S Leclercq

S Thouy

E Rossignol

Progress in understanding processes of underlying occupational accidents on the level based on case studies

Keywords: prevention, occupational accident, accident on the level, balance

The purpose of this work was to further our knowledge of contexts surrounding accidents on the level in occupational situations with a view to proposing suitable actions for the prevention of these accidents. The study, undertaken at three establishments belonging to a national rail transport company, was based on quantitative and qualitative analysis of accident-onthe-level data available at the establishments concerned, typology of these accidents, interviews with victims and activity analysis. Understanding accidents on the level through building scenarios makes it possible to consider the relevance of prevention actions such as workplace or environmental design/remediation and machine access system design. Moreover, it also makes it possible to consider curtailing the injury-causing aspect of the physical environment by reducing its "aggressiveness". Finally, we discuss the prospects emerging from this work in the research and study field.

Introduction

The frequency and seriousness of accidents on the level in occupational situations are well documented. In France, national health insurance fund CNAM (Caisse Nationale d'Assurance Maladie) has reported the following 2002 statistics for all its insured companies, representing approximately 18 million employees: 168,165 accidents on the level with days lost (22% of occupational accidents with days lost), of which 9,466 led to permanent disablement. A total of 8,135,691 days were lost through temporary disablement (23% of days lost through temporary disablement) resulting from accidents on the level and 14 such accidents were fatal (Travail et Sécurité, 2004). [START_REF] Leamon | Occupational slips and falls : more than a trivial problem[END_REF] amassed some 91,828 falls on the level (10% of occupational accidents involving a population of approximately 6 million workers). The average cost of each of these accidents was $4,363 and their combined cost represented 13% of the total cost of all occupational accidents. [START_REF] Mcnamee | Injuries after falls at work in the United Kingdom and Sweden with special reference to fractures in women over 45[END_REF] reveal that, since 1990, falls on the level have represented 25% of all occupational accidents declared in the United Kingdom. The disparity in figures concerning this risk may be explained by the multifactor nature of occupational accidents, which makes it possible to classify them into several different categories [START_REF] Anderson | Accident data in the new Swedish information system on occupational injuries[END_REF] and by different definitions or the lack of definition of these accidents [START_REF] Lortie | Reporting and classification of loss of balance accidents[END_REF][START_REF] Leclercq | Prevention of so-called "accidents on the level" in occupational situations : a research project[END_REF].

Moreover, relevant literature employs varied terminology to designate these accidents without always defining them explicitely. The literature focuses on "slips, trips or falls on the level" [START_REF] Health | Watch Your Step, Prevention of Slipping, Tripping and Falling Accidents at Work[END_REF], "accidents on the level" (Caisse Nationale d'Assurance Maladie, 1985), "falls" [START_REF] Leamon | Occupational slips and falls : more than a trivial problem[END_REF], "falls on the level" (Balance et al., 1985), "underfoot accidents", i.e. accidents in which the first unforeseen event is an interaction between the casualty's foot and a support [START_REF] Manning | The incidence of underfoot accidents during 1985 in a working population of 10000 Merseyside people[END_REF] or again "slips" [START_REF] Gronqvist | Serious occupationnal accidents caused by slipping[END_REF]. The lack of definition for targeted accidents and the multi-factorial nature of accidents lead to a different possible classification for each of them. For example, accidents occurring on stairs are sometimes classified as "accidents on the level" in some French companies, the "falls from a higher level" category being reserved for high-level falls (e.g. fall from a work cradle or cherry picker). At other companies, accidents occurring on stairs can be listed as "falls from a higher level", especially when there is no risk of falling from a very high level.

The prevention field features relatively few research studies and practical measures despite the importance of the human and financial issue represented by accidents on the level in occupational situations. [START_REF] Leclercq | Prevention of so-called "accidents on the level" in occupational situations : a research project[END_REF] refers to a number of curbs to preventing these accidents: they are considered minor and commonplace, and are not viewed as "trade/occupation-related accidents", the victim's behaviour is often the only factor questioned, there are no aids specific to controlling this risk and, finally, the risk of an accident on the level is diffuse, i.e. these accidents involve generally only one victim. Their impact is therefore less than that of an infrequent accident involving several victims. In-company awareness therefore represents an essential link in the prevention chain, whose aim must be, in particular, to consider the risk of an accident on the level as an occupational risk in its own right and to prompt analysis of these accidents to ensure their prevention.

The accident situation on which prevention efforts are currently focused is the slip, when the individual is walking on slippery floors (greasy floors within work premises or floors covered by snow or ice outside premises). Recommended actions (laying of antislip floor covering inside premises or wearing of anti-slip shoes) are designed to prevent a slip or make it more controllable. In this area, research and studies hinge around methodology for measuring slip resistance (Leclercq, 1999a;[START_REF] Courtney | The measurement of slipperinessan international scientific symposium[END_REF].

Nevertheless, a foot slipping on a greasy floor, when a person is walking within work premises does not trigger every accident on the level. Evidence is indeed provided by statistical studies based on occupational accidents amassed on databases [START_REF] Strandberg | The dynamics of slipping accidents[END_REF][START_REF] Strandberg | Ergonomics applied to slipping accidents[END_REF][START_REF] Manning | The incidence of underfoot accidents during 1985 in a working population of 10000 Merseyside people[END_REF][START_REF] Gronqvist | Serious occupationnal accidents caused by slipping[END_REF][START_REF] Kemmlert | Slips, trips and falls in different work groups with reference to age[END_REF][START_REF] Leclercq | Serious falls on the level in occupational situations[END_REF] or, again, by more clinical studies focusing on in-depth analysis of accidents on the level [START_REF] Bentley | Slip, trip and fall accidents occurring during the delivery of mail[END_REF][START_REF] Leclercq | Systemic analysis of so-called "accidents on the level" in a multi trade company[END_REF].

Unfortunately, prevention specialists remain powerless in the face of these many other accidents on the level. INRS has conducted a research project in this area; its ultimate aim is prevention of accidents on the level in occupational situations, i.e. prevention of accidents triggered by balance disturbance during work not performed "at height". The project scope includes ground surfaces both without changes of level and with changes of level, such as sidewalk curbs, steps or slopes and the work presented here falls within this scope. It is intended to broaden knowledge of the circumstances surrounding accidents on the level at work based on systemic accident analyses [START_REF] Leclercq | Prevention of Falls on the Level in Occupational Situations: A Major Issue, a Risk to be Managed[END_REF][START_REF] Leclercq | Prevention of so-called "accidents on the level" in occupational situations : a research project[END_REF]. The work situation can effectively be considered as a system with four components: the individual, the task he performs, the equipment he uses and the environment in which he walks [START_REF] Monteau | Essai de classement des risques professionnels et des actions de prévention[END_REF]. Accident-on-the-level factors identified from accident analyses or the literature are linked to all system components [START_REF] Leclercq | Prevention of Falls on the Level in Occupational Situations: A Major Issue, a Risk to be Managed[END_REF]. They reflect disturbances within a component or during interaction between different components. Within the scope of this project, accidents on the level will be characterised as a whole, for example, by a combination of accident factors.

Indeed, these accidents are rarely analysed in depth by companies and knowledge concerning them is therefore often limited to "immediate" accident factors involving a component of the physical environment (e.g. an obstruction or slippery floor). Whilst this environmental accident factor is necessary to accident occurrence, it is insufficient on its own. This is why a work situation often reveals isolated accident-on-the-level factors (an obstruction, rapid displacement speed, etc.), which do not contribute to creating an accident. It is a combination of risk factors that causes an accident-causing situation [START_REF] Monteau | dérives" et autres imprudences[END_REF].

With particular consideration for the diversity of accident situations causing accidents on the level (Bentley andHaslam, 1998, 2006;[START_REF] Leclercq | Systemic analysis of so-called "accidents on the level" in a multi trade company[END_REF], systemic analyses appear unavoidable for achieving the twin objective of identifying work situations likely to cause accidents on the level at a company and understanding what factors, arising at a given moment during the implemented activity in the working environment (in the term's widest possible meaning) and under work situation constraints, prevent the individuals from avoiding disturbance of their physical balance.

For the company concerned, these analyses thus allow prevention field priorities to be derived and knowledge of contexts in which these accidents occur to be amassed within a perspective of generalising certain results.

Method

The approach implemented in this study conducted at three establishments is globally the same as that adopted by the same authors at another company [START_REF] Leclercq | Systemic analysis of so-called "accidents on the level" in a multi trade company[END_REF].

Brief description of establishments

This study was conducted in 3 out of 300 establishments belonging to a national rail transport company. Each of these 3 establishments fulfils different missions and employs different trade/occupations.

-Establishment A groups together 913 employees, including 486 working in the commercial development sector (selling of rail tickets, passenger reception and safety) and 352 working in the production and transport sector (train marshalling manoeuvres, operating points, etc.). 76 employees work in other sectors within the establishment.

-Establishment B groups together 1057 employees around 4 activity sectors: the train drivers sector employing 653 train drivers, the logistics sector with 60 employees working on maintenance, certain engine breakdowns and engine movements, the engine maintenance sector with 150 employees and a sector employing office workers and cleaning staff .

-Establishment C grouped together 602 employees in 1999 and 2000, and 717 employees in 2001 and 2002. These employees check tickets and ensure certain services on trains as well as passenger safety. 279 employees who work in main line services, 167 in inner suburban line services and 156 in 1999 and 2000, or 271 in 2001 and 2002, in home station-based regional line services.

Analysis of accident-on-the-level information available at the company

Description of initial injury data.

Every occupational accident, whether it led to days lost or not, is recorded by the company and is associated with a number of variables, some of which are systematically, and others infrequently, included in the accident record. These data are:

-the accident date and time, -the victim's identity and age, -the accident geographical site and location, -the accident class (accident on the level, handling accident, road accident, etc.), -the type of work performed and the victim's activity at the time of the accident, -the tools and personal protective equipment used, -the climatic and environmental conditions, -the chronology of events leading to the accident, -the accident consequences (injury and number of days lost).

Most accidents on the level covered by the definition provided in the introduction to this paper are listed in the "accident on the level" or the "falls from a higher level" categories. In this company, no occupational accident caused a fall from a great height.

Our study therefore focused on these two occupational accident categories, to which we will refer to as "accidents on the level" in the remainder of this document. Most of the commuting accidents are categorized as "accidents on the level". They might have different causes and consequences to accidents on the level that occur at work sites.

However, if all accidents on the level that occur at work sites in a multi-trade company are considered, these do not have the same causes and consequences either. In this study, accidents on the level occurring at work sites and commuting accidents are considered together because company was concerned with preventing accidents on the level as a whole. Commuting accidents represented less than 10% of accidents taken into account. Moreover, within the scope of in-depth accidents analyses, potential specific characteristics of commuting accidents will be disclosed. Investigations were held at the three establishments in 2002 and 2003. At each establishment, available information on accidents on the level with and without days lost, which had occurred during the four years preceding the investigation, was analysed. Although a more sizeable volume of data would have been available, earlier information was not used for two reasons: firstly, for fear of distancing ourselves from current reality (constantly changing work situations) and secondly, in order to be able to acquire the maximum amount of information concerning the victims' past accidents. As far as prevention is concerned, accidents on the level with and without days lost have been taken into account, except for the quantitative analyses as explained in the next paragraph.

[Insert table 1 about here] 2.2.2. Quantitative analysis. For each establishment, the relative proportions were first calculated, represented by accidents on the level and days lost because of these accidents amongst all occupational accidents and corresponding days lost. A frequency index was also calculated for accidents on the level with days lost.

Frequency indices were then calculated within each establishment according to age and trade/occupation, to observe whether some populations are more affected than others by these accidents. Indeed, unlike many categories of occupational accidents, every person is effectively exposed to the risk of an accident on the level. However, previous research has shown that populations could be more or less affected, depending on their age or trade/occupation [START_REF] Buck | Slipping, tripping and falling accidents at work : a national picture[END_REF][START_REF] Kemmlert | Slips, trips and falls in different work groups with reference to age[END_REF][START_REF] Gauchard | Falls and working individuals : role of extrinsic and intrinsic factors[END_REF][START_REF] Leclercq | Systemic analysis of so-called "accidents on the level" in a multi trade company[END_REF]. The frequency index was calculated using the following formula:

Frequency index = (number of accidents on the level in the population concerned during N years x 10 3 )/(number of persons in population concerned x N)

Only accidents on the level with days lost were used for calculating frequency indices because it was assumed that accidents on the level without days lost could be less systematically declared and that, in this case, the number of accidents listed might be less representative of the real number. In establishments A and B, there were few between values was tested at a confidence level greater than 90%. Real confidence levels (greater than 90%) will be referred to and not the threshold value considered for the test (90%).

When information was available concerning the time periods that had passed between accident occurrence and the last rest day and between accident occurrence and starting work, the number of accidents with days lost were represented according to these time periods.

Search for recurrent accident scenarios. Accidents on the level occur in

varying circumstances and this is why recurrent accident scenarios and the resulting average number of days lost at each establishment were sought. For this, accident typology was first established based on the victim's activity at the time of the accident and the accident location. This information is very often recorded and represents two essential components of the work situation [START_REF] Monteau | Essai de classement des risques professionnels et des actions de prévention[END_REF]. Only some of the accidents amassed could be used for this typology, which provided several accidentcausing situations (cf. table 1). For the others, there was no victim activity at the time of the accident and/or accident location. Secondly, analysis of some accident-causing situations were further clarified by interviewing victims to build up the logical chain of events leading to injury. This latter process was performed using the causal tree method [START_REF] Monteau | dérives" et autres imprudences[END_REF]. This method allows phenomena generated by the work situation and contributing to the accident to be sought. This search starts from the injury itself (the first known event) and works its way back systematically, step by step, asking the following questions for each known event: "What event was required for it to appear?" -"Was another event required?", until a "no" answer is given. Questioning enables events to be interlinked. Examples of causal trees have been given within the context of a previous study [START_REF] Leclercq | Systemic analysis of so-called "accidents on the level" in a multi trade company[END_REF]. For each situation considered, the number of accidents on the level and the number of victims interviewed by an ergonomist are given in Section 3.2. The time from the injury was variable but less than 4 years, the period considered for the study. The three establishments combine more than seven trades. For each one, the work activity was observed and work activity-related verbal information was obtained from operatives to enhance the understanding of the presence and role of certain accident factors and to highlight accident on the level potential factors. Observations and work activity-related verbal information were guided by hypotheses derived from previous analyses (quantitative analysis, causal trees). The frequency or type of movement and the premises or environment, in which movements occurred and which provide a potential link with accidents on the level, will be evoked through accident scenarios. Based on the knowledge from causal trees and data relating to employee activity, an accident scenario was built up for each accident-causing situation. This scenario consolidates elements of understanding in relation to various accidents on the level and will form a basis for discussion around possible prevention paths. The numerous accident-causing situations meant that only some of them could be analysed in depth. The chosen situations were the most frequent or serious (according to the number of days lost) accident-causing situations.

3.Results

Quantitative analysis

For each establishment, table 1 shows the number of accidents on the level during the 4 years, the workdays lost as a result of these accidents, the relative proportion of accidents on the level and days lost due to these accidents in relation to all occupational accidents and days lost, along with a frequency index for accidents on the level with days lost. This table confirms that accidents on the level represent effectively a large proportion of occupational accidents with days lost and that their seriousness at establishments A and B, evaluated from the proportion of days lost, is greater than the average seriousness of accidents with days lost. It also shows that employees at establishment C sustained nearly three times the number of accidents on the level with days lost as establishment A and B employees.

[Insert table 1 about here]

For each establishment, again, figures 1 and 2 represent the frequency index for accidents on the level with days lost according to the victim's age and trade/occupation respectively. These figures show that, depending on their age, employees were not equally affected by accidents on the level with days lost. This was observed at all three establishments.

Figure 1 shows that, at establishment A, employees aged between 18 and 24 years and those aged between 45 and 54 years sustained more accidents on the level with days lost than employees aged between 25 and 34 years. The difference of the order of 10‰ is significant at a confidence level exceeding 95%. At establishments B and C, the youngest employees were most affected by accidents on the level with days lost: at establishment B, a difference of approximately 15‰ between employees in the 18 -25 age bracket and other employees was calculated. The frequency indices are different at a confidence level exceeding 93%. At establishment C, a difference of between 40‰ and 80‰, depending on the age bracket and years considered, between employees in the 18 -35 age bracket and other employees was calculated. The frequency indexes are different at a confidence level exceeding 95%.

Figure 2 reveals that, at establishment A, production-transport sector employees sustained more accidents on the level with days lost. The difference is of the order of 7‰, which is statistically significant at a confidence level exceeding 90%. At establishment B, the logistics and engine maintenance sectors were more affected than the train drivers sector or the office workers and cleaning staff sector. Differences between frequency indices, of the order of 10‰, are significant at a confidence level exceeding 90%, except between the train drivers and logistics sectors for which the difference is not significant. At establishment C, guards/inspectors on home station based regional trains sustained more accidents on the level with days lost. In years 1999 and 2000, the differences between the frequency index for this unit and those for the other two units were 13‰ and 30‰ respectively. These differences are significant at a confidence level exceeding 95%. In years 2001 and 2002, only the 28‰ difference between the frequency index for this unit and that for main lines is significant at a confidence level exceeding 95%.

[Insert figures 1 and 2 about here] For establishments B and C, figure 3 represents the number of accidents on the level that occurred in relation to the number of days that passed between accident occurrence and the last rest day. For establishment B, figure 4 represents the number of accidents on the level that occurred in relation to the number of hours that passed since starting work. Only these graphs could be drawn up from the available data. They highlight greater occurrence of accidents on the level on days following a rest day and during the first hours after starting work.

[Insert figures 3 and 4 about here]

Search for recurrent accident-on-the-level scenarios

For each establishment, table 2 displays an accident-on-the-level typology based on the victim's activity at the time of the accident and the accident location as well as the average number of days lost for each type of accident on the level. This table shows the wide diversity of accident-on-the-level contexts, not only within several establishments, but also within a single establishment. Accident-causing situations, displayed in bold print in table 3, were analysed in depth because of the large number of resulting days lost or their frequency. These analyses were based on summarised information concerning these accidents complemented by information provided by interviews with the victims concerning a limited number of accidents as well as activity analysis. This made it possible to propose an accident scenario for each situation; these scenarios are described below.

[Insert table 2 about here]

In the case of establishment C, combined analysis of employee activity and accidents led us to modify accident typology, grouping together accidents sustained by employees inspecting home station-based regional trains. These accidents would in fact be closely linked to the type of train considered. Amongst the 69 accidents included in this typology, employees from this unit sustained 27. 15 accident reports stated that these accidents occurred just before train departure (6 accidents when carriage doors were closing, 7 when climbing up into the train and 2 when walking on the platform) and 5 accidents resulted from slipping on a running board.

The employee is on the platform, supervising passenger movement just before departure of this type of train. He whistles to warn them that the train is going to leave and that they should stop climbing aboard. He climbs onto the train and activates the system closing all the carriage doors except for the door to the compartment, in which he himself is standing. The train moves away and the employee watches the passengers remaining on the passing platform. He remains ready for anything (a passenger jumping on after train departure) and he can stop the train at any time. As soon as the train has gathered speed, the employee closes his own door.

The access system for these trains, shown in figure 5, conditions the employee's activity. The central vertical bar (excluded from some carriages) and the running board outside the doors make it possible for passengers to climb on to the running board and hold on to the vertical bar after train departure. Moreover, the doors do not always close fully and they can sometimes be reopened. This is why it happens that passengers try to force an entry at the last moment, even when the train is already moving. The guard's/inspector's attention is therefore focused "for a long time" on movement of passengers to ensure their safety. Furthermore, the running board is exposed to climatic conditions and its construction does not allow drainage of water, which can freeze in winter and may be a cause of slipping. On other trains, the running board can be protected by the carriage doors or be retractable. Finally, the carriage door opening and closing system shown in figure 5 is subject to malfunctions. Accident reports state that the guard's/inspector's door in fact closes when he has actuated closure of all doors except his own. Observations have revealed that, in some cases, the employee places his foot against the door to prevent it from closing in front of him. The potential impact of the access system on the occurrence of accidents on the level could explain the fact that the guards/inspectors working on these trains sustained the greatest number of accidents on the level.

[Insert figure 5 about here] Scenario 1 -Establishment A -Accidents occurring when walking on a flat surface (7 accidents -5 interviews). A person walks at normal speed at his normal workplace or on his/her usual way to/from work. The ground surface has become temporarily slippery (due to climatic conditions or presence of wet cleaning product). Caught unawares by the unexpected slipperiness of the floor, the victim's foot slips uncontrollably on the floor (accelerated slip over an excessive distance). The person's balance is disturbed and he/she falls.

Comments. In 5 cases out of 7, the accident occurred around the time at which the victim started work (between 30 minutes before and 2 hours after). Collected information offers no explanation and victims are not generally subjected to a time constraint. It may be possible to explain the fact that these accidents were more frequent around at which victims started work as follows: between home and workplace, people are often confronted by variations in the state of the ground surface on which they walk, i.e. damp pavements, ice sheets, etc. The risk of being caught unawares by a slippery ground surface or an obstruction is therefore higher.

Scenario 2 -Establishment A -Accidents occurring when walking on a surface featuring a small obstruction (12 accidents -6 interviews). A person walks at normal speed in a familiar place, which is not his/her usual workplace. The ground surface features a small obstruction that the victim does not see because he/she is looking elsewhere (another person to whom he/she is talking, the car to which he/she is walking, etc.). The victim trips, but recovers his/her balance.

Comments. At least three accidents occurred in a corridor, in which there was a single step (2 cases) or a different step (non-uniform step height, which decreases over its whole width -1 case).

Scenario 3 -Establishment A -Accidents occurring when going up or down stairs (16 accidents -5 interviews). A person at his/her usual workplace or on his/her normal way to/from work goes down stairs often at high speed (to avoid the rain or to save time, when on the move; time he/she wants to devote to other tasks). The stairs are in bad condition or are slippery. The person's foot slips on a step. The slip is neither long not necessarily accelerated, but the victim, whose foot slides into space, falls.

Comments. These accidents are similar to scenario 1 slips in terms of familiarity with location, the fact that the person was on the move without pursuing any other physical activity and the accident (fall) consequences. They differ due to a probably less critical degree of slipperiness, but the risk of falling is greater in scenario 3, not only because of the small step area and the descending activity, but also because persons were sometimes moving fast.

Scenario 4 -Establishment B -Accidents occurring when climbing up onto or down from engine running boards (44 accidents -6 interviews).

The employee climbs down, whilst facing the engine. He is not in any particular hurry and he is familiar with both the work location and situation. The running board steps are one above the other like ladder rungs. They are flush with, or project only little from, the engine. The distance between steps often varies and the height of the step nearest the ground depends on the location at which the engine is stopped. If this is alongside a platform, the height of the bottom step will be reduced by the height of the platform above ground. If this is alongside the space between lines or a track, the height of the bottom step will be greater. It can be of the order of 60 centimetres. The handrails held by the employee, when climbing up or down are approximately 120 centimetres from the ground. This height is again variable depending on the type of engine. The employee cannot see where he places his foot. His three of four climbing holds (both hands, one or two feet) are almost in a vertical plane. His two feet are close together on steps, whose depth is sometimes of the order of only 15 centimetres. His balance is somewhat unsteady and the higher the bottom step and handrails, he more has to pull up with his arms, when climbing up, and to falling down by supporting part of his weight with his arms. When the bottom step is high, he lets his hands slide over the handrails when climbing down and releases them as soon as his foot touches the ground. The higher the bottom step, the greater impact on his foot, when it touches the ground. Thus, on uneven ground (e.g. coarse ballast), the higher the step and the more uneven the ground, the greater the likelihood of an ankle injury. This is what happens in some accident cases. In others, the employee feels shoulder pain at the instant his foot touches the ground. In this case, it would seem likely that the forces in his arms, when climbing up onto or down from the engine, have contributed to this injury.

Comments. Employees complain of handrail slipperiness. They exert regularly forces on their arms to lift or restrain their bodies, when climbing up or down respectively.

Handrail slipperiness therefore inhibits them on occasion, whilst at other times they let their hands slide over the handrails to complete their descent from high bottom steps.

Scenario 5 -Establishment B -Accidents occurring when walking on lines or tracks (21 accidents -9 interviews). Train drivers sustained four accidents, which shared common features and occurred during inspection prior to train departure. Following these analyses, we therefore propose two scenarios, of which the more detailed (scenario 5b) corresponds to these four accidents. Scenario 5a. The situation is of an emergency nature. The employee walks hurriedly on the line or track. He does not see the obstruction, often because he could not see it (concealed obstruction, night-time, attention focused on other situation elements). He trips or twists his ankle; he falls in one case out of three. Scenario 5b. The employee performs an inspection prior to train departure. The situation is unusual (e.g. emergency stop, problem detected during inspection); the employee is worried, even panicking (case of one 25 year-old employee with less than 3 years experience, confronted by a breakdown he has never encountered). He is worried about delaying train departure and he walks hurriedly along the train or on the line. He does not see the obstruction on, or unevenness in, the ground. He trips or twists his ankle; he falls in one case out of two, sometimes onto the engine.

Scenario 6 -Establishment C. Accidents sustained by employees on a train type with an access system shown in figure 5 (27 accidents -4 interviews).

Scenario 6a (3 accidents). After whistling train departure, the employee walks on the platform to climb aboard the train. His attention is focused on movement of passengers, whose safety he must ensure. He trips in a hole or on a difference in level on the platform or he misses the step, when trying to climb onto the train.

Scenario 6b (1 accident).

The employee slips on the running board, when climbing down from the train in the rain.

Comments. Employees in this unit are often required to initiate train departure (train stopping every 4 minutes on some lines) and are often exposed to these accidents. Most recruits start in this unit, which is characterised by its young personnel.

With regard to the second category of accidents occurring when climbing up onto, or down from, running boards, employees working on inner suburban trains sustain these more frequently (15 victims out of 22). They occur more often when climbing onto the train (14 cases) than when climbing down from the train (8 cases). The above paragraphs concerning the work situations of employees working on home station-based regional trains provide data for understanding these results. Comments. In this accident case, we may ponder the part played by carrying bags in accentuating balance disturbance. Moreover, this accident reveals a seriousness factor (contributing to injury aggravation): the gap between the platform and the train.

Amongst the 22 accidents that occurred when climbing onto or down from a train, 6 accident reports refer to the employee's leg or trunk slipping down between the platform and the train. Height and gap between platform and train are in fact highly variable depending on the station and even at the same station and there may be differences in level on the platform or curved rail tracks.

The other two interviews concerning accidents occurring when climbing onto, or down from, running boards were not triggered by balance disturbance: the employee, carrying his bags with work equipment, felt a pain in his back or hip, when he climbed onto, or down from, a stationary train.

Scenario 8 -Establishment C. Accidents occurring when walking down a moving train (14 accidents -4 interviews). When walking down a moving train, an employee's balance is disturbed by a sudden movement of the train and/or he trips on an item of luggage. In both cases, injuries result from the fact that the individual collides with his immediate environment.

Comments. In both cases, the employee's balance was disturbed due to luggage in the central corridor. Passengers leave their luggage here notably because luggage racks are full. In some carriages, steel bars have been installed under seats to prevent bags being placed or trapped there. So, some bags are left in the central corridor. In other cases, passengers want to keep their luggage near them for fear of theft. A lone guard/inspector does not always dare have the corridor cleared by passengers.

The employee who lost his balance collided with his immediate environment in two accidents. In the first case, this involved a bench seat in a carriage, in which these were not in line, obliging the guard/inspector to move "obliquely" in the central corridor and resulting in impacts to the legs, when the employee's balance is unexpectedly disturbed.

In the second case, the accident involved a luggage rack. The employee standing in front of the guard's/inspector's compartment had his balance disturbed by a sudden movement of the train. He collides with the luggage rack in front of the guard's/inspector's compartment. This luggage rack was originally fitted with metal parts that could be raised to allow stowage of bulky bags. These parts were missing and the remaining structure featured projecting corners at head and lower back levels.

Discussion

In-depth analyses of accidents on the level conducted by INRS [START_REF] Leclercq | Serious falls on the level in occupational situations[END_REF]Tissot and Leclercq, 2005;[START_REF] Leclercq | Systemic analysis of so-called "accidents on the level" in a multi trade company[END_REF] have permitted a clear definition of accidents to be prevented. The term "accident on the level" has been substituted for the more restricting term "fall on the level" because many occupational accidents triggered by balance disturbance have not resulting in the victim falling [START_REF] Leclercq | Systemic analysis of so-called "accidents on the level" in a multi trade company[END_REF]. As examples, we can also refer to scenarios 4 and 5 of this study (cf. § 3.2.), which describe cases in which the victim did not fall, yet a sprain resulted from disturbance of his/her balance.

This study confirms both the necessity and the insufficiency of an approach exclusively centred on preventing a slipping event. Work carried out to prevent slipping is essential because it provides a specific response to one category of accidents on the level, resulting most often in a fall. Accidents on the level triggered by the foot slipping on the ground (cf. scenarios 1 and 3) are effectively the only ones to have systematically caused the victim to fall. On the other hand, this research remains insufficient because other events may trigger accidents on the level: a trip (scenario 2), brutal impact of the foot on uneven ground (scenario 4), acceleration or deceleration of a means of transport (scenario 8). The authors therefore consider that notion of balance disturbance corresponds better to the triggering event with respect to these accidents. Finally, only consideration of the accident in relation to its dynamics, i.e. upstream of the triggering event, allows us to foresee with greater breadth actions in the prevention field. Understanding of accidents on the level provided by building scenarios enables us to study the suitability of the following prevention actions: design/alteration of rooms/environments (scenarios 2 and 7), design of engine access systems (scenarios 4 and 6). These scenarios also allow us to envisage reducing the injury-causing aspect of the environment (scenarios 7 and 8). It should indeed be remembered that injury seriousness could be curtailed, in particular, by reducing the "aggressiveness" of the immediate physical environment [START_REF] Leclercq | Prevention of so-called "accidents on the level" in occupational situations : a research project[END_REF].

Frequency indices calculated in relation to age or trade/occupation exercised by the victim show that individuals were not equally affected by accidents on the level. These results back up the hypothesis according to which factors governing these accidents should be sought, in particular, in work situation characteristics (experience, activity constraints, equipment used, etc.).

At two establishments out of the three examined in the present study, the youngest employees were also affected more than others by accidents on the level with days lost.

At establishment C, the difference amounts to 80‰ depending on the ages considered.

In this establishment, junior employees (in general the youngest) start their career by working on home station-based regional trains. Observations have led to questioning equipment characteristics in relation to accidents on the level. These characteristics indeed condition employee activity (see Figure 5 and Scenario 6).

The observation according to which young employees were more affected by accidents on the level at most of the studied establishments and in a previous study on accidents on the level in a multi-trade company [START_REF] Leclercq | Systemic analysis of so-called "accidents on the level" in a multi trade company[END_REF]) cannot be generalised. Based on national statistical data provided by the Health and Safety Executive (HSE), [START_REF] Buck | Slipping, tripping and falling accidents at work : a national picture[END_REF] have shown that the frequency level of "slips, trips and falls on the level" occurring in occupational situations increases with employee age (between 16 and 60 years). [START_REF] Kemmlert | Slips, trips and falls in different work groups with reference to age[END_REF] have observed from analysis of 1600 "slips, trips and falls" at work that persons older than 45 years sustain more of these accidents than younger persons. On the other hand, Bentley

and Haslam (1998) have observed no significant age effect on the occurrence of falls amongst postmen. The results of the present study reveal that reasons which could explain the higher aged-related frequency of accidents on the level in occupational situations should be sought elsewhere than in the susceptibility to falls of elderly persons; a susceptibility that is indeed apparent in the non-working population older than approximately 70 years (Pyykö et al., 1990;[START_REF] Alexander | Postural control in young and elderly adults when stance is perturbed : kinematics[END_REF][START_REF] Perrin | Mécanismes de l'équilibration humaine[END_REF]. [START_REF] Laflamme | Aging and occupational accidents. A review of the literature of the last three decades[END_REF] have analysed the relation between age and occupational accidents based on literature published in the 30 years preceding their study, with the aim of understanding the contradictory results reported therein. The authors observe that large scale studies, which assume that work-related risk factors are identical for all age groups (highly questionable), can give results in conflict with studies conducted at a more refined level, often within a specific activity sector. Greater age usually leads to a reduction in physical and mental capacities, but it also prompts more efficient use of resources due to experience. Moreover, as age increases, injuries are more serious and recovery slower in the event of an accident. The age/accident relationship is subject to many factors, which cannot be taken into account in studies of a statistical nature. [START_REF] Laflamme | Aging and occupational accidents. A review of the literature of the last three decades[END_REF] refer to the difference in risk exposure according to age group as a factor that could explain the differences in agerelated accident occurrence. These results point out that even within the same trade/occupation (train guard/ticket inspector), employee exposure varies effectively according to the type of carriage and, in particular, the access system equipping the carriages in which these employees work. A study of the impact of living experience under a winter climate, including snow and ice, on slip occurrence, conducted by [START_REF] Gao | Slips and falls on ice and snow in relation to experience in winter climate and winter sport[END_REF], stresses the difficulty in distinguishing the age factor from the experience factor. [START_REF] Cellier | Effects of age and level of work experience on occurrence of accidents[END_REF] studied the effects of age, work experience and their interaction on accident occurrence at several food sector companies. This work revealed the need to identify, as closely as possible, work situation constraints and personal characteristics both in terms of age and experience, when studying the impact of these different factors on the occurrence of accidents, in particular, accidents on the level. The authors also conclude that mechanisms operating in accident occurrence can be identified through clinical analyses of these same accidents. This type of analysis conducted within the scope of the present study makes it possible effectively to explain a number of unexpected results based on more detailed knowledge of the work situations concerned. For example, we observed that accidents on the level sustained by train guards/inspectors when climbing onto or down from running boards most frequently occur when entering a train than when leaving a train. This result is unexpected in the sense that literature concerning accidents occurring when going up or down stairs reports a higher accident frequency when descending [START_REF] Cohen | An analysis of occupational stair accident patterns[END_REF][START_REF] Nagata | Occupational accidents while walking on stairways[END_REF][START_REF] Jackson | An In-Depth Investigation of 40 Stairway Accidents and the Stair Safety Literature[END_REF] and that the stair descending activity is more dangerous, in particular because of the acceleration due to gravity and height of a fall in the event of balance disturbance [START_REF] Templer | The staircase : Studies of Hazards, Falls and Safer Design[END_REF][START_REF] Roys | Serious stair injuries can be prevented by improved stair design[END_REF]. In the particular situation involving guards/inspectors working on the type of carriage shown in figure 5, activity analysis revealing that the employee's visual attention is focused on passenger movement, when he climbs onto the train, makes it possible to understand the frequency of accidents on the level that occur when climbing onto running boards. It should be stressed that, amongst sensorial information contributing to balance adjustment, visual data is the only information that allows the risk of balance disturbance to be anticipated before entering a risk zone (obstruction, slippery floor, etc.). Vestibular and proprioreceptive information reflect balance disturbance only when it occurs [START_REF] Patla | Understanding the roles of vision in the control of human locomotion[END_REF]Leclercq, 1999c). Accident-on-the-level scenario 6, referred to above, also reminds us that visual data is information that is shared between the task to be performed and the implicit task of maintaining physical balance and that, in some situations, this information can be lacking in relation to maintaining balance. In the scenario 2 case, the individual overlooks the obstruction because it is unexpected (a step in the corridor) and, moreover, the individual doesn't find himself in his usual workplace. It would seem that he does not focus his attention of the environment because he assumes that the corridor within the premises is a flat walking surface.

Certain aberrations therefore appear when designing or altering the premises because of concerns for easiness, cost saving or aesthetics. The implicit argument that prevails in these situations is that a step is visible and can thus be anticipated. It is forgetten that the individual cannot always see the step and that he often makes assumptions in an unfamiliar environment; for example, he assumes that "a walking surface in a corridor is flat".

Accidents on the level sustained by train drivers when climbing down from running boards (scenario 4) are totally different to accidents on the level sustained by train guards/inspectors, when climbing up onto or down from running boards (scenarios 6a and 7). In the prevention field, paths to progress emerging from an understanding of accident processes differ according to situations.

The results also indicate that accidents on the level occurred most frequently on days following a rest day and during the first hours at work. This observation, recurrent at several establishments and already expressed in a previous study [START_REF] Leclercq | Systemic analysis of so-called "accidents on the level" in a multi trade company[END_REF], deserves more systematic investigation and analysis. The literature dealing with accidents on the level provides no hint of an explanation to these results. In relation to occupational accidents in general, [START_REF] Davezies | Mythiques tricheries du lundi[END_REF] explains the higher frequency of occupational accidents on Monday by the change in rhythm, which imposes on an individual a higher burden than that sustained under steady operation.

The present study involved three establishments and a diversity of processing operations: quantitative and qualitative analysis of data on past accidents on the level; accident on the level typology; interviews; activity analysis. This processing diversity was made necessary by the desire to consider the heterogeneous combination of accidents on the level, which occurred in work situations that were themselves different.

The analyses are descriptive (especially the quantitative data processing) and some of them could not be carried out in depth, in particular with regard to activity analysis because detailed analysis of multiple tasks is unfeasible in this study. In the authors' view, the major limit affecting the whole of this work resides in the empirical nature of the adopted approach. For example, accidents on the level were classified in this study based on the activity at the time of the accident and the accident location. It should be emphasized that, within the same trade/occupation (train guard/ticket inspector), an additional criterion determining the guard's/inspector's activity turned out to be essential to characterising accidents on the level. This third criterion was the type of carriage, on which the guards/inspectors worked. Enhanced knowledge and concepts concerning these accidents and their prevention should lead to optimisation of their necessary classification and more generally to approaches that could be less empirical.

Conclusion

In 2004, Leclercq and Thouy highlighted the diversity of circumstances surrounding accidents on the level within a multi-trade company, the diversity of accident factors and the recurrent nature of certain accident-on-the-level scenarios. The study presented here points out a greater diversity of accident-on-the-level contexts within several establishments and, in particular, different contexts within a same trade. This diversity of accident-on-the-level contexts dictates the importance of being able to group accidents together. Although these accidents are widespread in terms of place of occurrence, equipment used and activity developed, some of them present similarities in their progression and can give rise to similar prevention measures. A diagnostic stage prior to devising suitable prevention actions seems to be necessary. Provision of diagnostic data implies possessing sufficient understanding of phenomena.

Analyses performed at the company concerned have enabled multiple possible paths to progress in the prevention of accidents on the level to be proposed. Beyond this objective, amassing of results from detailed analyses ensures progress in understanding these accidents by accrediting or invalidating assumptions concerning, for example, the impact of age or experience. Results collected and discussed here plead in favour of a systemic approach of these accidents to get a better understanding of the reasons for their occurrence, which is subjected to numerous accident factors combinations.

The challenge represented by accident-on-the-level prevention is major at both human and financial levels. Today, research and practice are incompatible with the risk involved. This work reveals a number of risk factors associated with the occurrence of these accidents in occupational situations. It preserves an investigative character, given the small number of studies in the field of these accidents and the diversity of circumstances covered by them. It appears necessary to continue research with the aim of characterising more accurately and exhaustively accident scenarios, for example, and of investigating the degree of generalisation of these scenarios or, again, of explaining the reasons for the unsuitability of personal balance control strategy in these situations.

In particular, this study confirmed that in some occupational situations, individuals are unable to cope with a potential balance disturbance because their visual attention is directed to their task at hand and lack of control of balance. From a methodological standpoint, tools and methods for analysing accidents in general have been used. It would also be helpful to make progress in this field, for example by developing a diagnostic approach. Table 1: Period and number of accidents considered at each establishment.

Table 2: Relative proportion of accidents on the level and days lost due to these accidents with respect to all occupational accidents and days lost, along with frequency indices for accidents on the level with days lost, for each of the 3 establishments considered.

  personnel movements during the 4 years considered and the age-and trade/occupationbased population distributions, taken into account for calculating these indices, are those corresponding to the years 2001 and 2003 respectively. Regarding establishment C, 120 employees joining at the start of 2001 led to calculating these frequency indices firstly for years 1999 and 2000 and secondly for years 2001 and 2002. Age-and trade/occupation-based population distributions taken into account for calculating frequency indices are those for years 2000 and 2002 respectively. Frequency index values were statistically compared using a proportional comparison test. The difference

  Scenario 7 -Establishment C. Accidents occurring when climbing onto or down from train running boards (7 accidents -22 less 15 considered in previous scenarios -3 interviews). Only one accident out of three is triggered by balance disturbance: the employee climbing onto the train at a place where the track is curved. He held onto the left handrail with his left hand and held his two bags one above the other with his right hand. He missed the step (unusual gap between platform and running board) and his trunk slipping down between the platform and the train.

  TISSOT, C. and LECLERCQ, S., 2005, Textual analysis of 459 accounts of serious falls on the level in occupational situations. Additional contributions to coded variable analysis. In Contemporary Ergonomics, D. Bust and P. T. Mc Cabe (Ed.), pp.404-408 (London: Taylor and Francis, 2005).TRAVAIL etSECURITE, 2004, Statistiques technologiques 2002, travail et sécurité, -août, pp 46-51.

Figure 1 :

 1 Figure 1: Frequency indexes for accidents on the level with days lost according to

Figure 2 :

 2 Figure 2: Frequency indices for accidents on the level with days lost according to

Figure 3 :

 3 Figure 3: Number of accidents on the level with respect to number of days passed

Figure 4 :

 4 Figure 4: Number of accidents on the level with days lost with respect to number of

Figure 5 :

 5 Figure5: Access system on home station -based regional trains.

Table 3 :

 3 Accident-on-the-level typology based on activity at time of accident and accident location, average number of days lost associated with each type of accident on the level. Average number of days lost excluded for two types of accident on the level because of excessive heterogeneousness of the number of days lost associated with accidents in these classes.

	Establish-ment	Period studied	Number of accidents on the level with days lost recorded	Number of days lost counted	Number of accidents on the level lost recorded without days	Number of accidents on the level considered for typology development
	A	1998 to	55	2372	33	70 (incl. 44 accidents with
		2001				days lost)
	B	1999 to	70	1703	44	91 (incl. 61 accidents with
		2002				days lost)
	C	1999 to	109	2170	28	69 (incl. 50 accidents with
		2002				days lost)

outside premises 11 8 30 Climbing up onto and down from train running boards 22 17 19

	Walking in a moving train	14	8	17
	Operating train doors	10	7	16
	Going up and down stairs	12	10	11
	40

Acknowledgements

This work was made possible thanks to the cooperation of the French national rail transport company. The authors extend their warmest thanks to this company and, in particular, to its many representatives, without whom this work could not have been performed. The help of company managers involved in safety and that of employees interviewed were especially appreciated.


A Batailly

Q Agrapart

A Millecamps

Experimental and numerical simulation of a contact induced rotor/stator interaction inside an aircraft engine high-pressure compressor

Keywords: rotor/stator interaction, rubbing, nonlinear dynamics, unilateral contact dynamics, friction, abradable coating, clearance consumption

The development of a predictive numerical strategy for the simulation of rotor/stator interactions is a concern for several aircraft engine manufacturers. As a matter of fact, modern designs of aircraft engines feature reduced operating clearances between rotating and static components which yields more frequent structural contacts. Subsequent interaction phenomena (be it rubbing events, modal interaction or whirl motions) are not yet fully understood. For that reason, experimental data obtained from set-ups dedicated to the simulation of such interactions are scrutinized and are key in: (1) increasing the knowledge of the interaction phenomena and (2) allowing for a calibration of the numerical models with realistic events. In this contribution, the focus is made on an experimental set-up in Snecma facilities. It features a full-scale high-pressure compressor stage and aims at simulating contact induced interactions between one of the blades (slightly longer than the other ones) and the surrounding abradable coating that is deposited along the casing circumference. For this experimental set-up, it is found that the witnessed interaction involves a single blade-thus it should be analyzed as a sequence of rubbing events-and more specifically its first torsional mode, which is its second free-vibration mode. The focus is made both on the presentation of the experimental set-up and on the confrontation with the numerical results. Numerical results are analyzed by means of adaptative signal processing techniques and the consistency between numerical results and experimental observations is underlined both in time and frequency domains. In particular, the numerical strategy developed for Snecma is shown to predict very accurately the nature of the interaction as wear patterns obtained experimentally and numerically are a match. This numerical/experimental confrontation is the first attempt to calibrate a sophisticated numerical strategy with experimental data acquired within the high-pressure compressor of an aircraft engine for the simulation of rotor/stator interactions. Contrary to previous studies carried out within the low-pressure compressor of an aircraft engine, this interaction is found to be non-divergent: high amplitudes of vibration are experimentally observed and numerically predicted over a very short period of time. The ability of the numerical strategy to predict torsion induced interactions opens avenues for further analyses in turbine stages and with more sophisticated models including mistuned bladed disks and multi-stage components.

Simulation numérique et expérimentale d'un cas d'interaction rotor/stator dans le compresseur haute-pression d'un moteur d'avion

A. Batailly1 , Q. Agrapart2 , A. Millecamps 2

Résumé

Le développement d'une stratégie numérique prédictive pour la simulation d'interactions rotor/stator est un enjeu majeur pour plusieurs constructeurs de moteurs d'avion. En effet, les moteurs les plus récents possèdent des jeux de fonctionnement réduits entre les composants tournants et statiques ce qui favorise inévitablement l'apparition de contact entre ces structures. Les phénomènes d'interaction pouvant résulter de ces contacts (qu'il s'agisse de simples contacts frottants, d'interaction modale ou de mouvements de précession) ne sont pas encore bien compris. Pour cette raison, les données obtenues expérimentalement sont analysées avec minutie, notamment pour : (1) accroître la compréhension de ces phénomènes et, (2) renforcer le caractère prédictif des outils numériques existants en permettant leur calibration. Dans cet article, on s'intéresse à un banc d'essai mis en place par la société Snecma, composé d'un étage de compresseur haute-pression à échelle une dont une des aubes, légèrement en sur-longueur, entre en contact avec le revêtement abradable déposé sur le carter. Il est observé que le phénomène d'interaction n'implique qu'une seule aube-et doit ainsi être analysé comme un évènement de contact frottant aube/carter-et plus particulièrement son premier mode de torsion, soit son second mode de vibration propre. L'article porte sur la présentation du banc ainsi que sur une confrontation avec des simulations numériques. Les résultats numériques sont analysés par le biais de méthodes adaptatives de traitement du signal. La cohèrence entre les observations expérimentales et les résultats numériques est mise en évidence dans les domaines temporel et fréquentiel. En particulier, la stratégie numérique développée pour Snecma prédit de façon très précise la nature de l'interaction puisque les motifs d'usure prédits dans le matériau abradable correspondent aux observations expérimentales. Cette confrontation expérimental/numérique est une première tentative de calibration de cette stratégie numérique au sein d'un compresseur haute-pression. L'interaction observée est non-divergente: de hautes amplitudes de vibration sont observées expérimentalement et numériquement uniquement sur un court laps de temps. La possibilité de prédire numériquement de telles interactions permet d'anticiper l'analyse d'interactions dans d'autres composants du moteurs telles que les turbines.

Mots-clés

INTRODUCTION

The need for the improvement of modern aircraft engines efficiency motivates designers to consider reduced clearances between rotating and stationary components. As a result, structural contacts may occur in non-accidental configuration. Subsequent vibratory phenomena may involve a single blade-it is then usually referred to as rubbing [1, 2, 3]-, a full bladed disk and the surrounding casing-with possible modal coincidence yielding very high amplitudes of vibration [START_REF] Schmiechen | Travelling wave speed coincidence[END_REF][START_REF] Batailly | Assessment of reduced models for the detection of modal interaction through rotor stator contacts[END_REF]-or precessional shaft motions such as backward or forward whirl motions [START_REF] Muszynska | Whirl and whip-rotor/bearing stability problems[END_REF][START_REF] Sinha | Rotordynamic analysis of asymmetric turbofan rotor due to fan blade-loss event with contact-impact rub loads[END_REF][START_REF] Salvat | Two-dimensional modeling of shaft precessional motions induced by blade/casing unilateral contact in aircraft engines[END_REF][START_REF] Parent | Whole engine interaction in a bladed rotor-to-stator contact[END_REF]. One usual strategy employed by engine manufacturers in order to reduce clearances involves the deposition of an abradable coating [START_REF] Borel | The wear mechanisms occurring in abradable seals of gas turbines[END_REF] along the casing contact surface. This coating acts as a sacrificial material that is worn out in the case of blade/casing contact. However, even when this coating is employed, interaction phenomena have been reported [START_REF] Millecamps | Influence of thermal effects during blade-casing contact experiments[END_REF]. Because they may threaten the engine components' structural integrity, these interactions have been the focus of many recent studies, both numerical and experimental. In particular, the numerical simulation of rubbing events has been the centre of attention in a significant number of recent publications [START_REF] Shen | Numerical analysis of a rub-impact rotor-bearing system with mass unbalance[END_REF][START_REF] Arzina | Vibration analysis of compressor blade tip-rubbing[END_REF][START_REF] Turner | Airfoil deflection characteristics during rub events[END_REF][START_REF] Williams | Simulation of blade casing interaction phenomena in gas turbines resulting from heavy tip rubs using an implicit time marching method[END_REF][START_REF] Legrand | Numerical investigation of abradable coating removal through plastic constitutive law in aircraft engine[END_REF][START_REF] Ma | Vibration response analysis of bladedisk dovetail structure under blade tip rubbing condition[END_REF][START_REF] Yuan | Compact-impact analysis of a rotating geometric nonlinear plate under thermal shock[END_REF][START_REF] Almeida | Experimental analysis of dynamic interaction between a centrifugal compressor and its casing[END_REF]. In this contribution, the focus is made on an experimental test bench set-up in Snecma facilities. It features a full-scale high-pressure compressor stage and aims at simulating contact induced interactions between one of the blades (slightly longer than the other ones) and the surrounding abradable coating that is deposited along the casing circumference. For this experimental set-up, it is found that the witnessed interaction involves a single blade. Indeed, adjacent blades are also instrumented but feature negligible amplitudes of vibration. The localization of the interaction to the longer blade is also corroborated by the absence of significant amplitudes of vibration on the disk. Thus, this interaction phenomenon should be analyzed as a sequence of rubbing events and the focus is made on the identification of the blade response modal content, more specifically its first torsional mode, which is its second free-vibration mode.

The first section of the article describes the test bench and the targeted interaction scenario, it also provides details regarding the blade instrumentation as the type of gauges used during the experiment is briefly exposed. Experimental results and observations are then listed in the second section of the article. Following sections are related to the numerical investigations carried out in order to numerically predict the experimental results. The third section recalls the key points of the employed numerical strategy. Finally, in the last section, numerical results are sucritinised both in time and frequency domains and confronted with experimental observations. Note Amplitudes are normalized with respect to reference data for confidentiality purposes.

EXPERIMENTAL SET-UP Test bench

The experimental set-up presented in this article is part of the European research program NEWAC1 dedicated to the reduction of CO 2 emissions and fuel consumption. More specifically, the experimental set-up is used to investigate the interaction between a full scale bladed disk of an axial high-pressure compressor and its surrounding casing. The bladed disk of interest is machined from a single solid piece of material, also named blisk. It is composed of titanium alloy blades and is rotationally driven by an electric motor via a flexible shaft and sealed bearings. The blisk is surrounded with a thick simplified casing on which a thick abradable coating is thermally sprayed, see Fig. 1. Adjustment screws and laminated shims allow for compensating angular misalignment and concentricity deviation between rotor and stator. The alignment is a crucial point of the experimental set-up, it has been the subject of an accurate calibration at different angular speeds to eliminate parasitic vibratory phenomena.

The rotor is cantilevered at the end of the shaft, in such a way that it allows space for an additional infrared thermal imaging camera, and easy access to the casing for clearance adjustment. The test bench is housed in a vacuum chamber in order to avoid aerodynamic effects. All tests are conducted at room temperature. Because the blisk is investigated separately from downstream and upstream stages, the applied boundary conditions are significantly different than the ones within an actual engine. Yet, these differences are assumed to be negligible which is confirmed a posteriori since the disk dynamics does not play a significant role in the witnessed interaction.

Blade of interest

For all blades, clearances at rest are identical from the leading edge to the trailing edge before the engine starts. Because of centrifugal effects, this clearance configuration is slightly modified at the interaction speed. Yet, numerical predictions of the blade profile at the interaction speed suggest that the clearance variation from the leading edge to the trailing edge becomes less than 2.5 % of the initial clearance at rest. With such a small difference between leading edge and trailing edge clearances, it is not expected to favor torsion modes when contact occurs. As underlined in previous work [START_REF] Batailly | Conjectural bifurcation analysis of the contact-induced vibratory response of an aircraft engine blade[END_REF], clearance configuration may play a critical role in the arisal of rotor-stator interactions, but the impact of clearance configuration for our test bench goes beyond the scope of this study.

As mentioned above, one of the blades is slightly longer than the other ones (additional length is about 1 % of the blade height) so that only this blade may impact the surrounding abradable coating deposited on the casing. The clearances are adjusted so that contacts only occur between the longer blade and the abradable coating once the targeted interaction speed Ω c has been reached. In order to better apprehend the vibratory behaviour of the blade during the interaction, it is instrumented with two dynamic strain gauges located on its pressure side as depicted in Fig. 2. Each gauge position and orientation is carefully chosen in order to isolate the first bending mode (1B) and the first torsional mode of the blade (1T). Accordingly, these gauges will be referred to as gauge 1B and gauge 1T in the following. The acquisition frequency of these gauges, f a = 12, 500 Hz is chosen in order to accuratekly capture the modal participation of the blade first two free-vibration modes.

In order to ensure that the measured interaction is located on the longer blade only, the two adjacent blades and the disk are also instrumented with dynamic strain gauges. Also, it is assumed that the casing-which is particularly thick-is perfectly rigid thus casing amplitudes of vibration are not monitored.

Interaction scenario

The Campbell diagram pictured in Fig. 3 shows the evolution of the blade first three eigenfrequencies (the first bending mode 1B, the first torsional mode 1T and the second bending mode 2B). The dashed lines refer to the eigenfrequencies of the real blade measured experimentally at rest. The solid lines refer to the eigenfrequencies of the blade numerical model used in the following. The increase of the computed eigenfrequencies (solid lines only) over the angular speed range of interest is a consequence of centrifugal effects that are accounted for in the modal analysis. Small differences visible between the eigenfrequencies of the numerical model and the experimental blade stem from possible experimental measurements uncertainties as well as the simplification of the boundary conditions used for the numerical model. The latter is simply clamped on its root with a perfectly rigid disk, this is consistent with the aforementioned observation that the disk features negligible amplitudes of vibration.

It can be seen in Fig. 3 that the experimental angular speed Ω c corresponds to the intersection of the 22 nd engine order line with the frequency line of the first torsional mode. It is also located in the vicinity of the intersection of the 8 th engine order line with the frequency line of the first bending mode. The same observations stand for the numerically predicted angular speed Ω * c for the numerical blade model. The crossing between an engine order line and an eigenfrequency line is typically an area of interest when investigating the vibratory response of a blade subjected to unilateral contacts conditions. As a matter of fact, this crossing implies the following relation between the eigenfrequency of the blade f i and the angular speed Ω:

f i = kΩ, k ∈ N (1)
meaning the blade can oscillate exactly k times over its i th free-vibration mode during one revolution. The blade is thus excited over a sub-harmonic of its i th eigenfrequency which may lead to a resonance in a highly nonlinear context [START_REF] Batailly | Numericalexperimental comparison in the simulation of rotor/stator interaction through bladetip/abradable coating contact[END_REF][START_REF] Batailly | Conjectural bifurcation analysis of the contact-induced vibratory response of an aircraft engine blade[END_REF]. The targeted angular speed Ω c is approached step by step as shown in Fig. 4(a). A first acceleration quickly brings the rotor to 90 % of Ω c over the first 70 seconds of the experiment. Then, the angular speed is progressively increased and Ω c is reached around t = 508 s as pictured in Fig. 4(b). The first contacts are detected around t = 508 s and self-sustaining interaction stops by itself at t = 620 s. Between these two instants, which define the time window of interest in this article, the vibratory behaviour of the blade is scrutinized in the next section.

EXPERIMENTAL RESULTS AND OBSERVATIONS

Blade vibratory response Acquired time signals

The time signals obtained from gauges 1B and 1T (normalised strains) are respectively pictured in Fig. 5 and Fig. 6. Significant amplitudes of stresses are measured for t ∈ [508 ; 625] s right after the first blade/casing contacts have occurred. Small variations of these amplitudes are noticeable for t ∈ [508 ; 615] s and a sudden very large increase of these amplitudes takes place around t = 615 s.

Frequency analysis for gauge 1T

The Hilbert spectrum of the time history plotted in Fig. 6 is depicted in Fig. 7. A colour code-from white for negligible amplitudes to dark blue for highest amplitudes-indicates which frequencies are dominant in the blade vibratory response over time. At the beginning of the interaction, soon after the first contacts occur, for t ∈ [505 ; 535] s, non negligible amplitudes are found in the vicinity of the first three free-vibration modes of the blade (1B, 1T and 2B). However, for t ∈ [535 ; 620] s, significant amplitudes are solely located around the first torsion mode 1T. During this period of time, the evolution of the colour of the line around mode 1T is consistent with the overall increase of amplitude seen in Fig. 6.

Abradable wear

The surface condition of the abradable coating is scrutinized after the interaction using a coordinate measuring machine. Wear levels are measured using the following resolution: 180 equidistant points in the tangential direction and 44 equidistant points in the axial direction. Measured data yield the wear map depicted in Fig. 8: a colour code from light blue (no wear) to dark blue (maximum wear) is used. Black areas correspond to locations that could not be measured due to the use of anchored capacitive sensors in the casing, see Fig. 1. The wear map underlines that the coating underwent severe wear during the interaction. 22 lobes are evenly located along the casing circumference in front of the blade leading edge and trailing edge. This observation is in agreement with the time domain analysis carried above. Remarkably, no significant wear level is measured in front of the middle of chord. This observation is consistent with a close inspection of the blade tip as it vibrates along its first torsion modes: indeed, it is numerically predicted (but not detailed in this article for the sake of brevity) that the blade-tip/casing clearance in front of the middle of chord does not significantly vary as the blade vibrates along this free vibration mode. When having a close look at the angular location of the wear lobes, one may notice a small yet significant angular shift between the wear lobes at the trailing edge and at the leading edge meaning that blade casing contacts did not occur at the same time on the leading edge and the trailing edge. Based on this observation and the time domain analysis carried out previously. One can only speculate that contacts must have happened two times (one contact at the leading edge followed by one contact at the trailing edge) per oscillation of the blade along its first torsional mode.

NUMERICAL MODELLING

The employed numerical strategy has been developed for Snecma [START_REF] Legrand | Numerical investigation of abradable coating removal through plastic constitutive law in aircraft engine[END_REF][START_REF] Batailly | Numericalexperimental comparison in the simulation of rotor/stator interaction through bladetip/abradable coating contact[END_REF]. This strategy has been extensively detailed in previous contributions and is only briefly recalled here for the sake of brevity. It is based on an explicit time-marching algorithm [START_REF] Carpenter | Lagrange constraints for transcient finite element surface contact[END_REF] with a Lagrange multiplier-based contact treatment procedure ensuring that no penetration can occur between the impacting components. When contacts occur between a blade and the abradable coating, a plastic consitutive law [START_REF] Legrand | Numerical investigation of abradable coating removal through plastic constitutive law in aircraft engine[END_REF] is used for modeling abradable coating removal. While this strategy allows for accounting for a full bladed disk and a flexible casing, it is here limited to a single blade and the casing is assumed perfectly rigid in agreement with experimental observations.

Numerical framework

Same as suggested by experimental measurements, the casing is slightly ovalized. Numerically, this ovalization is used in order to initiate contacts with the blade. Numerical simulations are carried out with a constant angular speed Ω. At t = 0 s, the casing is perfectly circular and there is no penetration between the blade and the casing. Blade tip displacements due to centrifugal effects and casing ovalization are progressively accounted for during the first 20 % of the simulation where contact is initiated. After this point, the casing ovalization remains constant and the blade is free to interact with the abradable coating and the rigid casing.

The parameters of the simulation are as follows:

-friction is accounted for, µ = 0.15, this coefficient is used for both blade/abradable coating contacts and blade/casing contacts if the abradable is locally fully removed. This value was obtained from both numerical and experimental investigations, -the amplitude of the casing ovalization is equal to the blade/casing clearance at rest, -the abradable coating is 5 mm thick, -simulations are carried out over 100 blade revolutions, -the blade modal damping coefficient is ξ = 5 • 10 -4 , -empirical values are considered for the mechanical properties of the abradable coating, these values were obtained from the calibration of the employed numerical strategy with various experimental results [START_REF] Mandard | Interacting force estimation during blade/seal rubs[END_REF][START_REF] Cuny | Experimental measurement of dynamic forces generated during short-duration contacts: application to blade-casing interactions in aircraft engines[END_REF] : -Young's modulus is E = 20 GPa, -the plastic modulus is K = 5 GPa, -the yield limit is σ y = 1.5 Pa. The numerical convergence of the simulations has been checked with respect to the time step, the number of abradable elements as well as the blade reduction parameter but is not detailed here for brevity.

Targetted angular speed range

In order to numerically predict a rotor/stator interaction, it is first required to estimate the angular speed range over which it may occur. The Campbell diagram plotted in Fig. 3 combines the eigenfrequencies of the physical model and the numerical model. It appears that the targetted interaction is anticipated for Ω * c < Ω c since the eigenfrequency of the first torsional mode is slightly lower than the one of the physical model.

However, the stiffening effect stemming from unilateral contact constraints typically shifts the predicted interaction speed toward higher values. For that reason, numerical simulations are carried out over an angular speed range Ω ∈ [0.97Ω * c ; 1.10Ω * c ] centered on a higher value than Ω * c . This angular speed range also corresponds to Ω ∈ [0.9Ω c ; 1.02Ω c ]. In total, 101 simulations are carried out over this angular speed range, the angular speed increment between two simulations is δΩ = 0.0012Ω c .

Modelling

The reduced-order model of the blade used for the simulation is computed from a 3D industrial finite element model clamped on its foot. Eight nodes are used along the blade tip in order to manage contact with the abradable coating and the casing. A Craig-Bampton based component mode synthesis method [START_REF] Batailly | Numericalexperimental comparison in the simulation of rotor/stator interaction through bladetip/abradable coating contact[END_REF] is used in order to obtain a numerically efficient model. The reduced-order model contains 115 degrees of freedom, its convergence with respect to the inital finite element model with respect to the reduction parameter has been carefully checked, its first eigenfrequencies are shown in Fig. 3.

NUMERICAL RESULTS

Wear maps

At the end of each simulation, a wear profile is obtained in front of each of the eight boundary nodes along the blade tip. When put side by side, these profiles draw a wear map on which the wear level is represented by a colour code from white (no wear) to black (maximum wear). Such wear maps are pictured in Fig. 9(a) and in Fig. 9(b) respectively for the leading edge and the trailing edge. Several wear patterns are noticeable over the angular speed range :

for Ω ∈ [0.900Ω c ; 0.902Ω c ]: at the very beginning of the angular speed range, significant wear levels with 9 lobes are numerically predicted. This interaction is likely to be related to the crossing of the ninth engine order line with the first bending mode frequency line, see Fig. 3 around 0.856Ω c . Indeed, it has been previously observed that bending initiated interactions typically feature high amplitudes of vibration and deeper wear lobes than torsional initiated interactions [START_REF] Batailly | Conjectural bifurcation analysis of the contact-induced vibratory response of an aircraft engine blade[END_REF]; for Ω ∈ [0.907Ω c ; 0.909Ω c ]: a narrow band of wear is noticeable, the relatively small wear levels combined with the localization of this phenomenon indicates that it may be related with a high frequency mode (bending or torsion) or potentially a sub-harmonic of a high frequency mode. The impossibility to distinguish a precise number of lobes makes it arduous to analyze further; for Ω ∈ [0.941Ω c ; 0.950Ω c ]: a narrow interaction area with a high number of lobes is predicted, zoom over this area are provided for both the leading edge and the trailing edge in Fig. 9(c) and in Fig. 9(d). These figures allow for a precise count of the number of lobes: 22 wear lobes evenly spaced along the casing circumference are visible. The colour code indicates that higher wear levels are observed at the leading edge; for Ω ∈ [0.983Ω c ; 0.993Ω c ]: eight lobes of relatively low depth are predicted; for Ω ∈ [0.993Ω c ; 1.05Ω c ]: eight deep lobes are predicted through out this angular speed range, which coincidates with the crossing of the eighth engine order line with the first bending mode frequency line in Fig. 3. The analysis of such interaction goes beyond the scope of this article, the reader may refer to [START_REF] Batailly | Numericalexperimental comparison in the simulation of rotor/stator interaction through bladetip/abradable coating contact[END_REF] for more details regarding bending induced interactions. In the following, all the numerical results are related to Ω * c,a = 0.946Ω c .

Wear profiles at the critical speed Ω * c,a

Numerical/experimental confrontation

The wear profiles obtained from the leading edge to the trailing edge for Ω * c,a are combined in Fig. 10. The colour code used in Fig. 10 is identical to the one used in Fig. 8 and is relative: from highest wear levels pictured in dark blue to lowest wear levels in light blue. The comparison of Figs. 8 and 10 underlines that numerical results and experimental observations are consistent in terms of relative wear levels: in both cases the deepest lobes are located at the leading edge (LE) while the lobes at the trainling edge (TE) are thinner and less deep. Also, both experimental observations and numerical results suggest low to no wear facing the middle of chord. Finally, the same angular shift is found between the location of the wear lobes in front of the trailing edge and the location of the wear lobes in front of the leading edge.

All these observations confirm the fact that a linear-in the sense that the small perturbation framework is considered-finite element blade model used for the numerical simulations provides an accurate approximation of blade/casing clearances.

Frequency analysis

The radial displacement of the blade leading edge is pictured in Fig. 11. Soon after blade/abradable coating contact is initiated-around t = 0.05 s-significant amplitudes of vibration are predicted. These amplitudes remain steady over a few revolutions before they are amplified and reach a maximum for t = 0.55 s. At this precise moment, the blade did about 70 revolutions. Soon after, for t > 0.6 s, the amplitude of vibrations starts decreasing due to structural damping which indicates that contact has been lost.

The Hilbert spectrum of the time history depicted in Fig. 11 is shown in Fig. 12. Similarly to the Hilbert spectrum pictured in Fig. 7, significant amplitudes of vibration are represented in a time/frequency plane using the following colour code: from white (negligible amplitudes) to dark blue (highest amplitudes). Overall, two lines appear in this spectrum : (1) the first one is located slightly above the frequency of the first torsional mode 1T, it features the highest amplitudes, and (2) a second one is located slightly above the frequency of the second torsional mode 2T. This spectrum underlines that the numerically predicted interaction is almost essentially related to torsional modes as only negligible amplitudes are visible around the frequency of the first bending mode. A close inspection of the Hilbert spectrum highlights that the location of the highest amplitudes of vibration are slightly over the frequencies of the torsional modes for t < 0.6 s because of contact stiffening. As a matter of fact, for t > 0.6 s, after contact has been lost, these amplitudes are found closer to the frequency lines as contact stiffening disappeared.

The comparison of the Hilbert spectrum in Fig. 12 with the spectrum pictured in Fig. 7 underlines that the numerically predicted interaction seems to be consistent with experimental observations. From a numerical standpoint, and in agreement with experimental data, it is predicted that the interaction starts along bending modes but related amplitudes of vibration are so small that they are hardly visible in Fig. 12 for t ∈ [0 ; 0.12] s.

However, numerical simulations fail to predict any participation of the second bending mode 2B. The participation of a mode or another in the transient phase following the first contacts is closely related to the contact configuration. It is possible that the idealized configuration used numerically is responsible for this approximation. Regarding the strong participation of the second torsional mode 2T, this numerical prediction cannot be confronted to experimental data since the eigenfrequency of this mode is higher than the acquisition frequency of gauges 1B and 1T. Because experimental and numerically predicted wear profiles are consistent, the participation of this mode in the blade dynamics on the experimental set-up cannot be ruled out.

CONCLUSION

This article describes experimental and numerical investigations carried out for the simulation of a rotor/stator interaction case within the high-pressure compressor of an aircraft engine. This interaction case is initiated by structural contacts between a blade and the surrounding casing on which is deposited an abradable coating. Because only one blade of the bladed disk is involved in the interaction, the interaction may be qualified as a rubbing event. Experimental observations underline the domination of the first torsional mode-which is the second free-vibration mode of the blade-in the blade vibratory response. It is remarkable that the simulated interaction does not diverge: it does not lead to blade failure contrary to bending induced interactions previously observed.

A reduced-order model based numerical strategy is employed for the simulation of this rubbing event. The confrontation of numerical results with experimental observations highlights the relevance of this strategy. In agreement with experimental observations, numerical results feature (1) a blade dynamics dominated by bending modes following the first contacts before torsional modes become dominant, (2) an abradable coating significantly worn out with 22 wear lobes in front of the blade trailing edge, 22 deeper lobes in front of the blade leading edge and practically no wear in front of the middle of chord, and (3) a non-divergent type of interaction motion where the blade amplitudes of vibration decrease at some point. While the number of wear lobes may have been predicted based on the considered engine order, it is yet unelucidated why this particular engine order leads to an interaction while others don't. Work is in progress in order to get a better understanding of the physical phenomena at play (including thermal effects) as well as a clarification with respect to the role of the first bending mode in the arisal of an interaction.

ACKNOWLEDGEMENT

Thanks go to Snecma for its technical and financial support. x(t)

Figure 1 .

 1 Fig. 3

Figure 2 .

 2 Figure 2. detailed view of the longer blade, location of gauges

Figure 3 .

 3 Figure 3. Campbell diagram : experimental values at rest ( ) and numerical modal analysis ( )

  angular speed (rad/s) (b) zoom over the time window of interest

Figure 4 .Figure 5 .

 45 Figure 4. evolution of the bladed disk angular speed during the simulation

Figure 6 .

 6 Figure 6. time signal acquired with gauge 1T

Figure 7 .Figure 8 .

 78 Figure 7. Hilbert spectrum of the time signal obtained from gauge 1T

Figure 10 .

 10 Figure 10. numerically predicted wear profiles on the casing (the vertical dashed lines () underline the angular shift between the location of leading edge and trailing edge lobes)

Figure 11 .Figure 12 .

 1112 Figure 11. leading edge radial displacement

-Department of Mechanical Engineering, École Polytechnique de Montréal, P.O. Box 6079, Succ. Centre-Ville, Montréal, Québec, Canada H3C 3A7

-Safran Aircraft Engines, site de Villaroche, Moissy-Cramayel, 77550, France

-Département de génie mécanique, École Polytechnique de Montréal, P.O. Box 6079, Succ. Centre-Ville, Montréal, Québec, Canada H3C 3A7

-Safran Aircraft Engines, site de Villaroche, Moissy-Cramayel, 77550, France

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