B Van Don

M Van Ratingen

F Bermond

C Masson

INRETS P Vezin

D Hynd

TRL, UK C Owen

INSIA, Spain L Martinez

; S Knack

BASt R Schaefer

BIOFIDELITY IMPACT RESPONSE REQUIREMENTS FOR AN ADVANCED MID-SIZED MALE CRASH TEST DUMMY

The frontal crash is still an important contributor to deaths and serious injured resulting from road accidents in Europe. As the Hybrid-III dummy used in crash tests is over two decades old, the European Enhanced Vehicle-safety Committee is studying the potential for a new test device. Key is the availability of a well-defined set of requirements that identifies the minimum level of biofidelity required for an advanced frontal dummy. In this paper, a complete set of frontal impact biofidelity requirements, consisting of references, description of test conditions and corridors, is presented.

INTRODUCTION

Every year in Europe, more than 42 000 people are killed in road accidents, and over 1,7 million injuries are caused, of which several thousand give rise to severe disability [START_REF] Eu | Priorities in EU road safety[END_REF]. The frontal crash is still a significant contributor to these numbers, accounting for an estimated 40-66 percent of the impacts that cause severe and fatal injuries to car occupants [START_REF] Eevc | Recommended Requirements for the Development and Design of an Advanced Frontal dummy[END_REF]. Hence occupant protection in frontal impact remains an important subject of research for regulatory bodies, research institutes and car manufacturers. In particular within the European Enhanced Vehicle-safety Committee (EEVC) protection in frontal impact has been subject of strong collaboration, considering ways to further improve the level of protection offered to the European car user.

In view of this, the Steering committee of the EEVC has directed a mandate to establish requirements for an advanced frontal impact dummy which should be used to assess injury risk in frontal impact and could replace the existing adult male Hybrid-III crash test dummy. This mandate has provided the start of WG 12. Considering other research programs in the area, especially the Advanced Anthropomorphic Test Device (AATD) and subsequent THOR development programs of the US Department of Transport -National Highway Safety Administration (NHTSA), the EEVC has decided to focus on the development of a worldwide acceptable frontal dummy.

Further enhancement of the biofidelity of frontal impact dummies is considered essential, especially for improved interaction with the vehicle interior and various restraint systems. For this, an up to date and agreed set of frontal impact biofidelity requirements must be available. For side impact dummies, such a set of requirements has recently been proposed by IHRA (IHRA, 2003). Accordingly the EEVC has derived a comprehensive set of frontal impact requirements that defines the minimum level of biofidelity required for an advanced adult male frontal dummy from a European perspective. The aim of this paper is to present the requirements for each part of the human body and to provide information why the respective test conditions were selected.

METHODOLOGY

Achieving desirable humanlike response is probably the most difficult part of the design of a dummy. Mechanical characteristics, such as the stiffness of the dummy at the points at which it is struck, and where it is likely to strike the vehicle, should be similar to those of similar parts of the human body. This means that the dummy should inflict damage on the vehicle similar to that found from human impacts in accidents. Similarly the dummy should deform where struck in a representative manner as particularly specified for each body component. Biofidelity impact response requirements should therefore reflect the loading conditions in real life accidents and be meaningful in terms of the injuries observed for each body region.

With the above in mind, a frontal accident survey has been undertaken based on the two largest European accident databases available: the database of the UK Co-operative Crash Injury Study (CCIS) and the German database of the Medical University of Hanover (MUH) which is financed since 30 years by BASt. The objective of the study was to prioritize injuries for different body regions of belted drivers and front seat passengers in frontal accidents and to attribute the injuries to the airbag, seatbelt and/or the internal vehicle structures. A list of injury types of the most severe and most frequent injured body parts was derived to identify dummy performance and instrumentation needs.

Report obtained from EEVC web site -www.eevc.org van Don 2 Following the accident survey, a critical evaluation of biomechanical data from experiments with surrogates was carried out. This included consideration of the latest biomechanical test results generated in the EC sponsored FID project (FID, 2000). The aim of the work was to study for each body region the source data available and assess their appropriateness and completeness as biomechanical design target. In this selection process, preference was given to tests in which the subjects did not sustain fractures or severe injuries (AIS<3), exceptions tolerated were deemed acceptable. Furthermore, data should be of high quality, accepted by experts in the field and, more importantly, well documented. Also, priority was given to human data -animal data was only used in the case that animal to human scaling data were available -and to dynamic above static data, as the dynamic condition better represents the actual condition in which both human and dummy are being exposed in the vehicle. Finally, preference was given to rigid above padded impacts and to contact impact tests, which provide force-deflection responses.

RESULTS

Accident Survey

The data used were from frontal accidents (11-1 o'clock), focusing on car to car and car to obstacle impacts, EES < 80 kph and vehicles not older than 1990. Occupants were 12 years or older and belted (driver and passenger). Table 1 shows the comparison of the two databases, categorized for driver and passenger and airbag deployment and not fitted/no deployment. It can be observed that the CCIS database includes more airbag cases. Further study showed that the overall accident severity of MUH data seems less than in CCIS cases, which is due to the different sampling strategy used for both databases. This difference added to the different size of the databases and different EES calculation applied, made that the databases could only be analyzed independently.

One part of the analysis examined the maximum injury to each body region for all occupants. This provided information on the frequency and severity of the injuries in the samples by body region. For example, Figure 1 shows the comparison of the CCIS sample distribution for AIS2+ injuries of restrained drivers, with and without airbag deployment.

Figure 1. Distribution of AIS2+ injuries per body region for restrained drivers (CCIS)

Table 2 shows the AIS 2+ injury distribution of the MUH data set for those body regions, which will directly be addressed by frontal airbags. On the whole, in spite of in part small case numbers, the analyzed accidents from CCIS and MUH databases demonstrate that head and thorax are the body regions of vehicle occupants injured most severely and most frequently in frontal accidents. In accidents with airbag deployment, the injury severity of most body regions decreases considerably but remain relatively important nevertheless.

Further analysis was carried out to identify the main types of injury for the various body parts in the two samples. Severe head injuries mostly are brain injuries without fractures, but also severe skeletal injuries occurred to the head of belted drivers in accidents without airbag deployment. Severe thoracic injuries are mainly organic injuries and fractures to the ribs. In particular for drivers the percentage of fractures to the feet in accidents with Report obtained from EEVC web site -www.eevc.org van Don 3 airbag deployment increases considerable. Furthermore, significant face injuries were found for drivers in accidents without airbag deployment, and for front seat occupants in accidents with airbag deployment.

Regarding the requirements for an advanced frontal dummy, the accident study has pointed out that head and thorax area are the most important part to protect. Injury assessment for these body regions, as well as for the abdomen, may require different criteria for different types of contact (e.g. steering wheel, belt or airbag) and hence the responses of these body areas should be appropriate and sensitive to different loading conditions. Injuries to the lower extremities need more attention than given so far.

Biofidelity Requirements

A short summary of the literature review, the data on which the requirements are based, and the variables for which the requirements are defined are discussed below. The test procedures and the biofidelity requirements are described in more detail in the annex.

_Face. For the definition of the biofidelity requirements of the face it is of important to define the requirements for the different areas of the face (maxilla, zygoma, frontal bone) as well as for the whole face (simulating a driver site airbag deployment), and for an impact within the direction 0±30 degrees.

In most studies concerning the face the objective has been to define fracture loads. Only a few studies has dealt with below AIS<3 injury level. On the data of the latter studies, except from the tests in which added impacts were performed, biofidelity requirements can be based. Padding materials act as mechanical filters therefore it is better to avoid their use in order to characterize the response of the face independently of the impactor properties. [START_REF] Nyquist | Facial Impact Tolerance and Response. 30th Stapp Car Crash Conference[END_REF] and [START_REF] Allsop | A. Facial Impact Response -A Comparison of the Hybrid-III Dummy and Human Cadaver[END_REF][START_REF] Allsop | Force/deflection and fracture characteristics of the temporo-parietal region of the human head[END_REF] published force-deflection data, which were quite variable, primarily due to the large differences in the initial low-stiffness region of the concaveupward response curves. The variability was stated to be due to the deformation of the soft tissues of the face and the deflection of the nasal bone, factors that can vary greatly between subjects. Above loads of 0.25 kN the response stiffness and the subject exhibit force deflection behavior that were more consistent. [START_REF] Melvin | Facial injury assessment techniques[END_REF] proposed two response corridors for impactor force time histories for facial impact. The first corridor, for impact to the nose with a rigid cylinder, is based on the results of the tests of [START_REF] Nyquist | Facial Impact Tolerance and Response. 30th Stapp Car Crash Conference[END_REF], in which only the nasal bone was fractured. A second corridor was defined for full-face impacts with a flat surface, based on tests performed with fresh human specimens.

Recently many oblique face impact tests have been performed (Cesari et al., 1989[START_REF] Adria | Transport RTD Programme of the 4 th Framework Programme[END_REF][START_REF] Bermond | Human Face Response at an Angle to the Fore-aft Vertical Plane Impact[END_REF], Bruyere et al., 2000). In the ADRIA (1998) study response corridors were defined for the resultant head CG for the impact to the frontal bone, zygoma and the maxilla. It should be noted that the corridor defined for the maxilla was based on tests including AIS=3 injuries. [START_REF] Bermond | Human Face Response at an Angle to the Fore-aft Vertical Plane Impact[END_REF] provided response corridors for impactor force and resultant acceleration of the head CG versus impact velocity for the frontal bone, zygoma, and the maxilla. It should be noted that at higher impact velocities AIS>3 occurred. Because preference should be given to tests in which the subjects did not sustain fractures or severe injuries (AIS>3), the ADRIA (1998) is used to base biofidelity requirements instead of the data obtained by [START_REF] Bermond | Human Face Response at an Angle to the Fore-aft Vertical Plane Impact[END_REF].

To evaluate the biofidelity of the face, four tests are proposed. The defined biofidelity requirements are based on the studies of [START_REF] Nyquist | Facial Impact Tolerance and Response. 30th Stapp Car Crash Conference[END_REF], Melvin and Shee (1988) for the frontal impact condition. For the oblique impact condition, the requirements are based on the test performed in the ADRIA project (1998). For the frontal impact condition, requirements have been defined for the following variable: The impactor force versus time response. For the oblique impact condition, requirements have been defined for the resultant head CG acceleration vs. time.

_Head. In the literature various studies can be found in which head impacts were performed. For example to look at injury mechanism [START_REF] Canaple | Identification of Head Injury Mechanisms associated with Reconstruction of Traffic Accidents[END_REF] or the influence of head protection [START_REF] Newman | A New Biomechanical Assessment of mild traumatic Brain Injury[END_REF]. Both studies did not provide suitable data to base biofidelity requirements on. Data on head impact has also been obtained by Stalnaker et al. 1977, Rizitti et al., 1997, Rojanvanich et al., 1991, Troseille et al., 1992[START_REF] Ward | Dynamic Biofidelity of the Part 572 and Hybrid III anthropomorphic Test Dummy Heads[END_REF]. However based on the lack of detailed description of the test set-up or due to the small number of tests performed, biofidelity requirements for the head could not be based on data obtained in these studies. [START_REF] Melvin | AATD system Technical Characteristics, Design Concepts and Trauma Assessment Criteria, Task E-F Final Report[END_REF] has provided the most complete and detailed overview of suitable data and test protocols. In this study two corridors were defined. One corridor for a non-fracture head impact test at low impact speed (2.0 m/s) and one for a test at high impact speed (5.5 m/s). The corridors were based on the data of UMTRI [START_REF] Prasad | Head. Review of Biomechanical Impact Response and Injury in the Automotive Environment[END_REF] and [START_REF] Hodgson | Head Impact Response[END_REF] and Report obtained from EEVC web site -www.eevc.org van Don 4 are suitable for frontal, as well as lateral and rear impact tests.

To evaluate the biofidelity of the head, one test and two requirements are proposed. The requirements are based on the data of UMTRI [START_REF] Prasad | Head. Review of Biomechanical Impact Response and Injury in the Automotive Environment[END_REF] and [START_REF] Hodgson | Head Impact Response[END_REF], adopting the proposed corridors by [START_REF] Melvin | AATD system Technical Characteristics, Design Concepts and Trauma Assessment Criteria, Task E-F Final Report[END_REF]. Requirements have been defined for the peak impact force versus impact duration relationship. _Neck. In literature, two sets of performance requirements have been defined to evaluate the frontal flexion/extension performance of the neck. The first set is based on volunteer and cadaver sled tests performed by Mertz and Patrick (1971) and [START_REF] Patrick | [END_REF]. The second set of requirements is based on volunteer sled tests carried out at the Naval BioDynamics Laboratory (NBDL) in New Orleans [START_REF] Ewing | Human head and neck response t impact acceleration[END_REF][START_REF] Ewing | The effect of the initial position of the head and neck on the dynamics response of the human head and neck to -Gx impact acceleration[END_REF][START_REF] Ewing | The effect of duration, rate o onset and peak sled acceleration of the dynamic response of the human head and neck[END_REF][START_REF] Ewing | Multi-axis dynamic response of the human head and neck to impact acceleration[END_REF][START_REF] Grunsten | The mechanical effects of impact acceleration on the unconstrained human head and neck complex[END_REF].

As of the moment the most complete set of neck data available is the data obtained at the Naval BioDynamics Laboratory in New Orleans (NBDL data), therefore the newly defined biofidelity requirements defined were based on this data set.

To define a complete set of biofidelity requirements for the neck, the performance should be evaluated globally (in other words with regard to the environment like airbag etc) and locally focussing on the performance of the neck with regard to occurring injuries and defined injury criteria.

To evaluate the biofidelity of the neck, one test condition for the frontal and one for the oblique impact condition were defined. Requirements have been defined for the following variables, for the frontal as well as the oblique impact condition: The peak head center of gravity (CG) displacement w.r.t the sled; timing of peak head CG displacement w.r.t the sled; peak first thoracic vertebra (T1) displacement w.r.t to the sled; timing of peak T1 displacement w.r.t to the sled; peak T1 rotation w.r.t the sled; timing of peak T1 rotation w.r.t sled ; peak force at the OC joint; timing of peak force at the OC joint; peak moment at the OC joint and; timing of peak moment at the OC joint _Shoulder. Biomechanical research concerning the behavior of the shoulder during frontal impact conditions is rare. L' Abbé et al. (1982) andCesari et al. (1990) performed thorax belt loading compression tests. No special attention was paid to the shoulder behavior and no corridors, to be used for evaluation of the biofidelity of the shoulder, were/can be defined on the data obtained. [START_REF] Vezin | Human response to a frontal sled deceleration[END_REF] has performed post mortem human subjects (PMHS) sled tests with and without airbag, and defined biofidelity requirement corridors for the shoulder. It should be noted that the corridors are based on a maximum of three tests, therefore further testing should be performed to refine the corridors provided. Requirements have been defined for the left and right acromion resultant acceleration, and; left and right upper humerus resultant acceleration versus time _Spine. Biomechanical data, which properly define the kinematics of the spine under external loading of the whole body, is rare. Data, which provide the response of isolated segments of the spine, is available. However, it is difficult to extrapolate this information to how the spine would behave when it is integrated with the complete thorax and pelvis. [START_REF] Vezin | Human response to a frontal sled deceleration[END_REF] developed biofidelity requirement corridors based on PMHS sled performed with and without airbag. Requirements have been defined for the sacrum, T1, T8 and T12 resultant acceleration versus time relationships.

_Thorax. Many studies, with the emphasis on the biofidelity aspect of the thorax have been published. To asses the thorax performance, impactor tests [START_REF] Kroell | Impact Tolerance and Response of the Human Thorax. 15 th Stapp Car Crash Conference[END_REF]Stalnaker et al. 1973;Nusholz et al. 1985[START_REF] Bouquet | Thoracic and Pelvis Human Response to Impact[END_REF], belt-loading tests (L'Abbe et al , 1982;[START_REF] Backaitis | [END_REF]St-Laurent, 1986 Cesari and[START_REF] Césari | Behaviour of human surrogates thorax under belt loading[END_REF][START_REF] Riordain | A Comparison of the Hybrid III and Cadaver Thorax Response under diagonal Belt Loading[END_REF]; and sled tests [START_REF] Patrick | Forces on the Human Body in Simulated Crashes[END_REF][START_REF] Patrick | Cadaver Knee, Chest and head Impact Loads[END_REF]Yoganandan et al., 1991&1993, Kallieris , 1994[START_REF] Vezin | Human response to a frontal sled deceleration[END_REF]) have been performed.

Regarding the impactor tests the current thoracic biofidelity testing relies heavily upon the published work of [START_REF] Kroell | Impact Tolerance and Response of the Human Thorax. 15 th Stapp Car Crash Conference[END_REF], despite it being over 20 years old. Due to the large number of cadavers tested, almost 50, in a variety of test configurations it is clear to see that valuable data can still be obtained from these results. In the paper by [START_REF] Lobdell | Impact response of the Human Thorax[END_REF] Kroell proposed performance corridors for low speed (4.92 m/s) and high speed (7.14 m/s) impactor tests. The tissue thickness and obliqueness of the impact etc. and muscle tone was taken into consideration. Kroell's work was conducted as a continuation of the work presented by [START_REF]Deflection of the Human Thorax Under Sternal Impact[END_REF] where the same experimental test set-up had been used but the impact velocities were lower and aortic pressurization was not done. Stalnaker et al (1973) used a similar test configuration with the impact velocity varying from 5.35-6.71 m/s. No performance corridors were developed. The Stalnaker data represents a relaxed individual and provided data representing impactor penetration Report obtained from EEVC web site -www.eevc.org van Don 5 whereas the Kroell corridors represented sternal deflection. [START_REF] Neathery | Analysis of Chest Impact Response Data and Scaled Performance Recommendations[END_REF] has re-examined the data of Kroell and Stalnaker. And concluded that, the Kroell and Stalnaker data could not be considered as a common database. [START_REF] Neathery | Analysis of Chest Impact Response Data and Scaled Performance Recommendations[END_REF] proposed response corridors for the 5 th , 50 th and 95 th percentile crash dummies similar to those proposed by Kroell, but scaled by the equations developed in the study. [START_REF] Nusholtz | Thoraco-Abdominal Response to Steering Wheel Impacts[END_REF] has reported on a series of steering wheel impact tests. The focus of the research was on the trauma to the soft-tissue organs surrounded by the thoracic cage, as well as the kinematic response of the thoracic cage. [START_REF] Bouquet | Thoracic and Pelvis Human Response to Impact[END_REF] also performed impactor tests to the sternum. Two tests were performed on each PMHS, first at a sub-injury level test (impact speed 3.5m/s) followed by an injury level test (impact speed ~ 5.8m/s). Corridors for the force-time history during the sub-injury test were presented. However it should be noted that these corridors are based only on two tests, therefore not providing a good representation. In all of the above studies frontal impactor tests have been performed. Data with regard to oblique impactor tests to the thorax are rare. However [START_REF] Yoganandan | Impact biomechanics of the human thorax-abdomen complex[END_REF] defined response corridors for the overall lower ribcage to oblique impact conditions.

Regarding the belt-loading tests. L'Abbe et al (1982) has performed early dynamic and static belt loading to examine the thoracic deflection characteristics of human volunteers in comparison with the Hybrid-III. [START_REF] Backaitis | Chest deflection characteristics of volunteers and Hybrid III dummies. Biomechanics of impact injury and injury tolerances of the thoraxshoulder complex[END_REF] re-analyzed the data of L' Abbe et al. (1982) focussing on the deflections of the mid-clavicle, mid-sternum and the 7 th rib. Continuing on the work of L' Abbe et al. (1982) , [START_REF] Césari | Behaviour of human surrogates thorax under belt loading[END_REF] reported tests with 13 cadavers and a Hybrid III dummy in the same test configuration. [START_REF] Riordain | A Comparison of the Hybrid III and Cadaver Thorax Response under diagonal Belt Loading[END_REF] also used a similar test set-up. The paper included the results on 13 cadavers from the 1990 paper and extended the data with seven cadavers using a high mass (76.1 kg) impactor. Continuing on previous work Césari and Bouquet (1994) presented the results of an additional 9 nine PMHS. From the above, it would appear that the data of belt loading tests performed with cadavers/volunteers lying in supine position should be used to define biofidelity requirements. The loading pattern would be more representative of the real world than the Kroell test. However a limitation of these tests is that the back of the cadaver is fully supported against a rigid table, which could compromise rib deflection. A second limitation of these tests is that the test apparatus is not available/missing, therefore the tests can not easily be reproduced.

Regarding the sled tests. A few studies have been performed to evaluate the performance of the thorax in sled test. [START_REF] Patrick | Forces on the Human Body in Simulated Crashes[END_REF][START_REF] Patrick | Cadaver Knee, Chest and head Impact Loads[END_REF] performed a series of sled test with unrestrained embalmed cadavers to determine the human tolerance of the head, chest and knee impact based on skeletal fracture. Four tests with different velocities were performed, in only two tests no fractures were observed. No performance corridors were defined. Yoganandan et al. (1991) performed horizontal sled impact tests (impact velocity 14 m/s, 16 G). A three-point belt was used to restrain the PMHS. Due to the occurrence of the multiple fractures the data set has not been used for defining biofidelity requirements. In a further paper, [START_REF] Yoganandan | Thoracic Biomechanics with air bag restraint. 37 th Stapp Car crash Conference[END_REF] an additional sled tests (impact velocity 9 or 13 m/s) were presented. The following restraint system configurations were used: air bag with knee bolster (AK), air bag with lap belt (AL) and air bag with three point belt (A3). It was seen that the biomechanical response of the human thorax was very different between the air bag with three-point belt loading compared to the air bag with knee bolster and airbag with lap belt restraint combinations. The data set has not been used for defining biofidelity requirements. [START_REF] Kallieris | The performance of active and passive driver restraint systems in simulated frontal collisions[END_REF] performed frontal impact sled tests (impact velocity 48-55 km/h, 17 G). The following conditions restraint configuration were used: a 3point belt, a driver side air bag and knee bolster, or a 3-point belt with supplemental driver side air bag. Biomechanical responses and the thoracic deformation contours and deflection time histories are also available in the paper. Also due to the high number of thoracic injuries occurring the data set has not been used to define the biofidelity requirements on. [START_REF] Vezin | Human response to a frontal sled deceleration[END_REF] has performed sled tests using a sled velocity and deceleration pulse representative to the real world crashes. Tests with only seatbelt and with seatbelt and airbag were performed. Performance corridors were defined for significant variables. More data, bas the corridors on, will become available in future projects.

To evaluate the performance of the thorax three tests are proposed. The requirements are based on the data of [START_REF] Kroell | Impact Tolerance and Response of the Human Thorax. 15 th Stapp Car Crash Conference[END_REF], [START_REF] Yoganandan | Impact biomechanics of the human thorax-abdomen complex[END_REF] and [START_REF] Vezin | Human response to a frontal sled deceleration[END_REF]. For the Kroell and Yoganandan tests, requirements have been defined for the force deflection relationship; for the Vezin test for the upper and lower sternum resultant acceleration versus time relationships.

Report obtained from EEVC web site -www.eevc.org van Don 6 _Abdomen. Nahum and Melvin (1998) have published an extensive review concerning the human abdomen biomechanics, clinical data, injury mechanism and tolerance levels. It could be seen that many studies are concerned with the biofidelity of the abdominal area for side impact conditions, only a few studies focus on the frontal impact conditions. No tests have been performed to evaluate the different impact responses caused by the asymmetric distribution of the internal organs in the abdominal cavity. Only the abdominal impact response is known for the upper and lower part without any distinction between the left, central and right region. Cavanagh et al. (1986) and [START_REF] Begeman | Steering Assembly Impacts Using Cadavers and Dummies[END_REF] performed sled tests to evaluate the abdomen behavior during frontal impact. Cavanagh et al (1986) performed low (mean 6.1 m/s) and high impact velocity (10.4 m/s) tests. Force-deflection corridors for the lower abdomen at the two impact velocities were generated. [START_REF] Begeman | Steering Assembly Impacts Using Cadavers and Dummies[END_REF] performed impact tests using rigid steering columns with a soft wheel and an energy-absorbing column with rig and soft wheels. [START_REF] Nusholtz | Thoraco-Abdominal Response to Steering Wheel Impacts[END_REF] performed a PMHS test to determine the thoracoabdominal response with a deformable steering wheel impact. These tests were complex and it was difficult to determine the load paths, therefore the responses are not adequate to base the biofidelity requirements on. Later tests performed by [START_REF] Nusholtz | Abdominal Response to Steering Wheel Loading[END_REF] were less complicated. Impacts were performed with a ballistic pendulum fitted with a simulated rigid steering wheel assembly. A forcepenetration corridor for the upper abdomen was presented.

Belts loading tests using porcine cadavers for evaluation of the abdomen have been performed by [START_REF] Rouhana | Assessing Submarining and Abdominal Injury Risk in the Hybrid III Family of Dummies[END_REF]. Comparisons were made between the force-deflection curves from porcine cadavers, human cadavers and living porcine subjects. Observed differences were used to extrapolate the human cadaver force deflection to living human force deflection data.

To evaluate the biofidelity of the abdomen three tests are proposed. The requirements are based on the data of [START_REF] Nusholtz | Abdominal Response to Steering Wheel Loading[END_REF]), Cavanagh et al. (1986) and [START_REF] Rouhana | Assessing Submarining and Abdominal Injury Risk in the Hybrid III Family of Dummies[END_REF]. For the [START_REF] Nusholtz | Abdominal Response to Steering Wheel Loading[END_REF] and [START_REF] Cavanaugh | Lower Abdominal Tolerance and response[END_REF] tests and the [START_REF] Rouhana | Assessing Submarining and Abdominal Injury Risk in the Hybrid III Family of Dummies[END_REF] tests, requirements have been defined for the force versus penetration relationship.

_Femur/Knee. Only a few studies in which the femur/knee performance has been evaluated at subinjury or at moderate injury level (AIS<3) have been published. [START_REF]Impact response and tolerance of the lower extremities[END_REF], [START_REF] Horsch | Cadaver and Dummy knee impact response[END_REF], [START_REF] Nusholtz | Impact response and injury of the pelvis[END_REF] and [START_REF] Haut | Insult to the human cadaver patellofemoral joint: effects of age on fracture tolerance and occult injury[END_REF] all performed sub-injury level tests. However, [START_REF]Impact response and tolerance of the lower extremities[END_REF] performed padded impacts, the test set-up of [START_REF] Nusholtz | Impact response and injury of the pelvis[END_REF] is considered too complicated and [START_REF] Haut | Insult to the human cadaver patellofemoral joint: effects of age on fracture tolerance and occult injury[END_REF] performed only isolated knee flexion tests.

Within the FID project (2000) tests similar to tests of [START_REF] Horsch | Cadaver and Dummy knee impact response[END_REF] have been performed No response corridors have been provided by [START_REF] Horsch | Cadaver and Dummy knee impact response[END_REF]. However within the FID project corridors have been defined.

To evaluate the biofidelity of the femur/knee two tests are proposed. One with impact velocity of 2.8 m/s and the other with 4.0 m/s. For the 2.8 m/s condition requirements have been defined for the following variables: the knee impactor force versus time, the femur acceleration versus times and the iliac crest acceleration versus time. For the 4.0 m/s condition requirements have been defined for the following variables: knee impactor force versus time and the femur acceleration versus time.

_Lower Leg. Most studies concerning the lower leg performance have been aimed especially at the determination of injury mechanisms and criteria for the lower leg. The data obtained in these studies is not adequate to base biofidelity requirements on. [START_REF] Yoganandan | Dynamic Axial Tolerance of the Human Foot-Ankle Complex[END_REF] performed pendulum impact test on unembalmed human cadaver legs at the Medical College of Wiscounsin (MCW). Of the 26 tests performed, 13 resulted in fracture. This dataset was combined with data obtained at Calspan and Wayne State University. Age and axial force were found to be the most discriminating variables that define the risk function. [START_REF] Kuppa | Axial Impact Characteristics of Dummy and Cadaver Lower Limbs[END_REF] analyzed Yoganandan's data to characterize the dynamic response of the lower leg. Estimated values for the stiffness and the damping coefficient for the human lower leg were derived. [START_REF] Crandall | Biomechanical Response and Physical Properties of the Leg, Foot and Ankle[END_REF] reported on a series of static and dynamic tests carried out by Renault Biomechanical Research Department and the University of Virginia (UVA). Data for the volunteers were taken from a previously work by Hirsch and White (1965). [START_REF] Portier | Dynamic Biomechanical Dorsiflexion Responses and Tolerances of the Ankle Joint Complex[END_REF] published a detailed description of the dynamic tests performed. [START_REF] Manning | Dynamic Response & Injury Mechanisms in the Human Foot and Ankle and an Analysis of Dummy Biofidelity[END_REF] performed a series of subinjuries heel (2 and 4 m/s) and toe impact (2,4 and 6 m/s), to assess the effect of active muscle tension. Comparative tests were performed on braced and umbraced volunteers and PMHS with an artificial Achilles tension applied. The test procedure used was based on the EEVC " Tibia and foot certification tests". The data of Manning et al.

Report obtained from EEVC web site -www.eevc.org van Don 7 (1998) has been developed into performance corridors for each impact condition and different specimen (PMHS and volunteer) by [START_REF] Wheeler | Biofidelity of Dummy Legs for use in Legislative Car Crash Testing[END_REF].

In addition to injury generating impact tests, [START_REF] Mcmaster | Biomechanics of Ankle and Hindfoot Injuries in Dynamic Axial Loading[END_REF] has performed articulation tests. A basic articulation test was conducted. The ankles were tests through dorsiflexion to plantarflexion, plantarflexion to dorsiflexion, eversion to inversion and inversion to eversion. This study provided information concerning the range of motion of the ankle joint.

From the above overview of studies it can be stated that most of the data is unsuitable for basing biofidelity requirements on for the following reasons: unclear test conditions for reproduction (i.e for [START_REF] Yoganandan | Dynamic Axial Tolerance of the Human Foot-Ankle Complex[END_REF], [START_REF] Kuppa | Axial Impact Characteristics of Dummy and Cadaver Lower Limbs[END_REF], [START_REF] Portier | Dynamic Biomechanical Dorsiflexion Responses and Tolerances of the Ankle Joint Complex[END_REF]), injury generating tests (i.e. for [START_REF] Portier | Dynamic Biomechanical Dorsiflexion Responses and Tolerances of the Ankle Joint Complex[END_REF][START_REF] Mcmaster | Biomechanics of Ankle and Hindfoot Injuries in Dynamic Axial Loading[END_REF]), complex test set-up [START_REF] Mcmaster | Biomechanics of Ankle and Hindfoot Injuries in Dynamic Axial Loading[END_REF]), or static tests [START_REF] Crandall | Biomechanical Response and Physical Properties of the Leg, Foot and Ankle[END_REF]).

To evaluate the biofidelity of the lower leg two tests are proposed based on the corridors provided by [START_REF] Wheeler | Biofidelity of Dummy Legs for use in Legislative Car Crash Testing[END_REF]. For the toe impact condition the requirements have been defined for the following variables: the pendulum acceleration versus time; the tibial force versus time and the bending moment versus time. For the heel impact condition requirements have been defined for the following variables: pendulum acceleration versus time and tibial force versus time.

DISCUSSION

This paper presents a complete set of frontal biofidelity requirements that defines the minimum level of biofidelity required for an advanced adult male frontal dummy in Europe. The current set contains only those test conditions for which test specifications are well defined and corridors are established. Moreover, the set addresses the key body regions taking into account the various types of contact that may occur (belt, airbag, steering wheel, facia) according to the accident study.

The National Highway Safety Administration has defined a set of biomechanical response requirements for the THOR advanced frontal dummy (NHTSA/GESAC, 2001). A comparison of both sets of requirements indicates that the two sets do not lie far apart, increasing the potential for harmonization between the regions. For instance, the biofidelity requirements for the head, abdomen and femur/knee are identical between the two sets. The differences between EEVC and NHTSA proposed requirements are most notable for the neck -the oblique versus lateral NBDL impact condition-and the lower leg/ankle and foot for which NHTSA/GESAC defines a series of tests in the dynamic heel impact condition [START_REF] Kuppa | Axial Impact Characteristics of Dummy and Cadaver Lower Limbs[END_REF], dynamic dorsiflexion [START_REF] Crandall | Biomechanical Response and Physical Properties of the Leg, Foot and Ankle[END_REF], quasi static inversion [START_REF] Petit | Quasi-static Characterization of the Human Foot-Ankle Joints in a Simulated Tensed State and Updated Accidentological Data IRCOBI[END_REF], quasi static eversion [START_REF] Crandall | Biomechanical Response and Physical Properties of the Leg, Foot and Ankle[END_REF][START_REF] Petit | Quasi-static Characterization of the Human Foot-Ankle Joints in a Simulated Tensed State and Updated Accidentological Data IRCOBI[END_REF], quasi static dorsi-flexion [START_REF] Crandall | Biomechanical Response and Physical Properties of the Leg, Foot and Ankle[END_REF], quasi static plantar flexion (Paranteau et al., 1996), quasi static internal/external rotation [START_REF] Siegler | The Three-Dimensional Kinematics and Flexibility Charactersistics of the human Ankle and Subtalar Joints -Part I: Kinematics[END_REF]) and dynamic inversion/eversion [START_REF] Jaffredo | Cadaver Lower Limb Dynamic Response in Inversion-Eversion[END_REF]. Recognizing that the requirements presented in this paper form a minimal set, it must be further investigated whether such a large set of tests for the lower leg/ankle and foot can be justified.

CONCLUSIONS

This paper proposes a set of biofidelity responses requirements for an advanced adult male frontal impact crash dummy. The set consists of requirements for the face, head, neck, spine, thorax, abdomen, femur/knee and the lower leg/ankle/foot complex, and defines the minimal level of biofidelity that is required for this dummy. The set of biofidelity requirements proposed in this paper will be used to compare the biomechanical performance of existing adult male frontal impact crash test dummies, including the Hybrid-III and THOR-alpha dummies. Comparing the presented set with the biomechanical response requirements for the THOR advanced frontal dummy shows that there is good possibility for harmonization of requirements through IHRA, and hence, for the test device that meets of these requirements.

ANNEX: Description of Test Procedures and Biofidelity Requirements

FACE

_Face test 1 is based on the data of [START_REF] Nyquist | Facial Impact Tolerance and Response. 30th Stapp Car Crash Conference[END_REF]. The test is a horizontal guided impactor test (impact velocity 3.6 m/s). Using an impactor with a horizontal steel cylinder (mass 32 kg, diameter 25 mm). The dummy needs to be positioned with the anterior-posterior axis of the head horizontal and the sagittal plane vertical. Impact is performed to the nose.

Instrument the pendulum with a load cell, to be able to obtain the impact force versus time relationship. Filter all response data according to the requirements of SAE Recommended Practice J211.

The impactor force versus time response has to be within the corridor shown in Figure 1. _Face test 2 is based on the data/corridor provided by Melvin and Shee (1988). The test is a full-face impact test, using a flat disk horizontal guided impactor (mass 13 kg, diameter 152 mm) impact velocity 6.7 m/s. The dummy needs to be positioned with the anterior-posterior axis of the Report obtained from EEVC web site -www.eevc.org van Don 11 head horizontal and the mid sagittal plane vertical. The extremity of the impactor is to impact the midpoint of the line joining the two maxilla plates on the face.

Instrument the pendulum with a load cell, to be able to obtain the impact force versus time relationship. Filter all response data according to the requirements of SAE Recommended Practice J211 with Class 1000 for the head data and Class 180 for the impactor data.

The impactor force versus time response should be within the corridor shown in Figure 2. 1998). The test prescribed is a 3.84 m/s, 30 degree angled horizontal impact test to the frontal bone using a rigid impactor (17 kg, diameter 25 mm). The dummy needs to be positioned with the anterior-posterior axis of the head horizontal, its mid sagittal plane vertical and adjusted to obtain an impact direction 30°-angled from the mid sagittal plane of the head.

Instrument the dummy's head with a tri-axial accelerometer located at the head CG and place a load cell between the major part of the impacting mass and the extremity. The influence of the extremity mass on the load measurement must be corrected. Filter all response data according to the requirements of SAE Recommended Practice J211.

The resultant head CG acceleration has to be within the corridor shown in Figure 3

Head

The head test is a frontal impact test, impact velocity 2.0 m/s and 5.5 m/s using a rigid flat surface impactor (mass 23.4 kg, diameter 15.2 mm). The dummy needs to be seated upright without support.

Instrument the dummy's head with a tri-axial accelerometer located at the head CG. Place a load cell on the impactor. Filter all response data according to the requirements of SAE Recommended Practice J211.

The peak impactor force for 2.0 m/s and 5.5 m/s impact respectively should be corridors provided by [START_REF] Melvin | AATD system Technical Characteristics, Design Concepts and Trauma Assessment Criteria, Task E-F Final Report[END_REF] as shown in Figure 5. Neck _Neck test 1 is a full dummy 15-G frontal impact sled test. The sled pulse used should be similar to the NBDL sled pulse. Fasten a rigid seat, functionally similar to the one used by Ewing and Thomas (1977), to the HyGe sled, facing the direction of sled travel. The dummy needs to be positioned with the anterior-posterior axis of the head horizontal and its mid sagittal plane vertical. Use a restrain system similar to the restrain system used by Ewing and Thomas (1977).

Instrument the dummy with tri-axial accelerometers at the head CG and T1. Use cameras to record the photographic markers of the head CG, OC and T1. Filter all response data according to the requirements of SAE Recommended Practice J211.

Report obtained from EEVC web site -www.eevc.org

van Don 12

The following responses should be within the corridors provided. The peak T1 displacement and timing of the peak T1 displacement with respect to the sled, the peak CG displacement and timing of the peak CG displacement with respect to the sled, peak flexion of the head and the timing of the peak flexion of the head. _Neck test 2 is a full dummy 11-G oblique impact sled test. Fasten a rigid seat, functional similar to the one used by Ewing and Thomas (1977), to the HyGe sled at an angle of 45 degrees from the forward facing direction. Attach a vertical light padded wooden board against the seat to restrict upper torso rotation and to support the torso during sled translation. The dummy needs to be positioned with the anterior-posterior axis of the head horizontal and its mid sagittal plane vertical. Use a restrain system similar to the restrain system used by Ewing and Thomas (1977).

Instrument the dummy with triaxial accelerometers at the head CG and T1 Use cameras to record the photographic markers of the head CG, OC and T1. Filter all response data according to the requirements of SAE Recommended Practice J211.

The following responses should be within the corridors provided. The peak T1 displacement and timing of the peak T1 displacement with respect to the sled, the peak CG displacement and timing of the peak CG displacement with respect to the sled, peak flexion and the timing of the peak flexion of the head, and the peak twist and timing of the peak twist of the head. (Kallieris, 2001) and only set belt as restraint system (4 kN force limiting system).

Instrument the dummy with an tri-axial accelerometer at the upper and lower left and right arm. Filter all response data according to the requirements of the SAE Recommended Practice J211.

The following responses should be within the corridors provided. Left and right acromion and upper humerus resultant acceleration versus time. (Kallieris, 2001) and only set belt as restraint system (4 kN force limiting system).

Instrument the dummy with a tri-axial accelerometer at T1, T8 and T12. Filter all response data according to the requirements of the SAE Recommended Practice J211. The following responses should be within the corridors provided. The sacrum T1, T8 and T12 vertebra resultant acceleration versus time. [START_REF] Kroell | Impact Tolerance and Response of the Human Thorax. 15 th Stapp Car Crash Conference[END_REF]. The test is an impactor test (mass 23.4-kg, contacting interface 152-mm diameter wooden block with a 12.8-mm edge radius. Impact applied horizontally with the contact surface perpendicular to the direction of loading and Report obtained from EEVC web site -www.eevc.org van Don 15 centred mid-sagittal over the fourth costal interspace at the sternum. Impact velocities 4.3 and 6.7 m/s. Dummy positioned such that the surface of the thorax is line with the impactor centerline is vertical with the longitudinal centerline of the impactor at the same height as the mid-sternum and guided in the mid-sagittal plane of the subject.

Instrument the pendulum with two uniaxial accelerometers and a velocity sensor and the dummy with chest deflection sensor.

The force-deflection response for the 4.3 m/s and the 6.7 m/s impact velocity conditions should be within the well-known corridors based on the published work of [START_REF] Kroell | Impact Tolerance and Response of the Human Thorax. 15 th Stapp Car Crash Conference[END_REF] see Figure 22. Two uniaxial accelerometers rigidly mounted on the impactor with its axis of measurement collinear with the impact direction. The dummy will be instrumented with T12 triaxial accelerometer. The displacement and velocity of the spine and pendulum for the principal direction will be determined using the concept of a moving frame. Filter all response data according to the requirements of SAE Recommended Practice J211.

The upper abdomen force versus penetration response has to be within the well-known corridor provided by [START_REF] Nusholtz | Abdominal Response to Steering Wheel Loading[END_REF].

Figure 29. Force vs. Penetration

_Abdomen test 2 is a frontal impact test defined for the lower abdomen. The impact face of the pendulum consists of a rigid bar with above defined dimensions rigidly mounted on the pendulum. The effective mass of the pendulum should be 32 kg, impact speed 6.1 m/s for low impact and 10.4 m/s for high impact speed. The dummy needs to be position in an upright sitting position, with no back support and its legs and the arms raised. Impact point is located at the level of the L3 vertebra.

Two uni-axial accelerometers rigidly mounted on the impactor with its axis of measurement collinear with the impact direction. The dummy will be instrumented with an uni-axial accelerometer rigidly mounted in the rearward part of the spine and its measuring axis collinear with impact direction. Filter all response data according to the requirements of SAE Recommended Practice J211.

The lower abdomen force versus penetration response has to be within the well-know corridor provided by [START_REF] Cavanaugh | Lower Abdominal Tolerance and response[END_REF]. Femur/Knee _Femur/knee test 1 is a frontal impact test using a rigid flat face impactor (mass 5 kg, diameter 75 mm). The dummy needs to be seated on a rigid surface in front of the pendulum. The pelvis and the lower torso free to translate rearward such that no loading will occur on the pelvis due to the back support. Impact velocity 2.8 and 4.0 m/s.

Instrument the pendulum with one uni-axial accelerometers. Instrument the femur of the dummy with a load cell and four tri-axial accelerometers.

The knee impact versus time response, results femur acceleration versus time response and the iliac crest versus time response for the impact velocity of 2.8 m/s conditions should be within the corridors provided in Figure 33-37. _Lower leg test 1 is a pendulum impact to the toe, impact speed 6 m/s. The impactor comprises of a horizontal cylinder (mass 1.25, diameter 50 mm) and a lightweight support arm. The test procedure is based on the EEVC 'Tibia and Foot Certification The lower leg is attached via the knee clevis to a rigid back-plate. The foot is orientated such that the second metatarsal pointed vertically upwards, with the ankle maintained at 90° (neutral position). The line joining the knee clevis joint and the ankle is horizontal and the sole of the foot is vertical. The foot is fitted with a 3 mm thick PVC to represent the sole of a shoe. Laterally the center of the cylinder is aligned with the axis of the second metatarsal.

Data are acquired at a sampling frequency of 20 kHz and filtered as recommended in SAE J211/1. The lower leg is instrumented with a lower and upper tibial load cell. The impactor is instrumented with a single axis accelerometer.

For toe impacts at 6 m/s: the pendulum acceleration, the tibial force and the tibial bending moment should be within the corridors defined.

Report obtained from EEVC web site -www.eevc.org van Don 18 Only the position and the speed of the impactor has been changed. The EEVC foot certification procedure defines a height of 62 mm for heel impacts for a foot length of 265 mm. The height at which the cylinder impacted the dummy foot, will be scaled according to overall dummy foot length. Laterally the center of the cylinder is aligned with the axis of the second metatarsal.

Data are acquired at a sampling frequency of 20 kHz and filtered as recommended in SAE J211/1. The lower leg is instrumented with a lower and upper tibial load cell. The impactor is instrumented with a single axis accelerometer.

For heel impacts at 4 m/s: the pendulum acceleration and the tibial force and pendulum force should be within the corridors defined. Report obtained from EEVC web site -www.eevc.org

Figure 1 .

 1 Figure 1. Pendulum Force vs. Time.

Figure 2 .

 2 Figure 2. Pendulum Force vs. Time _Face test 3 is based on the data/corridor provided in ADRIA (1998). The test prescribed is a 3.84 m/s, 30 degree angled horizontal impact test to the frontal bone using a rigid impactor (17 kg, diameter 25 mm). The dummy needs to be positioned with the anterior-posterior axis of the head horizontal, its mid sagittal plane vertical and adjusted to obtain an impact direction 30°-angled from the mid sagittal plane of the head.

Figure

  Figure 3. Resultant Head CG Acceleration vs. Time (frontal bone impact) _ Face test 4 is identical to face test 3, only the seat height should be adjusted such that impact to the zygoma instead of the frontal bone occurs. Instrumentation is similar to the instrumentation described by face test 3.

Figure 4 .

 4 Figure 4. Resultant Head CG Acceleration vs. Time (Impact to the Zygoma)

Figure 5 .

 5 Figure 5. Peak impactor force vs. impact duration.

  is a frontal impact sled test at 50 km/h with a sled deceleration close to the ECE R44-03 regulation (Child's restraint regulation), Report obtained from EEVC web site -www.eevc.org van Don 13 corresponding to a maximum deceleration of 22 G's and seat belt (4 kN load limiting system) and airbag as restraint system Instrument the dummy with an tri-axial accelerometer at the upper and lower left and right arm. Filter all response data according to the requirements of the SAE Recommended Practice J211.The following responses should be within the corridors provided. Left and right acromion resultant acceleration versus time, left lower and upper humerus resultant acceleration versus time and the right upper and lower humerus resultant acceleration versus time.

FigureFigure 9 .

 9 Figure 6. Left acromion resultant acceleration versus time

Figure

  Figure 10. Left acromion resultant acceleration versus time

Figure 14 .

 14 Figure 14. Sacrum resultant acceleration vs. time

Figure

  Figure 18. Sacrum resultant acceleration vs. time

Figure 22 .

 22 Figure 22. Force deflection corridor _Thorax test 2 is based o the data of Yogananadan et al. (1997). The test is an impactor test (mass 23.4 kg, diameter 150 mm) to the right antero-lateral thorax, impact velocity 4.3 m/s. The lower extremities are stretched out horizontally, upper extremities are extended, back of the torso is unsupported, and the torso rotated 15º from right to left.The force-deflection response, the force-time history and the deflection time history response should lie within the corridors provided by[START_REF] Yoganandan | Impact biomechanics of the human thorax-abdomen complex[END_REF].

Figure 23 .

 23 Figure 23. Force-time corridor

Figure

  Figure 25. Upper sternum resultant acceleration vs. time

Figure

  Figure 27. Upper sternum resultant acceleration vs. time

Figure 30 .

 30 Figure 30. Force vs. penetration _Abdomen test 3 is a seatbelt-loading test. The dummy needs to be positioned upright with the legs outstretched, and will be loaded about the mid abdominal region (approximately level of the umbilicus).. The device to achieve seatbelt loading should provide a peak-loading rate of approximately 3 m/s and an penetration speed-time history.The load versus penetration response should be within the corridor provided in Figure31.

Figure 31 .

 31 Figure 31. Load vs. Penetration for (Seatbelt Loading Condition) _Abdomen test 4 is a surrogate airbag-loading test. The device used includes a pneumatic firing mechanism that accelerates a lightweight (approximately 1-kg) aluminum impactor

Figure 32 .

 32 Figure 32. Load vs. Penetration (Surrogate Airbag Loading Condition)

  Figure 33. Knee impact force vs. time

Figure

  Figure 38. Pendulum Acceleration vs. Time (Toe Impact)

Figure

  Figure 41. Pendulum acceleration vs time (Heel Impact)

Table 2 . Distribution of AIS2+ injuries per body region for restrained frontal occupants (MHH)

 2

	Percentages of AIS 2+ Injuries	Injury caused by	Head	Neck Injured Body Region Thorax Other
		Airbag	14.8		1.9
	with Frontal Airbag (n = 54 Injuries)	Steering Wheel Body Movement Belt other / unknown	3.7 3.7		5.6 1.9 3.7 1.9
		TOTAL	22.2	0.0	14.8	63.0
		Steering Wheel	3.1		1.5
		Belt	0.1		2.6
	without	Body Movement	0.7	1.2	0.4
	Frontal Airbag	Windscreen	1.5		0.1
	(n = 740 Injuries)	other	3.2	0.1	1.1
		unknown	24.7	2.2	7.8
		TOTAL	33.4	3.5	13.5	49.6

Table 1 . Comparison of accident databases

 1

	All EES < 80	CCIS	MUH
	Driver -no airbag	151	747
	Driver -airbag	152	57
	Passenger -no airbag	87	281
	Passenger -airbag	10	11

ACKNOWLEDGEMENT

This work is carried out under the FID "Improved Frontal Impact Protection through a World Frontal Impact Dummy" contract, funded by the European Commission under the Transport RTD Programme of the 5 th Framework Programme, Project Nr. GRD1-1999-10559. Project Duration: January 2000-End of June 2003. EEVC members are: M. van Ratingen (TNO Automotive, chair), F. Bermond (INRETS, secretary), D. Hynd (TRL), R. Schäfer (BASt), M. Svensson (Chalmers University), G. Gertosio (FIAT) and L. Martinez (INSIA). Industry advisors are: K. Bortenschlager (Audi AG), H. Öhrn (Volvo), M. Page (CEESAR) and H. Leyer (VW).


T Domenech
email: trystan.domenech@mines-paristech.fr

B Vergnes

E Peuvrel-Disdier

Nanocomposites polyoléfine/argile lamellaire: lien entre procédé et structure multi-échelle

Keywords: Nanocomposites, argile organophile, extrusion bivis, procédé, dispersion, nanostructure, rhéologie

La réalisation de nanocomposites par mélange d'argile lamellaire dans une matrice polymère à l'état fondu est généralement faite par extrusion bivis corotative. L'objectif de cette opération est de disperser les agglomérats d'argiles, d'exfolier les feuillets et de distribuer ces charges d'épaisseur nanométrique au sein de la matrice. La forte surface spécifique des argiles est ainsi développée, de même que l'interface polymère/argile. Dans le cas des polyoléfines, un troisième matériau est souvent ajouté en tant que compatibilisant entre la matrice apolaire et la surface polaire des argiles. Le procédé d'extrusion bivis peut être adapté selon le résultat recherché via de nombreux paramètres : profil de vis, vitesse de rotation, débit d'alimentation des doseurs, températures de régulation le long du fourreau. Nous avons étudié l'effet de ces paramètres procédé sur la structure des nanocomposites aux échelles micrométrique et nanométrique. Des échantillons sont prélevés pour chaque condition opératoire testée et le niveau de dispersion de l'argile est évalué en utilisant différentes techniques de caractérisation : la microscopie électronique à balayage permet de déterminer le nombre d'agglomérats d'argile par unité de surface ainsi que leur distribution de taille. La diffraction de rayons X permet de remonter à la distance interfoliaire et ainsi de connaître le niveau d'intercalation des argiles. La rhéométrie dynamique permet d'évaluer le niveau d'exfoliation des feuillets d'argile grâce à un modèle de Carreau-Yasuda avec seuil d'écoulement. Les résultats montrent que la dispersion de l'argile est défavorisée par une haute température de fourreau, notamment du fait de la dégradation thermique de la matrice. L'augmentation du rapport « vitesse de rotation des vis / débit d'alimentation » entraîne une diminution du nombre et de la taille des agglomérats présents au niveau de la microstructure, ainsi qu'un phénomène d'exfoliation plus important au niveau de la nanostructure.

Introduction

Depuis les travaux d'une équipe de recherche de Toyota à la fin des années 80 [START_REF] Fukushima | Synthesis of an intercalated compound of montmorillonite and 6polyamide[END_REF], l'utilisation d'argiles lamellaires en tant que charges dans des matrices polymères a suscité une attention particulière. En effet, le haut facteur de forme ainsi que la forte surface spécifique développée par ces argiles permettent d'obtenir une amélioration significative des propriétés de la matrice, telles que les propriétés mécaniques (résistance aux chocs et à la traction), la résistance au feu, l'imperméabilité ou encore les propriétés barrières, et ce pour de très faibles taux de charges (moins de 5 % en masse). Ces matériaux nanocomposites sont notamment utilisés dans l'industrie automobile où leurs performances mécaniques et leurs propriétés barrières sont exploitées afin de remplacer certaines pièces métalliques, permettant ainsi une réduction du poids des véhicules. Les premiers nanocomposites à base d'argiles lamellaires ont été réalisés par mélange à l'état fondu dans des matrices polyamide 6 (nylon), avec des interactions matrice-charge très favorables à la délamination des feuillets. Cette amélioration des propriétés de la matrice par l'incorporation d'argiles lamellaires a ensuite été testée dans des matrices polypropylène, moins onéreuses que le nylon. Les interactions polypropylène-argile étant nettement moins favorables à la dispersion des feuillets, l'utilisation d'agents compatibilisants s'est imposée.

Bien que le mécanisme de dispersion des argiles lamellaires au cours du mélange à l'état fondu soit difficile à observer du fait de la taille des feuillets d'argile, il est décrit dans la littérature comme une combinaison des contraintes de cisaillement sur les agrégats d'argile ainsi que de la diffusion des chaînes polymères entre les galeries d'argile [START_REF] Fornes | Nylon 6 nanocomposites: the effet of matrix molecular weight[END_REF].

Dans ce travail, nous nous intéressons à la réalisation de nanocomposites par extrusion bivis et en particulier au lien entre les conditions opératoires et la morphologie des nanocomposites aux échelles micrométrique et nanométrique, en s'appuyant sur différentes techniques de caractérisation. Tous les mélanges de cette étude ont été réalisés sur une extrudeuse bivis corotative ThermoFischer PTW24, de taille laboratoire (D = 24 mm et L/D = 40). Les profil de vis utilisés sont représentés à la Figure 1. L'alimentation matière est assurée par un doseur pondéral.

Matériaux, méthodes expérimentales et analyses

Les paramètres opératoires sont la vitesse de rotation des vis N, le débit d'alimentation Q et la température du fourreau T f . Nous nous sommes particulièrement intéressés à l'influence de la vitesse de rotation des vis, de la température du fourreau ainsi que de la configuration du profil de vis sur la structure des nanocomposites. Dans tous les cas, le débit Q est fixé à 3 kg/h. Les échantillons sont prélevés en sortie de filière avant d'être compressés à 180°C sous forme de disques de 25 et 50 mm de diamètre et de L'analyse rhéologique montre que le comportement des nanocomposites diffère de celui de la matrice, principalement aux basses fréquences. En effet, on observe l'apparition d'un plateau du module élastique, ainsi qu'une augmentation de la viscosité dans la zone terminale. La littérature attribue ce comportement de type pseudo-solide à la formation d'un réseau de percolation des charges d'argiles exfoliées [START_REF] Galgali | A rheological study on the kinetics of hybrid formation in polypropylene nanocomposites[END_REF]. Lertwimolnun et Vergnes [START_REF] Lertwimolnun | Influence of compatibilizer and processing conditions on the dispersion of nanoclay in a polypropylene matrix[END_REF] ont montré que le comportement rhéologique des nanocomposites peut être modélisé par une loi de Carreau-Yasuda avec contrainte seuil : 5). Le niveau d'exfoliation est également augmenté de manière considérable en passant de PV1 à PV2, quelle que soit la vitesse de rotation des vis utilisée, avec un facteur proche de 10 sur les contraintes seuil (Figure 6).

    1 0 0 1 m a a                 (2)

Conclusions

Des nanocomposites PP/PP-g-MA/argile ont été réalisés par extrusion bivis. L'influence des conditions d'extrusion sur la structure des matériaux obtenus a été étudiée. La dispersion des charges d'argile au niveau de la microstructure est améliorée lorsque la vitesse de rotation des vis N augmente et lorsque la température de consigne du fourreau diminue. L'étude de la nanostructure a révélé une configuration à la fois intercalée et exfoliée dans tous les cas. La distance interlamellaire semble peu sensible aux conditions opératoires, ce qui a déjà été mis en évidence dans d'autres études [START_REF] Lertwimolnun | Effect of processing conditions on the formation of polypropylene/organoclay nanocomposites in a twin screw extruder[END_REF][START_REF] Lertwimolnun | Influence of screw profile and extrusion conditions on the microstructure of polypropylene/organoclay nanocomposites[END_REF].

En revanche, le niveau d'exfoliation est influencé par les conditions d'extrusion. En particulier, la dégradation de la matrice polymère due à des températures matières élevées semble nettement défavoriser l'exfoliation.

L'étude menée sur la configuration du profil de vis confirme qu'il est possible d'améliorer la qualité de mélange dispersif en ajoutant des zones plus restrictives, notamment avec des éléments malaxeurs décalés en angles de 90°.

Figure 2 :

 2 Figure 2 : Influence de la vitesse de rotation sur le nombre des agglomérats d'argile Le diamètre équivalent moyen en nombre des agglomérats d'argile reste quasiment constant ( µm D n 2 26   ), quelles que soient les conditions opératoires. Cependant, la distribution de taille des agglomérats n'est pas la même selon la température du fourreau : on observe une réduction importante de la taille maximale des agglomérats en changeant la température de 245 à 180°C. La figure 3 montre des clichés représentatifs des morphologies observées, pour une vitesse N = 600 rpm et deux températures du fourreau, de 245 et 180°C.

Figure 3 :

 3 Figure 3 : Influence de la température du fourreau sur la taille des agglomérats

où  0 Figure 4 :

 04 Figure 4 : Évolution de la contrainte seuil en fonction de N pour T f = 180°C et 245°C La figure 4 représente l'évolution de  0 en fonction de N pour chacune des deux températures du fourreau. Lorsque T f = 180°C, l'augmentation de N entraîne une légère augmentation du niveau d'exfoliation. Pour ces conditions de procédé, les charges d'argile ont un effet renforçant sur la matrice, étant donné que la viscosité plateau des nanocomposites est plus de deux fois supérieure à celle de la matrice. En revanche pour T f = 245°C, on constate une importante diminution de la contrainte seuil. Les valeurs de  0 sont inférieures à celle de la matrice à partir de N = 300 rpm, ce qui met en évidence un phénomène non négligeable de dégradation de celle-ci au cours du procédé d'extrusion. Cette dégradation est visiblement néfaste pour l'exfoliation des argiles dans la matrice, peut être à cause de l'éventuelle dégradation concomitante de l'intercalant et du compatibilisant. La température du fondu mesurée en sortie de filière est toujours supérieure à la température de consigne du fourreau, étant donnée l'importante dissipation visqueuse. La température matière peut ainsi atteindre 260°C lorsque la consigne du fourreau est de 245°C.

Figure 5 :

 5 Figure 5 : Influence du profil de vis sur la taille des agglomérats d'argile (T f = 180°C)

Figure 6 :

 6 Figure 6 : Evolution de la contrainte seuil en fonction de N pour les deux profils de vis (T f = 180°C)

PV1 PV2 Sens de l'écoulement Zone de convoyage Zone de fusion Zone de malaxage PV1 PV2 Sens de l'écoulement Zone de convoyage Zone de fusion Zone de malaxage

  1,5 mm d'épaisseur. Chaque échantillon est analysé sur une aire A 0 = 10,3 mm 2 . On peut ainsi remonter à la distribution de taille des agglomérats (particules dont le diamètre équivalent est supérieur à 10 µm) à partir des clichés MEB. On définit également le rapport d'aires A r entre la somme des aires occupées par les agglomérats d'argile A MMT et l'aire analysée A 0 :

	valeur de la température de consigne du fourreau. Il
	apparaît également qu'une diminution de la
	La morphologie à l'échelle microscopique est caractérisée par microscopie électronique à balayage (MEB Philips XL 30 ESEM). Les échantillons sont polis mécaniquement jusqu'à une rugosité de surface inférieure à 5 µm et métallisés par une couche Au/Pd d'environ 10 Å. L'analyse d'image permet de déterminer les diamètres équivalents des agglomérats d'argile présents au sein de la matrice température de consigne du fourreau entraîne une diminution significative du nombre d'agglomérats pour des conditions similaires en vitesse. Cela peut s'expliquer par une augmentation des contraintes de cisaillement, due à des niveaux de viscosité plus élevés dans le cas de températures plus faibles. Ces diminutions en nombre d'agglomérats traduisent une dispersion des charges d'argiles à des échelles polymère. 0 MMT r A A   A Le niveau d'intercalation des nanocomposites (i.e. (1) l'augmentation de la distance interlamellaire d 001 des argiles) est caractérisé par diffraction de rayons X avec un Philips X'Pert PRO utilisé en mode réflexion. La source de rayons X employée est la comportement rhéologique des nanocomposites est sensible à leur état de dispersion, en particulier à leur niveau d'exfoliation [3, 4]. Les mesures de rhéométrie oscillatoire ont été réalisées avec un rhéomètre à déformation imposée ARES (TA Instruments) en utilisant une géométrie plan-plan de 50 mm de diamètre et un entrefer de 1 mm. Toutes les mesures rhéologiques ont été faites dans le domaine de viscoélasticité linéaire à une température de 180°C. Deux profils de vis ont été sélectionnés afin de mieux comprendre comment améliorer la capacité d'un profil à produire un mélange dispersif. Comme l'indique la figure 1, le profil PV1 a une zone de malaxage constituée de plusieurs séries d'éléments malaxeurs bilobes décalés de +30° tandis que le profil PV2 contient une première zone de malaxage dont les éléments sont décalés de +60° suivie de trois zones restrictives ayant des malaxeurs décalés de 90°. Dans un premier temps, nous analysons l'influence de la température du fourreau et de la vitesse de rotation des vis sur la structure des nanocomposites pour le profil de vis PV1, puis nous nous intéresserons à l'influence du profil de vis. Figure 1 : Profils de vis utilisés 3. Résultats et discussion 3.1 Effet des conditions opératoires sur la microstructure L'influence des conditions opératoires sur le nombre inférieures à celle considérée dans le cas des raie K  du cuivre de longueur d'onde = 1,54 Å. Plusieurs études ont mis en évidence que le agglomérats (D équivalent > 10 µm).
	d'agglomérats d'argile est représentée sur la figure
	2. Une nette tendance à la diminution du nombre
	d'agglomérats est observée lors de l'augmentation
	de la vitesse de rotation des vis N, quelle que soit la

  3.2 Effet du profil de vis sur la microstructureLe nombre d'agglomérats tend à diminuer lorsque N augmente, avec des valeurs similaires dans les deux cas. En revanche, il est évident que la taille des agglomérats est sensiblement réduite en utilisant le profil PV2, ce qui montre sa meilleure capacité à entraîner la rupture des agglomérats d'argile comparativement à PV1 (Figure

Remerciements

Cette étude a été effectuée dans le cadre du projet Nanotough, soutenu par l'Union Européenne (7 ème PCRD).


est destinée au dépôt