat (rigid or with a soft cushion) with the feet unsupported. When using experimental data from a third party, personalizing an HBM is challenging, as the test subject geometry is typically not available. This makes comparisons between simulation and experiment difficult to interpret if the model's geometry and mass distribution deviate from the test subject. Also, the test conditions were very simplified and different from real seating conditions. In a more recent study looking at the comfort of seat cushion, [START_REF] Yadav | Analysis of seat cushion comfort by employing a finite element buttock model as a supplement to pressure measurement[END_REF] validated their buttock-thigh partial FE model simplified from the 50th male full body model from the Global Human Body Model Consortium against the pressures. Twenty participants were asked to sit on different cushions put on a flat table. The participant's feet and back were unsupported. Again, the test conditions were over-simplified, and the corresponding loadings were likely unrealistic. Moreover, seat configurations vary in real life depending on use conditions (e.g., at the office or during transportation). To assess the effect of seat design, an HBM should also be validated under more realistic seating conditions covering a large range of existing seat configurations. As the HBMs give the possibility of estimating internal loading, it is also desirable to verify the soft tissue compression as well, especially under the ischial tuberosities (ITs). As far as we know, none of the existing models simulated the soft tissue displacement away from the ischium once seated, which was observed with an open MRI system in recent years by several researchers [START_REF] Sonenblum | 3dimensional buttocks response to sitting: A case report[END_REF][START_REF] Sonenblum | 3D anatomy and deformation of the seated buttocks[END_REF][START_REF] Sonenblum | Measuring the impact of cushion design on buttocks tissue deformation: An MRI approach[END_REF][START_REF] Wang | A case study on the effects of foam and seat Pan inclination on the deformation of seated buttocks using MRI[END_REF]. More importantly, a human model should be developed within an identified application context. Application specific adaptation of the model is often needed. Existing HBM models for seating comfort evaluation are not open source, thus limiting access to model adaptations, improvements, and comparisons with other models. Therefore, we believe that there is a need to propose an open source HBM, which should be validated under real seating conditions for seating comfort application.

A seat is designed to accommodate multiple body sized persons. As a first step to provide a virtual seating assessment tool, we recently morphed the PIPER open source child model (PIPER Child model), initially developed for impact simulation (www.PIPERproject.org), into an adult male model [START_REF] Liu | Referral patterns and treatment delays in medulloblastoma: A large academic proton center experience[END_REF]. This paper will present how the morphed adult model was further adapted and validated using the experimental data collected from the same adult male on a reconfigurable seat. The choice of a subject specific modelling approach was done to enable the comparison of measurements without the bias of geometrical and mass differences. Both shear and normal components of contact forces, and pressure parameters, such as contact area, peak pressure, and pressure profiles, will be compared between FE simulations and experimental results. In addition, we will preliminarily analyze the effects of soft tissue sliding on soft tissue compression under the ITs. Beyond this work, other body sized models could be generated using existing personalizing tools such as the PIPER ones (www. PIPER-project.org).

This paper will first present the adult-sized FE full-body HBM, then the experimental data used for validation and finally the comparison between simulations and experimental observations. The limitations of the model and its future improvements will also be discussed.

2 Development of an adult male HBM for comfort

Model development and adaptions

Developing an HBM is a time-consuming and complex process. Therefore, we morphed the PIPER Child model [START_REF] Beillas | Development and performance of the PIPER scalable child human body models 14th int[END_REF] into an adult-sized model [START_REF] Liu | Referral patterns and treatment delays in medulloblastoma: A large academic proton center experience[END_REF]. The morphing target was a male aged 40 years old, 1.74 m in stature, and 79.5 kg in weight (see Supplementary Appendix SA1 for more detailed anthropometric dimensions). Different types of data were collected on the target person (Figure 1). The target person's spine and pelvis geometric models (Figure 1B) in a seated position with a seat pan to backrest angle (SP2BA) of 106 °(Figure 1A) were obtained from an MRI study by [START_REF] Beillas | The effects of posture and subject-tosubject variations on the position, shape and volume of abdominal and thoracic organs[END_REF]. His external body shape was scanned in a similar seating condition as the MRI study with a hand-held laser scanner (Figure 1D) in the present study. For the rest of the bones, we used the full-body skeleton of a male with a similar body size from the PIPER open-source repository (www.piper-project.org, subject ID LTE605), which was segmented from the CT scans in a supine posture (Figure 1C). The shape of the ribcage and the location of the scapulae of the target person were manually palpated in a standing posture (Figure 1F). We assumed the positions of these bones were symmetric, only the right side of the subject was palpated: 11 points along the clavicle, 26 points on the ribcage, 17 points on the scapula and 6 points on the sternum, especially showing the outlines of the rib cage, clavicle and scapula. The full skeleton model from the PIPER subject ID LTE605 was then manually positioned and the bones others than the spine and pelvis were morphed interactively using LS-PREPOST until the shape was close to the palpated points and fitted within the skin envelope. Then, this complete skeleton was used as a target for the child morphing.

Once the full skeleton and skin shape were defined for the target person in a seated posture, we used the batch Kriging option of the PIPER software to facilitate the morphing. When morphing the child model into our target male adult, at first, only the node coordinates were changed and the mesh was kept the same (number of elements, connectivity). For the bones as well as for the regions away from the regions of interest, a same mesh was kept. For the regions of interest for comfort application such as the buttocks, we refined the mesh size of the soft tissue under the sitting bones with an element size of around 10 mm (Figure 1H). Then, the mesh was simplified in regions of limited interest (e.g., internal organs) and made symmetrical. As the internal organs of the abdomen (solid organs) and brain are away from the region of interest for seating, they were removed to save computational time. They were replaced by a constant pressure volume (linear_airbag_fluid) and their mass was distributed onto the envelope. This approach was already used for some parts of the abdomen and is used in other models (e.g., [START_REF] Beillas | Effect of contact separation on the abdominal response to impact of a human body model[END_REF]. To maintain the posture, we constrained the following bones into a single rigid body so that no relative movements were allowed between them: the skull and cervical spine, the calf and foot (left and right), the upper limb bones including the humerus, radius, ulna, hand (right and left bones). Altogether, this reduced the number of deformable elements in the model from about 488,000 to 395,000. As there were different targets for the skin and bones, the soft tissue thickness changed between the child and the adult. For the soft tissue below the IT, as no direct measurement was available from the target subject, a thickness of 40 mm was selected in line with the range from [START_REF] Wang | A case study on the effects of foam and seat Pan inclination on the deformation of seated buttocks using MRI[END_REF]. The adult HBM is shown in Figure 1G.

Regarding the structure of the model, most initial choices in the original child model were kept. Joints for which there is only an interest in terms of kinematic contribution are modelled using 6 degrees of freedom beams (e.g., lumbar and thoracic spine, elbow, knee and ankle). Others are modelled using contacts, ligaments and capsules (e.g., shoulder, hip). These choices are expected to be compatible with the current application, i.e., they allow model positionning and postural change as well as load transfer between skeletal structures during simulation. The simple model used for the spine is also compatible with the choice that the load in the intervertebral discs is not investigated. Most soft tissues are attached to their bone using coincident nodes. This helps with compressive load transfer while keeping the model simple. Exceptions include 1) the scapula which can slide on ribcage 2) tissues near the joints (e.g. shoulder, hip, elbow, knee) which can slide with respect to the skeleton as a direct attachment would stiffen the response and 3) the tissues near the ITs as it was attempted to simulate the muscle sliding observed in some recent MRI studies [START_REF] Sonenblum | 3D anatomy and deformation of the seated buttocks[END_REF][START_REF] Sonenblum | Measuring the impact of cushion design on buttocks tissue deformation: An MRI approach[END_REF][START_REF] Wang | A case study on the effects of foam and seat Pan inclination on the deformation of seated buttocks using MRI[END_REF]. Finally, the muscles and adipose tissues were lumped together in a homogeneous isotropic material called flesh as in the original model. This part is mainly loaded in compression. As the model is passive, muscle lines of action were not modelled and the beams initially representing them in the neck were removed. The skin was simulated with a 1 mm thick shell elements with a deformable material.

Concerning material properties, the bones were simplified as rigid bodies and soft tissue properties relevant to the interaction with the seat were also adapted. The soft tissue properties of the child model's tabular law (MAT_SIMPLIFIED_RUBBER/ FOAM) were selected to be stable and adequate for high rate and high stress conditions encountered in crash. The initial child law was found too stiff for the low rate and gravity loading associated with seating comfort. Their properties were changed to a low modulus Neohookean model as in [START_REF] Janák | Personalization of human body models to simulate obese occupants in automotive safety[END_REF] based on the results from [START_REF] Comley | The compressive response of porcine adipose tissue from low to high strain rate[END_REF]. Their material properties are summarized in Table 1.

To enable soft tissue sliding under the ITs, the method implemented and tested in [START_REF] Beillas | Effect of contact separation on the abdominal response to impact of a human body model[END_REF] for another model was used. First, the soft tissues surrounding the ITs were separated and offset from them with a small arbitrary distance of 0.2 mm. Then, a contact allowing sliding while maintaining the distance between the two components was defined (tiebreak contact with option 4). Frictionless coefficients of friction (COF) between the soft tissues and the ischial bone were used. To study the effect of soft tissue sliding on soft tissue compression, a model without muscle sliding was also developed for comparison.

The mass distribution was aligned with [START_REF] Huang | Modeling of human model for static pressure distribution prediction[END_REF] by adjusting the density of soft tissues and bones. The head has 7.4% of the full body mass. For the upper limbs, the proportion is 4.4% for each side. The trunk and lower limbs make 51.5% and 16.2% of the body mass, respectively.

Model validation

Experiment with four seat configurations

To validate the model, an experiment was carried out with a reconfigurable experimental seat (Figure 2). The experimental seat can simulate different seat configurations thanks to 13 motorized adjustments and measure all contact forces [START_REF] Beurier | A new multi-adjustable experimental seat for investigating biomechanical factors of sitting discomfort[END_REF]. The participant was the target male for the model development. Two wooden flat rectangular plates covered by two different 50 mm thick foam cushions were used as seat pan (550 × 550 mm) and backrest (620 × 565 mm). To measure the contact pressures, two pressure mats (XSENSOR, X3 PRO V6, Canada) were attached to the foam with double-sided tape and clips. Four seat configurations (Figure 2) were tested by varying the seat pan angle (SPA) from 0 °(horizontal) to 15 °with the step increase of 5 °while the seat pan to backrest angle (SP2BA) was kept at 100 °corresponding to the seating configuration used in the MRI study by [START_REF] Beillas | The effects of posture and subject-tosubject variations on the position, shape and volume of abdominal and thoracic organs[END_REF]. They were selected to represent the existing seats used in transportation. For each configuration, the seat pan length was adjusted to keep the distance between the front seat edge and the knee hollow at around 40 mm. The participant could adjust the foot support height for comfortable seating.

Foam cushion property test and validation

The MAT_57 (Mat Low Density Foam) material model of LS-DYNA (Livermore Software Technology Corporation in Livermore, CA, United States) was used to simulate the seat and backrest cushions. To use this model, material density, compression curve, tensile Young's modulus, hysteretic unloading factor (HU), and shape factor for unloading (SHAPE) need to be known. The decay constant for modeling creep in the reloading (BETA) was set to the default value (0.0). The ISO standard (ISO:2439) was applied for the compression test of a 50 × 50 × 50 mm sample. The test was performed on an INSTRON machine with a constant compression velocity of 100 mm/min and stopped at 80% compression with respect to its initial thickness. For the tensile test, the ATSM 3574-17 standard was applied with a 12.5 × 25 × 100 mm specimen at a constant loading speed of 500 mm/min until its failure.

The HU and SHAPE factors were identified with the help of a FE foam model simulating the compression test (Supplementary Appendix SA2). By trial and error, we found that the simulated and experimental compression curves matched well with SHAPE and HU combinations of (8 and 0.65) and (5 and 0.65) for the seat pan and backrest cushions, respectively. The material property parameters of the seat and backrest foams are summarized in Table 1.

After identifying the foam parameters, the seat pan and backrest foam cushion properties were validated by simple compression tests with three different masses before applying them to seating simulations with the adult HBM. It was found that a scaling factor of 1.15 should be applied to the experimental stress-strain curves for both seat pan and backrest cushions (see Supplementary Appendix SA3).

Simulations with the adult HBM

The four experimental seating conditions were simulated with the developed adult HBM (Figure 3). The LS-DYNA R11 MPP explicit solver was used on 36 cores of a machine equipped with AMD EPYC 7F72 processors. The time step was 2 µs. Relaxation was used to help limit contact force vibrations. The results (pressure, forces, etc.) were gathered for a duration of 500 ms of seating process, which took about 1 h of elapsed time for each simulation.

Model prepositioning and boundary conditions

To facilitate comparisons, the seat pan and backrest cushions were simulated with a 609.6 × 609.6 × 50 mm block, the same as the pressure mats (48 × 48 sensors for each) used in the experiment for comparison purposes. The two cushions were meshed with hexahedron elements of 12.7 × 12.7 × 12.5 mm. The bottom surface of both cushions was fixed, while the footrest was set to rigid and fixed at the position of corresponding experimental conditions.

In the past, different coefficients of friction (COFs) were used between the human body and seat, with values varying from 0.1 to 0.6 [START_REF] Grujicic | Seat-cushion and soft-tissue material modeling and a finite element investigation of the seating comfort for passenger-vehicle occupants[END_REF][START_REF] Du | Simulation of the interaction between driver and seat[END_REF][START_REF] Al-Dirini | Development and validation of a high anatomical fidelity FE model for the buttock and thigh of a seated individual[END_REF][START_REF] Moerman | Engineering on the importance of 3D, geometrically accurate, and subject-specific finite element analysis for evaluation of in-vivo soft tissue loads[END_REF][START_REF] Yadav | Analysis of seat cushion comfort by employing a finite element buttock model as a supplement to pressure measurement[END_REF]. In this study, the surfaceto-surface contact was defined for the backrest and seat pan contacts with a COF of 0.1. A COF of 0.4 was used for the footrest and foot contact as suggested by [START_REF] Derler | Analysis of factors influencing the friction coefficients of shoe sole materials[END_REF].

For simulating the four seating conditions, a same initial posture prior to loading was used except for a global rotation around the lateral y-axis and the ankle joint angle (Figure 3). The HBM was rotated so that its back and thighs were parallel to the seat back and seat pan. The ankles were rotated so that the foot bottoms were parallel to the foot support surface. A gravity loading of 9.81 m/s 2 was applied.

Comparison parameters

The contact forces on the seat pan (Fx_SP, Fz_SP), footrest (Fx_ FS, Fz_FS), and backrest (Fx_SB, Fz_SB) were compared between simulations and experiments in both the global (GCS) and local (LCS) coordinate systems (Figure 2B). The sums of all vertical forces including body weight (Sum_Fz) and horizontal forces (Sum_Fx), which should be zero, were verified. In addition to the contact forces, pressure parameters were compared, including contact area (CA), mean pressure (MP), peak pressure (PP), and pressure proportions of four contact areas on the seat pan (P I to P IV ) (Figure 4). Due to uncertainty in pressure measurements, we applied a correction factor (fcorr) using the trial specific correction method proposed by [START_REF] Zemp | Seat pan and backrest pressure distribution while sitting in office chairs[END_REF]. The correction factors for the seat pan and backrest were calculated by comparing the integration of the pressure map over the contact area with the normal force applied on the seat pan or on the backrest measured with load cells. Two pressure profiles were defined

FIGURE 4

Definition of the four seat pan contact regions (P I to P IV ) from the lateral pressure profile of pressure distribution. X beg is the first row that the buttocks contacting the sensor mat, X end is the last row that the thighs contacting with the sensor mat X max is corresponding to the row of the peak pressure, X mid1 the middle between X max and X end , and X mid2 the middle between X mid1 and X end .

in the sagittal plane by summing matrix columns (lateral profile, SOC) and in the frontal plane by summing matrix rows (frontal profile, SOR). Four contact regions on the seat pan were defined from the lateral pressure profile of the seat pan.

To characterize the internal soft tissue deformation, particularly underneath the ITs, two regions of interest (ROI) were defined using two cylinders with a diameter of 20 and 50 mm (Figure 5) as used by [START_REF] Wang | A case study on the effects of foam and seat Pan inclination on the deformation of seated buttocks using MRI[END_REF]. In addition to the mean soft tissue thickness, the tissue volume reduction (R) in ROIs under the IT was computed using the loaded volume (once seated) and the initial volume before loading as follows: 4 Results

Overall, simulated contact forces matched well with experimental values and followed same trends when changing seating conditions (Figure 6). Their comparisons are summarized in Table 2. The average differences of the vertical forces were 16.6, -15.5 and 3.0 N respectively on the seat back (Fz_SB_G), seat pan (Fz_SP_G) and foot support (Fz_FS_G), which are very small compared to body weight (784.8 N). Simulated horizontal forces (Fx_SB_G, Fx_SP_G, Fx_FS_G) were also very close to experimental values, but slightly higher in absolute value with an average difference of -22.3, 20.5, and 6.3 N for the seat back, seat pan, and foot support. This also resulted in a slightly higher shear force on the seat pan (Fx_SP_L) with an average difference of 17.3 N. Note that both simulated and experimental values of Sum_Fx and Sum_Fz (including body weight), which should be zero, were less than 10 N.

For the pressure parameters (Table 3), the simulated pressure proportions in the four regions (P I to P IV ) defined in Figure 4 had a difference of 5.2%, -1.4%, -13.9%, and -3.1% with respect to the experimental ones. The differences in terms of contact area (CA), peak pressure (PP), and mean pressure (MP) were respectively 5.8%, -6.4%, and 1.2% for the seat pan on average. For the backrest, relative larger errors were observed due to the small magnitude of the pressures. The relative difference (D%) reached 20.2% (12258 mm 2 ), 62.1% (2.1 kPa), 3.4% (0.1 kPa) compared to experimental values for CA, PP, and MP on average.

Soft tissue deformations under the ITs were also calculated for the four seating conditions. The soft tissue thicknesses simulated with and without considering soft tissue sliding are compared in Table 4. Allowing soft tissue sliding reduced the soft tissue thickness under the IT from 17.7 to 15.4 mm and from 23 to 20.5 for the 20 and 50 mm ROIs, with corresponding soft tissue volume

FIGURE 5

Two regions of interest (ROI) at the ITs defined using two cylinders with a diameter of 20 and 50 mm, illustrated with the unloaded HBM for the SPA5 configuration. The cylinder axes were perpendicular to the seat pan surface and centered at the ischium point closest to the seat. The cylinders were positioned so that the external circle of its upper surface was in contact with the ischium.

FIGURE 6

Comparison between simulated and measured shear and normal forces on the seat pan (Fx_SP_L, Fz_SP_L), backrest (Fz_SB_L, Fx_SB_L) and foot support (Fx_FS_G, Fz_FS_G) for the four test conditions (SPA0, SPA5, SPA10, and SPA15). TABLE 2 Comparison between simulated and experimental contact forces (N) as well their differences (D) for the four seat conditions characterized by seat pan angle (SPA). Are shown the forces in the seat symmetry plane applied on the seat back (SB), seat pan (SP) and foot support (FS) in both global (G) and local (L) coordinate systems defined in Figure 2. Sum_Fx, Sum_Fz are the sums of all horizontal forces and vertical forces including body weight.

Seat

Discussion

In this paper, we compared the contact forces and pressurerelated parameters simulated using the adult HBM with those obtained experimentally from the person whose data was used for the model development. Four seat configurations, with the seat pan angle varying from 0 °to 15 °and seat-to-back angle fixed at 100 °, were tested.

Contact forces

Results show that the adult HBM could correctly simulate the contact forces on the backrest, seat pan, and foot support with an average error of less than 22.3N and 15.5 N in the horizontal and vertical directions, which is small considering the body weight (784.8 N). However, simulated horizontal forces on the three contact surfaces were systematically higher than experimental results, leading to a systematically higher shear force on the seat pan (Fx_SP_L). [START_REF] Wang | Effects of seat parameters and sitters' anthro-pometric dimensions on seat profile and optimal compressed seat pan surface[END_REF] investigated the relationship between seat contact forces and perceived discomfort. They found that a seat configuration with a self-selected seat pan angle reduced shear force on the seat pan. For the seat configuration SPA0 tested in the present work, the seat back angle was 10 °and the seat pan was horizontal. For a seat back angle of 10 °, the preferred seat pan angle was about 5.9 °on average according to [START_REF] Wang | Effects of seat parameters and sitters' anthro-pometric dimensions on seat profile and optimal compressed seat pan surface[END_REF]. Compared to a horizontal seat pan, the shear force on the seat pan (Fx_SP_L) could be reduced from 8.56% to 4.61% of body weight, a reduction of about 4% of body weight, representing a shear force of about 32 N on the seat pan surface for the target person in the present study. Due to the relatively small shear force compared to the normal one, especially on the seat pan surface, a small difference in shear may be of importance for discomfort perception.

Higher simulated horizontal forces may be due to an inappropriate choice of boundary conditions including loading, initial model positioning, coefficients of friction of different contact surfaces as well as due to the effects of passive models. Sitting was simulated by releasing the HBM, which was pre-positioned slightly above the seat without considering muscle activation in postural control. However, this simulation did not accurately represent how a person would sit on a seat. When a person sits into a seat, they will put their buttocks on the seat first, using their hands pressing the armrests or other fixed objects (e.g., steering wheel for car seat) to guide the body movement, and then move their back to the backrest. One or two small repositioning motions are often necessary if they are not comfortably seated initially. Repositioning not only re-adjusts the posture, but also may reduce the shear forces between body and seat. However, small repositioning is challenging to simulate by just releasing the body. This was not addressed in previous simulation studies either. Therefore, in the future, a more realistic seating process should be explored for HBM simulations.

The body seat interface in the present study was complex as the foam cushion was covered with an XSESNOR pad. The participants only wore shorts without any cloth on the upper body, which may have resulted in a COF difference between the seat pan and backrest contact surfaces. Possible effects of COFs on contact forces also need to be studied.

Pressure parameters

In terms of contact area, peak, and mean pressure, the simulation matched well with the experiment for the seat pan, i.e., the magnitudes of their differences were limited. The global contact area was always higher by simulation (5.8% on average, Table 3). The simulated contact areas under the thighs were however smaller than the experiment values if we compare the simulated and experimental pressure distributions (Supplementary Appendix SA4). One reason could be that the material properties of soft tissues might not be appropriate. According to [START_REF] Scott | The effects of body position on the material properties of soft tissue in the human thigh[END_REF], the apparent soft tissue material properties could be significantly different even for the same subject from different positions. The soft tissue of the adult HBM was simulated with an isotropic homogeneous Neohookean material not accounting for regional difference of properties, the presence of muscles, their fascia, or the possible sliding between structures. Another reason might be related to the initial state of soft tissues. Both soft tissue thickness and initial strain states in a sitting posture prior to loading are not known and there is limited data available in the literature. For the region under the ITs, the soft tissue thicknesses in the ROI of 20 mm and 50 mm were adjusted to 39 and 41 mm based on the observations from four males using an open MRI by [START_REF] Wang | A case study on the effects of foam and seat Pan inclination on the deformation of seated buttocks using MRI[END_REF]. The muscle tension, which is expected to be limited while sitting, is not known either.

As far as the pressure parameters of the seat back are concerned, much larger differences in contact area (SB_CA) and shape between simulation and experiment were observed compared to seat pan pressure parameters (Table 3; Supplementary Appendix SA4). The lower back was in contact with the seat in the simulation while this was not observed in the experiment in any of the four seat configurations. One reason could be that the HBM was positioned too backward on the seat pan, and that the actual subject's trunk-tothigh angle was larger than the 100 °used to pre-position the model. This makes the back less parallel to the seatback surface, resulting in a single-location contact in the upper back region. A larger contact area on the seat back may be one of the reasons for a lower backrest mean pressure (MP_SB). The simulated backrest peak pressure was 2.1 kPa larger than the experiment on average. The peak pressure occurred in the lower scapula area with the presence of sharp edges and very thin soft tissues. As explained in the model development, there was no ribcage and scapula in the MRI geometry information of the target subject. Target information was obtained by palpating the subject in a standing posture (Figure 1F). This could lead to errors in the shape and position of the ribcage and scapulae in the seated posture. In addition, muscle activity, which was not simulated, may play a more important role in human backrest interaction than in the human seat pan interaction due to the need to maintain the posture.

The pressure mapping system used in the present study had an accuracy of ±10% of scale for a range of 1.4-27 kPa according to the product description by the manufacturer. Similar to the correction method used by [START_REF] Zemp | Seat pan and backrest pressure distribution while sitting in office chairs[END_REF], we applied a correction factor to match measured normal forces on the seat pan and backrest. Because of this, comparing the simulated and measured pressure values needs to be done with care. We think it is more important that simulated pressures follow the same trends when changing seating conditions, which is the case for the present study.

Soft tissue deformation

Soft tissue deformation (especially those below the ITs) is considered to be one of the major contributing factors to discomfort and injury [START_REF] Ceelen | Compression-induced damage and internal tissue strains are related[END_REF]. Using an upright MRI system, some researchers recently observed that the gluteus muscle was displaced from the IT once seated [START_REF] Sonenblum | 3dimensional buttocks response to sitting: A case report[END_REF][START_REF] Sonenblum | 3D anatomy and deformation of the seated buttocks[END_REF][START_REF] Sonenblum | Measuring the impact of cushion design on buttocks tissue deformation: An MRI approach[END_REF][START_REF] Wang | A case study on the effects of foam and seat Pan inclination on the deformation of seated buttocks using MRI[END_REF]. However, to our knowledge, none of the existing models for comfort considered soft tissue sliding. In the study by [START_REF] Wang | A case study on the effects of foam and seat Pan inclination on the deformation of seated buttocks using MRI[END_REF], the FOAM condition, corresponding to a seat configuration with SPA of 7 °and SP2BA of 105 °using a foam of 50 mm on the seat pan, was close to the condition SPA5 in the present study. They observed that four male participants had a tissue volume reduction R varied from 54.8% to 69.6% for 50 mm ROI and from 60.2% to 74.7% for 20 mm ROI. For SPA5, simulated R values were 49.1% and 58.5% for ROIs of 50 mm and 20 mm using the model with soft tissue sliding. Without considering soft tissue sliding, we obtained smaller soft tissue compressions, which were 42.4%, and 51.5% for ROIs of 50 mm and 20 mm. Compared to the MRI observations from four subjects [START_REF] Wang | A case study on the effects of foam and seat Pan inclination on the deformation of seated buttocks using MRI[END_REF], the simulation showed the same variation trend when changing seating condition, but lower soft tissue compression ratio. The value R with sliding in ROI 20 mm was 57.6% for an experimental range between 60.2% and 74.7% observed from four subjects. Therefore, considering the large subject-to-subject variability, the simulation value can be considered close to the experimental range although on the lower side. The reasons for the subject-to-subject variability (despite similar initial thickness for some of them) are unknown but would be helpful to understand the modelling requirements. In the meantime, a sensitivity study on the initial soft tissue thickness, initial strain in the tissues due to gravity and material properties could certainly be of interest in the future. While the simulation of the sliding may not be perfect in the current implementation, the tissue sliding may therefore help simulate the tissue response in the ischial region and, from a modelling standpoint, help prevent the use of excessively soft material properties in the region to match the external compression.

Human body modeling

The level of anatomical details to be modeled depends on application. The open source approach enables future changes and evolutions in the structures using the model as a basis including by other researchers. The current modeling choices have to be considered as starting choices reflecting the first intended applications in transportation. This includes the simulation of the interaction with the seat pan and the seat back including reclined seating configuration. Partial buttock thigh models developed by [START_REF] Verver | A finite element model of the human buttocks for prediction of seat pressure distributions[END_REF][START_REF] Yadav | Analysis of seat cushion comfort by employing a finite element buttock model as a supplement to pressure measurement[END_REF] are unable to simulate the interaction with the seatback. Therefore, it is important to include the trunk and spine in the model. However, as we are not necessarily focusing on the internal loads in the spine, the modelling was simplified using either articulated vertebrae or rigid segments. This can be changed if needed.

Concerning the muscles, it was hypothesized that for relaxed, static seating (including reclined), the muscular activity would play a limited role, and that differences of muscular activities would be small between seating configurations. Hence, the muscles activity was not modelled and active muscles that were present in the neck of the original model were removed. This assumption could evolve depending on application (e.g., dynamic comfort, etc.).

The passive contribution of the muscles and other soft tissues was modelled but it was assumed that small internal differences in structures (e.g., separating fiber bundles, fascia) would not affect the forces (including shear), or pressure as long as the overall stiffness of the combined structure was modelled. The possible sliding of the structure was considered near the ischium based on recent observations by [START_REF] Wang | A case study on the effects of foam and seat Pan inclination on the deformation of seated buttocks using MRI[END_REF] and others.

In the present study, a same initial posture prior to loading was used to simulate the four seating conditions. As explained in Section 2.1, the initial curvature of spine prior to loading was derived from the target subject's MRI data in a seating posture from our previous study [START_REF] Beillas | The effects of posture and subject-tosubject variations on the position, shape and volume of abdominal and thoracic organs[END_REF]. In that study, the seat pan to seat back angle was 106 °, which is slightly more than in the current study (100 °for all conditions). However, the seat was only covered with a very thin foam while a 50 mm foam was used in the current study. The two seated postures are therefore expected to be similar, and no adjustment was made prior to the simulations. Only a global rotation around the lateral y-axis was applied so that the model's back and thighs were parallel to the seat back and seat pan surfaces for the four seat configurations. It is interesting to see that the loading by gravity resulted in a rearward pelvis tilt and a slight change of curvature in the lumbar and thoracic regions after loading (Figure 7). Recall that the

FIGURE 7

Comparison of simulated spine curvatures before and after loading for SPA0 and SPA15 in the seat coordinate, after alignment with respect to the skull and the pelvis.

cervical vertebrae and skull were constrained together as a rigid body and only the rest of spinal joints were allowed to rotate during loading.

Implication in developing a seating (dis) comfort assessment tool

Since the proposition of the conceptual model of sitting comfort and discomfort by [START_REF] Zhang | Identifying factors of comfort and dis-comfort in sitting[END_REF], it is well accepted that comfort and discomfort are two distinct concepts. Discomfort is associated with biomechanical factors such as posture, soft tissue deformation, and pressure distribution, while comfort is associated with feelings of relaxation and wellbeing. We, as many researchers, believe that discomfort could be explained by some objective parameters including pressure and other biomechanical parameters. [START_REF] Zemp | Are pres-sure measurements effective in the assessment of office chair comfort/discomfort? A review[END_REF] made an extensive review on the question whether pressure measurements are effective in the assessment of office chair comfort/ discomfort. Though the question cannot be definitively answered due to the limited availability of data, they suggest that the peak pressure of the seat pan, the pressure distribution of the backrest and the pressure pattern changes (seat pan and backrest) appear to be reliable for quantifying comfort or discomfort. The adult model developed in the present study is just a first step, further developments and more research is needed to achieve a virtual seating comfort assessment tool. To better represent a target sitter population, models of various sizes have to be generated. The present adult model could be used as a reference using a morphing tool as proposed in PIPER. More importantly, relevant biomechanical evaluation criteria are still missing. This requires investigations, which establish the relationship between subjective feeling and objective biomechanical parameters.

Limitations

First, as a first validation study, only four seat configurations were tested with the seat pan angle varying from 0 °to 15 °while the seat pan to backrest angle was fixed to 100 °. Therefore, in the present study, the same spinal curvature including the pelvis was used to define the model's initial sitting posture. More reclined seating conditions for relaxing postures need to be validated.

Secondly, boundary conditions including model's prepositioning, loading process, and COFs, need to be further investigated. Measuring the seated body position by palpating anatomical landmarks could also be helpful in defining a more realistic model initial position for simulation.

Thirdly, as already mentioned, the model's trunk including the scapulae and ribcage may need to be further improved. An appropriate trunk model will be important for the seats with an increased ability to recline, as more body weight will be supported by the seat back. In the present study, we focused more on the model's adaptation to study the loading of the body by the seat pan. As the developed adult HBM will be open-sourced and accessible to other researchers, further model improvement will be facilitated.

Fourthly, the seat pan and backrest cushions were tested under well-controlled compressions by three masses for validating the identified foam properties (Supplementary Appendix SA3). Simulated compressions were higher than experimental values for two higher masses. A scaling factor of 1.15 was therefore applied to the initial foam stress-strain curves for a better match. The exact source of this discrepancy is unknown. Further investigation is needed to understand its causes and to characterize foam properties. Nevertheless, we verified that a 15% difference on the foam properties had almost no effect on soft tissue compression at the IT area and very limited effect on contact forces and pressure parameters. The maximal difference in contact forces was less than 7 N, while the differences in peak pressure were -0.9 kPa (-6.4%) and 2.1 kPa (62.1%) for the seat pan and backrest cushions.

Lastly, the effect of soft tissue material properties is not investigated. A sensitivity study is needed to understand how these parameters affect the responses to the loading when seated. These effects should also be put in the perspective of variations expected for a large population of seat users.

Conclusion

Developing an adult HBM is time-consuming and collecting all the required data from a same target person in a seated position is difficult. In this work, we developed a mid-sized male adult full-body model by morphing the PIPER Child model with target information from different sources. To the best of our knowledge, this is the first validation study of an adult HBM against contact forces and pressure parameters in several seat configurations used in offices and transports. We also introduced soft tissue sliding under the ITs for the first time in a model, which resulted in a higher soft tissue compression in agreement with recent observations using an upright MRI by several researchers. To facilitate its reuse and improvement, the developed adult HBM, called PIPER adult comfort (version 1.0), will be released at the time of publication under the same opensource license as the PIPER Child model (GPL v3 license with open science and liability clauses) at the PIPER repository (http://piperproject.org/).

FIGURE 1

 1 FIGURE 1 Data sources for model development: (A) Experimental seating set-up-used in the original MRI study by Beillas et al. (2009)-was reused for the body scan, (B) Partial skeleton of the target subject in the target sitting posture from Beillas et al. (2009), (C) Full skeleton in supine posture of the subject LTE605 from the open PIPER datasets (www.PIPER-project.org), (D) Body scan in the same sitting posture as in (A), (E) Processed target skin surface in the target position after smoothing and anonymizing, (F) Palpating the scapular position with the subject in standing, (G) Morphed adult-sized full-body model (H) Detail of the buttock's region near the ischial tuberosity with a higher mesh density (about 10 mm for mesh size).

FIGURE 2

 2 FIGURE 2 Experimental set-up (A) and definition of the global (GCS) and local coordinate systems (B).

FIGURE 3

 3 FIGURE 3Pre-positioned adult HBM for the four experimental seating conditions (SPA0, SPA5, SPA10, and SPA15).

TABLE 1

 1 Material properties.

	Materials	Type	Density (kg/mm 3 )	Parameters
	Bones	Rigid (MAT_020)	1.70E-06	Young's modulus	6 GPa
				Poisson's ratio	0.3
	Soft tissue	Neohookean (MAT_077)	9.00E-7	Poisson's ratio	0.499
				Shear modulus	Mu1 3E-6 GPa
					Alpha1 2.0
	Skin	Elastic (MAT_034)	1.00E-6	Young's modulus	2.5E-3 GPa
				Poisson's ratio	0.45
	Seat pan foam	low desnity foam (MAT_057)	5.00E-8	Tensile Young's	1.04E-4 GPa
				HU	0.65
				SHAPE	8
	Backrest foam	low desnity foam (MAT_057)	4.90E-8	Tensile Young's	8.75E-5 GPa
				HU	0.65
				SHAPE

TABLE 3

 3 Comparison between simulated and experimental pressure-related parameters for the four seat conditions characterized by seat pan angle (SPA), including contact area (CA), peak pressure (PP) and mean pressure (MP) on the seat pan (SP) and back (SB). P I to P IV are the pressure proportions in the four regions of seat pan pressure defined in Figure4. Fcorr_SB and fcorr_SP are the correction factors of the measured pressures on the seat back and seat pan.

	Seat		CA_SB	PP_ SB	MP_ SB	CA_ SP	PP_ SP	MP_ SP	P I	P II	P III	P IV	fcorr_SB fcorr_SP
			(mm 2 )	(kPa)	(kPa)	(mm 2 )	(kPa)	(kPa)	(%)	(%)	(%)	(%)
	SPA0	Exp	40,968	3.2	1.9	168,225	13.4	4.1	36.7	42.4	13.4	7.5	1.76	1.38
		Sim	46,935	4.9	1.9	176,774	13.1	4.2	39.2	42.7	11.3	6.7
		D	5,968	1.8	0.0	8,548	-0.2	0.0	2.5	0.3	-2.1	-0.7
		D%	14.6	55.6	1.6	5.1	-1.7	0.6	6.5	0.8	-18.8	-10.9
	SPA5	Exp	49,355	3.5	2.1	165,484	14.1	4.2	40.4	40.9	12.1	6.5	1.69	1.48
		Sim	65,968	5.4	2.1	174,354	13.2	4.2	39.1	42.4	11.5	6.9
		D	16,613	1.9	0.0	8,871	-0.9	0.0	-1.3	1.5	-0.6	0.4
		D%	34	54.4	-1.0	5	-6.2	0.0	-3.2	3.4	-5.3	6.0
	SPA10	Exp	71,451	3.3	2.0	165,967	14.7	3.9	37.2	43.1	13.8	6.0	1.81	1.54
		Sim	78,387	5.8	2.3	176,451	12.2	4.0	42.2	39.8	11.4	6.7
		D	6,935	2.5	0.3	10,484	-2.5	0.0	5.0	-3.3	-2.4	0.7
		D%	9.7	76.2	15.6	6.3	-17.3	0.5	11.9	-8.3	-21.2	10.3
	SPA15	Exp	80,806	3.8	2.5	158,871	12.1	3.8	40.9	42.5	11.5	5.1	1.67	1.64
		Sim	100,322	6.2	2.4	168,871	12.2	3.9	43.5	41.8	10.4	4.3
		D	19,516	2.4	0.0	10,000	0.2	0.1	2.5	-0.7	-1.1	-0.8
		D%	24.2	62.5	-1.3	6.3	1.4	3.9	5.8	-1.6	-10.5	-17.7
	All	Exp	60,645	3.4	2.1	164,637	13.6	4.0	38.8	42.2	12.7	6.3	1.73	1.51
		Sim	72,903	5.6	2.2	174,113	12.7	4.1	41.0	41.7	11.1	6.2
		D	12,258	2.1	0.1	9,476	-0.9	0.0	2.2	-0.5	-1.6	-0.1
		D%	20.2	62.1	3.4	5.8	-6.4	1.2	5.2	-1.4	-13.9	-3.1

TABLE 4

 4

	ROI 20 mm	ROI 50 mm

Comparison of simulated soft tissue thicknesses (mm) under the ischial tuberosity before (T_preload) and after loading (T_loaded) for the four seat conditions. The simulations with (Y) and without (N) considering tissue sliding are compared. The average tissue thicknesses were calculated in two ROIs defined in Figure

5

.

Frontiers in Bioengineering and Biotechnology frontiersin.org

Funding

The SL of this work had a scholarship from the China Scholarship Council (grant number 202006020192).

Data availability statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.

Ethics statement

The studies involving human participants were reviewed and approved by IFSTTAR (French Institute of Science and Technology for Transport, Development and Networks, now University of Gustave Eiffel) ethical committee. The patients/participants provided their written informed consent to participate in this study.

Author contributions

SL and PB constructed the finite element model. SL performed the study and wrote the first draft of the manuscript. XW was responsible for the conception of the work. LD was the supervisor of

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher's note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fbioe.2023.1170768/ full#supplementary-material


Guy Monteil

Xavier Roizard
email: xavier.roizard@ens2m.fr

Fabien Gréban

Hamid Makich

Description of an in-situ method to measure the punch wear during blanking

Keywords: wear, punch, blanking, thin layer activation

Introduction

In the blanking process, the main problems are linked to the final appearance and the functionality of the blanked parts. In the field of electronic industry, the quality of the blanking process is closely linked to the presence of the burrs, which can be responsible for electric failures. In order to avoid such problems, it is necessary to predict the growth of the burrs during the blanking process.

As this growth has been proved to be related to the wear of the blanking tools and mainly to the punch wear, [1,2] ,i ti s therefore needed to monitor the punch wear all along the blanking process. The various methods available for this purpose, like the measurement of the blanked profile [3,4] or of the worn contour of the punch, [5 -10] suffer major limitations.

In order to obtain a quantitative wear indicator, one must either wait until the end of the process or dismantle the blanking tool during the blanking process at regular intervals.

The first method requires a long time to be done and additionally, can be very expensive. The second one is questionable because when dismounting and remounting the tool, the punchdie clearance must be set at the same value because it influences dramatically the blanking conditions. [3] Consequently, efforts have been undertaken to overcome these limitations by setting up a technique which allows the punch wear to be measured directly on the punch without dismounting the blanking tool. The only available and sufficiently accurate method is the thin layer activation.

This article presents the application of the thin layer activation to the in situ wear measurement of a punch during blanking.

Thin Layer Activation Technique

The thin layer activation technique has been used in a wide range of wear measurements. [11 -18] In most of the cases encountered in literature, it has been applied in large systems. [19,20] Its advantage is the ability to measure the wear continuously [20] or quasicontinuously. [START_REF] Natter | Etude de l'usure et de l'écaillage de pièces de réacteur d'avion par la technique d'activation en couches superficielles[END_REF] In this last method, as the measured activity is proportional to the square root of the distance care has to be taken to precisely reposition the counting detector.

The principle of this method is to realize a selective activation by means of a particle beam on a precise area of the part to be monitored. This activated volume of material then emits radiations, which can be measured by means of a photo detector. The intensity of these emitted radiations is proportional to the remaining activity of the source, i.e. the activated part. As the residual activity is coming from the only remaining activated material, the difference between the original activity and the measured one is an indication of the removed quantity of the material. Consequently, if one is able to transmute the material of the punch on a selected area and ensure a proper set of measuring conditions, it will be possible to monitor the punch wear.

Thin Layer Activation of the Punch

The practical case under study is the blanking of electronic lead frames. The particular conditions of this process are linked to the dimensions of the blanked parts and the very limited amount of wear even after a million produced parts. For these reasons, the

The purpose of this study is to develop a new method in order to measure the punch wear on a real stamping press during blanking. This in situ measuringmethod has been used formeasuring the wear of tungsten carbide punches during blanking of thin copper alloy sheet steels (approximately 1/10 mm). As the quality of the final parts is largely influenced by the dimensions of the punch, the wear of the punch as low as a fewmicrometers must be avoided and consequently measured. So the difficulties to measure such a small wear amount on a high speed real industrial process (800 p/min) associated with the small overall dimensions of the punch is a great challenge. None of the existing measuring techniques (mainly geometrical) can ensure a continuous wear measurement without dismantling the blanking punch. The wear measurement technique developed for this study is the thin layer activation technique. The working area of the punch has been activated by means of a particles beam. The decay of the activity of the punch allows the punch wear to be monitored. Care has been taken to obtain a precisemeasurement of the punch wear during blanking. The results of the measurements of punch wear on an industrial stamping press are presented for various working conditions. For example, the influence of the punched material and the amount of lubrication upon the wear kinetics are described.

measurement of the wear of the tools proves to be very difficult and traditional dimensional measurements are not suitable. These conditions are also a great challenge for the thin layer activation technique because as the parts are small, the punch is also very small and the small amount of worn material requires a high precision in the final wear measurement.

The punch to be activated is made of tungsten carbide-cobalt alloy. For a set of reasons, the tungsten element has been chosen. The punch has been exposed to a beam of deutons. Tungsten is then transmuted into various species according to their isotopic abundance.

Only two created species are available for measuring the wear of the punch because of their half-lives, 183 Re and 184 Re. The decay of the activity of small species with half-lives allows the measurement to be made during the blanking of a million parts (three weeks) without interference. In order to limit the detrimental consequences of the activation of the punch, precautions have been taken to minimize the level of activity generated on the punch to limit the toxicological risks associated with the handling of the parts. Thus with the help of previous measurements of the maximum wear depth expected after a million blanked parts the activation volume has been set to a height of 50 µm and a depth of 100 µm on the cylindrical punch usedforthisstudyasshowninFig.1.

As mentioned previously, a cylindrical punch has been chosen because on such a form the operating conditions at the contact are the same all along the cutting edge of the punch. So the wear distribution is uniform on the activated zone of the punch and then it is allowed to obtain a quantitative result like a wear volume or a wear depth. A measuring set up where the scintillation detector measuring the γ -r a y si sv e r yc l o s et ot h e irradiated element has been designed since the measured intensity decreases with the square of the distance between the source and the detector (depending of solid angle). Precautions have been taken (Fig. 2) to ensure a constant position of the detector (±3.5 µm) when measuring at each stop of the press (every 10 000 parts).

The measured data represent the activity with respect to the number of blanked parts. In order to convert the loss of activity measured on the punch into a worn volume of material, a preliminary calibration has been done by activating a stack of tungsten sheets of 6-µm thickness under the same conditions as the punch. With this reference material, it is allowed to measure the remaining activity while pulling out the sheets one by one in order to give rise to a curve of the relative activity according to the thickness of the tungsten sheet stack (Fig. 3).

Given the linear law of the decay of the activity with respect to the thickness by means of the sheet stack it can be concluded that on the first 50 µm the activity is constant. Consequently, after the corrections described above, the percentage of lost activity can be directly related to the percentage of worn volume. The natural decay of the activity with respect to time of the tungsten has also been subtracted from the measured loss of activity of the punch, giving rise to the value of the lost activity only coming from the wear of the punch. At least a correction has been introduced also because of the variation of the distance detector/source associated to the thermal expansion of the press.

Results

The blanking tests have been driven on a 50 tons BRUDERER press with the operating conditions listed in Table 1. In order to evaluate the efficiency of this technique, it has been decided to check out the effect of changing various parameters of the blanking process on the wear kinetics. Two operating parameters have been studied, namely, the blanked material and the lubrication conditions. The wear kinetics is calculated from the slope of the linear regression of the punch wear volume versus number of stroke curve upon 50 000 blanked parts.

Figure 4 illustrates the results of the influence of the blanked material on the worn volume with respect to the number of blanked parts. Two copper alloys have been chosen because they have been previously identified as exhibiting very different behaviors on a tribometer test (material A and B). Material A is a copper alloy free of precipitates and material B is a copper alloy with a high level of precipitates. Three different phases can be identified: the first one and the last one which have the same slope and correspond to material A, and the second showing a higher level of the slope in relation to the change for material B known to be more abrasive. The wear volume is nearly 3 times higher with material B as shown by the slope value.

In addition, as the wear kinetics monitored for material A before and after the material change is the same it can be concluded that the change in wear kinetics can be attributed to the only effect of material B.

In order to study the influence of the lubrication conditions, material A has been blanked with two different oil feeding quantities: on one hand the oil flow has been reduced by 40% and on the other hand the amount of lubricant has been increased by 40%. In order to check the only effect of these lubrication conditions, the standard lubrication conditions have been monitored before and after these tests.

The results from Fig. 5 show that the thin layer activation is able to discriminate between various conditions of lubrication: it can be seen that an amount of lubrication reduced by 40% gives rise to an increase of the wear kinetics of the punch of 1.5. On the contrary, when lubrication is increased by 40%, the wear is lowered by a factor greater than two.

Conclusions

A blanking punch has been activated selectively on its cutting edge. By a calibration carried out with a stacking of tungsten sheets, it has been allowed to calculate the volume of wear of the punch corresponding to the loss of activity. From the measurements of the loss of activity of the punch, it has been shown that the influence of the parameters of the blanking process, like the nature of the blanked material or the amount of lubricant, can be quantified. This method exhibits a good precision, a high sensitivity to a change in the tribological conditions. There is no other method available which can measure the wear of the tool during blanking where there is no necessity to dismount the blanking tool and to consequently change the geometry of the blanking tool. This method has also been applied for the first time in the field of blanking of very small parts.

This method is now available for a systematic study of the influence of the blanking parameters on the wear of the tools with a reduced number of blanked parts, typically about 1-5% of the number of blanked parts between two sharpening.

Figure 1 .

 1 Figure 1. Schematic representation of the activated punch.

Figure 3 .Figure 2 .

 32 Figure 3. Relative activity with respect to the thickness decrease of the tungsten sheet stack.

3 )Figure 4 .

 34 Figure 4. Influence of the blanked material on the punch worn volume (materials A and B).

3 )Figure 5 .

 35 Figure 5. Influence of the lubrication conditions on the punch worn volume (material A).

Table 1 .

 1 Technical data of the blanking test

	Punch material	WC/Co	Blanked material	Copper alloy
	Punch geometry	Cylindrical	Sheet thickness	0.254 mm
	Punch diameter	3.7 mm	Lubrication	3.3 g/m 2
	Clearance per side	20 µm	Cadency	500 parts/min