SIMULATION OF A COLLISION BETWEEN PASSENGER CAR AND CHILD PEDESTRIAN

The Department of Forensic Experts in Transportation at the Faculty of Transportation Sciences performed a second set of dynamic passive safety tests of a passenger car (M1 category Škoda Octavia II) in a child pedestrian collision. The initial and test conditions were similar to those of the first set of tests in September 2009 (Škoda Roomster). The deformations of the contact zones on the frontal vehicle surface were analyzed by a 3D scanning technology (3D handy scanner). Head, thorax and pelvic resultant acceleration, acceleration of knee joint in sagittal direction and contact force on the femoral structure of the dummy (P6 dummy, 1.17m; 22kg) were measured. The aim of these tests is to provide a detailed description of pedestrian kinematics and comparison of primary and secondary impact seriousness.


INTRODUCTION
Pedestrian safety is nowadays one of the very important criteria in case of the vehicle safety evaluation.Vehicle certification standards are based on the vehicle frontal part testing with impactors which represent certain body parts of an adult pedestrian.The risk of the impact consequences for children is only tested by the head impactor test [1].The reason for this stems from many different scientific studies.They proved that the head is the most frequently injured body part in adults as well as in child pedestrians.This conclusion is more significant in case of child pedestrians as it is demonstrated by the two independent sources in Table 1.A head injury within the frame of polytrauma is usually a predictive injury from the fatality rate point of view.This fact is the predictive factor for the current child pedestrian passive safety certification methodology.
In order to obtain more information on the child pedestrian injury spectrum, a detailed analysis of patients hospitalized at the Anaesthetic Resuscitation Clinic in Motol´s Faculty Hospital in Prague in the period from 1996 to 2007 was performed, and contributed to ascertaining the rate and injury seriousness of other children's body parts (see Table 1).Based on the forensic expert's experience, cases which caused similar consequences were selected.
The Faculty of Transportation Sciences performed the second set of three dynamic passive safety tests of a passenger car (category M1 -Škoda Octavia II) vs. child pedestrian collision.The tests were performed at different impact speeds (10; 20; 30 kmph), analogous to the first set of tests made in September 2009 with Škoda Roomster.The deformations of the contact zones on the frontal vehicle surface were analyzed by 3D scanning technology.Head, thorax and pelvic resultant acceleration were measured on a child dummy P6.The dummy was modified due to the demand on higher number of measuring areas than in the case of the original P6 dummy, which is intended for child restraints testing.The left upper leg was equipped with two strain-gauge halfbridges on the femoral skeleton for the contact force measurement.One uniaxial accelerometer was installed in the knee area for the measurement of acceleration in the sagittal direction.
The initial and test conditions were similar to those of the tests in 2009.The acceleration measuring was made by new equipment.

EXPERIMENT 2.1 Conditions
With respect to the technical specifications and the possibility of the comparability with the previous measurement, the following initial conditions were formulated: a) collision of a passenger car (M1 category), b) P6 dummy, (6 years; 1.17m; 22kg) which was adapted for the test -mentioned above.(Note: There is no child dummy which is specified for fullscale pedestrian -vehicle crash tests).c) dummy position: the dummy was facing the approaching vehicle, heel standing in the longitudinal axis of the vehicle (see Figure 1), d) proposed collision speeds: 10; 20; 30 kmph, e) the vehicle is starting to break at the moment of the crash contact.Dummy instrumentation (see Figure 2) -head: 3-axis accelerometer, directions x, y, z, 1,000g range, -thorax: 3-axis accelerometer, directions x, y, z, 1,000g range, -pelvic region: 3-axis accelerometer, directions x, y, z, 500g range, -knee joint: 1-axis accelerometer, direction x, 500g range, -upper leg: femoral skeleton -two strain-gauge halfbridges, uniaxial state of stress.

Impact speed 22.4kmph
Time of the first contact of the dummy with the vehicle: t 9 s201 = ms.

Impact speed 30.6 kmph
Time of the first contact of the dummy with the vehicle: t 3 s301 = ms.

Injury criteria -head: HPC and 3ms
The head performance criterion is defined by the following formula: where a = resultant acceleration in g, t1 a t2 = time points, which determine the beginning and end of a time interval, where the HPC value is maximal.HPC limit value is 1,000.According to the US standard FM-VSS 208 "Occupant crash protection" HPC15 limit val-   ue in case of a 6-year-old child reaches 700 [6].HPC measured values are presented in Table 1.
The 3ms criterion is applicable not only to the head performance, but also to other body segments.The limit value for head is 80g.Criterion interpretation: Acceleration higher than 80g must not act longer than 3ms.
According to the US standard FMVSS 208 "Occupant crash protection" HPC15 limit value in case of a 6-year-old child reaches 60g [6].3ms measured values (see 3ms injury criteria -thorax The limit value of this criterion in case of thorax is 60g.According to standard ECE 44 "Child restraints systems" limit value in case of a 6-year-old child reaches 55g [7].Measured values (see Table 3): amax injury criteria -pelvic The maximum acceleration value must not exceed 130g (see Table 4).

Femur injury criterion -contact force
The bending femur tolerance is not strictly defined.In case of adult femur the following bending limits are frequently published: 1.5kN to 4kN.Levine (2002) [5] published the bending limit value till rupture 3.92kN  for men and 2.58kN for women.Yamada (1970) published the maximum bending limit till specimen rupture in relation to the donor's age [8,9].In the group from 20 to 39 years the limit is ca.2.8kN in case of 260mm [2] femur cortical bone cross-sectional area and bending strength 212N/mm [2].In children group of around 6 years Yamada published the same level of bending strength, femur has higher level of plasticity and is able to absorb more energy till rupture, crosssectional area of cortical bone is smaller [8,10].For the measured values see Figure 9.

3D scanning -3D data digitalization
3D scanning is a process of data digitalization; the goal is to express the real object in a virtual (mathematical) way.This method of digitalization is able to record space or solid effectively.
The result of 3D digitalization is "a point cloud" where the position of every single point is detected by a 3D scanner.This type of application in connection to a formulated task allows to record car body damage after a crash test.
Method requirements: mobility of device, limited time for scanning (max.15 minutes for one scanning series), scanning accuracy (in 0.01mm), reliability of the device, data quality, non-contact scanning, out- door performance, variable lightning conditions, availability of scanned object position change, scanning interruption, "easy" data processing, real time result visualization (data verification).With respect to the facts mentioned above, Handyscan type MAXScan from CreaForm was chosen for this application.The advantage of this type of scanner is the possibility of a relative motion of the scanner and the scanned object.The scanner identifies the position markings on the scanned object and two cameras record the laser intersection, which is projected on the object.
In case of the car body deformation scanning, parts on vehicle front (bumper, hood, fenders, front grill) were covered with reflex targets.The original vehicle frontal parts were fitted appropriately and scanned because of the consequent comparison with those that were damaged by the crash test.The 3D analysis is based on the 3D surfaces comparison.
The results from test 101/201/301 (12.2 kmph/22.4kmph/30.6 kmph) show, that the dummy head impact caused plastic deformation of the hood of 13.2 mm/23.7mm/20mm(depth), rear central part of the hood was deflected in test 101 on the average by 1.5mm.Dummy head contact point is demonstrated by the dark area on the deformation map, lifted area of the hood in test 201/301 reached a maximum of 6.7 mm/8.8mm.

Head injuries
Neither Head Performance Criteria (HPC), nor 3ms injury criteria limit value was exceeded in the primary head impact for all the performed tests.The head contacted the car bonnet behind the WAD1000 line.
The values of the biomechanical criteria are several times higher for the secondary impact than for the primary one.The limit value for the secondary impact was only exceeded in case of test No. 301 (30.6 kmph); the 3ms criterion was in this case exceeded by 10%.According to the US standards (FMVSS 208 "Occupant crash protection"), the value of HPC15 exceeded also the defined limit (limit 700) for a 6-year-old child.
Based on the test and video analysis, and the analysis of the secondary contact with the road surface, it is obvious that neither HPC value nor the 3ms criteria represent objectively the seriousness of the secondary impact.The reason for this lies probably in the mechanism of dominating flexion and extension motion in the neck spine and head skidding on the road surface.This conclusion corresponds with the previous experiments made in 2009 and with literature cited below [9,11].

Thorax injuries
The limit value of 3ms criteria for a 6-year-old child thorax (55g according to EHK 44) was not exceeded in any performed test.This value is close to the limit in test No. 301 for the primary impact.For the secondary impact, there is no critical acceleration because of the kinematics of the pedestrian after the collision.The secondary contact took place mostly via head and neck.

Pelvic area injuries
The maximal acceleration limit amax 130g was not exceeded in any performed test for the primary or for the secondary impact.The pelvic area is the point of the first contact with the car front end, which can be clearly seen from the graphic presentation of the acceleration and video records made by a high-speed camera.The highest acceleration values for the pelvis area were measured at the primary contact.There is a presumption of abdominal organs contusion and risk of pelvic fracture (symphysis pubic).The pelvic and knee area were the most loaded parts of the body within the experimental series.

Knee injuries
The limit value of the maximal acceleration for knee (170g) was exceeded in test No. 201 and 301 (primary impact).Injury of knee joint or a fracture of a crus (on epiphysis or metaphysis) can be expected.

Femur contact force
The limit value of the maximum contact force on the femoral skeleton is not exactly defined [10,12].On the basis of the research, we can say that the average biomechanical limit for the contact force was exceeded at the primary impact in test No.301.In this case a femur fracture can be predicted.The impact force on the femoral skeleton was calculated from the axial strain with the knowledge of the material properties.

Secondary impact remarks
HPC seems to be an indicator of the secondary impact seriousness regarding the fact that in all the tests it reached higher values than in the primary contact with the vehicle frontal part.An interesting observation is that in case of other body parts the results were inverse -the primary impact was the one with the more serious consequences regarding the biomechanical criteria values calculated for the dynamic impacts of certain body parts in the direct interaction with the vehicle frontal part.

Source ( 1 )
GIDAS/German In-depth Accident Study, file range N = 188 Source (2) IHRA/PS Accident Data Source (3) Clinical study made by author, patients of Anaesthetic Resuscitation Clinic in Motol´s Faculty Hospital between years 1996 -2007, Range of patients file N = 146 Note: Source (3): A total of 479 injuries Measured quantities -real vehicle speed, vehicle acceleration (3D), -acceleration time flow of the dummy (according to the dummy instrumentation), -contact force time flow in femur, -high speed video recording, -dimensional characteristics of the process (initial and final location of colliding object), 3.1 Test No. 101, impact speed 12.2 kmph Time of the first contact of the dummy with the vehicle: t 15 s101 = ms.

Figure 9 -
Figure 9 -Femur contact force time course

Figure 10 -Figure 12 -
Figure 10 -Knee acceleration time course in x direction

Table 5 -
Maximum femur contact force

Table 6 .Table 6 -
Maximum knee acceleration in x direction