influence of pop-up hood systems on brain injuries … · 20 mm corresponds to the most severe...

IRCOBI Conference - Madrid (Spain) - September 2006 253

INFLUENCE OF POP-UP HOOD SYSTEMS ON BRAIN INJURIES

FOR VULNERABLE ROAD USERS

Rikard Fredriksson1, Ola Boström

1, Liying Zhang

2, King Yang

2

1Autoliv Research, Sweden

2Wayne State University, USA

ABSTRACT

In order to decrease head injuries to pedestrians and other vulnerable road users, a pop-up hood,

also called Active Hood, was developed and is presently in production. The focus of development has

been on the HIC criterion and skull fractures. However, several studies have shown that brain injury is

also common, in all types of traffic injuries.

The aim of this study was to investigate the influence of the gap between the car hood and

underlying hard structures on brain injuries. Free-flying head component tests and full-scale dummy

tests were performed. Linear and rotational head acceleration was measured and the Wayne State

University Head Injury Model (WSUHIM) was used to calculate the risk of brain injury.

A 100 mm gap considerably reduced head rotational acceleration compared to smaller under-hood

distances. It also reduced strain in the WSUHIM model. This study indicated that the Active Hood

provides an adequate counter-measure to both skull fractures and brain injuries.

Keywords: Pedestrians, head injury, component tests, full scale tests, finite element method.

PEDESTRIAN-FRIENDLY CARS have been mandated in European Union (EU) and Japan since

2005, when new pedestrian directives came into force for all new car models. The EU directive

includes requirements for child head impact to the hood and for leg impact to the bumper. The

Japanese directive includes requirements for child and adult head impact to the hood. EuroNCAP has

also tested pedestrian protection performance of cars since 1999. The legal and rating tests use

different head impact speeds, but are all based on a car-to-pedestrian impact velocity of 40 km/h,

covering 75 % of all AIS2+ pedestrian impacts according to a global estimate by IHRA (2002) and

approximately 50 % of all AIS3+ injuries (IHRA 2003).

The new pedestrian head requirements have led to the development of pedestrian-friendly hoods

with increased under-hood distances or deployable hoods that will produce this additional distance in

the case of a car-to-pedestrian impact (Nagatomi et al, Fredriksson et al, EuroNCAP). The most

rigorous requirements exist in EuroNCAP with a HIC 1000 requirement for the entire hood at a head

test velocity of 40 km/h. This requires a free distance of at least 80-100 mm under the hood.

Legal and rating tests use the HIC criterion for evaluating head protection performance. The HIC

criterion is based on risk of skull fracture only, accounting for linear accelerations of the head. But

head injuries comprise more than skull fractures. Focal and diffuse brain injuries occur as well. Brain

injuries are frequent in traffic accidents, (Gennarelli et al 1987). Gennarelli claimed in 1985, with the

exception of skull fracture and epidural haematoma, virtually every known type of head injury could

be produced by angular acceleration. Otte (1999) and Bockholdt and Schneider (2003) also indicated

that brain injuries were common in pedestrian accidents. DiMasi (1995) showed that pure translational

acceleration of the head would induce minimal strain, while a pure rotational acceleration would

produce considerably greater strain. A combination of translational and rotational acceleration would

induce more strain than rotational acceleration alone (DiMasi 1995).

When a pedestrian-to-car impact occurs, the pedestrian typically falls over the hood and the head

impacts the hood, scuttle, A-pillar or windscreen at speeds approximating the car impact speed. It is

also likely that extensive rotation of the brain occurs in this type of impact, along with high linear

accelerations. Therefore the aim of this study was to evaluate the influence of pop-up hood systems on


brain injuries for vulnerable road users, by means of headform component tests and full-scale tests and

evaluation of those tests by a head injury model which calculates brain strain.

METHOD

HEADFORM TESTS The Hybrid III 50th percentile head was chosen for the headform

component tests. Because of its human-like shape impact conditions can be found where the impact

force is not directed through the centre of gravity, thus inducing rotation of the head. This is different

to the pedestrian headforms which are rotationally symmetric and rotation can only be induced by a

friction force.

Instrumentation The head was instrumented with a 12-accelerometer array. This array had one 3-

axial accelerometer at the cranial centre of gravity and one 3-axial accelerometer at a fixed distance

from the centre of gravity along each of the axes. Endevco 7267A accelerometers were used. The

accelerometer array was mounted on a plate with shape and weight similar to the upper neck load cell.

In total the headform including all parts and instrumentation attached had the same weight and centre

of gravity as the standard Hybrid III head equipped with a standard accelerometer and upper neck load

cell. The head acceleration data was filtered with CFC1000 and the rotational acceleration data was

filtered with CFC600.

Launcher A linear impactor propelled the headform. It is normally used for linear guided airbag

tests where a body part impacts the car interior with airbags. The hood was placed vertical with the

front pointing down while the linear impactor induced a horizontal motion upon the headform (Figure

1). This was intended to simulate head motion when the hood is placed in a real car environment. We

assumed gravitational forces small enough to be negligible since the impact deceleration in this case

was expected to reach high values.

Figure 1. Linear Impactor with Headform and Hood

Impact Angle One headform holding device was made to produce an initial angle of 30 degrees

from the head Z axis, compared to a pure lateral impact. Thus, the head hit the hood with the foremost

part of the side of the head. Another device was made to position the head at an initial angle of 30

degrees around the X-axis (horizontal longitudinal), again compared to a pure lateral impact. In this

case the upper part of the head was impacted first. Both holding devices had a surface on which the

skull base rested. In the latter test case the head was rotated around the X-axis necessitating small

magnets on the holding device to keep the head from falling off prior to the test. In both cases a

support surface was mounted behind the head to prevent it from rotating during the acceleration phase

of the machine. These orientations were chosen for two reasons. First, they represented well typical

impact conditions seen in previous full-scale dummy tests. Secondly, they were expected to induce the

largest rotation to the head, a so called “worst-case” condition.


The above mentioned test configurations will hereafter be called “forehead tests” and “upper head

tests”. Figure 2 illustrates this explaining how the headforms would be oriented prior to impact to the

hood of a real car.

1. Forehead tests 2. Upper head tests

Figure 2. Headform Test Impact Configurations

Hood and Impact Conditions The head impact speed was 11.1 m/s (40 km/h) in all tests with a

variation less than +/- 0.2 m/s. 40 km/h head impact speed was chosen since it is considered to be the

average speed for severe (AIS3+) pedestrian injuries (see background section). It also represents the

most severe conditions from legal and rating tests.

The hoods were standard Saab 9-5, model year 1999, an “Executive car” according to EuroNCAP

classification. A real hood is neither completely flat nor horizontal. The impacted surface of the hood,

in this case, had an angle of 3 degrees from the vertical (car X-axis) and 3 degrees from the lateral

horizontal (car Y-axis).

The selected impact point was above a hood section consisting of a single sheet of metal, distanced

from reinforcing structures. A rigid steel plate was mounted under the hood on a device with an

adjustable distance. The distance between the hood and the rigid plate varied between 20 and 100 mm.

20 mm corresponds to the most severe conditions in a standard hood today (engine top, suspension

etc.) for a car not developed for the new EU pedestrian directive. 100 mm was chosen to correspond to

a distance underneath an Active Hood system. Most pop-up hood systems lift between 100 and 150

mm which should be added to the minimum 20 mm necessary for a margin. However, pop-up hoods

usually only lift in the rear, which results in decreasing distances when moving forward along the

hood. Since we looked at the adult impact condition (that is rear part of the hood) 100 mm seemed a

reasonable average distance.

Test Plan This study consists of three test phases, “forehead tests”, “upper head tests” and “full-

scale tests” which will be described in next section. See test details in Table 1.


Table 1. Test Plan Impactor Head angle Z Head angle X Distance under hood

(degrees) (degrees) (mm)

Test phase 1. Forehead tests

Test 1 HIII head 30 0 20



Test phase 2. Upper head tests






Test 9 Polar II Standard hood

Test 10 Polar II Active Hood

Test phase 3. Full-scale tests

FULL-SCALE TESTS

Two full-scale tests were performed with a dummy and a car buck, with identical conditions apart

from the hood. In the first test the hood was equipped with a standard hood, while in the second test an

identical hood was used but equipped with Active Hood actuators.

Car A Saab 9-5, model year 1999, car body was cut behind the B-pillars and placed on a sled

equipped with brakes and wheel suspension (Figure 3 left). The test speed was 40 km/h, which is the

basis of all legal and rating tests, with brakes fully applied at time of first impact (leg to bumper). The

engine was removed and replaced with a rigid steel plate to correspond to a “worst case” scenario for

pedestrians (Figure 3 right). This implied a varying distance underneath the hood from 10 to 60 mm.

The distance was approximately 60 mm when measured from the hood top surface with varying hood

thickness depending on reinforcements.

The Active Hood actuators were fired at 30 ms after first impact, positioning the hood at

approximately 60 ms after impact. The hood was lifted about 100 mm at the rear.

Figure 3. Car on Sled and Car with Hood Removed.

Dummy A Polar II pedestrian dummy was used (Akiyama et al 2001). It was placed at the

centreline of the car with the right-hand side of the dummy being impacted (Figure 4). The dummy

was placed in a walking position with left leg forward, whereby the impacted leg was rearwards.

Hands were tied in front of the body, firstly to increase the repeatability of the tests and secondly to

simulate a worst case scenario where the arms could not “protect” the dummy from head impact.

The dummy shoulders were supported before test by a steel wire attached to a pyrotechnic release

device. This device was triggered 10 ms before first dummy to car contact. It was fully released in a

few milliseconds and the dummy was thereby standing by itself at the time of impact.


Figure 4. Polar II Dummy in Pre-impact Condition

Impact Conditions Tests were performed to produce the desired impact location in the centreline

215 mm from the rear end of the hood (Figure 5 left). This was achieved by adjusting the “ground

level” for the dummy. The dummy was finally placed 95 mm lower than normal ground level to

enable this impact location on the hood.

The impact point consisted of a single sheet of metal but 40 mm forward of a reinforcing structure.

The free distance under the hood surface was 60 mm at the point of impact and 10 mm under the

reinforcement structure (Figure 5 right).

Figure 5. Head Impact Location on the Hood

BRAIN INJURY PREDICTION USING A FINITE ELEMENT MODEL OF HUMAN HEAD

The head injury model developed by Wayne State University (WSUHIM), was used to evaluate the

risk of brain injury for a selection of tests. The model features fine anatomical details including the

scalp, skull with an outer table, diploë, and inner table, dura, falx cerebri, tentorium, pia, sagittal sinus,

transverse sinus, cerebral spinal fluid (CSF), hemispheres of the cerebrum with distinct white and gray

matter, cerebellum, brainstem, lateral ventricles, third ventricles, and bridging veins. The facial model

consists of facial bones, nasal cartilage, temporal mandibular joint, ligaments, soft tissue and skin

(Zhang et al. 2001). The whole model is made up of over 315,000 elements and uses 15 different

material properties for various tissues of the head (Figure 6). It has been subjected to rigorous

validation against available cadaveric intracranial and ventricular pressure data, the relative

displacement data between the brain and the skull, and facial impact data (Zhang et al. 2001).

60

mm

40

mm

10 mm Steel

plate

Impact point

Hood

Reinforcement

structure


Figure 6. Wayne State University Head Injury Model (WSUHIM)

Recently, the WSUHIM was utilized to investigate the mechanisms of the concussion sustained by

American football players using on-field accident data obtained from the National Football Leagues

(Zhang et al. 2003, Viano et al. 2005). Subsequently, the model was excised to estimate the brain

responses of the Indy race car driver during severe frontal, side and rear crash (Zhang et al. 2004).

The model was further applied to simulate the brain response using the data from the reconstruction of

the real-world automobile accidents and related the model prediction to the actual injury sustained by

the occupant (Franklyn et al. 2005). It was proposed that the localized strain and strain rate variables

were the most relevant brain injury indicators for the concussion and axonal injury seen in real-world

accident.

Brain Injury Risk Assessment In this study, the brain injury risk associated with the impact of

the head to the hood was assessed using the tissue strain response predicted by the WSUHIM. The

tissue level injury threshold used was 0.35 first principal strain proposed for the mild traumatic brain

injury (Zhang et al. (2003, Viano et al. 2005). In addition to assess the peak strain response occurred

within the brain the extent of the brain experiencing the given strain was evaluated using the

Cumulative Strain Damage Measure (CSDM) (Bandak and Eppinger, 1994). This parameter suggested

that an accumulated volume of brain (in %) exceeding tolerable principal strain was related to diffuse

axonal injury. The CSDM was monitored at the strain levels of 0.35 and 0.15.

For each test method we selected the “extreme” tests, meaning the largest and smallest under-hood

distance for evaluation by the WSUHIM. This resulted in six tests to be evaluated. The cases selected

were Test 3, Test 8 and Test 10 for the active hood (100 mm distance or more), and Test 1, Test 5 and

Test 9 for standard hood (between 20 and 40 mm), see Table 1. To simulate the head impact with the

hood, the three translational and three rotational accelerations at the cg of the head through the skull

were used to drive the WSUHIM. The total percentage of the brain volumes calculated at the 0.35 and

0.15 strains were compared between the impacts with pop-up hood and standard hood to estimate the

injury risk.

RESULTS

HEADFORM TESTS In the “Forehead Tests” (Tests 1 to 3), the head impacted with an initial

angle around the vertical (Z) axis of 30 degrees. In all three tests however the X-rotation was greater

than the Z-rotation both for angular acceleration and velocity. The X-rotation accelerations and

velocities are shown in Figure 7. There was a clear first acceleration peak for the initial impact with

the hood surface, followed by a second peak when the head bottomed out towards the rigid steel plate.

This second peak was therefore at different time delays because of the different under-hood distances


of 20, 60 and 100 mm. The 20 mm test gave the highest and earliest peak while the 60 mm test

produced a delayed, lower lower peak value. The 100 mm test resulted in no second peak indicating

that there was no bottoming out in this test. The rotational velocity curves also show a decrease with

increasing under-hood distance, although not as large decrease as in rotational acceleration.

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60 mm

100 mm

Figure 7. X-rotational Acceleration and Velocity in Forehead Tests

In the “Upper Head Tests” (Tests 4 to 8), the head had an initial orientation of 30 degrees rotated

about the horizontal X-axis. Figure 8 shows the five tests performed similarly at 20, 40, 60, 80 and 100

mm under-hood distance. Additionally, there was a clear first peak visible where the head impacted

the hood surface, followed by a second peak at the contact with the rigid plate. The magnitude of the

second peak was further delayed and decreased with increased under-hood distance. The highest

value, at 20 mm, was above 80 000 rad/s2, while the 100 mm test gave a value of less than 10 000

rad/s2. The 100 mm test, once again, did not produce a second peak, indicating there was no bottoming

out, whereby the maximum value was thus produced in the first peak. The rotational velocity for these

tests showed a similar trend. The peak was higher and earlier the smaller the under-hood distance was.

The highest value was 67 rad/s for 20 mm and the lowest value 20 rad/s for 100 mm.

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Figure 8. X-rotational Acceleration and Velocity in Upper Head Tests

FULL-SCALE DUMMY TESTS A comparison was made between the two full-scale tests with

the Polar II dummy. The dummy head impacted the rear of the car hood centreline at 112 ms for the

Active Hood and at 115 ms with the standard hood. The HIC15 values were 1567 for the Active Hood

test and 3020 for the standard hood. The head impacted with the upper part of the forehead first which

produced a rotation of the head most visible around the Z axis. When observing dummy data however,

the X rotational accelerations were the greatest, followed by Z rotation.

In Figure 9 the X-rotational acceleration and velocity from both tests are shown. The peak values

are similar but the standard hood is slightly higher at 14 900 rad/s2 compared to 14 000 rad/s2 with the

Active Hood. Regarding rotational velocity there was a larger difference between the two tests. The

peak values were 79 rad/s for the standard hood and 61 for the Active Hood. For lower cut-off

frequency filters and corresponding brain injury evaluation see following sections.


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Active Hood

Standard hood

Figure 9. Head Rotational X Acceleration and Velocity in Dummy Tests

SUMMARY PEAK VALUES The peak rotational acceleration and velocity values were

selected and summarized (Table 2). Peak rotational accelerations were filtered with alternative filters,

CFC1000, CFC600 and CFC180.

Table 2. Peak Values and HIC for All Tests

Peak rotational acceleration (rad/s2)

Test Lab test

no Test type

Under-hood distance (mm)

CFC 1000

CFC 600

CFC 180

Peak rotational velocity (rad/s)

HIC15

Test 1 T-06006259 Forehead 20 56100 52400 30900 57 9293

Test 2 T-06006260 Forehead 60 33200 29600 17000 45 4123

Test 3 T-06006261 Forehead 100 15600 14700 11500 32 416

Test 4 T-06011167 Upper head 20 108000 85700 37800 67 23166

Test 5 T-06011170 Upper head 40 60700 56100 26000 62 13826

Test 6 T-06011169 Upper head 60 26600 22800 11900 43 4123

Test 7 T-06011171 Upper head 80 14500 12300 9000 37 1465

Test 8 T-06012852 Upper head 100 8700 8000 6700 20 506

Test 9 T-04026902 Full-scale Std. hood 15569 14900 13188 79 3020

Test 10 T-04026900 Full-scale

Active Hood 15017 14000 11338 61 1567

COMPARISON FILTERS In this analysis, the CFC600 filter was used thus far for the rotational

acceleration. Comparison was made for one test with different filtering of the data (Figure 11 left).

The CFC600 filter appeared to provide sufficient information while eliminating the worst noise, but

the CFC180 filter eliminated some information such as the first peak in the curve below.

Comparison was also made with the Margulies curves. All three filtering modes presented similar

trends, but of differing magnitude (Figure 11 right). Since only the peak rotational acceleration was

affected by filtering, the difference in strain seems to be larger with the CFC180 filtering than the

CFC600 and the CFC1000.


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(ra

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Strain 0.05

Strain 0.10

Strain 0.15

Strain 0.20

CFC1000

CFC600

CFC180

Figure 11. Comparison of Different Filtering

COMPARISON TO MARGULIES STRAIN CURVES The peak angular acceleration and

velocity values were selected and presented in a graph with rotational velocity on the x-axis and

rotational acceleration on the y-axis. This graph has been proposed by Margulies and Thibault (1992)

along with injury thresholds corresponding to different strain values of the brain (Figure 12). While

the rotational acceleration values decreased considerably with larger hood gaps in the headform tests,

the rotational velocity values did not decrease to the same extent. In the full-scale tests there was an

opposite trend. The decrease then was greater in rotational velocity compared to rotational

acceleration. The CFC180 filter was used for the rotational acceleration peak values in all cases.

0

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0 50 100 150 200 250 300Peak angular velocity (rad/s)

Peak

an

gu

lar

accele

rati

on

(ra

d/s

2)

Strain 0.05

Strain 0.10

Strain 0.15

Strain 0.20

Forehead 20 mm Test 1



Upper head 20 mm Test 4





Full-scale Active Hood Test 9

Full-scale Std. hood Test 10

Headform forehead tests

Headform upper head tests

Full-scale Polar 2 tests

Figure 12. All Tests Compared to Margulies Strain Curves

WAYNE STATE UNIVERSITY HEAD INJURY MODEL (WSUHIM) SIMULATIONS For

each test method we chose the “extreme” tests for evaluation by the WSUHIM, meaning the largest

and smallest under-hood distance. For the upper head tests, the 20 mm test (Test 4) was judged not

usable for simulation due to the quality of the signal, so in that case the 40 mm test (Test 5) was

chosen instead. Also in this test the signal had some vibrations after the peak value so the simulation

was stopped after 13 ms. The six tests chosen were Forehead Tests 1 and 3, Upper Head Tests 5 and

8, and finally Full-scale Tests 9 and 10.


The peak strain values were calculated as an average of four elements in the model. The 20/40 mm

tests (1, 5 and 9) resulted in a peak strain of 0.64 ~ 0.72 while the 100 mm tests (3, 8 and 10), reached

a lower value of 0.36 ~ 0.44 strain.

Figure 13 shows the calculated percentage of the brain volume which exceeded the threshold strain

levels for 0.35 and 0.15. In the 20/40 mm under-hood distance tests (left in figure), 73 ~ 82 % of the

brain volume experienced a strain of at least 0.15 and 22 ~ 42 % of the brain volume experienced a

strain of 0.35. In contrast, in the 100 mm under-hood tests, a strain level of at least 0.15 was

experienced by 35 ~ 44 % of the volume and a strain of 0.35 was only reached by 2 ~ 5 % of the brain

volume.

Standard Hood Active Hood

Fo

reh

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Test 1

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Figure 13. CSDM Evaluation by WSUHIM Model

DISCUSSION

It is obvious that the neck could give an influence to the head motion. However, it was estimated

that the influence would be small in these cases. Evaluating the tests it showed that the maximum

rotational acceleration and velocity both occurred at a very early part of the angular displacement. For

the small under-hood distances the peak values were reached before 2 degrees of rotation, and the


angle for peak rotational acceleration and velocity increased with the under-hood distance but in all

cases it was less than 15 degrees. If the neck would have any influence, it would have more influence

the larger the head angle, which proved to occur for the larger under-hood distances, and likely

decrease the peak values in those cases. That would make the difference in peak values even greater

between the small and large under-hood distance tests.

In a headform test series with this car model in 2001, it was shown that when the hood was lifted

100 mm at the rear, the HIC15 values in the adult area resulted in values below 800 in all tested points

(Fredriksson et al, 2001). Those test points were chosen from EuroNCAP tests with the same car

where they had resulted in HIC15 values between 800 and 7 000 with the standard hood. In the

headform tests in this study the hood was mounted at four points with a rigid steel plate at different

distances behind the head impacting point. At the start of this study a few standard pedestrian

headform tests were performed to demonstrate that the hood behaved in the same manner as in the car

environment. In both cases a HIC15 value of circa 550 with 100 mm under-hood clearance was seen

which is realistic according to our experience. In a film analysis, the hood also appeared to behave

realistically.

In this test series, for practical reasons, one head impact angle of 30 degrees was chosen in two

different impact conditions. This angle was expected to induce the largest rotation to the head, but in a

future study different angles could be chosen to study the influence of impact angle.

The tests of different under-hood distances showed a small but clear first rotational acceleration

peak when the head hit the hood surface and a second peak when the hood reached the rigid surface.

This rigid surface represented motor parts positioned at different distances under the hood in real cars.

When the under-hood distance was at least 100 mm, there was no second peak in any test in our test

series. This kept the rotational acceleration low in the 100 mm cases since the first impact to the hood

surface was rather soft.

For both component tests and full-scale tests, X-rotation was greatest, even when the headform was

pre-rotated only around the Z axis. Gennarelli et al (1987) performed a study of the directional

dependence of the DAI. It showed that X-rotation resulted in the longest traumatic coma, followed by

Z-rotation, and lastly Y-rotation. A difference between test methods was that dummy tests resulted in

a greater difference in rotational velocity for different under-hood distances, while the headform tests

resulted in a greater difference in rotational acceleration for different under-hood distances. In the

dummy test, the shoulder impact differed between the two cases because of the hood difference,

possibly resulting in differing head impact characteristics. This could explain the difference to

headform tests.

The headform test method appeared to be a robust test method displaying the same trend of

decreasing rotational acceleration values with increasing under-hood gaps, for all three filters tested.

The CFC600 filter appeared suitable for studying rotational acceleration curves due to its capability of

maintaining considerable detail while eliminating most of the noise. However, we believe that the

brain is slow in responding to sudden motion changes and therefore opted for the CFC180 filter for

calculating peak values in rotational accelerations in this paper.

HIC15 values from all tests were compared with peak rotational acceleration and velocity values as

well as CSDM final values (values can be found in Table 2 and figure 13). In some cases a correlation

could be found while in other cases no correlation could be seen. Further tests at different head angles

should be performed to further study this correlation.

Very high HIC values were measured for the small under-hood distances. It is likely that this would

result in skull fractures in a real head, and it is also very likely that this would influence the rotation of

the head. But also if these extreme tests would be removed from the study the same trend is visible.

CONCLUSIONS

For 40 km/h head impact the headform results showed a large reduction in rotational acceleration

and a modest reduction in rotational velocity for increasing under-hood distance. The full-scale tests

showed a moderate decrease in rotational acceleration and a larger decrease in rotational velocity for

increased under-hood distance. The Wayne State FE brain model predicted a considerable reduction in

strain with increased under-hood distances. For the small under-hood clearance a strain of 0.35 was

experienced by 22-42 % of the brain while with the large under-hood distance only 2-5 % of the brain

volume experienced a strain of 0.35 some time during the event. Altogether, this indicated a reduction


of risk for brain injuries if the under-hood distance was increased. This could be achieved by

designing the car with additional space between the hood and the parts underneath or by equipping the

car with a deployable hood creating this distance in an impact with a vulnerable road user.

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influence of pop-up hood systems on brain injuries … · 20 mm corresponds to the most severe...

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