Pedestrian protection - Pedestrian in collision with personal car

Transcription

1 Pedestrian protection - Pedestrian in collision with personal car Authors: Jiří Svoboda, Ing; Zdeněk Šolc, Ing. Czech Technical University in Prague, Faculty of Mechanical Engineering, Department of Automotive and Aerospace Engineering, Technická 4, Praha 6, Czech Republic Tel:

2 Summary The paper is aimed to the area of pedestrian protection. The object of the research is the pedestrian in collision with front end of the personal car. The target is to summarise current approach to problem pedestrian protection, discuss prepared pedestrian protection directive and its benefit of pedestrian protection in contrast to mathematics simulation of collision between pedestrian and vehicle. The contact points of different parts of the human body on front vehicle end are determined on the basis of the statistics data concerning pedestrian - vehicle collision occurred in Europe. The courses of typical collision at different velocity are derived. The statistic data are considered as assumption for next research. The mathematic simulation for different vehicle front-end shapes and different velocity is carrying out to confirm statistic results and shove influence response of different initial conditions to the pedestrian kinematics. Introduction Road accident statistics indicate that a significant proportion of casualties involve pedestrians and cyclists who are injured as a result of contact with a moving vehicle, notably passenger cars frontal structures. Most accidents take place in urban areas where serious or fatal injuries can be sustained at relatively low speeds, particularly in the case of children. Although there is a clear case for the implementation of measures to separate pedestrians from vehicular traffic and, where this is not feasible, to reduce the speed of traffic, there is nevertheless scope to mitigate the severity of injuries to pedestrians by improving the frontal structures of vehicles. Accident statistic The official statistics of road accidents originate on the basic of police recordings. They contain both common information like number of the light or serious injured, killed pedestrian in certain period and more detail information like region, date, time and cause of accident. Picture 1 shoves an example of the accident statistic. It is percent share of the killed pedestrian relates to over all the killed persons in road traffic in different European countries. The data shoves how series problem pedestrian protection is. The number of the killed pedestrian is between % while number of collision of pedestrian with car represents 4-8% from the over all number of the traffic accidents. The result indicates to high risk of pedestrian death in collision with car. Majority of the collision ware occurred in urban areas. Number of the killed pedestrians in urban areas was 68% from over all number of the killed pedestrians. Number of injured pedestrians in urban areas is much more significant than number of pedestrians injured outside them (91% serious injured and 95% slight injured) The data contained in the official accident statistics are not enough for research of pedestrian behaviour during impact. It is necessary to know collision velocity, contact points of different part of pedestrian body on vehicle structure etc. Car producers and independent institutions have created their own statistics that contained these data. Information obtained from databases of syndicate VW and DEKRA is used at research. The databases contained: Collision velocity in urban areas as well as outside them Contact points of different part of pedestrian body on vehicle body List of pedestrian injury occurred and its number List of pedestrian injury occurred leading to death and its number

3 The bumper and the front edge of bonnet is the most often impacted areas on car body however impact of pedestrian to the bonnet top or the windscreen of car is the most often cause of injury. The head in contact with the structure is usually occurred in those cases. According to the accident statistics the head injury is appeared to be one of the most often and serious injury that are occurred at accidents. Contact point and contact velocity of pedestrian head impacting vehicle and pedestrian plays significant role in injury origin and its severity. Description of typical collision injury origin The kinematics of pedestrian movement towards vehicle is necessary to understand to be one able to clear up injury origin. The kinematics is dependent on vehicle shape, initial position of pedestrian and collision velocity. The collision is possible to divide into two basic phases: primary and secondary collision. Strike of the pedestrian by vehicle is a reason of the injury during the primary collision while the injury during the secondary collision is caused by contact with ground. Both phases contribute to origin of injury. Contribution of the second phase to total injury is appeared to be more significant. Analysis and studies of this problem displays existence of four basic injury mechanism types. Each type is typical for different shape of the vehicle front end, velocity and initial pedestrian position. 1. Passenger car, low and middle velocity This is the most frequent type of collision. The primary collision is possible to divide into three phases. The pedestrian is struck by the front of a car in the first phase. The primary contact is from the front bumper which strikes pedestrian s leg in area of calf and knee. The pedestrian s thigh or pelvis is strikes by the leading edge of bonnet (front edge of bonnet) at the second phase. The exact location of these contacts depends on the relative heights of the pedestrian and of the vehicle part. The pedestrian then rotates about the leading edge until the third phase occurred. Strikes of arms, head and chest to the bonnet top or windscreen are occurred during the third phase. The head impacts windscreen considered collision of pedestrian with large-capacity car or compact car having short bonnet. The impact of the head to the bonnet occurs in case of impact to bigclass car. The most critical is impact of the head in area of windscreen bottom edge. The body structure in this area is very hard. This variant is typical for middle-class cars having middlelong bonnet. 2. Passenger car, low and middle velocity The pedestrian is hit by the area of vehicle front end which is near to the body corner. The pedestrian falls over the wing aside. Often contact of the pedestrian head and the vehicle body structure does not occur in this case. 3. Passenger car, high velocity In this case pedestrian role-over around horizontal axis (somersault) happens and usually flies over the vehicle. It is caused by high level of kinetic energy of rotational movement of pedestrian upper part around horizontal axis that occurs after impact of vehicle to pedestrian in high velocity. 4. Lorry, bus and van Impact of over-all pedestrian body to vehicle front wall. Risk of run over a pedestrian after its fall down on ground is high in this case. Testing by means of impactors Method suggested in prepared pedestrian protection directive is often used within car laboratory tests. Working Group 10 of the European Experimental Vehicles Committee has published a report setting out proposal for suitable test procedures to assess the pedestrian friendliness of the front of passenger cars. This proposal establishes biomechanical performance standards covering the three areas of the human body where injuries are most often sustained: the leg,

4 the upper leg and the head. The proposed test procedures involve projecting suitably instrumented impactors, which represent each of the three parts of the body mentioned above, against the areas of the vehicle which give rise to injury, namely the bumper, the bonnet leading edge and the bonnet top (picture 4) The committee suggested using three different tests: Test of the bonnet top by model of head (head impactor) different for adult and child Test of the front edge of bonnet (BLE bonnet leading edge) by model of thigh (thigh impactor) Test of the bumper upper edge (BRL bumper reference line) by model of lover extremity (lower extremity impactor) Test of bumper Two cylinders covered by foam connected together by deformation element simulating the knee represent the human lower extremity (mass 13,4 kg) The model strikes the front bumper at velocity 40 km/h. The BRL is divided to three zones. The test is required to be carrying out in the each zone. Criteria: bending angle in knee less then 15, displacement in horizontal axes of deformation element of two cylinders less then 6 mm, acceleration at upper part of model (representing shin) less then 150 g. Test of front edge of bonnet Cylinder covered by foam represents the human thigh. Three dilatation strips are placed on cylinder to measure of bending moment. The cylinder is supported at both ends. Both supports are equipped by strength sensor to be possible measure the support strength. Test parameters: velocity km/h, mass kg, angle between vertical axes and cylinder axes The BLE is divided to three zones. The test is required to be carrying out in the each zone. Criteria shear load of impactor less then 4 kn, bending moment less then 220 Nm. Test of bonnet Semi-sphere covered silicon rubber represents the human head (adult diameter 150 mm, mass 4,8 kg; child diameter 115 mm, mass 2,5 kg). The three axes acceleration sensor is placed in model of the head. Test parameters: velocity 40 km/h, the head velocity towards bonnet contains angle 65 with horizontal axis in the adult case and 50 in the child case. Three zones on bonnet are defined. The zones for accessing of adult head injury risk are different from the zones for accessing of child head injury risk. Three tests are required to be carrying out in each zone. Criteria: HIC less then Tests results of contemporary passenger car Three passenger cars of different class have tested by the lower extremity impactor and the thigh impactor and ten cars of different class and producers have tested by the head impactor. Test of the bumper: values of observed quantities ware considerably exceed the marginal values of the each criterion in most cases. It seams to be the most difficult to meat demand of maximum angle 15 between the tight and the shin. The maximum values exceed marginal values up to 200% in this case. Test of the front edge of bonnet: Measured bending moments and forces often are exceeded marginal values by more than 100% and 50% respectively. Test of the bonnet top: measured value of HIC (1000) are exceeded in 86% cases of tests carried out by impactor representing child head and 58% respecting adult head. The results do not contain tests of lateral edges of bonnet. The euroncap has carried out tests of cars according the method similar to the method suggested in the prepared pedestrian protection directive. Twelve family cars and twelve

5 small family cars are accessed by means of test criteria. Every tested car are not meat the criteria of tests. Two stars (from four) is average assessment of the cars. The results shove no significant difference in the pedestrian load during collision with different class car. Tests using figurines Laboratory tests Up to this time the figurine Hybrid II 50% man has been the most often used figurine to crash test pedestrian vehicle. Extent of impact velocities km/h was determined on above mentioned statistic results. The most often occurrence of collisions was in urban areas where velocity limit is 50 km/h. Technical arrangements leading to pedestrian protection are effective to velocity 50 km/h. Observed parameters: trajectory, velocity and acceleration of body parts (most often Head and chest), contact points and velocity individual parts of figurine on vehicle body. Performance standard given by the Head Injury Criterion is the most often used for predicting of head injury: HIC = ( 2 1) a t t t () t. dt ( t 2 1) < 1000 The HIC is determined by tracing resultant acceleration signal. The deceleration values are considered in g s. Time frame 15 ms is considered in case of contact head with hard structure. In the no contact case the time frame is 36 ms. Another criteria traced during collision: Criterion of a 3ms the injury criterion is computed by tracing the resultant linear acceleration signal using a time window with a width of 3 ms. The highest acceleration level with a duration of at least 3ms is called criterion of a 3ms. Thorax Trauma Index definition of TTI for 50 th percentile dummy with a mass of 75 kg is: TTI = 0,5.( RIBg + T12g) RIB g maximum absolute value in g s of the 4 th and 8 th rib on struck side in lateral direction T12 g - maximum absolute value in g s of the 12 th thoracic vertebra in lateral direction The result of crash test of the Fiat Cinquecento is worth a mention. The test has carried out by DEKRA Accident Research in the work frame of testing of different car class and year of manufacture. Contact of head with windscreen in third phase of primary collision is typical for modern compact cars. At first sight it can be considered less pedestrian friendly than impact to bonnet top but measured values of head deceleration in contrast of them measured when head strikes bonnet indicate different result. Because of the low rigid bonnet fits the high rigid engine structure placed under during deformation the resultant deformation of bonnet is lower than windscreen deformation. Larger deformation of windscreen in comparison with bonnet is important factor of less deceleration peak. Table 1 contains list of tested cars and points of head contact with car structure Mathematics simulation max

6 The Madymo code is used for simulation of behaviour of figurine during collision with the passenger car. The database of mathematical models of figurines used in automotive branch is implemented in the code. The Madymo is chosen due to its intention on solving problems in field of passive safety at high level of precision. Due to it is the most frequent the first type of collision is modelled (chapter: Description of typical collision injury origin). According the statistic the most accidents take place in urban areas therefore the behaviour of figurine is observed at low velocities to 50 km/h. Extra attention is focused to the velocity 40 km/h to be possible compare the simulation results with the results obtained from tests and appreciate them in contrast to the method pedestrian protection directive. The model of collision between pedestrian and passenger car consists of three systems and interactions between them: System of roadway inertial coordinate system System of pedestrian System of car Following factors are considered at physical analysis of collision: Pedestrian Shape Mass, centre of mass, moments of inertia of each body part Resistance against motion in joints Motion state (position, velocity of each body part) Car Shape Stiffness and hysteresis of contact surfaces Driving state (velocity and acceleration) Roadway Stiffness Model of pedestrian Multibody technique is used for modelling of the figurines. Surface of figurine is created by ellipsoids to resultant shape corresponds with shape of pedestrian. Each body (defined - mass, centre of mass and moment of inertia) represents certain part of human body and each joint represents any important joint in human body or its group (four joints created spine). Characteristics (stiffness, hysteresis etc.) are defined in each joint to limit motion of connected bodies. The figurine Hybrid II model of 50% man is used in simulations to be possible compare results with laboratory tests and validate model according them. Simulations using the model of 6 th year child are carried out as well. The modified database model of 50% is used because original database figurines are designed for simulation of passenger during frontal impact. The results obtained by original figurine models are not corresponded with reality. Initial posture and contacts interactions are modified and any new contact interactions are defined. Model of car The simulations are carried out with three cars: Škoda Fabia, Škoda Octavia and Fiat Cinquecento. One body represents the car. Surface of car is created by ellipsoids to resultant shape corresponds with shape of real car. The deformation characteristic is added to each ellipsoid according which part the ellipsoid represents. Forward motion (velocity and acceleration) is forced to the car. Driving state of car at tine of figurine hitting is intensive braking (deceleration 6,5 ms -2 )

7 Simulation description and observed parameters Fiat cinquecento The simulation of collision between figurine Hybrid II and car is carries out at velocity 37 km/h. The figurine is positioned its left side to car and lower extremities are in step position. The goal of the simulation is to verify the model of figurine and justify its modifications mentioned above. The model is verified by comparison with experimental results obtained from real crash test carried out by the DEKRA Accident Research. Observed parameters: during simulation are coincident with published results of the test. They are: over all movement of figurine, course of head velocity, vector diagram of velocities at contact of head with windscreen, values of any injury criteria. Discussion of results: Contact of head occurred in area of bottom part of windscreen likewise at crash test (mentioned above). The courses of head velocity measured during simulation are in picture 2. The table 3 shoves comparison of injury criteria. The comparison of velocity courses and velocity vector diagrams as well as injury criteria shoves acceptable coincidence of test and simulation results. Simulation of collision pedestrian to the Škoda Fabia The mathematic model of collision between pedestrian and the Škoda Fabia is based on the previous model. The shape and contact characteristic are changed to be corresponding to modelled car Škoda Fabia. The simulations are carried out at initial velocities 10, 20, 30, 40 and 50 km/h to be possible observe velocity effect upon the point of head contact. Two types of figurine are considered: the figurine Hybrid II 50% man verified by previous simulation and figurine 6 th year old child modified on basic of previous experiences with the Hybrid II. Observed and assessed parameters: Evaluating of complete collision history Tracking of head contact point on vehicle. Contact velocities of the head having significant influence on severity of injury Courses of the head velocity and acceleration Injury criterion (HIC, SI, TTI etc.) Simulation of collision pedestrian to the Škoda Octavia Simulation conditions are coincident to them mentioned in previous section the simulation of collision pedestrian to the Škoda Fabia. But there is one difference: the simulations are only carried out at velocity 40 km/h. The scope of simulation is comparison with method pedestrian protection directive. Assessment of results Impact of the pedestrian to the car does not significantly influence movement of the car. The points impacted by the head of Hybrid II on car body of the Škoda Fabia are situated near bottom windscreen edge (picture 3). This area is very stiff. It leads to high level of load resulting high head injury risk. Windscreen of the Škoda Fabia is impacted by the head at speed 40 km/h while the top margin is impacted on Škoda Octavia bonnet due to its longer front end (picture 4). The head of figurine 6 th year old child impacts the bonnet near the bonnet leading edge. There the structure stiffness is much more higher than the bonnet stiffness as well as in case of windscreen edge stiffness. The areas of the head contact both adult and 6 th child are different of them suggested in the pedestrian protection directive for testing (picture3).

8 Trip of figurine s legs is possible observe during collision of the adult figurine Hybrid II at higher velocities. The trip causes rotation about the leading edge until the head strikes to the windscreen or bottom windscreen edge. The higher velocity of the car the higher velocity of the rotation motion that leads to higher contact velocity of the head. It is the contact velocity that is the important factor determining magnitude of injury. While the figurine of 6 th year child is rather taken on the front end of car. The level of head load is not so high compared to adult figurine. In this case the thorax, stomach and pelvis bear a majority of load. Therefore the dominant injury is not injury of head but above all injury of the thorax and pelvis (table 2). The neck injury risk is possible observe from the results. Discussion of results According to the simulations results the calculated contact points of head are not situated to the zones tested according to the method described in pedestrian protection directive (pictures 3 and 4). Calculated contact velocities both the adult head and the child head are less than velocities set down by the method (picture 4). The difference is particularly momentous at contact velocity of the child head. The injury criteria and/or load of the thorax and the spine should be investigated for assessing the child friendliness of the front of passenger car. According simulation and laboratory test results the windscreen is often impacted because of a test of the windscreen should be prescribed in pedestrian protection directive (picture 3 and table 2). The shape of the front car-end plays significant role in kinematics of figurine during collision. Height of the bonnet leading edge influences significantly point of the adult head contact. Higher leading edge causes shift of the head contact point towards front of the bonnet and avoiding contact with stiff structure around bottom windscreen edge. The front panel slope is very important factor too. Conclusion At the present the problem of pedestrian protection is paid a big attention. Road accident statistics indicate that a significant proportion of casualties involve pedestrians and cyclists who are injured as a result of contact with a moving vehicle. Most accidents take place in urban areas where serious or fatal injuries can be sustained at relatively low speeds. The collision is possible to divide into two basic phases: primary and secondary collision. Namely the second phase is influenced by many factors and it is difficult to determine any dependence. Although contribution of the second phase to total injury is significant it is the first phase of collision that is paid attention in this study. According to the statistic are determined four basic injury mechanism types that can occur during the primary phase. Each of them is typical for different shape of the vehicle front end, velocity and initial pedestrian position. The most frequent is the first type which kinematics is following. The front bumper strikes pedestrian s leg in area of calf and knee. Than the pedestrian s thigh or pelvis is stricken by the leading edge of bonnet. After it the pedestrian rotates about the leading edge until arms, head and chest strike to the bonnet top or windscreen. The first type is studied by means of mathematics simulation. The simulations are carried out for three cars representing different car class. The influence of car front-end shape, initial velocity and pedestrian posture on kinematics of pedestrian motion during collision is studied. The paper gives an introduction to the method of the prepared pedestrian protection directive used for assessing of the vehicle structure friendliness to pedestrian. The proposal establishes biomechanical performance standards and method its testing covering the three areas of the human body where injuries are most often sustained: the leg, the upper leg and

9 the head. According statistic data the head injury is the most often cause of death therefore the study is largely focussed on assessing of head kinematics, point of contact and load. The mathematics simulation and crash tests shoves that points of head contact mostly are not situated in the areas tested by method pedestrian protection directive. Modification of the front structure to be friendlier to adult head (meeting demand of bending angle in knee less then 15 and displacement in horizontal axes less then 6 mm when striking by bumper) could leads to high load measured on children figurine. While the head and the spine are the most endangered parts of adult body the high level of thorax load is occurred in addition in case of child collision. The biomechanical performance standard for thorax should be implemented into the directive. The test of windscreen and its bottom edge are not included in the directive. The study shoves they are the most often impacted parts by adult head. Calculated contact velocities both adult head and child head are less than velocities set down by the method therefore the real load of head impacting bonnet is less than the measured by means of impactor. Meeting the biomechanical performance standard demanded by the contemporary directive proposal could not leads to more pedestrian friendly car structure. Modification of the car front structure (gap between bonnet and stiff structure under, extend of front etc.) is necessary for achieve the directive criteria. Increase of the purchase price is estimated by 13%. Because of the study does not evidence unambiguous effect to pedestrian protection the extra charges caused by modification of car structure could not be justified. Large modification of front structure cannot be applied. Reasons for it are demands on layout of front end, driver s view, aerodynamics quality, behaviour of structure during frontal crash test etc. Higher friendliness of car structure to pedestrian could be ensure by the energy absorbing bonnet and front panel in case of collision with child and impact of adult thorax or pelvis and pedestrian airbag for protection of the adult head against impact to the windscreen and/or the windscreen edge. Research in both fields is in process at our department. Car Contact velocity Point of the first head contact (initiation of production) BMW 318 (07/1997) 44 km/h Bonnet Porsche 924 (11/1979) 41 km/h Windscreen Nissan Micra (04/1988) 33 km/h Windscreen Fiat Cinquecento (07/1993) 39 km/h Windscreen VW T4 (10/1993) 40 km/h Windscreen bottom edge Nissan Micra (11/1993) 34 km/h Windscreen Mazda 121 (07/1994) 43 km/h Windscreen Renault Twingo (04/1994) 38 km/h Windscreen bottom edge Table 1: List of tested cars by DEKRA and points of head contact with car structure HIC Head a 3ms Pelvis - a 3ms Thorax - a 3ms Test g 39 g 27 g Simulation ,4 g 40,4 g 29,6 g Table 3: comparison of injury criteria obtained from test and simulation. Velocity HEAD PELVIS CHEST

10 HIC a 3ms a 3ms a 3ms TTI adult g 13,7g 40,9g 40,2 child 51 20,2g 29,4g 15,5g 32,2 adult 86 34,4g 19,4g 32,7g 31 child 265,4 47g 49,8g 16,1g 23 adult 602,3 75,4g 36,4g 31,4g 49,8 child 758,2 94,1g 56,2g 51,2g 49,3 adult 1439,5 84,5g 50,2g 48,6g 35,6 child ,7g 180,4g 48,7g 97,4 adult 1095,1 105g 61,7g 72,7g 132,2 child ,9g 110,3g 95,1g 123,6 Table 2: injury criteria obtained from simulation of collision: pedestrian Škoda Fabia % S H CZ A FIN Year Picture 1: The share of pedestrians of all road users killed in traffic collision Velocity (m/s) Chest velocity Head velocity Thorax velocity Picture 2: Velocity of body parts obtained by simulation, comparison with test result

11 Child Adult Tested section Picture 3: The areas of the head contact both adult and 6 th child. The number means initial speed. V vehicle =37,7 km/h V head = 44,5 km/h V = 20,5 km/h V impactor = 40 km/h Child V impactor V vehicle Adult V vehicle V V vehicle =35,8 km/h V head = 47,2 km/h V = 35 km/h V impactor = 40 km/h V head V impactor V V head Section for adult head form Section for child head form Bonnet leading edge Bumper reference line Picture 4: The contact points on Škoda Octavia comparison with pedestrian protection directive