Mohamed Kharrat
email: mohamed.kharrat@insa-rouen.fr

Vincent Placet

Emmanuel Ramasso

Lamine Mohamed

M L Boubakar

Influence of damage accumulation under fatigue loading on the AE-based health assessment of composite materials: Wave distortion and AE-features evolution as a function of damage level

Keywords: A. Carbon fiber, B. Fatigue, B. Fracture, D. Acoustic emission

Influence of damage accumulation under fatigue loading on the AE-based health assessment of composite materials : wave distortion and AE-features evolution as a function of damage level

Introduction

During the last decades, an increasing use of Organic Matrix Composites (OMC) has been noted in several application fields such as aerospace, civil engineering, and automotive industry due to the improvement of their mechanical properties [START_REF] Berthelot | Composite materials: mechanical behavior and structural analysis[END_REF][START_REF] Miracle | Introduction to composites[END_REF][START_REF] Mouton | Organic materials in civil engineering[END_REF]. In order to ensure the structural integrity of OMC structures in real time, continuous condition monitoring techniques have been developed, in particular based on Acoustic Emission (AE) [START_REF] Harris | Fatigue and accumulation of damage in reinforced plastics[END_REF][START_REF] Berthelot | Acoustic emission in carbon fibre composites[END_REF][START_REF] Unnthorsson | Acoustic emission based fatigue failure criterion for CFRP[END_REF][START_REF] Eaton | Acoustic emission source location in composite materials using Delta T Mapping[END_REF]. When the structure is subjected to solicitations, a stress field is generated leading to microstructural changes. A transient elastic energy is then released, producing ultrasonic waves that propagate through the material. This wave propagation involves a surface vibration that can be measured using appropriate sensors. The form and characteristics of the generated transient signals are directly dependent on the local fracture mechanisms, such as delamination, matrix cracking, fiber/matrix debonding, fiber breakage and fiber pull-out. Each damage mechanism is characterized by a specific acoustic signature. The AE technique can be used to discriminate between these damage mechanisms by analyzing the AE signals using appropriate pattern recognition methods, based on prior knowledge of their acoustic signatures.

A pattern recognition method requires the extraction of the relevant AE features from the AE hits (such as amplitude, energy, frequency content). Groups of AE features, representing damage mechanisms, are then created using either supervised [START_REF] Averett | Comparative post fatigue residual property predictions of reinforced and unreinforced poly (ethylene terephthalate) fibers using artificial neural networks[END_REF][START_REF] Maillet | Damage monitoring and identification in SiC/SiC minicomposites using combined acousto-ultrasonics and acoustic emission[END_REF], unsupervised [START_REF] Sause | Quantification of failure mechanisms in mode-I loading of fiber reinforced plastics utilizing acoustic emission analysis[END_REF][START_REF] Doan | An unsupervised pattern recognition approach for AE data originating from fatigue tests on polymer-composite materials[END_REF][START_REF] Ramasso | Unsupervised consensus clustering of acoustic emission time-series for robust damage sequence estimation in composites, Instrumentation and Measurement[END_REF] or partially-supervised [START_REF] Ramasso | Health assessment of composite structures in unconstrained environments using partially supervised pattern recognition tools[END_REF] classification methods. The common bedrock of these methods is the use of the AE features while assuming the acoustic signatures unchanged during the fatigue life of the material. However, the acoustic signatures of the damages may evolve due to several factors. Among the factors of influence that have been relatively well studied in the literature, we can mention: sensors technology and associated electronics [START_REF] Higo | The general problems of AE sensors[END_REF][START_REF] Barbezat | Acoustic emission sensor properties of active fibre composite elements compared with commercial acoustic emission sensors[END_REF][START_REF] Sause | Finite element modeling of conical acoustic emission sensors and corresponding experiments[END_REF], the coupling between the sensor and the material [START_REF] Theobald | Couplants and their influence on AE sensor sensitivity[END_REF][START_REF] Geisse | Improving the coupling reproducibility of piezoelectric transducers[END_REF], the distance between the acoustic source and the sensor [START_REF] Scholey | Design considerations for the acoustic emission testing of large composite specimens[END_REF][START_REF] Aggelis | Effect of plate wave dispersion on the acoustic emission parameters in metals[END_REF][START_REF] Aggelis | The influence of propagation path on elastic waves as measured by acoustic emission parameters[END_REF], the physical properties of the material including material symmetries and wave propagation characteristics (dispersion, attenuation, etc.) [START_REF] Prosser | The propagation characteristics of the plate modes of acoustic emission waves in thin aluminum plates and thin graphite/epoxy composite plates and tubes[END_REF][START_REF] Verbis | Wave dispersion and attenuation in fiber composites[END_REF][START_REF] Lowe | Properties of guided waves in composite plates, and implications for NDE[END_REF][START_REF] Asamene | Influence of attenuation on acoustic emission signals in carbon fiber reinforced polymer panels[END_REF]. The geometry of the specimen is also an important parameter involving the edge effect, the dimensions of the specimens, as well as the interactions with existing discontinuities (holes, rivets, etc.). They potentially lead to wave dispersions and distortions of the AE waveforms [START_REF] Scholey | Quantitative experimental measurements of matrix cracking and delamination using acoustic emission[END_REF][START_REF] Sause | Influence of internal discontinuities on ultrasonic signal propagation in carbon fiber reinforced plastics[END_REF]. Particularly, the effect of damage accumulation during mechanical loading on the AE features could be substantial. It has been demonstrated that the captured waveforms during tests performed on cementitious materials [START_REF] Carpinteri | Influence of damage in the acoustic emission parameters[END_REF] and bearings [START_REF] Al-Dossary | Observations of changes in acoustic emission waveform for varying seeded defect sizes in a rolling element bearing[END_REF] have been altered by the damage growth and therefore, the AE features have been influenced. Actually, the discontinuities emerging in the material due to damage disrupt the wave propagation modes and could therefore contribute to a distortion in the detected AE transients emitted from defects. This phenomenon is besides exploited in the nondestructive testing field for damage assessment using Acousto-Ultrasonics (AU).

The AU technique has been developed since several decades as a nondestructive tool for the evaluation of the mechanical properties and the quantification of damage in composite materials. The principle of the technique consists in the excitation of the monitored structure by a particular wave packet emitted from an ultrasonic transducer; a receiver is used to collect the signals transmitted through the damaged area of the material. The recorded waveforms are acquired and processed using an AE-system [START_REF] Vary | The Acousto-Ultrasonic approach[END_REF][START_REF] Kharrat | Non-destructive evaluation of anchorage zones by ultrasonics techniques[END_REF]. The distortion of the waveforms is then studied based on the impact of damage on their AU parameters (AUPs), relatively to the healthy state of the structure. A. Vary has used the AU for the assessment and quantification of damages in composites materials [START_REF] Vary | An ultrasonic-acoustic technique for nondestructive evaluation of fiber composite quality[END_REF][START_REF] Vary | Acousto-Ultrasonics[END_REF].

Russell-Floyd and Phillips [START_REF] Russell-Floyd | A critical assessment of acousto-ultrasonics as a method of nondestructive examination for carbon-fibre-reinforced thermoplastic laminates[END_REF] have investigated the detection of deliberately introduced defects in unidirectional carbon-fiber/PEEK laminates by AU and have established the level of confidence with which various defects may be detected in comparisons with healthy laminates. Kwon and Lee [START_REF] Kwon | Acousto-ultrasonic evaluation of adhesively bonded CFRP-aluminum joints[END_REF] have studied the correlation between the amount of artificial defects and the AUPs in adhesively bonded joints of carbon-fiber reinforced plastic (CFRP) laminates and aluminum plates. Peng et al. [START_REF] Peng | Integrated experimental and numerical investigation for fatigue damage diagnosis in composite plates[END_REF] have used a network of piezoelectric actuators and sensors to characterize the effect of delamination and matrix cracking appearing during cyclic fatigue tests in composites plates on three features of the wave signals: amplitude, correlation coefficient and phase change. Maslouhi et al. [START_REF] Maslouhi | Acousto-ultrasonic signal classification to evaluate high temperature degradation in composites[END_REF] have used the AU to evaluate unidirectional carbon epoxy composite panels exposed to thermal solicitations. The effect of degradation of the specimens on the AUPs has been quantified leading to classifying them according to the thermal state of degradation. Loutas and Kostopoulos [START_REF] Loutas | Health monitoring of carbon/carbon, woven reinforced composites. damage assessment by using advanced signal processing techniques. part i: Acoustic emission monitoring and damage mechanisms evolution[END_REF][START_REF] Loutas | Health monitoring of carbon/carbon, woven reinforced composites: Damage assessment by using advanced signal processing techniques. part ii: Acoustoultrasonics monitoring of damage development[END_REF] have assessed the damage evolution in woven carbon/carbon composites during cyclic loading tests through extracted AUPs capable of monitoring the progressively developing damage.

All these studies and many others employ AU as an active control method to monitor the evolution of damages in the material. Moreover, the input signal characteristics are usually not directly related to the AE signatures of damages. As far as the phenomenon of the waveform distortion has been considered and exploited in AU, it has not been taken into account in AE, neither in signal processing nor in data analysis for discriminating damage mechanisms. To the authors' knowledge, the issue of the distortion of the acoustic signatures of fracture mechanisms in composite materials under the effect of damage accumulation and the subsequent impact on the AE features has not been addressed previously. The quality and relevance of the hits' classification (based on the AE features) could be so greatly impacted, leading to an erroneous structural health assessment. The experimental work presented in this paper focuses on characterizing the influence of damage accumulation in composite materials under cycling fatigue solicitation on the acoustic signatures of the local fracture mechanisms.

The evolution of the AE features as a function of the degree of damage is quantified in order to take into account this evolution in the pattern recognition algorithms and properly perform the classification of the AE events.

Considering the high difficulties that would be encountered to produce and master local fractures in composites, artificial AE sources are preferred, as it is usually performed in AU. The challenge here is twofold:

• Develop artificial AE sources able to generate AE hits having similar characteristics at the sensor level as those obtained from the local fracture mechanisms.

• Ensure the reproducibility of the generated signals in order to allow the comparison between the detected waveforms at different stages during the lifetime of the material.

According to the literature, several methods can be used to generate AEs in composites. The American Society for Testing and Materials (ASTM) has issued a standard guide for determining the reproducibility of the AE sensor response [START_REF]Standard Guide for Determining the Reproducibility of Acoustic Emission Sensor Response[END_REF] and has recommended three signal sources: a gas-jet, an impulsive source produced by breaking a pencil lead, and an electrically driven ultrasonic transducer.

The gas-jet method is performed without a mechanical contact and can be used as a calibration source and to simulate an AE event on the surface of the material [START_REF] Mcbride | Helium gas jet spectral calibration of acoustic emission transducers and systems[END_REF][START_REF] Hutchison | Excitation and spectral calibration of acoustic emission systems[END_REF][START_REF] Prateepasen | Comparison of artificial acoustic emission sources as calibration sources for tool wear monitoring in single-point machining[END_REF]. Nevertheless, this method can be affected by multiple environmental factors such as temperature and humidity. The pulsed laser is a second contactless method that has been employed to simulate an acoustic emission from crack growth by generating a dipole stress field within the structure [START_REF] Scruby | A laser-generated standard acoustic emission source[END_REF][START_REF] Hutchins | Laser generation as a standard acoustic source in metals[END_REF][START_REF] Davies | Laser-generated ultrasound: its properties, mechanisms and multifarious applications[END_REF]. It is relatively expensive, requires strict safety considerations and produces low power (to avoid surface damage), thus, weak AE signals.

The Hsu-Nielsen source [START_REF] Hsu | Characterization and calibration of acoustic emission sensors[END_REF], namely Pencil Lead Break (PLB), has been widely employed as an artificial AE source using the fracture of a brittle graphite lead in a suitable fitting [START_REF] Gary | On the far-field structure of waves generated by a pencil lead break on a thin plate[END_REF][START_REF] Hamstad | Acoustic emission signals generated by monopole (pencil lead break) versus dipole sources: finite element modeling and experiments[END_REF][START_REF] Prosser | Accurate simulation of acoustic emission sources in composite plates[END_REF][START_REF] Gupta | Detecting damage transitions in laminated composites using simulated acoustic emission[END_REF]. This break generates an intense acoustic signal -quite similar to a natural AE source -that sensors detect as a strong burst. Usually, this test aims at checking the sensor's coupling and detectability, as well as the accuracy of the source location setup. Practically, the operator presses the lead of a mechanical pencil tightly against the surface of the material. The surface undergoing this pressure is locally deformed. When the lead breaks, the accumulated energy is abruptly released leading to a microscopic displacement of the surface. An acoustic wave is therefore generated and propagates into the structure. The produced waveform is quite similar to that generated by a damage. Hamstad [START_REF] Hamstad | Acoustic emission signals generated by monopole (pencil lead break) versus dipole sources: finite element modeling and experiments[END_REF] has reported that PLB sources performed on either the top surface or the edge surface of a plate are well represented by interior buried out-of-plane monopole or in-plane monopole sources. Besides, in-plane PLB sources on a plate edge provide displacement signals with relative modal (A 0 and S 0 modes) intensity distributions much closer to those from buried dipoles at the same depths than do surface out-of-plane PLBs. Moreover, another studies have pointed out that PLB is characterized by a fairly good reproducibility, since it depends on several parameters such as the applied force, the angle of orientation and the free length of the lead [START_REF] Gorman | AE source orientation by plate wave analysis[END_REF][START_REF] Boczar | Determining the repeatability of acoustic emission generated by the Hsu-Nielsen calibrating source[END_REF][START_REF] Sause | Investigation of pencil-lead breaks as acoustic emission sources[END_REF].

An AE source can also be simulated using an ultrasonic transducer by generating AE waves on the surface of the material. Due to the vibration of a piezoelectric element, a local deformation of the material's surface is produced and transmitted into the structure in the form of an acoustic wave.

The principle of the AU can be used here to reproduce an AE source using a transmitting transducer instead of a real damage. Contrary to conventional AU configuration, the input signal of the ultrasonic transducer used as a transmitter was optimized in our work to obtain damage-like AE signatures at the sensor level. The input-signal was tuned, so that, to obtain damage-like AE waveforms. The AU has the advantage of allowing the control of the input-signal, so that, it can be adapted to obtain a damagelike AE waveform. The generated signals can also be reproduced perfectly. The measured signals can then be processed using the AE system in order to assess the potential distortions. However, some parameters have to be taken into account, such as the type of the transducer used as transmitter, the strength of the emitted signal, the coupling and the contact surface between the transducer and the structure. H.L. Dunegan has reported [START_REF] Dunegan | An alternative to pencil lead breaks for simulation of acoustic emission signal sources[END_REF] the possibility to generate signals similar to those produced by a PLB in plates using an ultrasonic transducer as a transmitter having a miniature small aperture.

In this work, an experimental procedure is conducted by creating artificial AE sources in order to reproduce and simulate different damage-like AE sources. An ultrasonic transducer and a PLB are used as artificial AE sources in CFRP specimens undergoing cycling tensile fatigue tests. The artificial AE sources are chosen due to their ease of implementation and manipulation. Their reproducibility is studied and compared by successively emitting a certain number of hits at several instants during the test. An AE sensor is used to collect the signals, from which the hits are extracted and the AE-features are then determined using an appropriate algorithm [START_REF] Kharrat | A signal processing approach for enhanced acoustic emission data analysis in high activity systems: Application to organic matrix composites[END_REF].

The evolutions of the waveforms as a function of damage development are quantified in order to guide the pattern recognition tools towards an efficient classification of the AE events according to the damage mechanisms. It should be mentioned that our basic idea, in this work, is not to establish a quantitative relationship between material/experimental parameters and AE-features evolution as a function of damage, nor to use such a relationship to implement supervised clustering approaches.

The limitations of such approaches have been clearly pointed out these last years in the literature related to AE monitoring of composite materials. The purpose here is rather to give guidance toward improving the pattern recognition algorithms in order to be able to consider the waveforms evolutions with damage accumulation in the AE-data classification through unsupervised approaches.

Materials and methods

Purpose of the study

The issue raised in this study and the followed methodology are shown in Figure 1. When defects accumulate during cycling loading on composite materials, the AE waveforms of damage mechanisms could be distorted. This eventuality is studied with a quantification of its impact on the AE features.

The idea is to assess the distortion of acoustic signatures of damages during fatigue tests. An acoustic signature of a particular damage could be simulated using an artificial AE source. While damages progress in the material, a hit is generated by an artificial acoustic source (representing an emerging or developing defect) at several instants during the test and waveforms picking is performed using an AE sensor. The evolution of these measured waveforms is then evaluated. It should be pointed out that the artificial AE sources are created in undamaged areas of the material and the propagation of their transmitted AE hits through a cumulative damaged area is studied here. The zone of the material in the vicinity of the artificial AE sources could be affected by microscopic fractures during the tests, however this cannot be visually verified.

Mechanical testing

The tested material is a ±45 • biaxial carbon fiber-reinforced epoxy resin polymer, composed of eight plies stitched with a textured polyester yarn. The specimens have the dimensions 250 × 25 × 3.5 mm.

In order to determine the composite material properties, several quasi-static (QS) tensile tests were performed on identical healthy specimens at a speed of 2 mm/min. Other specimens were also subjected to tensile-tensile fatigue tests at different loading levels ranging from 50% to 70% of the ultimate tensile strength (UTS). The profile of solicitation is sinusoidal with a frequency of 5 Hz and a stress ratio R = 0.1 (where R = σ min /σ max ). In order to assess the damage development during the tests, two extensometers were mounted to measure the longitudinal and transverse strains (gauge lengths equal to 50 mm and 12.5 mm, respectively). The loading was interrupted at regular time intervals during the tests with 57% and 70% UTS loading levels, so as to allow generating AE waveforms at different health states of the material.

Fatigue degradation and modelling

Stiffness degradation has been widely considered by researchers for fatigue damage modelling and life predictions [START_REF] Harris | Fatigue and accumulation of damage in reinforced plastics[END_REF][START_REF] Wharmby | Observations on damage development in fibre reinforced polymer laminates under cyclic loading[END_REF]. Concerning composites exhibiting significant nonlinear tensile behavior (due to fiber rotation, plastic deformation of matrix, etc.), the evolution of the dynamic stiffness does not often provide accurate means to assess the damage development. Petermann and Schulte [START_REF] Petermann | The effects of creep and fatigue stress ratio on the long-term behaviour of angle-ply CFRP[END_REF] have reported that the mean strain is more meaningful for describing the fatigue degradation process. Other researchers [START_REF] Plumtree | Damage evolution in a [±45] 2S CFRP laminate under block loading conditions[END_REF][START_REF] Montesano | Elevated temperature off-axis fatigue behavior of an eight-harness satin woven carbon-fiber/bismaleimide laminate[END_REF][START_REF] Albouy | Influence of matrix ductility on the high-temperature fatigue behavior of off-axis woven-ply thermoplastic and thermoset laminates[END_REF] have also employed the mean strain in damage accumulation modelling instead of the young modulus, since it provides indications about the three stages of damage development. The mean strain is determined from the maximum and minimum strains for a particular loading cycle N such as

ǫ mean (N ) = ǫ max (N ) + ǫ min (N ) 2 . ( 1
)
The damage development can be assessed experimentally using a damage index D expressed as a function of the mean strain ǫ mean (N ) as follows:

D(N ) = ǫ mean (N ) -ǫ mean (o) ǫ mean (N f ) -ǫ mean (o) , (2)
where ǫ mean (o) and ǫ mean (N f ) are the initial and final mean strains respectively, and N f is the number of cycles to failure at the corresponding applied loading level. The variable D is employed in the phenomenological models that characterize the residual rigidity and fatigue life of materials in terms of macroscopically observable properties [START_REF] Degrieck | Fatigue damage modeling of fibre-reinforced composite materials: Review[END_REF][START_REF] Böhm | A phenomenologically based damage model for textile composites with crimped reinforcement[END_REF]. Several models have been developed to predict the remaining life of composite structures [START_REF] Adam | A power law fatigue damage model for fibre-reinforced plastic laminates[END_REF][START_REF] Halverson | Fatigue life of individual composite specimens based on intrinsic fatigue behavior[END_REF][START_REF] Subramanian | A cumulative damage model to predict the fatigue life of composite laminates including the effect of a fibre-matrix interphase[END_REF]. In this study, the analytical damage model introduced by Mao and Mahadevan [START_REF] Mao | Fatigue damage modelling of composite materials[END_REF] is used. It consists in a normalized accumulated damage model for describing the damage growth during all three stages of load cycling exhibited by the material. This model has the following form:

D(N ) = q N N f m1 + (1 -q) N N f m2 , (3)
where q, m 1 and m 2 are parameters dependent on the material and the applied stress level. They can be determined by fitting the analytical damage index D in Eq. 3 to the experimental one represented using Eq. 2.

Creation of artificial AE sources

During a cyclic loading test, various types of damages appear with different AE signatures. Some hits detected by the sensor during the test are shown in Fig. 2. The aim was to reproduce an AE source able to generate acoustic signals with similar characteristics as the real detected AE hits at the sensor level. Artificial AE sources were hence used in order to generate acoustic signals in the material with different shapes representing some typical AE hits at the sensor level. The signals were generated at several instants during the fatigue life of the material starting with a healthy specimen until its total failure. The acoustic sources were reproduced by PLBs and an ultrasonic transducer used as a transmitter, according to Fig. 3. In order to verify the reproducibility of the generated signals, five waveforms were emitted successively after each interruption of the loading throughout the test. The hydraulic system of the testing machine was stopped in order to avoid any perturbation caused by the background noise. It should be mentioned that it is not a matter here of assigning the acoustic signatures to the damage mechanisms, but just to generate acoustic waveforms with similar characteristics (time and frequency shapes, amplitudes and durations) as the real acoustic signatures of damages in composites. The artificial AE sources were applied near the upper clamping jaw of the testing machine, far from the expected damaged area of the specimen. The zone of application could be likely partially damaged, however this was not visually confirmed.

Waveforms generation using Pencil Lead Breaks

The PLBs were applied on three different boundaries of the specimen: two opposite surfaces (Faces 1 and 2) and the edge (Face 3). This allows generating hits with different shapes representing out-ofplane and in-plane AE sources, which could be close to the real AE sources. Indeed, a PLB applied on the top surface of a composite plate generates out-of-plane AE sources; whereas, if it is applied on the edge surface of the plate, in-plane AE sources are expected [START_REF] Gary | On the far-field structure of waves generated by a pencil lead break on a thin plate[END_REF][START_REF] Hamstad | Acoustic emission signals generated by monopole (pencil lead break) versus dipole sources: finite element modeling and experiments[END_REF][START_REF] Prosser | Accurate simulation of acoustic emission sources in composite plates[END_REF][START_REF] Gorman | AE source orientation by plate wave analysis[END_REF]. For the three considered application surfaces, the PLBs were performed almost at the same distance from the AE sensor (see Fig. 3(a)).

Waveforms generation using an ultrasonic transducer

The transducer used as a transmitter was mounted on "Face 1" in an undamaged area throughout the tests, according to Fig. 3(b). The input signal of the transducer was created using a waveform generator (Tabor Electronics 5064, 100 MS/s) and a power amplifier (Tabor Electronics 9100 A, with a fixed gain of 50). It was an impulse-like signal optimized in terms of its shape, duration and amplitude to obtain transient bursts at the sensor level similar to those generated by a PLB. The input signal is shown in Fig. 4(a) at the output of the waveform generator and in Fig. 4(b) at the output of the power amplifier. The transducer used as a transmitter is a Micro 80-type sensor with an operating frequency-range of [200-900 kHz] and a resonant frequency of 325 kHz.

Signal acquisition

The signal acquisition is ensured by an AE system with the characteristics mentioned in Table 1.

The AE sensor was similar as the transducer used as a transmitter. Both were provided by Mistras

Group. An example of the waveforms detected by the AE sensor when the ultrasonic transducer was used as an artificial AE source is shown in Fig. 4(c). Some samples of hits detected after performing PLBs on the top surface and the edge of the specimen are shown in Fig. 5. The detected waveforms, when using both artificial AE sources, are quite similar in terms of shape, amplitude and duration to those detected with real damages (see Fig. 2). These artificial AE sources also allow generating waveforms with diversified characteristics in time and frequency domains, as shown subsequently.

Experimental procedure and results

In the following, the composite material used in this study is firstly characterized to offer complete information about its tensile and fatigue behaviors. The material properties are determined after QS monotonic tests. Fatigue cycling tests at different loading levels are carried out in order to emphasize the validity of the subsequent results of wave distortions for any loading level.

3.1. Monotonic-tensile and fatigue behaviors of the composite material

Figure 6 shows the stress over the longitudinal and transverse strains measured during the monotonic tensile tests for four identical specimens. The tensile response consists in an elastic-ductile behavior with a quasi-linear response up to a yield point at 55-65 MPa, followed by an inflexion of the tensile curve above this point up to failure. The mean longitudinal strain to failure is approximately 16.9% (Table 2). This non-linear response can be related to the time-dependent behavior and plastic strains of the polymeric constituents, and also to some damage mechanisms such as fiber matrix debonding and rotation of fiber bundles mainly due to necking. These damages and bundles reorientation as a function of longitudinal strain can be observed during the tests (Fig. 1(a), right) and using fractography.

The fatigue cycling tests at different loading levels allowed the Wöhler curve to be plotted (see Fig. 7). It shows the maximum applied stress over the number of cycles to failure. The square markers (at 35%, 40%, 57% and 70% UTS) indicate the tests performed continuously until the specimen's total failure (except for 35% and 40% where the tests were stopped before the rupture of the specimens).

The triangle markers (at 57% and 70% UTS) are related to those with multiple interruptions during the fatigue test in order to allow generating acoustic waves. Measurements of the temperature variation at the surface of the tested specimen (caused by cyclic loading) using infrared thermography have shown a change in the heat dissipation when the fatigue test was interrupted.

Figure 8(a) shows the mean longitudinal strains obtained with different loading levels over the number of cycles. A comparison between the damage indexes experimentally determined using the mean strain-based method of Petermann and Schulte [START_REF] Petermann | The effects of creep and fatigue stress ratio on the long-term behaviour of angle-ply CFRP[END_REF] and those predicted using the analytical model of Mao and Mahadevan [START_REF] Mao | Fatigue damage modelling of composite materials[END_REF] over the cycle ratio is presented in Fig. 8(b). The damage index curves are described by three stages: the first one is characterized by a rapid stiffness degradation;

followed by a gradual decrease in stiffness throughout the majority of the fatigue life; and finally the damage accumulation causes a significant decrease in stiffness prior to failure.

Hereafter, the test performed with an applied stress level of 70% UTS is taken as an example to conduct the study. Under this loading level, the material failed after 4144 cycles with 17 interruptions.

Effect of cumulated damages on the PLB-generated waveforms

Figure 9 shows the detected waveforms and their corresponding scalograms at three different damage levels, when using a PLB as AE source on "Face 1". It can be remarked that the signals' shape is distorted under the effect of cumulated damage. Indeed, the discontinuities emerged in the material, and caused by the initiation and development of defects, engender a wave dispersion which modifies the waveforms. The scalograms indicate an evolution in the frequency domain of these waveforms with the damage level. The emergent defects can produce a mode conversion: Other wave propagation modes appear, such as flexural modes, with different frequency spectra, as can be observed in Fig. 9.

It should be mentioned here that the waveforms' distortions could be partly caused by a distortion of the artificial AE source. In fact, the zone of application of the PLB could be microscopically or partially damaged. That way, the generated waveform would be already distorted before propagating in the damaged area of the material. When an AE source is simulated using a PLB on "Face 2", similar results as previously are obtained. For the sake of convenience, the corresponding results are not presented.

When applying PLBs on the edge of the plate ("Face 3"), the detected waveforms are quite different from those previously treated, as we can notice in Fig. 10. The footprint of high-frequency modes (in the range of [100-500 kHz]) is more present here, when an in-plane AE source is simulated, comparing to the case of an out-of-plane AE source. Gradually, these modes are damped with the damage development until a total extinction in the last measure before failure.

An important impact of the cumulated damage on the AE features is observed. Figures 11 and12 show some selected features where their evolution depending on the damage index can be noticed. The cumulated damage during fatigue cycling clearly affects the AE features in both "Face 1" and "Face 3"

cases. The variability of the features is relatively high, since it would be influenced by the various parameters affecting the breaking of the pencil-leads, as mentioned in the introduction. Besides that, some important remarks can be drawn by comparing the AE-features obtained from both material surfaces. In the case of "Face 1", some energy-related AE-features, like PAC-Energy, duration and amplitude are larger than those obtained with "Face 3". Out-of plane waves are found to be more energetic than in-plane waves. In contrast, frequency centroid and peak frequency are higher in the case of "Face 3". In-plane waves are thus characterized by high-frequency contents comparing to out-of-plane waves, which was already remarked in the scalograms.

Effect of cumulated damages on the transducer-generated waveforms

When using an ultrasonic transducer as AE source on "Face 1", the detected waveforms at three different damage levels and their corresponding scalograms are shown in Fig. 13. The shape of the AE signals is noticeably distorted, and the scalograms exhibit an evolution in their frequency spectra under the effect of cumulated damage. As mentioned previously, a part of these distortions might be engendered by an eventual degradation of the material in the vicinity of the artificial AE source. We can point out that the signals are characterized by relatively high frequencies (above 200 kHz). They are then greatly weakened with damage accumulation and toggle to lower frequencies (as for the PLB case). The AE-features computed from the detected hits are evolving during the damage development, as shown in Fig. 14. Their variability is much lower than that obtained using PLB. The waveforms emitted by the transducer are almost identical since their generation is completely mastered. Thus, it can be concluded that the transducer used as a transmitter has a better reproducibility than the PLB. It allows an accurate description of the wave distortion and the evolution of the AE features as a function of damage level.

The behavior of some AE features is difficult to predict, mainly with a non-monotonic evolution.

Hence, their tendencies could not be faithfully monitored during the fatigue life of the material, with the aim of compensating the effect of damage accumulation on the AE features.

Evolution of the AE-features diagrams with damage development

The pattern recognition approaches are used to distinguish the possible natural clusters/classes associated to the damage mechanisms. Figure 15 shows different representations, in the AE features space, of the data-point associated to the same hit detected after simulating an AE source by an ultrasonic transducer during the damage accumulation. It can be remarked that the evolving position of this hit covers a wide area, which could considerably affect the identification of the natural clusters/classes. A decrease of more than 30 dB in amplitude and about 1.5 ms are observed when moving from the data-point 1 (no damage) to the data-point 17 (total failure). A great change in the frequency spectrum can also be remarked: the weighted peak frequency undergoes a drop of about 150 kHz during the fatigue life of the material.

Therefore, it would be of interest to integrate consideration of these waveforms' evolutions in the data clustering approaches. Our idea is to use unsupervised or semi-supervised clustering approaches that are able to adapt the form of the clusters along different directions in the AE features space, in order to follow and consider the features evolution with damage accumulation. This can be achieved using specific algorithms that are not based on compactness and circularity of clusters in features space.

Generally, clustering algorithms applied on AE signals are based on the Euclidean distance, which suppose cluster compactness and circularity in the original feature space, such as K-means, fuzzy C-means and a variant of Self-Organizing Maps. Some validity indices used to estimate the number of clusters are also based on this particular distance. The validity of this hypothesis can be clearly questioned in tests, such the one presented in our study, in which damages cumulated in the composite and the associated discontinuities, induces a distortion over time of the transients propagating through the material. The capacity of some unsupervised techniques that we have developed recently will be illustrated in a forthcoming paper. Using such approaches, even if the clusters are initially compact, their prototypes can move in the feature space and their shapes may change as a function of the damage level in the composite. These techniques are partially described in previous papers [START_REF] Doan | An unsupervised pattern recognition approach for AE data originating from fatigue tests on polymer-composite materials[END_REF][START_REF] Ramasso | Unsupervised consensus clustering of acoustic emission time-series for robust damage sequence estimation in composites, Instrumentation and Measurement[END_REF].

In order to enhance data-driven phenomenological physics-models with better prediction capabilities, it is thus of paramount importance to develop specific clustering algorithms dedicated to AE signals originating from composite materials. These signals are unevenly time-spaced and consist in transients ordered chronologically with increasingly emergent damage mechanisms. In addition, these transients are propagating in a non-stationary media during the test. The amount of uncertainty on the characteristics of acoustic sources deduced from those statistical models should also be quantified, since results are highly dependent on multifarious parameters and unknown factors.

Conclusions

This study focused on the influence of damage accumulation during cyclic fatigue loading on the acoustic signatures of fracture mechanisms in carbon/epoxy composites. Specifically, the propagation through a cumulatively damaged area of a specific acoustic signature created in an undamaged area was assessed. Reproducible artificial AE sources were created by PLBs and an ultrasonic transducer used as a transmitter in undamaged areas on different surfaces of the material in order to generate various damage-like acoustic signatures. The waveforms detected at the sensor level were characterized as similar in time and frequency domains to those obtained with real local fracture mechanisms in composites. During the fatigue life of the material, identical AE hits were generated using the artificial sources at different instants. At each time, the AE waveforms were measured at the same location by an AE sensor. It was remarked that the waveforms issued from both artificial AE sources were clearly distorted in the time and frequency domains due to the damage accumulation. The AE features exhibited a relatively important evolution as a function of damage level. The behavior of some AEfeatures was sometimes difficult to predict, since their evolutions were non-stationary during the fatigue life of the material. Although the noticed distortions were patently engendered by the propagation of the AE waveforms through a damaged area, a distortion at the very AE source can be at the origin of a part of the waveform's distortions. An eventual damage in the vicinity of the AE source can contribute to the distortions. The case of an AE source generated in a damaged area deserves also to be investigated. This issue is being studied by the authors and will be addressed in a future work.

Even if the conclusions drawn in this study were obtained for a particular stress level, they can be generalized for any loading level since the evolutions of the damage index as a function of the cycle ratio are similar.

The major contribution of this paper was to underline -on the basis of one representative testthat the AE features evolution due to damage cumulated during mechanical loading can be significant, and clearly deserves to be taken into account in pattern recognition techniques. It should be mentioned that it was not a matter, in this work, to establish a model able to predict the features evolution as a function of damage, but to give guidance to improve the pattern recognition algorithms by using specific unsupervised or semi-supervised clustering approaches able to adapt the shapes of clusters along several directions in the features spaces during the progression of damage. This kind of approaches is being developed by the authors and will be addressed in a future study.

and by BPI France and the Franche-Comté Region through the project "Smart Composites". The authors would like to thank the MEMS team belonging to the department of Applied Mechanics for providing instruments for the waveform generation and for its technical assistance.

Appendix

The AE-features studied in this paper are defined below as implemented in the used algorithm [START_REF] Kharrat | A signal processing approach for enhanced acoustic emission data analysis in high activity systems: Application to organic matrix composites[END_REF].

• Counts: The number of times the AE signal s(t), of length L, exceeds the preset threshold T H:

Counts = L t=1 I s(t)≥T H , (4)
where I x is an indicator function equal to 1 if the condition x is true.

• Duration [µs]: The time between the start and the end of the AE hit:

Duration = L • ∆t. ( 5
)
where ∆t is a sampling period.

• Amplitude [dB]: A logarithmic measure of the maximum voltage of the AE hit expressed as:

dB AE = 20 log 10 ( A max A 0 ) -P G, (6)
where A max is the peak amplitude of the AE hit, A 0 is a reference amplitude equal to 1µV at the sensor (before amplification), and P G is the preamplifier gain.

• PAC-Energy (E P AC ) [µV s]: PAC-Energy is the sum of the signal envelope over the duration and converted into counts at 100 kHz/V:

E P AC = t |H[s(t)]| • ∆t • 100 • 10 6 , (7)
where H is the envelope -computed by the Hilbert Transform -of the signal above the threshold.

• Frequency Centroid (f ctr ) [kHz]: It is computed from the FFT magnitude as a weighted average of the FFT magnitude:

f ctr = f M F F T (f ) • f f M F F T (f ) , (8)
where M F F T (f ) is the FFT magnitude at a frequency f . Both the lower and the upper bounds of the sum may be adapted according to the application.

• Peak Frequency (f peak ) [kHz]: It is the peak of the power spectrum of the AE hit:

f peak = arg max f M F F T (f ). (9)
• Partial Power features (P P k ) [%]: They are derived from the power spectrum of the AE hit and reported as a percentage in a given user-defined frequency band

I k = [f k 1 , f k 2 ]: P P k = 100 • f ∈I k P S(f ) g∈∪ k I k P S(g) , (10)
where P S is the value of the power spectrum and k is ranging from 1 to the total number of frequency bands.

Partial power 1: f 1 = 0 kHz; f 2 = 150 kHz.

Partial power 2: f 1 = 150 kHz; f 2 = 300 kHz.

Partial power 3: f 1 = 300 kHz; f 2 = 500 kHz.

Partial power 4: f 1 = 500 kHz; f 2 = 1 MHz.

• Weighted Peak Frequency (WPF) [kHz]: It is computed from the frequency centroid and peak frequency as follows:

W P F = f peak • f ctr (11)
List of Figures

Figure 1 :Figure 2 :

 12 Figure 1: Purpose of the study. (a) The raised issue: potential evolution of the acoustic signatures of local fracture mechanisms during cyclic loading; and (b) the proposed methodology to quantify the AE wave distortion under the effect of damage accumulation.

  Ultrasonic transducer used as a transmitter.

Figure 3 :

 3 Figure 3: Test configuration for generating AE damage-like signals using artificial sources.

Figure 4 :

 4 Figure 4: Artificial acoustic source created by an ultrasonic transducer used as a transmitter.

Figure 5 :Figure 6 :Figure 7 :Figure 8 :Frequency

 5678 Figure 5: Pencil Lead Break signals detected at the AE sensor level mounted on "Face 1" of a healthy composite material.

Figure 9 :Frequency

 9 Figure 9: Waveforms distortion with cumulated damage when using a PLB on "Face 1" as AE source.

Figure 10 :Frequency

 10 Figure 10: Waveforms distortion with cumulated damage when using a PLB on "Face 3" as AE source.

Figure 13 :Figure 15 :

 1315 Figure 13: Waveforms distortion with cumulated damage when using an ultrasonic transducer as AE source.

Table 1 :

 1 Pencil Lead Break signals detected at the AE sensor level mounted on "Face 1" of a healthy composite material. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 6 Stress-strain curves obtained from the QS monotonic tests performed on four identical composite specimens, where ǫ l and ǫ t are the longitudinal and transverse strains respectively. Each color represents a test performed on one specimen. . . . . . . . . . . 28 AE source. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 13 Waveforms distortion with cumulated damage when using an ultrasonic transducer as AE source. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 14 Evolution of the AE-features with cumulated damage when using an ultrasonic transducer as AE source. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 15 Modification of the AE-features diagrams of a detected hit during the damage accumulation. AE source reproduced using an ultrasonic transducer. Data point 1 represents D = 0, and data point 17 represents D = 1. . . . . . . . . . . . . . . . . . . . . . . . . . 37 AE system setup parameters.

	1	Purpose of the study. (a) The raised issue: potential evolution of the acoustic signatures
		of local fracture mechanisms during cyclic loading; and (b) the proposed methodology
		to quantify the AE wave distortion under the effect of damage accumulation. . . . . . . 23
	2	Some AE signatures of different real damages appeared during a cyclic fatigue test
		performed on the composite. Artificial AE sources are used to reproduce damage-like
		AE signatures with similar characteristics as the real damage-related signatures. . . . . 24
	3	Test configuration for generating AE damage-like signals using artificial sources. . . . . 25
	4	Artificial acoustic source created by an ultrasonic transducer used as a transmitter. . . . 26
	5
	7	Wöhler curve determined after fatigue cycling tests performed on the composite material
		at different loading levels at a frequency of 5 Hz and a stress ratio R = 0.1. Continuous
		loading: 35%, 40%, 57% and 70% UTS; with interruptions: 57% and 70% UTS. . . . . 29
	8	Damage evolution with cyclic loading: (a) mean longitudinal strain for different load-
		ing levels over the number of cycles, (b) experimental damage index and numerical
		prediction using the Mao-and-Mahadevan's damage model. . . . . . . . . . . . . . . . . 30
	9	Waveforms distortion with cumulated damage when using a PLB on "Face 1" as AE

source. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 10 Waveforms distortion with cumulated damage when using a PLB on "Face 3" as AE source. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 11 Evolution of the AE-features with cumulated damage when using a PLB on "Face 1" as AE source. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 12 Evolution of the AE-features with cumulated damage when using a PLB on "Face 3" as

Table 2 :

 2 Average material properties determined after quasi-static tensile tests.

	Material properties	Average values
	Ultimate tensile strength	100 ± 6.2 MPa
	Longitudinal strain to fracture 16.9 ± 3.5%
	Young modulus	11.1 ± 1.4 GPa

Acknowledgments

This work was supported by the Laboratory of Excellence ACTION through the program "Investments for the Future" managed by the National Agency for Research (references ANR-11-LABX-01-01)

List of Tables


Taib Arif

Guorui Wang

Rana N S Sodhi

Guillaume Colas

Tobin Filleter
email: filleter@mie.utoronto.ca

Role of chemical vs. physical interfacial interaction and adsorbed water on the tribology of ultrathin 2D-material/steel interfaces

Keywords: Nanotribology, MoS2, Graphite, Water adsorption, Physical interaction, 440C Steel, Interfacial Chemistry, Chemical Interaction

come

Role of chemical vs. physical interfacial interaction and adsorbed water on the tribology of ultrathin 2D-material/steel interfaces

Introduction

Friction is an everyday phenomenon that costs humanity billions of dollars annually [START_REF] Holmberg | Influence of tribology on global energy consumption , costs and emissions[END_REF]. Mechanical components such as gears and bearings are often manufactured using different steel alloys depending on the application (e.g. 440C-steel for space and automotive applications), which are prone to wear, energy loss and corrosion caused by friction [START_REF] Berman | Few layer graphene to reduce wear and friction on sliding steel surfaces[END_REF]. One approach to minimize friction is coating the contacts with lubricants. Microscale-MoS2 and graphite are well recognized solid lubricants that consist of 2D-structures (2D-MoS2 and graphene) at the atomic scale. Moreover, ultrathin 2D-MoS2 and graphene have also been shown to be exceptional solid lubricants due to their unique structures including weak interlayer interaction (van der Waals) [START_REF] Ru | Interlayer friction and superlubricity in bilayer graphene and MoS2/MoSe2 van der Waals heterostructures[END_REF] and high in-plane strength [START_REF] Bunch | Impermeable atomic membranes from graphene sheets[END_REF][START_REF] Castellanos-Gomez | Elastic properties of freely suspended MoS2 nanosheets[END_REF].

Ultrathin 2D-materials have demonstrated the ability to lubricate multiscale contacts, ranging from nanoscale to macroscale [START_REF] Berman | Few layer graphene to reduce wear and friction on sliding steel surfaces[END_REF][START_REF] Fasihi | Effect of graphite and MoS2 based solid lubricants for application at wheel-rail interface on the wear mechanism and surface morphology of hypereutectoid rails[END_REF][START_REF] Chen | Effect of structure on the tribology of ultrathin graphene and graphene oxide films[END_REF][START_REF] Motozuka | Interfacial effect between graphite and iron substrate on basal plane orientation and lubricity of graphite[END_REF][START_REF] Zhao | An investigation on the tribological properties of multilayer graphene and MoS2 nanosheets as additives used in hydraulic applications[END_REF]. Few layers of graphene and graphite are reported to reduce the coefficient of friction (COF) between microscale 440C-steel/steel and graphite/iron-oxide contacts from ~0.91 to ~0.15 while preventing tribo-corrosion and wear. [START_REF] Berman | Few layer graphene to reduce wear and friction on sliding steel surfaces[END_REF][START_REF] Motozuka | Interfacial effect between graphite and iron substrate on basal plane orientation and lubricity of graphite[END_REF] Similarly, MoS2 additives can coat microscale-steel/steel interface with ultrathin-MoS2 sheets resulting in the reduction of friction by ~50% and wear by 5-9 times [START_REF] Kalin | Mechanisms and improvements in the friction and wear behavior using MoS 2 nanotubes as potential oil additives[END_REF]. Furthermore, 2D-materials can overcome some of the environmental dependency to lubricate contacts as compared to their microscale structures. In dry conditions, graphite is known to exhibit high friction while graphene remains in an ultra-low friction regime [START_REF] Berman | Graphene: A new emerging lubricant[END_REF][START_REF] Arif | Effect of Humidity and Water Intercalation on the Tribological Behavior of Graphene and Graphene Oxide[END_REF]. Thick-MoS2 coatings are sensitive to oxidation from water molecules and atmospheric oxygen, resulting in significant deterioration of the desired tribological performance. However, ultrathin-MoS2 can remain in a low friction regime even with some oxidation and presence of water. It is only after oxidation along the basal plane in excess of ~9.6 at.% that ultrathin-MoS2 loses its ultralow friction regime [START_REF] Curry | Impact of Microstructure on MoS2 Oxidation and Friction[END_REF][START_REF] Arif | Understanding the Independent and Interdependent Role of Water and Oxidation on the Tribology of Ultrathin Molybdenum Disulfide (MoS2)[END_REF].

In the absence of water, both 2D-MoS2 and graphene exhibit friction that is dependent on the sliding counter-surface material (e.g. silicon and silver) despite both the 2D-materials having similar sliding mechanisms [START_REF] Zeng | Probing the difference in friction performance between graphene and MoS2 by manipulating the silver nanowires[END_REF][START_REF] Khac | Quantitative assessment of friction characteristics of single-layer MoS 2 and graphene using atomic force microscopy[END_REF][START_REF] Vazirisereshk | Origin of Nanoscale Friction Contrast between Supported Graphene, MoS2 , and a Graphene/MoS2 Heterostructure[END_REF]. This dependency on counter-surface can result in either one of the 2Dmaterial to be more lubricious. At the nanoscale, the dependency of these 2D-materials on counter-surface and the influence of water at specific engineering relevant interfaces is largely unknown due to the commercial unavailability of nanoscale counter-surfaces. This motivates the need for understanding the influence of the specific counter-surface material, such as bearing, gears and valve-grade 440C steel, on the mating interface at the nanoscale. In this work, we take an atomic force microscope (AFM) based approach to compare the tribological properties of ultrathin-graphite and ultrathin-MoS2 (few layers) against a custom fabricated 440C-steel counter-surface in the presence of varied ambient humidity. First, we identify the surface chemistry on the steel counter-surface and its influence on the interfacial interaction with the respective ultrathin 2D-materials (i.e. few layers). Second, we gain insights on the influence of counter-surface interaction on the sliding and adhesion behavior and identify the role of water at differently interacting interfaces.

Experimental Procedure

Sample Preparation, AFM Cantilever Fabrication and Characterization. Samples were prepared using a MoS2 crystal (Graphene Supermarket) and highly oriented pyrolytic graphite (SPI supplies) by mechanically exfoliating ultrathin sheets on to an n-doped silicon wafer substrate. As the friction behavior of 2D materials including graphene and MoS2 is reported to plateau at approximately 4 layers of thickness and exhibit similar sliding regime to that of bulk material [START_REF] Lee | Frictional Characteristics of Atomically Thin Sheets[END_REF], ultrathin sheets with 5-13 layers (1.7 -4.3 nm for ultrathin-graphite and 3 -7.8 nm for MoS2) were used here to minimize the effect of substrate and thickness dependence for both the materials. The wafers were cleaned prior to exfoliation using ethanol first and then followed by methanol in an ultrasonic bath. The AFM cantilevers were fabricated by attaching 440C-steel (Sandvik) powder beads onto a tipless cantilever (APPNano) using PC-Super epoxy under a customized micromanipulator-optical microscope setup. Prior to attaching the bead, the 440C steel bead powder was cleaned in a Bransonic (M1800H) ultrasonic cleaner using ethanol and methanol for 10 min each. The powder was then suspended on a glass slide and dried in an oven (Cole-Parmer Model 281A) for 30 min at 40°C. Minimal epoxy was applied on the cantilever surface, with any excess epoxy being wiped before the bead is attached. The cantilever is submerged in methanol to wash away epoxy residue off the bead surface for 1 min. The cantilever was then imaged using a scanning electron microscope (Hitachi SU3500) to measure the 440C-steel bead diameter of ~12.5 μm. To ensure the tip geometry doesn't influence the results, we used the same customized cantilever while ensuring experiments were performed in a wear free regime. The maximum normal load only up till 90 nN to ensure it is well within this regime. X-ray photoelectron spectroscopy (XPS) characterization and oxide thickness analysis were performed on the 440C-steel bead powder to identify and estimate the thickness of the surface oxides using an ESCALAB 250Xi (Thermo Fisher Scientific -East Grinstead, UK). ESCALAB 250Xi is equipped with monochromatic Al Kα X-ray source. Survey spectra were collected (pass energy (PE) -100 eV) using an Al Kα X-ray (nominal spot size -400 µm) followed by the spectral regions of interest at higher resolution (PE -20 eV) from which the composition (Rel. At.%) was obtained. All data processing was performed with the system's software (Avantage v. 5.962). XPS characterization on MoS2 sample was also performed to study the influence of water on MoS2 oxidation (Figure 3d). Lastly Raman characterization on mechanically exfoliated MoS2 and HOPG were performed using a Bruker Senterra dispersive microscope (Supporting information; Figure S5).

Tribology Measurements. Surface imaging, friction force microscopy (FFM) and adhesion tests were performed using an Asylum Research MFP 3D atomic force microscope (AFM) system. FFM and adhesion experiments were performed using the 440C-steel beaded cantilever. The normal and torsional stiffness were obtained using the Sader's method [START_REF] Sader | Calibration of rectangular atomic force microscope cantilevers[END_REF][START_REF] Green | Normal and torsional spring constants of atomic force microscope cantilevers[END_REF] where the normal stiffness was measured to be ∼4.3 N/m. Lateral sensitivity was obtained using the test probe method [START_REF] Cannara | Lateral force calibration in atomic force microscopy: A new lateral force calibration method and general guidelines for optimization[END_REF] using a cleaved potassium bromide block, whereas the normal sensitivity of the cantilever was acquired from the slope of the normal voltage-displacement curve by deflecting the cantilever against a clean silicon wafer. The same steel bead cantilever was used to compare the 2D-materials and minimize any influence of bead topography and defects. A dedicated AFM cantilever mount was used to minimize the error of cantilever orientation misalignment. FFM was performed by scanning the AFM cantilever laterally (90° scan angle) at scan rate of ∼5 µm/s. Friction force was calculated by taking half the difference between with trace and retrace lateral signals and multiplied by the lateral sensitivity and lateral stiffness of the cantilever. For FFMcycle testing, scans were done over the same area repeated to observe any change in the friction while maintaining a normal load of 90 nN. interfacial shear strength (ISS) measurements were performed by using the generalized Maugis-Dugdale model fitting procedure proposed by Carpick et al. based on the FFM (friction vs. normal load) data in dry conditions [START_REF] Carpick | A General Equation for Fitting Contact Area and Friction vs Load Measurements[END_REF]. The fitting considered a local asperity on the 440C-Steel tip in contact with the surface which had a radius measured as ∼1.3 µm by using a reverse imaging procedure reported in the literature [START_REF] Pajovic | Work of Adhesion Measurements of MoS2 Dry Lubricated 440C Stainless Steel Tribological Contacts[END_REF]. The tip was reverse imaged using the same cantilever holder used for FFM and adhesion tests to ensure the cantilever had the same inclination angle. This ensures that the topography for the area of contact is acquired during reverse imaging. The contacting asperity was identified and the contact radius of the asperity was measured by emulating the geometrical interpenetration of the bead onto the topography of the 2D material surface. This approach was adopted from a previously reported study [START_REF] Pajovic | Work of Adhesion Measurements of MoS2 Dry Lubricated 440C Stainless Steel Tribological Contacts[END_REF]. Details of the ISS procedure are provided in the SI (Supporting Information; Section S2) [START_REF] Carpick | A General Equation for Fitting Contact Area and Friction vs Load Measurements[END_REF]. Adhesion experiments were performed with a maximum normal load of 90 nN and dwell time of 1s.

Humidity Control and Water Contact Angle (WCA) Measurement.

A custom-built environmental chamber was used to control the humidity locally around the contact by controlling the ratio of in-flowing wet and dry nitrogen gas. The wet nitrogen gas was obtained by passing 99.9% purity N2 gas through a water bubbler. The environment was constantly monitored using an embedded humidity sensor (Honeywell HIH 4000) and allowed to stabilize to ensure minimal drift. The humidity deviation reported herein is the experimental deviation acquired during tests while the sensor has a reported systematic error of ± 3.5% [24]. Water contact angle (WCA) measurements were performed on mechanically exfoliated ultrathin-graphite and MoS2 samples by drop-casting a water droplet (6 μl) and analyzed using a customized microscope setup (Supporting Information; Figure S4).

Results and Discussion

Characterization of 440C-steel bead counter-surface

A customized 440C-steel bead AFM tip was fabricated and analyzed by reverse AFM imaging to measure the surface roughness and identify the contact point (Figure 1a). Surface chemical characterization was performed using X-ray photoelectron spectroscopy (XPS) on the 440C-steel countersurface which identified the presence of Fe-oxides, with traces of Mn-and Cr-oxides coming from the steel alloying elements (Figure 1b). Fitting of the Fe2p peak indicated the presence of FeO and Fe2O3, whereas the Cr2p peak identified Cr2O3, Cr(OH)3 and CrO. Mn-oxides in the form of MnO, MnO2 and Mn2O3 were also identified on the surface. An oxide thickness of 5.6 nm was measured on the countersurface by comparing the relative intensities of the Fe2p and Fe3p peaks (Supporting Information S1).

Friction and adhesion measurements under dry conditions

Friction and adhesion were measured using friction force microscopy (FFM) for ultrathin-MoS2 and ultrathin-graphite (5-13 layers) in contact with the steel bead tip under dry conditions by purging a localized AFM chamber with 99.99% purity N2 (Figure 2 & Table 1). Both friction and adhesion were found to be higher for the steel/MoS2 interface compared to the 440C-steel/ultrathin-graphite interface as summarized in Table 1. In particular, at the maximum normal load of 90 nN, friction on MoS2 was measured to be ~21 times higher with a lateral force of 42.0 ± 1.2 nN compared to 2.0 ± 0.2 nN on ultrathin-graphite. The ISS of the two interfaces was measured by fitting the non-linear friction vs. normal load plot (Figure 2) using the generalized Maugis-Dugdale model proposed by Carpick et al. [START_REF] Carpick | A General Equation for Fitting Contact Area and Friction vs Load Measurements[END_REF], yielding an ISS of 249 ± 25 MPa between 440C-steel/MoS2 and 168 ± 35 MPa between 440C-steel/ultrathingraphite. These ISS values for 440C-steel/2D-materials reported herein are in similar order of magnitude to the other counter-surface (silicon) typically reported in the literature (~140 -173MPa) [START_REF] Chen | Effect of structure on the tribology of ultrathin graphene and graphene oxide films[END_REF][START_REF] Liu | Interlayer shear strength of single crystalline graphite[END_REF].

Furthermore, adhesion on MoS2 was measured to be ~1.6 times higher with the pull off force of 46 ± 2 nN compared to 29 ± 1 nN on ultrathin-graphite (Table 1). Such a trend in pull off force is also in accordance with the obtained adhesive force calculated by fitting friction vs. normal load curves. The comparatively higher friction, ISS and adhesion for the MoS2 suggests a stronger interfacial interaction at the 440C-steel/MoS2 interface (Table 1 & Table 2) than that for the 440C-steel/ultrathin-graphite interface. According to the classification of the interaction at the interfaces based on interfacial distance or cut off energy, previous studies have reported a physical interaction between steel oxides (Fe-, Cr-and Mn-oxides) with ultrathin-graphite [START_REF] Restuccia | Tribochemistry of graphene on iron and its possible role in lubrication of steel[END_REF][START_REF] Sharifi | Formation of nitrogendoped graphene nanoscrolls by adsorption of magnetic γ-Fe2O3 nanoparticles[END_REF][START_REF] Susi | Core level binding energies of functionalized and defective graphene[END_REF][START_REF] Singh | Graphene based materials: Past, present and future[END_REF][START_REF] Zhou | Magnetite/graphene nanosheet composites: Interfacial interaction and its impact on the durable high-rate performance in lithium-ion batteries[END_REF][START_REF] Dabaghmanesh | The formation of Cr2O3 nanoclusters over graphene sheet and carbon nanotubes[END_REF][START_REF] Yamada | Functionalized graphene sheets coordinating metal cations[END_REF][START_REF] Wu | High-Energy MnO2 Nanowire/Graphene and Graphene Asymmetric Electrochemical Capacitors[END_REF]. In the case of MoS2 samples, a stronger interaction is expected since all the steel oxides (Fe-, Cr-and Mn-oxides) are reported to interact preferentially with the sulfur atoms (chemically) on MoS2 and hence can contribute to a more resistive sliding and adhesive behavior with the 440C-steel counter-surface [START_REF] Didziulis | Applications of Surface Science to Solid Lubricants[END_REF][START_REF] Lince | Chemical Interaction of Mn With the MoSs (0001) Surface Studied by High-Resolution Photoelectron Spectroscopy[END_REF][START_REF] Yue | Functionalization of monolayer MoS 2 by substitutional doping : A first-principles study[END_REF][START_REF] Li | Electronic and magnetic properties of 3d-metal trioxides superhalogen cluster-doped monolayer MoS 2 : A first-principles study[END_REF][START_REF] Smyth | Contact metal-MoS2interfacial reactions and potential implications on MoS2-based device performance[END_REF][START_REF] Lince | Photoelectron Spectroscopic Study of The Interaction of Thin Fe Films With The MoS2 (0001) Surface[END_REF][START_REF] Mos | First principles prediction on the interfaces of Fe/MoS2 , Co/MoS2 and Fe3O4/MoS2[END_REF]. Furthermore, the 440C-steel surface mainly consists of Feoxides (Figure 1b) which are reported to interact strongly via ionic bond like interactions at the Feoxide/MoS2 interface [START_REF] Mos | First principles prediction on the interfaces of Fe/MoS2 , Co/MoS2 and Fe3O4/MoS2[END_REF]. Note that, to confirm the differences of interfacial properties between ultrathin-MoS2 and ultrathin-graphite are not attributable to a tip change or other instrument changes, AFM measurements were performed with the same tip tests but tests were done in different orders, by alternating between the ultrathin-graphite and MoS2 samples. Fig. 2. Friction force as a function of normal load curve for 440C-steel/ultrathin-graphite and 440Csteel/MoS2 including the generalized Maugis-Dugdale fitting model [START_REF] Carpick | A General Equation for Fitting Contact Area and Friction vs Load Measurements[END_REF]. Note the break in the friction force axis as MoS2 exhibits significantly higher friction at all normal loads. Table 1. Experimental sliding and adhesion (FAdhesion) comparison of 440C-steel/MoS2 and 440Csteel/ultrathin-graphite interfaces under dry (RH7-9%) conditions with a maximum normal load (FN) of 90 nN. Sliding comparisons include friction force (FFriction) and interfacial shear strength (ISS). FAdhesion,Fit is the pull off force acquired from the generalized Maugis-Dugdale fitting model on the friction data [START_REF] Carpick | A General Equation for Fitting Contact Area and Friction vs Load Measurements[END_REF].

Steel/MoS2

Steel

Cr-oxides

Chemical [START_REF] Didziulis | Applications of Surface Science to Solid Lubricants[END_REF][START_REF] Yue | Functionalization of monolayer MoS 2 by substitutional doping : A first-principles study[END_REF][START_REF] Li | Electronic and magnetic properties of 3d-metal trioxides superhalogen cluster-doped monolayer MoS 2 : A first-principles study[END_REF][START_REF] Smyth | Contact metal-MoS2interfacial reactions and potential implications on MoS2-based device performance[END_REF] Physical [START_REF] Dabaghmanesh | The formation of Cr2O3 nanoclusters over graphene sheet and carbon nanotubes[END_REF] Mn-oxides Chemical [START_REF] Didziulis | Applications of Surface Science to Solid Lubricants[END_REF][START_REF] Lince | Chemical Interaction of Mn With the MoSs (0001) Surface Studied by High-Resolution Photoelectron Spectroscopy[END_REF][START_REF] Yue | Functionalization of monolayer MoS 2 by substitutional doping : A first-principles study[END_REF][START_REF] Li | Electronic and magnetic properties of 3d-metal trioxides superhalogen cluster-doped monolayer MoS 2 : A first-principles study[END_REF] Physical [START_REF] Yamada | Functionalized graphene sheets coordinating metal cations[END_REF][START_REF] Wu | High-Energy MnO2 Nanowire/Graphene and Graphene Asymmetric Electrochemical Capacitors[END_REF]

Role of water on the interfacial behavior of 440C-steel/ultrathin-graphite and 440C-steel/MoS2

Next, the role of ambient humidity in both interfacial systems was studied (Figure 3). In the case of 440C-steel/ultrathin-graphite interface, the sliding behavior was found to be minimally influenced by water between RH7 -40%, where the friction force was observed to only increase from ~2.0 ± 0.3 nN to ~3.0 ± 0.2 nN (Figure 3a). However, a further rise in humidity past RH40% increased friction by almost three-fold to ~6.0 ± 0.3 nN as compared to the dry condition. This behavior was repeatable for varying normal loads (Supplementary Information; Figure S1) for two different samples. Adhesion between the 440C-steel/ultrathin-graphite interface at varying humidity was found to increase from 29.0 ± 1 nN (RH9%) to 140.0 ± 2 nN (RH71%), however, adhesion was only observed to increase by an appreciable value for humidity over RH40% (Figure 3b). No significant interlayer nor subsurface water intercalation is expected for the ambient conditions studied herein [START_REF] Arif | Effect of Humidity and Water Intercalation on the Tribological Behavior of Graphene and Graphene Oxide[END_REF][START_REF] Arif | Understanding the Independent and Interdependent Role of Water and Oxidation on the Tribology of Ultrathin Molybdenum Disulfide (MoS2)[END_REF], therefore the increasing pull off force with humidity is attributed to water meniscus build-up from capillary condensation around the contact and water adsorption [START_REF] Arif | Effect of Humidity and Water Intercalation on the Tribological Behavior of Graphene and Graphene Oxide[END_REF][START_REF] Peng | Study of nanotribological properties of multilayer graphene by calibrated atomic force microscopy[END_REF]. The overlapping friction-adhesion trend for ultrathin-graphite is suggestive that the increase in friction with humidity between 440C-steel/ultrathin-graphite is primarily due to water adsorption/meniscus build-up. This is consistent with previously reported transitioning interfacial behavior on ultrathin-graphite in contact with other counter surfaces as a result of humidity where water adsorption was considered to play the dominant interfacial role [START_REF] Arif | Effect of Humidity and Water Intercalation on the Tribological Behavior of Graphene and Graphene Oxide[END_REF][START_REF] Arif | Understanding the Independent and Interdependent Role of Water and Oxidation on the Tribology of Ultrathin Molybdenum Disulfide (MoS2)[END_REF].

In the case of MoS2, the presence of water was instead observed to reduce friction between the 440C-steel/MoS2 interfaces from ~42.0 ± 1 nN to ~22.0 ± 3 nN between RH7-44% (Figure 3a). The decreasing friction trend for 440C-steel/MoS2 interface as a function of humidity was repeatable for 5 datasets, averaging ~43% reduction (Supporting Information; Figure S2a). The average friction force between 440C-steel/MoS2 is notably higher than that of 440C-steel/ultrathin-graphite across the whole humidity range. Adhesion experiments between 440C-steel/MoS2 showed a sharp increase in pull off force from ~46.0 ± 2 nN (RH10%) to ~465.0 ± 10 nN (RH25%) which occurred at a lower humidity than the transition observed for ultrathin-graphite (Figure 3c). The transition of interfacial adhesion behavior for ultrathin-MoS2 herein at ~RH25% is consistent with the observation for microscale-MoS2 coatings [START_REF] Erdemir | Solid lubricants and self-lubricating films[END_REF] as well as other counter-surfaces for ultrathin-MoS2 [START_REF] Arif | Understanding the Independent and Interdependent Role of Water and Oxidation on the Tribology of Ultrathin Molybdenum Disulfide (MoS2)[END_REF]. For example in the case of SiO2 countersurfaces, the higher adhesion to ultrathin-MoS2 at a lower humidity (RH25%) as compared with that for ultrathin-graphite (>RH40%) was attributed to the comparatively stronger water-MoS2 interaction resulting from its tri-layer atomic structure [START_REF] Arif | Understanding the Independent and Interdependent Role of Water and Oxidation on the Tribology of Ultrathin Molybdenum Disulfide (MoS2)[END_REF][START_REF] Levita | Graphene and MoS2 interacting with water: A comparison by ab initio calculations[END_REF][START_REF] Luan | Wettability and friction of water on a MoS2 nanosheet[END_REF][START_REF] Ortiz-Young | The interplay between apparent viscosity and wettability in nanoconfined water[END_REF][START_REF] Tocci | Friction of water on graphene and hexagonal boron nitride from Ab initio methods: Very different slippage despite very similar interface structures[END_REF]. Furthermore, strength of the capillary force is also reported to be dependent on surface wettability [START_REF] Binggeli | Influence of capillary condensation of water on nanotribology studied by force microscopy[END_REF]. An increase in surface hydrophobicity weakens the influence of capillary force acting on the counter-surface [START_REF] Binggeli | Influence of capillary condensation of water on nanotribology studied by force microscopy[END_REF]. Water contact angle (WCA) measurements (Supporting information; Figure S4) show a more hydrophilic MoS2 (∼80°) as compared to graphene (∼91°). This is suggestive of a stronger capillary force acting between the 440C-steel/MoS2 than 440Csteel/ultrathin-graphite. The overall pull-off force herein is also generally higher between the 440Csteel/MoS2 interface as compared to the 440C-steel/ ultrathin-graphite interface which would contribute to an overall higher friction force (Figure 3). In stark contrast with the well-known macroscale behavior of thick-MoS2 coatings, ultrathin-MoS2 coatings are observed to exhibit a reduction of friction with increasing humidity (Figure 3a). Oxidation of thick-MoS2 form staggered MoO3 structure, which disrupts the 2D van der Waals sliding and hence results in higher friction and wear with increasing humidity [START_REF] Curry | Impact of Microstructure on MoS2 Oxidation and Friction[END_REF][START_REF] Arif | Understanding the Independent and Interdependent Role of Water and Oxidation on the Tribology of Ultrathin Molybdenum Disulfide (MoS2)[END_REF][START_REF] Zhao | The Tribological Mechanism of MoS2 Film under Different Humidity[END_REF]. The ultrathin pristine-MoS2 sheets present herein with aligned layers resist oxidation when exposed to water. Figure 3d compares the oxygen content of pristine MoS2 sample before and after exposure to high humidity ambient conditions (RH80%). XPS characterization shows that exposure of mechanically exfoliated ultrathin-MoS2 sheets underwent minimal oxidation, as oxygen was observed to increase from ∼4 at.% to ∼6 at.%. Some oxygen is expected to be present, since the edges and defect sites are passivated with oxygen and dissociated water molecules.

One thing should be noted that, the humidity dependence of friction for 440C-steel/MoS2 here is opposite to our previous study, where the SiO2 bead was used for friction testing [START_REF] Arif | Understanding the Independent and Interdependent Role of Water and Oxidation on the Tribology of Ultrathin Molybdenum Disulfide (MoS2)[END_REF]. Therein, amorphous SiO2 counter-surface interacts weakly with MoS2 by van der Waals interaction, hence water adsorption is dominating the interfacial behavior by increasing the friction. When the MoS2 was oxidized intentionally by heat treatment, the friction was seen to further increase. In contrast, in the present study, steel countersurface exhibits a stronger interaction with the sulfur atoms, while the water is shown not to oxidize the MoS2. Interestingly, the friction and adhesion behavior for the 440C-steel/MoS2 interface do not overlap (i.e. the major variations occur at different humidity), suggesting that there is an alternate mechanism other than oxidation and meniscus effects that contributes to the tribological behavior of 440C-steel/MoS2 herein at nanoscale. Studies which have observed opposite friction and adhesion trends report alternative interfacial mechanisms dominating the behavior, such as normal load, interfacial electronic interaction and tip-sample distance [START_REF] Hasz | Experiments and simulations of the humidity dependence of friction between nanoasperities and graphite: The role of interfacial contact quality[END_REF]. To gain further insight into the role of water on the 440C-steel/MoS2 sliding interface, FFM over multiple sliding cycles was performed at RH9% and RH42% (Figure 4) while ensuring a wear free interfacial sliding regime (Supporting Information; Figure S2 andS3). Note that, the formation of chemical bonding at 440C-steel/MoS2 interface does not necessarily cause the wear failure. While the interfacial energy of metal-oxide/MoS2 (~0.1 J/m 2 ) is comparable to the adhesion energy between MoS2 layers (~0.174 J/m 2 ), [START_REF] Yadav | Interfacial Interactions and Tribological Behaviour of Metal-Oxide/2D-Material Contacts[END_REF][START_REF] Sanchez | Mechanics of spontaneously formed nanoblisters trapped by transferred 2D crystals[END_REF] the contact area is six orders lower than the MoS2 flake area, thus making the debonding and wear unlikely. Similar to earlier results (Figure 3), friction at RH42% was found to be lower than RH9% (first cycle). At low humidity (RH9%), friction as a function of cycle shows no significant change (grey band). The steady cyclic friction behavior at RH9% indicates an unchanged sliding contact since very minimal water is expected to adsorb on the MoS2 surface resulting in a direct interaction between the 440C-steel/MoS2 surface. At high humidity (RH42%), there is more surface water adsorption [START_REF] Arif | Understanding the Independent and Interdependent Role of Water and Oxidation on the Tribology of Ultrathin Molybdenum Disulfide (MoS2)[END_REF] which can sandwich and separate the 440C-steel/MoS2 interface (440C- steel/water/MoS2). The thickness of interfacial water continuously increases with rising humidity by weakly adsorbing on the 440C-steel surface forming a liquid like structure [START_REF] Barthel | Humidity Effects on Friction and Wear Between Dissimilar Metals[END_REF]. At RH42%, it takes several cycles (~4 cycles) for friction to stabilize (grey band) indicating a change in sliding contact from the removal of adsorbed water before exhibiting friction of the 440C-steel/MoS2 interface that is similar to the RH9% measurements (Figure 4). The reduction in friction at RH42% and the friction-cycle behavior infers that the presence of water between the 440C-steel/MoS2 interface can reduce friction and act as a temporary protective film. Adsorbed water between 440C-steel/MoS2 rubs off during sliding as water is weakly bonded on both the 440C-steel and MoS2 surfaces [START_REF] Levita | Graphene and MoS2 interacting with water: A comparison by ab initio calculations[END_REF][START_REF] Barthel | Humidity Effects on Friction and Wear Between Dissimilar Metals[END_REF]. It was earlier established (Table 2) that oxides (Fe-, Cr-, and Mn-oxides) on the 440C-steel surface interact strongly (chemically) with the sulfur atoms in MoS2 hence separation of the 440C-steel/MoS2 interface by water suppresses the strong interfacial interaction resulting in lower friction force. Once the temporary water protective film is removed with the increase in the number of cycles, friction stabilizes and converges into the grey band regardless of the humidity (Figure 4 & Supporting Information; Figure S2b). This work focuses on understanding the tribology of both ultrathin-graphite and ultrathin-MoS2 sliding in contact with a 440C-steel beaded counter-surface. XPS revealed the surface chemistry of the 440C-steel counter-surface which consists primarily of Fe-oxides and with appreciable regions of Mnand Cr-oxides from the alloying elements. The presence of the Fe-, Cr-, Mn-oxides on the 440C-steel surface influence the interfacial interaction regime when in contact with ultrathin-graphite (physical bonding) and MoS2 (chemical bonding) surfaces. The stronger chemical like interaction between 440Csteel/MoS2 led to higher friction, ISS and adhesion as compared to the weaker physical interaction between 440C-steel/ultrathin-graphite at all levels of humidity. Water was observed to play a contrary role as it was found to increase both friction and adhesion for the physically interacting interface (440C-steel/ ultrathin-graphite) yet was observed to reduce friction for the chemically interacting interface (440Csteel/MoS2). By sandwiching between the 440C-steel/MoS2 interface, adsorbed water molecules suppress the strong interfacial chemical interaction and acts as a temporary lubricating film. The short-lasting effect of water on ultrathin-MoS2 is unique to its size scale as thick-MoS2 coatings are known to be sensitive to humidity [START_REF] Zhao | The Tribological Mechanism of MoS2 Film under Different Humidity[END_REF]. Lastly, the nanoscale tribological behavior of ultrathin-MoS2 is oppositely influenced by water as compared to the macroscale-MoS2 against 440C-steel. This is attributed to the impact water has on MoS2 at different scales (macroscale vs ultrathin) and the interfacial mechanism dominating the behavior. At the nanoscale, the direct presence of water molecules within the contact change the surface interaction and dominate the sliding behavior whereas at macroscale it is the oxidizing role of water which changes the chemical properties of the interface material that dominates the tribology.

Fig. 1 .

 1 Fig. 1. (a) Reverse AFM image of 440C-steel bead tip and MoS2 topography. Top-right Inset: SEM image of a 440C-steel beaded tip. b) High resolution XPS spectra on 440C-steel bead powder identifying the presence of Fe, Cr, and Mn oxides and their relative ratio with respect to Fe-oxide.

Fig. 3 .

 3 Fig. 3. (a) Average friction force comparison of MoS2 and ultrathin-graphite against a 440C-steel bead counter-surface as a function of humidity. Normal load of 90 nN was applied. (b) Effect of humidity on adhesion between 440C-steel/ultrathin-graphite. (c) Effect of humidity on adhesion between 440Csteel/MoS2. (d) High-resolution XPS of O1s oxygen peak of pristine-MoS2 and high humidity exposed MoS2.

Fig. 4 .

 4 Fig.4. Normalized friction force between 440C-steel/MoS2 at RH9% and RH42% as a function of scan cycles with normal load of 90 nN. Friction was normalized relative to the minimum data point (friction for 1 st cycle at RH42%). "# of cycles" represents the number of times the same area was scanned repeatedly. The grey background highlights the friction range once the water film is removed between the 440C-steel/MoS2 contact. Bottom-left inset: Schematic of 440C-steel/MoS2 interface during the 1 st -cycle at RH42% where water separates the 440C-steel/MoS2 interface. Bottom-center inset: Schematic of 440Csteel/MoS2 interface during 5 th -cycle at RH42% after water is rubbed off the MoS2 surface. Bottom-right Inset: MoS2 topography indicating the area of interest (dashed black square) and MoS2 thickness (solid red line).

Table 2 .

 2 Summary of the interfacial interaction regimes between different oxides on the 440C-steel surface with respect to MoS2 and ultrathin-graphite.

	/ ultrathin-graphite

Acknowledgements

T.A. and G.W. contributed equally to this work. The authors acknowledge financial support from the Ontario Ministry of Research and Innovation Early Researcher Award; the Erwin Edward Hart Endowed Professorship; the Natural Sciences and Engineering Research Council of Canada (NSERC); and the Canada Foundation for Innovation (CFI).


Johnny Beaugrand
email: johnny.beaugrand@reims.inra.fr

Arnaud Day

Vincent Placet

Colloque Fatigue et durabilité des composites biosourcés 1 INFLUENCE DU VIEILLISSEMENT AU CHAMPS DES TIGES DE CHANVRE SUR LES PROPRIETES DES FIBRES INDUSTRIELLES

Keywords: Fibres de chanvre, Rouissage, Propriétés physico-chimique, Propriétés mécaniques

RESUME

Ce travail évalue l'impact d'un rouissage non-contrôlé du chanvre au champ, et du vieillissement climatique associé, sur un large spectre de propriétés physico-chimiques et mécaniques des fibres extra-xylémiennes. Cette thématique s'inscrit plus largement dans un objectif industriel de suivi qualitatif des fibres de chanvre. Les résultats montrent que cette exposition aux intempéries et aux micro-organismes induit une diminution de la proportion massique en pectines et facilite l'individualisation des fibres transformées industriellement. Elle induit également un changement de couleur, une augmentation de la rugosité de surface, une diminution du taux de cristallinité de la cellulose et des propriétés de rigidité et de résistance en traction à l'échelle des fibres élémentaires et des fibres techniques.

INTRODUCTION

Après la fauche, les tiges de chanvre sont laissées quelques jours ou semaines au champ jusqu'au séchage des pailles permettant alors leur pressage. Durant cette période plus ou moins longue selon les conditions météorologiques, la qualité des pailles peut être fortement modifiée comme ce fut le cas pour une partie des récoltes de l'année 2013 des pailles de chanvre en France. En effet, l'exposition prolongée à l'humidité du sol et à la chaleur favorise le développement de micro-organismes dont les activités enzymatiques peuvent conduire à une dégradation partielle de la matière végétale [START_REF] Ribeiro | Microbial diversity observed during hemp retting[END_REF]. Néanmoins, ce rouissage peut également être volontaire afin d'initier la séparation des tissus corticaux riches en fibres des autres tissus de la tige et ainsi faciliter la séparation et l'extraction des fibres sur les lignes de transformation industrielle (Liu et al. 2015a, 2015b[START_REF] Pickering | Optimising industrial hemp fibre for composites[END_REF]). Ainsi, ce travail propose ainsi de caractériser l'influence de ce vieillissement au champ des pailles sur les propriétés physico-chimiques et mécaniques des fibres extra-xylémiennes transformées industriellement. Les mesures ont été effectuées sur 2 lots de fibres transformées sur la ligne de défibrage de la chanvrière de l'Aube (La Chanvrière) issues de deux lots de pailles, l'un correspondant à des pailles récoltées après la coupe, l'autre à des pailles récoltées après cinq semaines après la coupe et laissées sur le sol du champs. Les deux lots de fibres ont été caractérisés à l'aide d'un large spectre de techniques expérimentales, incluant de la spectrophotométrie, des analyses morphologiques, microscopiques, biochimiques (HPLC, DRX), thermogravimétriques, mécaniques, de surface (EDX, rugosité), et hygroscopiques (DVS).

RESULTATS

Les résultats ont montré que le vieillissement des pailles au champ pour le temps d'exposition considéré induit une diminution de la largeur moyenne des éléments fibreux après transformation industrielle et une diminution en substances pectiques. Ces résultats confirment l'influence positive du rouissage sur la séparation et la décohésion des éléments fibreux, attribuée à la dégradation du ciment pectique à l'interface entre les fibres. Nous avons également observé un changement de couleur (du jaune vers le gris foncé), une augmentation de la rugosité de surface (1,4 à 2,05 m), une augmentation de la température de décomposition pour la troisième région de perte de masse. Contre toute attente, ce vieillissement n'influence pas la capacité hygroscopique des fibres testées, malgré la dégradation significative des hémicelluloses (Cf. Tab. 1). Une diminution des propriétés de rigidité et de résistance en traction à l'échelle des fibres élémentaires et des faisceaux de fibre a également été observée (Cf. Fig. 1).

Fibres non-rouies

Fibres rouies

Glucose (% de la matière sèche) 69.8±2.3 68.8±1.3

Monosaccharides hors-glucose (% de la matière sèche) 10.9±0.2 9.7±0.5

Lignines Klason (% de la matière sèche)

CONCLUSION

Les résultats montrent que le vieillissement des pailles de chanvre au champ pendant un rouissage induit une modification significative de certaines propriétés physico-chimiques et mécaniques des fibres industrielles. Ces modifications peuvent contribuer à accroitre la variabilité des lots de fibres et pénaliser, in fine, leur valorisation économique dans des marchés nécessitant une régularité qualitative d'approvisionnement tel que les industries des composites.

Fig. 1

 1 Fig.1 Fonction de distribution cumulative de la résistance (gauche) et de la rigidité (droite) en traction des fibres élémentaires et des faisceaux de fibres chanvre rouis et non rouis